Assessment of global total column water vapor sounding using a spaceborne differential absorption radar

Abstract

The feasibility of using a differential absorption radar (DAR) to retrieve total column water vapor from space is investigated. DAR combines at least two radar tones near an absorption line, in this case a water vapor line, to measure humidity information from the differential absorption “on” and “off” the line. From a spaceborne platform, DAR can be used to retrieve total column water vapor by measuring the differential reflection from the Earth's surface. We assess the expected precision, yield, and potential biases of retrieved total column water vapor values by applying an end-to-end radar instrument simulator to near-global weather analysis fields collocated with CloudSat measurements. The approach allows us to characterize the DAR performance across a globally representative dataset of atmospheric conditions including clouds and precipitation as well as different surface types. We assume a hypothetical spaceborne G-band radar with pulse compression orbiting the Earth at 405 km with a 1 m antenna, equivalent to a footprint diameter of 850, and 500 m horizontal integration. The simulations include the scattering effects of rain, snow, as well as liquid and ice clouds, spectroscopic uncertainties, and uncertainties due to the initial assumed water vapor profile. Results indicate that using two radar tones at 167 and 174.8 GHz with a transmit power of 20 W ensures that both pulses will be detected with a signal-to-noise ratio greater than 1 at least 70 % of the time in the tropics and more than 90 % of the time outside the tropics and that total column water vapor can be retrieved with a precision better than 1.3 mm.