During the California Land-falling Jets Experiment (Dec. 1996 - Mar. 1997), scientists from the NOAA Environmental Technology Laboratory (NOAA/ETL) deployed a vertically pointing S-band radar near the crest of the coastal mountains near Cazadero, California. This radar was equipped with a signal coupler to prevent receiver saturation during the heavy rainfall events that occurred in this climatologically flood-prone coastal region during a strong El Nino. Analysis of the radar reflectivity and Doppler velocity profiles measured with the radar revealed a warm rain process that had previously been undetected in this region. A rainfall process partitioning algorithm based on the S-band radar profiles determined that warm rain contributed 37% to the total rainfall observed at this coastal mountain site when the rainfall rate exceeded 1 mm/hr. The shapes of the warm rain profiles are markedly different than the shapes of the profiles obtained in stratiform precipitation enhanced by the seeder-feeder process. In particular, a bright band feature is absent in both the Doppler velocity and radar reflectivity profiles obtained in warm rain, indicating that ice microphysics does not play a significant role in generating precipitation-sized particles. For comparable rain rates, the radar reflectivity measured near the surface in the warm rain was as much as an order of magnitude (or 10 dBZe) lower than the radar reflectivity measured at the same altitude in bright-band rain. This points to a marked difference in the microphysical properties of the two types of rain and invalidates quantitative precipitation estimation using a single Z-R relationship. The shallow nature of the warm rain, (typically less than 4 km AGL), the topography of the West Coast, and the siting of the NEXRAD network, make it difficult, if not impossible, to detect warm rain in the coastal zone.

This paper describes the warm rain process and focuses on its identification using the Doppler velocity and radar reflectivity profiles obtained with the NOAA/ETL S-band radar. Results of the rainfall process partitioning algorithm will also be discussed.