num_below {cmsafops}

Number of timesteps below a threshold.

Description

This function counts the number of timesteps below a certain threshold for each grid point of a dataset (x <= thld).

Usage

num_below(
  var,
  thld = 0,
  infile,
  outfile,
  nc34 = 4,
  overwrite = FALSE,
  verbose = FALSE,
  nc = NULL
)

Arguments

var

Name of NetCDF variable (character).

thld

Threshold (numeric).

infile

Filename of input NetCDF file. This may include the directory (character).

outfile

Filename of output NetCDF file. This may include the directory (character).

nc34

NetCDF version of output file. If nc34 = 3 the output file will be in NetCDFv3 format (numeric). Default output is NetCDFv4.

overwrite

logical; should existing output file be overwritten?

verbose

logical; if TRUE, progress messages are shown

nc

Alternatively to infile you can specify the input as an object of class ncdf4 (as returned from ncdf4::nc_open).

Value

A NetCDF file including the manipulated data fields of infile is written. Standard output precision is 'integer'.

See Also

Other temporal operators: cmsaf.detrend(), cmsaf.mk.test(), cmsaf.regres(), num_above(), num_equal(), timavg(), timmax(), timmean(), timmin(), timpctl(), timsd(), timsum(), trend_advanced(), trend()

Examples

## Create an example NetCDF file with a similar structure as used by CM
## SAF. The file is created with the ncdf4 package.  Alternatively
## example data can be freely downloaded here: <https://wui.cmsaf.eu/>

library(ncdf4)

## create some (non-realistic) example data

lon <- seq(5, 15, 0.5)
lat <- seq(45, 55, 0.5)
time <- seq(as.Date("2000-01-01"), as.Date("2010-12-31"), "month")
origin <- as.Date("1983-01-01 00:00:00")
time <- as.numeric(difftime(time, origin, units = "hour"))
data <- array(250:350, dim = c(21, 21, 132))

## create example NetCDF

x <- ncdim_def(name = "lon", units = "degrees_east", vals = lon)
y <- ncdim_def(name = "lat", units = "degrees_north", vals = lat)
t <- ncdim_def(name = "time", units = "hours since 1983-01-01 00:00:00",
 vals = time, unlim = TRUE)
var1 <- ncvar_def("SIS", "W m-2", list(x, y, t), -1, prec = "short")
vars <- list(var1)
ncnew <- nc_create(file.path(tempdir(),"CMSAF_example_file.nc"), vars)
ncvar_put(ncnew, var1, data)
ncatt_put(ncnew, "lon", "standard_name", "longitude", prec = "text")
ncatt_put(ncnew, "lat", "standard_name", "latitude", prec = "text")
nc_close(ncnew)

## Count the number of timesteps below a threshold of each grid point  
## of the example CM SAF NetCDF file and write the output to a new file.
num_below(var = "SIS", thld = 300, infile = file.path(tempdir(),
 "CMSAF_example_file.nc"), outfile = file.path(tempdir(),
 "CMSAF_example_file_num_below.nc"))

unlink(c(file.path(tempdir(),"CMSAF_example_file.nc"), 
 file.path(tempdir(),"CMSAF_example_file_num_below.nc")))