@BeginModule
@NewPage
@Name      = EcaGsl
@Title     = Thermal Growing season length index
@Section   = Climate indices
@Class     = Climate index
@Arguments = ifile1 ifile2 ofile
@Operators = eca_gsl

@BeginDescription
Let @file{ifile1} be a time series of the daily mean temperature TG, and @file{ifile2} be a land-water mask.
Within a period of 12 months, the thermal growing season length is officially defined as the number of days between:
@BeginItemize
@Item = frist occurrence of at least nday consecutive days with TG $>$ @math{T}
@Item = first occurrence of at least nday consecutive days with TG $<$ @math{T} within the last 6 months
@EndItemize
On northern hemispere, this period corresponds with the regular year, whereas on southern hemispere, it starts 
at july $1^{st}$. Please note, that this definition may lead to weird results concerning values TG = @math{T}: 
In the first half of the period, these days do not contribute to the gsl, but they do within the second half.
Moreover this definition could lead to discontinuous values in equatorial regions.

The numbers @math{nday} and @math{T} are optional parameter with default @math{nday} = 6 and @math{T} = 5@celsius. 
The number @math{fland} is an optional parameter with default value @math{fland} = 0.5 and denotes the fraction of 
a grid point that have to be covered by land in order to be included in the calculation. A further output variable 
is the start day of year of the growing season. Note that TG have to be given in units of Kelvin, whereas @math{T} 
have to be given in degrees Celsius.

The date information of a timestep in @file{ofile} is the date of the last contributing timestep in @file{ifile}.
@EndDescription
@EndModule


@BeginOperator_eca_gsl
@Title     = Growing season length index
@Parameter = [nday] [T] [fland]
@EndOperator


@BeginParameter
@Item = nday
INTEGER  Number of consecutive days (default: nday = 6)
@Item = T
FLOAT    Temperature threshold (unit: @celsius; default: T = 5@celsius)
@Item = fland
FLOAT    Land fraction threshold (default: fland = 0.5)
@EndParameter


@BeginExample
To get the growing season length of a time series of daily mean temperatures use:
@BeginVerbatim
   cdo eca_gsl tgfile maskfile ofile
@EndVerbatim
@EndExample
