32nd Conference on Broadcast Meteorology/31st Conference on Radar Meteorology/Fifth Conference on Coastal Atmospheric and Oceanic Prediction and Processes

Wednesday, 6 August 2003: 9:50 AM
Raindrop size distributions and radar bright bands in California coastal orographic storms
Brooks E. Martner, NOAA/ETL, Boulder, CO; and A. B. White and S. Y. Matrosov
Poster PDF (304.8 kB)
CALJET's profiling radar observations of precipitation in northern California's coastal mountains during the strong El Nino winter of 1997-98 revealed new insights about microphysical properties of the region's orographically-forced precipitation. Data from S-band precipitation profilers showed that, although these storms extended above and below the freezing level, they often did not exhibit the melting layer radar bright band (BB) that is usually characteristic of midlatitude winter storms. Yet these non-bright band (NBB) situations contributed substantially (28%) to the region's record-breaking winter precipitation that year, even though they were generally shallow and commonly passed beneath the coverage of the nearest NEXRAD radars.

PACJET collected new S-band profiler data in 2003 in the same area, this time augmented with raindrop disdrometer and polarimetric scanning radar measurements. One S-band profiler was again located at the same coastal mountain site near Cazadero, CA, and another was located about 12-km away on the coastline at Fort Ross, CA. Joss-Waldvogel raindrop disdrometers accompanied both S-bands to document the raindrop size distributions at the ground. An X-band polarimetric weather radar at Fort Ross scanned the sky above these sites to infer hydrometeor identities aloft and to map echo coverage as the storms approached from the ocean.

The preliminary 2003 disdrometer and S-band data are being partitioned into rainfall periods with and without a bright band. Analysis of the combined data from the Fort Ross site corroborates the earlier CALJET diagnosis based on radar data alone, in that the appearance of a bright band is usually associated with relatively sparse concentrations of larger raindrops at the surface, whereas the NBB periods have much greater concentrations of small drops. The NBB rainfall is less intense but more steady and prolonged, and it usually falls from regions with shallower echo tops. The X-band data show that a bright band can appear, vanish, and reappear in a matter of minutes. We will present a case study illustrating the 2003 observations and a conceptual model that explains differences between the NBB and BB cases and their rapid reversals. We will also use the scanning X-band radar at Fort Ross to demonstrate that it is commonplace for extensive rain echoes in NBB situations to arrive at the coastline beneath the coverage of the regional NEXRAD radars, thereby eluding the attention of NWS forecasters.

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