Relationships between subtropical warm precipitation features and lower tropospheric water vapor

Subtropical precipitating shallow cumulus are important not only for local vertical moisture transport, but also for the large scale horizontal transport of moisture into the tropics. Previous studies suggest that increases in lower and midtropospheric water vapor due to shallow convection could result from increases in cloud frequency, increases in cloud water, increases in cloud areal extent, increases in cloud depth, or decreases in precipitation efficiency. The Tropical Rainfall Measuring Mission (TRMM) Precipitation Feature (PF) database is used to explore the relationship between subtropical precipitating convection and lower tropospheric humidity. The PF database is first used to identify warm precipitating convection, defined here as any PF with infrared brightness temperatures greater than 273 K and echo top heights less than 5 km. The climatological mean, interannual, and seasonal variability in subtropical shallow PF characteristics, such as size, height, rain rate, and frequency, are first examined. The relationship between shallow PF characteristics and tropospheric humidity is then explored using TRMM Microwave Imager (TMI) column water vapor estimates and ERA-Interim reanalysis humidity profiles. Preliminary results show a strong correlation between lower tropospheric water vapor and warm rain feature frequency, height, and rain rate, however feature sizes lag increases in water vapor. Anomalies in subtropical lower tropospheric water vapor are most correlated with the height of shallow precipitating convection. Further analysis using other datasets to include subtropical non-precipitating shallow convection is also planned.