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ATSC 3032 Skew 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 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. Aerological diagrams • Hydrostatic balance • Ideal gas law • Hypsometric equation Aerological diagrams: different types emagram Stuve pressure p R d c p temperature 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) HLCL ground Applications 1. Determine the height of the base of cumulus clouds, given surface observations of T and Td : H T T T T d LCL d Td 2. Determine the cloud base temperature: d 8 T cloudbase T surface 10 H LCL wet-bulb potential temperature 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) Applications 1. Layer thickness (between po and p) Dz Dz = 100 DT 2. Precipitable water 3. Chinook (Föhn) effect west Cascade Mountains east 4. subsidence 5. Turbulent mixing, mixed layer (stratus), MCL Oakland Conserved or not conserved? Radiational DT T Td Tw q qe or qw qe* q or r RH Evaporation/ condensation Ascent/descent Conserved or not conserved? Radiational DT Evaporation/ condensation Ascent/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 Case II: d qe* < 0 dz Absolutely stable Conditionally unstable Absolutely unstable equilibrium level benign severe LFC no convection convective inhibition Typical wet-season tropical sounding Conditional instability: d qe* < 0 dz Potential instability Potential instability: dq e dz or dq w dz Lifting a potentially unstable layer Latent instability WLR: wet-bulb lapse rate deep convection source layer Stability indices Significant level indices e.g. UW sounding site • 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 • KINX: K index =T • SWET: Sweat index or severe weather threat - the higher the better, for severe 850 +Td 850 - 2T thunderstorms probable when TOTL>50 850 + Td 850 500 (°C) the higher the better, -T 500 -(T-Td)700 (°C) the higher the better 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 PARCEL indices Lifted index uses: Actual sfc temp or Estimated max sfc temp or Mean mixed-layer temp (note: always use virtual temp!) Showalter index SI=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 K CAPE=1974 J/kg CIN=-24 J/kg LCL= 900 mb LFL= 836 mb