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Water vapor (Sobrino et al., 2003)

 

Water vapor is estimated using the method developed by Sobrino et al. (2003). This method is based on the attenuation of surface reflected solar radiation and clouds in near infrared due to water vapor. To this end, water vapor absorbing bands centered at 0.905, 0.936 and 0.94 μm (bands 17, 18 and 19 respectively) in addition to a water vapor transparent band centered at 0.865 μm (band 2) are used as follows:

 

W = 0.192 W₁₇ + 0.453 W₁₈ + 0.355 W₁₉

where

W₁₇ = 28.449 G₁₇² - 54.434 G₁₇ + 26.314

W₁₈ = 27.884 G₁₈² - 23.017 G₁₈ + 5.012

W₁₉ = 19.914 G₁₉² - 26.887 G₁₉ + 9.446

and

where W is atmospheric water vapor (g.cm^-2), and L₁₇, L₁₈, L₁₉ and L₂ are at sensor radiances (RAD-TOA) for MODIS bands 17, 18, 19 and 2 respectively.
In the case of night-time acquisitions, such method cannot be implemented due to the lack of solar radiation, therefore the water vapor estimates during the previous day are reprojected to a lat/lon grid, averaged and reprojected back to the night-time acquisition configuration for further calculations.

 

 

Reference

 

Sobrino, J. A., El Kharraz, J. & Li, Z. L. (2003). Surface temperature and water vapour retrieval from MODIS data, International Journal of Remote Sensing, 24, 5161-5182.