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Sensitivity of WRF microphysics to aerosol concentration. Azusa Takeishi. 1/15. 1. Introduction. Effect of aerosols is one of the most uncertain factors in climate Direct effect/ Indirect effects/ Semi-direct effect Aerosol – Deep convection - PowerPoint PPT Presentation
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Azusa Takeishi
SENSITIVITY OF WRF MICROPHYSICS TO
AEROSOL CONCENTRATION
Eff ect of aerosols is one of the most uncertain factors in climate
Direct eff ect/ Indirect eff ects/ Semi-direct eff ect
Aerosol – Deep convectionEven though the areal coverage of deep convection is
small, its vertical motion is a part of local-/regional-/large-scale motion
It is important to understand the eff ect of aerosols on deep convective clouds and assess its climatic eff ects
1. INTRODUCTION
1/15
1. INTRODUCTIONHOW DOES DEEP CONVECTION CHANGE WITH AEROSOL
LOADING?
Rosenfeld et al. 2008
Cloud-aerosol
interaction
2/15
Weather Research and Forecasting (WRF) model Idealized simulation of quarter-circlular shear
supercellResolution: 2km (hori.), 500m (vert.)Maximum heating (3K) in the lower troposphere at the
center of the domain (200km*200km) -> trigger convection
3 hours of simulation with open boundary conditions
2. METHODS
-10 -5 0 5 10 15 20 25 30
-4
-3
-2
-1
0
1
2
3
4
750m
1250m
1750m
250m
4250m3250m 5250m6250m7250m
u [m/s]
v [m
/s]
3/15
Left: Skew-T log-P diagram of initial profile
Right: Hodograph of initial wind profile
Microphysics schemes: 2-moment schemes were chosen
2. METHODS
Morrison scheme (Morrison et al, 2005)
- 6 categories of hydrometeors (water vapor, cloud, rain, ice, snow, graupel/hail)- Fixed number concentration of
cloud droplets (default = 250 ) ↓
multiply the number of cloud droplets by 0.2, 0.5, 1 (control),
2, 3, 4, 5, and 6
Milbrandt-Yau scheme (Milbrandt and Yau, 2009)- 7 categories of hydrometeors (water vapor, cloud, rain, ice, snow, graupel, hail)- Calculate the number of CCN
using w, P, and T↓
multiply the number of CCN by 0.2, 0.5, 1 (control), 2, 3, 4, 5,
and 6Simulating 2 pristine cases + 1 control run + 5 polluted cases - How does the precipitation change? Why?
4/15
Accumulated precipitation slightly decreases with increased aerosols
But this decrease is quite small: ~5%
Moreover, this decreasing trend disappears when the horizontal resolution is set to 1km, when the heating is changed to 2K or 4K, and with graupel
3. RESULTS [MORRISON SCHEME]
0:000:10
0:200:30
0:400:50
1:001:10
1:201:30
1:401:50
2:002:10
2:202:30
2:402:50
3:000
0.5
1
1.5
2
2.5
3
3.5
4
50
125
250(control)
500
750
1000
1250
1500
Time
Dom
ain-
aver
aged
acc
umul
ated
pre
cipita
tion
[mm
]
5/15
Downdraft shows this decrease in precipitation intensity
Updraft does notNo interaction
between downdraft and updraft
3. RESULTS [MORRISON SCHEME]
0:000:10
0:200:30
0:400:50
1:001:10
1:201:30
1:401:50
2:002:10
2:202:30
2:402:50
3:000
5
10
15
20
25
30
35
40
45
50
55
60
-30
-27.5
-25
-22.5
-20
-17.5
-15
-12.5
-10
-7.5
-5
-2.5
0
50125250(con-trol)50075010001250150050125250(con-trol)
Time
Max
imum
ver
tical
vel
ocity
[m/s
]
Minim
um vertical velocity [m
/s]
No robust aerosol effect found in the simulations with Morrison scheme
6/15
Drastic decrease in accumulated precipitation
Almost 80% decrease in precipitation, when we compare the most pristine case and the most polluted case
Robust feature, even when the horizontal resolution or the perturbation is modified (2K or 4K)
3. RESULTS [MILBRANDT SCHEME]
0:000:10
0:200:30
0:400:50
1:001:10
1:201:30
1:401:50
2:002:10
2:202:30
2:402:50
3:000
0.5
1
1.5
2
2.5
3
3.5
4
0.2*control
0.5*control
control
2*control
3*control
4*control
5*control
6*control
Time
Dom
ain-
aver
aged
acc
umul
ated
pre
cipita
tion
[mm
]
7/15
Both downdraft and updraft show the invigoration in the storm system
Almost symmetric change implies the interaction between downdrafts and updrafts
3. RESULTS [MILBRANDT SCHEME]
0:000:10
0:200:30
0:400:50
1:001:10
1:201:30
1:401:50
2:002:10
2:202:30
2:402:50
3:000
5
10
15
20
25
30
35
40
45
50
55
60
-30
-27.5
-25
-22.5
-20
-17.5
-15
-12.5
-10
-7.5
-5
-2.5
0
0.2*control0.5*controlcontrol2*control
Time
Max
imum
ver
tical
vel
ocity
[m/s
]
Minim
um vertical velocity [m
/s]
Robust aerosol effect found in the simulations with Milbrandt-Yau schemeBut why?
8/15
WHICH HYDROMETEOR CONTRIBUTES?
Decrease in precipitation
RAIN
CLOUD
ICE SNOW GRAUPEL HAIL
Stop melting of (i) Hail (ii) Hail and graupel(iii) Hail, graupel, and snow
and see how frozen precipitation changes.
Melt
9/15
FROZEN PRECIPITATION
-> Decrease in graupel is the major reason for the decrease in total precipitation
0.2*control
control 3*control 6*control0
0.5
1
1.5
2
2.5
Control runs
liquidfrozen
CCN concentration
Accum
ula
ted p
recip
itati
on
0.2*control
control 3*control 6*control0
0.5
1
1.5
2
2.5
(i) No Melting of hail
liquidfrozen
CCN concentration
Accum
ula
ted p
recip
itati
on
0.2*control
control 3*control 6*control0
0.5
1
1.5
2
2.5
(ii) No melting of graupel & hail
liquidfrozen
CCN concentrationAccum
ula
ted p
recip
itati
on
0.2*control
control 3*control 6*control0
0.5
1
1.5
2
2.5
(iii) No melting of snow, graupel, & hail
liquidfrozen
CCN concentrationAccum
ula
ted p
recip
itati
on
10/15
GRAUPEL
11/15
CCN=0.2*control CCN=0.5*control CCN=control CCN=2*control
CCN=3*control CCN=4*control CCN=5*control CCN=6*control
[g/kg]
Domain-average graupel mixing ratio
WHY DOES GRAUPEL DECREASE?
Graupel forms when snow collects cloud droplets (riming) Only when riming rate > deposition rate Riming rate of snow is dependent on (i) Total number of snow (ii) Total number of cloud droplets(iii) Size spectrum of snow(iv) Size spectrum of cloud droplets
In pristine cases, cloud droplets are larger, soriming rate > deposition rate
-> production of graupel -> fast/effi cient precipitation
In polluted cases, cloud droplets are small, sodeposition rate > riming rate
-> production of snow -> slow/ineffi cient precipitation
12/15
SNOW
13/15
CCN=0.2*control CCN=0.5*control CCN=control CCN=2*control
CCN=3*control CCN=4*control CCN=5*control CCN=6*control
[g/kg]
Domain-average snow mixing ratio
Interaction between aerosols and deep convection is still not well understood
Simulations with Morrison scheme showed a low sensitivity to the aerosol concentration (4.9% reduction in precipitation) and this change is not robust
Simulations with Milbrandt scheme showed a high sensitivity to the aerosol concentration (79% reduction in precipitation) and this decrease is robust
Realistic simulations are necessary: types of aerosols, aerosol concentration, ideally 7 hydrometeors, inclusion of all processes, etc.
4. CONCLUSION
14/15
Thompson schemeChange humidity profi leAdd radiationConstrain simulations with observational dataUse of WRF-Chem
5. FUTURE WORK
15/15