Effect of dust on rainfall

1. Effect of dust on rainfall
over the Red Sea coast from WRF-Chem simulations
- Sagar Parajuli
March, 2022

2. A few questions to ponder
• What is dust?
• Is dust good or bad?
• Is there a difference between
natural and anthropogenic
dust?

3. Interesting facts about dust/aerosols
• Sand dunes are dynamic system
that reorganize constantly --
moving at the rate of about 8-12
feet per year.
• Sand from sand dunes cannot be
used for construction.
• Dust is the smallest building block
of entire physical existence that
can be physically measured (dust
emission/deposition can be
measured directly)
• The Solar System was formed from
a large, rotating cloud of
interstellar dust and gas called the
solar nebula

4. • Without dust or aerosols
there would be no rainfall1,
can you imagine?
• Measurements in high
Cirrus clouds show that
Mineral dust dominantly
forms IN2
• Dust from China can affect
precipitation in California,
USA3
2Cziczo et al., 2013
Interesting facts about dust/aerosols
3Creamean et al., 2013
1J. Aitken (18th century)

5. Why dust is good?
• Dust cycle is an important part of
the Earth System.
• Dust scatters and absorbs SW and
LW radiation thus regulates climate
and environment
• Mineral dust acts as a fertilizer (P,
Fe) for terrestrial/marine
ecosystem1.
• Dust is a key player of
long-term climate
(paleoclimate).
1[Koren, et al., 2006]

6. Why dust is good?
• Dust affects rainfall: modifies cloud
microphysical properties by forming
IN/CCN.
• Most dust particles are coarse in size, so
dust is not a big problem itself for air-
quality.
• People have lived in the desert regions
since ages, natural dust/sand was not an
air-quality threat.

7. When is dust a problem?
• Dust became an air-quality issue after
increased human activity, which
generated anthropogenic dust
• Anthropogenic dust generated from
agricultural/disturbed lands and
construction: finer particles and may
contain environmental pollutants
• Its ageing, or interaction with other
pollutant aerosols and gases makes it a
threat.
• Anthropogenic pollutants typically
have higher concentration of PM2.5.
• Dust can act as a medium to transmit
human/plant diseases1.
1[Kellogg and Griffin 2006]

8. When is dust a problem?
• Dust storms affect daily life activities,
hinders air/ground traffic operation.
• Affects the efficiency of solar panels and
wind turbines.
• Corrodes historical monuments and
buildings
• Anthropogenic dust can affect coral
health: bleaching
• Wind erosion removes soil nutrients (e.g.
soil organic carbon)1 thus affects
agricultural productivity
• Severe dust storms can bring economic
disaster: dust bowl of the 1930s.
1[Li et al., 2008]

9. Pollution is a regional
environmental problem
• Anthropogenic dust in Tigris-
Euphrates region affects air quality
in the downwind region: e.g.,
Kuwait, UAE, Saudi Arabia
• Depleted Uranium was used heavily
in the Tigris-Euphrates region
during Gulf War (1991).
• The U.S. Navy’s mesoscale
meteorological system COAMPS1
was used for real-time dust forecasts
during Operation Iraqi Freedom
(OIF) in 2003.
• Regional cooperation is essential for
mitigating the adverse effects of air
pollution
1Coupled Ocean/Atmosphere Mesoscale Prediction System
Tigris-Euphrates region

10. Health aspects of air pollution
• Long-term exposure of
PM2.5 and finer particles
can cause severe
cardiovascular diseases
• COVID-19 deaths are
associated with the level
of background air
pollution
1https://www.latestgkgs.com/science-technology-5467-a
2https://academic.oup.com/cardiovascres/article/116/14/2247/5940460?login=true

11. Dust effect on rainfall
• Water is essential for plant
and animal life
• Rainfall affects the
distribution of surface and
ground water resources,
which are constantly
declining over the MENA1
1Joodaki et al., 2014
why study?

12. • Irregular rainfall patterns: causes
floods and droughts
• Also increase water-borne diseases
such as malaria and diarrhea2
• Understanding how dust affects
the regional rainfall pattern is
crucial, particularly in regions
with high atmospheric dust loads
such as MENA
Dust effect on rainfall:
why study?
2Trinh et al., 2020

13. WRF-Chem model domain
The study area over the Red Sea coast (d03) showing nested domains (d01
and d02) used in WRF-Chem model simulations.

14. Summary of WRF-Chem model
configuration
• Cloud-resolving simulations at 1.5 × 1.5
km – thanks to Shaheen supercomputer
• Simulations carried for the month of
August from 2006-2015
• We calibrated dust amount and size
distribution to obtain realistic AOD and
aerosol sizes
• MOSAIC 8-bin sectional aerosol scheme
• Double-moment microphysics scheme
(Morrison)
• Boundary conditions from high-
resolution ECMWF operational analysis
with SST updates

15. Model experiments
The total effect (ΔTot), indirect effect (ΔIndir), and direct effect (Δdir) of dust is
then given by,
Δtot = F1-F2 (1)
Δindir = F3-F4 (2)
Δdir = ΔTot-ΔIndir = (F1-F2)-(F3-F4) (3)
Aerosol
species
Experiments with both direct
and indirect effects
Experiments with indirect
effects only
F1
all_aer
F2
no_dust
F3
all_aer, no_direct
F4
no_dust,
no_direct
Dust yes no yes no
Sea salt yes yes yes yes
Anthrop-
ogenic
yes Yes yes Yes

16. Validation
Abha Field Campaign
August 2009
Micropulse
Lidar (MPL)

17. Results

18. How does the model perform?

19. How does the model perform?
Daily-
accumulated
rainfall
Diurnal wind
profile at
KAUST.

20. How does the model perform
KAUST lies
slightly out of
the ‘Rain Zone’.

21. How does model perform?

22. Dust effect on rainfall
• Over the study region,
rainfall is very much
predictable
• The daily maximum
rainfall occurs exactly
at 15:00 UTC (6pm
local time), at the
same place -- over the
mountains.
• It is because the
moisture availability is
governed by sea
breezes which are
recurring feature.

23. Rain processes over the Red Sea coast
Diurnal rainfall
due to warm-
cloud processes
Profile of cloud water mixing ratio

24. CCN and aerosol size
comparisons
• Comparison with data from
Abha field campaign: August
2009
• CCNs are generally
overestimated by up to a
factor of two.

25. CCN comparisons
with VIIRS Satellite
data
• The # of activated CCN in a convective
cloud base is a function of cloud drop
effective radius Re, which can be
retrieved from a satellite imager with
high-resolution wave bands1.
• Similarly, the cloud base updraft
speeds is a linear function of cloud-
base height2.
• Again, the model overestimates CCNs
• Although large, difference is
reasonable for microphysical
parameters given their high
uncertainty
VIIRS satellite-retrieved CCN # concentrations, August 2015
1Rosenfeld et al., 2014 2Yue et al., 2019

26. Dust contribution on CCN # concentration
• There is up to ten-fold increase of CCN, making
dust the major contributor
• Although overestimated, CCN #s much closer to
observations with dust.
• Rainfall amount well simulated -- CCN is not a
limiting factor for rain formation.
• Cloud growth does not strongly depend on CCNs,
unlike in other aerosol-limited areas1
CCN # concentrations at 0.2%
at a cloud-level height (570
hPa) averaged at 15:00 UTC for
August, 2015
1Koren et al., 2014

27. Dust effect on
radiative balance
• Indirect effect very small,
total effect governed by
direct effect
• Decreases SW radiation
and increases LW
radiative fluxes at the
surface
• Δtot (top row)
• Δindir (middle row)
• Δdir (bottom row)

28. Dust effects
• 2-m air temp.
• 10-m winds
• 2-m water vapor
mixing ratio
• Black dots:
statistically
significant areas
• Δtot (top row)
• Δindir (middle row)
• Δdir (bottom row)

29. Direct effects
• Warming and high
moisture over the
lands due to enhanced
breeze circulation
• Δtot (top row)
• Δindir (middle row)
• Δdir (bottom row)
direct effects only
experiments
direct effects only but
without SW absorption
(SW refractive index
changed from 0.003 to 0)

30. Physical mechanisms: direct effect
• Well understood
• Dust induces land surface cooling (warming) by SW scattering (absorption)
• Land cooling (warming) ultimately weakens (strengthens) the sea breeze circulation, thus
reducing (increasing) the landward moisture transport.

31. Physical mechanisms:
indirect effects
• Indirect effects have higher
uncertainty because aerosol
effects on clouds and rainfall
process less understood
• During the early 21st century,
research showed that aerosols
suppress rainfall1
• Current understanding: aerosols
suppress rainfall initially but
enhances during the later stage of
the convection, through a process
called aerosol invigoration2
2(Koren et al., 2012)
1(Rosenfeld et al., 2000)

32. Results: Dust effects on rainfall
Normal rainfall events Extreme rainfall events
Indirect
effect
(%)
Direct
effect
(%)
Total
effect
(%)
Indirect
effect
(%)
Direct
effect
(%)
Total
effect
(%)
4.76 -5.78 -1.02 4.54 1.51 6.05
Yes Yes Yes Yes No Yes
• Dust enhances extreme rain events but suppresses normal rain events.
• Consistent with previous studies (e.g., Alizadeh-Choobari, 2018; Li et al.,
2011): dust increases (decreases) rainfall in high (low) rainfall conditions.
Significant?

33. Summary
• In summary, dust enhances rainfall for extreme-rainfall
events but suppresses rainfall for normal-rainfall events.
• For normal-rainfall events, the suppressing direct effect is
strong and significant, which is governed by the weakening
of the sea breezes in response to SW cooling by dust.
• The extreme rainfall events are governed by diverse synoptic
processes and breezes do not play significant role in the
effect.

34. Broader implications
• Dust and dust storms are considered as problems from
an air quality perspective, but our study highlights their
positive contribution in modulating rain
• Managing of regional water resources1, in the context of
Saudi Green Initiative – planting 10 billion trees
• Understanding of extreme rainfall events: minimize the
risk of heavy flooding and loss of public property
• Implications for cloud seeding -- regional rain
enhancement efforts. Presence of background dust
aerosols must be considered in the dusty regions.
1(Mostamandi et al., 2020)

35. Why?
Where?
When?
Who?
What?
Which?
Questions