Stability, Thunderstorms and Severe Weather

1
Stability, Thunderstorms and Severe Weather

• Discussion 4/4/08
• Dan Hartung

2
Lifting Condensation Level (LCL)

• Level at which a parcel lifted from the surface
  would reach saturation (I.e. level were
  temperature dewpoint temperature)
• LCL will refer to the height above the ground at
  which the cloud bottom is located
• Review DALR T decreases at 10C / km
• Also, Td decreases at 2C / km

3
Level of Free Convection (LFC)

• Level at which a parcel lifted from the surface
  would be warmer than its environment
• Above this level, air is able to freely convect,
  or ascend without resistance to the tropopause

Equilibrium Level (EL)

• Level at which a parcel is no longer warmer than
  its environment, usually is the tropopause
• Above the tropopause, environmental temperature
  increases with height in the stratosphere
• Corresponds to cloud top height

4
12






EL
10
Tparcel
8
Tenv
Height (km)
6
LFC
4
LCL
2
Td
30
20
10
0
-10
-20
-30
-40
Temperature (?C)
5
Thunderstorms
6
Ingredients for Thunderstorms
MoistureInstabilityLifting MechanismShear
7
1. Moisture

• Thunderstorms need abundant moisture as the
  energy source to drive their circulationsHigh
  moisture gt High Td gt Low Tdd gt Low LCL gt Low LFC
  gt Less lifting needed for parcels to reach level
  of free convection

8
2. Instability

• When a parcel is lifted from the surface and
  finds itself in an environment where the parcel
  is warmer than its environment, the atmosphere is
  said to be unstableThis is a situation in which
  a negative bouyancy force helps to accelerate the
  parcel upward

9
On the typical summer day, the atmosphere will be
conditionally unstable.Stable for unsaturated
parcelsUnstable for saturated parcelsSurface
(unsaturated) parcels will not be able to rise on
their own.Some mechanism must raise the
parcel until it reaches saturation (LCL) and then
past a level at which it is warmer than its
surroundings (LFC).
3. Lifting Mechanism
10
Different types of Lifting Mechanisms
Sufficient Surface Heating Convergence of Low
Level Winds Topography (Orography such as
mountains, etc.) Outflow boundaries Fronts and
Drylines
11
4. Shear

• Change in either wind speed or direction with
  height
• 2 types
• Directional shear (direction of winds change with
  height)
• Speed shear (same direction of winds, but speed
  changes with height).

4 km
2 km
6 km
sfc
DIRECTIONAL
6 km
4 km
2 km
sfc
SPEED
12
Purpose of shear

• Vertical shear allows for a tstorm to continue
  to take up moisture. This makes the tstorm
  circulation able to continue thriving by
  displacing the downdraft from the updraft
• Without shear, the downdrafts cut off the
  moisture source of the thunderstorm (updraft)
• Thunderstorms can intensify and last longer

13
Air mass thunderstorm (no shear)
14
Severe Weather
15
Severe Weather Criteria
-Wind in excess of 50 knots (58mph)-Hail gt
0.75in diameter-Tornado
16
Wind
-Must be 58mph wind gust or stronger Most intense
winds are generated when downdrafts hit the
surface and spread out (aka downbursts or
microbursts). - Have been reported to exceed 120
mph (EF 0 tornado)
17
Hail
-Must be 0.75 in (nickel size) diameter or larger
(pea and marble size hail are not considered
severe) - Hail is formed when ice chunks get
caught up in a tstorms updraft allowing ice
chunks to grow until they are heavy enough to
fall to the sfc - Largest hail stone ever
reported 7 inches in diameter (volleyball size!!)
18
Tornado
Oklahoma City TornadoMay 3, 1999
Wind 318mph!

• Definition violently rotating column of air in
  contact with the ground and extending from a
  cumulonimbus cloud

19
Supercells
-Most dangerous of all storms -Downbursts and
flash flooding can occur -Mesocyclone-
thunderstorm with deep, rotating updraft -Can
produce large hail (2) -Weak to violent
tornadoes (EF 0-EF 5 on Enhanced Fujita Scale) -
Need directional wind shear
20
Supercells cont.
21
Mesocyclone
22
Overshooting Top
23
Now lets take a look at some examples in the real
world
24
(No Transcript)
25
Time lapses .. You tube is actually good for
observing weather..http//www.youtube.com/watch?
v25clyNbWi5ohttp//www.youtube.com/watch?v_cl0a
w87LqAfeaturerelatedhttp//www.youtube.com/wat
ch?vhJYVscnhTLoNR1