Collocated UHF and Ka-band radar measurements for rain profile retrievals at ARM SGP facility

Description

The U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Climate Research Facility sites host an unequalled number of active and passive collocated sensors. In this study, Ka-band (35-GHz) and UHF-band (0.915-GHz) Doppler radar observations of the atmospheric column are combined to characterise profiles of clouds and precipitation microphysics.

Particular attention has been made to ensure that the 0.915-GHz wind profilers provide an accurate Rayleigh reflectivity profile reference. They were recently reconfigured in a vertically pointing mode for the observation of vertical velocities in convective clouds. Their sampling strategy cycles every 8 seconds through two interlaced modes: a long pulse mode with high sensitivity and a maximum range of 15 km and short pulse mode giving a high vertical resolution of 150 m up to 9 km. However, the real-time estimation of the signal-to-noise ratio (SNR) of the ARM wind profilers can lead to biases in the estimation of the radar reflectivity. Due to its large beam width, the measured spectra are very wide and can extend over the whole Nyquist interval in convective situations. In such conditions, the recovery of the hydrometeor signal from the background noise is troublesome and requires the estimation of the noise floor from clear air echoes. Furthermore, as a consequence of the high sensitivity of the long mode, the receiver can saturate at short ranges in case of heavy precipitation. Such saturated reflectivities have to be identified and replaced by short mode non-saturated measurements. Finally, the two modes can be merged to provide a full profile of reflectivity every 8 seconds.

Then, our approach makes use of the resulting non-attenuated reference reflectivity of the wind profiler and of the attenuated reflectivity of the new Ka-band radar (KAZR), to estimate the attenuation caused by rain at Ka-band. Rainfall rate profiles are then retrieved by taking advantage of the nearly linear relation between specific attenuation and rainfall rate at Ka band. Particular care has been devoted to precisely calibrate the two systems and to match their beams. Several cases occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E), which took place at the ARM Southern Great Plains (SGP) site in April–May 2011, are used to test this method and to compare the results with collocated disdrometer measurements.