Analysis of an ex-tropical cyclone after its reintensification as a warm-core extratropical cyclone

Abstract

Ex-hurricane Lili reintensified as an extratropical cyclone before travelling across the data-rich region of the British Isles on 28 October 1996. The cyclone centre passed close to a Mesosphere-Stratosphere-Troposphere (MST) radar, providing continuous profiles of wind etc. which were used to evaluate diagnostics from the mesoscale version of the operational UK Meteorological Office Unified Model. The paper presents a mesoanalysis of the mature extratropical cyclone using model output together with radar and satellite observations. The combined analysis reveals a vertically extensive warm core three-quarters surrounded by a low-level jet reaching over 40 m s−1. There was an associated eye, relatively free of cloud and partly surrounded by a hook cloud producing extensive heavy rain, which was itself encircled by cooler dry-intrusion air. The stratospheric part of the dry intrusion (and its potential-vorticity (PV) anomaly) descended within a tropopause fold around the cloud hook generally to below 400 hPa, with small pockets penetrating significantly lower. The cyclone's reintensification as an extratropical cyclone was related to its interaction with the stratospheric PV anomaly. This interaction commenced immediately after the decay of the strong moist ascent and associated deep column of diabatically generated positive PV that had characterized the earlier tropical-cyclone phase. Following reintensification, the dry-intrusion air entered the eye region of the extratropical cyclone over a deep layer. The mesoscale model represented many aspects of the cyclone structure well but it underestimated the dryness of the dry-intrusion air entering the eye. The MST radar vividly depicted the region of moist boundary-layer air responsible for the hook cloud rising up into the region of the lowered tropopause.