A Comparative Study on the Effect of Upstream Trough on Intensity Changes of Two Types of Tropical Cyclones during Extratropical Transition

Abstract

Based on the China Meteorological Administration (CMA) tropical cyclone (TC) database and the reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF), the post-transition intensifying tropical cyclones (ITC) and weakening tropical cyclones (WTC) which landed in China and underwent extratropical transition (ET) are discussed in this paper. The TCs with upper-level trough are selected in order to identify different effects of the trough on the ITC and the WTC. The dynamic composite analysis is applied to explore their structure characteristics, environment fields and dynamic diagnosis. Results show that affected by the South Asia high and the subtropical high, the trough of ITC (WTC) extends from northwest to southeast (northeast to southwest) in the ET process. Thus, the zonal wind shear of ITC drops off after ET due to the approach of trough and its northwest-southeast direction, while the zonal and total wind shears of WTC continue to increase because of the steering westerly flow at the upper level. In terms of the ITC, the cold air carried by the upper-level trough intrudes into TC inner area and mostly encircles the TC center, making the ITC characterized by a warm seclusion. While for the WTC, the cold air only wanders on the northwest side of TC without further intrusion. The upper-level divergence is also in favor of the ITC by the pumping influence. According to the diagnostic analysis of moist potential vorticity, the moist baroclinity can lead to changes in vertical vorticity to some extent. The vertical vorticity budget analysis further indicates that there is stronger and wider positive vorticity advection in the upper troposphere near the TC center for ITC. The contribution of the baroclinic term to the growth of vertical vorticity is more significant in ITC than WTC but it is also deeply influenced by the strength of upper-level trough.