@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
DOUBLE   Temperature threshold (degree Celsius, default: T = 5° Celsius)
@Item = fland
DOUBLE   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
