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ATSC 3032 Skew t diagrams, and static stability sources: -handout text -online module called “Skew T mastery”Skew T mastery

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1. Aerological diagrams Radiosonde (or rawinsonde) data –Maps –Vertical profiles Instrument contains: Hygristor, thermistor, aneroid barometer, and radio transmittor At 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.

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Aerological diagrams Hydrostatic balance Ideal gas law Hypsometric equation

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Aerological diagrams: different types emagram

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Stuve temperature pressure

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Stuve

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Skew T log p

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

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2. using a skew T

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Psychrometric chart RH = 100 e/e s = 100 r/r s [%] mixing ratio r: mass of water vapor mass of air q = 622 e/p [g/kg] e

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LCL (lifting condensation level) Applications 1.Determine the height of the base of cumulus clouds, given surface observations of T and T d : 2.Determine the cloud base temperature: ground LCL

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potential temperature wet-bulb potential temperature

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equivalent potential temperature saturated equivalent potential temperature

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

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1. Layer thickness (between p o and p) z = 100 T zz Applications

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2. Precipitable water

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3. Chinook (Föhn) effect

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west east Cascade Mountains

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4. subsidence

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

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Oakland

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Conserved or not conserved? Radiational TEvaporation/ condensation Ascent/descent T TdTd TwTw e or w e*e* q or r RH

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Conserved or not conserved? Radiational TEvaporation/ condensation Ascent/descent Tnnn TdTd ynn TwTw nyn nny e or w nyy e*e* nnn q or ryny RHnnn

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3. stability

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stability

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Local vs non-local stability

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Conditional vs absolute stability d dz < 0 e* e* Case II:

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Absolutely stableConditionally unstableAbsolutely unstable

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benign severe convective inhibition LFC equilibrium level no convection

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d dz < 0 e* e* Conditional instability: Typical wet-season tropical sounding

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Potential instability Potential instability: or

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Lifting a potentially unstable layer

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Latent instability WLR: wet-bulb lapse rate deep convection source layer

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Stability indices

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Significant level indices WB0: Wet bulb zero, T w = 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

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Lifted index uses: Actual sfc temp or Estimated max sfc temp or Mean mixed-layer temp (note: always use virtual temp!) PARCEL indices

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Showalter index SI=T 500 -T p,850

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PARCEL indices LIFT: Lifted index (°C) must be negative LI = T 500 – T parcel,near-sfc [a 50 mb deep mixed layer is often used] LFTV: lifted index, but T v is used. SHOW: Showalter index (°C, as LI but starts from 850mb) must be negative SHOW = T 500 – T parcel,850 CAPE: Convective available potential energy - should be over 500J/kg CAPV: CAPE using T v CINS: Convective inhibition (external energy) - ideally 100-300 J/kg CINV: CIN using T v CAP: Cap strength (C) T env –T parcel @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 (T p -T env ) is maximum

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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/.5BSHR 2 ) BSHR: Bulk shear value (magnitude of shear over layer), shear calculated between 1000-500 mb or 500 m –6000 m AGL

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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, T d =22°C, T 500 =-7°C, calculate T v at the surface, and the lifted index LI based on both T and T v. –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, T w, r, r s, e, e s, RH, , w, e *, e, 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)

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LIFT=-7 K CAPE=1974 J/kg CIN=-24 J/kg LCL= 900 mb LFL= 836 mb

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