Meso-β-scale Pressure Dips Associated with Typhoons

Using a mesoscale model, the formation process of a pressure dip, which was characterized by a rapid decrease and subsequent increase in surface pressure lasting less than an hour, was investigated. A simulated pressure dip accompanied by Typhoon Zeb was closely related to warm potential temperature anomalies in the lower troposphere. As the typhoon moved into the midlatitude westerlies, the inflow of a dry air mass into the moist region of the typhoon occurred to the west of the typhoon center. Then, a downdraft developed due to evaporation and sublimation. Below the melting levels where there were fewer hydrometeors, however, evaporation was insufficient to offset the adiabatic warming. As a result, warm potential temperature anomalies were created in the lower level, resulting in the formation of a pressure dip. The features of a pressure dip associated with other typhoons observed over Japan were also examined. The features were summarized as follows: 1) the pressure dip was observed only on the western side of the typhoon center at a distance of 50–300 km from the center, 2) the pressure dip was accompanied by a sudden cessation of rainfall, and 3) all typhoons with a pressure dip were found during boreal autumn when large-scale environmental conditions were characterized by westerlies with a trough and a dry air mass to the west of Japan at upper levels and fronts at lower levels. The formation process of the simulated pressure dip in other typhoons was found to be similar to that of Typhoon Zeb. The present study suggests that pressure dip is an inherent feature of the asymmetric structure of a typhoon undergoing transition to a extratropical cyclone.