Airflow and Hail Growth in a Severe Northern High Plains Supercell

Abstract

Air and particle trajectory calculations using internal motions from Doppler radar observations are used to identify kinematic feature and hail growth processes operating in a supercell storm that occurred on 2 August 1981 in southeastern Montana. As the Rock Springs storm moved rapidly east-southeastward across the Cooperative Convective Precipitation Experiment (CCOPE) observational network, it produced a significant hailfall composed mostly of 2 to 3 cm diameter hailstones. Some diameters were as large as 6 to 10 cm. At least one funnel cloud was sighted, and there was extensive crop and property damage. In the hail growth model 1 to 5 cm hailstones were readily produced from frozen drops in the size range 50 ?m to about 1 mm. Most of the hail apparently grew from frozen droplets that originated within either the upwind stagnation zone southwest of the main updraft core or the overhanging radar echo ahead of the updraft. Potential hailstone embryos entered the stagnation zone from a flanking cloud line associated with the surface gust front. Graupel particles grown in this weak updraft region were carried by the southwesterly airstream either into the hook echo or into the forward overhang. Hailstones and graupel particles in both branches were found to shed water drops as they traveled northward and descended below the melting level along the shoulders of the main updraft. This resulted not only in a substantial rainfall, but also in a source of droplet embryos especially within the forward overhang. Considerable numbers of hailstones were produced, since many of these shed drops passed westward through the main updraft. The route through the forward overhang or embryo curtain is the familiar particle recycling path which was proposed as the one that enabled large hailstones to grow in the Wokingham storm in England and the Fleming storm in northeastern Colorado. Indeed most of the hailstones ranging in diameter from 1 to 2 cm were produced in the Pock Springs storm along a path similar to this one. However, some of the largest hailstones originated in a narrow band behind (west of) the updraft axis near the center of rotation of the middle level mesocyclone. Trajectories such as these were instrumental in the development of the hook echo and several produced 3 to 5 cm hail that fell out close to where the embryos were initialized. Recycling did not happen in the model; all initialized particles followed simple up-and-down trajectories through the supercooled region. Hail from this large, severe supercell was produced along several paths, some similar to trajectories proposed for other storms and some different. Overall, the hail growth is more complicated in this well-documented storm than in any of the simple idealizations previously proposed.