Chapter 3Mesoscale Processes and Severe

Convective Weather

Meteorology 515/815

San Francisco State University

Spring 2006

Christopher Meherin

Mesoscale Phenomena—Severe Convective Weather

• Tornadoes

• Hail storms

• Heavy winds

• Flash floods

Synoptic Scale Flow

• Initiates mesoscale storms

• Affects their evolution

• Influences their environment

A Variety of Mesoscale Processes are Involved in Severe

Weather

• Environmental preconditioning

• Environmental triggering

• Storm initiation

• Feedback of convection on the environment

What is the mesoscale?

• A fixed geometrical scale (Fujitia 1963, 1981; Ogura 1963; Orlanski 1975)

• Dynamical considerations (Ooyama 1982; Emanuel 1986; Doswell 1987)

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)

Mesoscale preconditioning/triggering

processes for severe weather

• Local effects

• Advective effects

• Dynamical effects

Examples of Local Preconditioning processes

• Boundary layer processes– Nocturnal inversion

• Terrain effects– Modification of hodograph

• Surface effects– Evaporation & heating

Examples of Advective Preconditioning processes

• Differential advection– destabilization

• Convergence lines– fronts

• Moisture advection– Increase CAPE; lower LFC

Examples of Dynamical Preconditioning processes

• Secondary circulations– jets

• Gravity currents, waves– Localized reduction of CIN

• Mesoscale instabilities– Boundary layer processes

Examples of Local Triggering processes

• Boundary layer circulation– thermals

• Terrain effects– Orographic lifting

• Surface effects– Sensible/latent heat flux

Examples of Advective Triggering processes

• Convergence lines– Gust fronts

• Boundary intersections– Tripple point

Instability of the atmosphere

• Effects of buoyancy

• Effects of dry air aloft

• Effects of wind shear

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

Parcels, soundings, and deep convective instability

• Thunderstorms

• CAPE/CIN

• Lapse rate stability/instability

• Moist/dry layers aloft

• Warm/cold advection patterns

Environmental factors can alter the development of storms

• Mesoscale perturbations– Local orography– Low level jets– Weaker CIN

Effects of dry air aloft enhances evaporation increasing strength

in

• Outflow boundries

• Squall lines

• Bow echoes

• Dry microbursts

Mesoscale mechanisms for environment preconditioning

• Instability

• Shear

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

Draw backs in applying indices in forecasting

• Obtaining a representative sounding

• Shear profiles modified by mesoscale phenomena

• Variability in storm evolution by convection

This section will consider three preconditioning processes

• Locally preconditioning

• Advective preconditioning (later sections)

• Dynamic preconditioning (later sections)

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

Terrain effects

• Hills

• Ridges

• Escarpments

• Mountain Ranges

Three classifications have been assigned to these effects

• Mechanical lifting to the LFC

• Thermally generated circulations

• Aerodynamic effects

Thermally generated circulations

• Hail storms

• Tornadoes

• Flash floods

• Heavy winds

Flash floods are examples of mechanically forced upslope

flow• Low level jets

• Weak flow at midlevels

• Moderate to large CAPE

• Low level inversion

Locations of storms producing flash floods determined by

• Interaction of outflow boundaries with terrain

• Orographic lift

• Other mesoscale features

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

The most common terrain effects are located in

• Isolated mountains or hills

• Mountain ranges

• Mountain Islands

Surface affects on environmental preconditioning

• State of soil (dry vs. wet)

• Heterogeneities in surface conditions (dry land adjacent to wet land)

Wet soil more conducive to convection when

– Latent heat flux increases CBL q in afternoon– Cape enhancement– If cap is weak, convection explodes

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

Land surface can also produce circulations leading to convection

• Terrain roughness

• Wetness of terrain

• Albedo

• Vegetation cover

• Snow cover

• urbanization