Controls on Wintertime Non-brightband Rain Rate and Frequency in California’s Coastal Mountains

Non-brightband (NBB) warm rain is an important precipitation process along California’s coastal mountains and a key component of flood forecasting and water resource management. NBB rain often forms entirely below the NEXRAD 0.5° beam, causing difficulty identifying its presence and precipitation amount. However, the microphysical growth of NBB rain is not very complex, given the existence of only a single orographic cloud layer and single growth mechanism, collision-coalescence. This simplicity, and NOAA/ESRL/PSD’s profiler network allow for an advanced understanding of NNB rain intensity and frequency. This study focuses on four profiler locations in California’s Sonoma and Mendocino counties; two on mountains and two in valleys. A suite of observations includes vertically pointing precipitation profilers, disdrometers, and surface observations, along with nearby wind and temperature profiles and GPS-derived IWV. These datasets were used to identify the meteorological conditions where NBB rain formed and to what intensity.