1. Chapter 3 Mesoscale Processes and Severe Convective Weather Meteorology 515/815 San Francisco State University Spring 2006 Christopher Meherin

2. Mesoscale Phenomena—Severe Convective Weather Tornadoes Hail storms Heavy winds Flash floods

3. Synoptic Scale Flow Initiates mesoscale storms Affects their evolution Influences their environment

4. A Variety of Mesoscale Processes are Involved in Severe Weather Environmental preconditioning Environmental triggering Storm initiation Feedback of convection on the environment

5. What is the mesoscale? A fixed geometrical scale (Fujitia 1963, 1981; Ogura 1963; Orlanski 1975) Dynamical considerations (Ooyama 1982; Emanuel 1986; Doswell 1987)

6. Dynamical considerations λ=NH÷ƒ Where λ is the Rossby radius of deformation N is the Brunt-Viaisala frequency H is the scale height (~10 to several hundred km) And ƒ is the coriolis parameter (2Ω sin φ)

7. Mesoscale preconditioning/triggering processes for severe weather Local effects Advective effects Dynamical effects

8. Examples of Local Preconditioning processes Boundary layer processes –Nocturnal inversion Terrain effects –Modification of hodograph Surface effects –Evaporation & heating

9. Examples of Advective Preconditioning processes Differential advection –destabilization Convergence lines –fronts Moisture advection –Increase CAPE; lower LFC

10. Examples of Dynamical Preconditioning processes Secondary circulations –jets Gravity currents, waves –Localized reduction of CIN Mesoscale instabilities –Boundary layer processes

11. Examples of Local Triggering processes Boundary layer circulation –thermals Terrain effects –Orographic lifting Surface effects –Sensible/latent heat flux

12. Examples of Advective Triggering processes Convergence lines –Gust fronts Boundary intersections –Tripple point

13. Instability of the atmosphere Effects of buoyancy Effects of dry air aloft Effects of wind shear

14. What is buoyancy? The acceleration of gravity times the fractional density difference between a parcel of air and its environment Gathering information from soundings is difficult

15. Parcels, soundings, and deep convective instability Thunderstorms CAPE/CIN Lapse rate stability/instability Moist/dry layers aloft Warm/cold advection patterns

16. Environmental factors can alter the development of storms Mesoscale perturbations –Local orography –Low level jets –Weaker CIN

17. Effects of dry air aloft enhances evaporation increasing strength in Outflow boundries Squall lines Bow echoes Dry microbursts

18. Mesoscale mechanisms for environment preconditioning Instability Shear

19. Effects of Wind Shear Indices The Bulk Richardson number combines the effects of buoyancy and shear R=CAPE÷0.5ū 2 Ū is defined as the difference between the density weighted mean windspeed taken over the lowest 6 km and an average surface wind speed taken over the lowest 500 m R>30 multicell storms 10<R<40 supercell storms

20. Draw backs in applying indices in forecasting Obtaining a representative sounding Shear profiles modified by mesoscale phenomena Variability in storm evolution by convection

21. This section will consider three preconditioning processes Locally preconditioning Advective preconditioning (later sections) Dynamic preconditioning (later sections)

22. Local processes—vertical mixing in the boundary layer Day time heating (this process depends on several factors which either restrict or promote convection –Strength/depth of morning inversion –Sky cover –Surface wetness

23. Terrain effects Hills Ridges Escarpments Mountain Ranges

24. Three classifications have been assigned to these effects Mechanical lifting to the LFC Thermally generated circulations Aerodynamic effects

25. Thermally generated circulations Hail storms Tornadoes Flash floods Heavy winds

26. Flash floods are examples of mechanically forced upslope flow Low level jets Weak flow at midlevels Moderate to large CAPE Low level inversion

27. Locations of storms producing flash floods determined by Interaction of outflow boundaries with terrain Orographic lift Other mesoscale features

28. Aerodynamic effects F=U÷NH equation descries whether flow is blocked to go around or forced above F is defined as the Froude number N is defined as the stratification (represented by Brunt Vaisala frequency) U is the incident flow speed H is the height of the barrier F<1 flow is blocked goes around barrier F>1 Flow goes over a barrier

29. The most common terrain effects are located in Isolated mountains or hills Mountain ranges Mountain Islands

30. Surface affects on environmental preconditioning State of soil (dry vs. wet) Heterogeneities in surface conditions (dry land adjacent to wet land)

31. Wet soil more conducive to convection when –Latent heat flux increases CBL q in afternoon –Cape enhancement –If cap is weak, convection explodes

32. Dry soil more conducive to convection when When strong cap inversion exists Sensible heat flux errodes the cap Afternoon heating from the sun force temps to CT Parcels reach there CCL

33. Land surface can also produce circulations leading to convection Terrain roughness Wetness of terrain Albedo Vegetation cover Snow cover urbanization