Feedback of Deep Moist Convection to Its Near Environment as Diagnosed from Three-Dimensional Numerical Model Output. Results from an Early Experiment

Abstract

This paper presents preliminary results from an investigation into the feedback between convective storms and their new surroundings, using output from a three-dimensional anelastic cloud-scale model. Convective feedback budgets for heat, moisture and horizontal momentum are diagnosed from horizontally Reynolds-averaged governing equations, analogous to the theory of Anthes. There is limited horizontal scale separation between the active convection area and the averaging area, which at 30 km on a side is comparable to one grid cell of a typical mesoscale numerical weather prediction model. The simulation is run with an idealized midlatitude severe thunderstorm sounding. The resulting storm displays several supercell features. These include a vigorous erect large-diameter updraft that splits at lower levels, a vaulted weak echo region in the lower part of the main (right flank) updraft core, and a midlevel mesovortex couplet with cyclonic vorticity in the main updraft. The vertical profiles of the various budget terms show several findings of potential relevance to cumulus parameterization. The vertical eddy transport (flux divergence) is highly important to each budget; it significantly raises the height of the maximum apparent heat source and lowers the height of the maximum apparent moisture sink, and acts to reduce the net tropospheric vertical wind shear. At the same time, the horizontal eddy momentum transport and the mean horizontal pressure gradient force both act strongly to sharpen the tropopause-level jet, so that the net shear is little changed. The horizontal eddy transport is much less important to the heat budget, and remains negligible for moisture. Mean storage contributes significantly to the apparent source for each budget under consideration. Other terms derived by Anthes but ignored in existing cumulus parameterizations (the resultant of eddy storage, transport of mean fields by the eddy wind, and transport of eddy fields by the mean wind) become quite appreciable. These terms partially oppose the condensational heating and drying, and the processes that sharpen the tropopause-level jet.