Spaceborne differential absorption radar water vapor retrieval capabilities in tropical and subtropical boundary layer cloud regimes

Differential absorption radar (DAR) near the 183 GHz water vapor absorption line is an emerging measurement techniquefor humidity profiling inside of clouds and precipitation with high vertical resolution, as well as for measuring integratedwater vapor (IWV) in clear air regions. For radar transmit frequencies on the water line flank away from the highly attenuatingline center, the DAR system becomes most sensitive to water vapor in the planetary boundary layer (PBL), which is a region ofthe 5 atmosphere that is poorly resolved in the vertical by existing spaceborne humidity and temperature profiling instruments.In this work, we present a high-fidelity, end-to-end simulation framework for notional spaceborne DAR instruments that featurerealistically achievable radar performance metrics, and apply this simulator to assess DAR’s PBL humidity observationcapabilities. Both the assumed instrument parameters and radar retrieval algorithm leverage recent technology and algorithmdevelopment for an existing airborne DAR instrument. To showcase the capabilities of DAR for humidity observations in a variety of relevant PBL settings, we implement the instrument simulator in the context of large eddy simulations (LES) of 5different cloud regimes throughout the trade-wind subtropical-to-tropical cloud transition. Three distinct DAR humidity observationsare investigated: IWV between the top of the atmosphere and the first detected cloud bin or Earth’s surface; in-cloud water vapor profiles with 200 meter vertical resolution; and IWV between the last detected cloud bin and the Earth’s surface,which can provide a precise measurement of the sub-cloud humidity. We provide a thorough assessment of the systematic and random errors for all 3 measurement products for each LES case, and analyze the humidity precision scaling with along-trackmeasurement integration. While retrieval performance depends greatly on the specific cloud regime, we find generally that fora radar with cross-track scanning capability, in-cloud profiles with 200 m vertical resolution and 10-20% uncertainty can beretrieved for horizontal integration distances of 100-200 km. Furthermore, column IWV can be retrieved with 10% uncertaintyfor 10-20 km of horizontal integration. Finally, we provide some example science applications of the simulated DAR observations, including estimating near-surface relative humidity using the cloud-to-surface column IWV, and inferring in-cloudtemperature profiles from the DAR water vapor profiles by assuming a fully saturated environment.