Parameters for LCC and Geogrid

[afernandezody]

Hello,
My questions are related to the parameters for a LCC projection. According to the User Guide, the ref_lat and ref_lon variables specify the latitude and longitude for the center of the coarse domain. Then, you also have truelat1 & 2 (standard latitudes) plus stand_lon (standard longitude), which are described as LCC parameters. However, the LCC decriptions that I have seen use 4 parameters. For example a proj4 string could read something like:
"+proj=lcc +lat_1=33.0 +lat_2=45.0 +lat_0=40.0 +lon_0=-97.0 +datum=WGS84 +no_defs"
Let me parse my questions:
1 - Are we supposed to use the variable ref_lat (from namelist.wps) as the standard latitude (or latitude of origin) for lat_0 in addition to being the center of the coarse domain?
2 - If so, and for a hypothetical case where ref_lon and stand_lon are different (e.g. ref_lon=-78.0, ref_lat = 40.0, stand_lon=-97 ), would that mean that the center of the domain can be displaced (versus the projection) in the east-west direction but not in north-south direction?
3 - Is it possible to use False_easting or False_northing with Geogrid?
Thanks.

 

[mgduda]

I'm not that well versed in map projections, but I can offer a few comments that might help:

1) Taking the WPS namelist option ref_lat to be the lat_0 value in the proj-string seems reasonable to me.
2) My guess is that the stand_lon namelist option should correspond to lon_0 in the proj-string. How to handle a ref_lon value that differs from a stand_lon value would probably depend on exactly what you're trying to do.
3) No, there is no option to specify a false easting or a false northing directly in geogrid.

Conceptually (and based on what I can remember from when we wrote the geogrid program some years ago) the setup of the coarse domain in geogrid (specifically, using a Lambert conformal conic projection), goes something like the following:

1) First, truelat1 and truelat2 are used to define the shape of the projection cone.
2) The stand_lon value is then used to define the meridian that is parallel to the y-axis, and the projection cone is split along the meridian opposite (+180 degrees) of the stand_lon.
3) The units on the projection are scaled by the nominal grid distance (dx and dy; from meters to grid cells, effectively).
4) A false easting and false northing equal to the negative of the coordinates of the south-west (i.e., the (1,1)) grid cell are applied, so that that projection coordinates (1,1) now locate the lower-left / south-west grid cell in the coarse domain.

So maybe it's almost the case that the ref_lat and ref_lon values imply a certain false easting and false northing?