Transcript Airmass Thunderstorms - Kelvin K. Droegemeier

Airmass Thunderstorms
METR 1004: Introduction to Meteorology
Adapted from Materials by Dr. Frank Gallagher III
and Dr. Kelvin Droegemeier
School of Meteorology
University of Oklahoma
Thunderstorm

Definition: By definition, a thunderstorm is a local
storm, invariably produced by a cumulonimbus cloud,
that always is accompanied by lightning and
thunder. It usually contains strong gusts of wind,
heavy rain, and sometimes hail. Meteorologists often
use the word "convection" to describe such storms in
a general manner, though the term convection
specifically refers to the motion of a fluid resulting in
the transport and mixing of properties of the fluid. To
be more precise, a convective cloud is one which
owes its vertical development, and possibly its origin,
to convection (upward air currents).
Thunderstorm Climatology

At any given time there are an estimated 2000
thunderstorms in progress, mostly in tropical and
subtropical latitudes. About 45,000 thunderstorms
take place each day. Annually, The U.S. experiences
about 100,000 thunderstorms. About 16 million
thunderstorms occur annually around the world!
Thunderstorm Climatology
Thunderstorm Climatology
Thunderstorm Climatology
Courtesy H. Brooks, National Severe Storms Laboratory
Thunderstorm Climatology
Courtesy H. Brooks, National Severe Storms Laboratory
Key Ingredients for Thunderstorms
Static Instability, as
measured by the
Convective Available
Potential Energy
(CAPE)
Theoretical peak
updraft speed is
sqrt(2xCAPE)
Key Ingredients for Thunderstorms
Vertical Wind Shear;
Change in wind
speed and/or
direction with height;
Severe storms need
strong veering of
wind with height and
strong increase in
speed
Key Ingredients for Thunderstorms

Mechanism to trigger the instability
– Front
– Terrain
– Dryline
– Daytime heating
– Landmass inhomogeneities
Air Mass Thunderstorms






First studied just after World War II
Many commercial and military aircraft
accidents
Newly developed radar was exploited for
weather studies
The Thunderstorm Project
Resulted in first life cycle of a thunderstorm
Air mass thunderstorms are also referred to
as “Garden Variety!”
Conditions of Formation of Air
Mass Thunderstorms
 Conditional
instability
 Warm, moist air near the ground
 Localized source of lift (usually
thermally driven)
 Weak or no environmental vertical
wind shear
Wind Shear

Definition:
– The change in the direction or speed of the wind
over a distance.

Vertical Wind Shear
– The change with height in the direction or speed of
the horizontal wind.

Low wind shear indicates little change in
direction or speed of the wind over a
distance.
Weak Wind Shear
Height
7 kts
6 kts
7 kts
6 kts
There is very little
change in the speed
or direction of the
wind with height.
Air Mass Thunderstorms
Three Stages of Air Mass Thunderstorms
Cumulus Phase

Development of towering
cumulus
– Region of low level convergence
– Warm moist air
– Updraft driven by latent heating



Nearby cumulus may merge
to form a much larger cloud
Dominated by updraft
Mixing and entrainment occur
in the updraft
Cumulus Phase
c David Shohami
c William Zender (2001)
Entrainment
Entrainment is the process by which
saturated air from the growing cumulus
cloud mixes with the surrounding cooler
and drier (unsaturated) air.
 Entrainment causes evaporation of the
exterior of the cloud and tends to
reduce the upward buoyancy there.

Mature Phase
Precipitation, formed
by the Bergeron cold
rain process, begins to
reach the ground.
 The precipitation drags
some of the
surrounding air down
creating the
downdraft.

Airmass Thunderstorm
Airmass Thunderstorm
Downdraft
 The
downdraft is the descending
column of air in a thunderstorm.
 Created and maintained by three
processes
– Evaporational cooling of entrained air
– Downward drag caused by falling
precipitation
– Evaporational cooling of the air below
the cloud base
Downdraft


When the downdraft reaches the ground, it
spreads out in all directions.
The leading edge of this cold, often gusty
wind is called the outflow boundary or
gust front.
Gust Front Shelf Cloud
National Severe Storms Laboratory
Downdraft

The outflow boundary behaves like a cold front:
– Strong wind shift (speed and direction)
– Much colder air behind the gust front
– Acts as a location for additional lift for future storm
development.
New Storm
Mature Phase





The mature phase represents
the peak intensity of the storm.
Updrafts and downdrafts are
about equal in strength.
Precipitation is typically heavy
and may contain small hail
Gusty winds result from the
downdraft spreading out on the
ground.
The anvil, or cloud top, begins
to turn to ice, or glaciate.
Dissipating Phase



Eventually the downdraft
overwhelms the updraft and
convection collapses – because
the cloud is vertically-oriented
Precipitation becomes lighter and
diminishes.
Cloud begins to evaporate from
the bottom up often leaving
behind an “orphan anvil.”
– Cirrus Spissatus
cumulonimbogenitus
Air Mass Thunderstorms





Usually weak (but can produce heavy rain in
a short period of time).
Usually not severe
Usually move slowly (weak winds aloft)
Often develop and dissipate in less than one
hour
Form in a weakly sheared environment and
thus have a BUILT-IN SELF-DESTRUCT
MECHANISM that guarantees a short lifetime
Hazards of Air Mass
Thunderstorms
Heavy Rain
 Hail

– Usually not terribly large
– May be numerous

Downbursts or Microbursts
– Exceptionally strong downdrafts that, when
they hit the earth, may have potentially
destructive winds associated with them.
Downbursts and Microbursts

Microburst
– An anomalously strong, concentrated downdraft
that produces a pocket of dangerous wind shear
near the ground over an area of 4 km or less in
horizontal extent.
– Very short lived (last for 3-8 minutes)
– Very small and isolated (city block)

Associated with cumulonimbus clouds
– Can have heavy rain (Wet microbursts)
– Can have vanishing sprinkles (Dry microbursts)
Microburst
Microburst
Microburst
Dry and Wet Microbursts
Dry Microbursts





A microburst with little or no precipitation.
Very dry air is located beneath the cloud
base.
Hydrometeors falling into the dry air will
evaporate causing a pool of cold air just
below cloud base.
This cold pool descends rapidly forming the
dry microburst.
Often you can’t detect them until it is too
late.
Dry Microburst
Typical Morning Sounding
Typical morning
sounding for a dry
microburst.
The nocturnal
inversion is easliy
destroyed. The dry
air below 550 mb is
conducive for
microbursts.
Typical Dry Microburst
Sounding
Very Dry Layer
Below 500 mb.
Wet Microbursts






Microbursts associated with moderate or
heavy precipitation.
Some dry air above cloud top gets entrained
in the top of the thunderstorm.
This dry air mixes with cloud air causing
some evaporation of the cloud.
Evaporational cooling will form a pool of cold
air near the top of the cloud.
This cold pool descends and adds to the
downdraft to form a microburst.
Often there is a “rain gush” coincident with
the microburst.
Wet Microburst
Wet Microburst Sounding
Elevated Dry
Layer
Wet Microburst
Wet Microburst
Microburst
Damage
Detection of Microbursts

Visually
Visual Detection
Detection of Microbursts

Doppler Radar (Airport and Aircraft)
– Best when precipitation is present
– Terminal Doppler Weather Radar (TDWR)
Detection of Microbursts

LLWAS
– Low level wind shear alert system
– A network of wind sensors positioned
around the airport.
– Does not detect elevated microbursts or
microbursts that are between sensors.
Microbursts and Aviation

Microbursts are extremely hazardous to
low-flying aircraft because of
– Low airpseed
– Proximity to the ground
– “Dirty” aerodynamic configuration (flaps
out, gear down)
– Difficulty of visual microburst detection
– Rapid onset and short duration
Microburst
Glide Slope
Runway
Microburst
Glide Slope
Runway
Microburst
Glide Slope
Runway
Microburst
Glide Slope
Runway
Microburst
Glide Slope
Runway
Flight of Eastern 902
Flight of Eastern 66
Number of Fatalities
Fatalities Associated with Aviation
Wind Shear Accidents
154
150
136
136
115
Wind Shear R&D
100
Pilot Training
38
50
1
TDWR
0
0
'65-'69 '70-'74 '75-'79 '80-'84 '85-'89 '90-'94 '95-'98
Year of Accident