ATSC 3032 Skew t diagrams and static stability

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

pressure Stuve temperature

Stuve

Skew T log p

Fig 1 d. 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 : 2. Determine the cloud base 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)

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 q e* 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 q e* 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 q e* < 0 dz

Absolutely stable Conditionally unstable Absolutely unstable

equilibrium level benign LFC convective inhibition severe no convection

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

Potential instability: or

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 • WB 0: Wet bulb zero, Tw = 0°C ideally 7 -9, 000 ft 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 - 2 T thunderstorms probable when TOTL>50 • KINX: K index =T • SWET: Sweat index or severe weather threat - the higher the better, for severe storms, SW>300 SWET= 12*Td 850 +20*(TOTL-49) + 2*U 850 +U 500 +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 850 + Td 850 -T 500 (°C) the higher the better, -(T-Td)700 (°C) the higher the better

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=T 500 -Tp, 850

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 Tv is used. SHOW: Showalter index (°C, as LI but starts from 850 mb) must be negative SHOW = T 500 – T parcel, 850 CAPE: Convective available potential energy - should be over 500 J/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 (T p-Tenv) is maximum

Wind parameters • • STM: Estimated storm motion (knts) from 0 -20, 000 ft AGL layer, spd 75% of mean, dir 30 deg veer (to the right) from mean wind. HEL: Storm relative helicity 0 -10, 000 ft AG (total value) SHR+: Positive shear magnitude 0 -3000 m AG (sum of veering shear values) SRDS: Storm relative directional shear 0 -3000 m AG (directional difference of storm relative winds) EHI: Energy helicity index (prop to positive helicity * CAPE) BRN: Bulk Richardson number 500 -6000 m AG (BRN = CAPE/. 5 BSHR 2) 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, T 500=-7°C, calculate Tv 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, 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