Joint Centre for Mesoscale Meteorology (JCMM)
JCMM Internal Report No.80:Analysis of an ex-tropical cyclone after reintensifying as a warm-core extratropical cyclone by K A Browning, G Vaughan and P Panagi
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 an MST radar providing continuous profiles of wind etc which were used to evaluate diagnostics from the mesoscale version of the operational Met. 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 42 ms-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 anomaly) descended generally below 400 hPa, with small pockets penetrating significantly lower, within a tropopause fold around a cloud head at the leading edge of the hook cloud. The cyclone's reintensification as an extratropical cyclone and the development of the cloud head were closely related to its interaction with the stratospheric PV anomaly. This interaction began after the decay of the strong moist ascent and deep diabatic PV anomaly that had characterised the earlier tropical cyclone phase. The tropospheric part of the dry intrusion entered the eye region of the extratropical cyclone over a deep layer and may have been responsible for the decay of the cyclone after its rather short reintensification phase. The mesoscale model represented many aspects of the cyclone structure well but it underestimated the true dryness of the dry-intrusion air entering the eye. The MST radar vividly showed the region of boundary-layer air, which was responsible for the cloud had, rising up into the region of the lowered tropopause.