Coastal Orographic Rainfall Processes Observed by Radar during the California Land-Falling Jets Experiment

Radar and rain gauge observations collected in coastal mountains during the California Land-Falling Jets Experiment (CALJET) are used to diagnose the bulk physical properties of rainfall during a wet winter season (January–March 1998). Three rainfall types were clearly distinguishable by differences in their vertical profiles of radar reflectivity and Doppler vertical velocity: nonbright band, bright band, and hybrid (seeder–feeder). The contribution of each rainfall type to the total rainfall observed at the radar site (1841 mm) was determined by a new, objective algorithm. While hybrid rain occurred most often, nonbrightband rain (NBB rain) contributed significantly (28%) to the total. This paper focuses on characterizing NBB rain because of the need to document this key physical process and because of its impact on Weather Surveillance Radar-1988 Doppler (WSR-88D) precipitation surveillance capabilities. NBB rain is a quasi-steady, shallow rain process that does not exhibit a radar bright band, that occurs largely beneath the melting level, and that can produce rain rates exceeding 20 mm h−1. Composite vertical profiles were produced for NBB rain using 1417 samples and brightband rain using 5061 samples. Although the mean rain rate for each composite was 3.95 mm h−1, at all altitudes NBB rain had systematically weaker equivalent radar reflectivity (e.g., 20.5 dBZe vs 28.5 dBZe at 263 m above ground level) and much smaller Doppler vertical fall velocities (e.g., 2.25 m s−1 vs 6.25 m s−1 at 263 m) than did brightband rain. The reflectivity–rain-rate (Z–R) relationship for NBB rain (Z = 1.2R1.8) differs significantly from that of brightband/hybrid rain (Z = 207R1.1). The meteorological context in which NBB rain occurred is described through case studies and seasonal statistics. NBB rain occurred in a wide variety of positions relative to frontal zones within land-falling storms, but three-quarters of it fell when the layer-mean, profiler-observed wind direction at 1250 m MSL (the altitude of the composite low-level jet) was between 190° and 220°. The importance of orographic forcing during NBB rain, relative to all rain events, was indicated by a stronger correlation between upslope wind speed and coastal rain rates at 1250 m MSL (r = 0.74 vs r = 0.54), stronger low-level wind speeds, and wind directions more orthogonal to the mean terrain orientation.