The graphic shows the temperature anomaly in the US for the month of July (actually July 1 – 27). Parts of Kansas/Oklahoma experienced temperatures a full 4°C higher than normal over the month, while there was a cool anomaly in the Pacific Northwest.
The map uses an equal area projection (Albers) with horizontal and vertical scales in meters.
R aficionados will recognize use of the ggplot2 R package. We are experimenting with ggplot2 for weather map-making at Biospherica™ .
Data from CFSv2 Analysis.
Daily 2m temperature maps are available from operational CFSv2 here (tmp2m.l.gdas.201107.grib2). These correspond to initial conditions used by the CFSv2. To find a mean temperature for July, an average was taken over 4xdaily 6h forecast records from this grib2 file. There are probably many good ways to do this. One way is to first interpolate from gaussian to latlon grid, then use the wgrib2 -netcdf to produce files which can be read directly by R’s raster package. To get a temperature anomaly the monthly 2m temperature climatology needs to be subtracted. This is available here (flxf06.cfsr.fclm.m07.1982.2010.grb2).
The temperature anomaly raster map can be projected from latlon to another PROJ4 map projection using raster::projectRaster. The resulting July global temperature anomaly raster object is called tmp2m.ras. USA country maps (level 0 & 1) are from gadm. These are reprojected and called USA.poly & USA.poly1. Finally, with tmp2m.ras, USA.poly and USA.poly1 objects in R memory, ggplot2 was used to produce the map as follows:
x.min <- bbox(USA.poly)[1,1]
x.max <- bbox(USA.poly)[1,2]
y.min <- bbox(USA.poly)[2,1]
y.max <- bbox(USA.poly)[2,2]
tmp2m.ras <- crop(tmp2m.ras, extent(x.min,x.max,y.min,y.max))
#mask tmp2m.grd using country outline
USA.ras <- rasterize(USA.poly, tmp2m.grd)
tmp2m.ras <- mask(tmp2m.ras,USA.ras)
#convert data to dataframe for ggplot
tmp2m.ps <- rasterToPoints(tmp2m.ras)
df <- data.frame(tmp2m.ps)
colnames(df) <- c("easting","northing","tmp2m")
USA.poly <- fortify(USA.poly,region="ISO")
USA.poly1 <- fortify(USA.poly1,region="ID_1")
#now the pretty stuff
g <- ggplot(df,aes(x=easting,y=northing)) + scale_x_continuous(limits=c(x.min,x.max)) + scale_y_continuous(limits=c(y.min,y.max))
g <- g+geom_tile(aes(fill=tmp2m),alpha=0.6)+scale_fill_gradientn(name=expression(paste(degree,"C",sep="")),colours=rev(brewer.pal(9,"RdYlBu")))
g <- g+coord_equal()
g <- g+stat_contour(aes(z=tmp2m,colour=..level..),size=0.5, bins=4)+scale_colour_gradient(name=expression(paste(degree,"C",sep="")))
g <- g + geom_path(data=US.poly, aes(x=long,y=lat,group=group), colour="grey40", alpha=1.0,size=0.5)
g <- g + geom_path(data=USA.poly1, aes(x=long,y=lat,group=group), colour="grey80", alpha=1.0,size=0.3)
g <- g + opts(title = "USA Temperature July 2011")