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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