PRECIPITATION FROM A MARITIME CLOUD LAYER WITH VERY-LOW DROPLET CONCENTRATIONS

In very clean air the total concentrations of droplets in maritime str atiform clouds can be very low (Squires, 1958a, b). Interestingly, the se clouds sometimes precipitate. In this note we describe such a case and the meteorological conditions that accompanied it. The measurement s were made aboard the University of Washington's Convair C-131A aircr aft about 100 km off the Pacific Northwest coastline from about 2000 t o 2200 UTC on 2 December 1991. The instrumentation aboard the Convair C131-A for the measurement of aerosol and cloud particles has been des cribed by Hobbs et al. (1991). The synoptic conditions at 850 hPa (the standard pressure level closest to the cloud layers we studied) are s hown in Fig. 1. Air that originated from the sub-tropics was approachi ng the Washington Coast after being rotated anticyclonically around a surface high-pressure center (10 hPa) located about 1000 km west of no rthern California. Consequently, the clouds that we sampled formed in an air mass that had a very long trajectory over the Pacific Ocean.