NLOADF(1)

NAME
nloadf - compute ocean loads at a site, or give the grid used to do so

SYNOPSIS
nloadf stname lat long ht modelfile greenfile g | l [polyfile [+ | -] ]

nloadf stname lat long ht modelfile greenfile m [dist] [polyfile [+ | -] ]

DESCRIPTION

Nloadf is a program for computing ocean tidal loads at a location, given (1) a model of the ocean tides specified on a latitude-longitude grid; (2) a Green function for the response of the earth (which also includes the information about the intervals of integration); (3) a file specifying the distribution of land and sea; and (4) a ``polygon file'', which specifies what regions are and are not to be included in the convolution. Items (1-3) are required, the first two being given on the command line and the third being hardwired into the program. Item (4) is optional, and when present is specified on the command line.

The arguments on the command line are (with R for required, O for optional):

stname
    (R) an ascii string givening the station name; it must not contain any blanks or be longer than 80 characters.

lat (R) the North latitude (in decimal degrees) of the station.

long
    (R) the East longitude (in decimal degrees) of the station.

ht  (R) the elevation (in meters above or below sea level) of the station.

modelfile
    (R) the name of a file specifying the tidal height for a global of regional model of a particular tidal constituent, in ``standard form''. (See the supplementary documentation for a fuller description).

greenfile
    (R) the name of a file specifying the Green function for a particular Earth model, along with the distance range(s) and step size(s) to be used in the convolution, and whether or not the land-sea file is to be used to determine if ocean is present or not. (See the supplementary documentation for a fuller description).

g, l, m
    (R) determines the nature of the output. If g or l, the loading results are written to standard output. A g means that the phases are Greenwich phase, BUT with lags (the reverse of the usual phase convention in ocean tides). An l means that the the phases are in local phase at the site, again with lags negative. An m causes the program to write to standard output the coordinates of the corners of the cells used for the convolution, in a form (lat and long separated by impossible values) that is often suitable for plotting. This output is quite voluminous and should always be redirected to a file.

dist
    (O) If the m option is chosen, this is the maximum distance (in degrees) to which to cell coordinates will be given.

polyfile
    (O) The name of a ``polygon file'' which specifies regions into which points must (and/or must not) fall in order to be included in the convolution. By setting such files up correctly, it is easy to assure that a convolution done with a global model does not include a marginal sea for which a separate model is being used (and that the model of the marginal sea has no overlap with the global model). If a + or - is included following the name of this file, it overrides the instructions in that file for which polygons to include: if a + is given, all polygons in the file are included; if a -, all are excluded. (See the supplementary documentation for a fuller description).

OUTPUT

The program writes the results of its computations to standard output, along with a set of ``header lines'' which describe what went into the computation. These header lines are usually more voluminous than the results; experience has shown that it is easy to lose track of how a computation is done, so that it is worthwhile to make the results self-documenting. Both result and header lines are identified by a letter in the first column, lowercase for the result lines. The ones currently output are:

S   The station name and coordinates; format (3x,a40,3x,2f10.4,f10.0).

O   The tidal model: the Darwin symbol, Doodson number (in Cartwright-Tayler form), and a description of the model; format (2x,a4,5x,6i2,5x,a50).

G   The Green function. The first line describes the Earth model used; subsequent lines describe what radial spacing was used, and whether the detailed land-sea model, or the ocean model, was used to determine the land-sea boundary.

P   If a polygon file is used, the first line gives the description (from the polygon file); a second line is included if the + or - override is used.

C   This denotes information about the convolution. One line gives the time at which the computation was done; a second indicates how far away, and where, the closest nonzero load was found; and a third summarizes information on how many cells (if any) were found to be ocean from the land-sea model but had an amplitude of zero from the ocean model, even after trying to interpolate from cells nearby.

L   Describes the phase convention.

X   A terminator line (used by loadcomb) to indicate the end of the header lines.

g   The gravity load tide, amplitude and phase (as are all the loads), with the amplitude in microgal; format (10x,2f10.4). The convention is that positive accelerations are upwards; that is, they correspond to a decrease in the local acceleration of gravity.

d   The displacement load tides, alternating amplitude and phase, with the amplitude in millimeters; format (10x,6f10.4). The displacements are given in order east, north, and up; note that the displacements given by some other programs (and used in the MERIT and successor standards) are south, west, and up, as well as having lags positive.

t   The tilt load tides, alternating amplitude and phase, with the amplitude in nanoradians; format (10x,4f10.4). The tilts are given in order east and north, with positive tilt being that a plumb line would move in the direction given.

s   The strain load tides, alternating amplitude and phase, with the amplitude in nanostrain; format (10x,6f10.4). The strains are given in order east extension, north extension, and tensor shear ($ e sub EN$).

EXAMPLES
nloadf MDO1 30.6805 255.9849 2095 m2.csr3tr gren.gbav.std l

nloadf HAYS 42.617 -71.483 0 m2.bfund gren.gbav.std l poly.bfund +

nloadf HAYS 42.617 -71.483 0 m2.csr3tr gren.gbav.std l poly.bfund -

nloadf PFO 33.609 -116.455 1280 m2.csr3tr green.gbav.std m 20 poly.cortez -

nloadf PFO 33.609 -116.455 1280 m2.cortez green.gbav.std m 20 poly.cortez +

The first example shows the simplest (and usual) case where the convolution is done for a single tidal model. The next two lines show how a marginal sea (in this case the Gulf of Maine and Bay fo Fundy) would be included. On the second line the convolution is done for a model for this sea, with a polygon file being used to exclude any overlap with the global model; on the third a global model is used, with the polygon file set to exclude any overlap with the model for this sea. To get the final results, the outputs would have to be redirected to separate files and then added with loadcom. The fourth and fifth lines output the load-cell grids for a similar case; these are the lines used to produce the grids in Figures 1c and 1d of the supplementary documentation.

SEE ALSO
loadcomb

AUTHOR
D. C. Agnew