Chapter 3 Mesoscale Processes and Severe Convective Weather

1. Chapter 3Mesoscale 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÷0.5ū2 • 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