Verification of simulated DSDs and sensitivity to CCN concentration in EnKF analyses and ensemble forecasts of the 30th April 2017 tornadic QLCS during VORTEX-SE

Abstract:

The Verification of the Origin of Rotation in Tornadoes EXperiment-Southeast (VORTEX-SE) field program exists to investigate the development of tornadoes in the Southeast United States (SE-US) using a synergy of physical observations on multiple scales, numerical modeling, and social science investigations of societal vulnerability and response to tornado forecasts and warnings. Storms in the SE-US often evolve in different environments than those in the central Plains. Many poorly understood aspects of these differing environments may impact the tornadic potential of SE-US storms. Among these differences are potential variations in the cloud condensation nuclei (CCN) concentration owing to geographical proximity to maritime vs. interior continental sources. The relative influence of warm and cold rain processes is sensitive to CCN concentration, with higher CCN producing smaller cloud droplets and more efficient cold rain processes. Cold rain processes result in drop size distributions (DSDs) with relatively larger drops from melting ice compared to warm rain processes. Differences in DSDs impact cold pool and downdraft size and strength, that influence tornado potential. This study investigates the sensitivity of CCN on DSDs in the SE-US by comparing DSDs from ARPS-EnKF model analyses and forecasts with varying CCN concentrations to observed DSDs from portable disdrometer-equipped probes in a tornadic QLCS on 30 April 2017 during VORTEX-SE.

The ARPS-EnKF system is used with the NSSL triple-moment microphysics scheme. A forward operator, to simulate sampling from a Parsivel disdrometer, is applied to the model-predicted DSDs to account for instrument sampling effects. The simulated DSDs are then verified against observed DSDs. Simulations with CCN concentrations ranging from 100 cm^-3 (maritime) to 1500 cm^-3 (continental) are conducted to characterize the variability of DSDs and the model-simulated DSDs are verified against the disdrometer observations. The sensitivity of the DSD variability to CCN concentrations is evaluated. Preliminary results indicate continental CCN concentrations produce DSDs that align closer to the observed DSDs.