A New Scan Mode to Increase Clear-Air Data Coverage for Wind Analysis

By using long pulses with extended dwelling time, the radar measurement capability can be enhanced to detect weak echoes in the absence of precipitation and measure associated velocities from Bragg backscattering caused by temperature and moisture irregularities (sometime with insects and/or birds) in the atmospheric boundary layer (ABL). Such an enhanced capability may increase clear-air data coverage and provide useful information for detecting the ABL depth and lower-level wind convergence conducive for convective initiation (CI) in a pre-storm environment. To explore this potential capability and related benefits, a new scan mode with long pulses and low antenna rate is designed for the experimental (10.9 cm) S-band radar (named KOUN) at NSSL. In this mode, each volume scan completes three Doppler sweeps (at 0.5, 1.5 and 2.5 degrees) in 10.5 minutes. Using this scan mode, Doppler velocity data were collected from April 15 to 28 in the storm season of 2016. During this period, the atmospheric condition changed from clear to cloudy and then following by CI along the east side of a prefrontal dry-line (to the west of KOUN). In comparison with the velocity observations from the nearby operational KTLX radar, the newly collected velocity data showed increased coverage: from 1 km to 2 km in vertical depth and 100 km to 150 km in radial range under clear-air condition. However, since the PRF is limited to 455 Hz and thus the Nyquist velocity is only 12.4 m/s for this scan mode, the collected velocity data are severely aliased. The dealiasing techniques developed for operational WSR-88D radars are modified adaptively to process these data with some successes and some difficulties. The results will be presented at the conference.