d
θ
s
overshooting
θw
q
cirrus anvil
EL (Equilibrium level)
Γ
T
p
Td
cumulonimbus
LFC (Level of Free Convection)
LCL (Lifting Condensation Level)
T (skewed)
Sounding
Mixed Condensation Level (MCL)
1.
2.
3.
4.
Determine a layer which is unstable due to shear instability.
Criteria?
Ri<0.25
Draw a constant q line to represent the temperature sounding after
mixing.
Draw a constant q line to represent the dew point temperature (or the
mixing ratio) after the mixing.
The intersection of these two lines is the MCL.
Td
T
If top of the mixed layer
C
A
No MCL
Area A = Area B
Area C = Area D
D
B
Mixed Condensation Level (MCL)
1.
2.
3.
4.
Determine a layer which is unstable due to shear instability.
Draw a constant q line to represent the temperature sounding after
mixing.
Draw a constant q line to represent the dew point temperature (or the
mixing ratio) after the mixing.
The intersection of these two lines is the MCL.
Td
T
If top of the mixed layer
C
MCL
A
Area A = Area B
Area C = Area D
D
B
Convective Condensation Level (CCL)
The lowest level at which condensation will occur
as a result of convection due to surface heating.
When condensation occurs at this level, the layer
between the surface and the CCL will be
thoroughly mixed, the temperature lapse rate will
be dry adiabatic and the mixing ratio will be
approximately constant.
Soundings
Convective Condensation Level (CCL)
Convective Condensation Level (CCL)
CCL
Convective Condensation Level (CCL)
d
Γ
s
q
Td
p
......
.
..
..
.....
Convective Condensation Level (CCL)
Tc
T (skewed)
Convective temperature
Stability Indices
• Showalter Index
• Lifted Index
• K-Index
• Modified K-Index
• Vertical, Cross, and Total Totals indices
• Severe Weather Threat
• Convective Available Potential Energy
• Convective Inhibition
Stability Indices
Showalter Index (SI)
• Showalter Index
Lift 850 mb parcel by appropriate processes to 500 mb
and subtract its temperature from the observed 500
mb temperature. The smaller (more negative) the
number the more unstable the environment.
(a measure of thunderstorm potential and severity.
Especially useful when a shallow, cool layer of air below
850 mb conceals greater convective potential above)
SI = Te (500mb ) - Tp (from 850mb to 500mb )
Te
environmen t T
Td
environmen t dew point
Tp
air parcel T
(# ) pressure in mb
Temperatur e in o C
Showalter Index (SI)
d
s
q
T
p
Td
- T
T
500 mb
Air parcel at 500 mb
Sounding at 500 mb
850 mb)
T (skewed)
Showalter Index (SI)
>4
Thunderstorms unlikely
1–4
Thunderstorms possible – triggered needed
1 - -2
Increasing chance of thunderstorms
-2 - -3
High potential of heavy thunderstorms
-3 - -5
Getting scary
-5 - -10
Extremely unstable
< -9
Head for the storm shelter
Sturtevant (1995)
Showalter Index (SI)
Lifted Index
• Lifted Index
Like the SI but the parcel is defined by mixing the lowest
100 (or 50) mb to average q and q.
(A measure of the thunderstorm potential which takes
into account the low level moisture availability)
LI = Te (500mb ) - Tp (from mixed surfce layer to 500mb )
Lifted Index
d
s
q
T
p
Td
- T
T
500 mb
Air parcel at 500 mb
Sounding at 500 mb
100 mb
T (skewed)
Lifted Index
>0
Thunderstorms unlikely
0 - -2
Thunderstorms possible – triggered needed
-3 - -5
Thunderstorms probable
-5 - -7
Strong/severe thunderstorms. Tornadoes possible
-7 - -9
Move to Alaska
< -9
Yikes
Sturtevant (1995)
Lifted Index
K-Index
• K-Index
Attempts to include a measure of low level moisture
(Td(850mb)) and the depth of the moist layer by including
the 700 mb dew points depression. Large K means a lot of
moisture available to drive cumulus clouds.
K = Te (850mb ) - Te (500mb ) + Td (850mb ) - [Te (700mb ) - Td (700mb )]
K-Index
0 – 15
No thunderstorms
16 – 19
Thunderstorms unlikely
20 – 25
Isolated thunderstorms
26 – 29
Widely scattered thunderstorms
30 – 35
Numerous thunderstorms
36 – 39
Thunderstorms very likely
40+
100% chance of thunderstorms
Sturtevant (1995)
K-Index
Modified K-Index
• Modified K-Index
Replace the 850 mb T and Td with low altitude averaged values.
Mod K = Te * - Te (500mb ) + Td * - [Te (700mb ) - Td (700mb )],
where Te* = [Te (sfc) + Te (850mb) ]/ 2
Td * = [Td (sfc) + Td (850mb) ]/ 2
Vertical, Cross, and Total Totals Indices
• Vertical, Cross, and Total Totals indices
Here, the larger the number is, the more unstable the
atmosphere is. VT or CT >= 30 or TT > 60 indicates
moderate thunderstorms with the possibility of scattered
severe T-storms.
(A measurement of thunderstorm potential. Generally, the
value is higher if low-level moisture extends up through
the 850 mb level)
VT = Te (850mb ) - Te (500mb )
CT = Td (850mb ) - Te (500mb )
TT = VT + CT = Te (850mb ) + Td (850mb ) - 2Te (500mb )
TT
< 43
Thunderstorms unlikely
43 – 44
Isolated thunderstorms
45 – 46
Scattered thunderstorms
47 – 48
Scattered thunderstorms/isolated severe
49 – 50
Scattered T-storms/few severe/isolated tornadoes
51 – 52
Scattered-numerous T-storms/few-scattered
severe/isolated tornadoes
53 – 55
Numerous thunderstorms/scattered tornadoes
56+
You don’t wanna know…
Sturtevant (1995)
TT
Severe Weather Threat
• Severe Weather Threat (SWEAT)
This is a complicated index involving both buoyancy and
wind shear and a series of “ifs”.
SWEAT = Max[12Td (850mb ),0]+ Max[20(TT - 49),0]
+ 2 ff (850mb ) + ff(500mb) + 125(S + 0.2)
where ff = wind speed in knots, S = sin [dd(500mb) - dd(850mb) ]
and dd is the wind direction.
1. 130 < dd(850mb) < 250
NOTE : the whole shear term ≠0 if
2. 210 < dd(500mb) < 310
3. dd(500mb) - dd(850mb) > 0
4. ff(500mb) and ff(850mb) both > 15 knots
Severe Weather Threat (SWEAT)
< 272
Thunderstorms unlikely
273-299
Slight risk – general thunderstorms
300-400
Moderate risk – approaching severe limits
401-600
Strong risk – few severe T-storms/isolated tornadoes
601-800
High risk of severe –T storms/scattered tornadoes
801+
High wind damage, but not favorable for severe weather
Sturtevant (1995)
Convective Available Potential Energy (CAPE)
• Convective Available Potential Energy (CAPE)
The amount of energy a parcel of air would have if
lifted a certain distance vertically through the
atmosphere. CAPE is effectively the positive buoyancy
of an air parcel and is an indicator of atmospheric
instability, which makes it valuable in predicting severe
weather (J kg-1).
CAPE  Rd 
pEL
pLFC
T
p

 Te dlnp
Skew-T Log-P diagram
d
s
q
EL
Γ
T
p
Td
LFC
LCL
T (skewed)
CAPE and Vertical velocity (W)
• Vertical momentum equation:
dw dw dz dw
1 dw 2
T'


w
g
dt
dz dt dz
2 dz
Te
dw  2g
2
Tp  Te
Te
dz
T' Tp - Te
From hydrostatic balance and ideal gas law
dp
gdz  RTe
 RTedln p
p
CAPE and Vertical velocity (W)
 
WEL
WLFC
2
wEL

dw  2R 
2
pEL
pLFC
(Tp  Te ) dln p
EL: equilibrium level
pEL
2R  (Tp  Te ) dln p
p
LFC

2
wLFC

2
 wLFC
 2CAPE
Area between sounding and
air parcel in the skew-T
log-P diagram
Theoretically, the maximum w is at EL . However, in reality it is
below EL . Why?
2
wEL
 2CAPE
CAPE
convective cloud
CAPE  Rd 
pEL
pLFC
T
p

 Te dlnp
CAPE
Stratiform cloud
NO LFC
LCL
No CAPE
CAPE
< 300
300-1000
Very weak convection
Weak convection
1000-2500
Moderate convection
2500-3000
Strong convection
3000+
Very strong convection
Sturtevant (1995)
Convective Available Potential Energy (CAPE)
Convective inhibition (CIN)
• Convective INhibition (CIN)
A numerical measure in meteorology that indicates the
amount of energy that will prevent an air parcel rising
from the surface to the level of free convection.
(J kg-1).
CIN  Rd 
pLFC
pSFC
T
p

 Te dlnp
Convective inhibition (CIN)
CAPE 
 Rd 
pEL
pLFC
CIN
 Rd 
pLFC
pSFC
T
p
T
p

 Te dlnp

 Te dlnp
CAPE (+)
CIN (-)
GOES sounding
Processes affecting stability
• Diurnal surface temperature changes
• Differential temperature or moisture advection
• Differential rising or sinking motion:
w
z
stretching causes decreasing stability, compression
causes increasing stability.
• Others