Convective inhibition beneath an upper-level PV anomaly

Upper-level potential-vorticity (PV) anomalies reduce the convective stability of the troposphere through their impact on the vertical potential-temperature profile, thus reducing convective inhibition (CIN) and increasing convective available potential energy. Here, by contrast, we show the impact of a layer of stable air that was intrinsically linked with an upper-level PV anomaly and that increased CIN. This layer descended and tracked beneath the small upper-level PV anomaly, which in this case was a shallow upper-level trough. This low-humidity, relatively high-PV layer originated from the tropopause fold, generated by a breaking Rossby wave, which also produced the upper-level PV anomaly two days later. Despite conditions favourable for deep convection (as demonstrated by the development of a single storm), the CIN produced by this dry layer or lid was largely responsible for capping convection over much of southern England at around 2.5 km during the case presented here, which comes from the Convective Storm Initiation Project. Copyright (c) 2008 Royal Meteorological Society.