SAC-D and the Estimation of Water Vapor Over Land Surface
[12-Nov-2013] Epeloa, J., Meza, A., and Bava, J.A.
Presented at the 2013 Aquarius/SAC-D Science Team Meeting
A satellite-borne radiometer observing the Earth is sensitive to radiance which may emanate from both the surface and the atmosphere. Usually, a radiometer observation is expressed as a "brightness temperature" in kelvin. If the earth/sea surface were a black body and the atmosphere were transparent then the brightness temperature would equal the sea surface temperature (SST). Unfortunately, there is not any wavelength where such ideal conditions hold, so in practice satellite measurements of SST are made using regions of the electromagnetic spectrum where the sea surface emissivity is nearly unity (>0.95) and the atmosphere is relatively transparent (50 to 90% transmission of surface emission to the top of atmosphere [TOA]). The satellite SAC-D Aquarius takes on board a microwave radiometer (MWR) which is an 8 beam pushbroom, with a three channel Dicke radiometer that operates at 23.8 GHz(horizontal polarization) and 36.5 GHZ (vertical and horizontal polarization). This work estimates the water vapour (WV) over the ground earth using brightness temperatures at 37 Ghz and 24Ghz which are SAC-D's observables (currently calibrated to obtain WV over the sea surface). To calculate water vapor content in the atmosphere, a first order statistical regression was performed, where the water vapor value was the independent variable. The brightness temperatures for channels 24GHz (horizontal polarization), 37 Ghz (vertical polarization), and surface temperature were the dependent variables in the statistical regression. The water vapour and surface temperature data were obtained from radiosondes in the Southern of United States. The preliminary results shows that is possible to estimate the atmospheric water vapor from the 37 Ghz and 24Ghz brightness temperatures with a standard deviation less than 6 mm in a region around 100km from the radiosonde station.