Numerical Simulations of the 2 August 1981 CCOPE Supercell Storm with and without Ice Microphysics

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  • a Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin
  • | b Department of Meteorology, University of Oklahoma, Norman, Oklahoma
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Abstract

The Wisconsin Dynamical-Microphysical Model is used in two simulations of the 2 August 1981 supercell that passed through the Cooperative Convective Precipitation Experiment in southeastern Montana. The first simulation uses liquid water-only microphysics and is denoted as the liquid water model (LWM). The second includes both liquid water and ice microphysics and is designated as the hail category model (HCM). Results from the two simulations show that the inclusion of ice significantly alters the dynamics, kinematics, thermodynamics, and distributions of water in the storm, especially at the lower levels. Supercell features such as a rotating intense updraft, bounded weak-echo region, large forward overhanging anvil, and hooklike structure in the low-level rainwater field are present in both simulations. These features are generally more pronounced, however, and have a longer lifetime in the HCM.

Hail embryo and graupel particles make up more than 85% of the total hail mass during the steady-state phase in the HCM. Many of these particles are advected into the anvil regions away from the updraft and sublimate slowly. As a result, distributions of graupel and hail in the HCM cover a more extensive but less concentrated region than do the distributions of rainwater in the LWM. Heavier more localized precipitation in the LWM results in a stronger low-level downdraft and a faster-moving gust front than in the HCM. The LWM gust front propagates ahead of the low-level updraft, cutting off the warm, moist, low-level easterly flow into the storm that leads to complete dissipation of the cloud by the end of the 150-min simulation period. Conversely, less concentrated precipitation failing to the surface in the HCM results in a weaker downdraft and a slower-moving gust front. The gust front propagates with the low-level updraft, thus allowing the storm to remain in a quasi-steady state for the final 80 min of the simulation. Overall, there is slightly more total surface precipitation in the HCM due to the larger areal coverage of precipitation and slower movement of the storm.

Abstract

The Wisconsin Dynamical-Microphysical Model is used in two simulations of the 2 August 1981 supercell that passed through the Cooperative Convective Precipitation Experiment in southeastern Montana. The first simulation uses liquid water-only microphysics and is denoted as the liquid water model (LWM). The second includes both liquid water and ice microphysics and is designated as the hail category model (HCM). Results from the two simulations show that the inclusion of ice significantly alters the dynamics, kinematics, thermodynamics, and distributions of water in the storm, especially at the lower levels. Supercell features such as a rotating intense updraft, bounded weak-echo region, large forward overhanging anvil, and hooklike structure in the low-level rainwater field are present in both simulations. These features are generally more pronounced, however, and have a longer lifetime in the HCM.

Hail embryo and graupel particles make up more than 85% of the total hail mass during the steady-state phase in the HCM. Many of these particles are advected into the anvil regions away from the updraft and sublimate slowly. As a result, distributions of graupel and hail in the HCM cover a more extensive but less concentrated region than do the distributions of rainwater in the LWM. Heavier more localized precipitation in the LWM results in a stronger low-level downdraft and a faster-moving gust front than in the HCM. The LWM gust front propagates ahead of the low-level updraft, cutting off the warm, moist, low-level easterly flow into the storm that leads to complete dissipation of the cloud by the end of the 150-min simulation period. Conversely, less concentrated precipitation failing to the surface in the HCM results in a weaker downdraft and a slower-moving gust front. The gust front propagates with the low-level updraft, thus allowing the storm to remain in a quasi-steady state for the final 80 min of the simulation. Overall, there is slightly more total surface precipitation in the HCM due to the larger areal coverage of precipitation and slower movement of the storm.

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