Access CCMP ocean surface winds

Downloads the CCMP wind analysis from Remote Sensing Systems and returns it as an sf point object with one row per grid cell and time step — the same shape accessEnvDat(), accessFVCOM() and accessHYCOM() return, so matchData() joins it unchanged.

Usage

accessCCMP(
  vars,
  years = NULL,
  months = NULL,
  bounding_box,
  dates = NULL,
  frequency = c("daily", "6hourly"),
  hour = 12L,
  version = "v03.1",
  overwrite = FALSE
)

Arguments

vars: variables to read, from ccmp_variables()
years: years to read. Required unless dates is given.
months: months to read. Required unless dates is given.
bounding_box: named list with xmin, xmax, ymin, ymax, or an sf/sfc object. Longitudes negative west.
dates: the exact dates to read, as YYYYMMDD strings, YYYY-MM-DD strings, or Date objects
frequency: "daily" (the default) for one snapshot per day, or "6hourly" for all four steps. Neither is a mean.
hour: which UTC hour to take when frequency = "daily". Must be 0, 6, 12 or 18.
version: which CCMP version to read, from ccmp_versions()
overwrite: re-read days already cached

Value

one row per grid cell per time step, with YEAR, MONTH, DAY, an HOUR column when frequency = "6hourly", and a column per requested variable

Why reach for it, and why not

CCMP is the longest consistent surface wind record available here. It runs from January 1993 to within days of the present, six-hourly throughout, which is both longer and finer in time than the Copernicus L4 wind — that is monthly from mid-1994, or hourly only from 2007, with nothing daily in between.

Against that: CCMP carries no wind stress. The Copernicus product does, and stress rather than speed is what drives mixing and Ekman pumping. Stress cannot be recovered from these winds without choosing a drag coefficient, which is a modelling decision rather than a unit conversion. Where stress is the covariate you want, use the Copernicus wind.

The two are different analyses of the same quantity, so a wind from CCMP is not interchangeable with one from Copernicus even though both arrive in a UWND column. Say which you used.

It downloads the whole globe

RSS publishes CCMP as static files with no OPeNDAP endpoint, so there is no way to ask for a region. A day is one 33 MB global file however small the bounding box, and the subset is taken locally. A year is therefore about 12 GB of transfer to keep a few megabytes of it.

The extracted subset is cached, so this is paid once per day of data. A request for more than 30 days says what it is about to download before starting.

Six-hourly, and no mean to fetch

CCMP is an analysis at 00, 06, 12 and 18 UTC. There is no daily or monthly mean in the archive, so none is invented:

frequency = "daily" (the default) takes one snapshot, at hour.
frequency = "6hourly" returns all four steps, with an HOUR column.

A real mean is upscale_time()'s job, which keeps it visible:

steps <- accessCCMP(vars = c("UWND", "VWND"), frequency = "6hourly",
                    dates = "2010-06-15", bounding_box = bb)
daily <- upscale_time(steps, to = "day")

Note that a mean of UWND and VWND is not a mean WSPD. Averaging the components and taking the magnitude gives the net displacement of air; averaging the speed gives how hard it blew. On a day the wind reversed, the first is near zero and the second is not.

Longitude convention

CCMP is stored on a 0-360 grid, alone among the sources here. bounding_box is given negative west as everywhere else in this package, converted on the way in, and the returned coordinates are negative west too — so the result overlays the other sources without adjustment.

Examples

if (FALSE) { # \dontrun{
bb <- list(xmin = -70, xmax = -66, ymin = 41, ymax = 44)

wind <- accessCCMP(vars = c("UWND", "VWND", "WSPD"),
                   dates = unique(observations$date), bounding_box = bb)

matched <- matchData(observations, wind)
} # }