Convective Addiction

Skew-T Indicies

Posted on 05. Mar, 2010 by Ryan in Education

If you have ever looked at a forecast sounding and wondered what all of those numbers and letters were on the side of the graph, we have your answer. They are indices which allow forecasters to take a quick glance at different aspects of the atmosphere without having to compute all of the formulas by hand.

While the parameters that are computed along side of a skew-t can be very helpful for a quick glance at the overall sounding, they must be used carefully. You first need to understand how the parameters are calculated and how they play into the overall state of the atmosphere.

For example, let’s say a sounding has nearly 4,000 j/kg of CAPE. Sounds great, right? However, upon further inspection you also have a CAP that cannot be overcome simply by daytime heating. By looking at more than just CAPE and doing a bit more inspecting, you could save yourself an embarrassing drive into blue skies.

The following is a list of many of the typical parameters you will find on a forecast or real-time sounding and what they mean, there are more than are listed here too. If you would like more or specific ones added, please contact Ryan.

FRZ: Freezing Height-The pressure level at which the environment reaches 0 Celsius. This height is useful to determine if the development and growth of hail is likely. It is also used frequently in aviation forecasting to assess if ice build up is likely on aircraft. To find the freezing level, follow the 0 degree Celsius isotherm until it intersects the parcel line.

PW: Precipitable Water Value-The amount of moisture in the column of air through the troposphere. The value is calculated by taking the mass of the water vapor and measuring its depth as if it was placed on the earth’s surface. A “high” PW value is not universal by place or even season.

In the Midwest for example, a PW value of 1.5” in the summer may lead to minor flooding but generally the ground and streams can handle the runoff. In the Winter the same value could lead to major flash flooding with a frozen ground and snow melt only adding more water to tributaries.

RH: Relative Humidity-The relative humidity parameter is not the RH at the surface but the average of the column of air up to 500mb. Typically a sounding with a low RH (50 percent or less) is stable, and oppositely, a sounding that is very moist (greater than 85 percent) signifies that precipitation is likely with any lifting mechanisms.

MAXT: Max Temperature-Estimation of the afternoon high temperature by ‘mixing out’ the Planetary Boundary Layer (PBL) through the day. It is computer derived but is relatively easy to calculate by hand. Take the temperature from a morning sounding and follow the environmental temperature up 150mb (usually around 830mb in the central plains). Take the temperature at that point and force it down the surface via the dry adiabat line. This method works best on days where it is expected to be mostly sunny to allow maximum mixing.

TH: Thickness-There are many different thicknesses that forecasters use but soundings will give the 1000-500mb thickness. A quick look at the thickness can yield a forecaster important insight into the overall state of the atmosphere. Especially in the winter when forecasters watch the 540 line, traditionally representing the rain/snow line. In the summer thicknesses over 580 can prove difficult for organized severe weather episodes to thrive. This is an especially important parameter to use in concert with other indices such as lapse rates and lifting indices.

LCL: Lifted Condensation Level-A common parameter used to determine the height of a cloud base, but it can be a bit deceiving. The LCL is the cloud base from a parcel lifted from a certain level. Many times the computer derived LCL is the LCL that occurs by lifting a parcel from within the PBL. It is important to note that low LCL’s (<1000m) have been studied to have strong relationships with tornadoes.

LI: Lifted Index-Used to determine low-level stability in the atmosphere. The formula is simply LI=500mb temp – 500mb parcel temp and is a crude but effective way to quickly glance at stability. Negative values mean there is instability in the lower atmosphere, with values near -10 exhibiting large amounts of instability.

SI: Showalter Index- Essentially the same as LI but the parcel is lifted from 850 mb rather than the surface. This gives the forecaster a better view of instability in the middle troposphere also known as ‘elevated instability’. Values that are calculated can be compared by the same legend listed with LI values. If both LI and SI are largely negative numbers then there is a large amount of instability throughout the atmosphere.

TT: Total Totals-This is the first variable discussed so far that can be used to determine the actual strength of a storm from a sounding. Below is the actual formula for TT but it is basically a combination of cross totals (CT) and vertical totals (VT), which were not listed here since they typically do not appear on soundings. TT values below 45 are not conducive to thunderstorm development but above 55 scattered severe t-storms are likely.

TT= (T850-500)+(Td850-T500)

**This parameter uses specific heights to determine a storms strength/potential, it is the forecasters responsibility to analyze the sounding to see if moisture is sitting just above or below the 850mb level or surrounding environments from the sounding. Minor changes can radically alter the value that is produced.

KI: K Index-As opposed to TT which assesses storm strength, KI assesses overall convective potential. Values below 25 mean there is a low chance for convection and a number above 40 mean there is a high chance for convection. KI and TT used together can paint a picture of if and how severe storms will be.

KI= (T850 – T500) + (Td850 – Tdd700)

*While KI can give the forecaster insight to convective potential, it is important to assess any atmospheric caps, which would limit convective initiation.

SW: SWEAT Index-Stands for Severe Weather Threat Index. A comprehensive index which encompasses winds and instability throughout the lower troposphere to determine severe and tornadic potential. Values less than 300 have a low chance of being severe (nearly no chance when less than 100) and values above 450 have a strong possibility of become tornadic.

SWEAT= 12(850Td) + 20(TT – 49) + 2(V850) + (V500) + 125(sin(dd500 – dd850) + 0.2)

*NOTE: For the SWEAT formula to work TT less than 49 must be set to 0 and any negative term must be set to zero.

EL: Equilibrium Level- This is the level that a parcel will no longer rise freely through the atmosphere (assuming the air parcel has reached the LFC first). The EL is where the environmental temperature line meets the parcel line at the top of the CAPE bounds. It serves as a good estimation as to the height of a thunderstorm, excluding any overshooting tops.

CAPE: Convective Available Potential Energy-A very familiar parameter to all storm chasers, this is the pure amount of instability in the atmosphere. The area (in j/kg) is the area on a sounding where a parcel of air would be warmer than the surrounding environment. CAPE is the amount of energy that is available, but that does not mean it is always used. If a CAP or other environmental limitations prevent a parcel from reaching the LFC, the CAPE will not be utilized.

CINH: Convective Inhibition-Another well known parameter to chasers, this is the negative amount of buoyancy in the lowest levels of the atmosphere. Essentially CINH is the amount of a cap in the atmosphere. When values reach less than 50j/kg the CINH might be overcome shortly with some lifting mechanism present. Anything above 75 is considered a strong cap.

CAP: Cap-A true Cap is a warm wedge of air aloft that prevents deep convection from occurring. Simply put, a cap is the largest difference between the environmental temperature and a parcel temperature, when the parcel temperature is cooler than the environment. A strong cap is 2 degrees Celsius and a cap less than 1 degree is weak.

LFC: Level of Free Convection-The pressure level when an air parcel becomes as warm as the surrounding environment. This will represent the bottom intersection of your CAPE area and since the air parcel can rise buoyantly on its own, it will rise to the top of the CAPE area (EL). A lower LFC, as is also the case with LCL, has been linked to stronger and more frequent tornadoes in severe convective situations.

STM: Storm Motion and Speed-To determine what motion a storm will move and how fast (on average) it will be traveling, use the STM parameter. The direction will be in degrees and speed in knots. For example: STM: 145/55 would equate to a storm moving to the Southeast and moving 55knots.

HEL: Helicity-Combines speed shear and directional shear with height (or turning with height). This parameter may have a 1 or 3 next to it, which would indicate Helicity from 0-1km or 0-3km respectively. Values less than 300 indicate there is enough shear to maintain a supercell but higher values are strongly linked to tornadoes when deep and discrete supercells develop. Helicity tends to be maximized near fronts, strongly backed winds associated with low pressures, or along outflow boundaries.

EHI: Energy Helicity Index-This index was originally developed to combine the CAPE and Helicity values to create one useful parameter. To date it is one of the best parameters to predict where tornadoes are likely, when and if convection occurs. An EHI value above 1 indicates that tornadoes are possible and values above 4 indicate strong tornadoes are possible. Since this parameter only looks at Helicity and CAPE, a forecaster needs to examine the sounding for possible negative convective potentials, such as a strong CAP or EML. Extreme Helicity or CAPE in the absences of the other parameter can also skew the EHI value.

EHI = (CAPE * SR HEL) / 160,000

BRN: Bulk Richardson Number-Another parameter that combines CAPE and shear. The shear portion uses the average shear value between the surface and 500mb with the total amount of CAPE through the sounding used as well. The BRN value can help the forecaster compare when there is too much CAPE compared to shear and vise versus. A number in the teens is perfect for supercell development, while a number less than 10 indicates there is too much CAPE for the amount of shear in the atmosphere. A number greater than 50 indicates that there is too much shear for the amount of CAPE in the atmosphere.

BRN= CAPE / (0.5*(shear differential)^2)

-Ryan Wichman