S-band precipitation profiling radar and rain gauge observations by White et al. (2003) in the coastal mountains of northern California from the California Land-falling Jets (CALJET) experiment during the winter of 1997-98 revealed that a surprisingly large fraction (28%) of the seasonal rain there fell without a detectable radar melting-level brightband. These nonbrightband (NBB) precipitation periods were generally characterized by shallower echos and more enhanced orographic lifting than corresponding periods of brightband precipitation (BB). Although the averaged rain rates were nearly the same in the two situations, the NBB cases had weaker radar reflectivities and hydrometeor fall speeds, which suggests unusually high concentrations of small raindrops. The shallow NBB storm echo tops frequently resided beneath the coverage of the regional NEXRAD radars, yet the storms produced substantial rainfall rates (occasionally exceeding 20 mm/h) and accumulations capable of producing flooding.

In an extension of the earlier single-winter study, additional analysis of S-band profiler data from the same site for four winters are examined to assess interannual variations. Composite NCEP/NCAR re-analysis maps and GOES cloud-top temperature data are examined to evaluate the synoptic conditions that characterize periods of NBB precipitation and how they differ from periods with brightbands. The data set focuses on measurements near Cazadero, California, about 120 km northwest of San Francisco, during CALJET in the winter of 1997-98 (a very strong El Niño winter), and the ensuing Pacific Land-falling Jets (PACJET) experiments in the winter of 2000-2001 (moderate La Niña) and the winters of 2001-2002 and 2002-2003 (nearly neutral phase of the El Niño-Southern Oscillation).

NBB precipitation accounted for as little as 18% of the seasonal rain accumulation during the La Niña winter to as much as 50% in one of the near-neutral winters. The four-winter average of 35% was similar to that of the strong El Niño winter studied by White et al. (2003). Synoptic conditions favoring NBB rain exhibited several notable distinctions from those characterizing BB periods. During NBB periods, the left-exit region of the 300-mb jet core and underlying frontal surfaces of the land-falling storms were located farther north of Cazadero, hence providing less synoptically-forced lifting for the development of deep clouds over the profiler site, than during BB periods. The NBB rainfall was also associated with air over Cazadero that was considerably warmer (2-3°C) and somewhat more moist and stably stratified than BB periods. However, both regimes were potentially unstable at and below mountain-top level. The result was shallower clouds for NBB periods, but with the potential for producing significant rainfall amounts whenever aided by saturated upslope flow in the marine boundary layer.