mesoscale processes and severe convective weather chapter 3: severe convective storms c.a. doswell...
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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
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