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# icecdr

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The goal of icecdr is to download sea ice concentration data from the
NSIDC Climate Data Record.

## Installation

You can install the icecdr with:

``` r
install.packages("icecdr")
```

## Example

Use the `cdr_*` functions to download satellite-derived Antarctic or
Arctic sea ice concentration data at monthly or daily resolution. They
use version 5 by default but version 4 is also supported.

The basic usage downloads a NetCDF file in a temporary directory.

``` r
library(icecdr)

dates <- c("2020-01-01", "2023-01-01")
cdr_antarctic_monthly(dates, dir = "data")
#> Returning existing file.
#> [1] "data/995732f34533e77ab2bf32f847c84deb.nc"
```

By default, files are only downloaded if needed.

``` r
system.time(cdr_antarctic_monthly(dates, dir = "data"))
#> Returning existing file.
#>    user  system elapsed 
#>   0.031   0.000   0.031
```

There are four simple functions to download whole-domain data:

``` r
cdr_antarctic_daily()
cdr_antarctic_monthly()

cdr_arctic_daily()
cdr_arctic_monthly()
```

But the `cdr()` function exposes all arguments to download any crazy
subset

``` r
cdr(date_range = dates,
    # Data every 7 days
    date_stride = 7,
    resolution = "daily", 
    # Thin the grid by taking every other gridpoint
    xgrid_stride = 2,
    ygrid_stride = 2, 
    hemisphere = "north"
    )
```

The `cdr_fix()` function will fix the grid information to make it work
with [CDO](https://code.mpimet.mpg.de/projects/cdo) and standardise
variable names. This requires CDO installed in your system.

``` r
library(rcdo)
library(ggplot2)

extent_cdr <- cdr_antarctic_monthly(dates, dir = "data") |> 
  cdr_fix() |> 
  cdo_gtc(0.15) |>
  cdo_fldint() |> 
  cdo_execute() |> 
  metR::ReadNetCDF(c(extent = "aice")) 
#> Warning: Using CDO version 2.4.0 which is different from the version supported by this
#> version of rcdo (2.5.1).
#> ℹ This warning is displayed once per session.
#> Returning existing file.
#> No standard variable name found, returning unchanged file

extent_index <- sea_ice_index("south", "monthly", dir = "data") |> 
  data.table::fread() |>
  subset(time >= dates[1] & time <= dates[2])
#> Returning existing file.

extent_cdr |>
  ggplot(aes(time, extent)) +
  geom_line(aes(colour = "cdr")) +
  geom_line(data = extent_index, aes(colour = "sea_ice_index")) +
  labs(y = "Sea ice extent")
```

<img src="man/figures/README-unnamed-chunk-3-1.png" alt="" width="100%" />
