Skip to contents

Download hourly wind data from NCAR

Source: R/ncar.R
download_wind_data.Rd

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

Usage

download_wind_data(
  source = c("era5", "cfsr", "cfsv2"),
  level = "10m",
  xlim,
  ylim,
  years,
  months = 1:12,
  days = NULL,
  hours = NULL,
  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.

days

Integer vector of days of the month (1-31) to download, by the UTC date of each time step. NULL (the default) downloads all days. Months with none of the requested days (e.g. day 31 in a 30-day month) are skipped.

hours

Integer vector of hours of the day (0-23, UTC) to download. NULL (the default) downloads all hours; for example, hours = seq(0, 21, 3) downloads every third hour, which reduces download time and file size about threefold. Days and hours refer to the UTC date and time of each time step, within each requested month. (CFSR and CFSv2 monthly files hold hourly forecasts valid from 01:00 on the first through 00:00 on the first of the following month. With days or hours, time steps are assigned to their calendar month, so 00:00 on the first comes from the previous month's file; without them, each month's file is downloaded as is, as in the pre-built roses of download_wind_rose().)

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). Files for a subset of days or hours have names ending in a tag recording the selection (e.g. _d28_h23), so they are cached separately from whole months.

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; selecting hours with hours (e.g. every third hour) reduces both. The server limits the size of each request (about 100 MB), so large requests are split into several, by time, and reassembled.

Requires the ncdf4 package.

See also

wind_series() to load the files; wind_rose() to summarize them; download_land_mask() to download a matching land-water layer; download_wind_rose() for pre-built CFSR wind roses, which need no wind data downloads.

Examples

if (FALSE) { # interactive() && requireNamespace("ncdf4", quietly = TRUE)
# every third hour of 10 m ERA5 wind for summer 2020, Pacific Northwest
files <- download_wind_data("era5", xlim = c(-125, -115), ylim = c(42, 49),
                            years = 2020, months = 6:8, hours = seq(0, 21, 3))
rose <- wind_rose(wind_series(files))

# a single hour: Hurricane Katrina, 2005-08-28 23:00 UTC
f <- download_wind_data("era5", xlim = c(-99, -78), ylim = c(17, 35),
                        years = 2005, months = 8, days = 28, hours = 23)
katrina <- wind_field(wind_series(f))
}