Observing and Communicating Information About Nonbrightband Rain and Snow Level to Forecasters in the Western U.S.: Two Successful R2O Projects from NOAA's HMT

One of the early goals of the NOAA's Hydrometeorology Testbed (HMT; hmt.noaa.gov) was to identify the meteorological forcing for extreme rainfall events along the West Coast of the U.S. From observations using scanning and vertically-pointing radars along the coast and coastal mountains in Sonoma County just north of San Francisco it was observed that shallow clouds, often less than 3 km in depth and height, produced, on average, 34% of the cool-season rain and could produce rain rates capable of causing local flooding. This precipitation process was named Nonbrightband (NBB) rain because it occurs without the presence of a discernible bright-band melting layer, i.e., similar to warm rain. The NWS operational NEXRAD radars, given their location and scanning strategy, overshoot the NBB rain clouds and greatly underestimate the accompanying rainfall. A key demonstration of why this observing gap is important occurred in November 1994, when the city of San Francisco received its largest known 24-h rainfall. This event contained a large proportion of NBB rain, caused extensive urban flooding, and was virtually undetected by the closest NEXRAD radar. Several published studies also have shown that the microphysical nature of NBB rain is vastly different from rain that includes a brightband. These factors make it nearly impossible to accurately estimate NBB rain rates using conventional scanning radar techniques.

The vertically pointing S-band radar precipitation profiler (S-PROF) was designed and built by Earth System Research Laboratory (ESRL) engineers in the late 1990's to probe precipitation in the coastal mountains of Sonoma County for a research project called CALJET, a precursor to HMT. The S-PROF provides highly resolved (60-m vertically, 1 min. temporally) profiles of radar reflectivity and Doppler vertical velocity that can be used to identify and characterize NBB rain. The CALJET S-PROF deployment proved to be useful from both research and NWS operations perspectives, and four additional S-PROFs were built and deployed in subsequent field campaigns. More recently, ESRL engineers designed, built, and tested a much less expensive frequency modulated, continuous wave (FM-CW) S-band radar for a project with the California Department of Water Resources (CA-DWR). These radars provide similar information to the S-PROF and also are very useful for automated detection of the snow level (defined here as the altitude of peak radar reflectivity in the bright band) during precipitation. Ten of these new “snow-level radars” are being deployed at major watersheds across California for CA-DWR, and the snow level observations are being sent to NWS Western Region offices in the prescribed format for inclusion in the office's hydrologic database. The snow level is critical to runoff in mountainous watersheds because it determines the areal extent of the watershed that will be exposed to rain versus snow. ESRL's snow level product (derived from S-PROFs, snow-level radars, and Doppler wind profilers) has been used extensively by forecasters at the California/Nevada River Forecast Center, who otherwise struggle with verifying their snow level forecasts that are based mostly on numerical weather prediction guidance.