Large-Scale Aerosol Effect on Precipitation in the West African Monsoon

1
Large-Scale Aerosol Effect on Precipitation in
the West African Monsoon

• Jingfeng Huang1
• Chidong Zhang1
• Joseph M. Prospero1
• 1 RSMAS, University of Miami

2
Questions ? Objectives

• Question 1 In addition to the observational
  evidence from long term data (Huang et al. QJ),
  whether short term quality data (MODISTRMM) can
  also see the large-scale aerosol effect on
  precipitation?
• Question 2 If so, can we differentiate aerosol
  effects on different rain types (convective,
  stratiform, shallow), and can we differentiate
  effects from dust and smoke?
• Question 3 What are the consistency and
  discrepancies between observations and
  simulations?
• Question 4 How do we discriminate large-scale
  aerosol effect on precipitation from the wet
  deposition (a.k.a. wash-out effect of
  precipitation on aerosol)?

3
Outline
Observations Part 1 ClimatologyPart 2
Monthly analysisPart 3 Daily analysis
SimulationsPart 4 BC radiative effect Part 5
Washout effect
Summary
4
TRMM Precipitation Climatology
Total Surface Rain
Convective Rain
Stratiform Rain
3A12 mm/d Rainno rain
Convective Rain
Stratiform Rain
Shallow Rain
3A25 mm/h Rain
Deep convective rain contributes the most to the
total surface rain in the West African Monsoon
region it has much higher rain rate than
stratiform rain and shallow rain. Shallow rain is
more often off shore in the Gulf of Guinea.
5
MODIS Aerosol Climatology (2000-2007, deep blue
2003-2007)
Total AOD
Fine AOD
Coarse AOD
Fine Mode Fraction
Climatology (MODIS AOD and TRMM Precipitation)
In the West Africa Monsoon region, dust is more
predominant over land. There is well mixture of
dust and smoke over ocean in the Gulf of Guinea,
particularly in the boreal cold season (December
April).
6
Observational results, monthly
Total Surface Rain
Convective Rain
Stratiform Rain
Shallow Rain
The precipitation suppression associated with
African aerosol is most outstanding in boreal
cold season and more systematic suppression is
found with convective and stratiform rain.
Coarse AOD
Fine AOD
Both coarse AOD and fine AOD suppress
precipitation in this region, but with different
spatial patterns.
7
Observational results, monthly
From satellite observations, both long term and
short term quality data confirm There is
significant large-scale coherent variability of
aerosol and precipitation, particularly in boreal
cold seasons. (Huang et al., QJ, submitted
Huang et al., ERL, submitted.
TOMS vs. GPCP
MODIS vs. TRMM
8
Observations Sensitivity to daily rain rates
The aeosol-associated precipitation change is
sensitive to rain rates From north to south
(continental to maritime), between high and low
aerosol terciles, the changes in the PDF of
precipitation is significantly different.
Precipitation suppression in (C) comes from the
combination of (1) more non-precipitating days
and (2) reduced precipitating rain rates.
A
B
D
C
9
Observational results, daily
Pair Up aerosol and precipitation in different
places at different time
Daily TRMM Precipitation (3B42)
Backward Trajectory (1d, 2d, 3d) HYSPLIT, 20
evenly distributed starting points in the WAM
domain, starting level 1500 m or 3000 m
Daily MODIS AOD at endpoints

• Dusty/Smoky case in comparison to Clean case in
  winter
• PDF non-precipitating days increase from 35 to
  50
• Mean precipitation (precipitating days only)
  decreases from 1.82 mm/d to 1.29 mm/d

10
Satellite observations v.s. model simulatins
Precip. Diff. Composite from observations
Precip. Diff. Composite in forcing run
The satellite observation and model simulation
are consistent on the aerosol-induced large-scale
precipitation suppression in the WAM region, in
terms of both spatial pattern (more significant
in the central WAM domain) and seasonal cycle
(more significant in boreal cold season).
11
Model simulation High BC vs. Low BC
BC
PREC
Between high and low black carbon terciles,
convective rain, which is the major contributor
to total surface rain, is significantly
suppressed, with downward anomalies in the
vertical convective motion. This signifies the
suppression of deep convection in the WAM region
due to black carbon radiative effect.
PREC Convective
Omega 500 hPa
12
Model simulation Wet deposition vs. Composite
analysis
Aerosol wet deposition can be indicated by the
grid-to-grid correlation between BC and
precipitation in the reference run when there is
no any aerosol effect on precipitation. The
spatial pattern and seasonal cycle of aerosol wet
deposition is significantly different from the
large-scale co-variability of aerosol and
precipitation, observed from satellites or
simulated from models.
Aerosol wet deposition in reference run
Precip. Diff. Composite in forcing run
13
Summary

• Both monthly and daily data analysis confirm the
  large-scale inverse relationship between aerosol
  and precipitation in the WAM region.
• Satellite and simulation are consistent on that
  aerosol affects large-scale precipitation in the
  WAM region through suppressing deep convection.
• The observed precipitation change attributed to
  aerosols cannot be explained by aerosol wet
  deposition.