A numerical investigation of the effects of dry air aloft on quasi-linear convective systems

A high-resolution cloud model has been used to investigate the effects of relatively dry air above cloud base in the environment of quasi-linear convective systems. This study is motivated by the observation that dry air above cloud base is sometimes regarded as detrimental to overall convective intensity (e.g. in tropical convective environments), whereas the severe storms literature often describes low environmental humidity in the mid-troposphere as an ingredient for strong low-level cold outflow and severe straight-line winds at the surface. In an attempt to clarify the implications of a relatively dry environment above cloud base, this study examines the simulated sensitivity to a mid-tropospheric layer of dry air, with a focus on resulting changes in the intensity of the surface-based cold pool.

Squall line-type convective systems have been simulated under a wide range of CAPE and shear values, and sensitivity tests have been performed by comparing results with and without a 1.5 km deep layer of low relative humidity at various heights above cloud base. Using several different microphysical schemes, and with horizontal grid spacings of 400-1000 m, the results consistently indicate that dry air does not favor the production of strong low-level cold outflow in squall line convection, owing to the detrimental effects of dry air through entrainment into updrafts. When all other parameters are kept constant, drier environmental air above cloud base significantly reduces the rate of production of water and ice condensate, which in turn reduces the condensate mass that is available for the evaporation and melting processes that are important in moist downdraft generation.

The detrimental effect of reduced condensate mixing ratios on downdraft production is offset to a greater or lesser extent by the greater efficiency of evaporation in the drier environment. In the vast majority of the simulated convective systems, the enhancement of evaporation owing to drier ambient conditions does not overcome the decrease in evaporation owing to the reduced condensate mass, and consequently the total rain evaporation in downdrafts is decreased in the presence of dry air. Only in the mature phase of a squall line in a very high-CAPE environment does the rain evaporation rate within downdrafts become slightly enhanced by environmental dry air. However, the rate of ice melting within downdraft regions is not similarly enhanced by lower ambient relative humidity, so the contribution of ice melting to negative buoyancy production is always markedly diminished when dry air is present. The overall effect is that the total propensity for moist downdraft generation by phase change processes is either diminished or essentially unchanged in the presence of dry air.

The reduction of low-level cold outflow strength due to a layer of dry air in the environment is greatest under conditions of low CAPE, wherein the buoyancy of the updrafts is relatively small and entrainment of environmental air has a proportionately greater effect. At higher values of CAPE, the detrimental effect of dry air on cold pool intensity is reduced, although the total condensate mass production is still markedly diminished. The results also indicate that environmental dry air has a greater effect when it is located at lower altitudes just above cloud base, in which case the entrainment occurs early in the life of the updrafts, before the bulk of the precipitation formation has occurred.