Real-time detection of split fronts using mesoscale models and WSR-88D radar products

A structured methodology for detecting the presence of split cold fronts in an operational forecast environment is developed and applied to a case in which a split front passed over a region of cold air damming in the southea stern United States. A real-time mesoscale model and various products from the WSR-88D-including the velocity-azimuth display wind profile (VWP) and h odograph products, plus a thermal advection retrieval scheme applied to the VWP data-are used to study this split front and an associated convective r ainband that occurred on 19 December 1995. Wet-bulb temperature and vertical motion forecasts at 700 hPa from the mode l revealed the arc-shaped split front 300-500 km ahead of the surface cold front. As this midtropospheric front passed across the surface warm front a nd entered the cold air damming region, model vertical cross-section analys es showed that it created a deep elevated layer of potential instability. F urthermore, an ageostrophic transverse circulation associated with the spli t front provided the lifting mechanism for releasing this instability as de ep convection. Analysis of the absolute geostrophic momentum field provided greater understanding of the structure of the split front and a deep tropo spheric frontal system to its west that connected with the surface cold fro nt. An "S-inverted S" pattern in the zero isodop on WSR-88D radial velocity dis plays indicative of wind backing above wind veering suggested the presence of the split front in the observations (as did the hodographs). Detection o f the passage of the split front could be discerned from temporal changes i n the vertical profile of the winds. namely by the appearance of midlevel b acking of the winds in VWP time-height displays. Because of the subtlety of this backing and the need to be more quantitative, a temperature advection retrieval scheme using VWP data was developed. The complex evolving struct ure of the split front was revealed with this technique. Results from this retrieval method were judged to be meteorologically meaningful, to exhibit excellent time-space continuity, and to compare reasonably well with the fr ontal structures evident in the mesoscale model forecasts. The thermal adve ction scheme can easily be made to function in operations, as long as there is real-time access to level II radar data.