Validation of Numerical Hurricane Simulation Microphysical Fingerprints using Polarimetric Radar

Due to its ability to remotely sense extreme environments and its sensitivity to precipitation, weather radar has been an indispensable tool for observing hurricanes. The United States' Doppler radar network upgrade to dual-polarization from 2011-2013 provides new observations for estimating the microphysical properties of precipitation in tropical cyclones (TCs). Polarimetric radar observations can provide a targeted validation of the drop size distribution (DSD) by estimating the phase, size, and number concentration of precipitation at high resolution over a broad area. The radar upgrade allows for new assessments of the structure of TCs that approach the U.S. coastline to both improve our understanding of microphysical processes and validate the performance of numerical models.

The work to be presented investigates the use of these new observations using the cases of Hurricanes Arthur and Ana (both 2014) by comparing observed drop size distributions (DSDs). Hurricane Arthur formed in the Atlantic basin and impacted the southeast U.S. coast, while Hurricane Ana formed in the Central Pacific and impacted the Hawaiian Islands. We simulate Arthur and Ana with a quadruply nested WRF-ARW model with an innermost grid of ⅔ km resolution. The numerical simulations are used to test six different microphysical parameterizations with various levels of complexity, including single and double moment bulk schemes and an explicit spectral bin scheme. We then used a dual-polarization forward operator to simulate polarimetric radar variables from the WRF model output. Using the simulated horizontal and vertical reflectivity allows a comparison of the microphysical “fingerprint” of each simulation with the NEXRAD observations wherein the rain DSD is represented in a horizontal reflectivity -- differential reflectivity phase space. A detailed analysis of the microphysical properties of the simulations reveals that the Thompson aerosol-aware double-moment bulk scheme and the spectral bin microphysics (SBM) exhibit the best fidelity to the observed joint probability distribution of horizontal and differential reflectivity. The SBM also produced the most accurate intensity and lowest rainfall accumulation, but required much higher computational resources than the bulk schemes.