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.

Revisiting the synoptic-scale predictability of severe European winter storms using ECMWF ensemble reforecasts

“…Storm Gero is again an outlier with strong negative biases at days 5 and 8 (red triangles) but an even larger bias is found at day 7 for Lili (blue square). This storm formed in the tropics (Browning et al, 1998, see also Fig. 2), which is consistent with the poor predictability of the position during extratropical transition due to the difficulty to represent convective dynamics (e.g.…”

“…The storm took its origin in Hurricane Lili, which reached Europe after crossing the North Atlantic and undergoing extratropical transition (Browning et al, 1998). With the first method, the identified tracks start from the same location, because the storm is present in the reforecast at the time of initialization (Fig.…”

The Key Role of Diabatic Outflow in Amplifying the Midlatitude Flow: A Representative Case Study of Weather Systems Surrounding Western North Pacific Extratropical Transition

“…Another aspect of ET is its potential to cause highimpact weather (HIW) events such as heavy rain, strong winds, ocean swells, or wild fire conditions directly related to the transitioning TC (Sekioka 1959;Foley and Hanstrum 1994;Browning et al 1998;Harr and Elsberry 2000;Klein et al 2000;McTaggart-Cowan et al 2001;Atallah and Bosart 2003;Jones et al 2003). However, in remote regions away from the transitioning TC, the dispersion of downstream Rossby waves can trigger HIW as well Grams 2011;Archambault et al 2013;Keller and Grams 2014;Grams and Blumer 2015;Harr and Archambault 2016;Pantillon et al 2015).…”

Intensification of Hurricane Sandy (2012) through Extratropical Warm Core Seclusion

“…Evans and Hart (2003) note, that for some cases of ET, the lower-tropospheric warm core is retained. Examples of such cases include Hurricane Iris (1995; Thorncroft and Jones 2000) and Hurricane Lili (1996;Browning et al 1998;Agust ı-Panareda et al 2005). In the case of Hurricane Lili, detailed analysis by Browning et al (1998) showed how Lili evolved to a structure that resembled a warm core seclusion analogous to the final mature stage of the Shapiro and Keyser (1990) cyclone life cycle model.…”