Conditions for the occurrence of severe local storms

Abstract

The technique of relative-flow analysis on isentropic surfaces is used to examine the large-("synoptic"-) scale situations associated with selected severe local storms near southern England and over the mid-western U.S.A. (including the Horsham, Wokingham, and Geary storms whose behaviour has been described in several previous publications). The storms occur ahead of major troughs, in the vicinity of confluence-lines (usually recognised a8 cold fronts over western Europe but as "dry-lines'' over the U.S.A.), where an increase of wind with height favours the organisation and intensification of cumulonimbus convection. Extreme instability arises where small-scale convection is confined to a lowermost 1 or 2 km (leading to an abnormally high wetbulb potential temperature) beneath a plume of very warm air lying downwind of an extensive arid plateau (Spain or Mexico). The instability is released where the (backed) low-level flow eventually reaches the edge of the restraining plume aloft. It appears that the occurrence of severe local storms demands a peculiarly favourable combination of geographical features and atmospheric flow-pattern.

Central European tornado environments as viewed from a potential vorticity and Lagrangian perspective

“…Moreover, the elevated terrain with low soil moisture content westward of the plains supports strong heating of the air during summertime and the development of a mixed layer. This warmed air loses contact to the ground (elevated mixed layer) when it travels downstream over lower terrain and produces a strong capping inversion (Carlson and Ludlam, 1968). Finally, foehn effects might additionally induce dry mid-tropospheric conditions.…”

“…Finally, foehn effects might additionally induce dry mid-tropospheric conditions. In summary, the specific air flow conditions favour a vertical moist/dry contrast which is conducive for severe convection (Fawbush and Miller, 1954;Carlson and Ludlam, 1968;Palmén and Newton, 1969;Carlson et al, 1983).…”

Analysis of the pathways relating soil moisture and subsequent rainfall in Illinois

“…The energy balance equation is given by gne t -G = H + 2E. The turbulent sensible and latent heat fluxes both increase the moist static energy (MSE) of the boundary layer [Betts et al, 1996;Carlson and Ludlam, 1968] and mix the lower levels of troposphere, so quantities like mixing ratio and potential temperature are nearly constant throughout this mixed layer. MSE is very closely related to the moist entropy and the equivalent potential temperature 0E [Emanuel, 1994], and the terms moist static energy and boundary layer entropy (BLE) are used almost interchangeably.…”

Large‐scale potential vorticity anomalies and deep convection

“…As such, by studying cases associated with PV anomalies as we have done with CSIP and COPS, the accompanying dry layer will be of the upper-level type instead of a dry layer originating via differential advection, as suggested by Carlson and Ludlam (1968). Equally, though, there are currently no statistics on the origin of the capping inversion involved in Western European storms so, at this stage, we can do no more than highlight the gap in our knowledge and plan research for the future to fill that gap.…”

“…This makes our investigation particularly important, as there are very few cases in the literature that focus on the inhibition of convective storms. Indeed, the Carlson and Ludlam (1968) model, which describes the development of severe local storms in western Europe, shows the necessary capping inversion originating via differential advection involving a southerly airstream from lower levels over arid regions. This model is in contrast with our findings here, as well as those from Russell et al (2008Russell et al ( , 2009.…”