Saakeskus - Collection of Thunderstorm Related Functions

- NAME
- SYNOPSIS
- DESCRIPTION
- COMMAND LINE HELP
- PROVIDED FUNCTIONS

- dewpt(T,RH)
- ept(T,RH.P)
- epi(T850,RH850,T500,RH500)
- plcl(T2M,RH2M,PS)
- shear(U1,V1,U2,V2)
- srh1km(uVAR,vVAR)
- srh3km(uVAR,vVAR)
- tlcl(T2M,RH2M) - Temperature at LCL
- tsindex(CAPE180_0mb,CIN180_0mb)
- ttindex(T500,T850,RH850)
- wchill(T2M,U10M,V10M)

- EXAMPLES

- Computing dew-point temperature
- Wind chill
- Equivalent potential temperature at 850 hPa
- Potential Instability
- Pressure at LCL
- Total Totals Index
- Storm Relative Helicity
- Low Level Shear
- Schlenczek's Thunderstorm Index

- SEE ALSO
- AUTHORS
- COPYRIGHT

# NAME

Saakeskus - Collection of Thunderstorm Related Functions

# SYNOPSIS

  display dewpt(T,RH) - Dew-point Temperature

  display ept(T,RH,P) - Equivalent potential temperature

  display epi(T850,RH850,T500,RH500) - Potential Instability

  display plcl(T2M,RH2M,PS) - Pressure at LCL

  display shear(U1,V1,U2,V2) - Wind Shear

  display srh1km(uVAR,vVAR) - Storm-relative Helicity 0-1km

  display srh3km(uVAR,vVAR) - Storm-relative Helicity 0-3km

  display tlcl(T2M,RH2M) - Temperature at LCL

  display tsindex(CAPE180_0mb,CIN180_0mb) - Likelihood of T-storm Initiation

  display ttindex(T500,T850,RH850) - Total Totals Index

  display wchill(T2M,U10M,V10M) - Wind Chill Index (New)

# DESCRIPTION

This library implements the Suomen Saakeskus (Finnish Weathercenter) collection of Thunderstorm related functions originally developed by Stefan Gofferje as described in the following website:

http://www.saakeskus.fi/index.php

The majority of this expressions are from internet sources like the Wikipedia. Special thanks go to Oliver Schlenczek and Eerik Saarikalle invaluable help.

Here we implemented these functions by means of of gsUDF's, User Defined Functions written in the GrADS scripting language. This device requires the OpenGrADS extension b<gsUDF> documented at

http://opengrads.org/doc/udxt/gsudf

# COMMAND LINE HELP

Usage notice for most of these functions can be obtained at the GrADS command line. For example,

     ga-> display tlcl

produces

     Purpose: Temperature at Lifiting Condensation Level (LCL) [K]
     Usage:   display tlcl(t2m,in_rh2m)
              t2m = temperature at 2m [K]
              rh2m = relative humidity at 2m [%]

# PROVIDED FUNCTIONS

## dewpt(T,RH)

This function computes the dew-point temperature given the Temperature T in Kelvin and the relative humidity RH in percent.

## ept(T,RH.P)

This function computes the equivalent potential temperature in Kelvin given the Temperature T in Kelvin, the relative humidity RH in percent, and the pressure P in hPa.

## epi(T850,RH850,T500,RH500)

This function computes the Potential Instability (in Kelvin) given the temperature T (Kelvin) and relative humidity RH (percent) at 850 and 500 hPa. Low values of this index correspond to unstable areas.

## plcl(T2M,RH2M,PS)

This function computes the pressure at the Lifiting Condensation Level (in hPa or Pa) given the temperature T (Kelvin) and relative humidity RH (percent) at 2m above the ground, and the surface pressure PS (hPa or Pa).

## shear(U1,V1,U2,V2)

This function computes the magnitude of wind shear vector given the zonal (U) and meridional (V) wind components at 2 different vertical levels. The shear will be returned at the same units as the wind components, typically m/s.

## srh1km(uVAR,vVAR)

This function returns the Storm-relative Helicity at the lowest 1km of the atmosphere. On input, the name of the GrADS variable corresponding to the zonal (uVAR) and (vVAR). These variables must be defined at the following vertical levels: 1000, 950, 900, 850, 800, 750, 700, 650, 600, 550, 500, 450 and 400 hPa.

## srh3km(uVAR,vVAR)

This function returns the Storm-relative Helicity at the lowest 3km of the atmosphere. On input, the name of the GrADS variable corresponding to the zonal (uVAR) and (vVAR). These variables must be defined at the following vertical levels: 1000, 950, 900, 850, 800, 750, 700, 650, 600, 550, 500, 450 and 400 hPa.

## tlcl(T2M,RH2M) - Temperature at LCL

This function computes the temperature at the Lifiting Condensation Level (Kelvin) given the temperature T (Kelvin) and relative humidity RH (percent) at 2m above the ground.

## tsindex(CAPE180_0mb,CIN180_0mb)

This function computes the thunderstorm initiation index given the convective available potential energy CAPE and the convective inhibition parameter CIN at 180 hPa above ground, both in J/Kg. The thunderstorm likelihood can be determined from the value of this index:

        < -1     thunderstorm unlikely
       [-1,0)    thunderstorm possible
       (0,1]     thunderstorm likely
         > 1     thunderstorm very likely

## ttindex(T500,T850,RH850)

This function computes the Total Totals Index given the temperature T at 500 and 850 hPa, and the relative humidity RH at 850 hPa. The TT (Total Totals) is an index used to assess storm strength:

      <44     Convection not likely
      44-50   Likely thunderstorms
      51-52   Isolated severe storms
      53-56   Widely scattered severe
      >56     Scattered severe storms

## wchill(T2M,U10M,V10M)

This function computes the Wind Chill Temperature given the temperature at 2m and the zonal (U) and meridional (V) wind components at 10m above ground. This function uses the new formula adopted by the U.S. NWS in 2001.

# EXAMPLES

The following examples are based on forecasts from NCEP's Global Forecast System (GFS) available through the NOMAD OpenDAP server. Typically one would open such a dataset with a command like

     ga-> sdfopen http://nomads.ncep.noaa.gov:9090/dods/gfs_hd/gfs_hd20090331/gfs_hd_18z
     ga-> set gxout shaded

For comparison with the Suomen Saakeskus (Finnish Weathercenter) charts, select the following region/map parameters:

     ga-> set mproj nps
     ga-> set mpvals -10 51 48 80
     ga-> set lon -60 100
     ga-> set lat 43 90
     ga-> set mpdset hires

## Computing dew-point temperature

     ga-> d dewpt(tmp2m,rh2m)-273.16
     ga-> draw title Dew Point Temperature at 2m [C]
     ga-> cbarn

## Wind chill

     ga-> d wchill(tmp2m,ugrd10m,vgrd10m)-273.16
     ga-> draw title Wind Chill [C]

## Equivalent potential temperature at 850 hPa

     ga-> d ept(tmpprs(lev=850),rhprs(lev=850),850)-273.16
     ga-> draw title Equivalent Potential Temperature at 850 hPa [K]

## Potential Instability

     ga-> d epi(tmpprs(lev=850),rhprs(lev=850),tmpprs(lev=500),rhprs(lev=500))
     ga-> draw title Potential Instability [K]

## Pressure at LCL

     ga-> d plcl(tmp2m,rh2m,pressfc/100)
     ga-> draw title Lifiting Condensation Level [hPa]

## Total Totals Index

     ga-> d ttindex(tmpprs(lev=850),rhprs(lev=850),tmpprs(lev=500))
     ga-> draw title Total Totals Index

## Storm Relative Helicity

     ga-> d srh3km(ugrdprs,vgrdprs)
     ga-> draw title Storm Relative Helicity 3km

## Low Level Shear

     ga-> d shear(ugrdprs(lev=900),vgrdprs(lev=900),ugrd10m,vgrd10m)
     ga-> draw title Low Level Wind Shear: 900 hPa - 10m

## Schlenczek's Thunderstorm Index

     ga-> d tsindex(cape180_0mb,cin180_0mb)
     ga-> draw title Likihood of T-Storm Initiation

# SEE ALSO

- http://www.saakeskus.fi/index.php - Suomen Sääkeskus (Finnish Weathercenter)

- http://cookbooks.opengrads.org/index.php - A recipe demonstrating the use of these functions.

- http://opengrads.org/ - OpenGrADS Home Page

- http://opengrads.org/wiki/index.php - OpenGrADS User Defined Extensions

- http://www.iges.org/grads/ - Official GrADS Home Page

# AUTHORS

- Stefan Gofferje <stefan@saakeskus.fi> - original scripts

- Arlindo da Silva <dasilva@opengrads.org> - implementation as gsUDFs

# COPYRIGHT

These scripts have been placed in the Public Domain. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Saakeskus - Collection of Thunderstorm Related Functions