Development and Tropical Transition of an Alpine Lee Cyclone. Part I: Case Analysis and Evaluation of Numerical Guidance

Abstract

The development and tropical transition (TT) of a subsynoptic-scale cyclone in the Gulf of Genoa during the Mesoscale Alpine Project (MAP) demonstration of probabilistic hydrological and atmospheric simulation of flood events in the alpine region (D-PHASE) project is investigated using analyses and model simulations. Cyclogenesis occurs in association with the passage of a synoptic-scale trough and attendant surface cold front across the Alps on 15 November 2007. An embedded coherent tropopause disturbance (CTD) plays an important role in promoting the initial development of the lower-level vortex by simultaneously providing quasigeostrophic forcing for ascent and reducing the bulk column stability over warm Mediterranean waters. Persistent convection thereafter erodes the CTD as the storm transitions into a hurricane-like vortex.In addition to this upper-level forcing, a pair of diabatically generated lower-level cyclonic potential vorticity (PV) features associated with distinct flow regimes is potentially important to the cyclogenetic process in this case. The first, a warm surface potential temperature anomaly, is generated during cross-barrier flow by prefrontal upslope precipitation on the Alpine northside, followed by parcel descent in the lee. The second PV feature is a mountain-scale PV banner that extends southward from the southwestern tip of the Alps as the flow is deflected around the mountain chain.Numerical guidance for this case is evaluated on its ability to accurately depict the development and evolution of the cyclone. Comparison of a triply nested integration (grid spacings of 33, 10, and 2.5 km) with observations and analyses demonstrates that the model is capable of simulating the salient features of the event. Combining reliable guidance from high-resolution modeling systems with the paradigms of lee cyclone development and the emerging concepts of TT promotes an improved understanding of these potentially high-impact events.

A new methodology for identifying daughter cyclogenesis: application for the Mediterranean Basin

“…This scenario is in agreement with theories on lee cyclogenesis combined with the retardation effect of the Alps on a cold front (e.g. Tafferner and Egger, , Buzzi et al , ; McTaggart‐Cowan et al , , see Section above). The Gulf of Genoa is an example of a region where different processes for cyclonic formation act in concert and produce much more vigorous developments than the sum of each individual factor (Lim and Simmonds, , Kouroutzoglou et al , ).…”

“…The spatial distribution of parent cyclones in individual cyclogenetic areas (Figure ) indicates a tendency of the parents to concentrate in specific regions, a fact suggesting preferred cyclogenesis scenarios. For example, parents of Genoa lows are found mostly east to their daughters, implying that the cyclogenesis takes place within the northerly flow that crosses the Alps, a hypothesis in agreement with the analysis of Tafferner and Egger (), Buzzi et al () and McTaggart‐Cowan et al (). Parents of MCs formed over the Aegean Sea are located mainly to the west of the daughters, implying that the cyclogenesis takes place within the southerly flow ahead of the parent cyclones, i.e.…”

Tropical transition of a Mediterranean storm by jet crossing

“…Among the Mediterranean cyclones accompanied by upper-level anomalies and deep convection, about 15 mesocyclones have been observed over the last 20 years, preferentially in autumn (Fita et al, 2007). They share some similarities with tropical cyclones and a number of case studies have been made on these hurricanelike cyclones or medicanes (Mediterranean 'hurricanes'; Lagouvardos et al, 1999;Pytharoulis et al, 2000;Reale and Copyright c 2011 Royal Meteorological Society Atlas, 2001;Homar et al, 2003;Fita et al, 2007;Moscatello et al, 2008aMoscatello et al, , 2008bDavolio et al, 2009;Claud et al, 2010;McTaggart-Cowan et al, 2010;among others). A medicane exhibits a clearly defined eye, quasi-circular cloud patterns, strong surface winds sometimes reaching the intensity of a tropical storm, deep convection and the presence of a warm core.…”

Promising to tell the truth makes 8‐ to 16‐year‐olds more honest

“…Among the Mediterranean cyclones accompanied by upper‐level anomalies and deep convection, about 15 mesocyclones have been observed over the last 20 years, preferentially in autumn (Fita et al , 2007). They share some similarities with tropical cyclones and a number of case studies have been made on these hurricane‐like cyclones or medicanes (Mediterranean ‘hurricanes’; Lagouvardos et al , 1999; Pytharoulis et al , 2000; Reale and Atlas, 2001; Homar et al , 2003; Fita et al , 2007; Moscatello et al , 2008a, 2008b; Davolio et al , 2009; Claud et al , 2010; McTaggart‐Cowan et al , 2010; among others). A medicane exhibits a clearly defined eye, quasi‐circular cloud patterns, strong surface winds sometimes reaching the intensity of a tropical storm, deep convection and the presence of a warm core.…”