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Johnson, R. H., and B. E. Mapes, 2002: Mesoscale processes and severe convective weather. In Severe Convective Storms, C. A. Doswell III (Ed.), American Meteorological Society, 71-122.


INTRODUCTION

Severe convective weather events - tornadoes, hailstorms, high winds, flash floods - are inherently mesoscale phenomena. While the large-scale flow establishes environmental conditions favorable for severe weather, processes on the mesoscale initiate such storms, affect their evolution, and influence their environment. A rich variety of mesoscale processes are involved in severe weather, ranging from environmental preconditioning to storm initiation to feedback of convection on the environment. In the space available, it is not possible to treat all of these processes in detail. Rather, we will introduce several general classifications of mesoscale processes relating to severe weather and give illustrative examples. Although processes on the mesoscale are often intimately linked with those on smaller and larger scales, we will exclude from discussion those that obviously lie outside the mesoscale domain (e.g., baroclinic waves on the synoptic scale or charge separation in clouds on the microscale).

a. Definition of mesoscale

There are several definitions of "mesoscale," and even "scale," in common currency. Some use fixed geometrical scales (Fujita 1963, 1981; Ogura 1963; Orlanski 19750 while others are based on dynamical considerations (Ooyama 1982; Emmanuel 1986; Doswell 1987).

Ooyama (1982) defines mesoscale flows as those having a horizontal scale between the scale height H of the atmosphere and the Rossby radius of deformation λRNH/f, where N is the Brunt-Vaisala frequency and f the Coriolis parameter.1 By this definition, mesoscale phenomena occur on horizontal scales between ten and several hundred kilometers. This range generally encompasses motions for which both ageostrophic advections and Coriolis effects are important (Emmanuel 1986). In general, we apply such a definition here; however, strict application is difficult since so many mesoscale phenomena are "multiscale." For example, a ~100-km-long gust front can be less than ~1 km across. The triggering of a storm by the collision of gust fronts can actually occur on a ~100-m scale (the microscale). Nevertheless, we will treat this overall process (and others similar to it) as mesoscale since gust fronts are generally regarded as mesoscale phenomena.

b. Scope of paper

The range of mesoscale processes associated with severe weather is enormous. Therefore, to provide focus, we present a division of mesoscale processes according to whether they help to generate severe weather (termed preconditioning and triggering) or arise from the convection itself. Moreover, we will draw a distinction between preconditioning and triggering, much in the same way as Newton (1963) has done. Newton distinguished factors that precondition (destabilize) the environment, for example, an approaching upper-level trough, differential horizontal advection, low-level jets, from those that release the instability, such as rapid lifting by fronts, cold domes from thunderstorms, drylines, and topography (although slow quasigeostrophic lifting was also suggested as a possibility).

A list of common mesoscale preconditioning processes is provided in Table 3.1. In most instances, these mechanisms serve to gradually destabilize the environment and modify the wind shear profile, thereby setting the stage for severe weather. However, if the destabilization occurs rapidly enough, some of these processes may actually trigger convection, thus blurring the distinction between preconditioning and triggering.

The mesoscale processes in Table 3.1 have been subdivided into local, advective, and dynamical. Local preconditioning processes include boundary layer mixing and interactions of the atmosphere with geographically fixed features such as topography or gradients of surface properties. Advective processes involve the physical transport of air masses. Advection acts as an important preconditioning process in the prestorm environment (e.g., differential advection of cold air over warm, or the development and convergence of humid air masses). Mesoscale dynamical processes are harder to observe, as they often involve rapidly evolving motions in clear regions of the atmosphere. Events in one location can affect events in another location through the propagation of gravity waves that may travel faster than the wind at any level. Much of the unsaturated fluid dynamics of the atmosphere for which horizontal advection and rotation processes are secondary can be described as gravity wave processes, not just the rare cases of coherent propagating phenomena with a single, well-defined frequency and wavelength. These processes are important both in prestorm environments and inside severe storms. In the former situation, they can include secondary circulations associated with geostrophic adjustment, upper-level jets, and low-level jets. In addition, the atmosphere can be subject to mesoscale dynamical instabilities that may cause convective preconditioning. In this case, there is no "upstream precursor" for a weather development, as there is for the advective phenomena discusses above. Discussion of individual phenomena listed in Table 3.1 is given in section 3.3.

Specific processes involved in triggering convection are identified in Table 3.2. As mentioned earlier, some triggering and preconditioning processes are the same. For example, some cold fronts trigger convection everywhere along their leading edge, whereas others precondition the atmosphere by providing mesoscale lifting and moisture convergence. However, in general, the lifting required for triggering is much stronger than that for preconditioning, particularly when convective inhibition (CIN) is present. Probably the most common triggering mechanisms are advective in nature: convergence lines (gust fronts, cold fronts, sea/lake breezes, drylines) or boundary intersections (e.g., triple points, colliding gust fronts). Mesoscale dynamical processes are less common, but there are several instances where gravity waves or boundary layer rolls trigger convection. Superpositions of triggering processes are particularly effective at initiating storms (e.g., thermals along fronts, fronts intersecting terrain, rolls intersecting boundaries, etc.). These processes will be discussed in detail in section 3.4.

Once initiated, severe storms generate mesoscale phenomena that impact storm evolution as well as the growth of neighboring storms (Table 3.3). On the local scale, radiation and microphysics are two such processes. Microphysical effects acting on the mesoscale are key in downdraft and cold pool production, generation of microbursts and other high-wind events, generation of midlevel convergence due to melting, and lightning production.

Advance effects include particle advection, fall, and phase changes, which influence downdraft development and upscale growth of convection. Cold pool advective processes lead to cell regeneration and mesoscale convective system (MCS) evolution. Momentum transport and sloping flows are important factors in severe surface winds. Vortex tilting/stretching can generate vertical velocity, leading to supercell development or MCS mesovortices.

Severe storms have a number of important dynamical effects. Convectively generated gravity waves influence storm evolution and the development of neighboring storms. Mesoscale pressure fields produced by buoyancy and dynamic effects (e.g., shear/updraft interactions) impact supercell evolution and propagation. Baroclinic vorticity generation at gust fronts may play a role in tornadogenesis. These processes are discussed in detail in section 3.5.

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1 Ooyama (1982) notes that if the relative rotation and vorticity are increased in an area, f should be replaced by the geometric mean of the absolute vorticity and absolute angular speed.