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During the Northern Hemisphere Summer, the Asian monsoon is characterized by a large northward migration of the ITCZ. The largest excursion of the ITCZ and the associated precipitation takes over the Indian region between 60-100 E. During the season, intraseasonal oscillations (ISO) with time scales of 30-40 days (northward moving) and 10-20 days (westward moving) are very active over the monsoon region. Due to the low frequency nature of these ISOs, the phase and amplitude associated with them, may in principle, influence the seasonal mean monsoon. We analyse the outputs of the Unified Model at various horizontal and vertical resolutions to understand this problem. However, it depends on the models' ability to simulate the ISOs. We compare the simulations at various horizontal and vertical resolutions to gain some insight on the dependence of the simulation of ISOs on resolutions. After careful assessment on the simulations of the ISOs, we study their net effect on the simulation of the seasonal mean monsoon.

A numerical Eady model is used to determine how aspects of the initial perturbation alter subsequent development, particularly in terms of maximizing the growth in ener- gy. The initial growth is found to be dependent on the vertical and horizontal scale, tilt and location of the initial perturbation. The behaviour exhibited can be explained by considering the effect of the shear flow on simple perturbations in PV and boundary interactions. The ideas presented can be extended, beyond the quasi-geostrophic mod- els, to PE model experiments and the real atmosphere. These should find relevance in optimal modes and storm track seeding.

Aircraft data from the AASE I and II flight campaigns are analyzed with a view to de- riving constraints on the diffusion coefficient in the atmosphere. Trajectory models are used to identify filaments and their orientations in the data. The smallest identifiable one of these filaments, together with the knowledge of its orientation, will give an up- per bound to the diffusion coefficient.

The tropical Pacific ocean exhibits substantial variability on intraseasonal, seasonal and interannual timescales. Some of this variability, for example intraseasonal Kelvin waves, can be understood (at least to first order) as a comparatively simple ocean re- sponse to atmospheric variability. Other aspects, such as the instability waves that arise in the tropical eastern Pacific, depend more directly on intrinsic ocean processes. Still other aspects, such as the changes which occur during an El Nino cycle, manifest a fun- damentally coupled ocean-atmosphere mode of variability.

The accurate simulation of variability in the tropical Pacific ocean is a major challenge in ocean modelling. One of the problems is a lack of knowledge of the surface fields (windstress, heat flux and fresh water flux) that are needed to force an ocean model. In this context the recently available ECMWF Reanalysis (ERA) data are an invaluable resource offering 15 years of consistent (if not error-free!) high frequency surface fields. The ERA data are being used to force a general circulation model (MOMA) of the tropical Indian and Pacific ocean basins in order to investigate intraseasonal-to-in- terannual variability. The results of the simulations will be compared with newly avail- able observational data sets such as TAO and TOPEX-Poseidon.

As yet this work is in its early stages. Future studies will aim to: 1) Elucidate further the role(s) of ocean-atmosphere interactions in the variability of the tropical atmos- phere and tropical oceans. 2) Address the relationships between variability on different timescales.

Using pressure level and isentropic monthly mean diagnostics a comparison is made between ECMWF re-analyses and operational analyses. Attention is focused upon the diabatic heating, PV and mean meridional circulation.

Annual cycle integrations using ECMWFÕs operational forecast model (IFS Cycle 13R4) are described and the model behaviour is compared to that obtained using Cycle 12R1. The more recent version includes major changes to the physical parametrization schemes for orographic drag and cloud. Integrations using Eulerian and semi- La- grangian dynamics are compared.

The parallelisation of chemical transport models allows them to be run at resolutions which are not possible on conventional architectures. The SLIMCAT model has been parallelised and two advection schemes have been implemented: the original Prather scheme and a semi-Lagrangian scheme taken from ECMWFÕs IFS forecast model. We present the results of low and high resolution runs using both these advection schemes.

A simple modification of the pressure term in the standard shallow-water equations is presented that allows large-amplitude gravity waves to propagate significant distances without forming shocks. Such shock-formation of gravity waves is an unwanted fea- ture of the shallow-water system as far as its utility as a model for atmospheric flows is concerned, in which such shock-formation of gravity waves is entirely absent. The modification, which can be very easily incorporated in existing numerical shallow-wa- ter codes, consists of replacing the gradient of the depth field in the momentum equa- tion by the gradient of a nonlinear function of the depth field. The nonlinear steepening of simple one-dimensional gravity waves is then investigated using RiemannÕs theory of characteristics. Demanding that the modified shallow-water system should have the same linear wave properties as the standard shallow-water system, and that simple gravity waves can propagate in it without change in shape, a unique choice for the non- linear function is found. Some descriptive numerical integrations of one-dimensional gravity-wave propagation are presented, which illustrate the differences between the standard and the modified shallow-water gravity waves. Various further consequences of the modification are briefly explored, e.g. the unchanged form of potential vorticity and of potential-vorticity conservation, and the changed form of the expressions for pressure and total energy.

Observational studies have long shown the presence of ozone laminae in ozonesonde profiles. In particular the study of Reid and Vaughan (1991) showed that laminae are abundant near the edge of the polar vortex during the winter with a peak in occurrence at about 15km and typical vertical scale about 1-1.5km. Other studies based on obser- vations have emphasized the correlation between the vertical distribution of laminae and ozone loss rates, and the abundance of laminae following vortex disturbances.

Recently, high resolution model case studies (e.g. Orsolini et al. 1995) have demon- strated the link between ozone laminae and the formation of filaments of vortex air. The presence of laminae would therefore seem to be a good indicator of the transport of polar air to mid-latitudes.

We will present some recent high resolution modelling work to demonstrate how the formation and transport of vortex filaments can give rise to the laminae seen in ozonesonde profiles. In particular, we will focus on a particular period in February 1995 when a large filament produced laminae in a number of profiles during SESAME.

Reid, S.J., and G. Vaughan, Lamination in ozone profiles in the lower stratosphere, Q. J. R. Meteorol. Soc., 117, 1991.

Orsolini, Y.J., D. Cariolle and M. Deque, Filamentation and layering of an idealised tracer by observed winds in the lower stratosphere, Geophys. Res. Lett., 22, 839-842, 1995.

The TOMCAT and SLIMCAT off-line chemical transport models have been widely used for seasonal simulations of stratospheric chemistry. For example, when studying the winter/spring chemistry of the Arctic lower stratosphere the models are typically integrated from November to April. Such experiments are repeated for each year of in- terest. Obviously, it would be preferable to simulate many successive years in a single continuous model run. A longer run would remove uncertainties in the model chemical initialisation. Such multiyear simulations are also a prerequisite in the validation of the 3D model chemical schemes before future predictions can be made in coupled GCM experiments. Validating the stability of the model chemistry and dynamics over long time periods also increases our confidence in the details of the shorter, seasonal simu- lations.

I will describe how the SLIMCAT model has been developed for multiyear, full chem- istry runs. The standard TOMCAT chemical scheme has been extended to include the common CFC and halon species, and other long-lived source gases. The TOMCAT chemical scheme is now as comprehensive as standard 2D (latitude-height) schemes, without many of the approximations. I will show results from preliminary low-resolu- tion tests of the multiyear SLIMCAT runs. I will discuss the ÔclimatologyÕ of the 3D model chemistry scheme (compared to the standard 2D model schemes) and the ability of SLIMCAT to reproduce the correct stratospheric circulation.

Modelling the past climate by AGCMs can help to investigate the physical mechanisms of climatic change, to evaluate the capability of the models to simulate climatic chang- es, and to help to explain the geological data. The PMIP, endorsed by both IGBP/PAG- ES and WCRP/WGNE, aims to perform model-model and model-data comparisons of the results obtained from AGCMs with identical paleoclimatic conditions. The first two time periods being studied under the PMIP are the mid-Holocene (6000 years ago) and the Last Glacial Maximum (21000 years ago). Results from models participating in the PMIP will be presented. It is shown that models exhibit similar global sensitivity. All models show an enhanced summer monsoon precipitation over south Asia and north Africa in the 6K simulation, and cold and dry climate in the LGM simulations. The main features mimic the geological evidence.

The interannual variation of the Indian summer monsoon is analysed based on two 10 year simulations of the present day climate using the UGAMP GCM with different land surface parametrization schemes. A strong negative relationship is found between south Eurasian winter/spring snow cover (mass) and the amount of summer monsoon rainfall over India in both simulations. However, the significance of this relationship is dependent on the land surface parametrization scheme. This inverse relationship is the strongest over north India and the foothills of the Himalayas and is statistically signif- icant. The model simulation is in good agreement with observational studies. Compos- ite analyses suggests that weak winter/spring snowfall over south Eurasia is associated with a strong monsoon, characterized by strong southwesterlies over the Arabian Sea in the lower troposphere and heavy summer precipitation over north India and the foot- hills of the Himalayas. In contrast, heavy winter/spring snowfall delays the onset of the Indian summer monsoon through the feedback of snowmelt, soil moisture and evapo- ration processes, and is associated with weak summer precipitation over the two re- gions.

Studying the interaction of waves with the mean flow in the Stratosphere, with an aim to model the QBO within a three dimensional model. The UKMO Stratosphere-Mes- osphere model is used to represent the atmosphere from 16km to 80km. The bottom boundary is fed into the model from data or generated fields, and the resulting circula- tions studied. A range of waves can be forced, such as the Kelvin and Rossby-gravity waves of the original Holton-Lindzen model of the QBO, and their effects observed.

It has been a matter of concern for some time, e.g. McIntyre(1982), that the dynamics of the winter hemisphere in stratosphere-mesosphere models are complicated by the resonant excitation of free modes of the system. Prior to a warming event, an increased flux of wave activity is observed out of the modelÕs artificial lower boundary, and the disturbance fields show a large projection onto a linear mode of the system with a fre- quency close to that of the geopotential forcing, see Clark(1992, Phd thesis). As the structure of these free modes is dependent on the position of the modelÕs lower bound- ary, the qualitative behaviour of these models at the time of warming events may be sensitive to the level chosen. To test this hypothesis, preliminary results are given from winter hemisphere experiments in a sigma coordinate version of the SGCM, with a re- alistic troposphere and ground level orography. These results are compared with those from a pressure coordinate version with equivalent geopotential forcing at an artificial lower boundary.

The results of the 10-year run of the UGAMP model are analysed to study the varia- bility of the North Atlantic storm track. It is envisaged that a diagnostic package in- cluding Lagrangian diagnostics as well as the usual Eulerian diagnostics is necessary to analyse the data.

A first step towards the study of the storm track dynamics using a Lagrangian perspec- tive is the tracking of individual cyclones. Cyclone statistics, such as probability den- sity functions for genesis of the storms, mean speed etc. have been calculated for the cyclones occurring during the nine winters covered by the run. The cyclones have been identified and tracked using vorticity at 850mb. Comparisons of the cyclone statistics to standard diagnostics like temperature and momentum fluxes and transient kinetic energy are carried out to get further insight into mechanisms responsible for the vari- ability of the North Atlantic storm track.

The UGAMP model results will then be compared to the reanalysis data.

This study presents results from a years integration of an isentropic coordinate chem- ical transport model (SLIMCAT). The horizontal winds have been taken from the UKMO analyses. Vertical velocities have been calculated using a radiation scheme (MIDRAD). The model includes simple parametrized chemistry of CFCs, N2O, CH4, H2O, and O3. The seasonal variability of the tracer fields and correlations are analysed and compared to data from the UARS satellite. In addition to this a detailed study of the transport mechanisms in the tropical stratosphere will be presented.

Observational evidence has pointed to the presence of at least a partial barrier to trans- port in the subtropics which may be due to steepening isentropic gradients in PV. In this study we investigate the possible factors that affect the position and permeability of this barrier, using HALOE and other UARS measurements. In particular, factors de- termining the meridional shape of the isolines of water vapour mixing ratio in the sub- tropics and their dependence upon the phase of the QBO are studied.

In recent years much attention has been focused on the effect of deforestation in the tropics, particularly in Amazonia. The climatic impact of this land degradation has been the subject of a number of GCM experiments, which have yielded marked differ- ences in the regional surface field anomalies. Our work undertakes preliminary studies with a column model to investigate the significance of various surface parametrizations on the regional climate for both control and deforested scenarios. We use these sensi- tivity studies to aid the analysis of a GCM deforestation experiment using a modified version of the IFS model. Non-regional effects are also investigated.

The impact of solar variability on climate has been investigated using the T42L19 GCM in perpetual January and perpetual July modes. The solar effects are included both in increases in solar irradiance and in stratospheric ozone. Ozone changes have been prescribed both based on previous 2-D model runs and also from TOMS and SBUV data. In the perpetual January case at solar maximum a warming of the summer stratosphere was found to strengthen the stratospheric easterlies which penetrate into the tropical upper troposphere causing polewards shifts in the positions of the sub-trop- ical westerly jets, broadening the tropical Hadley cells and polewards shifts of the storm tracks. A simulation in which only solar irradiance was changed showed a much weaker response. The results of the perpetual July experiment will also be discussed and both runs compared with apparent observational evidence of solar-induced climate change.

We have run the UGAMP Stratosphere-Mesosphere Model (USMM) through the 1994-1995 winter. In most respects it compares well with the UKMO assimilated data, including the unusual morphology of the mid-January major warming which extended through the depth of the stratosphere.

Low Frequency variability of the zonal mean zonal wind in runs of a simple global cir- culation model is accounted for largely by alternating bands of easterly and westerly anomalies, termed ÒwobblersÓ, which propagate slowly polewards across the midlati- tudes. These have been interpreted in terms of nonlinear interaction between Rossby waves initiated in the mid-latitude storm zones and zonal flow anomalies.Timeseries of the zonal mean zonal wind from the AMIP and Y1 integrations of the UGAMP gen- eral circulation model are being analysed to reveal their characteristic low frequency behaviour, and it is planned to use these data in the wobbler theory of low frequency variability.Possibilities for future analysis are also discussed.

An isentropic transport event, in which southern midlatitude middle-stratospheric air is irreversibly transported deep into the tropics in late November/early December 1991, is seen in a range of UARS tracers. The broad features of the event are reproduc- ible in contour advection studies using UKMO assimilation winds. Isentropic trajecto- ry calculations from UKMO winds also show some cross-equatorial air transport from 30S over a 15-day period. The event appears to be associated with a PV anomaly con- nected with a remnant of the southern hemisphere winter vortex.

Several techniques have been explored for the identification of tropical convective complexes, in time series of satellite images. A technique based on smoothing the whole brightness temperature field, one based on smoothing only over identified con- vective complexes and one based on the smoothed shape of the complexes have been considered. These complexes are tracked and statistics derived. This approach can be used to derive short term climatologies (10 years) from the historical archive of satel- lite imagery held by ISCCP for mesoscale and synoptic scale tropical convective weather systems.

Such a climatology is presented for the Tropical African/Atlantic region and compared with large scale atmospheric parameters as well as to geographical features to improve our understanding of convection in the tropical African climate.

A simple two-level channel model is used to examine the effect of lower level forcing on various initial flows, crudely simulating a tropospheric jet stream with the baroclin- ic eddies associated with our weather systems.

For certain initial jets, a steady blocking-like response is produced, which remains sta- ble at large amplitudes. Using a different initial upper-level jet, the flow undergoes a low-frequency oscillation with a build up and breakdown of a large scale anomaly, but whose maximum amplitude is smaller than that of the previous case.

The low-frequency oscillation appears to be caused by a local instability, dependent on the magnitude of the anomaly excited. Recent work attempts to link this behaviour to the initial jet structure.

The stratification of the troposphere is a crucial parameter in determining the character of the large scale circulation. Latent heating in convective regions tends to force the stratification to be close to the moist adiabatic lapse rate, especially in tropical regions. Long wave radiation in clear air acts to cool the troposphere. Short wave heating of the surface and warm ocean currents act to destabilise the boundary layer. At the same time dynamical processes such as vertical heat transport by baroclinic waves act to maintain the observed stratification. A study of ECMWF analyses of the Global structure of stratification will be presented. This will show the variation in space and time of the stratification from seasonal time scales to synoptic time scales. Mechanisms forcing the stratification observed will be suggested, along with the effect that the observed stratification has on the dynamics. Feedbacks involving the stratification and the dy- namical transports of heat will be suggested and evaluated.

The effects of the solar cycle and the QBO on the northern hemisphere have been ex- plored using a modified version of the Kinnersley & Harwood two-dimensional isen- tropic model with three planetary waves. Some of the decadal oscillations seen in observational data appear in the model results and appear to emanate from the equato- rial wind data used to force the model QBO.

Over four years of observations from the Halogen Occultation Experiment (HALOE), which flies on the Upper Atmosphere Research Satellite (UARS), have been used to examine intra- and inter-annual variability in mesospheric water vapour. The observa- tions show that near the equator there is a clear semi annual oscillation (SAO) in mes- ospheric water vapour mixing ratio which propagates downwards with time. At mid latitudes the annual cycle in water vapour is more prevalent. Hemispheric differences in the behaviour of mid latitude water vapour are discussed, as are possible model sim- ulations that can aid interpretation of these features.

A version of the SGCM has been set up to simulate a simple storm track. This produces reasonably realistic Eulerian diagnostics. Hovmoeller plots of vÕ reveal that the storm track is intermittent in its behaviour, with periods of localized baroclinic activity alter- nating with periods of hemispheric baroclinic activity and quiescent periods. It is de- sired to investigate this low frequency behaviour further. However, because the Eulerian diagnostics are non-Galilean invariant, they cannot be used to monitor chang- es in the intrinsic eddy activity; they also generate a signal as a result of fluctuations of the mean flow. A scheme for Doppler correcting Eulerian eddy diagnostics will be pro- posed.

A simplified global circulation model is used to test a parametrization of the eddy tem- perature fluxes due to baroclinic waves based on the Eady linear theory. The parametri- zation is shown to give adequate results over a wide range of imposed parameters. However, the irregular low frequency fluctuations of temperature flux and baroclinicity within an individual run appear to be largely uncorrelated and therefore unparametriz- able. Hence, internal and externally forced changes of baroclinic activity have very dif- ferent signatures in terms of flux/baroclinicity relationships and could be distinguished.

A description of the main characteristics of the Climate version of the Unified Model will be presented. This will include an outline of the main physical parametrizations and dynamical routines in the model. Some standard diagnostics from this version of the model will be presented.

An ocean covered version of the Unified Model has been developed. The climatology of this model will be described. A number of Sea Surface Temperature profiles have been used as a lower boundary condition for the model. The response to these profiles will be highlighted.

Some preliminary results of a 20 year+ run of the U.K. Meteorological Office Strato- sphere-Mesosphere Model will be presented. In this simulation the same geopotential height forcing was cycled through each year. The run may be thought of as a single integration in which there is no external source of interannual variability, or as a series of annual runs with the same bottom boundary but with randomized initial conditions.

In its first guise the results are useful for characterising the natural variability inherent in the (modelled) middle atmosphere, as opposed to that originating from external stimuli. It is hoped that similar considerations will apply in the real atmosphere.

In its second guise it provides a (non-rigorous) constraint on the accuracy it is reason- able to expect from the UKMO SMM, and perhaps more generally.

The horizontal excursions of air parcels within the stratospheric surf zone exposes air to a variety of radiative environments. If the exposure to different radiative environ- ments is effectively random then the cross-isentropic motion becomes diffusive in the ensemble mean. This cross-isentropic, quasi-vertical diffusion can be called diabatic diffusion.

Diabatic trajectories have been integrated for ensembles of air parcels and it has been shown that ensembles exist for which the cross-isentropic motion is indeed diffusive. Within the surf zone the ensemble motion can be described by a lagrangian-mean de- scent and diabatic diffusion. For a surf zone ensemble of parcels initialised at 500K (~50hPa) the lagrangian-mean descent is about -0.3Kday-1 (-0.15mms-1) and the dif- fusion coefficient is about 2.6K2day-1 (50mm2s-1), where K refers to potential temper- ature.

We present simulations of the southern stratosphere using a mechanistic model (the USMM) to test our understanding of large-scale phenomena such as the evolution of the large-scale circulation during the final warming, including the variability in the temperature and wind fields and the interaction of vortices. These simulations are im- portant for the study of transport from polar latitudes to mid latitudes during southern winter and spring.

We present results from a 15 month Unified Model (UM) run recently carried out in collaboration between CGAM and the UKMO. We outline the capabilities of the UM by focusing on the model evolution and synoptic features such as warmings and vortex interactions, and by comparing the UM results with UKMO analyses.

Further progress has been made in using the method of four-dimensional variational data assimilation for the analysis of chemical trace species. The latest results will be presented. Progress is being made towards producing a self-consistent analyses of chemical species for January 1991. The use of this method to analyse chemical con- stituents is a world wide first for UGAMP.

Carbon aerosols are produced by all combustion processes. A combustion process of particular importance for our atmosphere is air traffic. A recent paper we have submit- ted to JGR investigates some possible effects of heterogeneous reduction of atmos- pheric constituents on carbon aerosols. Reduction of HNO3 and NO2 on carbon aerosols may be an important effect of increased air traffic which has not been consid- ered to date. It is shown that if HNO3 and NO2 are heterogeneously reduced on atmos- pheric amorphous carbon aerosols then a significant mid-latitude, lower stratospheric ozone loss mechanism could exist. This ozone loss mechanism is almost independent of temperature and does not require the presence of sunlight. The mechanism can op- erate at all latitudes where amorphous carbon aerosols are present. The relative impor- tance of the mechanism increases with night-length. The reduction of HNO3 on carbon aerosols could also be a significant renoxification process wherever carbon aerosols are present. Due to the very different soot levels in the two hemispheres this implies that there should be a hemispheric asymmetry in the role of these mechanisms.

Several simulations of the middle atmosphere carried out with a sophisticated 3D- model constrained by observations at the tropopause are used to show breaking gravity waves in the upper stratosphere and mesosphere have a significant role to play in the dynamics of the entire stratosphere.

The quality of the simulation in the mesosphere is discussed with reference both to ob- servations (particularly those carried out by the Improved Stratosphere and Meso- sphere Sounder, ISAMS), and to the physical parametrizations used. The influence of the upper boundary is also found to be significant.

General Circulation Models will be more and more used for coupled climatic simula- tions involving ice sheets. It is therefore of prime importance to evaluate the perform- ance of these models in simulating the mass balance and climate over ice sheets. Although the snow related parametrizations of the UGCM are relatively crude, the simulated Greenland and Antarctic climates are in good agreement with the available observations and the precipitation/accumulation pattern appears fairly reasonable. The simulated pressures above and around the ice sheets are in good agreement with the observations and the general circulation pattern seems to be correctly represented. The low level winds are well simulated given the resolution (T42) of the model. The biggest discrepancies between results and observations are found related to the surface tem- perature and energy budget. Systematic errors seem to be due to an inaccurate repre- sentation of the topography but the biggest imperfections appear to be related to the neglect of the snow physical properties.

The Madden-Julian Oscillation (MJO), or intraseasonal oscillation, is well established as one of the most dominant modes of variability in the tropical atmosphere, and is characterised by large scale, eastward propagating tropical convection and circulation anomalies with a timescale of 30-60 days. However, the basic mechanisms behind it remain unclear, and attempts to simulate it with numerical models ranging from the idealised to state-of-the-art GCMs have met with a range of (limited) success.

Based on an analysis of 10 year integrations from 15 different GCMs, the UKMO Uni- fied Model has one of the most realistic simulations of the MJO (Slingo et al., 1996). However, analysis of the MJO in these long integrations, and indeed in the real atmos- phere, is complicated by the seasonal cycle. As the characteristics of the MJO change substantially with the seasonal cycle, many studies have chosen one particular season (e.g. northern winter) to analyse the MJO. However, as the time scale of the MJO (30- 60 days) is not much less than one season, this can lead to sampling problems. Hence, an 1800 day integration of the UKMO UM was performed with constant (perpetual March) boundary conditions to give a ÒcleanÓ MJO signal uncontaminated by the sea- sonal cycle.

The simulated MJO has a timescale of 30 days, which, in common with other models, is shorter than the timescale of the observed MJO. A composite life cycle, based on more than 30 events, was constructed. The simulated MJO bears many similarities to the observed MJO, and is being analysed to identify the basic mechanisms behind it, in particular those responsible for the eastward propagation of the anomalies, and the oscillationÕs timescale.

Advection in baroclinic wave life cycles results in filamentary and spiral structures in tracers. The second life cycle (LC2) of Thorncroft, Hoskins and McIntyre (1993) is distinguished by the existence of an exceptionally persistent synoptic scale cyclonic vortex. In the vortex region the PV field exhibits a spiral whose number of turns in- creases at a rate given by the angular velocity at the vortex centre (approx. 2day-1). The self-similar nature of the spiral can be characterised by its Kolmogorov capacity or box counting dimension. The capacity of the spiral is found to be almost constant (approx. 1.4) during a period when the number of turns increases from 2 to 5. Another more tra- ditional measure of tracer structure is power spectra. Vassilicos and Hunt (1991) have derived a relationship between the spectral slope and the Kolmogorov capacity under the assumptions that the tracer field is piecewise constant, self-similar, isotropic and homogeneous. N-spectra for PV in the life cycles show two subranges with very dif- ferent spectral slopes, neither of which relate to the value of capacity. The high wave- number range has a very steep spectral slope which is related to the cross-filament PV distribution, which in turn is far more dependent upon model diffusion than the spiral structure itself. We conclude that the Kolmogorov capacity is a useful measure of self- similarity, whilst n-spectra do not convey information related to chaotic mixing by stretching and folding.

In numerical simulations of stratospheric transport and chemistry there are clearly-de- fined regions which can benefit from high spatial resolution integrations. These regions occupy a relatively small part of the computational domain; increasing the number of computational cells globally, so that these regions can be adequately captured, is not cost-efficient. Selective increase of the number of cells in the vicinity of these regions is the optimum way to discretize the domain. Given the time-dependent nature of the flow, the operation of the equation solver (as well as other related issues, like para- metrizations and chemistry) and the need to attain a very low associated overhead, this is not a trivial task. Potential gains, however, are substantial. Integrations at unprece- dented resolutions can be achieved (at a fraction of the cost of constant cell-width ones), which will explicitly capture a wider range of flow scales, thus increasing the accuracy of the results. Towards this end an adaptive grid refinement approach is de- scribed (with the non CFD expert in mind), which has been used extensively in the past for compressible, unsteady flow integrations. One of the techniqueÕs important features is its ability to accommodate temporal as well as spatial adaption, thus significantly re- ducing the problem of restrictive timesteps (related to the CFL stability condition).

Rodwell and Hoskins (1995) proposed a mechanism for the remote forcing of descent in the East Mediterranean region through a Rossby Wave response to Summer Mon- soon heating over Asia. This link is investigated using an ensemble of integrators from the Hadley Centre Climate Model.

Data from 4 integrations of the AGCM, run with SST forcing from 1948-1993, was used to investigate the seasonal behaviour and variability associated with ascent in the Asian Summer Monsoon region.

A strong signal of the two-day wave has been diagnosed in the mesosphere of the Ex- tended UGAMP GCM (EUGCM). Its structure, amplitude, and time of occurrence agree well with those of the observed two-day wave. Rival theories for the origin of the two-day wave suggest that it could be an unstable mode or a planetary normal mode. In the EUGCM it results from an instability of the summer easterly jet, but its global structure also has similarities to a Rossby-gravity planetary normal mode. It has a re- markable potential vorticity structure in wave 3 and sometimes wave 4 and higher wav- enumbers.

The gravity wave drag in the EUGCM is crucial for setting up the unstable mean state on which the two-day wave grows. An integration using Rayleigh friction instead has quite different mean winds and no two-day wave.

A three-dimensional tropospheric model of chemistry and transport, TOMCAT, has been developed at Cambridge University. It is an off-line grid point model which can be run using meteorological analyses or output from a GCM. It includes the Prather (1986) advection scheme which has been shown to preserve sharp gradients; a highly desirable feature in tropospheric chemistry models. In the first part of this paper the various components required to build a tropospheric chemistry model; tracer convec- tion, planetary boundary layer mixing, vertical diffusion, wet and dry deposition and chemistry will be briefly discussed.

TOMCAT includes the Tiedtke convective treatment of tracers. In order to validate this scheme the model has been run including 222-Radon and compared to measured pro- files. Physical quantities such as convective cloud amount and convective rainfall have also been compared to GCM output or observations, where possible. A boundary layer mixing scheme based on Holtslag and Boville (1993), which only requires meteoro- logical analyses as input, is also being developed and tested. Results from these com- parisons will be shown. The wet deposition scheme takes into account the solubility of a gas and the frequency of storm events as well as the temperature and specific humid- ity in a grid box (Giorgi and Chameides, 1985; Jonson and Isaksen, 1992). Runs using simple chemistry (NOx emissions converted to HNO3 using appropriate lifetimes) have been used to validate this scheme against available nitrate data. The dry deposi- tion of a gas is calculated as a function of surface type (forest, grass, snow, ice, ocean etc.) and time of year (Walcek et al., 1986; Valentin, 1990). Finally, a variety of chem- ical schemes have been developed to run in TOMCAT, ranging from methane oxida- tion up to complex hydrocarbon schemes. The different schemes are compared to one another and to results from IPCC (1995). Photolysis rates vary diurnally and are taken from a 2-D model. The complete model has been run for a period of several months using meteorological analyses from ECMWF. Chemical fields are initialised using 2- D model output. In the second part of this paper we present a comparison between model results and available surface observations of O3, CO, NOx etc. ozonesonde data and profiles from the CEC MOZAIC (Measurement of OZone by Airbus In-service airCraft) project. The model has been run using methane oxidation chemistry and emissions of CH4, CO, NOx and HCHO. A comparison between relative humidity data from ECMWF and MOZAIC has been carried out and will be presented along with results from a study assessing the sensitivity of tropospheric chemistry to water vapour concentrations. Comparisons between seasonally averaged O3 fields from the model and MOZAIC will be shown, differences highlighted and discussed in terms of the modelÕs tropospheric O3 budget and treatment of the tropopause. MOZAIC data can also be used in the interpretation and validation of particular event

Preliminary results from a case study of a stratospheric intrusion will be presented if time permits.

Experiments have been performed to analyse the interaction between the convective parametrization and enhancements to the surface fluxes used in the UGCM. There are gross similarities in the modelÕs response to enhancing the surface fluxes, in regions of low wind speed and convection, using both the Kuo and Betts-Miller (BM) convection schemes. In particular the zonally averaged tropical heating tends to collapse onto the equator. However, ensembles of spin-up experiments reveal differences in the initial response using the two parametrizations which lead to differing large-scale dynamical responses. It is interaction between the convection and large-scale flow which deter- mines the equilibrium response.

Although the BM closure does not explicitly involve the moisture budget, it is possible to diagnose the partitioning of available moisture in the scheme (a Òbeta parameterÓ), just as for Kuo. Experiments in which the beta parameter is modified in the Kuo scheme, including a form based on that diagnosed from the BM scheme, show that some aspects of the BM behaviour can be obtained using this Kuo scheme. However this is achieved by effectively overriding the Kuo moisture convergence criterion and imposing logic similar to that used in the BM scheme.

The very different PV and moisture characteristics of SH mid-latitude air injected from time to time into the Asian monsoon inflow may be able to trigger a break in rainfall. The fact that this mid-latitude air is associated with inherently chaotic activity in the southern hemisphere westerlies may imply an unpredictable element to the seasonal mean monsoon. However, on the shorter timescale of about a week to 10 days, there may be increased predictability if one considers the origin of the inflow. Results will be presented from the SGCM and the Met-Office Unified Model.

With the eruption of Mt. Pinatubo on the 15th of June 1991 large amounts of sulphur dioxide entered the tropical stratosphere. This was then converted to aerosol (sulphuric acid water droplets) a tracer measured by the Improved Stratospheric and Mesospheric Sounder (ISAMS) aboard the Upper Atmosphere Research Satellite (UARS). Contour advection and the SLIMCAT model are used to produce high resolution 3D sulphuric acid fields. By studying the movement of this long lived tracer, information on trans- port mechanisms in the lower stratosphere can be gained.

Results indicate that transport from the tropics varies with altitude. In the lower strat- osphere transport is dependent upon tropospheric effects, whereas at higher altitudes, the presence of planetary waves and displacements in the vortex edge provide the mechanisms responsible for the transport of air from the tropics. As well as isentropic flow, the diabatic circulation and sedimentation of sulphuric acid droplets have also been found to be important.

UGAMP model UGCM2.1 (19 levels, T42) is used to study the effect of changes in Antarctic polar ozone on the radiation balance, temperature structure and evolution of the polar vortex in the winter high latitude stratosphere.

The experiments have been carried out with prescribed ozone depletions of 25%, 50%, 75% and 99% between 60S and 90S and between 20mb and 200mb during the period 20 August to 20 November.

Preliminary results show that the ozone depletion causes cooling and a delay in the fi- nal warming and vortex breakdown. The length of the delay is determined by the mag- nitude of the ozone depletion and is up to 30 days. These results and further analysis of the spatial and temporal impacts of the ozone reductions will be presented.

Methane is an important greenhouse gas whose concentration is increasing in the at- mosphere. It also has important effects on the levels of other trace gases, such as car- bon monoxide and ozone. A 3D global model (TOMCAT) has been used in an attempt to obtain the strengths of the many different sources of methane, by comparing the model predictions with data from the NOAA/CMDL measurement network. Published databases were used to describe the majority of the sources, apart from two which were constructed here. Forcing files from ECMWF analyses of real winds were used to drive the transport of the trace gases.

The model predicts methane levels in marine environments fairly well, but does not perform well in locations which are influenced by large sources. The annual growth rate is predicted to be 30 Tg year-1, in good agreement with other estimates. This initial experiment used a total source strength of 500 Tg year-1, and did not include oxidation of methane by soils. This result implies the true methane source strength must be great- er than 500 Mt year-1.

The model is being improved, by using more accurate emission databases, and also to predict delta-13C values, as many sources have different proportions of 12-CH4 and 13-CH4.

It has long been known that there exists an annual cycle in tropical lower stratospheric temperatures, with highest temperatures occurring in July/August and lowest in De- cember/January. It is now thought that this seasonal variation is caused by a seasonal variation in the meridional circulation forced by extra-tropical planetary-wave break- ing. This talk will be concerned with the extent to which the observed forces (Eliassen- Palm flux divergences calculated from UKMO assimilated data) can produce the nec- essary tropical upwelling, both quantitatively and qualitatively. I will also consider just what dynamical ingredients are essential to produce the required circulation. For ex- ample, what is gained by including a realistic background angular momentum profile into the model? And how much of the seasonal variation of the meridional circulation is purely the result of the time-mean force interacting with the seasonally varying an- gular momentum profile?

Analyses of data from the SSU,HIRS and MSU radiometers on board the NOAA polar orbiting satellites are used to diagnose El Niqo-Southern Oscillation signals in the extratropical winter stratosphere for the period 1979-95. During El Niqo there appears to be an enhancement of the stationary eddies and a weakening of the polar night jet. Pairs of numerical simulations differing only in a prescribed lower stratospheric El Niqo anomaly are used to verify and further investigate the stratospheric response.

In this study we employ a simple dry general circulation model to simulate the upward propagation of planetary waves that are nonlinearly forced by unstable baroclinic ed- dies in the troposphere. The simulations are performed in a deep model domain in a forced/dissipative regime in which the tropospheric and stratospheric circulations are thermally relaxed towards zonally symmetric notional wintertime conditions. Long in- tegrations show the continual growth and decay of baroclinic eddies in the troposphere, with intermittent events in which there is significant propagation of long waves, some- times wave-1, sometimes wave-2, up into the stratosphere. In this experiment all long- wave variability in the stratosphere is a direct consequence of tropospheric forcing events associated with baroclinic eddies.

In order to gain further insight into those aspects of the tropospheric flow that are im- portant for the stratospheric long waves we carry out the following experiment. The nonlinear forcings of the different wave components are extracted from the numerical simulation and applied as artificial external forcings to a linear model, without baro- clinic eddies. The full forcing successfully reproduces the long-wave stratospheric re- sponse. Different spatial and temporal filters are applied to the forcing and the differences in the response are analysed. This shows that (i) only the low-frequency component of the nonlinear tropospheric forcing is important, (ii) the most important contribution to the forcing comes from the flow near tropopause level. Most important- ly, (iii) the variability of the response in stratosphere is not a consequence of variability in the longitudinally averaged flow in the troposphere, but is due the variability of the nonlinear forcing itself. This latter conclusion is in direct disagreement with some of the mechanisms for variability in stratospheric long waves that have been suggested in the past.

The analysis of the forced linear model experiments indicates that both the eddy vor- ticity flux and eddy heat flux fields from the full numerical simulation are required for the adequate reconstruction of the stratospheric response of each long wave. The re- quirement of two forcing fields for each planetary-scale zonal wavenumber makes it difficult to characterize the structure of the nonlinear wave forcing (i.e there my be sig- nificant cancellation between to two eddy forcing fields). This parallels the situation encountered for the Eulerian mean equations (e.g. Andrews et. al. 1987, eqs. 3.3). In analogy with the Eulerian mean case, we undertake a transformation of several prog- nostic variables in the forced linear system in order that all of the eddy forcing appears as a single term in one equation. This is done in analogy to the Transformed Eulerian Mean (TEM) formalism (Andrews and McIntyre 1976, 1978; Boyd 1976). We shall re- fer to this procedure as the Transformed Fourier Decomposition (TFD).

The TFD equations are then used for a series of experiments in which we reduce the complexity of the nonlinear wave forcing. The analysis indicates that the vertical struc- ture of the wave forcing is unimportant and is well represented as a vertically localized source near tropopause level. In addition, the TFD provides a single representation of the nonlinear wave forcing and its spatial and temporal characteristics are analysed.

If we are to diagnose the upwelling in the tropical lower stratosphere, then it is impor- tant that we can calculate the diabatic heating in this region with reasonable accuracy. In prognostic models, any inaccuracy could cause severe temperature biases. There is some confusion in the literature about the importance of, for example, cloudiness, on the heating rates. This talk will aim to illuminate the important controls, and might even let on how good or bad MIDRAD is in this region!

Results from the UGCM integration with the Betts/Miller convective adjustment scheme have been analysed to investigate the role of the extratropics in determining the distribution of tropical convection and its inherent timescales. The mechanisms by which the extratropics influence the modelÕs tropical convection have also been stud- ied. The validity of the modelÕs behaviour has been assessed by comparing the results with previous observational studies. Although there are probably many examples of the extratropics interacting with the tropics, this study will concentrate on the influence of upper tropospheric extratropical troughs in the Pacific waveguide, and of the East Asian cold surge events. The possible relationship between cold surge events and sub- sequent enhanced activity in the Pacific waveguide, as well as low frequency variation in the behaviour of the waveguide, will be described.

UGAMP uses a wide range of models as research tools and UGAMP needs to ensure that it can exploit all the high performance computing facilities available, both nation- ally and at the UK forecasting agencies, to run these models. Many of these facilities now include massively parallel computers. Access to the Cray T3D at Edinburgh has enabled UGAMP to examine the issues of porting these models to a massively parallel computer. The experience of running some of the UGAMP models (SGCM, SLIM- CAT and IFS) on the Cray T3D will be reported.

The COSMIC (COupled Stratosphere-Mesosphere Interactive Chemistry) model is a model of the middle atmosphere that is constrained to a prescribed tropospheric forc- ing and in which the radiation scheme uses ozone calculated by the photochemistry scheme. It has been developed from the UGAMP Stratosphere-Mesosphere Model (USMM) by coupling it to the TOMCAT off-line chemistry transport model using part of the OASIS code. The inclusion of a detailed ozone photochemistry in the USMM, including heterogeneous reactions on polar stratospheric clouds, will enable investiga- tion of the importance of feedbacks between perturbed chemistry and stratospheric dy- namics both in recent winters and for future scenarios. This talk will discuss the motivation for developing the new model, the model itself and our tests and initial sim- ulations with it.

The mid-latitude storm tracks are a major feature of the extratropical climate. The storm tracks exhibit considerable variability on intraseasonal, interannual and decadal timescales. Although much of this variability can be understood as an intrinsic proper- ty of the extratropical atmosphere, there are physical reasons to expect that some of the atmospheric variability may be related to variability in sea surface temperatures (SST) in the region of the Gulf Stream in the Atlantic and the Kurushio current in the Pacific. At the same time, one expects on the basis of Sverdrup balance that some of the vari- ability in SST in these regions will be related to variability in the patterns of windstress curl over the ocean basins. Since the patterns of windstress are significantly influenced by the mid-latitude storms the possibility exists for coupled ocean-atmosphere modes of variability.

If coupled modes of the above type exist and are important, a necessary but not suffi- cient condition is that there should be some consistent relationship between SST anomalies in the regions of the Gulf Stream and the Kuroshio and patterns of wind- stress curl over the N. Atlantic and N. Pacific basins. With this in mind, we are analys- ing the relationship between SST and windstress curl in the 45-year da Silva reanalysis of the COADS data set using a variety of techniques for the identification of coupled patterns.

We will discuss our recent investigations into how fluid-dynamical stirring and mixing can affect chlorine deactivation and ozone depletion in the middle-latitude lower strat- osphere. A variety of models of stirring and mixing have been employed.

Results will be presented from trajectory models with sophisticated chemical reaction schemes augmented by simplified representations of mixing, to illustrate the effects of mixing air parcels originating from different latitudes, and permitting a distinction be- tween chemical dilution (in which the timescale for homogenization of chemical mix- ing ratios is a few days but the timescale for removal of active, ozone-destroying, chlorine molecules is a few weeks) and chemical deactivation (in which the timescale for removal is reduced) by mixing.

Results from single layer models with detailed simulation of transport and mixing aug- mented by simplified chemical reaction schemes will be presented. The single layer models have been employed to advect tracers which are also allowed to react chemi- cally. This allows us to make a quantitative assessment of the sensitivity of ozone de- pletion to stirring and mixing that is generated by realistic atmospheric flows.

The rapid climate changes at the terminations marking the end of the last glaciation are poorly under- stood. This study uses the Cambridge 2-D atmospheric model to investigate the impact of a large ma- rine sediment slump or permafrost rupture releasing 4,000 Tg of CH4 into the high latitudes of the northern hemisphere, with particular emphasis being placed upon the evolution of atmospheric CH4 concentrations. The model-generated CH4 concentrations were converted into synthetic ice core sig- nals for 66N and 85S, representing Arctic and Antarctic cores respectively. A range of possible aver- aging times was considered. The results show that, irrespective of the site or enclosure time chosen, with the current sampling interval of ~300 years it is not possible to discount completely the existence of a spike due to the instantaneous release of 4,000 Tg CH4. Detection or refutation of a CH4 spike would however be probable if the sampling interval was reduced to around 50-100 years.

A coupled atmosphere-ocean GCM has been developed for use within the UGAMP community. It is based around the flexible coupling package OASIS developed at CERFACS in Toulouse, France. The atmosphere component GCM is the ECMWF IFS model. Currently we use version 13r4, with minor adaptations. The ocean component GCM is based on an early version of the MOMA model developed by the OCCAM group, with further substantial developments within UGAMP. The first production in- tegration of the full coupled GCM is now under way using a tropical version of the ocean model extending from 29S to 29N. Progress with this integration will be report- ed and some initial results presented.

In preparation for a coupled ocean-atmosphere model, a tropical domain ocean model has been spun-up using climatological winds and heat fluxes with some additional weak relaxation to climatology in sea surface temperature and sea surface salinity.

The spin-up integration has shown that the tropical domain model can simulate many features of the tropical Indo-Pacific oceans. For example, the structure of the ocean currents in the Equatorial Pacific compares well with observed values. However, some difficulties remain. The Tropical Instability Waves of the eastern equatorial Pacific are over-active, persisting into boreal spring when observations show that they cease. This problem is probably soluble through a judicious choice of eddy-mixing parameters. The modelÕs simulation of the Equatorial Atlantic, however, is very poor with large er- rors in the circulation and temperature of the ocean. This is possibly an effect of the artificial model boundaries at 29 South and 29 North on the strong Atlantic thermoha- line circulation. The simulation of other tropical ocean features and their comparison with observations will also be presented.

Much of the evidence for past climate change comes from fossil botanical evidence. The fossil plants are interpreted in terms of changes in temperature and precipitation but the methods are indirect. The process is like a classical inverse problem, where one observation (of a plant) gets used to try and deduce several different climate parame- ters. In general, the solution may not be unique.

An alternative approach is to use a biogeographical model to predict past vegetation from GCM output, and then directly compare the resulting biome predictions with fos- sil evidence. Further iterations of such an approach can examine the feedback between vegetation and climate. Results of such an approach will be shown for a number of pe- riods in the recent and distant past.

Westerly wind bursts are intense bursts of westerly winds that occur around, or west of the dateline, usually just south of the equator. They last for several days and are most frequently observed during the boreal winter and spring. The frequency of these west- erly windbursts increases during the El-Nino years and although some study has been done in conjunction with this phenomenon, westerly windbursts, on the whole, remain unexplained and little researched. A full diagnostic study of these westerly windbursts, using the ECMWF reanalyses is planned, to enable us to build up a climatology includ- ing, for example, their frequency of occurrence, spatial distribution, and duration. Then with this solid background, we hope to be able to study the relationship of these westerly windbursts with El-Nino and also the general atmospheric and oceanic intra- seasonal variability.

Solar Proton Events, (SPE), originating from solar flares, can interact with the EarthÕs atmosphere and cause ozone depletion catalytically through the production of reactive Odd Nitrogen and Odd Hydrogen species. This is largely restricted to the middle at- mosphere, over the polar cap regions but the altitude of penetration and ionisation rates are functions of the energy spectrum of the particles, which typically varies in the range 1-500 MeV. Solar Proton Events are sporadic but tend to be more frequent around solar maximum; 1989 was a particularly active year with the most intense event occurring in October. Proton data for 1989 SPE, from IMP-8 and GOES-7 satellites, have been analysed and analytical fits to the daily spectra have been calculated using various different numerical methods. We have found that the calculated NOx increases are sensitive to assumptions made concerning the method of interpolation of the proton spectra. For this reason, previous calculations may have led to overestimates of the ozone depletion and the long term effects of the proton events. The impact of the SPEs on the stratosphere are investigated using a 2-D model; their potential impact on an en- hanced carbon dioxide atmosphere are also presented.

The mass exchange rate of chemical species between the free troposphere and the sur- face below is governed by the physical process in the atmospheric boundary layer (ABL). A realistic modelling of ABL process is therefore crucial in a tropospheric chemistry transport model.

The ABL parametrization includes:

1. A prognostic determination of surface temperature based on the surface energy bal- ance, which includes the heat-transfer process in the ground for different type of sur- face vegetation.

2. A diagnostic determination of the convective velocity scale in the ABL, which is based on the surface sensible heat flux and surface latent heat flux.

3. A diagnostic determination of the height of the ABL, which will determine the ver- tical scale and structure of eddy diffusivity and local transport of chemical species due to dry convection in the ABL.

4. A coupling process between solar radiative heating at the ground (which depends on the cloud cover and radiative transfer process by trace gases), surface heat flux (sensi- ble heat and latent heat), the ABL, and the cloud transports (including boundary-layer stratocumulus clouds and deep cumulus clouds).

The one-dimensional propagation of a spectrum of gravity waves through a realistic middle atmosphere is investigated. An upward propagating initial, or ``launchÕÕ, ener- gy spectrum, of the shape suggested by Fritts and VanZandt, is prescribed in the lower stratosphere.

We model the propagation and breaking of the spectrum of gravity waves as follows. In the first place we use linear, conservative propagation including Coriolis and non- hydrostatic effects. In the second place, we use an empirical model of wave-breaking that is simply to impose as an upper bound the saturated portion of the spectral shape, usually incorporating a -3 power law for the vertical wavenumber dependence, and a 5/3 or a -1 power law for the intrinsic frequency dependence. The computed profiles of wave-induced force, as a function of altitude, are sensitive to the shape and total en- ergy of the launch spectrum as well as to the steepness of the saturation criterion. Fur- ther details are given in [1].

The effect of varying the asymptotic shape of the launch spectrum at the smallest ver- tical wavenumbers, modelled by varying the +s power law dependence for the smallest vertical wavenumbers is investigated for a number of representative cases. The total wave-energy profile E(z) predicted by the model as a function of altitude z is compared with an empirical profile derived by Fritts and Lu from observational studies, with an energy scale height of 2.3 times the density scale height H. For the cases considered so far, the values of s that give the best fit are in the range 1.25--1.75. If the energy scale height of 2.3 H describes typical reality then model calculations like these should pro- vide a new and promising approach to constraining values of s.

[1] Warner C.D., M.E. McIntyre, 1996: On the propagation and dissipation of gravity- wave spectra through a realistic middle atmosphere, J. Atmos. Sci., in press.

In this experiment we are attempting to establish whether the development of small scale features such as filaments from the polar vortex edge have an effect on the large scale features of the seasonal circulation in the northern hemisphere winter strato- sphere. We employ a three-dimensional mechanistic model (USMM) which we run at a number of horizontal resolutions from T21 to T106 for a period of several weeks in mid winter 94/95 when there is strong prescribed wave forcing from below. We com- pare the simulations with analyses from the UK Met Office.

The Atmospheric Chemistry Studies in the Oceanic Environment (ACSOE) project is a NERC community research project devised to expand the field measurement aspect of atmospheric chemistry studies in the U.K., while at the same time contributing to the International Global Atmospheric Chemistry project of the IGBP. The project will investigate the chemical processes affecting the concentration of trace gases in the troposphere over the ocean and the production of condensation nuclei and aerosol from sulphur emissions, focusing on the North Atlantic and surrounding shores. The project has three sub-programmes: the Oxidising Capacity of the Oceanic Atmosphere (OXI- COA) programme, the Aerosol Characterisation Experiment (ACE-2) and the Marine Aerosol and Gas Exchange (MAGE) programme. The OXICOA programme is the largest of these projects, focusing principally on the tropospheric Ozone budget and HOx, ROx and NO3 free radical chemistry in relatively clean conditions. A number of field campaigns are planned, including a full suite of measurements at and around Mace Head in Ireland during July 1996.

In support of the ACSOE/OXICOA measurements, modelling studies will be per- formed to assess the Ôoxidising capacityÕ of the air masses encountered in these cam- paigns. For the Mace Head campaign, the Cambridge Tropospheric Trajectory Model will be used in conjunction with calculated back trajectories to compare the expected chemical evolution of air masses arriving at the Mace Head site with the field meas- urements. Analysis of the discrepancies should lead to a better understanding of the en- vironmental conditions over the ocean, and the physical and chemical processes affecting them, whilst testing our current knowledge. In addition, the model will be used to calculate the concentrations of radical species for direct comparison with measurements, and to derive the formation and destruction terms in the ozone budget to classify the air masses encountered. A number of studies have already been per- formed to classify typical air mass types that may be expected at the site; on-site par- ticipation in the campaign in the second half of July will allow us to test the assumptions made about the physical and chemical processes involved, and to attempt to answer questions posed by the measurements.

Experiments with a two-level model have shown that a transition to a general circula- tion with an equatorial westerly jet can be forced by a sufficiently large zonal wave- number two tropical heating anomaly. Results from integrations of such a model with a tropical heating anomaly applied to different basic states will be presented. Particular attention will be paid to the extratropical response and the associated eddy fluxes.

The radiative forcing of the surface-troposphere system caused by stratospheric ozone depletion in the 1980s is calculated using observed values of change in ozone, from TOMS data, and temperature, from MSU data. The seasonal variation of the ozone and temperature trends produces strong seasonal and latitudinal variations in radiative forc- ing. The time difference between peak ozone loss and maximum temperature decrease shifts the peak negative forcing at the South Pole from October to November with net positive values in mid-latitudes in August and September. Positive values also occur throughout the year at low latitudes in both hemispheres. The globally and annually av- eraged net radiative forcing is about -0.04 Wm-2 between 1979 and 1990, much less negative than the about -0.1 Wm-2 previously reported using fixed dynamical heating model temperature changes. The ozone forcing is very sensitive to the vertical profile of temperature change but is unlikely to be more negative than -0.07 Wm-2.

New analytical functions have been found for longwave transmittance of the atmos- phere in the CO2 15micron and O 39.6micron bands. The functions are fitted to line by-line transmittance calculations as a function of path absorber amount, pressure and temperature.

The functions have been tested from 180-320K, 1096-3.355-10-4 mb, 9.946-10-9- 2.9273 g/cm2 for CO2 and 5.40-10-10-0.01305 g/cm2 for O3 which makes them suit- able for use throughout all 47 levels of the EUGCM2.1.

When compared to suitable transmission tables calculated by GENLN2 the errors are at most 3%. This maximum error occurs at the higher temperatures where the structure of the functions is more complicated.

These functions are intrinsically more accurate than previous methods since a ÔrealÕ temperature dependence, as opposed to a quadratic fit, is built into them.

Errors in heating rates calculated by the use of these functions are at most 0.1K/day in the lower stratosphere and 0.8K/day higher up when compared to GENLN2 calcula- tions.

The functions can also be calculated faster as they work in linear p and u space. Our previous method using look-up tables, and most other peopleÕs methods, require the use of the LOG function which is computationally slow and will not vectorize on the CRAY.

This means we now have one radiation scheme that can be used for the whole of the atmosphere in the EUGCM2.1, without the problems of flux matching, that is compu- tationally efficient and more accurate.

It is anticipated that use of the new scheme will remove some of the cold bias seen in the model stratosphere using the ECMWF code.

© 1996 Centre for Atmospheric Science/UGAMP, Cambridge University, UK. Editor: Glenn Carver. Web: http://www.atm.ch.cam.ac.uk/acmsu/. The UGAMP Newsletter is prepared using FrameMaker v5.