STORM
TRACKS
Moist Processes in Storm-Tracks and Large-Scale Temperature Structure
Moisture is a major contributor, through both its latent and radiative effects, to the distribution of heating in the extratropical troposphere. The latent heating occurs primarily in weather systems in the storm-tracks, whilst moisture transport by the weather systems helps to determine the distribution of moisture and therefore its radiative impact. Moisture thus plays a role in feedbacks between storm-track activity and the large scale structure of the extratropical atmosphere.
Idealised experiments have been performed using the Reading primitive-equation spectral model to investigate these moist storm-track processes. An aim of this work is to understand systematic differences in large scale temperature structure and storm-track activity seen in climate simulations by GCMs which differ only in their dynamical formulation. The fact that dynamical processes alone appear unable to explain these differences (Chen et al., 1997) points to the role of moisture in driving them, reinforcing results from the spin-up phase of GCM experiments by Blackburn (1997).
The idealised experiments build on the "dynamical core" framework of Held and Suarez (1994), which excludes orography and land-sea contrasts and which includes only linear diabatic and frictional effects (temperature relaxation to a prescribed zonal state and low-level Rayleigh drag). Extratropical jets and storm-tracks develop in long integrations and the zonal mean state remains different from the prescribed relaxation state because of potential vorticity fluxes by the weather systems. The experimental design is similar to that of James and James (1992), who used a simplified General Circulation Model (SGCM) to investigate ultra-low-frequency variability.
A simple interactive moisture cycle and associated heating have been added to the dynamical core framework, with idealised representations of surface evaporation and stratiform condensation, and initial integrations have been performed. The main observed features of the extratropical transport and sources and sinks of moisture are surprisingly well simulated.
The experiments highlight the impact of meridional moisture fluxes and associated latent heating on the large scale baroclinicity and on the strength of the weather systems (Figure 1). Such effects were originally seen in increased CO2 experiments by Manabe and Wetherald (1980) and have been analysed in terms of individual baroclinic wave lifecycles by, among others, Hall et al. (1994) and Pavan et al. (1999). Further experiments are being performed to investigate impacts on the tropopause.
References
Blackburn, M. (1997) Advection of water vapour and the cold polar tropopause bias in Eulerian GCMs. UGAMP Newsletter, 16, 8-9.
Chen, M., Rood, R. B. and Takas, L.E. (1997) Impact of a semi-Lagrangian and an Eulerian dynamical core on climate simulations. Journal of Climate, 10, 2374-2389.
Hall, N. M. J., Hoskins, B. J., Valdes, P. J. and Senior, C. A. (1994) Storm-tracks in a high resolution GCM with doubled carbon dioxide. Quarterly Journal of the Royal Meteorological Society, 120, 1209-1230.
Held, I. M. and Suarez, M. J. (1994). A proposal for the intercomparison of the dynamical cores of atmospheric general circulation models. Bulletin of the American Meteorological Society, 75, 1825-1830.
James, I. N. and James, P. M. (1992) Spatial structure of ultra-low-frequency variability of the flow in a simple atmospheric circulation model. Quarterly Journal of the Royal Meteorological Society, 118, 1211-1233.
Manabe, S. and Wetherald, R. T. (1980) On the distribution of climate change resulting from an increase in CO2 content of the atmosphere. Journal of the Atmospheric Sciences, 37, 99-118.
Pavan, V. L., Hall, N. M. J., Valdes, P. J. and Blackburn, M. (1999) The importance of moisture distribution for the growth and energetics of mid-latitude systems. Annales Geophysicae, 17, (to appear).
Mike Blackburn
CGAM, University of Reading
M.Blackburn@Reading.ac.uk
