cirrus anvil cumulonimbus T (skewed) LCL (Lifting Condensation Level) LFC (Level of Free Convection) EL (Equilibrium level) p overshooting CAPE 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 T TdTd  = constant q = constant A B C D Area A = Area B Area 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 layer T TdTd  = constant q = constant MCL A B C D Area A = Area B Area 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)

CCL

Convective Condensation Level (CCL) T (skewed) p Convective Condensation Level (CCL) 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)

T (skewed) p Air parcel at 500 mb 500 mb Sounding at 500 mb 850 mb) Showalter Index (SI) T - T

> 4Thunderstorms unlikely 1 – 4Thunderstorms possible – triggered needed Increasing chance of thunderstorms High potential of heavy thunderstorms Getting scary Extremely unstable < -9Head for the storm shelter Showalter Index (SI) Sturtevant (1995)

Showalter Index (SI)

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)

Lifted Index T (skewed) p 500 mb 100 mb Air parcel at 500 mb Sounding at 500 mb T - T

> 0Thunderstorms unlikely Thunderstorms possible – triggered needed Thunderstorms probable Strong/severe thunderstorms. Tornadoes possible Move to Alaska < -9Yikes Lifted Index Sturtevant (1995)

Lifted 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.

0 – 15No thunderstorms 16 – 19Thunderstorms unlikely 20 – 25Isolated thunderstorms 26 – 29Widely scattered thunderstorms 30 – 35Numerous thunderstorms 36 – 39Thunderstorms very likely % chance of thunderstorms Sturtevant (1995) K-Index

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)

< 43Thunderstorms unlikely 43 – 44Isolated thunderstorms 45 – 46Scattered thunderstorms 47 – 48Scattered thunderstorms/isolated severe 49 – 50Scattered T-storms/few severe/isolated tornadoes 51 – 52 Scattered-numerous T-storms/few-scattered severe/isolated tornadoes 53 – 55Numerous 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”.

Severe Weather Threat (SWEAT) < 272Thunderstorms unlikely Slight risk – general thunderstorms Moderate risk – approaching severe limits Strong risk – few severe T-storms/isolated tornadoes 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) 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 ).

Skew-T Log-P diagram T (skewed) p LCL LFC EL

CAPE and Vertical velocity (W) Vertical momentum equation: From hydrostatic balance and ideal gas law

CAPE and Vertical velocity (W) 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 EL: equilibrium level EL

CAPE CAPE = 3540 J/kg convective cloud

LCL NO LFC Stratiform cloud No CAPE CAPE

< 300Very weak convection Weak convection Moderate convection 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 ).

Convective inhibition (CIN) CAPE (+) CIN (-)

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