Definitions of the meteorological parameters
Total precipitable water is the depth of water in a vertical column of atmosphere which would be obtained if all the water vapor in that column were condensed to liquid. The unit of TPW is mm or kg/m2.
Layer precipitable water is the depth of water in a vertical column of atmosphere between two pressure levels which would be obtained if all the water vapor in that volume were condensed to liquid. The unit of LPW is mm or kg/m2.
K-index (George, 1960):
KI = (T(850) - T(500)) + TD(850) - (T(700) - TD(700)),
where T(x) is the temperature and TD(x) is the dew point temperature at x hPa pressure level. The unit of the K-index is °C.
The lifted and Showalter indices are defined as the difference between the temperature of an air parcel lifted up adiabatically to 500 hPa, and the temperature of its surroundings at 500 hPa. The indices have negative values when the lifted parcel arrives warmer than the environment, which indicates instability.
Showalter index (Showalter, 1947):
SHW = T(500) – T(lifted from 850 hPa to 500 hPa),
where T(500) is the temperature of the environment at 500 hPa pressure level.The air parcel is adiabatically lifted from 850 hPa to 500 hPa and its temperature is compared to that of the environment. The unit of the Showalter index is Kelvin or ºC.
Lifted index (Galway, 1956):
LI = T(500) – T(lifted from the ‘lowest 100hPa’ to 500 hPa),
where T(500) is the temperature of the environment at 500 hPa. The air parcel is lifted adiabatically from ‘inside’ the 100 hPa thick layer just above the surface to 500 hPa and its temperature is compared to that of the environment. The initial temperature and humidity of the virtual lifted parcel are the average temperature and humidity of this layer. The unit of the lifted index is Kelvin or ºC.
The definitions of K- and lifted indices are thoroughly explained in the ‘Global Instability Index’ EUMeTrain webcast ( http://www.eumetrain.org/resources/global_instability_index_2011.html ).
Note that for high mountain regions where the surface pressure is less than 850 hPa, BL, K- and Showalter indices are not defined.
| LI value | Stability |
|---|---|
| LI>0 | Stable |
| -3<= LI < 0 | Slightly Unstable |
| -6<=LI<-3 | Unstable |
| -9<=LI<-6 | Very Unstable |
| LI<-9 | Extremely Unstable |
Table 1 - Empirical relation between LI values (°C) and stability.
| Showalter value | Stability |
|---|---|
| SHW>0 | Stable |
| -3<= SHW < 0 | Slightly Unstable |
| -6<= SHW <-3 | Very Unstable |
| SHW <-6 | Extremely Unstable |
Table 2 - Empirical relation between SHW values (°C) and stability.
| K-index value | Thunderstorm probability |
|---|---|
| KI < 15 | ≈0% |
| 15 < KI <= 20 | < 20% |
| 20 < KI <= 25 | 20 – 40 % |
| 25 < KI <= 30 | 40 – 60 % |
| 30 < KI <= 35 | 60 – 80 % |
| 35 < KI <= 40 | 80 – 90 % |
| KI > 40 | > 90 % |
Table 3 - Empirical relation between KI values (°C) and thunderstorm probability.
Tables 1 and 2 show the empirically attained relation between the lifted index, Showalter index and stability. Table 3 shows the corresponding relation between the K-index and the probability of thunderstorms (Martinez, Romero and Li, 2012b). Note that weak convection is possible for positive Showalter index up to 2, or positive lifted index up to 3, if lifting is present ( http://www.crh.noaa.gov/lmk/soo/docu/indices.php). These thresholds are usually determined empirically and should not be regarded as fixed values – they may vary from season to season and region to region. A skilled local forecaster is required for a correct interpretation of the indices.