WATER VAPOR RETRIEVAL OVER

CLOUD COVER AREA ON LAND

Dabin Ji, Jiancheng Shi, Shenglei Zhang

Institute for Remote Sensing Applications Chinese Academy of Sciences, China

Outline

Introduction

Basic Theory

Water Vapor Retrieval

Validation

Introduction

Current status of Remote Sensing Water vaporAdvantage Disadvantage

Optical RSHigh precision and spacial resolution.

Only for Clear condition

Microwave

RS

OceanPenetrate cloud.Precision is 2.5mm.

Low spacial resolution

Land Penetrate cloud

a. Not very sensitive to water vapor due to high surface emissivity.

b. Low spacial resolution.c. No operational running

algorithm.

Introduction Current methods overland.

1. Neural Network.

2. Two frequency ratio of polarization difference of Tb ( TbΔ 18.7/ TbΔ 23.8, Deeter et al., 2007, Wang et al., 2010)

In this study:

Based on method TbΔ 18.7/ TbΔ 23.8 with improvement on surface emissivity estimation

Basis of Two Frequency ratio of Polarization DifferenceBrightness received by Microwave sensor[Jones, A. S, 1990; Liou K. N, 2004]

2, , ,

2,

(0) ( ,0) (1 )[ ( ,0)]

(1 )[ ( ,0)]

Sur atm atmv p v p v s v v p v s v

v p v s Space

Tb Tb t P Tb t P Tb

t P Tb

ε

ε

↑ ↑ ↓= + + −

+ −

, ,Surv p v p sTb Tε↑ =

0 ( ,0)( )

s

atm vv P

t PTb T P dP

P↑ ∂=

∂∫

0

2

( ,0)( )

[ ( ,0)]s

atm vv P

v

t PT PTb dP

t P P↓ ∂=

∂∫

ε: Surface emissivityt: TransmittanceP: PressureT: Temperature

Subscript v: Frequency,p: Polarization, s: Surface.

(1)

Polarization difference of Tb2

, , , , , ,

2, ,

( ) ( ,0) ( )[ ( ,0)]

( )[ ( ,0)]

Sur Sur atmv v V v H v V v H v s v V v H v s v

v V v H v s space

Tb Tb Tb Tb Tb t P t P Tb

t P T

ε ε

ε ε

↑ ↑ ↓∆ = − = − − −

− −

Ratio of ΔTb at two frequency2 2

18.7 18.7 18.7 18.718.7 18.7 18.72 2

23.8 23.8 23.823.8 23.8 23.8 23.8

( , )atm

s spacesatm

s space

T t t Tb t TbTbf T wv

Tb T t t Tb t Tb

ε εε ε

↓

↓

− −∆ ∆ ∆= ⋅ = ⋅∆ ∆ ∆− −

(2)

(3)

Understanding ： Δε18.7 and Δε23.8 are well correlated for land surface conditions. In previous study (Wang et al., 2010), the first part Δε18.7/Δε23.8 in Eq (3) is equal to a constant.

Basis of Two Frequency ratio of Polarization Difference

The second part on the right of Eq.(3) is a function of water vapor (wv).

Both TbΔ 18.7/ TbΔ 23.8 and TbΔ 36.5/ TbΔ 89 are more sensitive than single Tb.

TbΔ 36.5/ TbΔ 89 will not be used in the retrieval due to its sensitivity to cloud liquid water.

Basis of Two Frequency ratio of Polarization Difference

Sensitivity AnalysisUsing MODIS water vapor products (MOD07) under clear sky condition, the surface emissivities can be derivedRelationships of surface emissivity at frequency 18.7 and 23.8GHz estimated from the South Great Plain in USA.

V-POL H-POL V - H

Jan, 2007

Mean and variance of Δε18.7/Δε23.8 have significant change over seasons

The assumption of Δε18.7/Δε23.8 might result in a significant error in atmospheric water vapor estimation

V-POL H-POL V - H

July, 2007

Sensitivity Analysis

Water Vapor Retrieval with AMSR-E

From measurements of TbΔ 18.7/ TbΔ 23.8, if Δε18.7/Δε23.8

and Surface temperature (Ts) are known, then water vapor can be estimated.

Ratio of ΔTb at two frequency

2 218.7 18.7 18.7 18.718.7 18.7 18.7

2 223.8 23.8 23.823.8 23.8 23.8 23.8

( , )atm

s spacesatm

s space

T t t Tb t TbTbf T wv

Tb T t t Tb t Tb

ε εε ε

↓

↓

− −∆ ∆ ∆= ⋅ = ⋅∆ ∆ ∆− −

(3)

Estimation of Surface Temperature with AMSR-E

Ts is estimated using Eq.(4) [Holmes, T. R. H., 2008].

36.5 36.51.11 15.2, 259.8s V VT Tb Where Tb K= − > (4)

Tb36.5V is AMSR-E vertical polarization brightness temperature at frequency 36.5GHz

Water Vapor Retrieval with LUT

MODEL: 1-D Microwave Radiative Transfer Model (1DMRTM), (Olson, William S, 2001) used to build LUT for estimating emissivity.

Atmospheric profiles

Parameter of Microwave Radiometer

Surface temperature

Atmospheric parameter calculating

Eddington two stream calculation

Simulated Microwave Brightness Temperature

Cosmic T(2.7K)

Input

Calculate

Output

Estimation of Δε18.7/Δε23.8

Data used: §AMSR-E brightness temperature§MODIS-MYD07 (atmospheric profiles and surface temperature)

Estimating Δε18.7/Δε23.8 at clear sky. For cloudy, it’s estimated using 7-day average data.

Water Vapor Retrieval Summary

Outline of the retrieval:§Re-project AMSR-E TB and MYD07 into 0.25° -0.25° grid image.§Calculate TbΔ 18.7/ TbΔ 23.8 using AMSR-E TB.§Estimating Ts in cloudy condition using Eq. (4).§Estimating Δε18.7/Δε23.8 .

§Using LUT to estimate water vapor

Comparison with GPS Estimation

Water vapor in Jan. 2007 was retrieved over cloud cover area in South Great Plain of USA.

GPS retrieved water vapor from SuomiNet was used for comparison

Possible Errors

Uncertainty of Δε18.7/Δε23.8 in cloudy

condition due to precipitation event and

other events changing surface

emissivity.

1D atmospheric Microwave RT model

itself

Surface temperature estimation

Spatial scale difference of the

retrieved water vapor and GPS water

vapor.

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