Mesoscale Weather Systems within an Occlusion

Abstract

Precipitation systems within a mature extratropical cyclone are related to the mesoscale thermal and circulation fields aloft using data from Project Stormy Spring conducted by the Air Force Cambridge Research Laboratories. Precipitation systems were analyzed using radars and recording raingages, including a special mesoscale array; upper-air structures were deduced from a 10-site mesoscale rawinsonde network including serial soundings at 90-min intervals. Results show that most of the widespread precipitation, in conjunction with the cyclonic-scale vertical motions in frontal baroclinic zones, occurs in bands and groups of showers. A sub-synoptic core of cold dry air in the middle troposphere ahead of the surface occlusion was found to be subsiding and surpressing wide-spread cloudiness, while it was furnishing a large amount of potential instability. The cyclone-scale ascending motions then released the potential instability around this cold core and also above the warm frontal stable layer. The convection became aligned in bands roughly parallel to the wind shear in the convective layer. These bands included clusters of cells of more intense precipitation. The cyclonic-scale baroclinic zone associated with the synoptic fronts is made up of multiple mesoscale hyper-baroclinic zones which are shown to be related to the existence and production of potential instability and to precipitation bands and groups of cells.