WATER VAPOR RETRIEVAL OVER CLOUD COVER AREA ON LAND
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Transcript of WATER VAPOR RETRIEVAL OVER CLOUD COVER AREA ON LAND
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 RS High precision and spacial resolution. Only for Clear condition
Microwave
RS
Ocean Penetrate cloud.Precision is 2.5mm. Low spacial resolution
Land Penetrate cloud
①Not very sensitive to water vapor due to high surface emissivity.
②Low spacial resolution.③No operational running
algorithm.
Introduction Current methods overland.
1. Neural Network.
2. Two frequency ratio of polarization difference of Tb (ΔTb18.7/ΔTb23.8, Deeter et al., 2007, Wang et al., 2010)
In this study:
Based on method ΔTb18.7/ΔTb23.8 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
, ,Surv p v p sTb T
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
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
(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 ΔTb18.7/ΔTb23.8 and ΔTb36.5/ΔTb89 are more sensitive than single Tb.
ΔTb36.5/ΔTb89 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 derived
Relationships 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 ΔTb18.7/ΔTb23.8, 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
(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 LUTMODEL: 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:
①AMSR-E brightness temperature
②MODIS-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:
1.Re-project AMSR-E TB and MYD07 into 0.25°-0.25° grid image.
2.Calculate ΔTb18.7/ΔTb23.8 using AMSR-E TB.
3.Estimating Ts in cloudy condition using Eq. (4).
4.Estimating Δε18.7/Δε23.8.
5.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.
THANK YOU FOR YOUR ATTENTION!