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

Humidity

Ming-Jen Yang and Robert A. House Jr.

Mon. Wea. Rev., 123, 3175-3193

Hsiao-Ling Huang 2004/01/09

Introduction

• Squall line with 50-200 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 km2250 kmFine mesh=1 kmStretch 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 microphysicsFive types of water condensate are include: cloud water, cloud ice, rainwater, snow, and hail (Lin et al. 1983).

• Initial conditions1985/06/10/2331 UTC [Enid(END), Oklahoma]. T, TD, u, v

1985/06/10/2330 UTC [Pratt(PTT), Kansas]. Low-level moisture

1985/06/10~11

1985/06/10/2331 UTC

A 5-km deep, 170-km-long cold pool of

ΔΘ´= -6 K and Δqv´ = -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 10-11 June squall line.

Overview of the storm development

Hail is

turned off

INI; t = 7.5-8.5 h MAT; t = 10-11 h

DEC; t = 12.5-13.5 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

HL

L

L

Lc

w

Latent heating fields

Air parcel trajectories Trajectories of precipitation particles

t = 10-11 h

t = 10-11 h

Sensitivity tests

RunRun

time (h)Restart time Comments

CNTL 15 full physics; turn off hail generation processes after 6 h

HAIL 13 full physics; leave hail generation processes on after 6 h

NICE 14 no ice-phase microphysics

NEVP 12 CNTL at 3 h no evaporative cooling

NMLT 12 CNTL at 3 h no melting cooling

NSUB 12 CNTL at 3 h no sublimational cooling

DRYM 12 driver midlevel environment; see Fig. 1a

Hailstorm simulation (HAIL)

150 km 55 km

No ice-phase microphysics (NICE)

7K 4K

150 km 60 km

8 ms-112.2 ms-1

No evaporative cooling (NEVP)

5 ms-112.2 ms-1

No latent cooling by melting (NMLT)

W

S

12 ms-112.2 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.

10.8 ms-112.2 ms-1

Drier midlevel environment (DRYM)

S

W

12.2 ms-112.2 ms-1

Run Storm speed Storm orientation RTF flow structure

CNTL 12.2 ms –1 upshear tilt two Max. in the storm (8 ms-1)

HAIL 11 ms-1 upshear tilt one Max. in convective region

NICE 8 ms-1 upshear tilt one Max. in convective region

NEVP 5 ms-1 upright to downshear tilt a highly elevated RTF flow

NMLT 12 ms-1 less upshear tilt two Max. in the storm (6 ms-1)

NSUB 10.8 ms-1 less upshear tilt two Max. in the storm (7 ms-1)

DRYM 12.2 ms-1 more upright two Max. in the storm (3 ms-1)

Role of mesolows in the formation of the descending rear-to-front flow

Mature (t = 10-11 h) Late (t = 12.5-13.5 h)

Conclusions• The mass convergence associated with the ascendin

g 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.