Hi,all,

Im confused of DSTART, TIDE_START, TIME_REF in Time Stamps section.

Suppose im trying to do a M2 test and will use M2 harmony constant as the tide input and wanna do a test from May 20 -- May 30, 2005. How can I set those three variables?

(I was using Roomstools for creating the tidal input info at May 20,2005 with setting all those variables to be 0.0d0 but the result turned out to be around 3 hours phase shift. Ive no idea whether i made mistakes in Roomstools or the 3 variables here. Now Im trying to input just M2 harmony constant and try to set the 3 variables right to re-test the case.)

Thanks all

## How to set variables in Time Stamps section

### Re: How to set variables in Time Stamps section

Ah, tides. How are you providing the tidal boundary conditions and where do they come from? I am using tides from OTPS which I believe has a time origin of the year zero AD (if that makes sense). Your TIME_REF can be zero, but you need to know how much time is between that time and the tidal origin time.

Then there's the long-period corrections. The phase and amplitude of the eight main diurnal and semi-diurnal tides can be thought of as wobbling a bit about their base phase and amplitude. These wobbles can be computed with Mike Foreman's code, which some use to provide zeta and ubar as boundary conditions, rather than as tidal constituents. My version of ROMS has a stab at doing this internally but it's not part of the ROMS trunk.

Then there's the long-period corrections. The phase and amplitude of the eight main diurnal and semi-diurnal tides can be thought of as wobbling a bit about their base phase and amplitude. These wobbles can be computed with Mike Foreman's code, which some use to provide zeta and ubar as boundary conditions, rather than as tidal constituents. My version of ROMS has a stab at doing this internally but it's not part of the ROMS trunk.

### Re: How to set variables in Time Stamps section

Hi, Kate, thanks a lot for ur kind reply.

As for tide boundary condition input, Im using the tidal harmony constant from TPXO which I guess is the same with ur mentioned OTPX.

Actually Im really confused of the explaination of TIDE_START & TIME_REF in the input file. What do they mean?

As for tide boundary condition input, Im using the tidal harmony constant from TPXO which I guess is the same with ur mentioned OTPX.

Actually Im really confused of the explaination of TIDE_START & TIME_REF in the input file. What do they mean?

### Re: How to set variables in Time Stamps section

There's a terse description here. ROMS has a clock, which starts at some time origin time_ref. The tidal model also has a time origin tide_start. Those time origins are not necessarily at the same time. Here's what I use:
So my time origin is at the start of the 20th Century and the tidal one is something like 1900 years before that. My starting time for this run is about 101 years after my time origin.

Code: Select all

```
DSTART = 36893.0d0 ! days
TIDE_START = -693962.0d0 ! days
TIME_REF = 19000101.0d0 ! yyyymmdd.dd
```

### Re: How to set variables in Time Stamps section

Hey, Kate, that really helps a lot for me to understand those 3 variables and I will re-test my case. Thank you so much:)

- jivica
**Posts:**130**Joined:**Mon May 05, 2003 2:41 pm**Location:**The University of Western Australia, Perth, Australia

### Re: How to set variables in Time Stamps section

TIDE_START is used in the code for tidal phase lag (check it by tour self) in simple manner.

so If you have tidal phase lags calculated (using say t_tide or some other tools) for specific day say 20080610

and you MODEL TIME is for current time (say 20080620) thanyou have to specify the two times in order to get lagging of tidal info in the model ok.

(simply like for M2 zeta_m2(t)=amp_m2*cos(w_m2*(t-tide_start)+phase_m2) ...

this is just crude scheme to show philosophy, but once again use the source Luke...

Ivica

p.s. One could use amplitude and phase modulation as well by imposing in the upper form VUF like:

zeta_m2(t)=f_0*amp_m2*cos(w_m2*(t-tide_start)+phase_m2+VU)

did I get it right?

so If you have tidal phase lags calculated (using say t_tide or some other tools) for specific day say 20080610

and you MODEL TIME is for current time (say 20080620) thanyou have to specify the two times in order to get lagging of tidal info in the model ok.

(simply like for M2 zeta_m2(t)=amp_m2*cos(w_m2*(t-tide_start)+phase_m2) ...

this is just crude scheme to show philosophy, but once again use the source Luke...

Ivica

p.s. One could use amplitude and phase modulation as well by imposing in the upper form VUF like:

zeta_m2(t)=f_0*amp_m2*cos(w_m2*(t-tide_start)+phase_m2+VU)

did I get it right?