Vertical Structure of Local Fronts Observed in Kanto, Japan

This paper focuses on two cases of a local front that formed ahead of a synoptic cold front under a southwesterly inflow in the Kanto region. Vertical sounding data provided by an observation network of the Tsukuba Area Precipitation Studies (TAPS) enabled a detailed investigation of the mesoscale structure in the vicinity of the local front from its formation through dissipation. Despite some differences in synoptic features, the local front exhibited essential similarities in vertical structure between the two cases, as well as in horizontal structure. The local front, with WSW-ENE orientation, was well defined even in the daytime. A stable layer with a thickness of about 400 m formed on the cold side of the developed local front. As in the upper layer of warm inflow, southwesterly winds prevailed in this stable layer, except for the lowest 50-200 m. The maintenance mechanism of the local front is discussed. The evolution of the local front largely depends on the development of the stable layer on its cold side. In both cases, frontal formation began in the nighttime. A nocturnal cooling on the land surface contributed to forming a cold air mass on the northern side of the local front. In addition, the southwesterly inflow caused warming just above the surface-based cooling layer, which intensified the stratification. Furthermore, the observed vertical structure suggests that the locally modified circulation affected the frontal evolution. Two orographic effects on the circulation, 1) intensification of the inflow over the eastern part, and 2) downslope wind over the western part, both related to the mountains in the west of the Kanto region are expected. Especially, the downslope wind can reasonably explain the daytime persistence of the local front through the enhancement of stratification by adiabatic warming, and resultant influences on the mass transport.