How do I create an OCEAN file for my CMAQ simulation?
The CMAQ OCEAN file is used by the CCTM to define surfzone and open ocean grid cells in the calculation of sea salt emissions. The file contains two variables: OPEN, and SURF. The OPEN variable defines open ocean cells. It represents the fraction of an ocean grid cell’s area that is further than 50 m from the shoreline. The SURF variable defines surf zone cells. It represents the fraction of an ocean grid cell’s area that is within 50m of the shore, not including bays and inlets.
The Spatial Allocator can be used to generate an OCEAN file. The alloc_surf_zone_to_oceanfile script in the Spatial Allocator distribution is an example of how to create an OCEAN file using the allocate mode of the software. The inputs to the script are an I/O API grid description file and a Shapefile that defines the surf zone. The Shapefile contains an integer attribute called Type. Type 2 = land and Type 3 = surfzone. The Surrogate Tool converts this Shapefile to a netCDF OCEAN file on the modeling grid defined in the grid description file. There is a surf zone Shapefile for North America packaged with the Spatial Allocator (see the Download Sample Data table in the Spatial Allocator 3.6 downloads area under the CMAS Center software clearinghouse).
The biggest issue for generating an OCEAN file outside of North America is the surf zone Shapefile. This file needs to be created for each new region of the world where CMAQ is being applied. There is no standarized process for creating this Shapefile. Basically you have to mask the locations in the Shapefile within 50m of the land boundary as the surfzone by assigning the attribute Type = 3 to these areas on the map. See the North American Shapfile as an example.