Section 3.5, 3.5a, 3.5bOverview

For Storm-generated Mesoscale processes

1. Local Effects2. Advective Effects

Storm Generated Mesoscale Process

• Severe storms generate host of mesoscale effects

– Promote storm development, severity and longevity, or

– Weaken storms

Recall From Table 3.3

• Local effect includes:– Radiation– Microphysics

• Downdraft, cold pool production• Microburst generation

Local Effects

• Radiation

1. Cloud-radiative effects- may be important in the development of new storms

2. Longer life by enhancing mass circulation

3. Increase total precipiation

Local Effects

• Microphysics– Downdraft, cold pool production– Microburst generation

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 stor

ms

Microburst Generation

• Particle sizes are important in determining downdraft intensity.– Smaller raindrop has most conducive to stron

g downdrafts

• Also, in frontal rainbands, evaporation, sublimation, and melting can have the effect of enhancing the thermal contrast across cold fronts

Advective Effects (Overview)

• From table 3.3• Particle advection, fall and phase changes

– Downdrafts generation– Upscale growth

• Cold pool processes– Cell regeneration– MCS evolution

Continuing Overview

• Momentum transport/ sloping flows– Severe surface winds

• Vortex tilting/ stretching– Vertical velocity generation– (supercells, MCS mesovortices)

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

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

Schematic Plan View of Tornadic Thunderstorm near the surface

Special Case

Advection of Condensate

• Development of trailing stratiform regions of squall lines

Schematic Plan View

Momentum Transport

• Vertical transport of horizontal momentum helps to generate covergence at the leading convective line

Vortex Tilting

Tilting produce vertical vorticity in MCSs

Development of Mesovortices in MCSs

• Interaction between the downdraft and the ambient westerly shear.

• Another tilting involves perturbation shears

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.

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

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

• End