Vertical air motion uncertainties in profiling radar observations

 

Author

Christopher R Williams — University of Colorado, Boulder

Category

Dynamics/Vertical Motion

Description

Vertically pointing profiling radars operating at 50 MHz (VHF) are able to directly measure the vertical air motion even when precipitating cloud systems pass overhead. During deep convective rain, updrafts exceeding 10 m/s are commonly seen in the Darwin, Australia, VHF profiler data set. During stratiform rain, updrafts and downdrafts are on the order of +/- 20 cm/s. This study developed a Monte Carlo simulation to estimate the vertical air motion measurement uncertainty that complements the vertical air motions estimated for the Tropical Western Pacific-International Cloud Experiment (TWP-ICE). The Monte Carlo simulation was constructed by adding realistic noise to idealized Doppler velocity power spectra. For each idealized spectrum, the spectrum moments (signal-to-noise ratio (SNR), mean radial velocity, and spectrum width) were estimated for 1000 noisy spectra. The mean deviation from the expected value and the spread of the 1000 moments provides estimates of the measurement bias and uncertainty. The simulations confirmed previous work that measurement uncertainties increase with decreasing SNR. But interestingly, the simulations showed that measurement uncertainties increased with increasing spectrum width. The Monte Carlo simulation also showed that the uncertainties are dependent on the radar operating parameters. Using the VHF profiler operating parameters, it was found that for convective rain events with mean updrafts exceeding 5 m/s, the velocity uncertainty ranged from 0.1 to 0.6 m/s. For stratiform rain, the velocity uncertainty was less than 2 cm/s. In the future, the Monte Carlo simulations developed in this study can be used to estimate measurement uncertainties for other radars, including the MMCR.