Using Soundings For Severe Weather

Soundings and Adiabatic Diagrams forSevere Weather Prediction and Analysis

Review

Atmospheric Soundings Plottedon Skew-T Log P Diagrams Allow us to identify stability of a layerAllow us to identify various air massesTell us about the moisture in a layerHelp us to identify cloudsAllow us to speculate on processesoccurring

Why use Adiabatic Diagrams? They are designed so that area on thediagrams is proportional to energy. The fundamental lines are straight and thuseasy to use. On a skew-t log p diagram the isotherms(T)are at 90o to the isentropes (q).

The information on adiabatic diagramswill allow us to determine things such as: CAPE (convective available potential energy)CIN (convective inhibition)DCAPE (downdraft convective available potential energy)Maximum updraft speed in a thunderstormHail sizeHeight of overshooting topsLayers at which clouds may form due to various processes,such as:– Lifting– Surface heating– Turbulent mixing

Critical Levels on aThermodynamic Diagram

Lifting Condensation Level (LCL) The level at which a parcel lifted dryadiabatically will become saturated. Find the temperature and dew pointtemperature (at the same level, typically thesurface). Follow the mixing ratio up fromthe dew point temp, and follow the dryadiabat from the temperature, where theyintersect is the LCL.

Finding the LCL

Level of Free Convection (LFC) The level above which a parcel will be ableto freely convect without any other forcing. Find the LCL, then follow the moist adiabatuntil it crosses the temperature profile. At the LFC the parcel is neutrally buoyant.

Example of LFC

Equilibrium Level (EL) The level above the LFC at which a parcel will nolonger be buoyant. (At this point the environmenttemperature and parcel temperature are the same.) Above this level the parcel is negatively buoyant. The parcel may still continue to rise due toaccumulated kinetic energy of vertical motion. Find the LFC and continue following the moistadiabat until it crosses the temperature profileagain.

Example of finding the EL

Convective Condensation Level(CCL) Level at which the base of convectiveclouds will begin. From the surface dew point temperaturefollow the mixing ratio until it crosses thetemperature profile of sounding.

Convective Temperature (CT) The surface temperature that must bereached for purely convective clouds todevelop. (If the CT is reached at the surfacethen convection will be deep enough toform clouds at the CCL.) Determine the CCL, then follow the dryadiabat down to the surface.

Finding the CCL and CT

Mixing Condensation Level(MCL) This represents the level at which cloudswill form when a layer is sufficiently mixedmechanically. (i.e. due to turbulence) To find the MCL determine the averagepotential temperature (q) and averagemixing ratio (w) of the layer. Where theaverage q and average w intersect is theMCL.

Finding the MCL

Thermodynamic Diagrams andSevere Weather

What is Severe Weather?TornadoHail or 3/4inchWind 50 knots

Convective Available PotentialEnergy (CAPE) Remember: Area on a thermodynamicdiagram is proportional to energy. CAPE is also called buoyant energy. CAPE on a thermodynamic diagram is thearea between the parcel and the environmenttemperatures from the LFC to the EL CAPE is a measure of instability

CAPE

CAPE

Maximum Updraft Speed If we convert the potential energy of CAPEto a kinetic energy, we can get themaximum speed of any updraft that maydevelop.1 2CAPE PE KE mv2wmax 2CAPE

Convective Inhibition (CIN) CIN is NOT negative CAPE!!!!!! CAPE integrates from the LFC to the EL,CIN integrates from the surface to the LFC Is a measure of stability Reported as an absolute value

CIN

Overcoming ConvectiveInhibition A convective outbreak rarely occurs from surfaceheating alone! Triggering Mechanisms for T-Storms––––––frontsdry linessea-breeze frontsgust fronts from other thunderstormsatmospheric buoyancy wavesmountains

Cap Strength Very important for severe weather todevelop Too little or no cap: happy little cumuluseverywhere Too strong of a cap: nothing happens Just the right amount of a cap: SevereWeather

At the inversion* lookat the temperaturedifference between theparcel and theenvironment.

Shear vs. CAPE Need a balance between Shear and CAPEfor supercell development Without shear: single, ordinary, air massthunderstorm which lasts 20minutes If shear is too strong: multicellular t-storms(gust front moves too fast)

CAPE and Shear

Shear Just Right 2-D equilibrium: squall line developsAABB 3-D equilibrium:right moving and leftmoving supercellsLeft MoverRight Mover

Bulk Richardson Number (BRN)BRN CAPE1/22Uz(where U is a measure of the verticalzwind shear)

VHodographs Draw windvectors indirection theyare goingU This isopposite of howthe wind barbsare drawnWind speed

Example

Straight Line Shear Storm Splitting:700– R and L storm cellsmove with mean windbut drift outward1000850900500

Curved Hodograph Emphasizes one of the supercells– Veering (clockwise curve): right moving supercells warm air advection in northern hemisphere– Backing (counter clockwise curve): left moving supercells warm air advection in southern hemisphere7008509001000500300

Straight LineHodographCurvedhodograph

Helicity Can be thought of as a measure of the “corkscrew”nature of the winds.H velocity dotted with vorticity V ζ u (dyw - dzv) - v (dxw - dzu) w (dxv - dyu) Higher helicity values relate to a curvedhodograph.– large positive values-- emphasize right cell– large negative values-- emphasize left cells Values near zero relate to a straight linehodograph.

CAPE and Helicity Plainfield, IL tornado:CAPE 7000Helicity 165Energy Helicity:CAPE H )(EHI 160,000

Stability Indices

K Index This index uses the values for temperature (t) and dewpoint temperature (td), both in oC at several standard levels.K t850 - t 500 td850 - t700 td700K valueT-Storm Probability 150%15-20 20%21-2520-40%26-3040-60%31-3560-80%36-4080-90% 40 90%

Vertical TotalsVT T850 - T500 A value of 26 or greater is usuallyindicative of thunderstorm potential.

Cross TotalsCT T d850 - T500CTT-Storm Potential18-19Isolated to few moderate20-21scattered moderate, a few heavy22-23scattered moderate, a few heavy and isolated severe24-25scattered heavy, a few severe; isolated tornados26-29scattered to numerous heavy, few to scattered severe, a fewtornados 29numerous heavy, scattered showers, scattered tornadoes

Total Totals (TT)TT VT CT T850 T d850 - 2 T500TTT-Storm Potential44-45Isolated to few moderate46-47scattered moderate, a few heavy48-49scattered moderate, a few heavy and isolated severe50-51scattered heavy, a few severe; isolated tornados52-55scattered to numerous heavy, few to scattered severe, a fewtornados 55numerous heavy, scattered severe, scattered tornadoes

SWEAT (severe weather threat) IndexSWI 12D 20(T - 49) 2f8 f5 125(S 0.2)where: D 850mb dew point temperature (oC)(if D 0 then set D 0)T total totals (if T 49 then set entire term 0)f8 speed of 850mb winds (knots)f5 speed of 500mb winds (knots)S sin (500mb-850mb wind direction)And set the term 125(S 0.2) 0 when any of the following are not true1.850mb wind direction is between 130-2502.500mb wind direction is between 210-3103.500mb wind direction minus 850mb wind direction is positive4.850mb and 500mb wind speeds 15knots

SWEAT (severe weather threat) IndexSWI 12D 20(T - 49) 2f8 f5 125(S 0.2) 300Non-severe thunderstorms300-400Severe thunderstorms possible 400Severe thunderstorms, includingpossible tornados

Lifted Index (LI) Compares the parcel with the environmentat 500mb.LI (Tenv-Tparcel)500Lifted IndexThunderstorm Potential 20 to 2No convective activityShowers probable, isolatedthunderstorms possibleThunderstorms probable-2 to 0-4 to –2 -4Severe thunderstorms possibleSevere thunderstorms probable,tornados possible

Best Lifted Index– Uses the highest value of qe or qw in the lowertroposphere.– Use the highest mixing ratio value incombination with the warmest temperature. SELS Lifted Index– Use the mean mixing ratio and mean q of thelowest 100mb– If using a 12z sounding add 2o– Start parcel at 50mb above the surface

Showalter Index (SI) Compares a parcel starting at 850mb withthe environment at 500mb.SI (Tenv-Tparcel)500SIThunderstorm Possibility 3No convective activity1 to 3Showers probable, isolated thunderstorms possible-2 to 1Thunderstorms probable-6 to –2Severe thunderstorms possible -6Severe thunderstorms probable, tornados possible

Bulk Richardson NumberBRN CAPE½ (Uz2)Where Uz the vertical wind shear(averaged over 3-6km layer) In general: 15-40 favors supercell development 40 favors multicellular type storms Explains the balance between wind shear andconvective energy

Supercell Index Weights various parameters which areindicative of possible supercelldevelopment

Important Points to Remember Severe weather is more dependent ondynamical forcing than instability! No one parameter tells the full tale! 12z soundings usually predict afternoonconvection better than 00z soundingspredict evening convection.

Links p://avc.comm.nsdlib.org/cgi-bin/wiki.pl?Severe Weather /http://mocha.meteor.wisc.edu/table.12z.html

Level of Free Convection (LFC) The level above which a parcel will be able to freely convect without any other forcing. Find the LCL, then follow the moist adiabat until it crosses the temperatu