Sensitivity of Squall-Line Rear Inflow to Ice Microphysics and Environmental Humidity Ming-Jen Yang and Robert A. House Jr. Mon. Wea. Rev., 123, Hsiao-Ling Huang 2004/01/09

Introduction Squall line with km wide trailing stratiform precipitation regions are an important type of organized mesoscale convective system (MCS), which occur in both the Tropics and midlatitudes. A mesoscale storm-relative ascending front-to-rear (FTR) flow, transporting hydrometeors rearward from a leading- edge convective line to the trailing stratiform region. A mesoscale storm-relative rear-to-front (RTF) flow, descending through the stratiform region toward low levels in the leading convective region.

Zhang and Gao(1989) performed mesoscale model simulations of an intense midlatitude squall line indicating that the large-scale baroclinicity provided deep and favorable RTF flow within the upper half of the troposphere. Fovell and Ogura (1989) and Weisman (1992) showed that the RTF flow increased in strength with increasing environmental vertical wind shear and convective available potential energy (CAPE). This study use a high-resolution nonhydrostatic cloud model to perform six numerical experiments in order to determine the sensitivity of the storm structure to hydrometeor types, ice-phase microphysics, and environmental humidity.

Model description 314 km 2250 km Fine mesh=1 km Stretch grid is 1.075:1 Δ z = 140 m Δ z = 550 m Numerical model Compressible nonhydrostatic clod model. A 2D simulation (x-z). grid points: 455 (H) × 62 (V) domain: 4814 km(H)×21.7 km(V) Cloud microphysics Five types of water condensate are include: cloud water, cloud ice, rainwater, snow, and hail (Lin et al. 1983).

Initial conditions 1985/06/10/2331 UTC [Enid(END), Oklahoma]. T, T D, u, v 1985/06/10/2330 UTC [Pratt(PTT), Kansas]. Low-level moisture 1985/06/10~ /06/10/2331 UTC

A 5-km deep, 170-km-long cold pool of ΔΘ´= -6 K and Δq v ´ = -4 g kg -1

The control experiment (CNTL) A control run (CNTL) with full model physics for 15 h. The justification for turning off hail generation processes ( at t = 6 h) after the early stage is that there were very few hailstones in the mature or decaying stage of the June squall line.

Overview of the storm development Hail is turned off INI; t = hMAT; t = h DEC; t = h 28~ 36% 45~ 53%

Evolution of the squall-line structure Kinematic structure The rear inflow plays a crucial role in supplying potentially cold and dry midlevel air from the environment to aid in the production of the convective and mesoscale downdraft.

Evolution of the squall-line structure Thermal and pressure structure H H H L L L L c w

Latent heating fields

Air parcel trajectoriesTrajectories of precipitation particles t = h

Sensitivity tests Run Run time (h) Restart timeComments CNTL15full physics; turn off hail generation processes after 6 h HAIL13full physics; leave hail generation processes on after 6 h NICE14no ice-phase microphysics NEVP12CNTL at 3 hno evaporative cooling NMLT12CNTL at 3 hno melting cooling NSUB12CNTL at 3 hno sublimational cooling DRYM12driver midlevel environment; see Fig. 1a

Hailstorm simulation (HAIL) 150 km55 km

No ice-phase microphysics (NICE) 7K 4K 150 km60 km 8 ms ms -1

No evaporative cooling (NEVP) 5 ms ms -1

No latent cooling by melting (NMLT) W S 12 ms ms -1

No latent cooling by sublimation (NSUB) The latent cooling by evaporation and melting are the most important microphysical processes determining the structure and strength of rear inflow in the cloud-model simulations ms ms -1

Drier midlevel environment (DRYM) S W 12.2 ms -1

RunStorm speedStorm orientationRTF flow structure CNTL12.2 ms –1 upshear tilt two Max. in the storm (8 ms -1 ) HAIL11 ms -1 upshear tilt one Max. in convective region NICE8 ms -1 upshear tilt one Max. in convective region NEVP5 ms -1 upright to downshear tilt a highly elevated RTF flow NMLT12 ms -1 less upshear tilttwo Max. in the storm (6 ms -1 ) NSUB10.8 ms -1 less upshear tilttwo Max. in the storm (7 ms -1 ) DRYM12.2 ms -1 more uprighttwo Max. in the storm (3 ms -1 )

Role of mesolows in the formation of the descending rear-to-front flow Mature (t = h)Late (t = h)

Conclusions The mass convergence associated with the ascending FTR flow and descending RTF flow in the trailing stratiform region was crucial to the generation and maintenance of mesoscale updraft and downdraft. The most important latent cooling is produced by evaporative cooling of rainwater. The structure and strength of the rear inflow is sensitive to precipitating hydrometeor types, ice- phase microphysics, and the latent cooling of evaporation, melting, and sublimation.