SENSITIVITY OF SQUALL-LINE REAR INFLOW TO ICE MICROPHYSICS AND ENVIRONMENTAL HUMIDITY

Two-dimensional nonhydrostatic numerical simulations of a midlatitude squall line show that the rear inflow and related aspects of storm str ucture are sensitive to hydrometeor types, ice-phase microphysics, and the midlevel 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. Evaporat ion is the most important latent cooling process determining the struc ture and strength of the descending rear inflow and the mesoscale down draft. Latent cooling by melting snow does not initiate the mesoscale downdraft; however, it accounts for at least 25% of the strength of th e maximum of rear-to-front flow at the back edge of the storm during t he mature stage and enhances the strength of the mesoscale downdraft b y 22%. Mesoscale downdraft is initiated above the 0 degrees C level by sublimational cooling. With the environmental midlevel moisture reduc ed by half, mesoscale downdrafts 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 s torm, orientation, and hence the resultant weaker mesolow. These resul ts indicate that the descending rear inflow is in part a dynamical res ponse to the latent cooling processes in the trailing stratiform regio n of a squall-line-type mesoscale convective system.