Application of NASA Multi-Frequency Airborne Doppler Radar for Estimates of Hydrometeor Microphysical Properties

NASA Goddard Space Flight Center has developed three airborne radar systems which are the High-altitude Imaging Wind and Rain Airborne Profiler (HIWRAP) operating at Ku- and Ka-band, the Cloud Radar System (CRS) at W-band, and the ER-2 X-band Radar (EXRAD). These multi-frequency radar systems have been deployed to many NASA-sponsored field campaigns, including the Investigation of Microphysics and Precipitation for Atlantic Coast – Threatening Snowstorms (IMPACTS) that has provided observations critical to understanding the mechanisms of snowband formation, organization, and evolution. A combination of these three radars installed on the NASA ER-2 high-altitude aircraft provides four-frequency reflectivity and Doppler measurements from rain and snow during IMPACTS in 2020, 2022 and 2023.

To explore the full capability of this NASA multi-frequency radar system, a comprehensive study involving theoretical simulations of the dual-frequency ratio (DFR, defined as difference of radar reflectivities between two frequencies) and the differential Doppler velocity (DDV, defined as difference of the Doppler velocities between two frequencies) is carried out. Measured rain-drop/snow-particle size distribution (DSD/PSD) data, acquired from a variety of storm systems during NASA field campaigns, are used to compute radar reflectivities and Doppler velocities at frequencies from X- to W-band. The relationships between the radar and hydrometeor-size parameters obtained from the Gamma DSD/PSD models, assumed in the retrieval procedures, are compared with those computed directly from the measured DSD/PSD data to check the suitability of the DSD/PSD models in representing actual DSD/PSD and to assess the uncertainties associated with natural variations of rain/snow size spectra in estimates of hydrometeor properties.

Another important application for the multi-frequency airborne radar systems is to classify the hydrometeor phase state, i.e., identification of liquid, frozen and melting hydrometeors along precipitation profiles. Knowing the hydrometeor phase state is indispensable not only for active/passive remote sensing retrieval of cloud and precipitation microphysical properties but also for climate and weather prediction models that require accurate representation of the phase state in time and space. Our preliminary simulation study shows clear separations of snow and rain in the space comprised of DFR, DDV and radar reflectivity factors, leading to a potential means to distinguish liquid, frozen and melting hydrometeors.

In this study, measurements from the NASA ER-2 (X, Ku, Ka and W bands) radar systems during IMPACTS are used to test and validate multi-frequency techniques for the estimates of snow and rain parameters as well as for the identification of hydrometeor phase. The in situ microphysical measurements, taken from the NASA P-3 aircraft during IMPACTS, will be incorporated into our study for direct validation.