Extracting ArcGIS rasters from the HYCOM global ocean model's 4D netCDFs

THIS EXAMPLE IS UNDER CONSTRUCTION

The  Hybrid Coordinate Ocean Model, or HYCOM, is a sophisticated, high resolution system for simulating ocean physics. HYCOM is a set of equations refined over many years that describe the effects of the tides, winds, earth's rotation, and many other factors on the flow of water. Using supercomputers, the HYCOM team executes these equations at fine spatial and temporal scales to produce daily 3D snapshots of oceanographic variables such as temperature, salinity, and current velocity. At the time of this writing, HYCOM had been applied to global ocean simulations at 1/12º resolution and several ocean basins at higher resolution.

Advantages of HYCOM over satellite data

• HYCOM resolution is similar to satellite resolution. The 1/12º simulations have a cell size of approximately 8.9 km at the equator. This is not as good as the popular global SST products; NODC AVHRR Pathfinder 5.0 and MODIS have cell sizes of 4 to 5 km. But it is far better than the popular Aviso geostrophic currents product, which has a 37 km cell size.

• HYCOM images are cloud-free. Daily satellite images are often very, very cloudy.

• HYCOM is 3D. Satellite images only provide data for the ocean surface.

Disadvantages of HYCOM

• HYCOM is a model, not reality. While HYCOM has a high spatial resolution, is very sophisticated, has been under refinement for years, has been tested extensively against in situ measurements and satellite estimates, and uses assimilation to improve its accuracy, it is important to keep in mind that HYCOM is a model. If you have in situ or satellite data available, we recommend you compare it against HYCOM data and form your own opinion about whether HYCOM provides enough accuracy for your situation.
  
  Below is visual comparison of a relatively cloud-free GOES satellite SST image and a corresponding HYCOM SST image for the Gulf Stream. As you can see, the HYCOM image resembles the GOES image at a broad spatial scale, but the fine scale structure looks fairly different. Also, the HYCOM SST appears to be several degrees warmer than the GOES SST south of the Gulf Stream.
  
  These differences may or may not be important, depending on how the data are used. In showing this comparison, we make no recommendation one way or the other. We simply urge you to make your own comparison and decide for yourself. It is very difficult to model the fine scale structure and dynamics of the Gulf Stream, so it is not surprising to find fine scale differences. Better correspondance might be observed elsewhere, in less dynamic regions. Also, the HYCOM model shown here is a "free run" that simulated the global ocean without attempting to increase accuracy by assimilating in situ or satellite measurements. More recent HYCOM models that use assimilation will provide better results.

• HYCOM data are available for limited time ranges. At the time of this writing, global simulations were available from 2003 to the present, a north and equatorial Pacific simulation was avialble for 1979-2003, and a Gulf of Mexico simulation was availble from 2008 to the present.

• Global HYCOM simulations use a complicated coordinate system. As discussed below, global HYCOM simulations use three different coordinate systems, making it difficult to import these data into GISes as raster data. Most of the complexity of this example relates to this problem; please see below for details.

Downloading HYCOM data

HYCOM output resembles a time series of stacks of satellite images. At each time period, typically 1 day, there is one stack for each oceanographic variable. The stack represents the value of that variable at a series of depths, which typically are 0, 10, 20, 30, 50, 75, 100, 125, 150, 200, 250, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1750, 2000, 2500, 3000, 3500, 4000, 4500, 5000, and 5500 meters.

TO BE CONTINUED...