The mechanism by which the atmospheric boundary layer reduces the intensity of mid‐latitude cyclones is investigated. It is demonstrated that two alternative theories, Ekman pumping and the baroclinic potential vorticity ( PV ) mechanism, in fact act in union to maximize the spin‐down. Ekman pumping aids the ventilation of PV from the boundary layer, and shapes the resulting PV anomaly into one of increased static stability. PV inversion techniques are used to demonstrate how this anomaly reduces the coupling between the upper‐ and lower‐levels within the cyclone, reducing the growth rate.
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