Controls on Wintertime Nonbrightband Rain Rate and Frequency in California's Northern Coast Ranges.

Nonbrightband (NBB) rain is a shallow, orographic precipitation that does not produce a radar bright band as a result of melting ice crystals. However, NBB rain is not the same as warm rain, which excludes ice from being involved in the microphysical growth of precipitation. Despite this difference, NBB rain is often treated as warm rain in the literature, and past studies have mostly ignored the role of ice. Here, we use two wet seasons (2015/16 and 2016/17) at four precipitation-observing sites in the Northern Coast Ranges of California to show the role of echo top height and ice in determining NBB rain intensity. It was found that NBB rain was only absent of bright bands 32%–46% of the time depending on location of the site. Additionally, all NBB rain rates that exceeded 10 mm h−1 exhibited observable bright bands during the hour period. We also define growth efficiency (GE) as the ability of shallow rain clouds to produce raindrops larger than drizzle size (D > 0.5 mm). High-GE rain drop size distributions were composed of fewer small drops and more large drops than low-GE rain, which was mostly drizzle. High-GE rain occurred with echo top heights above the freezing level where rapid growth of precipitation was observed by radar. Echo tops that only extended 1 km or less above the freezing level suggested hydrometeor growth from mixed-phase processes, indicating that ice may be present in coastal precipitation at warmer temperatures than previously considered. [ABSTRACT FROM AUTHOR]