Identification of precipitation onset based on Cloudsat observations

- A new method to determine precipitation onset by CloudSat observations.
- Radar reflectivity threshold varies from −18 to −13 dBZ for precipitation onset.
- Cloud liquid water path threshold varies from 150 to 300 gm-2 for precipitation onset.

Observations of cloud vertical structure by Cloud Profiling Radar on CloudSat satellite provide a unique opportunity to globally identify the onset of precipitation. In this study, based on a conceptual model for an adiabatic cloud, a new method to determine the onset of precipitation in marine warm clouds is developed. The new method uses the slope of radar reflectivities near the cloud top, which gradually reverses its signs as drizzle occurs. By analyzing multiyear CloudSat data, it is found that globally the radar reflectivity threshold for precipitation onset varies from −18 to −13 dBZ with an average value of −16 dBZ. The corresponding liquid water path threshold for precipitation onset is also studied by analyzing satellite microwave observations collocated with CloudSat data. Results show that the liquid water path threshold is 190 g m−2 as a global mean, varying from 150 to over 300 g m−2 depending on regions.