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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.