Recipe: R program as OPeNDAP client

Back to G5NR Data Access Guide.

Problem

By accessing the collection inst01hr_3d_T_Cv via the OPeNDAP server

http://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km

we want to read the surface temperature data inside the box bound by latitudes 25^oN, 50^oN and longitudes -130^oW, -65^oW for 2006/Sep/18, 9z, compute its min/max and plot the data at the surface.

Solution

The following code has been tested with R version 3.1.2.

The metadata for the collection inst01hr_3d_T_Cv is available at

http://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv.info

Code

This code accesses the collection inst01hr_3d_T_Cv from the OPeNDAP server and reads a subset of the temperature data (all levels inside the bounding box specified above) and computes its max/min. It then plots the data at the surface (level=72).

NOTE:

1. Instead of reading a downloaded NetCDF-4 file, we read an OPeNDAP URL.
2. While in the downloaded file, the temperature variable appears in the uppercase (T), on the OPeNDAP server, this variable is in lowercase.
3. Via the OPeNDAP URL, we now have access to all times for which data exists. The hourly inst files are available starting at 2005/15/15, 2200z. Our desired time, 2006/09/18, 0900z is then the 11772^th step.

# Load Libraries
library(ncdf4)
library(rworldmap)
# Url
url <- "http://opendap.nccs.nasa.gov:80/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv"
nc <- nc_open(url)
# Bounding Box
imin <- round((-130.+180.)/0.5)-1
imax <- round(( -64.+180.)/0.5)-1
jmin <- round((  25 +  90.)/0.5) - 1
jmax <- round((  50 +  90.)/0.5) - 1
im <- imax-imin+1
jm <- jmax-jmin+1
lm <- 72
# Start and Count vectors
offset <- c(imin, jmin, 1, 11771)
count <- c(im, jm, lm, 1)
t <- ncvar_get(nc,"t",start=offset,count=count)
# Statistics
print(summary(t))
# Plot Data
mapGriddedData(t[1:im,1:jm,71],xlim=c(-130.,-65.),ylim=c(25.,50.))

Output

Running this R script > source("g5nr.R")

we get the text output

  Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
 190.7   221.2   248.1   248.7   275.3   305.8 

and the plot

See Also

1. File Spec: File:G5NR-Ganymed-7km FileSpec-ON6-V1.0.pdf
2. Recipe: Fortran program to read data from downloaded file