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Sensitivity of Squall-Line Rear Inflow to Ice Microphysics and Environmental Humidity

Ming-Hen YangDepartment of Atmospheric Sciences, University of Washington, Seattle, Washington

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Robert A. Houze Jr.Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

Two-dimensional nonhydrostatic numerical simulations of a midlatitude squall line show that the rear inflow and related aspects of storm structure are sensitive to hydrometeor types, ice-phase microphysics, and the mid-level environmental humidity. Without ice-phase microphysics, the model cannot produce realistic air motions or precipitation in the stratiform region. With the occurrence of heavy hailstones, there is no enhanced rear-to-front flow at the back edge of the storm, because of the weak midlevel mesolow in the narrow stratiform region. Evaporation is the most important latent cooling process determining the structure and strength of the descending rear inflow and the mesoscale downdraft. Latent cooling by melting snow does not initiate the mesoscale downdraft; however, it accounts for at least 25% of the strength of the maximum of rear-to-ftont flow at the back edge of the storm during the mature stage and enhances the strength of the mesoscale downdraft by 22%. Mesoscale downdraft is initiated above the 0°C level by sublimational cooling. With the environmental midlevel moisture reduced by half, mesoscale dowadrafts are 22% stronger, but the maximum of rear-to-front flow at the back edge of the system reaches only 38% of its mature-stage intensity, as a result of a more vertically upright storm orientation, and hence the resultant weaker mesolow. These results indicate that the descending rear inflow is in part a dynamical response to the latent cooling processes in the trailing stratiform region of a squall-line-type mesoscale convective system.

Abstract

Two-dimensional nonhydrostatic numerical simulations of a midlatitude squall line show that the rear inflow and related aspects of storm structure are sensitive to hydrometeor types, ice-phase microphysics, and the mid-level environmental humidity. Without ice-phase microphysics, the model cannot produce realistic air motions or precipitation in the stratiform region. With the occurrence of heavy hailstones, there is no enhanced rear-to-front flow at the back edge of the storm, because of the weak midlevel mesolow in the narrow stratiform region. Evaporation is the most important latent cooling process determining the structure and strength of the descending rear inflow and the mesoscale downdraft. Latent cooling by melting snow does not initiate the mesoscale downdraft; however, it accounts for at least 25% of the strength of the maximum of rear-to-ftont flow at the back edge of the storm during the mature stage and enhances the strength of the mesoscale downdraft by 22%. Mesoscale downdraft is initiated above the 0°C level by sublimational cooling. With the environmental midlevel moisture reduced by half, mesoscale dowadrafts are 22% stronger, but the maximum of rear-to-front flow at the back edge of the system reaches only 38% of its mature-stage intensity, as a result of a more vertically upright storm orientation, and hence the resultant weaker mesolow. These results indicate that the descending rear inflow is in part a dynamical response to the latent cooling processes in the trailing stratiform region of a squall-line-type mesoscale convective system.

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