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 (30.7ºN, 96.4ºW). 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.