Boundary-Layer Humidity Sounding in Cloudy and Clear Skies Using Differential Absorption Radar

Satellite-based measurements of water vapor have proven to be very useful for assimilation in numerical weather models, and are a critical resource for improving climate models. There are various microwave, infrared, near-infrared, and visible passive sensors currently in operation that provide measurements of vertical water vapor profiles or total column water vapor (TCWV), but each exhibit certain limitations that prohibit high-spatial-resolution measurements in all atmospheric conditions and for all surface types. For example, TCWV is measured with high-accuracy from the Orbiting Carbon Observatory-2 (OCO-2), but can only be measured in daytime and in clear-sky regions. Passive microwave radiometers can provide daytime and nighttime measurements of water vapor in the presence of clouds with coarse vertical resolution, but are typically limited to regions with an ice-free ocean surface. To fill these observational gaps, we are developing an active sounding instrument that utilizes the differential absorption radar (DAR) technique on the flank of the G-band water absorption line to remotely sense planetary-boundary-layer (PBL) water vapor with ubiquitous spatial and temporal coverage. The method provides the capability to vertically profile water vapor inside of PBL clouds and precipitation, and provide TCWV measurements in clear-sky areas using the radar returns from the surface.

We present ground-based measurements and humidity profile retrievals using a prototype DAR with tunable transmission from 167 to 174.8 GHz. The project is part of NASA’s Instrument Incubator Program, targeting specific planetary boundary layer (PBL) observables that were called out in the recent 2017 ESAS Decadal Survey. The chosen frequency band results in optimal sensitivity to humidity within PBL clouds and precipitation, and permits penetration through the whole atmosphere from an airborne or spaceborne platform. By applying a radar instrument simulator to CloudSat and ECMWF-aux products and subsequently retrieving TCWV, we investigate the accuracy, precision, and coverage achievable by a spaceborne DAR with realistic instrument parameters.