The dynamical consequences for tropopause folding of PV anomalies induced by surface frontal collapse

226

220

SUMMARYDynamical aspects of the life cycle of the winter storm 'Lothar' (24-26 December 1999) are investigated with the aid of the European Centre for Medium-Range Weather Forecasts analysis data and mesoscale model simulations. Neither of these datasets capture the full amplitude of the observed extreme pressure fall and surface wind speeds, but they do help identify a range of key dynamical and physical features that characterize the development of this unusual event. The analysis and interpretation is primarily based upon the evolution of the lower-and upper-level potential vorticity (PV) eld complemented by three-dimensional trajectory calculations.'Lothar' originated in the western Atlantic and travelled as a shallow low-level cyclone of moderate intensity towards Europe. This translation took place below and slightly to the south of a very intense upper-level jet and was accompanied by continuous and intense condensational heating that sustained a pronounced positive low-level PV anomaly (not unlike the concept of a 'diabatic Rossby wave'). No signi cant PV anomalies were evident at the tropopause level during this early phase of the life cycle. The surface cyclone intensi ed rapidly when the shallow cyclone approached the jet-stream axis. The circulation induced by the diabatically produced low-tropospheric PV anomaly on steeply sloping isentropic surfaces that transect the intense upper-level jet contributed signi cantly to the rapid formation of a narrow and deep tropopause fold. This stratospheric PV anomaly virtually merged with the diabatically produced ephemeral PV feature to form a vertically aligned tower of positive PV at the time of maximum storm intensity. A sensitivity study with a dry adiabatic hindcast simulation shows no PV-tower con guration (and only a very weak surface development) and con rms the primary importance of the cloud diabatic heating for the tropopause fold formation and the rapid 'bottom-up' intensi cation of 'Lothar'.A comparison of the anomalously high Atlantic sea surface temperatures in December 1999 with the watervapour source regions for the latent-heat release that accompanied the rapid intensi cation phase of 'Lothar' shows a close relationship. This is of importance when discussing the possible implications of climate variability and change on the development of North Atlantic winter storms.

Section: (A) Strong Upper-level Jetmentioning

confidence: 99%

Dynamical aspects of the life cycle of the winter storm ‘Lothar’ (24–26 December 1999)

Wernli

Dirren

Liniger

et al. 2002

226

220

“…For small Froude number, these additional terms are small compared with the rst term of the PV de nition (Eq. (11)), as they scale like the rst term multiplied by F 2 (Ziemiański 2000).…”

Section: Balanced Modelsmentioning

confidence: 99%

“…Instead, they use the standard NLB omega equation. Their model gives good accuracy for the diagnosis and prognosis of atmospheric ows, including also their divergent component (Davis and Emanuel 1991;Ziemiański 2000;Ziemiański and Thorpe 2000;Ziemiański and Thorpe 2002). However, the model is not fully consistent, formally, as the inclusion of the standard NLB omega equation is equivalent to the additional inclusion of the truncated vorticity equation into the model.…”

Section: Introductionmentioning

confidence: 99%

“…The equation is applied in an implicit way, so that the temperature tendency is approximated via a single time step of the model, performed with every iteration. It can be shown, however, that while the method is mathematically consistent and numerically stable for mesoscale weather systems, it becomes unstable if applied to systems of a synoptic scale in an atmosphere involving a realistic step change of PV across the tropopause (Ziemiański 2000).…”

Section: Introductionmentioning

confidence: 99%

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Nonlinear balanced models for stratified fluids conserving Ertel–Rossby PV

Ziemiański

Thorpe

2003

SUMMARYA derivation of consistent balanced models for strati ed uids conserving the Ertel-Rossby potential vorticity (PV) is presented. It is used to develop a set of balanced models incorporating new omega equations valid for ow regimes from synoptic scale to mesoscale. The models are developed for quasi-Boussinesq and real atmospheres, and for adiabatic and frictionless as well as non-conservative processes. The new omega equations allow the identi cation of additional forcing of the vertical velocity due to the ageostrophic and stability effects, and the modi cation of atmospheric response due to the non-Boussinesq effects. It is shown that the new omega equation, valid beyond the limit of small Rossby number, allows for a subjective analysis of the vertical-velocity forcing based on an approach involving the isentropic upgliding, Ertel-Rossby PV advection and boundary temperature advection.

“…At this point, it is of interest to mention a recent study by Ziemiański and Thorpe (2000), who extended the conceptual model of Hoskins et al (1985) by deriving the attribution of vertical motion to various PV anomalies within the nonlinear balancedequation framework. They showed that the vertical motion induced by the low-level PV anomalies could contribute to the process of tropopause folding, leading to a strong coupling of the low-and upper-level ows.…”

Section: Adiabatic Dynam Icsmentioning

confidence: 99%

The impact of various potential‐vorticity anomalies on multiple frontal cyclogenesis events

Zhang

Cheng

Gyakum

2002

SUMMARYThe in uences of various potential vorticity (PV) anomalies on multiple frontal cyclogenesis events are examined using 60 h simulations of a family of six frontal cyclones that developed over the western Atlantic between 0000 UTC 13 March and 1200 UTC 15 March 1992. The cyclogenesis events are characterized by pronounced low-level baroclinicity along a large-scale cold front moving over the warm ocean and by multiple propagating perturbations in the two PV rings on the cyclonic-shear side of an upper-level jet stream. It is found that the tracks of the frontal cyclones follow closely the distribution of the PV rings, whereas the cyclogenesis occurs in the cold-frontal zone, and in close proximity to the propagating PV perturbations in the central portion of an upper-level parent trough. A piecewise PV inversion reveals that the low-level thermal advection (or the bottom thermal anomaly) contributes the most to the depths of two major frontal cyclones, followed by latentheat release and the upper-level PV anomalies, whereas the opposite order is true for the remaining four weaker frontal cyclones.A series of sensitivity experiments is performed to test the impact of removing the upper-level PV anomalies individually, or collectively, on the development of a major frontal cyclone (MFC) as an initial-value problem. It is found that the MFC genesis and its nal intensity depend on how much its track departs from the upper-level PV perturbations. Removing a PV anomaly associated with a major short-wave trough delays the MFC genesis by 18 h and weakens its nal depth by 13 hPa. The largest departure occurs when two upstream PV anomalies are removed collectively, delaying the MFC genesis by 36 h and weakening its nal depth by 21 hPa. It appears that the MFC genesis in the rst 36 h depends more on the PV anomalies in an inner PV ring, whereas its subsequent development depends more on the PV reservoir in an outer PV ring.It is shown that most of the frontal cyclones can still develop in the absence of diabatic heating, indicating the important role of dry dynamics in controlling the multiple frontal cyclogenesis events. However, the 'dry' MFC fails to form a closed isobar when the trough-related PV anomaly is removed. The associated PV inversion and sensitivity simulation results suggest that even though the low-level thermal advection plays an important role in deepening the frontal cyclones, it is the upper-level PV anomalies that provide the necessary forcing for its ampli cation and the frontal cyclogenesis.It is concluded that the upper-level propagating multiple PV anomalies play a major role in triggering, but a secondary role in determining the nal depth of the multiple frontal cyclones. It is the subsequent growth of the bottom thermal anomaly that accounts for most of the nal depth of the frontal cyclones.