The Structure of an Atmospheric Warm Front and its Interaction with the Boundary Layer

The detailed structure of a warm front has been determined from JASIN radiosonde data. The warm front was the base of a sloping moist layer over 200 km wide and 100 mbar deep, within which the baroclinicity was reversed and both observed and geostrophic wind backed with height. The maximum warm baroclinicity occurred at the top of this layer, well within the warm air. The resulting differential advection was such as to maintain potential instability within the frontal zone. The warm air above the front exhibited a laminar structure with two main bands of moist air; accelerations were significant in this region. Subsidence occurred in the cold air beneath the front, although the mean vertical motion became upward before the frontal passage. The decrease in depth of the cold air boundary layer was due to both subsidence and advection. The warm front described in this paper showed many similarities to those of previous studies, in particular the alternation of high and low Theta _e layers in the warm air. Although acceleration terms were not negligible, most of the vertical wind variations have been shown to be essentially geostrophic in origin. Caused by the vertically mixed nature of the warm air frontal zone, the differential advection was such as to enhance the potential instability in the upper part of the frontal zone and hence promote further vertical mixing. Similar differential advection was associated with each of the two warm air moisture bands. Thus the warm air frontal zone, moisture bands, and dry layers represented the interleaving of warm sector air of differing origins. Mixing between the warm air frontal zone and the cold air occurred in a narrow `cold air frontal zone', which represented the frontal interface. However, for this relatively weak front, the largest density gradients, temperature inversions and wind shears occurred within the warm air in association with differential advection.