cirrus anvil

cumulonimbus

T (skewed)

dTT

LCL (Lifting Condensation Level)

LFC (Level of Free Convection)

EL (Equilibrium level)

p

θ wθ q

overshooting

CAPE

d s

Γ

Sounding

Mixed Condensation Level (MCL)1. Determine a layer which is unstable due to shear instability.

2. Draw a constant line to represent the temperature sounding after mixing.

3. Draw a constant q line to represent the dew point temperature (or the mixing ratio) after the mixing.

4. The intersection of these two lines is the MCL.

If top of the mixed layer

TTd

= constant

q =

cons

tant A

B

C

D

Area A = Area BArea C = Area D

No MCL

Criteria? Ri<0.25

Mixed Condensation Level (MCL)

1. Determine a layer which is unstable due to shear instability.2. Draw a constant line to represent the temperature sounding

after mixing.3. Draw a constant q line to represent the dew point temperature

(or the mixing ratio) after the mixing.4. The intersection of these two lines is the MCL.

If top of the mixed layerTTd

= constant

q =

cons

tant

MCLA

B

C

D

Area A = Area BArea C = Area D

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)

T (skewed)

p

ds

q

Convective Condensation Level (CCL)

dT

Γ

....... ...........

.

Tc 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)

)mb500 tomb850 from(T-)mb500(T=SI pe

Cin eTemperatur

mbin pressure )(#

T parcelair Tpoint dewt environmen T

Tt environmen T

o

p

d

e

T (skewed)

p

d s q

dT T

Air parcel at 500 mb

500 mb

Sounding at 500 mb850 mb)

Showalter Index (SI)

T - T

> 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

Showalter Index (SI)

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 . (A measure of the thunderstorm potential which takes into account the low level moisture availability)

)mb500 layer to surfce mixed from(T - )mb500(T=LI pe

Lifted Index

T (skewed)

p

d s q

dT T500 mb

100 mb

Air parcel at 500 mb

Sounding at 500 mb

T - T

> 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

Lifted Index

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.

[ ])mb700(T-)mb700(T-)mb850(T+)mb500(T-)mb850(T=K dedee

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

K-Index

Modified K-Index

[ ]

[ ][ ] 2/(850mb)T+(sfc)T = T

2/(850mb)T+(sfc)T = T where

,)mb700(T-)mb700(T-T+)mb500(T-Te=KMod

dd*

d

ee*

e

de*

de*

• Modified K-Index Replace the 850 mb T and Td with low altitude averaged

values.

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)

)mb500(T2-)mb850(T+)mb850(T=CT+VT=TT

)mb500(T-)mb850(T=CT)mb500(T-)mb850(T=VT

ede

ed

ee

< 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

TT

Severe Weather Threat

• Severe Weather Threat (SWEAT) This is a complicated index involving both

buoyancy and wind shear and a series of “ifs”.

[ ] [ ]0.2)+125(S + ff(500mb)+)mb850(ff 2+

0),49-TT(20Max+0),mb850(T12Max=SWEAT d

[ ]direction. wind theis dd and

dd(850mb)-dd(500mb)sin= S knots,in speed wind= ff where

knots 15 >both ff(850mb) and ff(500mb) 4.0 > dd(850mb)- dd(500mb) 3.

310 < dd(500mb) < 210 2.250 < dd(850mb) < 130 1.

if 0 ≠term shear whole the:NOTE

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).

dlnp TTRCAPE EL

LFC

pp epd

Skew-T Log-P diagram

T (skewed)

p

d s q

LCL

dT

T

LFC

EL

Γ

CAPE and Vertical velocity (W)

• Vertical momentum equation:

dzTTTgdw

T'Tgdz

dwwdzdw

dtdz

dzdw

dtdw

e

ep

e

2

21

2

2

ep T- T T'

plndRTpdpRTgdz ee

From hydrostatic balance and ideal gas law

CAPE and Vertical velocity (W)

CAPEw

plnd )TT(Rww

LFC

pp epLFCEL

EL

LFC

2

2

2

22

plnd )TT(Rdw EL

LFC

EL

LFC

pp ep

WW 22

Theoretically, the maximum w is at . However, in reality it is below . Why?

Area between sounding and air parcel in the skew-T log-P diagram

CAPEwEL 22

EL: equilibrium level

ELEL

CAPE

CAPE = 3540 J/kg

convective cloud

dlnp TTRCAPE EL

LFC

pp epd

LCL

NO LFC

Stratiform cloud

No CAPE

CAPE

< 300 Very weak convection

300-1000 Weak convection

1000-2500 Moderate convection

2500-3000 Strong convection

3000+ Very strong convection

CAPE

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).

dlnp TTRCIN LFC

SFC

pp epd

Convective inhibition (CIN)

CAPE (+)

CIN (-) dlnp TTRCIN

LFC

SFC

pp epd

dlnp TTRCAPE

EL

LFC

pp epd

GOES sounding

Processes affecting stability

• Diurnal surface temperature changes• Differential temperature or moisture advection• Differential rising or sinking motion:

stretching causes decreasing stability, compression causes increasing stability.

• Others

zw