Simulations of the potential vorticity structure and budget of fronts 87 IOP8

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AbstractDropsonde observations in the cold fronts sampled during the joint British‐French Fronts 87 Experiment along the west coast of Europe have shown that the dry and moist potential vorticity fields (qd and qe, respectively) have significant anomalies on the mesoscale. the processes associated with these anomalies during a 12 h period encompassing the observation time have been examined by simulating the qd and qe structures observed near the cold front of IOP8 using the Météo‐France mesoscale PERIDOT model, and by developing and using an Eulerian potential vorticity budget module. High‐resolution dropsonde, rawinsonde, and Doppler radar observations are used as verification of the simulated potential vorticity structures. Initial and boundary conditions are obtained from three different analysis systems.The simulated potential vorticity structures are sensitive to the initial conditions because the analyses over the data‐sparse Atlantic Ocean vary significantly from one analysis system to another. of the seven persistent mesoscale potential vorticity features identified in the observations, five were simulated reasonably well using at least one of the initial analyses, and examined with the potential vorticity budget. the low‐level, pre‐frontal, positive qd anomaly along the length of the cold front is maintained through a balance between Lagrangian source/sink terms, dominated by differential diabatic heating, and vertical advection within the main frontal updraught. This balance maintains the qd anomaly in a region of near‐zero qd tendency between areas of qd generation near the surface and qd destruction in the middle troposphere. A mid‐tropospheric region of negative qe directly behind the surface cold front appears to be produced by a weak but persistent generation by the solenoid term in the region of strong humidity gradients along the back edge of the mid‐tropospheric cloud shield. This position, and the model evolution, suggest that the solenoid‐created region of negative qe can lead to enhanced mid‐tropospheric updraughts through conditional symmetric instability or a combination of conditional symmetric instability and large‐scale forcing. the magnitudes of regions of negative qd and qe on the anticyclonic side of the upper‐level jet decrease throughout the simulation due to horizontal momentum diffusion, while a post‐frontal, low‐level, negative qe region is generated principally by temperature and moisture diffusion.