Mesoscale ProcessesAnd Severe Convective Weather

Chapter 3: Severe Convective Storms

C.A. Doswell III

Authors: Richard H. Johnson

Brian E. Mapes

Presenter: Rebecca S. Bethke

Fall 2007

Outline

Introduction Definition of Mesoscale Section Outlook

Processes Preconditioning vs Triggering the Atmosphere Processes Arising from Convection

Instability of Atmosphere to mesoscale convections Elementary Deep Convective Instability (buoyancy only)

Parcels, Soundings, and Deep Convective Instability Dry Air Aloft

Effects of Wind Shear

Mesoscale Mechanisms for Environmental Preconditioning Local Processes

Vertical Mixing, Boundary Layer Terrain Effects Surface Effects

Introduction:

What is Mesoscale?– The events: tornadoes, hailstorms, high winds,

flash floods – Aid Initiation of severe storms– Effect Storm Evolution– Influence Storm Environment

– Focus: general classifications of mesoscale processes associated with severe weather

Definition of Mesoscale

Occurring on horizontal scales between ten and several hundred kms, generally (Ooyama 1982)

Important motions– Ageostrophic advections– Coriolis effects

Division of Mesoscale processes

Preconditioning the environment– Processes gradually destabilize environment;

change wind shear profile1. Local: ABL mixing; interactions with topography/terrain

and those effects; etc

2. Advective: physical transport of air masses:eg, moving cold over warm air; and/or

development and convergence of humid air masses – fronts, drylines, Mt./valley breezes, etc

Division of Mesoscale processes

Triggering environment – launches severe convection– Advective are most common processes:

converging lines, boundary intersections

Lifting needed is stronger than mesoscale preconditioning effects

Mesoscale Processes

Processes initiated by severe storms: Affect storm evolution

: Affect nearby storms

– Local: downdrafts, microbursts, high wind events

– Advective: Particle advection, momentum transport

Instability of Atmosphere:Deep Convective Instability

Buoyancy– Buoyant cloudy air from lower levels responsible

for Severe Convection (density of air + water) Depends on temperature,

humidity, condensed water content at a given level

– Density of Parcel and Environment needs clarification-----

Buoyancy: 1. Parcels, Soundings,

Deep Convective Instability

Skew-T /log p diagrams– Buoyancy and Convective Available Potential

Energy (CAPE) can be assessed at each level for each potential lifted parcel, surface to 100mb ,

– or for the entire air column (ICAPE), – and for CIN

Parcel temperature is warmer than midtropospheric temp.

– indicating large amounts of potential buoyancy

- Note: capping inversion layer producing CIN (preventing atmosphere from overturning everywhere)

Buoyancy: 2. Dry Air Aloft

Can aid the evaporation of precipitation– And affect strength of downdraft and

cold outflows from convection

Downdraft buoyancy (DCAPE) can be assessed, potentially– However, it’s difficult to measure & interpret;

Dry, potentially dense air can speed up vigorous downdrafts but also drag on updrafts that entrain dry air

Wind Shear Effects

General parameters: R (bulk Richardson number) > 30 for multicell growth 10 < R < 40 for supercell storm growth

Until recently however, : Difficult to get representational sounding, and to assess

actual (realized) CAPE + Shear profile modified by: terrain effects, outflow

boundaries, other mesoscale effects+ Small mesoscale perturbations greatly affect storm

development= forecast trouble — and also implies small mesoscale

disturbance(s) may radically affect storm development

Mechanisms For Preconditioning:A. Local processes:

1. Vertical mixing in Boundary Layer

Daytime heating is a common example

Nighttime inversion wears off, clouds can form, thermals from boundary layer rise to LCL

• However, specific sounding features must be assessed

Virtual Potential Temp (C) soundings; Water vapor mixing ratio, (g/kg); Reflectivity – boundary layer height (line) and cloud base height.

Clouds grew as LCL of boundary layer was reached,

~ 2:00pm CST

Boundary Layer Evolution:August 16, 1995

Mesoscale Preconditioning:Terrain Effects

Topographic effects: three classifications (Banta 1990)

1. Mechanical lifting to the LCF

2. Thermally generated circulations:May initiate and develop hailstorms; tornadoes; flash

floods; and high winds with dry microbursts

3. Aerodynamic effects

Thermally generated circulations

Hailstorm example– Large-scale: large Mt. barriers create circulation

features that fluctuate diurnally setting up thermodynamic and wind profile

– Mesoscale: smaller topographic features produce thermally forced flows

allowing focal point for starting convection

Radar Echo Frequency 1100 MST, July 1981Northeastern CO

Vector-mean surface flow over CO plains, on summer radar climatology (dashed line [+10] is intermediate contour)

East-west ridges north and south of Denver

-Focal points for intense hailstorms in afternoon -Consists of: mesoscale and synoptic flow

Thermally generated circulations Example: Flash Floods

Flash flood areas: – western US: heavy rains, often start in afternoon– Asia: frequent flooding, windward side of Mt. ranges

during summer monsoon– Also in areas with more gentle topography when

combined with other features Associated with: low-level jets; weak midlevel

flow; moderate/large CAPE; and low-level inversion

Triggered by: terrain/outflow interactions, direct orographic lifting (& other mesoscale features)

Thermally generated circulations:Dry microbursts & high surface wind

Often occur in summer along Front Range of Rocky Mt.’s

See typical soundings for AM and PM over the High Plains (US)

Importance of Mt.’s: 1. Provide deep dry adiabatic layer, upper portions

made partly of advected mixed layers from the Mt.’s to the left

2. Generate the rain that is the mode of the initial downdraft

Aerodynamic Terrain Effects: Flow deflections and Blocking

They often influence the location and development of convection– Ex: Low level shear lines and midlevel vortices that

develop on the leeside Tibetan Plain, creating heavy rains

– Coexistence of meso and large scale topographic effects

Large-scale temperature gradient drives moist SW flow Mesoscale SE-ern corner (Gui Plateau) has low level flow

blocked; this creates a descending flow and cyclonic vorticity over the leeside basin

Surface Effects:

Parts that effect environmental preconditioning :1. Surface moisture content

- can enhance CAPE

2. Heterogenities in surface conditions- Can impact structure of elevated mixed layer, dryline,

ageostrophic flow, potentially unstable air under an inversion- ie, convective potential at dryline enhanced as moist air is

drawn westward and upwards, to top of the mixed layer (called Inland Sea Breeze, Ogura and Chen, 1977)

- Contributes to mesoscale variability of severe weather and cloudiness