McIDAS User's Guide
Version 2015.2
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This appendix defines some of the sounding, parcel, and stability parameters output by the HODO, UALIST, and UAPLOT commands. The surface parcel that appears in several descriptions uses the mean temperature and mixing ratio from the boundary layer defined by the DEPTH keyword in the commands. The environment is defined as the sounding's measured conditions (temperature, dew point, wind, etc.) at the corresponding level. Parcels that are lifted adiabatically are lifted dry adiabatically until saturation, and moist adiabatically thereafter.
The parameters are listed alphabetically below with the abbreviation used by the commands in parenthesis.
The stability parameters defined here are:
Convective Available Potential Energy (aka, Positive Area) is the amount of energy available to a parcel as it freely rises between the Level of Free Convection (LFC) and the Equilibrium Level (EL). The CAPE is non-zero only if an LFC exists. The default units for CAPE are J/kg.
Convective Inhibition (aka, Negative Area) is the amount of energy that must be supplied to a parcel for it to rise to the Level of Free Convection (LFC). Convective Inhibition is non-zero only if an LFC exists. The default units for CIN are J/kg.
The Convective Temperature is the temperature to which a surface parcel must be heated to allow it to freely convect. The CVT is found by first computing a surface parcel's equivalent potential temperature (theta-E). This is followed by adiabatically lifting the parcel to the point where its theta-E equals the environment's theta-E. From this point (called the Convective Condensation Level), the parcel is brought down to the surface pressure dry adiabatically. The temperature at the surface pressure is the CVT. The default units for CVT are degrees Celsius.
The Dew Point Temperature is the temperature to which a parcel must be cooled (at constant pressure) for saturation to occur. The default units for TD are degrees Celsius.
The Equilibrium Level (aka, Equilibrium Pressure) is the pressure at which a rising saturated parcel encounters negative buoyancy. Above that pressure, the parcel is cooler than its surroundings. The default units for EL are mb.
The Equivalent Potential Temperature is the Potential Temperature a parcel would have if all its moisture were condensed out, liberating the latent heat. The default units for ThE are degrees Kelvin.
The Forecast Maximum Temperature is found by displacing a parcel at the top of the boundary layer to the surface pressure dry adiabatically. It's best used with morning soundings. The default units for FMAX are degrees Celsius.
Helicity is a measure of the storm-relative streamwise vorticity advection. It gives an indication of the low-level shear in the atmosphere. Large values of helicity are correlated with supercellular thunderstorm development. The default units for HELI are m**2/s**2.
The K-Index is
a measure of thunderstorm potential based on lapse rate and the vertical extent
of the moisture content in the lower atmosphere.
The Level of Free Convection is the level at which the temperature of a surface parcel that's lifted adiabatically first becomes warmer than the temperature of the environment. The default units for LFC are mb.
The Lifted Condensation Level is the level at which a surface parcel lifted dry adiabatically becomes saturated. The default units for LCL are mb.
The Lifted Index is a measure of thunderstorm potential. It is found by lifting a surface parcel adiabatically to 500 mb. The difference between the 500 mb temperature and the lifted parcel's temperature is the LI.
The Mixing Ratio is a measure of the amount of water vapor in the air. The default units for MIX are g/kg (grams of water vapor per kilogram of dry air).
The Potential Temperature is the temperature a parcel would have if moved dry adiabatically to 1000 mb. The default units for Th are degrees Kelvin.
Precipitable Water is the sum of average mixing ratios across pressure layers up to and including 300 mb, i.e., w(ave) * DP, where w(ave) represents the average mixing ratio across the pressure layer (DP). The data used is from the interpolated sounding seen with the following command: UALIST station OPT=ALL. The default units for PW are mm.
The Severe WEAther
Threat Index (aka, SWEAT Index) is a measure of thunderstorm potential. It
is a composite index that combines 850 mb moisture, wind speed and direction,
the Total Totals Index, 500 mb speed and direction, and wind shear.
The Showalter Index is a measure of thunderstorm potential. It is equal to the difference between the sounding's 500 mb temperature and the temperature of the sounding's 850 mb parcel lifted adiabatically to 500 mb.
The Total Totals
Index a measure of thunderstorm potential. It is equal to the sum of the Vertical
Totals (VT) and Cross Totals (CT).
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