Structure of an Evolving Hailstorm, Part 1: General Characteristics and Cellular Structure

Abstract

The detailed structure and evolution of radar echoes observed in a multicellular hailstorm are analysed. General environmental conditions, overall radar echo development, and precipitation measurements are briefly discussed, but the analysis is mainly concerned with a particular event which was thoroughly observed by several different field facilities of the National Hall Research Experiment. This hailstorm, which evolved in a systematic and periodic manner, is the subject of four companion papers appearing in this issue. Overall storm characteristics are found to compare closely to earlier descriptions of multicell hailstorms occurring in the High Plains. The motion of the main system was to the right of the mean wind vector in the cloud layer. Cell velocity was along but less than the wind in the mid–troposhere. Propagation by new cell growth in a preferred location with respect to existing radar echoes dominated the motion of the overall system. Study of the formation and evolution of individual cells showed that discrete new echoes formed near the altitude of 7 km MSL (−12°C) on the storm's right forward flank 5 to 10 km ahead of existing echo components at approximately 15 min intervals. Each grew rapidly in intensity and height and by moving more slowly than the overall echo complex soon became the main storm component. Average lifetime of individual cells, including the period from visually perceptible turrets, to &ldquo:first echo”, to echo decay, was 45 min. Thus, as many as three cells were found to coexist in varying stages of development. The ascent rate of visual cloud turrets and the history of maximum radar reflectivity of individual cells after the appearance of first echo indicate that the longest in–cloud residence time available for particle growth to the largest observed hail sill (1.5 cm diameter) was between 30 and 35 min.