A Revised Bourgouin Precipitation-Type Algorithm (262)
Kevin Birk, National Weather Service Chicago, Romeoville, IL, IL
Eric Lenning, National Weather Service Chicago
Top-down precipitation-type forecast methods often use the 0°C threshold to distinguish warm melting layers from cold refreezing layers. The National Weather Service uses a version of the top-down technique in its ForecastBuilder tool and its National Blend of Models. When this version finds a warm layer aloft, melting potential is computed from the layer's maximum temperature (or wet-bulb). It assumes the layer depth and maximum temperature are proportional. It also assumes little melting occurs at temperatures <1°C, total melting occurs at temperatures >3°C, and partial melting occurs otherwise. It is understood that these assumptions are not valid in isothermal layers. If such a layer is present but undiagnosed, this version of the top-down technique generally predicts ice pellets even though freezing rain becomes much more likely. Given this shortcoming, a comprehensive method that accounts for both the depth and magnitude of warm and cold layers is desired for improved gridded precipitation-type forecasts.
The Bourgouin technique is a top-down method that calculates melting and refreezing energies (J/KG) in a sounding. These energies are proportional to the area of warm and cold layers and have no problem accounting for isothermal layers. Despite being developed from a small dataset, the Bourgouin technique is widely utilized in post-processing of NWP output. It also provides precipitation-type nomograms in the popular Bufkit program for analyzing NWP forecast soundings. However, recent analysis with a larger dataset suggests revisions are needed to the original technique. This is especially true for the function used for discriminating freezing rain versus ice pellets. This presentation describes the development of these new thresholds. It also suggests additional revisions to the original technique intended to make this an even more viable and valuable tool for gridded precipitation-type forecasts.