1. 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

2. Outline  Introduction  Basic Theory  Water Vapor Retrieval  Validation

3. Introduction Current status of Remote Sensing Water vapor AdvantageDisadvantage Optical RS High precision and spacial resolution. Only for Clear condition Micro wave RS Ocean Penetrate cloud. Precision is 2.5mm. Low spacial resolution LandPenetrate cloud ①Not very sensitive to water vapor due to high surface emissivity. ②Low spacial resolution. ③No operational running algorithm.

4. Introduction  Current methods overland. 1. Neural Network. 2. Two frequency ratio of polarization difference of Tb (ΔTb 18.7 /ΔTb 23.8, Deeter et al., 2007, Wang et al., 2010)  In this study: Based on method ΔTb 18.7 /ΔTb 23.8 with improvement on surface emissivity estimation

5. Basis of Two Frequency ratio of Polarization Difference Brightness received by Microwave sensor [Jones, A. S, 1990; Liou K. N, 2004] ε: Surface emissivity t: Transmittance P: Pressure T: Temperature Subscript v: Frequency, p: Polarization, s: Surface. (1)

6. Polarization difference of Tb Ratio of Δ Tb at two frequency (2) (3) Understanding ： Δε 18.7 and Δε 23.8 are well correlated for land surface conditions. In previous study (Wang et al., 2010), the first part Δε 18.7 /Δε 23.8 in Eq (3) is equal to a constant. Basis of Two Frequency ratio of Polarization Difference

7. The second part on the right of Eq.(3) is a function of water vapor (wv).  Both ΔTb 18.7 /ΔTb 23.8 and ΔTb 36.5 /ΔTb 89 are more sensitive than single Tb.  ΔTb 36.5 /ΔTb 89 will not be used in the retrieval due to its sensitivity to cloud liquid water. Basis of Two Frequency ratio of Polarization Difference

8. Sensitivity Analysis Using 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-POLV - H Jan, 2007

9. Mean and variance of Δε 18.7 /Δε 23.8 have significant change over seasons The assumption of Δε 18.7 /Δε 23.8 might result in a significant error in atmospheric water vapor estimation V-POL H-POLV - H July, 2007 Sensitivity Analysis

10. Water Vapor Retrieval with AMSR-E From measurements of ΔTb 18.7 /ΔTb 23.8, if Δε 18.7 /Δε 23.8 and Surface temperature (T s ) are known, then water vapor can be estimated. Ratio of Δ Tb at two frequency (3)

11. Estimation of Surface Temperature with AMSR-E T s is estimated using Eq.(4) [Holmes, T. R. H., 2008]. (4) Tb 36.5V is AMSR-E vertical polarization brightness temperature at frequency 36.5GHz

12. 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

13. Estimation of Δε 18.7 /Δε 23.8 Data used: ① AMSR-E brightness temperature ② MODIS-MYD07 (atmospheric profiles and surface temperature) Estimating Δε 18.7 /Δε 23.8 at clear sky. For cloudy, it’s estimated using 7-day average data.

14. 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 ΔTb 18.7 /ΔTb 23.8 using AMSR-E TB. 3. Estimating Ts in cloudy condition using Eq. (4). 4. Estimating Δε 18.7 /Δε 23.8. 5. Using LUT to estimate water vapor

15. 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

16. Possible Errors  Uncertainty of Δε 18.7 /Δε 23.8 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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