Section 3.5, 3.5a, 3.5b

1. Section 3.5, 3.5a, 3.5b Overview For Storm-generated Mesoscale processes Local Effects Advective Effects

2. Storm Generated Mesoscale Process • Severe storms generate host of mesoscale effects • Promote storm development, severity and longevity, or • Weaken storms

3. Recall From Table 3.3 • Local effect includes: • Radiation • Microphysics • Downdraft, cold pool production • Microburst generation

4. Local Effects • Radiation • Cloud-radiative effects - may be important in the development of new storms • Longer life by enhancing mass circulation • Increase total precipiation

5. Local Effects • Microphysics • Downdraft, cold pool production • Microburst generation

6. Downdraft, cold pool production • Occurs through evaporation and melting • Strength of cold pool is important to • Supercell’s behavior and longevity • Squall-line intensity and longevity • Baroclinic vorticity generation in tornadic storms

7. Microburst Generation • Particle sizes are important in determining downdraft intensity. • Smaller raindrop has most conducive to strong downdrafts • Also, in frontal rainbands, evaporation, sublimation, and melting can have the effect of enhancing the thermal contrast across cold fronts

8. Advective Effects (Overview) • From table 3.3 • Particle advection, fall and phase changes • Downdrafts generation • Upscale growth • Cold pool processes • Cell regeneration • MCS evolution

9. Continuing Overview • Momentum transport/ sloping flows • Severe surface winds • Vortex tilting/ stretching • Vertical velocity generation • (supercells, MCS mesovortices)

10. Cold pool processes • Are responsible for cell regeneration in multicell storms involves cold air outflow from convective downdrafts • As it spreads out over a large area and becomes shallow, then the regeneration stops

11. Downdraft Outflows • Also important in supercells where forward-flank downdrafts and rear-flank downdrafts produces storm-scale fronts • Special case: new mesocyclones may form at the occlusion

12. Schematic Plan View of Tornadic Thunderstorm near the surface

13. Special Case

14. Advection of Condensate • Development of trailing stratiform regions of squall lines

15. Schematic Plan View

16. Momentum Transport • Vertical transport of horizontal momentum helps to generate covergence at the leading convective line

17. Vortex Tilting

18. Tilting produce vertical vorticity in MCSs

19. Development of Mesovortices in MCSs • Interaction between the downdraft and the ambient westerly shear. • Another tilting involves perturbation shears

20. Development of Mesovortices in MCSs • Buoyancy forces act to generate front-to-rear or rear-to-front flows • Tilting of perturbation shears generated by the cold pool is important in the production of line-end vortex pairs in environments with weak-to-moderate shear.

21. Development of Mesovortices in MCSs • As squall lines mature, Coriolis effects become important in the development • Result: eventual evolution of many squall lines to an symmetric precipitation pattern

22. Effects of the plume of heat and moisture • Convective/advective warming of the midtroposphere is the primary mechanism for the generation of the midlevel rear-inflow jets in squall lines • Moistening increases buoyancy of secondary convection

23. End