Combining Differential Absorption Radar and Microwave Radiometer for Water Vapor Profiling in the Cloudy Trade-Wind Environment

Understanding atmospheric processes, such as e.g. cloud and precipitation formation, requires high-resolution water vapor and temperature profile observations particularly in the cloudy boundary-layer. As current observation techniques are limited by low spatial or temporal resolution, the potential of combining microwave radiometer (MWR) with differential absorption radar is investigated by analysing the retrieval information content and retrieval uncertainty. Two radar frequency combinations are analyzed: Ka- and W-band (KaW), available at e.g. Barbados Cloud Observatory, as well as a synthetic combination of G-band frequencies (167 and 175 GHz, G2), simulated using the Passive and Active Microwave TRAnsfer model PAMTRA.

The novel synergistic retrieval approach is based on an optimal estimation retrieval scheme. The absolute humidity profile is retrieved from the MWR K-band brightness temperatures, as well as the Dual-Wavelength Ratio (DWR) signal of the two radars. Evaluating a suite of radiosonde profiles measured at Barbados from 2018, adding the active KaW combination to K-band MWR brightness temperatures increases the information content for the retrieved profile from 3.2 to 3.4 degrees of freedom for signal (DoF). The usage of the higher G2 radar frequencies leads to higher Dual-Wavelength Ratios (DWRs), and, in combination with the MWR, to increased DoF (4.5), decreased retrieval errors, and a more realistic retrieved profile within the cloud layer. Information partitioning among MWR and the radars makes the synergy particularly beneficial: the profile below and within the cloud is restricted by the radar observations, whereas the water vapor above cloud top and the LWP are constrained by the MWR.

Based on selected case studies with single- as well as multi-layered clouds from the EUREC4A campaign, different retrieval configurations will be evaluated based on the resulting retrieval error, as well as the Degrees of Freedom. Tools for customizing the retrieval to the trade wind driven atmosphere will be analyzed by e.g. constraining the humidity profile to saturation within the cloud layer, or making use of a direct inversion approach of the differential attenuation signals.