R: Download and Cache an amsr File

download.amsr {oce}

R Documentation

Download and Cache an amsr File

Description

If the file is already present in destdir, then it is not downloaded again. The default destdir is the present directory, but it probably makes more sense to use something like "~/data/amsr" to make it easy for scripts in other directories to use the cached data. The file is downloaded with download.file(). Please read the ‘History’ section for important details on how download.amsr() and also read.amsr() have had be altered over the years, to deal with changes in the directory structure and file format on the server from which files are downloaded.

Usage

download.amsr(
  year = NULL,
  month,
  day,
  destdir = ".",
  server = "https://data.remss.com/amsr2/ocean/L3/v08.2",
  type = "3day",
  debug = 0
)

Arguments

`year`, `month`, `day`	a specification of the desired observation time. There are 3 choices for this specification. (a) If `year` is an object created by `as.Date()`, then that specifies the time, and so `month` and `day` are ignored. This scheme can be convenient for creating a sequence of images, starting at a particular date, because adding 1 to an object of class `Date` increases the time by 1 day, saving the user from having to know how many days are in any given month. (b) If `year` is an integer, then it is taken to be the year, and the user must also specify `month` and `day`, also integers. (c) If `year` is NULL (which is the default), then the focus is set to the most recent date, but this depends on the value of `type` (see next). If `type` is `"3day"`, `"daily"` or `"weekly"`, or just the first two of them if `type` is `"monthly"`. If these things are provided, then they just match exactly the values in the sought-after file on the remote server. If `year` is NULL, then `download.amsr()` constructs a URL that ought to be the most recent available file: 3 days prior to the present date (if `type` is `"3day"` or `"daily"`), the Saturday two weeks prior to the present date (if `type` is `"weekly"`), or two months in the past (if `type` is `"monthly"`).
`destdir`	A string naming the directory in which to cache the downloaded file. The default is to store in the present directory, but many users find it more helpful to use something like `"~/data/amsr"` for this, to collect all downloaded amsr files in one place.
`server`	A string naming the server from which data are to be acquired. See “History”.
`type`	character value indicating where to get the data. This may be `"3day"` (the default), for a composite covering 3 days of observation, which removes most viewing-path and cloud blanks, `"daily"` for a daily reading, `"weekly"` for a composite covering a week, or `"monthly"` for a composite covering a month. In the `"daily"` case, the data arrays are 3D, with the third dimension representing ascending and descending traces, but in all the other cases, the arrays are 2D.
`debug`	an integer specifying whether debugging information is to be printed during the processing. This is a general parameter that is used by many `oce` functions. Generally, setting `debug=0` turns off the printing, while higher values suggest that more information be printed. If one function calls another, it usually reduces the value of `debug` first, so that a user can often obtain deeper debugging by specifying higher `debug` values.

Value

download.amsr returns a character value holding the full pathname of the downloaded file.

History

Until 25 March 2017, the default server was "ftp.ssmi.com/amsr2/bmaps_v07.2", but this was changed when the author discovered that this FTP site had been changed to require users to create accounts to register for downloads. The default was changed to "http://data.remss.com/amsr2/bmaps_v07.2" on the named date. This site was found by a web search, but it seems to provide proper data. It is assumed that users will do some checking on the best source.

On 23 January 2018, it was noticed that the server-url naming convention had changed, e.g. ⁠http://data.remss.com/amsr2/bmaps_v07.2/y2017/m01/f34_20170114v7.2.gz⁠ becoming ⁠http://data.remss.com/amsr2/bmaps_v08/y2017/m01/f34_20170114v8.gz⁠

On 26 July 2023, it was noticed that the server-url naming convention had changed again, requiring not only the alteration of the default server value but also the addition of a new parameter named type. Worse yet – much worse – the file format is now changed from a gzipped format to a NetCDF format, and this will require a complete rewriting of read.amsr().

Sample of Usage

# The download may take up to about a minute.
f <- download.amsr(2023, 7, 27, destdir="~/data/amsr")
d <- read.amsr(f)
plot(d)
mtext(d[["filename"]], side=3, line=0, adj=0)

Author(s)

Dan Kelley