Saakeskus - Collection of Thunderstorm Related Functions

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

AUTHORS

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