R/download_chelsa.R

In EvolEcolGroup/pastclim: Manipulate Time Series of Palaeoclimate Reconstructions

#' Download the CHELSA modern observations.
#'
#' This function downloads monthly variables from the CHELSA 2.1 dataset.
#' These variables are saved in a format that can be read by
#' load_chelsa, and easily used for delta downscaling palaeoclimate
#' observations.
#'
#' Note that variables are named differently from WorldClim. "tas" is the mean
#' temperature ("tavg" in WorldClim), with "tasmax" and "tasmin" being equivalent
#' to "tmax" and "tmin".
#'
#' @param var	character Valid variables names are "tas", "tasmax","tasmin", and
#' "prec".
#' @param path	character. Path where to download the data to. If left NULL, the data
#' will be downloaded from the directory returned by [get_data_path()].
#' @returns TRUE if the requested CHELSA variable was downloaded successfully.
#'
#' @keywords internal

download_chelsa <- function(var, res, path = NULL, version = "2.1", ...) {
  if (!var %in% c("tas", "tasmax", "taxmin", "prec")) {
    stop('Valid variables names are "tas", "tasmax","tasmin", and "prec".')
  }

  if (is.null(path)) {
    path <- get_data_path()
  } else {
    if (!dir.exists(path)) {
      stop("the provided path does not exist")
    }
  }

  month_values <- sprintf("%02d", 1:12)
  chelsa_filenames <- paste("CHELSA", var, month_values, "1981-2010_V.2.1.tif", sep = "_")
  chelsa_urls <- paste0(
    "https://os.zhdk.cloud.switch.ch/envicloud/chelsa/chelsa_V2/GLOBAL/climatologies/1981-2010/tas/",
    chelsa_filenames
  )
  # create the destination directory if needed
  dir.create(file.path(tempdir(), "chelsa", var), showWarnings = FALSE, recursive = TRUE)
  # download the files
  curl::multi_download(chelsa_urls,
    destfile = file.path(tempdir(), "chelsa", var, chelsa_filenames)
  )
  # combined them into a single raster object
  combined_rast <- terra::rast(file.path(tempdir(), "chelsa", var, chelsa_filenames))
  # save it as netcdf file
  chelsa_nc_name <- paste0("chelsa21_30sec_", var, ".nc")
  terra::writeCDF(combined_rast, file.path(path, chelsa_nc_name),
    compression = 9
  )
}