Skew-T Analysis and Stability Indices to Diagnose Severe

Thunderstorm Potential

Mteor 417 – Iowa State University – Week 6Bill Gallus

Basic features on a skew-T chart

isotherm

Moist adiabat

Mixing ratio line

isobar Dry adiabat

Parameters that can be determined on a skew-T chart

• Mixing ratio (w)– read from dew point curve

• Saturation mixing ratio (ws) – read from Temp curve

• Rel. Humidity = w/ws

More parameters• Vapor pressure (e) – go from dew point up an

isotherm to 622mb and read off the mixing ratio (but treat it as mb instead of g/kg)

• Saturation vapor pressure (es)– same as above but start at temperature instead of dew point

• Wet Bulb Temperature (Tw)– lift air to saturation (take temperature up dry adiabat and dew point up mixing ratio line until they meet). Then go down a moist adiabat to the starting level

• Wet Bulb Potential Temperature (θw) – same as Wet Bulb Temperature but keep descending moist adiabat to 1000 mb

More parameters

• Potential Temperature (θ) – go down dry adiabat from temperature to 1000 mb

• Equivalent Temperature (TE) – lift air to saturation and keep lifting to upper troposphere where dry adiabats and moist adiabats become parallel. Then descend a dry adiabat to the starting level.

• Equivalent Potential Temperature (θE) –same as above but descend to 1000 mb.

Meaning of some parameters

• Wet bulb temperature is the temperature air would be cooled to if if water was evaporated into it. Can be useful for forecasting rain/snow changeover if air is dry when precipitation starts as rain. Can also give a rough idea of temperature within the cold pools of thunderstorms

• Equivalent Potential temperature – gives an idea of total energy in the atmosphere (sensible heat plus latent heat)

Conserved quantities

• For unsaturated lift, potential temperature and mixing ratio are conserved (which is why you move along dry adiabats and mixing ratios when lifting/sinking parcels on a skew-T chart)

• For satured lift, θE or θW are conserved so parcels follow a moist adiabat (line of constant θE or θW ), and thus dew point drops faster than for unsaturated lift, and temperature cools more slowly due to latent heat release

Other parameters to compute• Tc – Convective temperature – temperature air

has to be heated to for it to rise on its own and form deep convection

• CCL – Convective condensation level – level at which air becomes positively buoyant when Tc is reached

• LCL – Lifted Condensation Level – level where air first becomes saturated after lifting

• LFC – Level of Free Convection – level where air becomes positively buoyant after being lifted past LCL

More parameters • EL – Equilibrium Level – level where parcel no

longer is positively buoyant and acceleration would cease

• MCL – Mixing Condensation Level – levelwhere saturation occurs due to mixing of layer near ground

• CAPE – Convective Available Potential Energy – positive area in region where parcel is warmer than environment

• CIN – Convective Inhibition – negative energy where parcel is cooler than environment

More…

• Cap – informal term for warm layer of air, usually between 700 and 850 mb, which creates CIN and prevents convection from easily forming

• Overshooting top – go beyond EL to the point where it appears you’ve created enough negative area to balance the positive area in the CAPE region

LCLLFC

EL

CAPE

Overshooting top

Negative area

CIN

Stability changes

• Diabatic (radiation, evaporation, condensation)

• Advection of different lapse rate• Differential advection due to wind shear• Vertical motion (“potential instability” can

exist if currently stable lapse rates wouldbecome unstable due to lift – this usuallyhappens if lower part of layer is moist andupper part is dry)

Lift this layer 100 mb and see how lapse rate changes

Stability Indices

• LI (Lifted Index) = T(env@500mb) –T(parcel@500 mb).

• Lift an air parcel from the surface (requires useof skew-T)

• LI < 0 significant cu convection possible• -4 < LI < 0 showers• -6 < LI < -4 thunderstorms• LI < -6 severe thunderstorms (SPC has -6 to -9

is very unstable; < -9 extremely unstable)

LI here is about -4 since parcel temp at 500 is -12 and environ. Temp is -16.

SI (Showalter Index)

• Same as LI but lift the parcel from 850 mb• Index is mostly used in Midwest but not so much

elsewhere. Why? (elevated convection at nightdominates the Midwest – so lifting from 850makes more sense than from surface)

• SI 0 – 3 Thunderstorm possible but need trigger• SI -3 to 0 unstable, T.storm probable• SI -4 to -6 very unstable, svr.storm potential• SI < -6 extremely unstable, svr.storms likely

K Index

• K = (T850 – T500) + Td850 – DD700

• Because DD (dew point depression is subtracted out at 700 mb, and dry air there is helpful for severe weather (thus big DD), this index works best for general thunderstorm prediction and NOT severe storm prediction

K guidelines

K value Thunderstorm probability

< 15 0%15-20 <20%21-25 20-40%26-30 40-60%31-35 60-80%36-40 80-90%>40 Near 100%

TT (Total Totals)East Central/

WestWx

44 48 Iso. Storms (winds < 35 knts, hail < .5 inch)

46 52 Scatt. Storms (winds 35-50 knts, hail .5-1 in.)

48 55 Scatt. MDT, iso. SVR

50 58 Scatt. MDT, few SVR, iso Tornadoes

52 61 Scatt-numerous MDT, Scatt. SVR, few Tornadoes

56 64 Numerous MDT, Scatt. SVR, Scatt. Tornadoes

• TT = T850+Td850 -2*(T500)

• Works forsevere stormprediction

SWEAT (Severe WEAther Threat)

• SWI = 12*Td850 +20*(TT-49) +2*fff850+fff500 +125*(S + 0.2)

• fff is wind speed in knots• S = sin (dd500 – dd850) where dd is

direction in degrees, and S= -0.2 ifdd850 < 130 or > 250dd500 < 210 or > 310dd500 - dd850 < 0Both 850 and 500 speeds < 15 knots

SWI Interpretation

• This is a good index for helping withrotating storm/tornado forecasting since itincludes wind and wind shear information

• SWI > 300: high severe storm potential• SWI > 400: high tornado potential

HWBZ (Height of the Wet Bulb Zero)

• This is a standard procedure forforecasting large hail probability

• HWBZ between 2.1 – 2.7 km is ideal• HWBZ between 1.5 – 3.4 km is possible

range for large hail• HWBZ > 3.4 km can still yield large hail IF

very large CAPE exists

HWBZ calculation is an interativeprocedure. Try a point like 650 mb and compute Twthere. It looks like -8C. So, obviously HWBZ is lower down than that.

Try new point, like 700mb.

700mb Twstill is colder than 0 (about -6 C), so we need to try further down –maybe 740 mb?

Tw at 740 mb looks like exactly 0C, so this is HWBZ level.

Need to convert to a height value to apply the rule.

Usually, 740 mb will be around 2.6 km, so this is probably a good large hail sounding

BRN (Bulk Richardson Number)• BRN is used to predict storm type, and in

particular supercell potential• BRN = CAPE/(.5*∆U2) where∆U = Ave U in 0-6km layer – Ave U in 0-500 m layer ( U here is full wind, so components are needed for calculation)

BRN < 10 – too much shear and storms are ripped apart

BRN 15-35 – Severe weather likely with supercellsfavored

BRN > 50 – Multicells likely (less tornado potential)

EHI (Energy-Helicity Index)

• EHI = CAPE * SRH /166000where SRH is storm relative helicity

EHI > 2.5 supercells with tornado potential

Sounding from around time of tornadic thunderstorms

Sounding from near tornadic storms that hit Waterloo area