Severe convective weather: Microphysical insights from models and dual-polarization radar

Evan Kalina

NRC Research Associate, NOAA AOML Hurricane Research Division

Friday, Oct 28, 2016, 11:00 am
DSRC Room 2A305

Abstract

In situ microphysical data from severe weather events are rare, requiring numerical models and radar data to play key roles in our understanding of precipitation processes. In this two-part talk, 1) model data will be used to examine the effect of aerosol concentration on supercell thunderstorms, and 2) radar measurements will be used to determine the types and amounts of ice species in Hurricanes Arthur (2014) and Irene (2011).

In part 1, idealized supercell thunderstorms are simulated with the Weather Research and Forecasting (WRF) model at 15 cloud condensation nuclei (CCN) concentrations (100–10 000 cm-3) using four different environmental soundings. Changes in the microphysical process rates with CCN concentration are found to be negligible beyond CCN = 3000 cm-3. Changes in cold pool characteristics with CCN concentration are non-monotonic and highly dependent on the environmental conditions. Accumulated precipitation is enhanced (by up to 25 mm) in the most polluted cases near the updrafts, except for the drier RH sounding. The different responses for moist and dry soundings are mostly due to compensating changes in near-surface cooling from evaporating rain and melting hail as CCN concentration increases in the moist simulations. This compensating effect does not exist when the low-level RH is drier.

In part 2, dual-polarization radar measurements are used to map the ice water path (IWP) in Hurricanes Arthur (2014) and Irene (2011) at landfall. The IWP maps reveal that the total IWP and snow/graupel IWP reach up to 10 kg m-2 within convective precipitation in the rainbands and eyewall. Ice crystal IWP remains mostly < 4 kg m-2, with the largest values collocated with maxima in the total IWP. Histograms of the ice crystal-to-total IWP ratio reveal a gamma-like distribution that peaks from 0.1–0.2. The shape of the distribution is similar in convective and stratiform precipitation. Only weak precipitation (reflectivity < 20 dBZe) has a different IWP ratio distribution, in which the peak is located near 0.3 and ratios ≥ 0.8 are common. These larger IWP ratios are most prevalent in the “moat” regions of the tropical cyclones and at the edges of the rainbands, where ice crystals in the anvil are the dominant ice species.

Together, these results highlight the valuable contributions that numerical model experiments and new operational radar technology can make to our study of the microphysical and thermodynamic characteristics of extreme events.