pressure = NCEPatm_2(site_lat,site_lon,site_elv)
This function converts elevation (m) to atmospheric pressure (hPa) using the pressure-elevation relationship in the ICAO Standard Atmosphere:
Where is the molar weight of air, the acceleration due to gravity, and the gas constant, giving 0.03417 K m. The adiabatic lapse rate is taken to be 0.0065 K m.
The sea level pressure (hPa) and the sea level temperature K are obtained by interpolating the sample location onto global grids of annual mean sea level pressure and annual mean 1000 mbar temperature generated by the NCAR-NCEP reanalysis:
This function performs reasonably well for the deep southern latitudes, but it's not recommended for this purpose. The function
antatm.m (which is fit to actual station measurements) should do a much better job.
There is more discussion of how well this atmosphere approximation performs against actual station measurements in the main text of the paper. In particular see Figures 1 and 2.