Section 3.5, 3.5a, 3.5b Overview For Storm-generated Mesos cale processes 1. Local Effects 2. Advective Effects
Section 3.5, 3.5a, 3.5bOverview
For Storm-generated Mesoscale processes
1. Local Effects
2. 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