Abstract
Two rapidly growing, hail-producing storms observed in Alabama during the Microburst and Severe Thunderstorm project in 1986 were examined: the well-studied single-cell storm case on 20 July 1986 and a single cell within a multicellular storm on 6 July 1986. Both storms are examples of extremely efficient accretional growth processes that produced hail within 10 min. A simple hydrometeor classification algorithm based on multiparameter radar data was used to identify regions within the rain and snow portions of the storm volumes that included hail, graupel, and supercooled rain. By comparing the results of the simple hydrometeor classification algorithm to previous polarimetric analysis and modeling of the 20 July 1986 storm by other authors, the hydrometeor classification methodology for the 6 July 1986 storm was indirectly validated.
The microphysical development of hail and graupel was similar for both the single isolated cell storm and a cell within a multicellular storm. Rapid coalescence within updrafts with high liquid water contents quickly produced precipitation-sized drops that were lofted above the 0°C level and subsequently froze. These frozen drops became hail and graupel embryos and continued to grow by accretion. Supercooled rain was present only in the earliest stages of cell evolution lasting 8–12 min and extending 1–2 km above the 0°C level. Hail and graupel appeared several minutes after the first appearance of supercooled rain. Graupel was present at higher altitudes and encompassed a larger area of the storm than hail. Completion of the glaciation of the supercooled rain and the start of hail and graupel fallout occurred at nearly the same time.
Examination of volumetric statistics of the storms in terms of time–height frequency of hydrometeor type and contoured frequency by altitude diagrams (CFADs) of reflectivity and vertical velocity showed that the evolution of the storm kinematics and microphysics were closely coupled for individual cells. Individual cells can be described in terms of a single particle fountain. Previous studies had shown that in multicellular storms, the ensemble of particle fountains rapidly evolves toward microphysical characteristics indicative of dominant vapor depositional growth, characteristic of stratiform regions, even when strong updrafts are present. This study aided in clarifying that, in contrast to the ensemble of particle fountains, for individual particle fountains the kinematic and microphysical evolution are more closely coupled in time and that vapor depositional growth does not dominate in the individual cell until the updraft associated with the cell has weakened.
In the two cases examined, the combined effects of enhancement of the upper levels of the updraft by the latent heat released by glaciation, and the precipitation loading of the heavy falling particles at lower levels, acted to tear the cell apart at the middle. Previous studies have noted midlevel convergence and constriction of the cell associated with these effects. It is postulated that as a result of these factors, cells producing hail and graupel will hasten their own demise and will have on average shorter lifetimes as distinct cells compared to cells producing only rain.
Corresponding author address: Professor Sandra Yuter, Departmentof Atmospheric Sciences, University of Washington, Box351640, Seattle, WA 98195-1640. Email: yuter@atmos.washington.edu
