Airflow and Moisture Budget Beneath a Northeast Colorado Hailstorm

Abstract

A case study is presented of a persistent thunderstorm of moderate intensity which occurred in northeast Colorado, and which produced a light hailfall at the ground. The storm was intensively monitored by aircraft, radar, dropsondes, and surface and upper air networks involved in the National Hail Research Experiment. The present study emphasizes the measurements obtained by instrumented aircraft in examining the properties of the subcloud airflow. Large-scale analysis shows that the storm formed on a surface confluence line and subsequently moved toward the region of surface moisture. A brief radar analysis of the storm during the mature stage of its ?5-hr lifetime is presented, and identifies the general features as belonging to the category termed ?supercell? by previous workers. A precipitation analysis is carried out, and rain and hailfall are correlated with the track of the storm. Liberal use of a time-space conversion technique results in detailed mesoscale pressure, temperature, moisture and wind field analyses from a network of 22 remote meteorological stations. Surface divergence of mass and moisture is computed. Surface features are related to the position and structure of the radar echo. Wind, temperature and moisture data obtained by aircraft encircling the storm in the region below cloud base are presented. Emphasis is placed on the airflow with respect to the moving storm, and details of the subcloud circulation are examined. Analysis of relative streamlines, supported by the observed temperature and moisture structure, delineates distinct regions of inflow and outflow for the storm. Partitioning of the measured flux into inflow and outflow segments at the aircraft levels and at the surface results in estimates of the mass and moisture budgets for the storm. The ratio of rainout measured at the ground, 2 ? 109 gm sec?1, to the computed moisture influx, 13 ? 109 gm sec?1, results in a precipitation efficiency of only 15%. A physical basis (following a discussion presented by Marwitz) for correlating this low efficiency with the fairly high value of vertical wind shear which existed around the storm, ?5 ? 10?2 sec?1, is elaborated upon, and the present results are compared with those of previous investigators. The computed mass inflow, 2 ? 1012 gm sec?1, is rather large, but is shown to be compatible with other measurements made of the storm. The computations indicate that, in this case, only about half of the upward flux took place in the vicinity of the ?echo-free vault,? and attention is drawn to a secondary region of echo overhang where various measurements indicate that significant vertical transports also occurred. Based on the airflow and thermodynamic measurements, some ideas concerning the energy source that drives the inflow circulation are presented.