Use of a mixed-phase microphysics scheme in the operational NCEP Rapid Update Cycle

Scientists at NCAR (National Center for Atmospheric Research) and NOAA (National Oceanic and Atmospheric Administration) are collaborating to improve the mixed-phase, bulk microphysics scheme that is currently part of the operational RUC (Rapid Update Cycle) forecast model run at the NCEP (National Centers for Environmental Prediction, part of the USA National Weather Service). This is part of the general upgrade of the RUC discussed in a companion abstract submitted to this conference by Benjamin et al. An important motivation for this work (under partial sponsorship of the Federal Aviation Administration) is to provide better guidance to the Aviation Weather Center of NCEP for preparation of their icing forecasts.

This scheme, known colloqually as EXMOISG, incorporates explicit prediction of the mixing ratios of cloud water and cloud ice, as well as rain, snow and "graupel" (which is formed primarily by riming on ice or snow particles, or by freezing of raindrops as result of collisions with ice). In addition, an explicit prediction of number concentration of cloud ice is included.

This major upgrade of EXMOISG specifically addresses known problems with the present scheme, including too much graupel production at the expense of supercooled liquid water. We will present examples of EXMOISG performance in the developmental version of RUC at the Forecast Systems Laboratory (FSL), and changes in behavior that users of the RUC may note during this coming icing season.