Sensitivity of cloud microphysical property retrieval methods to size distribution

bi-modality: Theoretical considerations and potential implications

MODIS Science Team MeetingMay 7, 2012, Silver Spring, MD

Zhibo Zhang (UMBC), Steven Platnick (NASA GSFC), Andrew Ackerman (NASA GISS), Hyoun-Myoung Cho(UMBC)

Graham Feingold (NOAA ESRL), Robert Pincus (NOAA ESRL), Matthew Lebsock (JPL)

Monday, May 7, 12

Background

MODIS Operational re(2.1µm)

Monthly Mean Effective Radius of Liquid Water Cloud

CAM5-COSP MODIS simulation

Data courtesy of Jennifer Kay

Substantial difference between observation and GCM.

Monday, May 7, 12

Background

MODIS Operational re(2.1µm) MODIS Operational re(3.7µm)

Substantial difference between MODIS 2.1µm and 3.7µm.

Monthly Mean Effective Radius of Liquid Water Cloud

Monday, May 7, 12

Achievements of Year One

• Performed a global assessment of the difference between difference between MODIS re(2.1µm) and re(3.7µm) retrievals (Zhang and Platnick 2011 JGR)

• Developed a MODIS-simulator based on the combination of LES model with bin microphysics and radiative transfer models (both 1-D and 3D)

• Case studies on the effects of cloud horizontal inhomogeneity and drizzle on MODIS effective radius retrievals (Zhang et al. 2012 under revision)

• Collocated MODIS and CloudSat observations of MBL clouds

Monday, May 7, 12

Probability of Precipitation from collocated CloudSat

Assessment of the difference between MODIS re(2.1µm) and re(3.7µm)

Sub-pixel inhomogeneity indexPPL cloud Broken clouds

Zhang and Platnick 2011 JGR

r e(3

.7µm

)-r e

(2.1µm

)

Regime I:Non-precipitating, PPL clouds

Regime III:Drizzling cloud?

Reg

ime

II:H

ighl

y in

hom

ogen

eous

clo

uds

Whether and How can drizzle affect MODIS Re retrievals?Monday, May 7, 12

Probability of Precipitation from collocated CloudSat

Assessment of the difference between MODIS re(2.1µm) and re(3.7µm)

Sub-pixel inhomogeneity indexPPL cloud Broken clouds

Zhang and Platnick 2011 JGR

r e(3

.7µm

)-r e

(2.1µm

)

Regime I:Non-precipitating, PPL clouds

Regime III:Drizzling cloud?

Reg

ime

II:H

ighl

y in

hom

ogen

eous

clo

uds

Whether and How can drizzle affect MODIS Re retrievals?Monday, May 7, 12

Effects of precipitation (Bi-modal PSD)?

Insignificant impacts Significant impacts

Zinner et al. 2011 ACPZhang et al. 2012 JGR

(under review)Painemal et al. 2011 JGR

Nakajima et al. 2010a,bMinnis et al. 2004

Effects of precipitation on passive Re retrieval still remains unclear

LES-based study In-situ measurement Sensitivity Study

Monday, May 7, 12

Objective: Establish a theoretical understanding of how MODIS Re retrieval response to the drizzle

mode in PSD

Questions:1) Can we derive a “back-of-envelope” formula to estimate

the impact of drizzle mode on MODIS Re retrieval?2) whether and how can bi-modal PSD cause Re retrieval

difference?3) What are the potential implications?

Monday, May 7, 12

Theoretical Consideration: Simplification of the problem

R(re*,τ *) = R(rc ,rp ,τ c ,τ p )Real retrieval

Simplified retrieval

Assumptions: 1) Single-scattering albedo can be used as surrogate for reflectance for assessing retrieval process (to be justified by RT simulations)2) Cloud is optically thick so that Tau and Re retrievals are independent3) Cloud is homogenous (NO 3-D effect or vertical weighting)

ω c(re*) =ω (rc ,rp ,τ c ,τ p )

Monday, May 7, 12

Theoretical consideration: Linearization

10 15 20 25 30re [ m]

0.70

0.75

0.80

0.85

0.90

0.95

1.00

PSD

aver

aged

ω

ve=0.06ve=0.10ve=0.16ve=0.20linear fit

30 40 50 60 70 80 90 100re [ m]

0.4

0.5

0.6

0.7

0.8

0.9

1.0

PSD

aver

aged

ω

Mielinear fit

Radius [µm] Radius [µm]

PSD

Ave

rage

d Al

bedo

ω

a) cloud mode b) rain mode

2.1µm 3.7µm 2.1µm 3.7µm

ω c,λ =ω 0c,λ − kc,λre ω r ,λ =ω 0r ,λ − kr ,λre

Gamma distribution Truncated Exponential (Wood et al. 2005 JAS)

Effective radius Effective radius

kc,3.7µm = 0.0061µm−1

kc,2.1µm = 0.0018µm−1

kp,2.1µm = 0.0014µm−1

kc,3.7µm = 0.0021µm−1

1) Albedo decreases linearly with increasing Re 2) Decreasing rate depends on spectral and Re range3) Albedo-Re line is flatter in drizzle region than in cloud region

Monday, May 7, 12

Theoretical consideration: Formula for estimating effect of drizzle on

MODIS re retrieval

re,λ* = re,c 1+

RWPCWP

kp,λkc,λ

⎛

⎝⎜⎞

⎠⎟−τ pτ c

re,c −ω 0c,λ −ω 0 p,λ

kc,λ

⎛

⎝⎜⎞

⎠⎟

re,λ*

Retrieved effective radius (spectral dependent)

re,c Effective radius of cloud modeStrength of drizzle (in upper part (Tau

RT simulations

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

Cloud mode: re = 15µm τ ≈ 20 CWP = 300g /m2

Rai

n m

ode

r e [µ

m]

Rai

n m

ode

r e [µ

m]

Rai

n m

ode

r e [µ

m]

Rai

n m

ode

r e [µ

m]

RWP/CWP RWP/CWP

RWP/CWP RWP/CWP

re*(2.1µm) re

*(3.7µm)

DISORTRef

TheoAlbedo

26

24

22

20

18

16

14

Monday, May 7, 12

Effect of Bi-Modality on Polarization-based Re retrieval

144 146 148 150 152 154 156Scattering Angle [o]

-0.05

0.00

0.05

0.10

-P12

10 20 30 40 50Effective Radius [µm]

0.02

0.04

0.06

0.08

0.10

Effe

ctiv

e Va

rianc

e

Cloud mode Re=15µmCWP=300g/m2

Rain mode Re=80µm RWP

= 300g/m2

Combined

Retrieval cost function

Polarization-based retrieval ~ 15.5µm

Polarization insensitive to drizzle mode

Monday, May 7, 12

Effect of Bi-Modality on Polarization-based Re retrieval

144 146 148 150 152 154 156Scattering Angle [o]

-0.05

0.00

0.05

0.10

-P12

10 20 30 40 50Effective Radius [µm]

0.02

0.04

0.06

0.08

0.10

Effe

ctiv

e Va

rianc

e

Cloud mode Re=15µmCWP=300g/m2

Rain mode Re=80µm RWP

= 300g/m2

Combined

Retrieval cost function

Polarization-based retrieval ~ 15.5µm

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

0.2 0.4 0.6 0.8 1.0RLWC/CLWC

60

80

100

120

140

160

180200

Rai

n m

ode r e

[m

]

MODIS 2.1µm retrieval ~22µm Polarization insensitive to drizzle mode

Monday, May 7, 12

Potential Implications: GCM/MMF-Satellite comparison

Cloud mode re Cloud + Precip. re

MODIS re

Preliminary results:inclusion of precip. mode leads to better agreement between a

MMF simulation and MODIS

MMF data courtesy of Minghuai Wang

Monday, May 7, 12

Summary

• Observations show correlation between precipitation and MODIS cloud effective radius retrieval “anomaly” (drizzle effects?)

• A simple formula is derived to predict the impact of PSD bi-modality (i.e., drizzle) on MODIS cloud effective radius retrieval

• More investigations are underway

Monday, May 7, 12

Future Work• LES simulation of heavily drizzling clouds- LES-MODIS simulation- LES-polarimeter simulation• Regime by regime study of cloud microphysics

using collocated MODIS-CloudSat data

• Investigation of the implications for GCM-satellite comparison

• Investigation of air-borne observations?

Monday, May 7, 12

Thank You!

• Questions?

Monday, May 7, 12