user define function code UWAVE

[Cathy039]

subroutine uwave(v,a,pdyn,dpdyndz,surf,lpdyn,
1 lrecompute,luplocal,lupglobal,
1 lsurf,ndim,xcur,xintmed,
2 grav,density,elevb,elevs,
3 seed,nspectrum,freqwamp,
2 time,dtime,noel,npt,kstep,kinc)
C
include 'aba_param.inc'
C
dimension v(ndim),a(ndim),xcur(ndim),xintmed(ndim)
dimension time(2),freqwamp(2,nspectrum)

C
parameter(pi=3.14159265358979d0,two=2.0d0,abig=1.d36)
parameter (const2=2.d10, twopi = 2.d0*pi )
C
luplocal=0
lupglobal=0
if(lrecompute.ne.0) then
C Only stochastic analysis with UWAVE can have lrecompute=1
C The user must set other flags accordingly; see User's manual.
else
C For regular Aqua analysis with UWAVE, lrecompute=0 always
C
C
C Wave definition for a single Airy wave component:
C Phase angle of waves: in radians
phase=0.0d0
C
C Wave travel direction:
xdir=1.0d0
ydir=0.0d0
C
C Period, wavelength, wave number, wave height, frequency:
C
period=15.d0
waveln=1153.0709499453355d0
wavenum=twopi/waveln
wavehgt=50.d0
freq=twopi/period
C
if (lsurf.eq.1) then
C Calculate the instantaneous water surface only, no
C wave kinematics are required:

wtp=-freq*time(2)+phase
sn=xdir*xcur(1)
if (ndim.eq.3) sn=sn+ydir*xcur(2)
termt=wavenum*sn+wtp
surf=elevs-wavehgt*cos(termt)

This is an example from the abaqus documatation. My question is that the instantaneous surface surf, should it be just wavehgt*cos(termt), just like ϵ=ϵ_0*cos(kx-wt+φ_0).What does it mean when surf=elevs-wavehgt*cos(termt)?