The Influence of Midtropospheric Dryness on Supercell Morphology and Evolution

Abstract

This work studies the relationship between midtropospheric dryness and supercell thunderstorm morphology and evolution using a three-dimensional, nonhydrostatic cloud model. Environments that differ only in midtropospheric dryness are found to produce supercells having different low-level outflow and rotational characteristics. Thunderstorms forming in environments with moderate vertical wind shear, large instability, and very dry midtropospheric air produce strong low-level outflow. When this low-level outflow propagates faster than the midlevel mesocyclone, the storm updraft and low-level mesocyclone weaken. However, in environments with larger vertical wind shear or with higher-altitude dry midtropospheric air, the low-level outflow is not as detrimental to the supercell. This provides a possible explanation for why some environments that appear favorable for the development of strong low-level mesocyclones in supercells fail to do so. Downdraft convective available potential energy (DCAPE) is also investigated as one possible index for estimating potential downdraft strength. Trajectory analysis shows that the strongest downdrafts are subsaturated and diluted due to mixing between the downdraft and the surrounding environment. These significant violations of parcel theory make DCAPE a worse estimate for supercell downdraft intensity than convective available potential energy is for the updraft. A more sophisticated parameter is needed in order to determine downdraft intensity and low-level outflow strength within supercells.