Download Mesoscale Processes And Severe Convective Weather

Mesoscale Processes
And 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
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Processes Preconditioning vs Triggering the Atmosphere
Processes Arising from Convection
Instability of Atmosphere to mesoscale convections
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Elementary Deep Convective Instability (buoyancy only)
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Parcels, Soundings, and Deep Convective Instability
Dry Air Aloft
Effects of Wind Shear
Mesoscale Mechanisms for Environmental Preconditioning
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Local Processes
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Vertical Mixing, Boundary Layer
Terrain Effects
Surface Effects
Introduction:

What is Mesoscale?
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–
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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)
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Important motions
– Ageostrophic advections
– Coriolis effects
Division of Mesoscale processes
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Preconditioning the environment
–
Processes gradually destabilize environment;
change wind shear profile
1.
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
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Triggering environment
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–
launches severe convection
Advective are most common processes:
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converging lines, boundary
intersections
Lifting needed is stronger than mesoscale
preconditioning effects
Mesoscale Processes
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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
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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
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General parameters:
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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
Boundary Layer
Evolution:
August 16, 1995
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
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
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Large-scale: large Mt. barriers create circulation
features that fluctuate diurnally
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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 1981
Northeastern 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
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Thermally generated circulations
Example: Flash Floods
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Flash flood areas:
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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
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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.
2.
Provide deep dry adiabatic layer, upper portions
made partly of advected mixed layers from the Mt.’s
to the left
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
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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
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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
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2.
can enhance CAPE
Heterogenities in surface conditions
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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