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- Title: Multicell_Storms
- Type: Notes
- School: Oklahoma
- Course: METR 4433
- Term: Fall
Storms Multicell METR 4433: Mesoscale Meteorology Spring 2006 Semester Adapted from Materials by Drs. Kelvin Droegemeier, Frank Gallagher III and Ming Xue School of Meteorology University of Oklahoma Multicellular Thunderstorms So far we have discussed the structure of air mass or single cell thunderstorms. s We can think of these types of storms as a single cell where each cell is: s Independent Has a complete life cycle Has a life cycle of 30 minutes to an hour Is usually weak Multicellular Thunderstorms s We know that many thunderstorms can persist of longer periods of time. These storms are made up of many cells. Each individual cell goes through a life cycle but the group persists. These storms are called multicellular thunderstorms, or simply multicells Multicellular storms consist of a series of evolving cells with each one, in turn, becoming the dominant cell in the group. Multicell Storms s Multicell cluster storm - A group of cells moving as a single unit, often with each cell in a different stage of the thunderstorm life cycle. Multicell storms can produce moderate size hail, flash floods and weak tornadoes. Multicell Line (squall line) Storms - consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. Also known as squall lines, these storms can produce small to moderate size hail, occasional flash floods and weak tornadoes. s Multicell Storm Weather s Multicell severe weather can be of any variety, and generally these storms are more potent than single cell storms, but considerably less so than supercells, because closely spaced updrafts compete for low-level moisture. Organized multicell storms have higher severe weather potential, although unorganized multicells can produce pulse storm-like bursts of severe events. s Multicellular Thunderstorms With air mass storms, the outflow boundaries are usually too weak to trigger additional convection. In multicell storms, the outflow boundary does trigger new convection. Cell #1 Mature Multicellular Thunderstorms With air mass storms, the outflow boundaries are usually too weak to trigger additional convection. In multicell storms, the outflow boundary does trigger new convection. Cell #1 Mature Cell #2 Cumulus Multicellular Thunderstorms After about 20 minutes or so, the second cell becomes the dominant cell. Cell #1 is now dissipating, and a new cell (#3) is starting. Cell #1 Dissipating Cell #2 Mature Cell #3 Cumulus Multicellular Thunderstorms After about 20 minutes or so, the third cell becomes the dominant cell. This process may continue as long as atmospheric conditions are favorable for new convection. Cell #2 Dissipating Cell #3 Mature Cell #4 Cumulus Cell #1 Almost Gone Multicellular Thunderstorms s s s s A cluster of short lived single cells. Cold outflow from each cell combines to form a much larger and stronger gust front. Convergence along the gust front tends to trigger new updraft development. This is the strongest in the direction of storm motion. New cell growth often appear disorganized to the naked eye. s This illustration portrays a portion of the life cycle of a multicell storm. As cell 1 dissipates at time = 0, cell 2 matures and becomes briefly dominant. Cell 2 drops its heaviest precipitation about 10 minutes later as cell 3 strengthens, and so on. Life Cycle of Multicell Storms n-2 n n-1 n+1 s A closer view at T = 20 minutes (from in the earlier slide) shows that cell 3 still has the highest top, but precipitation is undercutting the updraft in the lower levels. New echo development is occurring aloft in cells 4 and 5 in the flanking line, with only light rain falling from the dissipating cells 1 and 2 on the northeast side of the storm cluster. The inset shows what the low-level PPI (plane-position indicator) radar presentation might look like. This storm appears to be unicellular but the several distinct echo tops tell us otherwise. s A Real Example of Multicell storm s Here is a real storm, with radar superimposed. Observe the physical similarities to the previous slide. This Texas Panhandle storm was non-severe. Looking northnortheast from about 20 miles. Note that the updraft numbering is reversed. Multicell Storm on Radar s Radar often reflects the multicell nature of these storms, as seen with the central echo mass and its three yellowish cores in the lower portion of this picture. Occasionally, a multicell storm will appear unicellular in a low-level radar scan, but will display several distinct tops when a tilt sequence is used to view the storm in its upper portion This one might also contain multiple cells s 4 cells Multicellular Thunderstorms s Conditions for development Moderate to strong conditional instability s Once clouds form, there is a significant amount of buoyant energy to allow for rapid cloud growth Little clockwise turning Low to moderate vertical wind shear s s Importance of Vertical Wind Shear Single cell Weak shear - storm is vertically stacked Outflow boundary may outrun the motion of the storm cell New storms that develop may be too far from the original to be a part of it s Multicell Weak to moderate shear keeps gust front near the storm updraft triggers new cells New development forms adjacent to the older cells and connects with the old cell A Schematic Model of a Thunderstorm and Its Density Current Outflow Downdraft Circulation - Density Current in a Broader Sense (Simpson 1997) Cell and Storm System Motion Multicellular Thunderstorms s s s s On the previous diagram, there are two arrows that show the cell motion and the storm motion. Notice that they are different. Why? New cells tend to form on the side of the storm where the warm, moist air at the surface is located. In the central Plains, this is often on the south or southeast side. 1 2 1 Average Wind Warm, Moist Surface Air (inflow) Cell Motion versus Storm Motion s Cells inside a storm (system) do not necessarily move at the same speed and/or direction as the overall storm system The storm system can move as a result of the successive growth and decay of cells It can also move because the cell motion Environmental winds can have significant influence on the cell and/or storm movement, but the storms do not s s s Multicellular Thunderstorms Individual cells typically move with the mean (average) wind flow s The storm system moves differently by discrete propagation s Multicell storms may last a long time. They constantly renew themselves with new cell growth. s Height (3-12km) The growth of a multicell storm Time (0-21min) Multicellular Thunderstorm Hazards s s s s s s Heavy rain -- Flooding Wind damage Hail Lightning Tornadoes -- Usually weak Multicell storms are notorious for heavy rain and hail Cell Generation in Multicell storms s s Before we discuss the cell regeneration in multicell storms, we will first look the gust front dynamics, which plays an important role in long-lasting convective systems We ll now begin to use some of what we learned in vorticity! A Schematic Model of a Thunderstorm and Its Density Current Outflow Downdraft Circulation - Density Current in a Broader Sense (Simpson 1997) At the surface, the cold pool propagates in the form of density or gravity current Laboratory Current Fresh Water Note lobe & cleft structures Salt Water Haboob in the Sudan (Lava flow on Surtsey, Iceland 1963) Other Geophysical Density Currents (Goff 1976, based on tower measurements) Schematic of a Thunderstorm Outflow Rotor Vorticity Dynamics s u v w v ( f ) t x y z The equation for relative vertical vorticity p p w v w u x y y x x z y z Stretching/ Convergence Local Derivative Of Relative Vorticity Advection Solenoidal Tilting Recall Absolute Vertical Vorticity f Vorticity Dynamics s The vector vorticity equation for absolute vertical vorticity Material Derivative Of Absolute Vorticity Stretching/ Convergence Tilting Solenoidal Friction Vorticity Dynamics Can derive the same equation for horizontal vorticity to apply to the horizontal rotor at the head of the gust front s In it, the solenoidal term is exactly the same form but acts in the horizontal s Now, recall circulation s Circulation Dynamics Definition of Circulation rr C g V dl rr r r ndA ndA Via Stokes Theorem C g V g g V dl r rr d dC d d r g g V dl ndA g ndA dt dt dt dt Circulation Dynamics Mitchell and Hovermale (1977) Gust Front Propagation s The low-level-inflow-relative speed of gust front often to a large extend determines the propagation of the storm system. This is almost certainly true for 2-D squall lines. Therefore the determination of gust front speed is important. Gust front/density currents propagate due to horizontal pressure gradient across the front created mainly by the density difference across the front. s Gust Front Propagation Speed How to Determine It? s For an idealized density current shown above, we apply simple equation du 1 p ' dt 0 x s What have we neglected? Friction, Coriolis effect, effect of vertical motion Now, to simplify the problem, let s look at the problem in a coordinate system moving with the gust front. In this coordinate system, the density current/gust front is stationary, and the front-relative inflow speed is equal to the speed of the gust front propagating into a calm environment. We further assume that the flow is steady in this coordinate system, a reasonably valid assumption when turbulent eddies are not considered. Therefore / 2 t 0 u u 1 p ' x 0 x u / 2 1 p ' x 0 x s s Gust Front Propagation Speed s Integrate the steady momentum equation along a streamline along the lower boundary from far upstream where u = U and p' = 0 to a point right behind the gust front where u=0 and p'= p: u 1 p ' u x 0 x u 2 / 2 1 p ' x 0 x U 2 p 2 p U 2 0 0 / t 0 s The above is the propagation speed of the gust front as related to the surface pressure perturbation ( p) associated with the cold pool/density current. Gust Front Propagation Speed s This equation is very. The contributions to the surface pressure perturbation from cold pool, upper-level heating, nonhydrostatic effect (vertical acceleration) and dynamic pressure perturbation can all be included. U p 2 p U 2 0 0 2 Gust Front Propagation Speed s If we assume that the p is purely due the hydrostatic effect of heavier air/fluid ( = 0 + ) inside the cold pool of depth h (other effects as listed in the previous slide are neglected), the above formula can be rewritten as (assuming pressure perturbation above the cold pool is zero): 2 g U 0 h because h 0 h dp ' dz g dz 0 dz 0 0 p g h 0 s s In this case, the speed of density current is mainly dependent on the depth of density current and the density difference across the front, not a surprising result. When other effects are included, the speed can be somewhat different. But it is generally correct to say that deeper and/or heavier (colder) density current/cold Laboratory Current Turbulent Laminar Laboratory Current Numerical Simulations Temperature Pressure Horizontal Wind Droegemeier and Wilhelmson (1987) Numerical Simulations Droegemeier and Wilhelmson (1987) Pressure perturbations ahead of the gust front s In the previous idealized model in the front-following coordinate, the inflow speed decreases to zero as the air parcel approaches the front from far upstream. There must be horizontal pressure gradient ahead of the gust front to ahead of the gust front and this positive pressure perturbation has to be equal to that produced by cold pool. We can rewrite the earlier equation as u 2 / 2 1 p ' x 0 x u2 p ' u2 p ' 0 C x 2 0 2 0 s s is constant along the streamline following the lower boundary which is a special form of the Bernoulli function (with the effect of vertical displacement excluded). u2 p ' 2 0 Numerical simulation of density currents showing the pressure perturbations associated with density current Pressure perturbations associated with rotors / rotating (KelvinHelmholtz) eddies Pressure perturbations associated with rotors / eddies s s Above the density current head there usually exist vorticity-containing rotating eddies. Most of the vorticity is generated by the horizontal density/buoyancy gradient across the frontal interface. Associated with these eddies are pressure perturbations due to another dynamic effect pressure gradient is need to balance the centrifugal force. The equation, called cyclostrophic balance and applied to tornadoes, is 1 p Vs2 n Rs s where n is the coordinate directed inward toward the center of the vortex and Rs is the radius of curvature of the flow. To overcome centrifugal force, pressure at the center of a circulation is always lower. The faster the eddy rotates and the smaller the eddy is, the lower is the central pressure. Pressure perturbation s in the head region and associated (rotor) circulation Effects of Surface Friction Free Slip Max Wind at Ground Surface Drag Max Wind Elevated (Nose) (Goff 1976, based on tower measurements) Schematic of a Thunderstorm Outflow Rotor Nose Reversal of Near-Surface Vorticity Solid Dashed Recall Life Cycle New Storm on Right n-2 n n-1 n+1 Model-Simulated 2D-Multicell Storm (Lin and Joyce 2001) Newest Cell Updraft solid, downdraft dashed Summary of life cycle Formation and maintenance of the gust front updraft (GFU) Rearward advection of the growing GFU Cutting off of the growing cell (c1) from the GFU by the upstream compensating downdraft Cell generation and coexistence of the growing (c2 and c3) and propagating (c1) cells Based on Lin et al 1998. Conceputal Model of Lin et al (1998) for Cell Regeneration In Lin et al (1998), the following processes are believed to repeat for cell regeneration (see previous illustration). s (i) Near the edge of the gust front, the gust front updraft is formed by the low level convergence ahead of the gust front near the surface. (ii) The upper portion of the gust front updraft grows by feeding on the midlevel inflow since the gust front propagates faster than the basic wind, creating mid level as well as low level convergence. (iii) The growing cell (C1) produces strong compensating downdrafts on both sides. The downdraft on the upstream (right) side cuts off this growing cell from the gust front updraft. (iv) The period of cell regeneration is inversely proportional to the midlevel, storm relative wind speed. s s s Numerical Simulations (M. Xue) s Multicell Storm Simulation Density Current simulation http://twister.ou.edu/DensityCurrent/LS.html http://cirrus.ou.edu/RKW/RKW54c2.ptew/RKW54c2 s Cell Regeneration theory of Fovell et al s s s s s Fovell and Tan (1998, MWR) also examined the cell regeneration problem using a numerical model They noted that the unsteadiness of the forcing at the gust front is one reason why the storm is multicellular . The cells themselves feed back to the overall circulation. The multicellular storm establishes new cells on its forward (upstream) side, in the vicinity of the forced updraft formed at the cold pool boundary, that first intensify and then decay as they travel rearward within the storm s upward sloping front-to-rear airflow. The cells were shown to be convectively active entities that induce local circulations that alternately enhance and suppress the forced updraft, modulating the influx of the potentially warm inflow. An explanation of the timing of cell regeneration was given that involves two separate and successive phases, each with their own timescales. Cell Regeneration theory of Fovell et al s Pressure field induced by perturbation buoyancy (derived from u and w momentum equations: ( B ') 2 p 'b z s Equation of the horizontal component of vorticity (in the x-z plane), neglecting friction, u w horizontal vorticityin y direction x d B ' dt x z s We call generation of horizontal vorticity by horizontal gradient of buoyancy the baroclinic generation of vorticity Schematic illustrating the effect of an individual convective cell on the storm s low-level circulation s s s s Panel (a) shows the BPGA (buoyancy pressure gradient acceleration) vector field associated with a finite, positively buoyant parcel. Panel (b) shows the full Fb field and the circulatory tendency associated with baroclinic vorticity generation. Panel (c) presents an analysis of the circulation tendency at the subcloud cold pool (stippled region) boundary. Panel (d) adds a positively buoyant region with its attendant circulatory tendency, illustrating the initial formation of a convective cell. The influence of transient cell s circulation on new cell generation s At first, the positively buoyant air created by latent heating within the incipient cell is located above the forced updraft, as depicted in Fig. 10d. The new cell s circulation enhances the upward acceleration of parcels rising within the forced updraft while partially counteracting the rearward push due to the cold pool s circulation. As a result, the forced lifting is stronger and parcels follow a more vertically oriented path than they would have been able to without the condensationally generated heating. s s The influence of transient cell s circulation on new cell generation s s s s s The influence of the transient cell s circulation depends on its phasing relative to the forced updraft. When the cold pool circulation dominates, the new cell and its positive buoyancy will be advected rearward. As it moves away from the forced updraft, the intensifying cell soon begins to exert a deleterious effect on the low-level lifting, as depicted in Fig. 10e. Instead of reinforcing upward accelerations in the forced lifting, the new cell is assisting the cold pool circulation in driving the rising parcels rearward. Thus, at this time, the forced lifting is weaker than it would have been in the absence of convection. As the cell continuing moving rearward, its influence wanes, permitting the forced updraft to reintensify as the suppression disappears. Summary of Cell Regeneration Theories s Examined closely, the two theories are more complementary than contradictory. Both examine the rearward movement of older cells and the separation of the cell from the new cells Lin et al focuses on the environmental conditions that affect the rearward cell movement and the associated cell regeneration. Fovell s work emphasizes cell and cold pool interaction and the associated gust-front forcing/lifting. The change in the gust-front lifting is considered to play an important role in modulating the intensity and generation of new cells at the gust front. Hence, Lin et al s work looks to external factors while Fovell et al s work looks to internal dynamics for an explanation of the multi-cellular behavior, so each could be looking at a different but complementary aspect of the problem. s s s
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