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Download hourly wind data from NCAR

ncar_download.Rd

Downloads gridded hourly wind data from the NCAR Geoscience Data Exchange (GDEX) for a bounding box and a set of months, saving one GeoTIFF file per month. No account is needed. Data are clipped to the bounding box on the server, so only the requested region is transferred.

Usage

ncar_download(
  source = c("era5", "cfsr", "cfsv2"),
  level = "10m",
  xlim,
  ylim,
  years,
  months = 1:12,
  time_stride = 1,
  dir = tempdir(),
  overwrite = FALSE,
  quiet = FALSE
)

Arguments

source

Data set to download:

  • "era5" (the default): ECMWF ERA5 reanalysis, 0.25 degree grid, 1940 to present (NCAR data set d633000).

  • "cfsr": NCEP Climate Forecast System Reanalysis, ~0.31 degree Gaussian grid, 1979 to 2010 (d093001).

  • "cfsv2": NCEP Climate Forecast System version 2, the operational continuation of CFSR, ~0.2 degree Gaussian grid, 2011 to present (d094001).

level

Height of the wind data. "10m" (the default) is wind 10 m above the ground, available for all sources. "100m" is available for ERA5 only. Pressure levels "1000hPa", "850hPa", "700hPa", "500hPa", and "200hPa" are available for CFSR and CFSv2 only.

xlim

Numeric vector of length 2 giving the western and eastern limits of the bounding box, in degrees. Longitudes can be given either in the -180 to 180 range or the 0 to 360 range, and output files use the same convention. Boxes crossing the prime meridian (in -180 to 180 coordinates, e.g. c(-10, 30)) or the antimeridian (in 0 to 360 coordinates, e.g. c(170, 200)) are supported.

ylim

Numeric vector of length 2 giving the southern and northern limits of the bounding box, in degrees, in the range -90 to 90.

years

Integer vector of years to download.

months

Integer vector of months (1-12) to download within each year. Defaults to all months.

time_stride

Integer. Download every time_stride-th hourly time step, starting with each month's first time step. The default, 1, downloads all hours; for example, 3 downloads every third hour and reduces download time and file size about threefold.

dir

Directory where monthly files are saved. Defaults to a temporary directory, which is deleted at the end of the R session; set it to a permanent location to keep the data and make use of caching across sessions.

overwrite

Logical. If FALSE (the default), months already downloaded to dir are skipped.

quiet

Logical. If TRUE, progress messages are suppressed.

Value

A character vector of file paths, one per month, in chronological order (returned invisibly if all files were already cached).

Details

Each output file holds one month of data in wind_series layout: all u layers followed by all v layers, named like "u 2005-08-28 23:00:00" (UTC), in m/s, oriented to true east and north, on a longitude/latitude grid. Combine the files into one series with wind_series(), which keeps the data on disk, and summarize it with wind_rose().

Downloads are cached: a month whose file already exists in dir is not downloaded again unless overwrite = TRUE, so an interrupted download can be resumed by rerunning the same call. Large requests (many years, or a large region) can take a long time and use substantial disk space; thinning hourly data with time_stride reduces both.

Requires the ncdf4 package.

See also

wind_series() to load the files; wind_rose() to summarize them; ncar_land() to download a matching land-water layer.

Examples

if (FALSE) { # \dontrun{
# every third hour of 10 m ERA5 wind for summer 2020, Pacific Northwest
files <- ncar_download("era5", xlim = c(-125, -115), ylim = c(42, 49),
                       years = 2020, months = 6:8, time_stride = 3,
                       dir = "~/wind_data")
ws <- wind_series(files)
rose <- wind_rose(files)
} # }