Transcript Stability Indices - UW-Madison Department of Atmospheric

Skew T Log P Diagram
AOS 330 LAB10
Outline
•
•
•
•
Identify different air masses
Buoyancy
Stability Assessment
Freezing Level
Air Masses
Air Masses
We can also identify air masses from sounding data. We
do this by looking for features characteristic of certain
environments.
For example, in arctic regions we generally see persistent
radiational cooling, especially in winter (Why?) This often
produces a very deep radiation inversion that can extend
from the surface to 700mb or so. Very cold surface
temperature and a deep isothermal or inverted surface
layer characterizes the arctic air mass, as we see in the
next slide.
Arctic Air Mass
Now, let’s go to the other extreme. The Tropics!
In particular, the tropical oceans and embedded
landmasses.
The tropical maritime air mass is typified by
the following:
1. A warm, moist boundary layer
2. A subsidence inversion in the mid levels (usually
around 700mb).
3. An approximately psuedoadiabatic lapse rate over a
deep layer
We see a typical sounding in the following slide.
Tropical Maritime Air Mass
What might we expect to see in the tropics from time
to time? And for that matter, what about in the midwest
during severe weather season? Yep, thunderstorms!
Thunderstorms are often called “deep convection”
and a hallmark of deep convection is a pseudoadiatic
temperature profile over the depth of the troposphere
(or nearly so) and near saturation conditions
throughout.
Note these features in the following slide.
Deep Convection
Mixed Layers
A mixed layer is produced by turbulence, which tends to
mix conservative tracers such as potential temperature
and momentum. Moisture is also mixed, although it may
not be mixed uniformly (often there may be a slight
decrease with height).
The most common mixed layer in the atmosphere is the
planetary boundary layer (PBL), which normally
occupies the lower kilometer or two of the atmosphere.
This feature is normally most well-defined in the late
Afternoon. In fact, at other times of day, it may not be
mixed at all. Why?
The Elevated Mixed Layer
Another type of mixed layer that we’ll be interested in
is the elevated mixed layer. This feature typically forms
over high terrain (the Rockies, the Mexican Plateau)
during the spring and summer. It is VERY important
in the severe weather process (it provides a “cap” to
the boundary layer).
Not all layers are mixed....
...and if they’re not, they’re known as “stratified layers.”
We usually think of stratified layers as those in which the
potential temperature and moisture have significant
vertical gradients (i.e. they cut across adiabats and
mixing ratio lines at high angles).
Let’s take a look at a sounding which depicts these
features.
Layers and Layers and Layers
What suggests a layer of cloud between 650 and 750 mb?
Buoyancy
Buoyancy
• Archimedes’s Principle
– Upward force exerted on object that is immersed
in a fluid is equal to the weight of the displaced
fluid.
– Buoyant force
FB  (  enV  M)g
 ( en  )gV
M

V
Buoyancy of air parcels
FB ( en  )gV en   
fB 

 
g
M
V
  
P

Rd Tv

P
en 
Rd Tv,en
T  T 
f B   v v,en g
 Tv,en 

Stability Indices
Stability Indices
• Stability indices usually can be calculated
using temperatures (and dew point
temperatures) from a few mandatory levels
from a radiosonde sounding.
• Purpose:


to obtain a number to provide some measure of
the overall stability of the atmosphere.
to evaluate the potential for severe weather /
predict thunderstorms to occur
However…
• Since they are only calculated using a few
levels of the sounding, many details on the
soundings are not accounted for.
• Sounding is a snapshot of the atmosphere.
Stability indices calculated from sounding
would not tell us how the atmosphere is going
to evolve later on.
Lifted Index (LI)
• Compares the parcel with the environment at 500mb.
• Takes into account both near surface moisture content and
static stability
LI = (Tenv-Tparcel)500
Lifted Index
Thunderstorm Potential
>+2
No convective activity
0 to +2
Showers probable, isolated thunderstorms
possible
-2 to 0
Thunderstorms probable
-4 to –2
Severe thunderstorms possible
< -4
Severe thunderstorms probable, tornados
possible
Lifted Index (LI)
Lifted Index
Atmospheric Stability
>+3
Stable
0 to +3
Weak convection possible if provided
strong lifting
-3 to 0
Marginally unstable
-6 to –3
Very unstable
< -9
Extremely unstable
Petty (2008)
• Best Lifted Index
– Uses the highest value of qe or qw in the lower
troposphere.
– Use the highest mixing ratio value in
combination with the warmest temperature.
• SELS (Severe Local Storms) Lifted Index
– Use the mean mixing ratio and mean q of the
lowest 100mb
– If using a 12z sounding add 2o
– Start parcel at 50mb above the surface
Showalter Index (SI)
• Compares a parcel starting at 850mb with the
environment at 500mb.
• Tells us instability aloft other than from near surface.
SI = (Tenv-Tparcel)500
SI
Thunderstorm Possibility
> +3
No convective activity
1 to 3
Showers probable, isolated thunderstorms possible,
need to provide it some sources of lifting
-2 to 1
Thunderstorms probable (but generally weak)
-6 to –2
Severe thunderstorms possible
< -6
Severe thunderstorms probable, tornados possible
Vertical Totals
VT = T850 - T500
• Account for static stability in between 850 to
500 hPa.
• A value of 26 or greater is usually indicative of
thunderstorm potential.
Cross Totals
CT =T d850 - T500
• Account for 850hPa low level moisture.
CT
T-Storm Potential
18-19
Isolated to few moderate
20-21
scattered moderate, a few heavy
22-23
scattered moderate, a few heavy and isolated severe
24-25
scattered heavy, a few severe; isolated tornados
26-29
scattered to numerous heavy, few to scattered severe, a few
tornados
>29
numerous heavy, scattered showers, scattered tornadoes
Total Totals (TT)
TT = VT + CT = T850 + T d850 - 2 T500
• Not as useful when all low level moisture lies
below 850 hPa
TT
T-Storm Potential
44-45
Isolated to few moderate
46-47
scattered moderate, a few heavy
48-49
scattered moderate, a few heavy and isolated severe
50-51
scattered heavy, a few severe; isolated tornados
52-55
scattered to numerous heavy, few to scattered severe, a few
tornados
>55
numerous heavy, scattered showers, scattered tornadoes
K Index
K = T850 - T 500 + Td850 – (T700 - Td700)
• Takes into account of the vertical distribution of moisture and
temperature.
• 700 hPa moisture, which is important for air mass thunderstorms
development.
K value
Air mass T-Storm Probability
<15
0%
15-20
<20%
21-25
20-40%
26-30
40-60%
31-35
60-80%
36-40
80-90%
>40
>90%
SWEAT (Severe Weather thrEAT) Index
SWI = 12D + 20(T - 49) + 2f8 + f5 + 125(S + 0.2)
where: D=850mb dew point temperature (oC)
(if D<0 then set D = 0)
T = total totals (if T < 49 then set entire term = 0)
f8=speed of 850mb winds (knots)
f5= speed of 500mb winds (knots)
S = sin (500mb-850mb wind direction)
And set the term 125(S+0.2) = 0 when any of the following are not true
1.
850mb wind direction is between 130-250
2.
500mb wind direction is between 210-310
3.
500mb wind direction minus 850mb wind direction is positive
4.
850mb and 500mb wind speeds > 15knots
SWEAT (severe weather threat) Index
SWI = 12D + 20(T - 49) + 2f8 + f5 + 125(S + 0.2)
<300
Non-severe thunderstorms
300-400
Severe thunderstorms possible
>400
Severe thunderstorms, including
possible tornados
Bulk Richardson Number
BRN =
CAPE
½ (Uz2)
Where Uz = the vertical wind shear
(averaged over 0.5-6km layer)
• In general: 15-40 favors supercell development
>40 favors single / multicellular type
storms
• Explains the balance between wind shear and
buoyancy strength
Supercell Index
• Weights various parameters which are
indicative of possible supercell
development
Convective Available Potential Energy
(CAPE)
• Amount of buoyant energy available as parcel accelerate
upward.
• It contains information about the overall stability of the
atmosphere.
• Does not only based on a few levels, but the whole profile in
the calculation.
CAPE value
<0
0 - 1000
1000 - 2500
2500 – 3500
3500 – 4000
Atmospheric Stability
Stable
Marginally unstable
Moderately unstable
Very unstable
Extremely unstable

CAPE
q parcel  qenv
 g  LFC
dz
qenv
EL
wmax  2CAPE
Convective Inhibition (CIN)
• The amount of energy required to lift the
parcel from surface to LFC.
• > 200 Jkg-1 (strongly inhibit convective
potential)
Important Points to Remember
• Severe weather is more dependent on
dynamical forcing than instability!
• No one parameter tells the full tale!
• 12z soundings usually predict afternoon
convection better than 00z soundings predict
evening convection.
Freezing Level
• Lowest level on a sounding in which 0 deg Celsius is found.
• Follow 0 deg C isotherm up, until it reaches the and crosses
the temperature profile.
Reference
• Petty, G (2008). A First Course in Atmospheric Thermodynamics,
Sundog Publishing.
• Potter and Coleman, 2003a: Handbook of Weather, Climate and
Water: Dynamics, Climate, Physical Meteorology, Weather Systems
and Measurements, Wiley, 2003
Links
• http://www.theweatherprediction.com/severe/indices/
• http://www.theweatherprediction.com/habyhints/315/
• http://www.spc.noaa.gov/exper/mesoanalysis/
• http://mocha.meteor.wisc.edu/table.12z.html