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Skew T Log P Diagram AOS 330 LAB10 Outline • • • • Identify different air masses Buoyancy Stability Assessment Freezing Level Air Masses Air Masses We can also identify air masses from sounding data. We do this by looking for features characteristic of certain environments. For example, in arctic regions we generally see persistent radiational cooling, especially in winter (Why?) This often produces a very deep radiation inversion that can extend from the surface to 700mb or so. Very cold surface temperature and a deep isothermal or inverted surface layer characterizes the arctic air mass, as we see in the next slide. Arctic Air Mass Now, let’s go to the other extreme. The Tropics! In particular, the tropical oceans and embedded landmasses. The tropical maritime air mass is typified by the following: 1. A warm, moist boundary layer 2. A subsidence inversion in the mid levels (usually around 700mb). 3. An approximately psuedoadiabatic lapse rate over a deep layer We see a typical sounding in the following slide. Tropical Maritime Air Mass What might we expect to see in the tropics from time to time? And for that matter, what about in the midwest during severe weather season? Yep, thunderstorms! Thunderstorms are often called “deep convection” and a hallmark of deep convection is a pseudoadiatic temperature profile over the depth of the troposphere (or nearly so) and near saturation conditions throughout. Note these features in the following slide. Deep Convection Mixed Layers A mixed layer is produced by turbulence, which tends to mix conservative tracers such as potential temperature and momentum. Moisture is also mixed, although it may not be mixed uniformly (often there may be a slight decrease with height). The most common mixed layer in the atmosphere is the planetary boundary layer (PBL), which normally occupies the lower kilometer or two of the atmosphere. This feature is normally most well-defined in the late Afternoon. In fact, at other times of day, it may not be mixed at all. Why? The Elevated Mixed Layer Another type of mixed layer that we’ll be interested in is the elevated mixed layer. This feature typically forms over high terrain (the Rockies, the Mexican Plateau) during the spring and summer. It is VERY important in the severe weather process (it provides a “cap” to the boundary layer). Not all layers are mixed.... ...and if they’re not, they’re known as “stratified layers.” We usually think of stratified layers as those in which the potential temperature and moisture have significant vertical gradients (i.e. they cut across adiabats and mixing ratio lines at high angles). Let’s take a look at a sounding which depicts these features. Layers and Layers and Layers What suggests a layer of cloud between 650 and 750 mb? Buoyancy Buoyancy • Archimedes’s Principle – Upward force exerted on object that is immersed in a fluid is equal to the weight of the displaced fluid. – Buoyant force FB (enV M)g (en )gV M V Buoyancy of air parcels FB ( en )gV en fB g M V P Rd Tv P en Rd Tv,en T T f B v v,en g Tv,en Stability Indices Stability Indices • Stability indices usually can be calculated using temperatures (and dew point temperatures) from a few mandatory levels from a radiosonde sounding. • Purpose: to obtain a number to provide some measure of the overall stability of the atmosphere. to evaluate the potential for severe weather / predict thunderstorms to occur However… • Since they are only calculated using a few levels of the sounding, many details on the soundings are not accounted for. • Sounding is a snapshot of the atmosphere. Stability indices calculated from sounding would not tell us how the atmosphere is going to evolve later on. Lifted Index (LI) • Compares the parcel with the environment at 500mb. • Takes into account both near surface moisture content and static stability LI = (Tenv-Tparcel)500 Lifted Index Thunderstorm Potential >+2 No convective activity 0 to +2 Showers probable, isolated thunderstorms possible -2 to 0 Thunderstorms probable -4 to –2 Severe thunderstorms possible < -4 Severe thunderstorms probable, tornados possible Lifted Index (LI) Lifted Index Atmospheric Stability >+3 Stable 0 to +3 Weak convection possible if provided strong lifting -3 to 0 Marginally unstable -6 to –3 Very unstable < -9 Extremely unstable Petty (2008) • Best Lifted Index – Uses the highest value of qe or qw in the lower troposphere. – Use the highest mixing ratio value in combination with the warmest temperature. • SELS (Severe Local Storms) Lifted Index – Use the mean mixing ratio and mean q of the lowest 100mb – If using a 12z sounding add 2o – Start parcel at 50mb above the surface Showalter Index (SI) • Compares a parcel starting at 850mb with the environment at 500mb. • Tells us instability aloft other than from near surface. SI = (Tenv-Tparcel)500 SI Thunderstorm Possibility > +3 No convective activity 1 to 3 Showers probable, isolated thunderstorms possible, need to provide it some sources of lifting -2 to 1 Thunderstorms probable (but generally weak) -6 to –2 Severe thunderstorms possible < -6 Severe thunderstorms probable, tornados possible Vertical Totals VT = T850 - T500 • Account for static stability in between 850 to 500 hPa. • A value of 26 or greater is usually indicative of thunderstorm potential. Cross Totals CT =T d850 - T500 • Account for 850hPa low level moisture. CT T-Storm Potential 18-19 Isolated to few moderate 20-21 scattered moderate, a few heavy 22-23 scattered moderate, a few heavy and isolated severe 24-25 scattered heavy, a few severe; isolated tornados 26-29 scattered to numerous heavy, few to scattered severe, a few tornados >29 numerous heavy, scattered showers, scattered tornadoes Total Totals (TT) TT = VT + CT = T850 + T d850 - 2 T500 • Not as useful when all low level moisture lies below 850 hPa TT T-Storm Potential 44-45 Isolated to few moderate 46-47 scattered moderate, a few heavy 48-49 scattered moderate, a few heavy and isolated severe 50-51 scattered heavy, a few severe; isolated tornados 52-55 scattered to numerous heavy, few to scattered severe, a few tornados >55 numerous heavy, scattered showers, scattered tornadoes K Index K = T850 - T 500 + Td850 – (T700 - Td700) • Takes into account of the vertical distribution of moisture and temperature. • 700 hPa moisture, which is important for air mass thunderstorms development. K value Air mass T-Storm Probability <15 0% 15-20 <20% 21-25 20-40% 26-30 40-60% 31-35 60-80% 36-40 80-90% >40 >90% SWEAT (Severe Weather thrEAT) Index SWI = 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 true 1. 850mb wind direction is between 130-250 2. 500mb wind direction is between 210-310 3. 500mb wind direction minus 850mb wind direction is positive 4. 850mb and 500mb wind speeds > 15knots SWEAT (severe weather threat) Index SWI = 12D + 20(T - 49) + 2f8 + f5 + 125(S + 0.2) <300 Non-severe thunderstorms 300-400 Severe thunderstorms possible >400 Severe thunderstorms, including possible tornados Bulk Richardson Number BRN = CAPE ½ (Uz2) Where Uz = the vertical wind shear (averaged over 0.5-6km layer) • In general: 15-40 favors supercell development >40 favors single / multicellular type storms • Explains the balance between wind shear and buoyancy strength Supercell Index • Weights various parameters which are indicative of possible supercell development Convective Available Potential Energy (CAPE) • Amount of buoyant energy available as parcel accelerate upward. • It contains information about the overall stability of the atmosphere. • Does not only based on a few levels, but the whole profile in the calculation. CAPE value <0 0 - 1000 1000 - 2500 2500 – 3500 3500 – 4000 Atmospheric Stability Stable Marginally unstable Moderately unstable Very unstable Extremely unstable CAPE q parcel qenv g LFC dz qenv EL wmax 2CAPE Convective Inhibition (CIN) • The amount of energy required to lift the parcel from surface to LFC. • > 200 Jkg-1 (strongly inhibit convective potential) Important Points to Remember • Severe weather is more dependent on dynamical forcing than instability! • No one parameter tells the full tale! • 12z soundings usually predict afternoon convection better than 00z soundings predict evening convection. Freezing Level • Lowest level on a sounding in which 0 deg Celsius is found. • Follow 0 deg C isotherm up, until it reaches the and crosses the temperature profile. Reference • Petty, G (2008). A First Course in Atmospheric Thermodynamics, Sundog Publishing. • Potter and Coleman, 2003a: Handbook of Weather, Climate and Water: Dynamics, Climate, Physical Meteorology, Weather Systems and Measurements, Wiley, 2003 Links • http://www.theweatherprediction.com/severe/indices/ • http://www.theweatherprediction.com/habyhints/315/ • http://www.spc.noaa.gov/exper/mesoanalysis/ • http://mocha.meteor.wisc.edu/table.12z.html