A potential vorticity perspective on the motion of a mid-latitude winter storm
Rivière, Gwendal ; Arbogast, Philippe ; Lapeyre, G. ; Maynard, Karine
The motion of mid-latitude surface cyclones relative to the jet streams is of particular interest because of the commonly observed strong and rapid deepening they undergo when they cross the upper-level jet axis. The purpose of the present study is to validate a recent theory that may explain this motion which is a generalization of the so-called beta drift in the mid-latitude baroclinic context. According to this theory, the key parameter controlling the movement of a surface cyclone across the mean tropospheric jet is the vertically averaged potential vorticity (PV) gradient associated with the jet. To test this theoretical result, numerical sensitivity experiments are performed using the Météo-France global operational forecast model ARPEGE-IFS for the particular case of the storm Xynthia (2628 February 2010). The control forecast, starting from the operational analysis almost 2 days before the storm hit France, represents the trajectory of the storm quite well, together with the deepening during the crossing of the upper-level jet axis. A PV-inversion tool is used to modify the vertically averaged PV gradient of the initial state. As expected from the theory, when the PV gradient is intensified, there is a quicker displacement of the surface cyclone toward the jet axis and the jet-crossing phase occurs earlier than in the control forecast. The opposite occurs for a reduced PV gradient. The interpretation is that an enhanced PV gradient reinforces the dipolar PV anomaly located at upper levels which, in turn, advects the surface cyclone faster towards the jet axis.
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