CHAPTER 3 JOHNSON AND MAPES Chapter 3 Mesoscale Processes and Severe Convective Weather RICHARD H. JOHNSON Department of Atmospheric Science. Colorado State University, Fort Collins, Colorado BRIAN E. MAPES CIRESICDC, University of Colorado, Boulder, Colorado REVIEW PANEL: David B. Parsons (Chair), K. Emanuel, J. M. Fritsch, M. Weisman, D.-L. Zhang 3.1. Introduction tion, mesoscale phenomena occur on horizontal scales between ten and several hundred kilometers. This Severe convective weather events-tornadoes, hail­ range generally encompasses motions for which both storms, high winds, flash floods-are inherently mesoscale ageostrophic advections and Coriolis effects are im­ phenomena. While the large-scale flow establishes envi­ portant (Emanuel 1986). In general, we apply such a ronmental conditions favorable for severe weather, pro­ definition here; however, strict application is difficult cesses on the mesoscale initiate such storms, affect their since so many mesoscale phenomena are "multiscale." evolution, and influence their environment. A rich variety For example, a -100-km-Iong gust front can be less of mesocale processes are involved in severe weather, than -1 km across. The triggering of a storm by the ranging from environmental preconditioning to storm initi­ collision of gust fronts can actually occur on a ation to feedback of convection on the environment. In the -lOO-m scale (the microscale). Nevertheless, we will space available, it is not possible to treat all of these treat this overall process (and others similar to it) as processes in detail. Rather, we will introduce s~veral mesoscale since gust fronts are generally regarded as general classifications of mesoscale processes relatmg to mesoscale phenomena. 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 b. Scope of paper discussion those that obviously lie outside the mesoscale domain (e.g., baroclinic waves on the synoptic scale or The range of mesoscale processes associated with charge separation in clouds on the microscale). severe weather is enormous. Therefore, to provide focus, we present a division of mesoscale processes a. Definition of mesoscale according to whether they help to generate severe weather (termed preconditioning and triggering) or There are several definitions of "mesoscale," and arise from the convection itself. Moreover, we will even "scale," in common currency. Some use fixed draw a distinction between preconditioning and trig­ geometrical scales (Fujita 1963, 1981; Ogura 1~63; gering, much in the same way as Newton (1963). ~as Orlan ski 1975) while others are based on dynamIcal done. Newton distinguished factors that precondItIOn considerations (Ooyama 1982; Emanuel 1986; Dos­ (destabilize) the environment, for example, an ap­ well 1987). proaching upper-level trough, differential horizontal Ooyama (1982) defines mesoscale flows as those advection, low-level jets, from those that release the having a horizontal scale between the scale height H instability, such as rapid lifting by fronts, cold domes of the atmosphere and the Rossby radius of deforma­ from thunderstorms, drylines, and topography (al­ tion AR = NHIf, where N is the B~nt-Vha"~salda fifr~­ though slow quasigeostrophic lifting was also sug­ quency and f the Coriolis parameter. By t IS e lll- gested as a possibility). A list of common mesoscale preconditioning pro­ cesses is provided in Table 3.1. In most instances, 1 Ooyama (1982) notes that if the relative rotation and vorticity are these mechanisms serve to gradually destabilize the increased in an area, f should be replaced by the geometric mean of environment and modify the wind shear profile, the absolute vorticity and absolute angular speed. thereby setting the stage for severe weather. However, 71 C. A. Doswell III (ed.), Severe Convective Storms © American Meteorological Society 2001 72 METEOROLOGICAL MONOGRAPHS VOL. 28, No, 50 TABLE 3.1. Mesoscale preconditioning processes for severe weather. Local Advective Dynamical Boundary layer processes Differential advection Secondary circulations • deepening the mixed layer • creation of capping inversion • geostrophic adjustment • deepening the moist layer • destabilization • jets • convergence along dryline • formation of deep, dry PBL (leading to Gravity currents, waves • nocturnal inversion, low-level jet microbursts) • cold pool lifting formation Convergence lines • localized reduction of CIN Terrain effects • fronts • modification of vertical shear • creation of convergence zones • drylines Mesoscale instabilities • development of slope flows • sealland/lake breezes Boundary layer processes • modification of hodograph • mountain/valley breezes • horizontal convective rolls Surface effects Moisture advection • inertial oscillation (low-level • evaporation, heating • increase CAPE, lower LFC jets) • surface, discontinuities • local cumulus moistening -soil moisture -roughness if the destabilization occurs rapidly enough, some of gravity wave processes, not just the rare cases of these processes may actually trigger convection, thus coherent propagating phenomena with a single, well­ blurring the distinction between preconditioning and defined frequency and wavelength. These processes triggering. are important both in pre storm environments and The mesoscale processes in Table 3.1 have been inside severe storms. In the former situation, they can subdivided into local, advective, and dynamical. Lo­ include secondary circulations associated with cal preconditioning processes include boundary layer geostrophic adjustment, upper-level jets, and low­ mixing and interactions of the atmosphere with geo­ level jets. In addition, the atmosphere can be subject graphically fixed features such as topography or gra­ to mesoscale dynamical instabilities that may cause dients of surface properties. Advective processes in­ convective preconditioning. In this case, there is no volve the physical transport of air masses. Advection "upstream precursor" for a weather development, as acts as an important preconditioning process in the there is for the advective phenomena discussed prestorm environment (e.g., differential advection of above. Discussion of individual phenomena listed in cold air over warm, or the development and conver­ Table 3.1 is given in section 3.3. gence of humid air masses). Mesoscale dynamical Specific processes involved in triggering convec­ processes are harder to observe, as they often involve tion are identified in Table 3.2. As mentioned earlier, rapidly evolving motions in clear regions of the some triggering and preconditioning processes are the atmosphere. Events in one location can affect events same. For example, some cold fronts trigger convec­ in another location through the propagation of gravity tion everywhere along their leading edge, whereas waves that may travel faster than the wind at any others precondition the atmosphere by providing me­ level. Much of the unsaturated fluid dynamics of the soscale lifting and moisture convergence. However, atmosphere for which horizontal advection and rota­ in general, the lifting required for triggering is much tion processes are secondary can be described as stronger than that for preconditioning, particularly TABLE 3.2. Mesoscale triggering processes for severe weather. Local Advective Dynamical Boundary layer circulations Convergence lines Gravity currents, waves • thermals • cold fronts Boundary layer horizontal convective rolls Terrain effects • gust fronts • orographic lifting • sea/lake breezes • thermal forcing • dry lines • obstacle effects Boundary intersections Surface effects • triple point • sensible/latent heat flux discontinuities • colliding fronts, sea breezes Combined lifting processes: thermals along fronts fronts intersecting terrain boundary layer rolls intersecting fronts, sea breezes, drylines gravity waves interacting with fronts, dry lines CHAPTER 3 JOHNSON AND MAPES 73 when convective inhibition (CIN) is present. Probably 3.2. Instability of the atmosphere to mesoscale the most common triggering mechanisms are advec­ convection: General considerations tive in nature: convergence lines (gust fronts, cold a. Elementary deep convective instability fronts, seallake breezes, drylines) or boundary inter­ (buoyancy only) sections (e.g., triple points, colliding gust fronts). Mesoscale dynamical processes are less common, but Severe convective weather owes its existence to the there are some instances where gravity waves or buoyant ascent of cloudy air whose source is in the boundary layer rolls trigger convection. Superposi­ lowest layers of the troposphere. This convective as­ tions of triggering processes are particularly effective cent occurs in regions with a temperature and humidity at initiating storms (e.g., thermals along fronts, fronts stratification that is typically stable toward small­ intersecting terrain, rolls intersecting boundaries, amplitude vertical displacements, but unstable toward etc.). These processes will be discussed in detail in large-amplitude upward displacements of low-level section 3.4. air. Here we review some definitions, measures, and Once initiated, severe storms generate mesoscale indices of deep convective instability, as tools for phenomena that impact storm evolution as well as the considering the effects of mesoscale processes on growth of neighboring storms (Table 3.3). On the local storm potential and organization. scale,