Publication Date:
2017-07-11
Description:
Deep convection is a multiscale process that significantly influences the budgets of heat, moisture and momentum. In global climate models the thermodynamic effects of convection are normally treated by parametrisation schemes, with a separate formulation for convective momentum transport (CMT). The transport modules for current thermodynamic and momentum parametrisations are upright entraining plume models that do not account for vertically tilted mesoscale circulations that characterise organised convection in sheared environments. The associated counter-gradient vertical transport of horizontal momentum fundamentally affects dynamical interactions between the convection and the mean flow. This study examines the CMT properties of simulated idealised mesoscale convective systems, including their sensitivity to horizontal resolution, domain size, and lateral boundary conditions. It is found that even for large domains, the horizontal gradient terms are important, especially the mesoscale pressure gradients that are neglected in CMT parametrisations. A nonlinear analytic model provides a dynamical foundation for the effects of convective organisation, including the role of the horizontal pressure gradient. It is found that a small computational domain adversely affects the convective organisation by generating artificially large compensating subsidence and an unrealistic evolution of the CMT. Finally, analyses of the cross-updraft/downdraft pressure gradients expose significant uncertainties in their representation in contemporary CMT parametrisation schemes.
Print ISSN:
0035-9009
Electronic ISSN:
1477-870X
Topics:
Geography
,
Physics
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