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Jean-Pierre Cammas and Daniel Ramond

Abstract

Diagnostic case studies of ageostrophic circulations in upper-tropospheric jet-front systems are presented, using the numerical analyses of the ECMWF model as data sources. The impact of the ECMWF analysis procedure on the retrieved ageostrophic fields is first discussed. Upper-level fields of the ageostrophic wind and its divergence are computed in the natural coordinates system in order to analyze the composition of the transverse (cross-stream) and the alongstream ageostrophic components that are induced by alongstream wind variations and curvature effects respectively. The analysis shows that the curvature effects can contribute in a predominant way to the upper-tropospheric divergence field in entrance or exit regions of jet streaks, so as to sometimes inhibit the expected transverse circulations. Several transverse indirect circulations are displayed in the exit regions of jet streaks. A case of lateral shift towards the anticyclonic side of the jet axis of an indirect circulation is discussed with respect to the conceptual approach based on the frontogenetic forcing in highly idealized two-dimensional flow configurations.

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Francois Lalaurette, Claude Fischer, and Jean-Pierre Cammas

Abstract

Both upper-air and surface frontogenesis have often been depicted as processes whose dynamics could he reduced to 2D balanced problems in which “self-sharpening” configurations could be highlighted.

This paper reports on a 3D adiabatic simulation of a baroclinic wave life cycle. Great care has been devoted to the vertical resolution, allowing for a good description of both surface and upper-air frontogenesis. The authors introduce a kinematic diagnostic (Q′ vector) that permits the identification of frontogenetic areas in such complex 3D flows where classical, low-Rossby number balance conditions can be violated. Relations and specificity with respect to frontogenetic forcing diagnostics are discussed. First, Q′ is used for surface frontogenesis, where it describes well the actual frontal activity, including the complex warm-frontal seclusion process. Upper-air frontogenesis is also investigated, both in terms of this kinematic diagnostic or in terms of potential vorticity displacements on isentropic surfaces. Both types of diagnostics clearly distinguish between dynamics of the entrance zone of the northerly jet—where 2D concepts may usefully be applied—and those of the strongly curved zone near the trough axis. Classical cyclogenetic terms (stretching and tilting) as well as the separation of ageostrophic circulations in terms of natural components of the wind also lead to a clear dynamical separation.

The cold front is shown to extend from the surface far into the troposphere. This is shown to be related to a singular property of the 3D flow. Parcels undergoing frontogenegis in the northwesterly upper-air flow are advected on top of those that were forced at the surface cold front in a southwesterly flow. The occurrence of a feedback proem between these upper-air frontogenesis processes and the surface ones is then investigated. Stepwise vertical profiles of horizontal diffusion are used to force local frontolysis. The resulting upper-air frontolysis, despite its local efficiency, does not have any remote effect on the surface front, whose frontolysis in turn has no effect on the upper-air front. The feedback process is thus not occurring in our simulation.

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Jean-Luc Sortais, Jean-Pierre Cammas, Xiao Ding Yu, Evelyne Richard, and Robert Rosset

Abstract

An example of coupling between an upper-level and a low-level jet–front system is analyzed using the mesoscale hydrostatic model SALSA. The case study chosen is the cold front sampled during the intensive observation period 2 of the Mesoscale Frontal Dynamic Project FRONTS 87 experiment (11 November 1987). Two prominent features of the cold front are a well-developed undulation and a frontal band ahead of the undulation.

Dynamic and diabatic processes of the coupling are investigated through the results of two numerical simulations: full physics and adiabatic. In particular, the roles of ageostrophic circulations are investigated through the decomposition of the ageostrophic wind into its isentropic components: the advective inertial and the diabatic inertial components, and the isallobaric component.

In both simulations it is shown that the low-level branch of the indirect and transverse ageostrophic circulation, associated with the exit region of the upper-level jet streak, is the origin of the cold-front undulation beneath the upper-level jet axis. Other effects of the coupling processes are diagnosed in the full-physics simulation: the intensification of the low-level jet, the formation of the frontal band ahead of the cold front on the left side of the low-level jet, the intensification of the convective system close to the ascending branch of the transverse indirect circulation, and the kinetic energy generation in the exit region of the upper-level jet streak.

An examination of the isentropic components of the ageostrophic wind shows that the upper-level branch of the indirect circulation is the inertial advective component of the ageostrophic wind, whereas the low-level branch is mainly isallobaric. Being caught up in its eastward motion by the faster propagating low-level jet, the low-level branch of the indirect circulation reinforces, by its 7 m s−1 ageostrophic component, the advection of heat and moisture toward the convective system close to the ascending branch of the indirect circulation. In that region, the diabatic inertial component of the ageostrophic wind grows at a rate smaller than that needed for mass adjustment between the thermal and wind fields, thereby leading to kinetic energy generation through the inertial rotation mechanism downstream and on the cyclonic side of the upper-level jet streak.

Dynamical and diabatic processes are shown to contribute to the blocking-in phase and to the development of synergistic interactions between the upper- and the low-level jet-front systems. The results of this study are summarized in a conceptual scheme of the coupling mechanism, including an explanation for the formation of the frontal band.

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Jean-Luc Baray, Yves Pointin, Joël Van Baelen, Marie Lothon, Bernard Campistron, Jean-Pierre Cammas, Olivier Masson, Aurélie Colomb, Claude Hervier, Yannick Bezombes, Sandra Banson, Christophe Duroure, Dany Hadad, and Frédéric Tridon

Abstract

The authors present a climatological analysis of tropospheric horizontal wind profiles and jet stream events using long series of wind profiles from two VHF profilers located in France: Lannemezan (2001–14) and Opme (1999–2014). A case study of jet stream and stratospheric intrusion of air into the troposphere that occurred in January 2013 is first described and demonstrates the capability of the VHF profilers to detect jet stream events. The climatology study over the two sites reveals the strongest values of seasonal wind during winter (21.4 m s−1 at 8.7-km height at Opme; 25.1 m s−1 at 9.6-km height at Lannemezan). A methodology based on the automatic detection of maximum winds on a decadal series of hourly wind profiles allows the detection of jet stream events and establishes its climatology for each site. A frequency analysis of jet stream events of westerly winds over 50 m s−1 presents a clear seasonality at the two sites, with a maximum in winter (3.5%–9.7% of hourly profiles) and a minimum in summer (near 1%). Cosmogenic radionuclides sampled at Opme also exhibit a clear seasonal variation with maximum in spring and minimum in the cold seasons; the 7Be/22Na activity ratio confirms stratosphere-to-troposphere exchanges for the studied cases. The mean interannual variability of the frequency of jet stream events is 1.5% in Opme and 2.9% in Lannemezan. Positive decadal trends are observed for the two sites: +1.6 ± 1.2% decade−1 for Opme and +2.4 ± 2.2% decade−1 for Lannemezan.

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Alain Joly, Dave Jorgensen, Melvyn A. Shapiro, Alan Thorpe, Pierre Bessemoulin, Keith A. Browning, Jean-Pierre Cammas, Jean-Pierre Chalon, Sidney A. Clough, Kerry A. Emanuel, Laurence Eymard, Robert Gall, Peter H. Hildebrand, Rolf H. Langland, Yvon Lemaître, Peter Lynch, James A. Moore, P. Ola G. Persson, Chris Snyder, and Roger M. Wakimoto

The Fronts and Atlantic Storm-Track Experiment (FASTEX) will address the life cycle of cyclones evolving over the North Atlantic Ocean in January and February 1997. The objectives of FASTEX are to improve the forecasts of end-of-storm-track cyclogenesis (primarily in the eastern Atlantic but with applicability to the Pacific) in the range 24 to 72 h, to enable the testing of theoretical ideas on cyclone formation and development, and to document the vertical and the mesoscale structure of cloud systems in mature cyclones and their relation to the dynamics. The observing system includes ships that will remain in the vicinity of the main baroclinic zone in the central Atlantic Ocean, jet aircraft that will fly and drop sondes off the east coast of North America or over the central Atlantic Ocean, turboprop aircraft that will survey mature cyclones off Ireland with dropsondes, and airborne Doppler radars, including ASTRAIA/ELDORA. Radiosounding frequency around the North Atlantic basin will be increased, as well as the number of drifting buoys. These facilities will be activated during multiple-day intensive observing periods in order to observe the same meteorological systems at several stages of their life cycle. A central archive will be developed in quasi-real time in Toulouse, France, thus allowing data to be made widely available to the scientific community.

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