Based on the analysis of idealized two- and three-dimensional cloud model simulations, Rotunno et al. (hereafter RKW) and Weisman et al. (hereafter WKR) put forth a theory that squall-line strength and longevity was most sensitive to the strength of the component of low-level (0‐3 km AGL) ambient vertical wind shear perpendicular to squall-line orientation. An ‘‘optimal’’ state was proposed by RKW, based on the relative strength of the circulation associated with the storm-generated cold pool and the circulation associated with the ambient shear, whereby the deepest leading edge lifting and most effective convective retriggering occurred when these circulations were in near balance. Since this work, subsequent studies have brought into question the basic validity of the proposed optimal state, based on concerns as to the appropriate distribution of shear relative to the cold pool for optimal lifting, as well as the relevance of such concepts to fully complex squall lines, especially considering the potential role of deeper-layer shears in promoting system strength and longevity. In the following, the basic interpretations of the RKW theory are reconfirmed and clarified through both the analysis of a simplified two-dimensional vorticity‐streamfunction model that allows for a more direct interpretation of the role of the shear in controlling the circulation around the cold pool, and through an analysis of an extensive set of 3D squall-line simulations, run at higher resolution and covering a larger range of environmental shear conditions than presented by WKR.

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“A Theory for Strong Long-Lived Squall Lines” Revisited

A dynamical basis is established for understanding the structure and transport properties of organized convection and for expediting its parametrization in large-scale models. A two-dimensional model provides an exact theory of the momentum transport by mesoscale convective systems and similar phenomena. the system-scale dynamics are supposed to dominate local processes and are modelled by a stationary triple-branch airflow regime consisting of a double-branch updraught and a downdraught. It is argued that a jump updraught is essential and has a fundamental effect on the momentum transport. the triple-branch model degenerates into a form of conservative density current; another limiting case consists of a propagating positive pressure jump of maximum amplitude-an overturning updraught but no downdraught. the functional relationship among dynamical parameters is determined by a characteristic regime equation derived from elementary nonlinear Lagrangian conservation properties and the volume integral of the horizontal-momentum equation.

The inflow to the archetypal jump updraught is constant. an example shows that the inclusion of shear in this region alters the detailed shape of the momentum flux profile but its fundamental character, namely the negative values of momentum flux for a system travelling in the positive x-direction, is retained. This result, together with recent numerical simulations, implies that low-level shear directly influences the initiation and evolution of convective elements, whereas the mature-state fluxes for which the system-scale flow organization and tilt is paramount is a product of the distribution of heat sources/sinks and deep tropospheric shear. the universal nature of the momentum flux profiles is explained in elementary terms by appealing to dynamical theory.

The physical basis of the model and the momentum flux profiles are validated by using published results. the archetype emulates the basic character of the mass and momentum fluxes by mesoscale convective systems. For example an upper-tropospheric flow deceleration is consistent with the observed effect of tropical cloud clusters on the mean flow but is distinct from the balanced response due to diabatic heating.

The theory is used to develop a dynamical approach to the parametrization of organized convective processes that have hitherto been neglected in global models. Mass and momentum fluxes are obtained from the archetypal model in an approach that is fundamentally different from the statistical or averaging approximations that characterize present techniques. the activation of the parametrization scheme is also studied. Mass-flux criteria are used to define an amplitude function for the mesoscale flux divergence to incorporate the flux laws into the large-scale equations. the work can be extended to include thermodynamic fluxes by using generalized conservation properties.

Organized convective systems : archetypal dynamical models, mass and momentum flux theory, and parametrization

Observed upper air soundings that occurred within 2 h and 167 km of derechos were collected and analyzed to document atmospheric stability and wind shear conditions associated with long-lived convective windstorms. Sixty-seven derechos, accompanied by 113 proximity soundings, were identified during the years 1983–93. Owing to the large variability of the synoptic-scale environments associated with derechos, each derecho was further divided into categories based on the strength of synoptic-scale forcing associated with each event. Derechos are shown to develop and persist in a wide range of shear and instability conditions. Although this range of shear and instability narrows when derechos are grouped by synoptic-scale forcing strength, considerable variation of values remains, primarily with the shear. These results suggest that ambient shear and instability values alone are not sufficient to distinguish derecho environments from those associated with nonsevere mesoscale convective system (MCS) e...

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Examination of Derecho Environments Using Proximity Soundings

Analyses of the African easterly jet (AEJ) are presented which are based on meridional transects of high-resolution dropsonde observations made during JET2000, an aircraft campaign conducted in the last week of August 2000. The observations have confirmed that the AEJ is closely defined by geostrophic balance. The baroclinicity between the extreme northern and southern profiles accurately determines the altitude of the jet core, while the location and morphology of the jet core correspond to a locally-defined geostrophic wind measure. The potential-vorticity (PV) structure has also been found to accord with theoretical expectations, with distinctive positive- and negative-PV anomalies equatorward and poleward of the jet core respectively.

The thermodynamic structure of the AEJ environment can be categorized into coherent layers. The monsoon layer is a humid zone connected to the land surface, extending northwards into the Sahel and increasing in depth towards the south. This layer is affected by the land surface on diurnal time-scales, through the growing convective mixed layer and through shallow cumulus clouds. Above the monsoon layer is the Saharan air layer (SAL), which can be identified as a layer of low static-stability and low PV. The SAL is deep where it merges with the Saharan boundary-layer in the north, and becomes thinner toward the south. It has been shown that the boundaries of the SAL can be approximated to good accuracy as adiabatic surfaces, meaning that the SAL comprises air which is adiabatically connected to the land surface via the Saharan boundary-layer. The upper region of the SAL is identified as a layer of high relative-humidity where altocumulus and stratocumulus layers are observed. Finally, the troposphere above the SAL is again almost pseudoadiabatic, with small baroclinicity which determines the closure of the AEJ core aloft. Through inspection of thermodynamic tracers, evidence of convective and lateral transport and exchange between these layers is also presented. Copyright © 2005 Royal Meteorological Society

Analysis of the African easterly jet, using aircraft observations from the JET2000 experiment

Recent severe weather events have prompted the European scientific community to assess the current understanding of convective processes with a view to more detailed and accurate forecasting. The initial development of convective cells remains one of the least understood aspects and one in which limited research has taken place. The important processes can be split into three main areas: boundary-layer forcing, upper-level forcing and secondary generation. This paper is a review of the mechanisms responsible for the initiation of precipitating convection in the United Kingdom; i.e. why convective clouds form and develop into precipitating clouds in a particular location.

The topography of the United Kingdom has a large influence on the initiation of convection. Boundary-layer forcings determine the specific location where convection is triggered within larger regions of potential instability. These latter regions are created by mesoscale or synoptic-scale features at a higher level such as dry intrusions and mesoscale vortices. Second-generation cells are those formed by the interaction of outflow from convective clouds with the surrounding environmental air. Large, long-lived thunderstorm complexes can develop when new cells are repeatedly triggered on one side of the system. Current and future field campaigns along with the development of high-resolution modelling will enable these processes to be investigated in more detail than has previously been achieved. © Royal Meteorological Society, 2006. Contributions by P. A. Clark and M. E. B. Gray are Crown Copyright.

A review of the initiation of precipitating convection in the United Kingdom

Squall lines stand out from ordinary cumulonimbus convection because of a special structure that dramatically extends their duration, precipitation output and area of influence. Time-dependent models have identified surface outflow boundaries and low-level environmental wind shear as the crucial elements in squall-line formation and/or maintenance. Steady-state models have shown how properties of the downdraught reservoir, or ‘cold pool’, must be matched to the far-field wind and thermodynamic profiles to sustain the convection. However, the dynamical role of the environmental shear and the nature of the interaction between the positively and negatively buoyant air have remained uncertain. For help with these issues, a time-dependent numerical model is developed with all moist processes either parametrized or based on ad hoc assumptions. the simplifications make it possible to distinguish more clearly than in the past between the formation and maintenance of squall lines and between the initial and disturbed far-field environments.

The convective updraught forms within an expanding region of disturbed flow separated from the undisturbed environment by ‘storm fronts’. Forced (or neutral) ascent occurs at a surface ‘gust front’ on the upwind side of the cold pool. We distinguish squall lines from ordinary convection by looking at the coherence between the forced and convective updraughts. It is found that subsidence over the cold pool disorganizes the updraughts, whereas a deep overturning circulation downshear from the gust front has an organizing effect. Hence, factors which allow the initial subsidence to occur ahead of the gust front, namely contrasting lower-and upper-level winds and large potential buoyancy, favour squall-line development. It is argued that, in cases with large convective potential energy, the deep mesoscale circulation is more important in the formation and maintenance of the line than either the vertical shear of the air reaching the cold pool or the strength of the forced updraught.

S. T. Garner

Papers

The Development of Organized Convection In A Simplified Squall-Line Model