Precipitation in the coastal zone can dramatically impact society, particularly when it occurs in large quantities and contributes to flooding. Northern California is a prime example of a coastal region that experiences these types of problems. The region has several hydrologically sensitive watersheds (e.g., Russian, Napa) in the midst of a large population base in the San Francisco and Sacramento metropolitan areas. Steep coastal orography has a distinct influence on the development, intensity and horizontal distribution of precipitation resulting from landfalling winter storms. However, the nature of this influence is not well understood, particularly as a function of the synoptic regimes embedded within each storm system. This study takes steps to bridge the gap in knowledge on this subject.

The ten largest rain-producing storm systems observed during the December 1997 to March 1998 California Landfalling Jets (CALJET) experiment are examined. Data from Doppler radars, wind profilers, balloon soundings and surface meteorological stations are used to characterize the synoptic regimes associated with the storm systems as well as the development processes, intensity and horizontal distribution of precipitation emanating from these systems. Two observing locations are emphasized: one in the coastal mountains north of San Francisco and another in the lee of the coastal mountains west of Sacramento.

At the coastal mountain site, the warm frontal regimes of landfalling storm systems produced the largest amount and longest duration of precipitation, with an average rate of 6.7 mm/hr. The most intense precipitation occurred during cold frontal regimes, with an average rate of 11.3 mm/hr. Based on Doppler radar and balloon sounding data, the levels of most rapid precipitation growth occurred around the -15 C level and just above the melting level. Growth in the intervening layer was slower, particularly during the warm sector and cold sector regimes. The relationship between rainfall rate and radar reflectivity indicates a similar slope but smaller intercept relative to the well known Marshall-Palmer relation, suggesting the presence of larger numbers of smaller raindrops. The intercept is particularly small during the warm frontal and warm sector regimes when a large amount of precipitation was associated with growth below the melting level, suggestive of a collision-coalescence process. At the site in the lee of the coastal mountains, there was on average only one-sixth the amount of precipitation that fell in the coastal mountains to the west. Comparison of vertical profiles of radar reflectivity at the two sites indicates weaker echoes and a somewhat lower bright-band in the lee. The relationship between rainfall rate and radar reflectivity is also different in the lee, exhibiting larger intercepts that closely correspond to the Marshall-Palmer relation.