The essential ingredients leading to the explosive growth stage of the European wind storm Lothar of Christmas 1999
Rivière, G. ; Arbogast, P. ; Maynard, K. ; Joly, A.
Key dynamical ingredients leading to the explosive growth stage of the extratropical wind storm Lothar (2426 December 1999) are identified by performing numerical sensitivity experiments using the MZetZeo-France operational model. This stage suddenly occurred when the surface cyclone crossed the upper-level jet in its exit region. The model is shown to capture quite well the whole process by starting the forecast 12 h before the start of the explosive growth stage. A first set of experiments consists of revisiting the role of humidity and other physical processes. A run suppressing latent heating does not exhibit any cyclone growth, similar to a previous study by other authors. However, a frictionless dry adiabatic run, i.e. when latent heating processes and dissipation terms are both suppressed, is shown to reproduce the timing and the location of the rapid intensification stage but not its intensity (which is overestimated). Moist processes are thus crucial because they compensate for the dissipation terms, but the vertical coupling between the surface cyclone and the upper-level jet can be reproduced and interpreted in terms of frictionless dry adiabatic interactions, at least for conceptual purposes. All the other numerical experiments have the full physics of the model and only differ in their initial conditions. The modification of the flow is performed using a recently developed potential vorticity inversion method.Upper-level high-frequency anomalies are shown to have only a moderate impact on the rapid deepening of the surface cyclone. Other experiments are conducted to look at the sensitivity to the strength of the upper-level jet, to the intensity of the low-level baroclinicity, to the location of the jet exit and finally to the shape, amplitude and location of the low-level cyclone. The location of the explosive growth stage depends primarily on the position of the low-frequency jet exit while its intensity and even its existence is strongly sensitive to the low-level background baroclinicity and to the properties of the incipient surface cyclone itself. Copyright c 2010 RoyalMeteorological ciety
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