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    Geography of the Alpine region including locations discussed in the text. The track of the GoG cyclone (Table 1) is shown with a solid line, with 0000 UTC and 1200 UTC positions marked by circles. Dates and times for the track are given as hour (UTC)/date, with the minimum central pressure from the CMC analysis (hPa) in parentheses. The regional orography is shaded in meters for reference with values as indicated on the grayscale bar.

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    Extent of the limited-area model domains used in the numerical modeling component of the study (section 4), identified with the tags used during the MAP D-PHASE project (CMCGEML, CMCGEMH/S). The model terrain (shaded in meters, as indicated on the grayscale bar) of both domains is shown. The locations of surface and upper-air stations referenced in this study are indicated, Payerne, Switzerland (LSMP); EDTZ; LIML; Stuttgart, Germany (EDDS).

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    Potential temperature on the DT (values <320 K plotted as indicated on the grayscale bar), DT winds (in this and other figures, wind barbs are plotted in kt with short, long, and pennant barbs indicating 5, 10, and 50 kt, respectively), and 925–850-hPa layer-average relative vorticity (solid contours at 1 × 10−4 s−1 intervals for values >1 × 10−4 s−1, not plotted where surface pressure is <900 hPa) at 12-hourly intervals from (a) 1200 UTC 14 Nov to (f) 0000 UTC 17 Nov 2007. The CTD is identified with a C, the Alpine banner with an S, and the precursor cyclone with a P on plots, as in Table 2.

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    Manual analysis of surface potential temperature (dashed contours every 2°C) and sea level pressure (solid contours every 2 hPa) at 1200 UTC 15 Nov 2007. The analysis is overlaid on traditional observations that show sky cover and wind (standard barbs in kt) at the center of the station model, and dewpoint temperature (°C), air temperature (°C), and sea level pressure (hPa) rotating from the bottom left to the top-right position of the station model.

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    Observed 2-m potential temperatures and winds (barbs plotted in kt) at Konstanz, Germany (solid line; station located on the windward side of the Alps) and Milan, Italy (dashed line; station located on the leeward side of the Alps) from 0000 UTC 13 to 0000 UTC 17 Nov. Station locations are identified in Fig. 2.

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    Lower-level (1000–900 hPa) frontogenesis (shading in K (100 km)−1 (3 h)−1 as indicated on the grayscale bar), potential temperature (solid contours at 2°C intervals), and winds from the NCEP analysis for (a) 1200 UTC 14 Nov and (b)1200 UTC 15 Nov. The first analyzed position of the GoG (1200 UTC 15 Nov) is plotted with a star. Fields are plotted only over water to avoid extrapolation under terrain.

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    Base radar reflectivity composite at hourly intervals from (a)–(f) 1200 to 1700 UTC 15 Nov 2007 (retrieved rain rates in mm h−1 as indicated on the color bar on each panel and at the bottom of the figure). The region of interest is outlined by a white circle in (a); also shown is a 200-km length scale for reference. These images were provided by ARPA-SIM as part of the MAP D-PHASE project.

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    Storm total lightning strikes recorded by the WWLLN (solid circles) between 1200 UTC 15 Nov and 1500 UTC 16 Nov. Time periods are plotted with different gray shades as indicated on the legend at the bottom of the plot. The track of the GoG cyclone from Table 1 is plotted with a gray line for reference, with dates and times as shown in Fig. 1.

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    Coupling index (computed as defined in section 2, with values <5 K plotted as indicated on the grayscale bar), deep-layer DT 850-hPa vector shear (regions of less than 20 kt of shear are outlined with a dashed line), and 925–850-hPa layer average relative vorticity (solid contours at 2 × 10−4 s−1 intervals for values >2 × 10−4 s−1) at 12-hourly intervals from 1200 UTC 14 Nov to 0000 UTC 17 Nov 2007. The CTD is identified with a C, the Alpine banner with an S, and the GoG cyclone with an L on plots (note that an arrow indicates the center of the GoG cyclone to avoid obscuring the feature with the annotation), as in Table 2.

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    Satellite imagery for the GoG cyclone. (a) Meteosat Second Generation (MSG) infrared imagery for 0600 UTC 16 Nov and (b) visible MSG imagery for 0912 UTC 16 Nov are shown, with the approximate domain of this image outlined with a black rectangle in (a). (c),(d) QuikSCAT-derived surface winds (black barbs) and observed winds (gray barbs). Midday QuikSCAT passes on 15 November are shown with 1200 UTC 15 Nov surface observations in (c), and early 16 November QuikSCAT passes are shown with 0600 UTC 16 Nov surface observations in (d). The GoG cyclone is identified with an L, as in Table 2.

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    Air parcel 12-h back trajectories computed from the GFS analysis with a base time of 0000 UTC 16 Nov. The 1000–500 hPa thickness (gray dashed lines at 2 dam intervals) and pressures along parcel paths from the GoG cyclone’s (a) warm-core region and the (b) warm sector of the thermal wave. The θ values for these parcels are shown for (c) the warm-core and (e) the warm sector, as indicated on the plots. (d) A cross section of θ (plotted with dashed lines at 4 K intervals) and PV (values as indicated on the color bar), taken along the arrow in (b) is shown. The 1000-m terrain contour is plotted with a thin black line for reference. The GoG cyclone is identified with an L and the CTD with a C, as in Table 2.

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    Zonal thickness anomaly (1000–700 hPa layer) from the regional zonal mean between 0° and 20°E (shaded anomaly with values as indicated on the grayscale bar, plotted only where surface pressures are >900 hPa), and 700-hPa winds for analyses valid at 12-h intervals from (a) 0000 UTC 14 Nov to (d) 1200 UTC 15 Nov 2007. Thickness anomalies computed from different mean states, including wider meridional bands and the long-term mean, produce qualitatively similar results. The orientation of the cross section shown in Fig. 14 is plotted in (c). The 1000-m terrain contour is plotted with a heavy dark line, and the GoG cyclone is identified with an L, following Table 2.

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    Air parcel 24-h back trajectories computed from the GFS analysis with base times of (a)–(c) 1200 UTC 14 Nov and (d)–(f) 1800 UTC 15 Nov 2007. (a),(d) Show the parcel paths, with parcel pressures as indicated on the color bar. (b),(e) The RHS shows the time evolution of parcel pressure and (c),(f) θ for parcels ingested by the GoG cyclone (blue, defined as parcels that are trapped by the cyclone’s circulation) and those escaping from the circulation (red). The 1000-m terrain contour is plotted with a heavy dark line in panels (a),(c). (a) Shows 24-h foreword trajectories until 1200 UTC 15 Nov, with a white box outlining the trajectory source region at 1200 UTC 15 Nov.

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    Cross sections of potential temperature (solid lines at 4 K intervals) and total condensate mass content (shading, with light and dark gray representing mass contents of 2 g m−3 and 6 g m−3, respectively) for (a) 0000 UTC 15 Nov and (b) 0000 UTC 16 Nov. The position of the cross section is shown in Fig. 12c.

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    Soundings from Payerne, Switzerland (LSMP, location identified in Fig. 2) taken at 0000 UTC 14 Nov (gray) and 0000 UTC 15 Nov (black). Observed temperatures and dewpoints are drawn with solid and dashed lines, respectively. The approximate orientation of the western section of the Alps near the sounding station is indicated in the lower right corner of the panel for reference.

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    (a) Air parcel trajectories and PV [values as indicated on the horizontal color bar in (a)] computed from the GFS analysis with a base time of 0600 UTC 15 Nov, centered on the PV banner emanating from the southwestern tip of the Alps. Trajectories are computed backward to 0600 UTC 14 Nov (−24 h) and forwards to 0000 UTC 16 Nov (+18 h). The layer-average 925–850-hPa potential vorticity at 0600 UTC 15 Nov is plotted as indicated on the vertical color bar in (a). (b),(c) Shown are the parcel pressures and PV, respectively, for the trajectories shown in (a). Trajectories identified as “ingested” by the GoG cyclone are plotted in blue, and those that “escaped” to the southeast are plotted in red in (b) and (c). (d),(e) Layer-average (925–850 hPa) potential vorticity (contours as indicated on the color bar) and winds in the immediate Alpine region at 0600 and 1200 UTC 15 Nov are shown. The 1000-m terrain contour is plotted with a solid line in (a),(d),(e), the GoG cyclone is identified with an L, and the banner position is indicated with an S, as in Table 2.

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    Vorticity banner development in an idealized configuration of the GEM model described in the text. For K values of (left) 0 and (right) 1 m2 s−1 at 0600 UTC 15 Nov: (a),(d),(b),(e) Layer-average (925–850 hPa) relative vorticity [(a),(d) shading, × 10−4 s−1 and heavy black contours at intervals of (a) 40 × 10−9 s−2 and (d) 20 × 10−8 s−2; there is a a single gray contour at half of these values for reference and this plot (as in all panels) does not have a 0 contour] and (a),(d) winds; (b),(e) cross sections of relative vorticity (contour intervals of 5 × 10−5 s−1) and (c),(f). cross sections of the tilting–stretching source term (contour intervals of 10 × 10−9 s−2 with ±1 × 10−9 s−2 contours shown for reference) along lines A–B in (a) and C–D in (d). The thin solid contour in (a),(b) is 1000-m terrain contour.

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    (a),(c),(e) Layer-average (925–850 hPa) PV [PVU, with shading as indicated on the grayscale bars, (a) with different shading increments consistent with increased lower-level PV values in the idealized tests] and winds for the NCEP analysis and idealized K = 1 m2 s−1 and K = 0 integrations are shown. (b),(d),(f) Cross sections of PV [PVU, as labeled on the thick black contours, with additional plotting intervals in (b) as noted above] along the lines indicated on the panel for the corresponding integration are shown in panels. The 1000-m terrain contour is plotted with a thin solid line in the left column of panels.

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    (a) Track of the GoG cyclone in the analysis and early control integrations (initialized 1200 UTC 14 Nov) as indicated in the legends. All realizations of the cyclone are tracked from 1200 UTC 15 Nov (diamonds) to 0000 UTC 16 Nov (circles). (b) Time series of minimum MSLP for the GoG cyclone from 1200 UTC 15 Nov to 0000 UTC 16 Nov.

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    Surface winds valid at 0900 UTC 15 Nov from the 15-h forecast of the early CMCGEMH/S integration (black barbs) and combined QuikSCAT and surface observations (gray barbs).

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    Zonal thickness perturbation (1000–700-hPa layer) from the model zonal mean (stippled anomaly with values as indicated on the horizontal grayscale bar) and mean 925–850-hPa PV (overlaid black contours and shading in PVU with values as indicated on the vertical grayscale bar) from the early CMCGEMH/S, valid at (a) 0000 UTC 15 Nov and (b) 1200 UTC 15 Nov. All fields are plotted only where surface pressures are >900 hPa and the 1000-m terrain contour (heavy black line) is provided for reference. The position of the GoG cyclone in the forecast is indicated with an L in (b), as in Table 2.

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    Forecast structure of the GoG cyclone from the early CMCGEMH/S integration with a 30-h lead time. (a) Thickness of the 1000–500-hPa layer (dashed contours at 2 dam intervals), 700-hPa winds, and sea level pressure below 1004 hPa (shading as indicated on the grayscale bar) are shown. (b) The composite reflectivity (dBZ as indicated on the grayscale bar) for the same time is shown. (c) A zonal cross section of PV (PVU, with shading as indicated on the grayscale bar) and θ (solid contours at 2 K intervals) through the center of the GoG cyclone at 0000 UTC 16 Nov is shown. (d) A similar cross section of simulated reflectivity (dBZ, with shading as indicated on the grayscale bar) and θe (solid contours at 2 K intervals) is shown. The cross-section line used for (c) and (d) is shown in (a), and the position of the GoG cyclone is identified in (b),(c), and (d) with an L, as in Table 2.

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    Forecast 0000 UTC 16 Nov θDT (K, with values as indicated on the color bar), winds on the DT (orange barbs), and 925–850-hPa layer-average-level relative vorticity (plotted with pink solid contours at 4 × 10−4 s−1 intervals for values >4 × 10−4 s−1, where surface pressure >900 hPa) from the (a) early Global, (b) CMCGEML, and (c) CMCGEMH/S integrations. The steering flow is computed from Global and CMC analysis grids and compared with the observed storm displacement [Table 1 in the panel (a),(d) inset]. Air parcel 24-h back trajectories ending near the storm center at 1800 UTC 15 Nov at 850 hPa from the (d) Global, (e) CMCGEML, and (f) CMCGEMH/S (f) integrations initialized at 1200 UTC 14 Nov. Parcel pressures along the trajectories are plotted as indicated on the color bars below the panels. The position of the GoG cyclone is identified with an L in (a),(b), and (c), as in Table 2, and the 300 K θDT contour from the analysis (Fig. 3d) is plotted for reference in (a).

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    Evaluation of vertical transports of horizontal momentum in the 24-h forecast (valid 1800 UTC 15 Nov) of the early CMCGEMH/S integration. (a) Profiles of the 8° storm-centered box-average westerly (solid) and southerly (dashed) wind components are shown in knots. (b) Profiles of the component mean vertical momentum fluxes within 2° of the GoG center at all points with a rain rate of ≥1 mm h−1. (c) The mean accelerations (momentum flux convergence) using the same set of points as in (b).

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

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  • 1 Numerical Weather Prediction Research Section, Meteorological Service of Canada, Dorval, Québec, Canada
  • | 2 Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York
  • | 3 Numerical Weather Prediction Research Section, Meteorological Service of Canada, Dorval, Québec, Canada
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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.

Corresponding author address: Ron McTaggart-Cowan, 2121 Trans-Canada Highway, Floor 5, Dorval, QC H9P 1J3, Canada. Email: ron.mctaggart-cowan@ec.gc.ca

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.

Corresponding author address: Ron McTaggart-Cowan, 2121 Trans-Canada Highway, Floor 5, Dorval, QC H9P 1J3, Canada. Email: ron.mctaggart-cowan@ec.gc.ca

1. Introduction

The rapid development of subsynoptic-scale cyclones in the Gulf of Genoa (GoG; Fig. 1) constitutes an important forecasting challenge for the western Mediterranean (Radinović 1965). One such event took place on 15 November 2007 during the Mesoscale Alpine Project (MAP) demonstration of probabilistic hydrological and atmospheric simulation of flood events in the alpine region (D-PHASE) project (Rotach et al. 2009), and within 12 h led to the generation of tropical storm–force winds near the islands of Corsica and Sardinia. This region is well known to favor the development of subsynoptic-scale cyclones (Buzzi and Tosi 1989; Trigo et al. 1999, 2002), which generally occur as an upper-level trough over western Europe that crosses the Alpine barrier (Bergeron 1928; Bjorkdal 1931; Smith et al. 1982; Pichler and Steinacker 1987; and others). With dense coastal populations and numerous shipping lanes, such strong wind events in the western Mediterranean can have a large impact on the region despite their relatively small horizontal scale (Lionello et al. 2006).

A proposed development mechanism for Alpine lee cyclogenesis (Egger 1972) is based on a two-layer conceptual model in which a pair of vertically coupled potential vorticity (PV) features exist upshear of the mountain barrier (Buzzi and Speranza 1983; Hoskins et al. 1985). Cold advection ahead of the lower-level feature limits development until blocking occurs on the upwind slopes of the high terrain. As the primary upper-level wave decouples and crosses the obstacle, it induces secondary development in the lee. Because of the blocking effect of the terrain, cold advection in the lee is minimized and static stability is reduced relative to the windward environment. Theoretical models based on development mechanisms such as this focus on normal mode baroclinic growth of perturbations superposed on a simplified basic state representative of flow in the Alpine region (Smith 1984; Pierrehumbert 1985; Speranza et al. 1985; Buzzi and Speranza 1986; Smith 1986; Trevisan and Giostra 1990); however, none of the pre-1990 theories were independently verified by Egger (1988) or Tafferner and Egger (1990).

Early simulations of Alpine lee cyclone development by primitive equation models suggested that the mesoscale details of the flow were responsible for the rapid intensification of the subsynoptic-scale vortex (Bleck 1977; Dell’Osso 1984; McGinley and Goerss 1986). This finding has been supported by the PV-based diagnostic study of Thorpe et al. (1993) and the modeling investigation undertaken by Aebischer and Schär (1998). The authors of the latter study developed a two-stage conceptual model of lee cyclogenesis consisting of lower-level diabatic PV generation followed by baroclinic coupling with the upper-level PV feature. Although Thorpe et al. (1993) and Aebischer and Schär (1998) agree that the creation of lower-level PV is important for lee cyclogenesis, they differ in their mechanisms for such development. Thorpe et al. (1993) suggest that differential friction at the southwestern corner of the Alps is responsible for increases in PV, whereas Aebischer and Schär (1998) cite the results of Smolarkiewicz and Rotunno (1989) to conclude that hydraulic jumps are capable of producing a PV maximum in the lee. Rotunno et al. (1999) note that a spatially invariant eddy diffusivity is sufficient to create PV banners from vertical vorticity generated by tilting as the flow interacts with the barrier. Conversely, Epifanio and Durran (2002) show that in a strongly nonlinear system the presence of strong vertical gradients of thermal diffusion in the hydraulic jump region are important for the diabatic generation of PV from the vertical vorticity described by Rotunno et al. (1999).

The thermodynamic feasibility of “Mediterranean hurricanes” is described by Emanuel (2005) using an axisymmetric model and potential intensity (Bister and Emanuel 1997) arguments to demonstrate the possibility for convectively driven cyclogenesis under an upper-level trough. This development pathway is closely linked to “cold low” cyclogenesis described by Businger and Reed (1989). The existence of hurricane-like features in the region is also studied by Ernst and Matson (1983), Mayengon (1984), Rasmussen and Zick (1987), Pytharoulis et al. (2000), Reale and Atlas (2001), and Moscatello et al. (2008). Because these systems generally form under conditions of upper-level troughing not normally considered conducive to tropical cyclogenesis, the potential of their conversion to tropical vortices following a tropical transition (TT; Davis and Bosart 2004) pathway represents an important aspect of their life cycles. During TT, convection sustained by moist enthalpy fluxes (Rotunno and Emanuel 1987) erodes the upper-level trough and reduces vertical wind shear in the cyclone’s environment. Hulme and Martin (2009) show that this process bears similarities to a midlatitude occlusion, particularly associated with the formation of a PV “hook” (treble clef) during TT. The connection between the case of Alpine lee cyclogenesis presented here and the expanding body of TT literature highlights the importance of the Mediterranean hurricane development pathway and provides additional insights into the life cycle of the storm.

The present study presents a diagnosis of the key features that influence the development of the GoG cyclone on 15 November 2007 and evaluates the ability of a set of control forecasts to simulate the evolution of the system. Section 2 describes the datasets and methodologies that are used during the investigation. A description of the cyclogenesis event follows in section 3. An evaluation of the forecast initialized 24 h before development is undertaken in section 4. This first part of the study concludes with a discussion of the findings in section 5.

2. Model, data, and diagnostics methodology

The analysis data used for this study comes primarily from the Canadian Meteorological Centre (CMC) Canadian Global Environmental Multiscale (GEM) 0.33° global analysis and forecasting system. McTaggart-Cowan et al. (2010, hereafter Part II) show that this resolution is sufficient to capture the influence of the Alps on the flow, but that it is insufficient to accurately represent the evolution of the GoG cyclone itself. For trajectory and frontogenesis (Miller 1948) calculations archived 0.5° final analysis data from the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) are used because of the availability of analyzed vertical motion. Air parcel trajectories are computed with 2-hourly subintervals using linear interpolation between 6-hourly analysis states and with half-hourly intervals for forecast model output. Although diabatic processes are not directly included in the Miller (1948) form of frontogenesis, the bulk effects of moist convection are represented through the upscale modifications to the mass and flow fields. Because gridded analysis fields are available at all mandatory pressure levels, the diagnostics and plots presented in this study use masking to limit the effects of subterranean extrapolations on the results.

Information from a variety of observational platforms are used to generate a set of storm positions at 3-hourly intervals from 1200 UTC 15 November to 1500 UTC 16 November (Fig. 1 and Table 1). These observing systems include: an Italian radar composite for the D-PHASE project [made available by the Agenzia Regionale Prevenzione e ambiente dell’Emilia-Romagna Servizio IdroMeteoClima (ARPA-SIM)], Meteosat Second Generation visible and infrared satellite imagery, and Quick Scatterometer (QuikSCAT) data. Data from the World-Wide Lightning Location Network (WWLLN; Jacobson et al. 2006) are used to evaluate the extent and intensity of deep convection associated with the GoG cyclone. Despite the increasing coverage of the WWLLN, the detection efficiency of the network over the Mediterranean was only about 20% at the time of the GoG cyclone, suggesting that the actual strike density was about 5 times as large as the results presented here (see Fig. 12 of Rodger et al. 2006).

The GEM model is used for all simulations in this study. The model uses semi-implicit time stepping and semi-Lagrangian advection. Côté et al. (1998) and Mailhot et al. (1998) provide detailed descriptions of the GEM model dynamics and physics formulations, respectively. Simulations are triply nested, beginning with a global ⅓° forecast integrated from CMC analyses. A limited-area version of the GEM model (identified as CMCGEML during MAP D-PHASE) is launched simultaneously with a grid spacing of 0.135°, using the global model outputs as boundary conditions (Fig. 2). A second nest is started 6 h later with a grid spacing of 0.0225°. This inner grid is coded CMCGEMH/S, a 3.5° southward shift of the CMCGEMH grid used for the D-PHASE project designed to improve the GoG cyclone’s placement within the domain. These nesting intervals, domains, and grid spacings were chosen to adequately represent the terrain-influenced dynamics while respecting the areal coverage and delivery time constraints of the D-PHASE project, and they are consistent with the resolutions of the other forecast systems run during the operational period (Rotach et al. 2009). The limited-area simulations are fully nonhydrostatic and employ the single-moment version of the Milbrandt and Yau (2005) grid-scale microphysics scheme. Synthetic reflectivity is computed within the microphysics scheme by assuming Raleigh scattering as for X-band radar. No convective parameterization is activated in integrations on the CMCGEMH/S domain.

Potential vorticity is computed as the vertical component following Ertel (1942) with a conversion metric consistent with the isobaric analyses used for the calculations. The dynamic tropopause (DT) is defined as the 2 PVU (1 PVU = 1 × 10−6 m2 s−1 K kg−1) surface throughout this study, and potential temperature θ on the DT is referred to as θDT. Modifications to the initial states of the GEM model are made using the piecewise PV inversion technique of Davis (1992). Unless otherwise noted, PV anomalies are computed relative to an Eady (1949) basic state as proposed by McTaggart-Cowan et al. (2006). The coupling index is calculated following Bosart and Lackmann (1995) as the difference between θDT and the equivalent potential temperature at 850 hPa. The steering flow is computed as suggested by Renard (1968), with an inner radius of 250 km and an annulus width of 500 km for the 1000–500-hPa layer.

The Froude number (FR = U/Nh) used in this study follows the formulation suggested by Reinecke and Durran (2008) for diagnosing deflection by a barrier in a flow with nonuniform stratification. A bulk estimate of the Brunt–Väisälä frequency (N = (g/θo)(θhθo)/h) uses θ at the surface θo and obstacle height θh (where the barrier height h = 2000 m is used here) to represent lower-level stability. The cross-barrier wind speed U is evaluated at both 850 hPa (FR850) and h (FR2000) for consistency with ongoing studies of Alpine blocking using a similar formulation of FR (O. Martius 2009, personal communication) and the calculations of Reinecke and Durran (2008), respectively. As noted by O’Handley and Bosart (1996) and Schumacher et al. (1996), the passage of upper-level disturbances across a barrier can limit the utility of a quantitative evaluation of FR. In this study, the evolution of FR is therefore used as a consistency check because flow blocking is demonstrated using trajectory analyses and diagnoses of thermal structures near the Alps.

3. Case description

The case of GoG cyclogenesis studied here occurred during the passage of an upper-level trough across the Alpine region, under strong northwesterly cross-barrier flow classified as the “Überströmungs” type by Pichler and Steinacker (1987). The period of interest extends from 1200 UTC 14 November to 0000 UTC 16 November 2007, covering the passage of the trough/front across the Alps and the initiation and rapid development of the lee cyclone in the GoG (GoG cyclone). The analyses presented here, however, extend beyond this period to include a description of the synoptic-scale flow leading up to and following the event. For reference, Fig. 1 identifies both locations described in the text and the track of the GoG cyclone (Table 1).

a. Analysis of the upper-level flow

The development of the GoG cyclone on 15 November is associated with the passage across the Alps of an upper-level trough and associated near-surface baroclinic zone. This section describes how the rapid nonlinear evolution of the trough structure and its southward extension over the Alpine region promote the development of the system.

A positively tilted upper-level trough lies over central Europe at 1200 UTC 14 November (Fig. 3a). As its leading edge moves over the Mediterranean, a quasigeostrophic (QG) forcing for ascent promotes the development of a broad, weak cyclone (identified with a P in Fig. 3a and listed with all other annotations in Table 2) west of Italy that moves rapidly eastward. Despite the broad nature of the trough, there are finer-scaled structures within it whose length scales are on the order of 250 km. The most noteworthy of these is a coherent tropopause disturbance (CTD) located at 55°N, 15°E at this time (labeled C in Fig. 3). Strong northerly cross-barrier flow behind the synoptic-scale trough axis occurs between 1200 UTC 14 November and 0000 UTC 15 November as the CTD, embedded in the same northerly flow, progresses rapidly toward the base of the trough. By 1200 UTC 15 November (Fig. 3c), the first time that the GoG cyclone can be identified in surface analyses (section 3b), the CTD has moved to a position over the western Alps, and its associated cyclonic circulation has disrupted the cross-barrier flow in the upper troposphere (an easterly wind barb near the Alpine crest is highlighted in white in Fig. 3c). The southward motion of the CTD leads to upward-increasing cyclonic vorticity advection that serves as QG forcing for ascent over the GoG. In response to this forcing, the first indication of the lower-level circulation in the objective analyses is present between Corsica and a long vorticity banner that stretches southward from the southwestern tip of the Alps (the latter is identified with an S in Fig. 3c).

The CTD has moved off the French coast by 0000 UTC 16 November and has coupled with the rapidly developing, symmetric near-surface vorticity maximum now located west of Sardinia (Fig. 3d and Table 1). Also at this time, the upper-level trough evolves into a cutoff feature, as the portion of the trough north of 50°N continues to progress eastward, whereas the base, including the embedded CTD, remains stationary over southern Europe and the Mediterranean. By 1200 UTC 16 November (Fig. 3e), the CTD has been weakened by persistent convection associated with the system (discussed in section 3b), although it remains phase locked with the lower-level vortex without an appreciable baroclinic tilt. The system continues to weaken through 0000 UTC 17 November (Fig. 3f), when the remnant CTD moves eastward as a ridge begins to build over western Europe.

b. Analysis of GoG cyclogenesis

The GoG cyclone develops on a shear line associated with the confluence of northerly flow to the west of the Alps and easterly flow on the southside of the Alps. This section describes how this zone of enhanced cyclonic vorticity is effectively converted into a subsynoptic-scale cyclone by local PV generation and coupling with the upper-level flow described in section 3a.

A surface analysis for 1200 UTC 15 November (Fig. 4) clearly demonstrates the influence of Alpine blocking on the regional environment. Surface potential temperatures θ of 2°–3°C in southern Germany (on the windward slopes of the Alps under conditions of northerly cross-barrier flow) contrast with values over 10°C in the Po Valley of northern Italy (Fig. 1). A comparison of θ and wind traces at Konstanz, Germany (EDTZ), and Milano, Italy (LIML; see Fig. 2 for station locations), is consistent with the evolution of the flow from unblocked on 13–14 November to strongly blocked on 15 November (Fig. 5). Despite differing diurnal ranges, the persistent cooling trend at EDTZ on 14 and 15 November leads to a cross-barrier potential temperature gradient of over 5°C by 1200 UTC 15 November. The effectiveness of Alpine blocking on 15 November is further confirmed by the strong sea level pressure gradient across the Alps (Fig. 4), with sea level pressures of over 1014 hPa in southern Germany and 1008 hPa in northern Italy. Also evident in this analysis is the presence of the mistral winds, blowing from the north between the Massif Central and the western Alps, over southern France, and into the western Mediterranean. Of particular interest in the analysis of Fig. 4 are the numerous reports of northeasterly winds at coastal stations on the GoG and across much of northern Italy (including LIML as shown in Fig. 5), a common feature associated with cyclogenesis in the GoG (Buzzi and Tibaldi 1978). This circulation represents the large-scale Alpine lee vortex under conditions of northerly split flow (Krishnamurti 1968) and is consistent with the southerly large-scale pressure gradient established by the precursor cyclone (P), which is still identifiable over Romania at this time.

The development of a confluence zone between the northerly mistrals and the easterly flow in the Alpine lee on 14 and 15 November is well depicted in both the NCEP analysis and available observations. The flow over the northern GoG transitions from northerly and northwesterly at 1200 UTC 14 November (Fig. 6a) to strongly confluent by 1200 UTC 15 November (Fig. 6b), concurrent with the development of a cold plume in the Mistrals to the west. Relative warmth over the GoG leads to strong lower-level confluent frontogenesis in bands extending northwestward from both Corsica and Sardinia (Fig. 6b). Ascent forced by warm advection in the western portion of this frontogenetic zone leads to the development of a line of convective elements visible from the Italian radar composite at 1200 UTC 15 November (white circle in Fig. 7a). Isolated regions with rain rates in excess of 5 mm h−1 and dense lightning activity are associated with the convection (Fig. 8), providing evidence for deep moist ascent. Between 1200 and 1800 UTC 15 November, cyclonic rotation of the individual convective elements around the low center (L) is evident in the radar sequence (Figs. 7a–f). In particular, an area of convective precipitation off the south coast of France at 1400 UTC (Fig. 7c) seems to enhance the cyclonic circulation around the center by promoting convergence at lower levels (not shown), thereby generating vorticity by stretching the background relative vorticity present along the shear line.

Also present over this period is a lower-level vorticity banner that extends from the southwestern tip of the Alps to western Sardinia on 15 November (labeled S on Figs. 9b,c). This narrow strip of vorticity resembles the PV banners identified by Aebischer and Schär (1998) as evidence for PV generation by the elevated terrain. The axis of the banner is located 200 km west of the vorticity center of the GoG cyclone at 1200 UTC 15 November (Fig. 9c); however, a description of its evolution over the next 6-h period is not possible given the temporal resolution of the analyses. Further discussion of this feature, and its potential impact on GoG development, will therefore be left until section 3c.2.

The evolution of the coupling index illustrates the importance of the CTD (C) in the development of the near-surface circulation (Fig. 9). As the CTD moves over the relatively warm waters of the Mediterranean—approximately 16°C, compared to the surface θ values north of the Alps of 2°–3°C shown in Fig. 4—between 1200 UTC 15 November and 0000 UTC 16 November (Figs. 9c,d) the coupling index drops below 0 K over much of the GoG. This has two important implications for development: 1) it reduces the bulk column stability over the region by upper-level cooling, and 2) it allows greater vertical interaction between PV anomalies (Hoskins et al. 1985). The first has a direct effect on the strength and persistence of the convection shown in Figs. 7, 8, and 10a,b, and the second assists the development of the near-surface vortex in response to the CTD’s circulation (Figs. 10c,d). The coupling index remains negative throughout the GoG’s lifetime, thereby ensuring that the near-storm environment remains favorable for a deep, moist, convectively driven mode of development (Figs. 8 and 9c–f).

In addition to the reduced coupling index, the GoG cyclone is embedded in a region of weakening deep-layer shear by 0000 UTC 16 November, an indication that TT is under way at this time (Fig. 9d). Moist convective ascent appears to result in a vertical redistribution of horizontal momentum and the reduction of shear in the column as the lower-level flow is accelerated and the upper level flow is decelerated (Montgomery and Shapiro 1993). This process is one of the key elements of the tropical transition process outlined by Davis and Bosart (2003, 2004), and in conjunction with PV superposition it appears to reduce shear values to below 20 kt (DeMaria 1996) in the area surrounding the developing vortex. The spatial and temporal scales of the global analyses preclude a detailed study of convectively driven vertical transports, and such an investigation will therefore be left for section 4. Despite this limitation, the analyzed shear values shown in Fig. 9 are supported by the 16 and 17 November soundings from Cagliari, Sardinia (LIEE), which show rapid shear reduction in association with a drop in the tropopause height to 350 hPa as the CTD crosses the region (not shown).

One of the signatures of TT is the development of a warm-core structure through sustained latent heat release associated with deep moist convection. Figure 11 shows that an analyzed warm core has developed by 0000 UTC 16 November (Fig. 11a) and that parcels entering the cyclone’s circulation experience a diabatic increase in θ of approximately 4 K in the 6-h period leading up to the analysis time (Fig. 11c). Further evidence of the impact of deep convection is the erosion of the upper-level PV flanking the lower-level vortex as the TT nears completion (Fig. 11d). Although the advection of cold air over the western Mediterranean by the Mistral winds leads to the development of a thermal wave across the region, the zonal track of parcels composing the warm sector of the wave (Fig. 11b) and the lack of sustained diabatic heating (Fig. 11e) along the trajectories indicate that the warm core of the GoG cyclone forms as a result of persistent convection rather than a frontal seclusion process (Keyser and Shapiro 1986; Hulme and Martin 2009).

Reduced shear, warm sea surface temperatures and a diabatically generated warm core indicate that the GoG cyclone underwent TT to form a hurricane-like vortex of the type described by Emanuel (2005). This assertion is supported by the symmetric form of the lower-level vorticity field in Fig. 9 and the cloud patterns shown in Figs. 10a and 10b. The primary energy source for such tropical cyclone–like vortices is typically related to enhanced, moist enthalpy fluxes from the warm, moist, underlying surface driven by strong near-surface winds [wind-induced surface heat exchange (WISHE); Rotunno and Emanuel (1987) and Bister and Emanuel (1997)]. This process requires wind speeds in excess of 20 kt to exist around the vortex, a condition clearly met by the GoG cyclone, as evidenced by QuikSCAT retrievals for 15 and 16 November (Figs. 10c,d) in which winds in the vicinity of the system (L) are seen to climb above tropical storm force (34 kt). Recent findings by Montgomery et al. (2009) suggest that convergence beneath isolated convective cells is sufficient to lead to vortex intensification in the absence of WISHE. Therefore, it is possible that such finescale structures within the circulation play an important role in the development of the hurricane-like vortex; however, these processes cannot be directly evaluated here given the relatively coarse resolution of the analysis dataset. The deep convection, eye-like feature in Fig. 10b and warm-core perturbation reminiscent of a tropical cyclone are maintained by the decreased upper-level stability and weak vertical shear shown in Fig. 9. These factors result in the rapid evolution of the GoG cyclone following a TT development pathway.

c. Leeside potential vorticity

Two components of the PV field have been identified by Aebischer and Schär (1998) as being potentially important during the early stages of lee cyclogenesis: 1) a surface θ anomaly and 2) a lower-level PV banner. It is shown that the former is generated in this case by both flow blocking (Radinović 1965; Egger 1972; Buzzi and Tibaldi 1978; Bleck and Mattocks 1984; Tafferner 1990) and upstream moist diabatic processes, whereas the latter is generated as the flow splits around the Alpine orography (McGinley 1982; Aebischer and Schär 1998; Rotunno et al. 1999; Epifanio and Durran 2002).

1) Surface potential temperature

Two distinct local flow regimes exist during the lead up to the development of the GoG cyclone. Between 0000 and 1200 UTC 14 November, the ridge-level winds (approximately 700 hPa for the 3000 m Alpine barrier) veer from a northwesterly to a northerly cross-barrier flow (Figs. 12a,b). A FR2000 near 0.4 at this time (Table 3, with FR850 between 0.5 and 0.6) appears to be sufficient to minimize the blocking of potentially cold air by the Alps (Fig. 5) because the majority of the flow rises up and over the barrier and descends in the lee (Smolarkiewicz and Rotunno 1989). Over this 12-h period, a pool of warm air begins to develop along the immediate leeward slopes of the Alps (Fig. 12b). As the cutoff develops on 15 November (Fig. 3), the 700-hPa flow veers to northerly (Fig. 12c) and eventually northeasterly (Fig. 12d) by 1200 UTC. The strength of the northerly component of the flow in the lee decreases, suggesting that cross-barrier flow has ceased; however, the near-surface warm anomaly persists in the protected region of weak flow over northern Italy (Figs. 12c,d).

Two factors contribute to the development and maintenance of the warm lower-level anomaly in the lee of the Alps: diabatic warming and cold frontal retardation. The dominant cross-barrier flow leading up to 15 November results in strong upslope flow on the northern side of the Alps as shown in Fig. 13a. Parcels originating below the Alpine ridge level over the eastern North Atlantic and western France rise rapidly to ridge-top level as they approach the barrier (Fig. 13b). Corresponding increases in parcel θ (Fig. 13c) are related to diabatic heat releases associated with heavy precipitation on the northside of the Alps, where snow accumulations between 50 and 120 cm were observed at high elevations. Total condensate mass contents of greater than 6 g m−3 on the windward slopes of the Alpine ridge are present in the 0000 UTC 15 November analysis as shown in Fig. 14a (the downstream condensate mass maximum is an isolated transient feature that appears in the cross section only at this time). As the flow descends in the lee, parcel θ values remain constant and a warm near-surface θ anomaly is generated (Figs. 12, 13c, and 14a).

The potentially cold air in the northerly flow behind the baroclinic zone (surface θ values near 2°C as shown in Fig. 4) is blocked by the Alps at between 0000 and 1200 UTC 15 November as FR2000 drops to 0.2 (Table 3, with FR850 decreasing to 0.1) and the flow transitions from “over” to “around” the barrier [Fig. 13d; a similar flow configuration was found by Buzzi and Tibaldi (1978) using trajectory analyses based on radiosonde data]. This change in the local flow regime is triggered by the drop in ridge-level wind speed and the presence of the frontal inversion at 650 hPa as the trough crosses the Alps (Figs. 14b and 15, as described by Chen and Smith 1987). Flow splitting occurs upstream of the Alps (Fig. 13d), with strong northerly winds advecting the cold postfrontal air over the Balkans and the western Mediterranean with little evidence of persistent diabatic heating (Fig. 13f). This results in the maintenance of the localized warm anomaly beneath ridge-top level in the lee of the Alps, as shown in Figs. 12 and 14b.

Although much of the warm air in the lower-level anomaly is advected southward toward Sardinia and Sicily (Fig. 13a) on 15 November, some of it is ingested directly by the developing system. As shown in Figs. 13b and 13c, approximately half of the ingested parcels rise from below 900 hPa and experience diabatic θ increases of 4°–6°C over the 24-h period leading up to 1200 UTC 14 November. The cyclonic flow implied by the presence of the warm anomaly (Bretherton 1966) is evident in Fig. 12c, where it enhances the northeasterly component of the flow in the northern GoG, thereby strengthening confluence in the cyclonic shear zone along which the cyclone forms (section 3b). In this way, the barrier-scale warm anomaly enhances background cyclonic vorticity, which is available for concentration through the stretching term of the vorticity equation in regions of ascent (not shown), thereby assisting in the development of the GoG cyclone.

2) Alpine PV Banner

Flow splitting around the Alpine orography on 15 November leads to the generation of counterrotating circulations in the lee of the kind shown by Buzzi and Tibaldi (1978) and Steinacker (1984). In this regime, a pair of cyclonic and anticyclonic wake vortices is shed by the southwestern and eastern corners of the barrier, respectively. The presence of a barrier-scale PV banner (Smith and Smith 1995) by 0600 UTC 15 November is confirmed in Fig. 16a, which shows a narrow strip of high-PV air extending southward from the southwestern tip of the Alps. Additionally, Fig. 16d suggests that an anticyclonic vortex is being shed from the eastern end of the Pyrenees at this time. Both banners extend over 400 km downstream and have widths that are consistent with the resolution of the gridded analyses, suggesting that their aspect ratio is similar to that of the banners described by Aebischer and Schär (1998).

As noted in the introduction (section 1), differential friction (Thorpe et al. 1993), tilting in the presence of dissipation (Rotunno et al. 1999), and hydraulic jump dynamics (Schär and Smith 1993; Aebischer and Schär 1998) have all been proposed as mechanisms for generating lower-level PV in the Alpine wake. In this case, the low FR and strong northerly lower-level flow channeled by the western Alps leads to rapid increases in parcel PV values from less than 1 PVU to well over 2 PVU in the 6-h period following 0000 UTC 15 November (Fig. 16c). A quick drop in PV values is evident for parcels that enter the boundary layer, which extends to 850 hPa in the 1200 UTC 15 November sounding from Agaccio, Corsica (not shown), and experience strong frictional dissipation; however, 90% the ingested parcels experienced a net increase in PV between 0600 UTC 14 November and 0000 UTC 16 November, and 40% retain values above 1 PVU as they enter the GoG cyclone’s circulation (Fig. 16b).

A series of simulations using an idealized configuration of the GEM model are used to evaluate the vorticity source in the Alpine banner. Results from this adiabatic, inviscid form of the model, using only a numerical diffusion of the type employed by Schär and Durran (1997), show that a vorticity maximum forms in the absence of dissipative processes by 0600 UTC 15 November for a global integration initialized at 1200 UTC 14 November (Fig. 17, left column). The tilting and stretching vorticity source terms (Bluestein 1992) are maximized at the southwestern corner of the Alps (Figs. 17a,c), leading to the development of strong vertical vorticity in a localized region (Figs. 17a,b) as shown in Figs. 2a and 2b of Rotunno et al. (1999). However, the reversibility of these processes results in no permanent modification to the flow and therefore no horizontally extended vorticity banner of the type seen in Figs. 3b and 3c. The application of constant eddy diffusivities of heat and momentum (K right column of Fig. 17) has an important influence on the development of the vorticity banner because it enables a permanent modification of the vorticity field (cf. Fig. 17d here and Fig. 2d of Rotunno et al. 1999; and Figs. 17e,f here and Figs. 6a,b of Rotunno et al. 1999).

In addition to the irreversible generation of vertical vorticity, the presence of a nonzero K permits the diabatic generation of PV from the enhanced vertical-vorticity structure. No appreciable PV banner is produced when K = 0 (Figs. 18c,d); however, an increase in K (Figs. 18e,f) permits the development of a PV field that is remarkably similar to the analysis given the large number of simplifications made to develop the idealized formulation of the model (e.g., the free slip lower boundary results in increased lower level PV throughout the domain). The structure of the PV banner in the latter case also compares well with the results of Rotunno et al. (1999) (cf. Fig. 18f here and Fig. 6d of Rotunno et al. 1999), suggesting that locally enhanced diffusivity in a hydraulic jump region downstream of the barrier of the type described by Epifanio and Durran (2002) is not essential to the generation of PV in this case.

Aebischer and Schär (1998) suggest that these banners serve as a source of lower-level PV for a developing lee cyclone in the GoG. The strong northerly component in the Mistral winds appears to keep the Alpine banner well west of Corsica and Sardinia and away from the incipient GoG cyclone (Fig. 16e); however, the influence of this PV banner on cyclogenesis will be assessed in greater detail during the attribution tests presented in Part II.

4. Evaluation of the control forecast

This section describes and evaluates the development of the GoG cyclone in a numerical prediction system designed for the MAP D-PHASE project (Rotach et al. 2009). The regions covered by the domains used for this study are shown in Fig. 2. Forecasts initialized at 1200 UTC 14 November are shown here and represent a lead time of 24 h for GoG cyclogenesis on the global and CMCGEML domains, with a 6-h nesting delay for the higher-resolution CMCGEMH/S integrations. This start time is denoted as “early” in Part II. A set of forecasts initialized 12 h later produce similar results, although the track and intensity of the GoG cyclone are slightly improved. For brevity, these “late” integrations will not be described in detail here. All forecasts end at 0000 UTC 16 November and serve as the baseline for testing presented in Part II.

Development of the GoG cyclone in the early sequence of integrations generally follows the pathway outlined in section 3b; however, the track of the system at all resolutions begins and ends too far to the southeast (Fig. 19a). This track error (up to 150 km by 0000 UTC 16 November in CMCGEMH/S) places the GoG cyclone too close to the Sardinian coast during the integrations. The cyclonic curvature of the track in both model and analysis, however, suggests that the interaction between the GoG vortex and the Mistral winds to the west was represented in the forecasts. Despite the error in the cyclone’s position and uncertainty associated with the representation of a poorly resolved feature in the analysis system, the minimum mean sea level pressure (MSLP) of the GoG cyclone in both the Global and CMCGEMH/S integrations agrees closely with estimates from the CMC analysis (Fig. 19b; Trigo et al. 1999 present a climatology of MSLP values for cyclones in the region). The CMCGEML nest predicts a GoG cyclone that is too deep (992 hPa, this error is discussed in detail in Part II) and overly broad in scale (not shown); however, because this integration is used primarily to provide boundary conditions for CMCGEMH/S, the effects of this error appear to be minimal.

The forecast wind field after 15 h of the CMCGEMH/S integration (valid 0900 UTC 15 November) shows that the model is capable of reproducing the salient features of the flow prior to the cyclone’s development (Fig. 20). The northerly and northwesterly Mistral winds west of Corsica and Sardinia are consistent with the scatterometer observations at this time. Additionally, the 20–30-kt northeasterly flow in the GoG is well represented in the CMCGEMH/S forecast; however, these winds extend too far westward in the northern GoG, a fact that may explain the initial westerly track direction bias throughout the forecast sequence (Fig. 19a). Unfortunately, no more scatterometer passes occurred until after 0000 UTC 16 November, so comparisons of the forecast and QuikSCAT-derived wind fields at times closer to the cyclone’s initiation and TT are impossible.

The evolution of the lower-level thermal and PV fields are well represented by the early CMCGEMH/S integration. In the 6-h forecast (valid 0000 UTC 15 November, cf. Figs. 12c and 21a), the blocked cold air on the windward side of the Alps is spilling into the western Mediterranean and a warm anomaly is present in the lee of the barrier and over the GoG. A disorganized PV banner is generated in flow past the southwest tip of the Alps (cf. Figs. 9b and 21a) and additional short banners are shed by individual Alpine peaks despite the weakening cross-barrier flow. By 1200 UTC (Fig. 21b), only the primary banner southwest of the Alps still exists, along with enhanced regions of PV near the GoG cyclogenesis location. In this 18-h forecast, the low-level thermal anomaly does not extend as far westward as in the analysis (Fig. 12d). Its role in the last hours leading up to development is therefore uncertain; this topic will be addressed during the attribution tests undertaken in Part II.

The structure of the forecast storm by 0000 UTC 16 November is shown in Fig. 22. The low center is elongated along the western Sardinian coast, likely as a result of interaction with the island’s orography. An isolated warm-core PV maximum has developed (Figs. 22a,c) over the secondary low-level center near the west coast of the island. This is also the region with the largest column-maximum synthetic reflectivity values (Fig. 22c), indicative of large diabatic heating rates and height falls near the surface. The convection both to the west of the center and over Sardinia is troposphere deep and occurs in an environment of lower-layer convective instability (Fig. 22d). In accordance with the conceptual model of TT (Davis and Bosart 2004), convection leads to the erosion of the CTD in both the CMCGEML and CMCGEMH/S integrations (Figs. 23b,c, with currently active cells notable in the CMCGEMH/S results as isolated elevated θDT values). The 36-h Global forecast initialized at 1200 UTC 14 November, however, does not represent the finescale structure of the CTD well nor show θDT increases associated with the GoG cyclone’s convection (Fig. 23a). This has an important impact on the forecast track of the cyclone because the reduction in downstream ridging leads to an increased westerly component in the steering flow (Fig. 23a and inset, with comparison to the analysis θDT provided for reference), leading directly to the eastward bias in the predicted track (Fig. 19a).

Although only a subjective comparison between the composite equivalent reflectivity values shown in Fig. 22c and the satellite imagery presented in Figs. 10a and 10b is possible, the structure of the condensed hydrometeor mass fields appears to compare well between the CMCGEMH/S integration and observations. The tongue of low reflectivities east of Sardinia in the model forecast has numerous embedded shallow ice-phase clouds with small particles that yield low simulated X-band reflectivities yet show up as warm cloud tops in Fig. 10a. Additionally, the fact that the graupel category in the Milbrandt and Yau (2005) microphysics scheme contributes most strongly to the forecast equivalent reflectivity values (not shown) is consistent with the presence of both deep convection (Fig. 10a) and electrification (Fig. 8).

The transition of the flow from over to around the Alpine barrier was shown in section 3b to be a key ingredient for cyclogenesis in this case. Trajectory analyses from the early forecast sequence are shown in Fig. 23 for direct comparison with Fig. 13d. In general, the paths of the parcels agree well with the analyzed trajectories; however, descent of air from 600 hPa and above northwest of the Alps in the CMCGEML and CMCGEMH/S integrations (Figs. 23e,f) is inconsistent with the generally flat trajectories in the analysis (Fig. 13d) and Global forecast (Fig. 23d). This error appears to be related to an overdeepening of the trough, which results in strengthened QG descent forcing behind the trough axis (not shown). Descent in this airstream reduces relative humidity values and enhances the “dry slot” east of the GoG cyclone (Fig. 22b). The degree of deflection by the Alpine barrier depends on the resolution of the mountain range in the forecast (cf. Figs. 23d,e,f), as fewer trajectories are able to cross the barrier on progressively higher-resolution domains. A discussion of the effects of grid spacing on the GoG cyclone’s evolution and energetics constitutes an important element of Part II of this study.

The vertical transport of horizontal momentum by convective updrafts and downdrafts was hypothesized in section 3b to be responsible for a rapid reduction in shear near the incipient GoG cyclone and represents an important component of TT dynamics (Fig. 9). The results of the CMCGEMH/S integration suggest that this process is active in this case (Fig. 24). Following the approach of Soong and Tao (1984), vertical fluxes of horizontal momentum (Fig. 24b) and vertical momentum flux convergence (Fig. 24c) show that the flow is being accelerated below 700 hPa and decelerated above. This vertical momentum redistribution in regions of active precipitation works to homogenize the wind profile, reducing deep-layer shear (Fig. 9), and enhancing the PV hook structure of the CTD (Hulme and Martin 2009).

5. Discussion

The development and TT of an intense, subsynoptic cyclone in the GoG on 15–16 November 2007 has been analyzed, and the quality of numerical guidance for the case been evaluated. Three key components of the cyclogenesis event have been identified: 1) a long-lived upper-level CTD embedded in a synoptic-scale trough, 2) a leeside surface θ perturbation, and 3) a primary orographic PV banner.

The passage of a CTD across the Alps (Fig. 3) leads to upward-increasing PV advection and stability reduction over the GoG (Fig. 9). Once it emerges over the warm Mediterranean waters, deep convection in both the cold northerly Mistral near-surface flow and the incipient GoG circulation lead to the rapid deformation and destruction of the CTD. This process is accompanied by vertical momentum redistribution that reduces vertical shear over the cyclone during TT, thereby allowing it to intensify as a Mediterranean hurricane (Emanuel 2005).

The near-surface circulation develops along a lower-level horizontal shear line that forms in the confluence zone between easterly winds over the GoG and the northerly Mistrals blowing off the coast of France. This intense band of cyclonic curvature vorticity serves as a focus for frontogenesis and stretching by the ascent forced by both the CTD and moist convection (Figs. 6 and 7). The incipient vortex is initially steered southwestward, and then it takes on a southeasterly track as the influence of the Mistrals increases after 2100 UTC 15 November (Fig. 1). Over this period, the winds around the GoG cyclone increase to tropical storm force and deep convection surrounds an eyelike feature as the system undergoes TT and develops a diabatically generated warm core (Figs. 10 and 11). The storm reaches its peak intensity early on 16 November west of Sardinia, then weakens rapidly as it makes landfall on the island.

The influence of the Alps on the development of the GoG cyclone is highlighted by the transition of the flow from over to around the Alpine barrier. Prior to the passage of the cold front associated with the synoptic-scale trough on 15 November (Fig. 4), air parcels rise from the Atlantic boundary layer and warm diabatically as precipitation falls on the Alpine northside (Figs. 13 and 14). Descent in the lee of the mountains leads to the formation of a warm surface θ anomaly, representing cyclonic boundary PV (Fig. 12). The cold-frontal inversion near ridge-top and veering midlevel winds induce a transition to flow “around” the Alps on 15 November. As parcels curve cyclonically around the southwestern tip of the barrier (Fig. 16), they acquire vertical vorticity through the tilting and stretching terms of the vorticity equation (Fig. 17). A set of simulations using an idealized form of the GEM model show that this vorticity is sufficient to create a primary PV banner in the presence of homogeneous dissipation (Fig. 18). In this case, the enhancement of PV in a hydraulic jump downstream of the barrier was not essential to the formation of the banner.

Numerical guidance from the forecasting system used at CMC during the MAP D-PHASE project was evaluated for its ability to simulate the essential elements of the GoG cyclone’s development. The role of the easterly flow in the GoG in enhancing confluence along a shear line extending from Sardinia to the French coast is clear in the model forecasts and compares well with QuikSCAT retrievals (Fig. 20). A system with an isolated warm core and upright PV maximum is predicted at 0000 UTC 16 November (Fig. 22) in a region of reduced shear that results from moist convective vertical transports of horizontal momentum, a key component of the TT process (Fig. 24). The symmetric near-core structure and extensive convective activity with large graupel contents are consistent with satellite and lightning observations (Figs. 10 and 8). Additionally, a comparison of parcel trajectories in the model forecasts and those from the analyses shows that “flow around” the barrier was accurately predicted on 15 November by the forecasts, suggesting that the Alpine influence on the regional environment is well represented at all levels of the forecast system (Fig. 23).

The diagnosis presented here identifies the key features involved in this case of GoG cyclogenesis (the CTD, surface θ anomaly, and orographic PV banner) and describes their evolution during an abrupt flow transition that preceded the event. The development and subsequent TT of the cyclone demonstrates that the challenges associated with the prediction of Alpine lee cyclogenesis combine elements traditionally associated with mountain meteorology (development of the incipient vortex in the Alpine wake) and tropical–extratropical interaction (moist convective development and TT). The combination of knowledge accumulated in each of these domains allows for a more thorough understanding of the GoG cyclone’s evolution than is possible from either field in isolation.

Acknowledgments

The authors thank Eyad Atallah, Jason Cordeira, and Antonio Speranza for useful discussions during the preparation of this study. Dave Shultz and three anonymous reviewers provided valuable feedback and guidance during the review process. We also appreciate the efforts of Kristen Corbosiero and Sergio Abarca who processed the WWLLN lighting presented here, and Mike Brennan who provided gridded QuikSCAT data. This study was motivated by the weekly academic year Friday afternoon map discussion led by two of the authors (Bosart and Galarneau) at the University at Albany/SUNY.

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  • Trigo, I. F., , G. R. Bigg, , and T. D. Davies, 2002: Climatology of cyclogenesis mechanisms in the Mediterranean. Mon. Wea. Rev., 130 , 549569.

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Fig. 1.
Fig. 1.

Geography of the Alpine region including locations discussed in the text. The track of the GoG cyclone (Table 1) is shown with a solid line, with 0000 UTC and 1200 UTC positions marked by circles. Dates and times for the track are given as hour (UTC)/date, with the minimum central pressure from the CMC analysis (hPa) in parentheses. The regional orography is shaded in meters for reference with values as indicated on the grayscale bar.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 2.
Fig. 2.

Extent of the limited-area model domains used in the numerical modeling component of the study (section 4), identified with the tags used during the MAP D-PHASE project (CMCGEML, CMCGEMH/S). The model terrain (shaded in meters, as indicated on the grayscale bar) of both domains is shown. The locations of surface and upper-air stations referenced in this study are indicated, Payerne, Switzerland (LSMP); EDTZ; LIML; Stuttgart, Germany (EDDS).

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 3.
Fig. 3.

Potential temperature on the DT (values <320 K plotted as indicated on the grayscale bar), DT winds (in this and other figures, wind barbs are plotted in kt with short, long, and pennant barbs indicating 5, 10, and 50 kt, respectively), and 925–850-hPa layer-average relative vorticity (solid contours at 1 × 10−4 s−1 intervals for values >1 × 10−4 s−1, not plotted where surface pressure is <900 hPa) at 12-hourly intervals from (a) 1200 UTC 14 Nov to (f) 0000 UTC 17 Nov 2007. The CTD is identified with a C, the Alpine banner with an S, and the precursor cyclone with a P on plots, as in Table 2.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 4.
Fig. 4.

Manual analysis of surface potential temperature (dashed contours every 2°C) and sea level pressure (solid contours every 2 hPa) at 1200 UTC 15 Nov 2007. The analysis is overlaid on traditional observations that show sky cover and wind (standard barbs in kt) at the center of the station model, and dewpoint temperature (°C), air temperature (°C), and sea level pressure (hPa) rotating from the bottom left to the top-right position of the station model.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 5.
Fig. 5.

Observed 2-m potential temperatures and winds (barbs plotted in kt) at Konstanz, Germany (solid line; station located on the windward side of the Alps) and Milan, Italy (dashed line; station located on the leeward side of the Alps) from 0000 UTC 13 to 0000 UTC 17 Nov. Station locations are identified in Fig. 2.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 6.
Fig. 6.

Lower-level (1000–900 hPa) frontogenesis (shading in K (100 km)−1 (3 h)−1 as indicated on the grayscale bar), potential temperature (solid contours at 2°C intervals), and winds from the NCEP analysis for (a) 1200 UTC 14 Nov and (b)1200 UTC 15 Nov. The first analyzed position of the GoG (1200 UTC 15 Nov) is plotted with a star. Fields are plotted only over water to avoid extrapolation under terrain.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 7.
Fig. 7.

Base radar reflectivity composite at hourly intervals from (a)–(f) 1200 to 1700 UTC 15 Nov 2007 (retrieved rain rates in mm h−1 as indicated on the color bar on each panel and at the bottom of the figure). The region of interest is outlined by a white circle in (a); also shown is a 200-km length scale for reference. These images were provided by ARPA-SIM as part of the MAP D-PHASE project.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 8.
Fig. 8.

Storm total lightning strikes recorded by the WWLLN (solid circles) between 1200 UTC 15 Nov and 1500 UTC 16 Nov. Time periods are plotted with different gray shades as indicated on the legend at the bottom of the plot. The track of the GoG cyclone from Table 1 is plotted with a gray line for reference, with dates and times as shown in Fig. 1.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 9.
Fig. 9.

Coupling index (computed as defined in section 2, with values <5 K plotted as indicated on the grayscale bar), deep-layer DT 850-hPa vector shear (regions of less than 20 kt of shear are outlined with a dashed line), and 925–850-hPa layer average relative vorticity (solid contours at 2 × 10−4 s−1 intervals for values >2 × 10−4 s−1) at 12-hourly intervals from 1200 UTC 14 Nov to 0000 UTC 17 Nov 2007. The CTD is identified with a C, the Alpine banner with an S, and the GoG cyclone with an L on plots (note that an arrow indicates the center of the GoG cyclone to avoid obscuring the feature with the annotation), as in Table 2.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 10.
Fig. 10.

Satellite imagery for the GoG cyclone. (a) Meteosat Second Generation (MSG) infrared imagery for 0600 UTC 16 Nov and (b) visible MSG imagery for 0912 UTC 16 Nov are shown, with the approximate domain of this image outlined with a black rectangle in (a). (c),(d) QuikSCAT-derived surface winds (black barbs) and observed winds (gray barbs). Midday QuikSCAT passes on 15 November are shown with 1200 UTC 15 Nov surface observations in (c), and early 16 November QuikSCAT passes are shown with 0600 UTC 16 Nov surface observations in (d). The GoG cyclone is identified with an L, as in Table 2.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 11.
Fig. 11.

Air parcel 12-h back trajectories computed from the GFS analysis with a base time of 0000 UTC 16 Nov. The 1000–500 hPa thickness (gray dashed lines at 2 dam intervals) and pressures along parcel paths from the GoG cyclone’s (a) warm-core region and the (b) warm sector of the thermal wave. The θ values for these parcels are shown for (c) the warm-core and (e) the warm sector, as indicated on the plots. (d) A cross section of θ (plotted with dashed lines at 4 K intervals) and PV (values as indicated on the color bar), taken along the arrow in (b) is shown. The 1000-m terrain contour is plotted with a thin black line for reference. The GoG cyclone is identified with an L and the CTD with a C, as in Table 2.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 12.
Fig. 12.

Zonal thickness anomaly (1000–700 hPa layer) from the regional zonal mean between 0° and 20°E (shaded anomaly with values as indicated on the grayscale bar, plotted only where surface pressures are >900 hPa), and 700-hPa winds for analyses valid at 12-h intervals from (a) 0000 UTC 14 Nov to (d) 1200 UTC 15 Nov 2007. Thickness anomalies computed from different mean states, including wider meridional bands and the long-term mean, produce qualitatively similar results. The orientation of the cross section shown in Fig. 14 is plotted in (c). The 1000-m terrain contour is plotted with a heavy dark line, and the GoG cyclone is identified with an L, following Table 2.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 13.
Fig. 13.

Air parcel 24-h back trajectories computed from the GFS analysis with base times of (a)–(c) 1200 UTC 14 Nov and (d)–(f) 1800 UTC 15 Nov 2007. (a),(d) Show the parcel paths, with parcel pressures as indicated on the color bar. (b),(e) The RHS shows the time evolution of parcel pressure and (c),(f) θ for parcels ingested by the GoG cyclone (blue, defined as parcels that are trapped by the cyclone’s circulation) and those escaping from the circulation (red). The 1000-m terrain contour is plotted with a heavy dark line in panels (a),(c). (a) Shows 24-h foreword trajectories until 1200 UTC 15 Nov, with a white box outlining the trajectory source region at 1200 UTC 15 Nov.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 14.
Fig. 14.

Cross sections of potential temperature (solid lines at 4 K intervals) and total condensate mass content (shading, with light and dark gray representing mass contents of 2 g m−3 and 6 g m−3, respectively) for (a) 0000 UTC 15 Nov and (b) 0000 UTC 16 Nov. The position of the cross section is shown in Fig. 12c.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 15.
Fig. 15.

Soundings from Payerne, Switzerland (LSMP, location identified in Fig. 2) taken at 0000 UTC 14 Nov (gray) and 0000 UTC 15 Nov (black). Observed temperatures and dewpoints are drawn with solid and dashed lines, respectively. The approximate orientation of the western section of the Alps near the sounding station is indicated in the lower right corner of the panel for reference.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 16.
Fig. 16.

(a) Air parcel trajectories and PV [values as indicated on the horizontal color bar in (a)] computed from the GFS analysis with a base time of 0600 UTC 15 Nov, centered on the PV banner emanating from the southwestern tip of the Alps. Trajectories are computed backward to 0600 UTC 14 Nov (−24 h) and forwards to 0000 UTC 16 Nov (+18 h). The layer-average 925–850-hPa potential vorticity at 0600 UTC 15 Nov is plotted as indicated on the vertical color bar in (a). (b),(c) Shown are the parcel pressures and PV, respectively, for the trajectories shown in (a). Trajectories identified as “ingested” by the GoG cyclone are plotted in blue, and those that “escaped” to the southeast are plotted in red in (b) and (c). (d),(e) Layer-average (925–850 hPa) potential vorticity (contours as indicated on the color bar) and winds in the immediate Alpine region at 0600 and 1200 UTC 15 Nov are shown. The 1000-m terrain contour is plotted with a solid line in (a),(d),(e), the GoG cyclone is identified with an L, and the banner position is indicated with an S, as in Table 2.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 17.
Fig. 17.

Vorticity banner development in an idealized configuration of the GEM model described in the text. For K values of (left) 0 and (right) 1 m2 s−1 at 0600 UTC 15 Nov: (a),(d),(b),(e) Layer-average (925–850 hPa) relative vorticity [(a),(d) shading, × 10−4 s−1 and heavy black contours at intervals of (a) 40 × 10−9 s−2 and (d) 20 × 10−8 s−2; there is a a single gray contour at half of these values for reference and this plot (as in all panels) does not have a 0 contour] and (a),(d) winds; (b),(e) cross sections of relative vorticity (contour intervals of 5 × 10−5 s−1) and (c),(f). cross sections of the tilting–stretching source term (contour intervals of 10 × 10−9 s−2 with ±1 × 10−9 s−2 contours shown for reference) along lines A–B in (a) and C–D in (d). The thin solid contour in (a),(b) is 1000-m terrain contour.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 18.
Fig. 18.

(a),(c),(e) Layer-average (925–850 hPa) PV [PVU, with shading as indicated on the grayscale bars, (a) with different shading increments consistent with increased lower-level PV values in the idealized tests] and winds for the NCEP analysis and idealized K = 1 m2 s−1 and K = 0 integrations are shown. (b),(d),(f) Cross sections of PV [PVU, as labeled on the thick black contours, with additional plotting intervals in (b) as noted above] along the lines indicated on the panel for the corresponding integration are shown in panels. The 1000-m terrain contour is plotted with a thin solid line in the left column of panels.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 19.
Fig. 19.

(a) Track of the GoG cyclone in the analysis and early control integrations (initialized 1200 UTC 14 Nov) as indicated in the legends. All realizations of the cyclone are tracked from 1200 UTC 15 Nov (diamonds) to 0000 UTC 16 Nov (circles). (b) Time series of minimum MSLP for the GoG cyclone from 1200 UTC 15 Nov to 0000 UTC 16 Nov.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 20.
Fig. 20.

Surface winds valid at 0900 UTC 15 Nov from the 15-h forecast of the early CMCGEMH/S integration (black barbs) and combined QuikSCAT and surface observations (gray barbs).

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 21.
Fig. 21.

Zonal thickness perturbation (1000–700-hPa layer) from the model zonal mean (stippled anomaly with values as indicated on the horizontal grayscale bar) and mean 925–850-hPa PV (overlaid black contours and shading in PVU with values as indicated on the vertical grayscale bar) from the early CMCGEMH/S, valid at (a) 0000 UTC 15 Nov and (b) 1200 UTC 15 Nov. All fields are plotted only where surface pressures are >900 hPa and the 1000-m terrain contour (heavy black line) is provided for reference. The position of the GoG cyclone in the forecast is indicated with an L in (b), as in Table 2.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 22.
Fig. 22.

Forecast structure of the GoG cyclone from the early CMCGEMH/S integration with a 30-h lead time. (a) Thickness of the 1000–500-hPa layer (dashed contours at 2 dam intervals), 700-hPa winds, and sea level pressure below 1004 hPa (shading as indicated on the grayscale bar) are shown. (b) The composite reflectivity (dBZ as indicated on the grayscale bar) for the same time is shown. (c) A zonal cross section of PV (PVU, with shading as indicated on the grayscale bar) and θ (solid contours at 2 K intervals) through the center of the GoG cyclone at 0000 UTC 16 Nov is shown. (d) A similar cross section of simulated reflectivity (dBZ, with shading as indicated on the grayscale bar) and θe (solid contours at 2 K intervals) is shown. The cross-section line used for (c) and (d) is shown in (a), and the position of the GoG cyclone is identified in (b),(c), and (d) with an L, as in Table 2.

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 23.
Fig. 23.

Forecast 0000 UTC 16 Nov θDT (K, with values as indicated on the color bar), winds on the DT (orange barbs), and 925–850-hPa layer-average-level relative vorticity (plotted with pink solid contours at 4 × 10−4 s−1 intervals for values >4 × 10−4 s−1, where surface pressure >900 hPa) from the (a) early Global, (b) CMCGEML, and (c) CMCGEMH/S integrations. The steering flow is computed from Global and CMC analysis grids and compared with the observed storm displacement [Table 1 in the panel (a),(d) inset]. Air parcel 24-h back trajectories ending near the storm center at 1800 UTC 15 Nov at 850 hPa from the (d) Global, (e) CMCGEML, and (f) CMCGEMH/S (f) integrations initialized at 1200 UTC 14 Nov. Parcel pressures along the trajectories are plotted as indicated on the color bars below the panels. The position of the GoG cyclone is identified with an L in (a),(b), and (c), as in Table 2, and the 300 K θDT contour from the analysis (Fig. 3d) is plotted for reference in (a).

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Fig. 24.
Fig. 24.

Evaluation of vertical transports of horizontal momentum in the 24-h forecast (valid 1800 UTC 15 Nov) of the early CMCGEMH/S integration. (a) Profiles of the 8° storm-centered box-average westerly (solid) and southerly (dashed) wind components are shown in knots. (b) Profiles of the component mean vertical momentum fluxes within 2° of the GoG center at all points with a rain rate of ≥1 mm h−1. (c) The mean accelerations (momentum flux convergence) using the same set of points as in (b).

Citation: Monthly Weather Review 138, 6; 10.1175/2009MWR3147.1

Table 1.

Track of the GoG cyclone at three-hourly intervals from 1200 UTC 15 Nov to 1500 UTC 16 Nov 2007. The primary observations used to determine the center position are indicated in the right-hand column.

Table 1.
Table 2.

List of annotations used throughout the text and figures.

Table 2.
Table 3.

Froude numbers computed from Stuttgart, Germany soundings (EDDS in Fig. 1).

Table 3.
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