Sensitivity of Ice-phase Radar Doppler Spectra to uncertainties in Ice Crystal Microphysical Properties

Description

The ARM program has deployed vertically pointing single and dual frequency Doppler radars for the purpose of characterizing the properties of clouds and precipitation. Of the many uncertainties in any radar retrieval algorithm of cloud and precipitation properties, the specific microphysical characteristics of the ice crystal habits stands out significantly as one of the largest sources. We explore that sensitivity in this study. This study requires two things. First, we must be able to distinguish between the properties of ice crystal habits. We characterize those properties using mass- and area-dimensional power laws derived from aircraft data collected from NASA and NSF-funded field programs where particle size distributions, bulk water, and cross sectional area were measured independently. Second, we require a means of quantifying how the spectrum of ice crystal habits influence the radar backscatter cross section and Doppler velocity. For this, we take T-Matrix backscatter cross sections calculated as a function of radar frequency, particle size, particle aspect ratio, and a discrete set of mass-dimensional relationships to derive a new backscatter-size relationship by scaling the Clausius-Mosotti factor in the radar backscatter-size relationship. With these tools, we then quantify the sensitivity of the Doppler spectra and Doppler moments to the degree of ice habit variability found in natural clouds. Specifically, we ask what the effective forward model uncertainty in calculated observables would be given this variability. Second, we ask whether dual frequency radar retrievals, that would utilize vertically pointing Doppler spectra, provide enough information to overcome this uncertainty. These results have implications for algorithm development in mixed phase and ice clouds observed at ARM’s permanent and mobile sites.