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The ability in assessment of Total Precipitable Water (TPW) is useful in the prediction of the amount of raining, dam over-flooding and the flood. To extract TPW, the algorithm of infra- red bands near the MODIS sensor images were used. The satellite TPW, was validated using radiosonde data. Due to the limitation of the algorithm implementation to the cloud free sky and stable atmosphere, the general atmospheric conditions in the satellite passing date were investigated using auxiliary curves produced by synoptic and higher level meteorological data. In this way, the calm and eddy free atmosphere were selected. Then MODIS images were supplied from Iran Space Agency for this satellite passage. Then the TPW data were estimated using radiosonde and thermodynamics equation. Then regarding the stability and lack of new air masses in the region for the selected days (using analysis of the ground data and atmospheric profiles), the TPW for the time of satellite passage was interpolated.For determination of and in the aforementioned algorithm, EVI and ENDVI indices were deployed. At the end, a regression between the TPW produced by satellite and the one calculated from the radiosonde. showed that for the Mehrabad weather conditions, the MODIS channels 18 and 19 are suitable. Using the ratio of the apparent reflectance in the water vapor absorption bands to the one in the non-absorbing band, the atmospheric water vapor transparency for each one of the water vapor bands was calculated. The TPW in the earth-sensor path was calculated by implementing MODIS infrared bands under different atmospheric conditions, taking into account sensor and zenith angles, and the water vapor transparency using band ratio technique.

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Total Perceptible Water (TPW) is an important parameter in climatology and weather forecasting and is directly related to any climate process. There are three approaches to estimate this parameter i.e. using radiosonde, using GPS and calculating from satellite images where the first two are localized and the last one can give an instant view of TPW in a vast region. The algorithm used for the TPW calculation from MODIS images is related to the ratio of the reflectance in a water vapor absorbing channel and the reflectance in a non-absorbing channel. Due to strong horizontal variation in the surface reflectance in non-absorbing channels, the retrieved TPW varies strongly from one pixel to its neighboring pixels while it is believed that the horizontal gradient of TPW is very weak. To solve this problem, a damping coefficient was added to the non-absorbing channel reflectance. It is found that this coefficient differs for different surface covers. The current work presents a procedure for calculating these coefficients. The results of a comparison between modified TPW and those extracted from GPS data showed a R² of 0.81 whilst this was about 0.67 for non-modified MODIS TPW.