A Revised Bourgouin Precipitation-Type Algorithm

Using temperature profiles obtained from upper-air observations or numerical weather prediction (NWP) models, the Bourgouin technique for determining precipitation-type calculates areas of positive melting energy and negative refreezing energy. The energies are proportional to the product of the mean temperature of a layer and its depth. Layers above 0°C consist of positive energy while those below 0°C consist of negative energy. Sufficient melting or freezing energy in a layer can produce a phase change. The Bourgouin technique therefore utilizes these energies to determine the likelihood of rain versus snow (in the case of a surface-based melting layer) and ice pellets versus freezing rain (in the case of an elevated melting layer). This technique, despite being developed from a relatively small dataset, has been widely utilized by operational forecasters and in post-processing of NWP output. However, recent analysis with a wider range of temperature profiles than in the developmental 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 will describe the development of these new thresholds and also suggest additional revisions to the original technique intended to make this an even more viable and valuable tool for precipitation-type forecasts.