Question about exact definition of CAM4 grid

[jfkok@ucar_edu]

Hello,<br /><br />I'm trying to match measurements taken at particular coordinates to the corresponding grid boxes for simulation results using CAM4. To do this, I need to know what exactly the numbers in the fields 'lat' and 'lon' in the output files correspond to, and I am unable to find any documentation on this. For instance, the first entry of lat and lon are -90 and 0, respectively. So I have two questions:

- For the longitude, I suspect the '0' corresponds to the western edge of the grid box. So in my 2.5x2 degree simulations, grid box 1 would span from 0 - 2.5 degrees longitude. Is that correct?

- The more puzzling question then concerns the latitude. The first entry for lat is -90, and the last one 90, with equally-spaced intervals in between. This seems to imply that the number refers to the poleward margin of the gridbox, so the northern margin for the NH, and the southern margin for the SH. Is this correct? If so, the problem then is that the two grid boxes right in the middle, -0.95 and +0.95 in the 2.5x2 degree simulation, must either be redundant, or half the size of the other grid boxes. Or is there something more complex going on?<br /><br />Thank you for your help with this!<br /><br />Jasper

 

[rneale]

All lat/lon physics grid points are assumed to be the central location of the gridpoint.The polar points are diffferent and as you say represents the area of half a grid point equatorward of the pole. The slat/slon points are the dynamics grid which is in quadrature with the lat/lon physics grid.