Calibrating ARM’s Cloud Radars: Process and Timeline
Authors
Karen Lee Johnson — Brookhaven National Laboratory
Nitin Bharadwaj — Pacific Northwest National Laboratory
Adam Theisen — Argonne National Laboratory
Bradley Isom — Pacific Northwest National Laboratory
Category
General topics – Clouds
Description
Radar calibration is a critical component of ARM’s overall quality assurance commitment. The ARM program owns seven vertically-pointing Ka-band ARM Zenith Radars (KAZRs) and eight dual-wavelength pairs of Scanning ARM Cloud Radars (SACRs). The radars are deployed in diverse and sometimes harsh climate regimes, with the expectation of continuous and autonomous operations. It is essential that radar reflectivities and other radar moments be as accurate and consistent as possible to fulfill the radar suite’s potential for improving microphysical profile retrievals and our understanding 3D cloud processes, via a consistent long-term data record.
The reflectivity calibration plan implemented for this collection of ARM cloud radars is described here along with its implications for the processing and availability of higher-order radar Value-Added Products (VAPs). Routine calibration processing also provides an opportunity for timely correction of possible errors in other radar moments or in recorded radar system measurements. Two types of calibration processes are described. The first is internal calibration, based on the traditional radar equation for volume targets. System operating parameters for both KAZR and SACR radars are continuously monitored at a number of points along the transmit and receive chains. The second type of calibration we discuss is made possible by routine SACR scans of trihedral corner reflectors installed near each system. During clear sky periods, comparing corner reflector returns with their known radar cross sections can provide an absolute calibration for SACR, which can in turn be applied to colocated KAZR reflectivities via comparison with vertical pointing SACR scans.
Radar system performance is now assessed every two months to determine whether calibrations have changed. After this analysis, radar calibration constants are developed for the two most recent months of data collected. The KAZRCAL VAP is run to apply the new KAZR calibrations for each site and make other corrections, if any. For the SACRs, a similar SACRCAL VAP will soon be available. Once data are calibrated, radar-based VAPs such as KAZRCOR, KAZRARSCL and SACRCOR can be processed using the calibrated and corrected radar observations.
