Interactions of Tropopause Depressions with an Ex–Tropical Cyclone and Sensitivity of Forecasts to Analysis Errors

Abstract

This paper focuses on the coupling between an ex?tropical cyclone and two preexisting mesoscale tropopause depressions (TDs). The TDs approached the cyclone from widely separated sources after becoming cut off from different upper-level troughs upstream. The first part of the paper combines Meteosat imagery with products from the limited-area version of the operational U.K. Meteorological Office Unified Model to reveal the 3D structure and evolution of the mesoscale features. Each TD was a potential vorticity (PV) maximum characterized by dry-intrusion air descending slantwise beneath an upper-level jet streak. Each TD generated its own cloud head and each is believed to have contributed to the deepening of the surface cyclone. Later parts of the paper identify errors in model forecasts and attribute them to analysis errors in the position of one of the TDs. Two methods are used to locate the analysis errors. Both are capable of being used in real time. The first is the identification via satellite imagery of an error in the model?s water vapor analysis. The second method, using singular vectors calculated for the ECMWF model, involves the identification of sensitive regions where any analysis error would be expected to grow rapidly. The regions highlighted by these two methods were broadly collocated. The present case is unusual in that the most sensitive region was in the upper troposphere. Forecast reruns made with the Met Office and ECMWF models after modifying the upper-level PV in the analysis showed some limited improvements.