Abstract
Abstract. Differential absorption radar (DAR) near the 183 GHz water vapor absorption line is an emerging measurement technique for humidity profiling inside of clouds and precipitation with high vertical resolution, as well as for measuring integrated water vapor (IWV) in clear-air regions. For radar transmit frequencies on the water line flank away from the highly attenuating line center, the DAR system becomes most sensitive to water vapor in the planetary boundary layer (PBL), which is a region of the 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 feature realistically achievable radar performance metrics and apply this simulator to assess DAR's PBL humidity observation capabilities. Both the assumed instrument parameters and radar retrieval algorithm leverage recent technology and algorithm development 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 (LESs) of five different cloud regimes throughout the trade-wind subtropical-to-tropical cloud transition. Three distinct DAR humidity observations are 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 three measurement products for each LES case and analyze the humidity precision scaling with along-track measurement integration. While retrieval performance depends greatly on the specific cloud regime, we find generally that for a radar with cross-track scanning capability, in-cloud profiles with 200 m vertical resolution and 10 %–20 % uncertainty can be retrieved for horizontal integration distances of 100–200 km. Furthermore, column IWV can be retrieved with 10 % uncertainty for 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-cloud temperature profiles from the DAR water vapor profiles by assuming a fully saturated environment.
Highlights
Cloud morphology and precipitation depend sensitively on the three-dimensional distributions of water vapor and temperature, especially within the planetary boundary layer (PBL) where most convective initiation occurs
Because the analysis framework is identical for all five cases, we provide the most detail in Sect. 3.1 in describing the GATE simulations, with more limited discussion in the sections for the remaining four cases
It is important to decide on a framework for conveying the independent aspects of (1) random uncertainty, which stems from reflectivity measurement error, and (2) systematic uncertainty, or bias
Summary
Cloud morphology and precipitation depend sensitively on the three-dimensional distributions of water vapor and temperature, especially within the planetary boundary layer (PBL) where most convective initiation occurs. Existing spaceborne sensors have limited ability to sample water vapor and temperature in the PBL with high spatial resolution, with increased difficulty inside of cloudy and precipitating volumes for passive infrared and microwave sounders (Wulfmeyer et al, 2015; Stevens et al, 2017; Sahoo et al, 2015). Active humidity sounding approaches, including differential absorption lidar (DIAL) and differential absorption radar (DAR), offer new potential spaceborne solutions for providing high-vertical-resolution water vapor profiles in clear-sky and cloudy regions, respectively (Nehrir et al, 2017), with a typical vertical resolution target of 200 m. The advantage of the active DIAL/DAR approaches is to constrain the vertical variability in water vapor desired by the decadal survey (NASEM, 2018). Because DAR is a relatively new measurement approach with limited instrument deployment (Roy et al, 2020, 2021; Cooper et al, 2020) and simulation (Lebsock et al, 2015; Millán et al, 2016; Battaglia and Kollias, 2019; Millán et al, 2020; Schnitt et al, 2020) heritage, there is a need to critically assess the measurement capabilities of notional spaceborne DAR systems with detailed radar simulations from an orbital altitude, in addition to continued assessment of observational capabilities from airborne platforms
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