89th American Meteorological Society Annual Meeting

Sunday, 11 January 2009
Drop-size distributions of storm types in southeast Texas
Phoenix Convention Center
Matthew Raper, Texas A&M University, College Station, TX; and C. Schumacher, L. J. Hopper Jr., and K. E. Brugman
Many properties of rainfall, such as rain rate (R), radar reflectivity factor (Z), and liquid water content (W), can be determined from the drop-size distribution (DSD) of a drop sample. Variations in the DSD and calculated parameters yield valuable insight into the microphysical origins of the rain within the clouds. This study uses a multi-year disdrometer data set to study DSD variations associated with different storm types in southeast (SE) Texas, a region affected by both tropical and extratropical influences. An underlying hypothesis of this study is that different large-scale forcing can affect the resulting storm dynamics and microphysics, leading to different DSDs at the surface.

Surface rainfall measurements were made by a Joss-Waldvogel RD-80 (J-W) disdrometer and two tipping bucket rain gauges located in College Station, Texas (30.7N, 96.4W). The instruments were installed and maintained by the Department of Atmospheric Sciences at Texas A&M University. The disdrometer data analyzed in this study is from 16 December 2004 19 September 2008. The resulting data set contains 2088 hours of disdrometer observations.

Disdrometer rainfall events are identified for analysis based on a variation of Steiner and Smith (2000). A minimum rain rate of 0.1 mm h-1 is used to identify the beginning and end of the event period. Periods with at least four hours of no precipitation are identified as separate events and a total rainfall accumulation of 2.5 mm is required for an event designation.

Using NEXRAD imagery and synoptic maps, the events are then classified based on their precipitation structure and large-scale forcing, with the goal of determining microphysical variations between storm types based on DSD variations. The mechanisms forming the droplets within the precipitating systems appear to differ based on a storm's synoptic environment and radar-observed structure, in part because of varying amounts of convective, stratiform (i.e., from deep convection) and non-convective (i.e., from weak, large-scale ascent) rainfall. Z-R relations are calculated for each storm event and averaged for each classification. Thus, these DSDs and Z-R relations can be used to link microphysical processes to storm organization, as well as provide a basis for studies of other variations within subtropical storms.

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