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Barotropic Model - Along-Track (T/P)
Ahmed Haider Ali and Victor Zlotnicki
DATA ACCESS
The barotropic model output, sampled along
the Topex/Poseidon track, and read software, are available at our ftp
site
Last cycle available:
281
Temporal extent: Sep 23, 1992 -
May 09, 2000
DATASET OVERVIEW
- This dataset is organized
along the Topex/Poseidon tracks, at the latitude-longitudes of our nominal
'grid', just as our residuals. It includes
the following parameters:
- TIME2: time of each point,
for pass j, position i along the pass.
- RES: sea level residual
in mm.
- IB: inverted barometer
from the T/P GDR, in mm. This value is derived from ECMWF pressure.
- BM: Barotropic Model 'sea
level', in 0.1 mm, at same time-lat-lon as res. Details below.
- BMIB: Barotropic Model
- IB in 0.1 mm. Because the model was forced with NCEP wind and pressure,
this IB is not the same as the field 'IB', from ECWMF.
- HFBM: High Frequency part
of BM, in 0.1 mm. Details below.
- HFBMIB: High Frequency
part of (Full Model - IB) in 0.1 mm
- All quantities have had their 1993-1996
time-average removed.
- The Along-track grid has fixed locations
for all cycles, 6.2 km spacing along-track. The lat-lons (also (3127,
254), but independent of time) are in a separate file, latlon.dat.
- The values of TIME2, RES and IB are exactly
the same as those in the alongtrack residuals
dataset (as are the lat-lon values).
- For details on the barotropic model itself,
see the description below.
DATASET VERSION
vBT10v2000-12-20-AKH. See residuals
description here.
Date of Last Update
2001-06-28
KNOWN PROBLEMS
For
residuals problems, look here.
The barotropic model version used here has
masked out the Mediterranean, Black and Caspian seas, where the T/P data
is otherwise good, and all shallow waters (Patagonian shelf, Grand Banks,
etc). Special tuning is needed to get those regions performing better
than inverted barometer alone.
The wind stress is derived from 1000 mbar,
not 10 m wind. In our experiments, this conversion matched T/P data better
than when 10 m winds were used, even though it is not defensible on first
principles, and the Kondo stress parameterization is for 10 m wind.
PROCESSING HISTORY
DATA FORMAT
Units, Data Type, Array
Dimensions
- TIME2(integer*4 time2(3127,254)):
time of each point. time2(i,j) corresponds to pass number j, position
i along the pass. In seconds past Jan 1, 1992.
- RES (integer*2 res(3127,254)):
sea level residual in mm. (res(i,j)=32767 is a missing value flag; the
normal range for res is -1500 to +1500 mm)
- IB (integer*2 ib(3127,254)):
inverted barometer from the T/P GDR, in mm. This value is derived from
ECMWF pressure.
- BM (integer*2 bm_res(3127,254)):
Barotropic Model 'sea level', in 0.1 mm, at same time-lat-lon as res.
- BMIB (integer*2 bm_res(3127,254)):
Barotropic Model - IB in 0.1 mm. Because the model was forced with NCEP
wind and pressure, this IB is not the same as the field 'IB', from ECWMF.
- HFBM (integer*2 bm_res(3127,254)):
High Frequency part of BM, in 0.1 mm.
- HFBMIB (integer*2 bm_res(3127,254)):
High Frequency part of (Full Model - IB) in 0.1 mm
Special Values
Value set to 32767 where data does not exist
or has been edited out
Residual values have 10,000 mm added in shallow
water
- To use deep data only, exclude values
h: abs(h) > 1500
- To use shallow also, if(abs(h) .gt.
1500 AND h .lt. 32767) h=h-10000
Data Organization
Each file corresponds to one cycle of exactly
3127x254 points. Each group of 3127 consecutive points corresponds to one
'pass'. There are 254 passes in a cycle. Each file contains equator crossing
time, the relative time (in sec) of of each 1 sec sample, and the sea level
residual in mm. Latitude and longitude values (3127x254, in degrees) are
in a separate file, common to all cycles.
Read Software
MODEL DESCRIPTION
The residuals themselves are described here.
Barotropic Model BT-PHA-1.0
* Code:
- Barotropic model of Ponte (1993, 1997, 1999),
as modified by Hirose et al (2000):
* subsurface no-slip condition
* topography from ETOPO5 (1.125 averages) over the unmasked oceans
* optimized friction parameter, -bu/H, b=2 cm/s
* Resolution: 1.125° x 1.125°
- Coverage/landmask: global, 75°S to 65°N, EXCEPT it does NOT
include Mediterranean sea, North Sea, Sea of Japan, smaller
enclosed seas and bays, or waters shallower than 1000m (such
as the Patagonian shelf, Indonesian Sea, Hudson Bay, etc)
* Forcing:
- Atmospheric pressure and 1000 mbar winds, from NCEP Reanalysis, NCAR
dataset DS090, http://dss.ucar.edu/datasets/ds090.0/ . These forcing files
are issued every 6 hrs.
- The 1000 mbar wind is converted to wind stress using the Kondo (1975)
coefficient.
- Both the pressure and wind components are time-filtered: at each grid
point a
two-pass 4-point (24 hr) average is applied, to remove effect of atmospheric
thermal tides.
* Inputs required:
- 6-hourly grids of atmospheric pressure, north wind stress, east wind
stress.
- time-invariant bathymetric grid
- time-invariant land-mask grid
- parameters file, includes time step, days to integrate, westernmost
longitude, southernomost latitude; flags on whether or not to write
grids and how often, whether to smooth in time and how much, Rayleigh
friction coeff (cm/s), Newtonian damping coeff (days).
- optional: topex sea level file. If the corresponding option is on, this
file
is read, the model is sampled at Topex time-lat-lon on-the-fly, and an
output file is generated for later statistical summary.
* Outputs:
- 1-hourly grids of output model sea level, sea level-IB
- 1-hourly grids of output water velocity (optional)
* Filtering:
- a 2-pass, 20-day bin average is applied to the 1-hour grids at each
lat-lon node, then substracted from them, to produce the 'high-frequency'
values.
* References:
Ali, A.H., V. Zlotnicki, N. Hirose, I. Fukumori, and R. M. Ponte: Effect
of different Wind Forcings on a barotropic ocean model's ability to fit
Topex and BPR data. AGU SF 12/2000. (preprint at http://oceans-www.jpl.nasa.gov/vz)
Hirose N., I. Fukumori, V. Zlotnicki and R. M. Ponte, High-Frequency
Barotropic Response to Atmospheric Disturbances: Sensitivity toForcing,
Topography, and Friction. Submitted to J. Geophys. Res., 2000. (preprint
at http://oceans-www.jpl.nasa.gov/vz)
Kondo J., Air-Sea Bulk Transfer Coefficients in diabatic conditions.
Bound. Layer Meteorol. 9, p91-112, 1975
Ponte, R. M., Variability in a homogeneous global ocean forced by barometric
pressure, Dyn. Atmos.Oceans, 18, 209-234, 1993.
Ponte, R. M., Nonequilibrium response of the global ocean to the 5-day
Rossby-Haurwitz wave in atmospheric surface pressure, J. Phys. Oceanogr.,
27, 2158-2168, 1997.
Ponte, R. M. and P. Gaspar, Regional analysis of the inverted barometer
effect over the global ocean using TOPEX/POSEIDON data and model results,
J. Geophys. Res., 104, 15 587-15 601, 1999.
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