Development and Tropical Transition of an Alpine Lee Cyclone. Part II: Orographic Influence on the Development Pathway

McTaggart‐Cowan, Ron; Galarneau, Thomas J.; Bosart, Lance F.; Milbrandt, Jason A.

doi:10.1175/2009mwr3148.1

Cited by 18 publications

(14 citation statements)

References 33 publications

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“…Therefore, mountains plausibly act as a reservoir of relatively high PV values that potentially reinforces cyclones. This hypothesis has been confirmed in a case study by McTaggart-Cowan et al (2010b). The authors performed several sensitivity tests on the dynamics of a medicane that developed close to the Alps.…”

Section: Dynamical Structure Of Cyclones: Conserved and Non-conserved Pvmentioning

confidence: 69%

“…filaments of diabatically produced PV at the wake of mountains (Rotunno et al 1999;Epifanio and Durran, 2002;Schar et al 2003;Flamant et al 2004). Especially when combined with the upper-tropospheric PV streamer, PV banners are suggested to reinforce cyclones (Tsidulko and Alpert, 2001;McTaggart-Cowan et al 2010a, 2010b. Therefore, the uniqueness of the region adds a higher degree of complexity to the processes implicated in Mediterranean cyclone development.…”

Section: Introductionmentioning

confidence: 99%

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A process-based anatomy of Mediterranean cyclones: From baroclinic lows to tropical-like systems

Flaounas¹,

Gray²,

Teubler³

2020

Preprint

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Abstract. In this study, we address the question of the atmospheric processes that turn Mediterranean cyclones into severe storms. Our approach applies on-line potential vorticity (PV) budget diagnostics and piecewise PV inversion to WRF model simulations of the mature stage of 100 intense Mediterranean cyclones. We quantify the relative contributions of different processes to cyclone development and therefore deliver, for the first time, a comprehensive insight into the variety of cyclonic systems that develop in the Mediterranean from the perspective of cyclone dynamics. In particular, we show that all 100 cyclones are systematically influenced by two main PV anomalies: a major anomaly in the upper troposphere, related to the baroclinic forcing of cyclone development, and a minor anomaly in the lower troposphere, related to diabatic processes and momentum forcing of wind. Among the diabatic processes, latent heat is shown to act as the main PV source (reinforcing cyclones), being partly balanced by PV sinks of temperature diffusion and radiative cooling (weakening cyclones). Momentum forcing is shown to have an ambiguous feedback, able to reinforce and weaken cyclones while in certain cases playing an important role in cyclone development. Piecewise PV inversion shows that most cyclones develop due to the combined effect of both baroclinic and diabatic forcing, i.e. due to both PV anomalies. However, the stronger the baroclinic forcing, the less a cyclone is found to develop due to diabatic processes. Several pairs of exemplary cases are used to illustrate the variety of contributions of atmospheric processes to the development of Mediterranean cyclones: (i) cases where both baroclinic and diabatic processes contribute to cyclone development; (ii) cases that mainly developed due to latent-heat release; (iii) cases developing in the wake of the Alps; and (iv) two unusual cases, one where momentum forcing dominates cyclone development and the other presenting a dual surface pressure centre. Finally, we focus on ten medicane cases (i.e. tropical-like cyclones). In contrast to their tropical counterparts – but in accordance with most intense Mediterranean cyclones – most medicanes are shown to develop under the influence of both baroclinic and diabatic processes. In discussion of medicane driving processes, we highlight the need for a physical definition of these systems.

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Section: Dynamical Structure Of Cyclones: Conserved and Non-conserved Pvmentioning

confidence: 69%

Section: Introductionmentioning

confidence: 99%

A process-based anatomy of Mediterranean cyclones: From baroclinic lows to tropical-like systems

Flaounas¹,

Gray²,

Teubler³

2020

Preprint

View full text Add to dashboard Cite

show abstract

“…2). Unless otherwise noted, PV anomalies are computed relative to an Eady (1949) basic state as proposed by McTaggart-Cowan et al (2006). This inner grid is coded CMCGEMH/S, a 3.58 southward shift of the CMCGEMH grid used for the D-PHASE project designed to improve the GoG cyclone's placement within the domain.…”

Section: Model Data and Diagnostics Methodologymentioning

confidence: 99%

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

McTaggart‐Cowan

Galarneau

Bosart

et al. 2010

Monthly Weather Review

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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.

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“…The model uses the procedure of a time-dependent adjustable orography (Benoit et al 2002;McTaggart-Cowan et al 2010) to improve the initialization of atmospheric fields in complex terrain. This procedure (known as ''growing orography'') consists of adjusting the orographic height over the first hours of integration in order to reduce the interpolation problems associated with abrupt changes of the topography at the beginning of the simulation.…”

Section: Conditionsmentioning

confidence: 99%

Wintertime Subkilometer Numerical Forecasts of Near-Surface Variables in the Canadian Rocky Mountains

Vionnet

Bélair

Girard

et al. 2015

Monthly Weather Review

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Numerical weather prediction (NWP) systems operational at many national centers are nowadays used at the kilometer scale. The next generation of NWP models will provide forecasts at the subkilometer scale. Large impacts are expected in mountainous terrain characterized by highly variable orography. This study investigates the ability of the Canadian NWP system to provide an accurate forecast of near-surface variables at the subkilometer scale in the Canadian Rocky Mountains in wintertime when the region is fully covered by snow. Observations collected at valley and high-altitude stations are used to evaluate forecast accuracy at three different grid spacing (2.5, 1, and 0.25 km) over a period of 15 days. Decreasing grid spacing was found to improve temperature forecasts at high-altitude stations because of better orography representation. In contrast, no improvement is obtained at valley stations due to an inability of the model to fully capture at all resolutions the intensity of valley cold pools forming during nighttime. Errors in relative humidity reveal that the model tends to overestimate relative humidity at all resolutions, without improvement with decreasing grid spacing. Wind speed forecasts show large improvements with decreasing grid spacing for high-altitude stations exposed to or sheltered from wind. However, no systematic improvement with decreasing grid spacing is found for all stations, which is similar to previous studies. In addition, the model's sensitivity at subkilometer grid spacing is investigated by evaluating the effects of (i) accounting for additional drag generated by subgrid orographic features, (ii) considering slope angle and aspect on surface radiation, and (iii) using high-resolution initialization for the surface fields.

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Development and Tropical Transition of an Alpine Lee Cyclone. Part II: Orographic Influence on the Development Pathway

Cited by 18 publications

References 33 publications

A process-based anatomy of Mediterranean cyclones: From baroclinic lows to tropical-like systems

A process-based anatomy of Mediterranean cyclones: From baroclinic lows to tropical-like systems

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

Wintertime Subkilometer Numerical Forecasts of Near-Surface Variables in the Canadian Rocky Mountains

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