cmsaf.mk.test {cmsafops}

Apply Mann-Kendall trend test.

Description

The function determines the trend from data of a single CM SAF NetCDF input file basing on a Mann-Kendall test.

Usage

cmsaf.mk.test(
  var,
  infile,
  outfile,
  nc34 = 4,
  overwrite = FALSE,
  verbose = FALSE,
  nc = NULL
)

Arguments

var

Name of NetCDF variable (character).

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 three data layers is written. One layer contains a measure for the significance of the calculated mann-kendall statistic (S). A very high positive value of S is an indicator of an increasing trend and a very low negative value indicates a decreasing trend. Another layer (Z) contains the calculated normalized test statsitic Z. A positive value of Z is an indicator of an increasing trend and a negative value indicates a decreasing trend.

See Also

Other temporal operators: cmsaf.detrend(), cmsaf.regres(), num_above(), num_below(), 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(10, 15, 0.5)
lat <- seq(50, 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(11, 11, 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)

## Determine the trend of the example CM SAF NetCDF file and write the
## output to a new file.
cmsaf.mk.test(var = "SIS", infile = file.path(tempdir(),
 "CMSAF_example_file.nc"), outfile = file.path(tempdir(),
 "CMSAF_example_file_mktrend.nc"))

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