18.2 Options for constructing the KMT field

Various ocean depth construction methods may be selected for use within MOM by enabeling options described below. Before being used, realistically derived depth fields must first be interpolated to the latitude and longitude coordinates for the domain defined by module grids. After the interpolation, ocean depth within the model is approximated as the nearest number of discrete vertical levels that comes closest to matching the interpolated ocean depth field. If option partial_cell is enabled, the deepest ocean cells have variable depth so the model ocean depth matches the interpolated ocean depth exactly. Refer to Chapter 26 for a discussion of partial cells. The resulting field of model levels is the base field defined at all latitude and longitude coordinates given by (xt_i,yt_jrow). One (and only one) of the following options for selecting the various forms of ocean depth fields must be enabled:

•  rectangular_box constructs a flat bottomed rectangular box with kmt _i,jrow = km (deepest level) for all interior points on the grid (i=2,imt-1 and jrow=2,jmt-1) while setting kmt _i,jrow = 0on all boundary cells. Enabling option cyclic turns the box into a zonally re-entrant channel by re-setting

  
  

  kmt_1,jrow = kmt_imt-1,jrow (18.5)

  kmt_imt,jrow = kmt_2,jrow (18.6)

  
  

  for jrow=1,jmt.

•  idealized_kmt constructs an idealized version of the earth's geometry. Continental features are very coarse^18.3 but map onto whatever grid resolution is specified by module  grids. They are built with the aid of subroutine setkmt which approximates continental shapes by filling in trapezoidal areas of with zero. Subroutine setkmt readily allows idealized geometries to be constructed. All that is required for input are the grid point coordinates, the coordinates of four vertices of a trapezoid, and a value for setting within the trapezoid. The bottom topography has a sinusoidal variation which is totally unrealistic. It is intended only to provide a test for the numerics in MOM. Since this option generates geometry and topography internally, no external data is required and therefore the DATABASE (explained in Chapter 3) is not needed. Typically, this option is useful when researching idealized geometries and topographies since arbitrary ones can be easily constructed. Also, this is useful when porting MOM to other computer platforms since no data files need be considered.

•  gaussian_kmt constructs a field derived from an analytical gaussian bump superimposed on a sloping ocean floor in a rectangular basin without geometry. The intent of this option is to provide a basis for regional idealized studies. The slope of the floor and scale of the bump can be set by modifying the USER INPUT section under the gaussian bump section in module topog.F. Further code modifications can easily turn the bump into a bowl or depression in the ocean floor if desired. This option requires no data and is therefore also highly portable. Results are interpolated to the resolution specified by module grids. The ocean bottom depth ht_i,jrow is specified as

  
  

  $$zw_{km} - b_0\cdot exp^{-((xt_i - xu_{imt/2})^2 + (yt_{jrow}-yu_{jmt/2})^2)/b_1^2)}$$ ht_i,jrow =

  $$slope\_x \cdot (xt_i - xu_1) - slope\_y \cdot (yt_{jrow} - yu_1)$$ - (18.7)

  
  

  where

  
  

  b₀ = zw_km / 2 (18.8)

  $$0.25\cdot min(xt_{imt-1} - xt_2,\;\;yt_{jmt-1} - yt_2)$$ b₁ = (18.9)

  $$slope\_x 10 \;meter/deg$$ = (18.10)

  $$slope\_y 10 \;meter/deg$$ = (18.11)

  
  

  and ht_i,jrow is discretized to the nearest model level to construct the field.

•  scripps_kmt reads the scripps.top $1^\circ$ x $1^\circ$ topography file (from the MOM DATABASE explained in Chapter 3) and interpolates to for the grid defined by module grids. Subroutine  scripp within module topog makes an educated guess as to whether resolution specified by module grids is coarser or finer than the native Scripps resolution. If finer, it does a linear interpolation from Scripps data, otherwise it uses area averaging of Scripps data to estimate the value of for the resolution required by module grids.

•  etopo_kmt reads the file ETOPO5.NGDCunformat_ieee which is a $1/12^\circ$ x $1/12^\circ$ topography dataset. The dataset may be purchased from the Marine Geology and Geophysics Division of the National Geophysical Data Center and is not included in the MOM DATABASE. Depths are interpolated to a field for the grid defined by module grids. Subroutine  etopo within module topog does the interpolation. If model resolution is finer than $1/12^\circ$, a linear interpolation from the dataset is used, otherwise area averaging over the dataset is used to estimate the value of for the resolution required by module grids.

•  read_my_kmt allows importing fields into MOM which have been exported from module topog using option write_my_kmt. One purpose of importing is to provide a hook for reading fields which have been constructed outside the MOM environment. Another is to allow direct editing of the field using a text editor as explained in Section 18.4.2. Still another use is when working with gigantic datasets like the $1/12^\circ$ ETOPO5. Bringing these gigantic datasets into a model run simply to compute a field can adversely affect model turn around time. Expecially when trying a new model setup which requires multiple attempts to get problems resolved. It is better to export the resulting field from a stand alone execution of module topog. Then just import into a model execution instead of the huge dataset.

Note that the field only needs to be constructed or imported into the model once at initial condition time. After that, it is incorporated as part of the restart file restart.dta. Regardless of how is generated, module topog produces a checksum which can be used to verify that the field produced by script run_topog is the one being used by MOM.