7A.5 Comparing Disdrometer Measured Raindrop Size Distributions from VORTEX-SE with Distributions from Polarimetric Radar Retrievals Using the Constrained Gamma Method

Monday, 28 August 2017: 5:00 PM
Vevey (Swissotel Chicago)
Jessica Bozell, Purdue Univ., West Lafayette, IN; and D. T. Dawson II, R. Tanamachi, and S. Frasier

Many aspects of microphysical variations within supercells are not well understood, even though they play a key role in storm dynamics and evolution. Raindrop size distributions (DSDs) provide a lot of insight into a storms microphysics, however, DSDs vary significantly throughout storms. Unfortunately, the National Weather Service’s radar network does not have the capability to directly observe DSDs, making retrieval algorithms based on advanced microphysical models and in-situ disdrometer observations necessary. If these small scale variations can be better characterized, and differences between convective regimes quantified, it will lead to improved algorithms for retrieving DSD parameters from radars as well as improved microphysical parameterizations within numerical weather prediction models. With better modeling of supercells, tornado predictions and warnings can be improved.

Gamma distributions are often used to model DSDs, since they empirically fit well, are relatively flexible, and are fairly easy to manipulate mathematically. One method to estimate the main parameters of the gamma DSD from polarimetric radar data is the constrained-gamma method. This method uses reflectivity (ZH), differential reflectivity (ZDR), and a disdrometer-derived empirical relation between the DSD slope and shape parameters (𝜇 − Λ relation) to retrieve the DSD parameters.

While disdrometers directly measure DSDs, they are severely limited in spatial coverage. In order to improve our understanding of the spatial variation of DSDs across supercells, methods such as the constrained-gamma method can be used to retrieve DSD parameters at high spatial resolution across an entire storm. In this study, we will apply the constrained gamma method to radar data collected during the 2016 and 2017 VORTEX-SE field program in order to retrieve normalized gamma distributions from polarimetric radar variables, leveraging the disdrometer data collected during the field program. Specifically, we will derive new 𝜇 − Λ relations from these disdrometer data and compare them to 𝜇 − Λ relations found in other studies. We will then apply these retrievals to characterize the spatiotemporal variation of DSDs for VORTEX-SE storms.

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