Results

Effect of mineral dust on cloud microphysics and precipitation of mesoscale convective systems

Bing Lin1, Qilong Min2, and Rui Li2, Everette Joseph3, Yongxiang Hu1, Vernon Morris3, Shuyu Wang2

1NASA Langley Research Center , 2State University of New York at Albany, 3NOAA Center for Atmospheric Sciences

Mineral dust & aerosol indirect effect

As giant CCN, dusts may enhance the collision and coalescence of droplets and

therefore increase warm precipitation formation and decrease cloud’s albedo.

But they may also reduce ice particle sizes due to abundant ice nuclei (IN), thus

increase lifetime and amount of ice clouds.

Some observations show that dusts suppress clouds and precipitation.

Objectives

Introduction

Approach

Terra

MODIS: Aerosol and cloud optical

properties

CERES: Radiation and climate

forcing

TRMM

TMI: Cloud water and

precipitation

PR: Precipitation

VIRS: Aerosol and cloud

optical properties

CERES: Radiation

Aqua, GOES and METSAT

Data

Summary

The probability density functions of rain water in convective and stratiform precipitation

regions for both dusty and dust-free sectors (a), and corresponding evaporation rates (b).

There are inconsistent evidences of aerosol indirect effects (AIE) on clouds and

precipitation, and not enough observations of the impacts of dusts on rainfall

internal structures. This study uses multi-platform and multi-sensor measurements

to investigate detailed physical processes of cloud microphysics and precipitation

that are affected by mineral dusts in a case of mesoscale convective systems.

separation of DS and DF regions

convection & stratiform precipitation

dynamic variations in precip. clouds (DF, DS)

changes in St. & Cu in DS regions

statistics for the DS and DF regions

dust indirect effects

Selected case

early March 2004

Sahara dust storm

mesoscale convection

dust-free (DF) and dusty (DS)

regions (or sectors)

Surface network: AMMA and AERNET

Relative Humidity (%)

0 20 40 60 80 100

0

2

4

6

8

10

12

Air Temperature (k)

220 240 260 280 300

Heigh

t (km

)

0

2

4

6

8

10

12

Diff_T (k)

-2 -1 0 1 2

Diff_RH (%)

-20 -10 0 10 20

Dust

Dust-free

Air temperature and relative humidity profiles in dusty and

dust-free sectors derived form AQUA AIRS/AMSU on

March 8 about 2:00 UTC

sector: dusty (DS) dust-free (DF)

convective clouds: weaker strong, variable

stratiform aloft: stronger weak, variable

bottom: similar more variable

Log10( Rain Water)

-2 -1 0 1

PDF(

%)

0

20

40

60

Log10(Rain Water)

-3 -2 -1 0 1

Evap

orat

ion R

ate

(km

-1)

0.0

.1

.2

.3

.4

Stratiform

Convective

Dust

Stratiform

Convective

Dust-free

(a) (b)

-4 ~ 0

Meteosat-8 RGB

composite visible image

at 09:12UTC

TMI precipitation

image on at 09:11

UTC

MODIS 03/08/04

0 ~ 4

Dusts transported up by the strong convection updraft acted as additional ice nuclei. Some of ice

particles grown under this dusty condition were carried up to upper levels by the strong convective

updraft and contributed to convective precipitation, and others were entered into the neighboring

stratiform region and slowly settled downward in the upper layer of the cloud system until they reach the

melting level.

Consequences of microphysical effects of dusts were shifting precipitation size spectrum from heavy

precipitation to light precipitation and suppressing precipitation. Dusts also enhanced evaporation

processes, which further reduced the precipitation reaching surfaces.

The microphysical processes of dust-cloud interaction had strong feedbacks to cloud

thermodynamics and altered the vertical gradient of heating profiles in both convective and stratiform

regions.

Cloud system adjusted itself for IN changes and resulted in a weak but long lasting cloud system with

increasing convective precipitation fraction and decreasing stratiform precipitation fraction.