ATSC 3032Skew t diagrams, and static stability

sources:-handout text-online module called “Skew T mastery”

1. Aerological diagrams• Radiosonde (or rawinsonde) data

– Maps– Vertical profiles

Instrument contains:Hygristor, thermistor, aneroid barometer, and radio transmittorAt the ground, a highly directional radio direction finding antenna is used to obtain the wind speed and direction at various levels in the atmosphere by tracking the radiosonde and determining the azimuth and elevation angles.

Aerological diagrams

• Hydrostatic balance• Ideal gas law• Hypsometric equation

Aerological diagrams:

different types

emagram

Stuve

temperature

p

d

c

R

p

pres

sure

Stuve

Skew T log p

Fig 1d. Elements of a tephigram. First, the 5 lines are shown separately, and then they are combined in the lower-right image.

2. using a skew T

LCL (lifting condensation level)

Applications

1. Determine the height of the base of cumulus clouds, given surface observations of T and Td :

2. Determine the cloud base temperature:

ground

8d

Tdd

dLCL

TTTTH

HLCL

LCLsurfacecloudbase HTT 10

potential temperature

wet-bulb potential

temperature

equivalent potential

temperature

saturated equivalent potential

temperature

wet bulb temperature:energy balance on the damp sock: LE = H LE = 6 u [esat(Tw)-e] H = 4 u [T-Tw](Regnault balance)

1. Layer thickness (between po and p)

Dz = 100 DT

Dz

Applications

2. Precipitable water

3. Chinook (Föhn) effect

west eastCascade Mountains

4. subsidence

5. Turbulent mixing, mixed layer (stratus), MCL

Oakland

Conserved or not conserved?Radiational DT Evaporation/

condensationAscent/descent

T

Td

Tw

q

qe or qw

qe*

q or r

RH

Conserved or not conserved?Radiational DT Evaporation/

condensationAscent/descent

T n n n

Td y n n

Tw n y n

q n n y

qe or qw n y y

qe* n n n

q or r y n y

RH n n n

3. stability

stability

Local vs non-local stability

Conditional vs absolute stability

d

dz< 0

qe*

Case II:

Absolutely stable Conditionally unstable Absolutely unstable

benign severe

convective inhibition

LFC

equilibriumlevel

no convection

d

dz< 0

qe*

Conditional instability:

Typical wet-season tropical sounding

Potential instability

Potential instability: dz

d edz

d wor

Lifting a potentiallyunstable layer

Latent instability

WLR: wet-bulb lapse rate

deep convectionsource layer

Stability indices

Significant level indices

• WB0: Wet bulb zero, Tw = 0°C ideally 7-9,000ft MSL, yet well below the FL

• PWAT: Precipitable water (mm) the higher the better

• LCL: Lifting condensation level (mb, from surface data) the lower the better

• TOTL: Total totals index =T 850 +Td 850 - 2T 500 (°C) the higher the better, thunderstorms probable when TOTL>50

• KINX: K index =T 850 + Td 850 -T 500 -(T-Td)700 (°C) the higher the better

• SWET: Sweat index or severe weather threat - the higher the better, for severe storms, SW>300

SWET= 12*Td850 +20*(TOTL-49) + 2*U850 +U500 +125*(0.2+sinf) where f= [wind direction 500 - wind direction 850 ]

U is expressed in kts and TOTL-49 is set to 0 if TOTL<49 • MLTH and MLMR: mean mixed layer (lowest 500 m) potential temp and

mixing ratio

e.g. UW sounding site

Lifted index uses:

Actual sfc temporEstimated max sfc temporMean mixed-layer temp

(note: always use virtual temp!)

PARCEL indices

Showalter indexSI=T500-Tp,850

PARCEL indices

• LIFT: Lifted index (°C) must be negative LI = T500 – T parcel,near-sfc [a 50 mb deep mixed layer is often used]

• LFTV: lifted index, but Tv is used.• SHOW: Showalter index (°C, as LI but starts from 850mb) must be negative

SHOW = T500 – T parcel,850

• CAPE: Convective available potential energy - should be over 500J/kg• CAPV: CAPE using Tv • CINS: Convective inhibition (external energy) - ideally 100-300 J/kg• CINV: CIN using Tv • CAP: Cap strength (C) Tenv –Tparcel @LCL - should be <5°C• LFC: Level free convection (LFCT and LFCT) (mb) - should be close to the LCL • EQL: Equilibrium level or level of neutral buoyancy (EQLT and EQLV)(mb) - should be

high• MPL: Maximum parcel buoyancy level (mb) - level where buoyancy (Tp-Tenv) is

maximum

Wind parameters

• STM: Estimated storm motion (knts) from 0-20,000ft AGL layer, spd 75% of mean, dir 30 deg veer (to the right) from mean wind.

• HEL: Storm relative helicity 0-10,000ft AG (total value) • SHR+: Positive shear magnitude 0-3000m AG (sum of veering shear

values) • SRDS: Storm relative directional shear 0-3000m AG (directional

difference of storm relative winds) • EHI: Energy helicity index (prop to positive helicity * CAPE) • BRN: Bulk Richardson number 500-6000m AG (BRN = CAPE/.5BSHR2)• BSHR: Bulk shear value (magnitude of shear over layer), shear

calculated between 1000-500 mb or 500 m –6000 m AGL

example mid-term questions

• As a rule of thumb, thunderstorms are possible when LI<0, and severe thunderstorms are likely if LI<-8. Assuming surface values T=32°C, Td=22°C, T500=-7°C, calculate Tv at the surface, and the lifted index LI based on both T and Tv. – Note that traditionally LI was calculated based on T, but the more correct procedure

uses Tv. The difference is small but not negligible!

• Using a given sounding on a tephigram, graphically determine, for an air parcel at 850 mb, the following: LCL, Tw , r, rs, e, es, RH, q, qw, qe

*, qe,

• Using a given sounding on a tephigram, graphically determine layers of:– absolute instability– conditional instability– potential instability– draw a parcel ascent path and shade the areas of

• positive energy (CAPE)• negative energy (CIN)

LIFT=-7 KCAPE=1974 J/kgCIN=-24 J/kgLCL= 900 mbLFL= 836 mb