Synoptic and topographic variability of northern California precipitation characteristics in landfalling winter storms observed during CALJET

Observations from northern California during the California Landfalling Jets (CALJET) experiment are used to examine the mean characteristics of precipitation and their variances as functions of synoptic and topographic regimes. Ten cases involving the landfall of extratropical cyclones are analyzed with radar and raingauge data collected at two sites: one in the coastal mountains north of San Francisco (CZD) and the other in the central valley just west of Sacramento (KDAX).

Aside from the melting-layer bright band, the most striking feature in the ten case composite vertical profile of radar reflectivity at CZD was a distinct change in slope about 2.5 km above the bright band. This “shoulder” is thought to represent a change in the growth rate of hydrometeors and occurs just below the -15°C level, an air temperature where diffusional growth rates for ice crystals maximize. Although the bright band was quite distinct, about one-third of the profiles in the composite did not exhibit this feature. These nonbrightband (NBB) profiles had a low-level slope where reflectivity increased with decreasing altitude, a structure suggesting that collision-coalescence was the primary growth process. The relationship between surface rainfall rate and low-level radar reflectivity implies that all profiles were composed of larger numbers of small drops than expected from a Marshall-Palmer drop-size distribution, a trend that was especially apparent for NBB profiles.

Synoptic variability of precipitation characteristics at CZD were examined by identifying five distinct regimes (cold sector, warm front, warm sector, cold front and cool sector) based on a simplified conceptual model. Mean brightband altitude was lowest during the cold-sector and cool-sector regimes, highest during the warm-sector regime and at intermediate levels during the warm-frontal and cold-frontal regimes. The shoulder remained approximately 2.5 km above the bright band in each regime. Rainfall intensity was highest during the cold-frontal regime and NBB rainfall was most common during the warm-frontal, warm-sector and cool-sector regimes.

Topographic variability of precipitation characteristics was investigated by comparing results at CZD and KDAX. Mean rainfall rates near KDAX were about half the magnitude of those at CZD and led to only about 20% of the accumulated rainfall. The mean KDAX brightband altitude was slightly lower than at CZD, likely in response to relatively cooler air being trapped in the central valley. A shoulder structure located about 2.5 km above the bright band was also evident in the KDAX profiles, suggesting that this feature is related to large scale dynamic, thermodynamic and microphysical processes rather than orographic effects. The relationship between surface rainfall rate and low-level radar reflectivity near KDAX closely followed a trend expected for a Marshall-Palmer drop-size distribution, implying the presence of relatively larger raindrops than observed at CZD and indicating that NBB rainfall occurs less frequently near KDAX