A Numerical Investigation of the Effects of Dry Air Aloft on Deep Convection

A three-dimensional cloud model was used to investigate the sensitivity of deep convective storms to dry air above the cloud base. In simulations of both quasi-linear convective systems and supercells, dry air aloft was found to reduce the intensity of the convection, as measured by updraft mass flux and total condensation and rainfall. In high-CAPE line-type simulations, the downdraft mass flux and cold pool strength were enhanced at the rear of the trailing stratiform region in a drier environment. However, the downdraft and cold pool strengths were unchanged in the convective region, and were also unchanged or reduced in simulations of supercells and of line-type systems at lower CAPE. This result contrasts with previous interpretations of the role of dry air aloft in the development of severe low-level outflow winds. The buoyancy-sorting framework is used to interpret the influence of environmental humidity on the updraft entrainment process and the observed strong dependence on the environmental CAPE. The reduction in convective vigor caused by dry air is relatively inconsequential at very high CAPE, but low-CAPE convection requires a humid environment in order to grow by entrainment. The simulated responses of the downdraft and cold pool intensities to dry air aloft reflected the changes in diabatic cooling rates within the downdraft formation regions. When dry air was present, the decline in hydrometeor mass exerted a negative tendency on the diabatic cooling rates and acted to offset the favorable effects of dry air for cooling by evaporation. Thus, with the exception of the rearward portions of the high-CAPE line-type simulations, dry air was unable to strengthen the downdrafts and cold pool. A review of the literature demonstrates that observational evidence does not unambiguously support the concept that dry air aloft favors downdraft and outflow strength. It is also shown that the use of warm rain microphysics in previous modeling studies may have reinforced the tendency to overemphasize the role of dry air aloft.