Analysis of Wet and Dry Microburst Simulations and the Importance of the Elevated Dry Layer on Wet Microburst Formation and Strength

Utilizing idealized wet microburst soundings created here, varying only in humidity profile, wet microburst producing thunderstorm cloud model simulations are examined to determine the importance of the elevated dry layer on the forcing and strength of wet microbursts. A dry microburst simulation is also analyzed to quantify significant differences in evolution and forcing between a wet and dry microburst. Dry microbursts are well understood, forced by negatively buoyant air, primarily due to thermodynamic cooling. Wet microbursts have been identified to form in humid, unstable environments, typically exhibiting an elevated dry layer of air. However, there is question as to the importance of the elevated dry layer on the forcing and strength of wet microbursts. In this study, environmental data were analyzed for several airmass thunderstorm events producing wet microbursts across humid regions of the United States during the warm season. Using this collection of environmental data as guidance, three idealized wet microburst environmental soundings were constructed using the Rawisonde Observation Program (RAOB). Each sounding exhibits an identical well-mixed boundary layer and thermal profile, along with a simple unidirectional wind profile, with small amounts of vertical wind shear, characteristic of an airmass thunderstorm-producing environment. Two of these soundings exhibit an elevated dry layer at varying altitudes, while the other contains no dry layer. These soundings were inputted into the Bryan Cloud Model, version 1 (CM1) with 50 meter horizontal grid spacing. Using visualization software, thermodynamic and dynamic information were interrogated from these simulations. The simulations indicate that the wet microburst resulting from the sounding with no dry layer is associated with the strongest downdraft speed, strongest surface outflow wind speed, and largest accumulated rainfall, though the wet microburst soundings containing an elevated dry layer also result in strong wet microbursts as well. Furthermore, it was determined that all simulated wet microbursts are forced primarily by hydrometeor drag, with thermodynamic cooling playing a much smaller role. These results indicate that the elevated dry layer need not be present to result in a strong wet microburst and indicate that hydrometeor drag is the primary forcing mechanism behind wet microburst formation in these simulations.