global and regional applications to make forecasts of the sea state, which can be used for many applications such as ship routing and offshore activities, and for the validation and interpretation of satellite observations. The relationship between wave model and satellite remote sensing becomes closer with every passing day. Satellite data were used to run the model as well as to validate the model forecast. Data from QuikSCAT and Topex was used to derive and validate the WAM model.
Trang 1MSc MT.Luong Van Viet, Eng Bui Chi Nam, BSc Le Anh Tuan.
Sub-institute of Hydro-Meteorology and Environment of South Vietnam
E-mail Address: sihymete@hcm.fpt.vn
ABSTRACT
Satellite observations (TOPEX Altimeter, QuikSCAT Scatterometer and Synthetic Aperture Radar (SAR)) were used as input data for wave models and also to validate wave model forecasts In this study, we made runs of a WAM model for the whole of year 2001 in order to highlight the ability of the WAM model to make accurate hindcasts of wave patterns using remotely sensed data for the model input To derive the wave model for the above period, satellite wind data of QuikSCAT Scatterometer was processed and analyzed The comparison of hindcast model parameters was made with Topex radar altimeter derived parameters For the validation, in-situ data collected at Bach Ho Oil Rig in the East Sea (South China Sea) was utilized The study results show adequate agreement between the WAM model derived parameters and the validation data indicating that the WAM model is suitable for hindcasting/forecasting wave dynamics
Trang 21 INTRODUCTION
At present, the WAM model is used operationally
in global and regional applications to make forecasts of
the sea state, which can be used for many applications
such as ship routing and offshore activities, and for the
validation and interpretation of satellite observations
The relationship between wave model and satellite
remote sensing becomes closer with every passing day
Satellite data were used to run the model as well as to
validate the model forecast Data from QuikSCAT and
Topex was used to derive and validate the WAM model
2 SATELLITE DATA
2.1 Surface Wind Data from QuikSCAT
NASA's Quick Scatterometer (QuikSCAT)7, was
launched on 6/19/99 Its orbit is Sun-synchronous, 803
km height and has a 98.6° inclination The SeaWinds
instrument (sensor) on the QuikSCAT satellite is a
specialized microwave radar that measures sea surface
wind speed and direction under all weather and cloud
conditions
The basic purpose of the scatterometer is to
provide sea surface wind speed and direction data It
measures the backscattered power (σo) of a signal
transmitted by the radar onboard the satellite and
returned from the ocean surface The surface winds are
obtained from the σo values by making use of empirical
relationships relating to wind and σo
SeaWinds uses a rotating dish antenna with two
spot beams that sweep in a circular pattern The antenna
radiates microwave pulses at a frequency of 13.4
Gigahertz across broad regions on Earth's surface The
instrument will collect data over ocean, land, and ice in
a continuous, 1,800-kilometer-wide band, making
approximately 400,000 measurements and covering
90% of Earth's surface in one day Its operational
objectives are to improve weather forecasts near
coastlines by using wind data in numerical weather and
wave-prediction models and to improve storm warning
and monitoring
2.1 Significant Wave Height from TOPEX Altimeter
Sensor
The Ocean Topography Experiment
(TOPEX/Poseidon)8 is a cooperative project between
the United States and France to develop and operate an
advanced satellite system to provide global sea level
measurements with unprecedented accuracy On
August 10, 1992, TOPEX/Poseidon was launched into
its orbit 1336 kilometers above the sea surface The sea
level data from TOPEX/Poseidon is being used to
determine global ocean circulation and to increase the
knowledge of the interaction between the oceans and
the atmosphere
The TOPEX altimeter is a dual frequency radar instrument which draws upon a long heritage of single-frequency altimeters extending back to SeaSat The primary channel for the altimeter is Ku-band (13.6 GHz), and the secondary channel is C-band (5.3 GHz) Inclusion of the secondary channel allows correction for propagation delays in the ionosphere, reducing a significant error source in the measurement The pulse repetition frequency is approximately 4500 Hz for the Ku-band and 1200 Hz for the C-band The antenna beam width is approximately 1.1 degrees for Ku-band and 2.6 degrees for C-band
The primary objective of the radar altimeter is to provide sea surface topography and significant wave height With the analysis of the shape of the returned pulse and normalized radar backscattering cross-section, it is possible to derive significant wave height (Hs) and surface wind speed from altimeter data5
2 EXPERIMENTS
In these experiments, wind data from QuikSCAT scatterometer was used as the input for the WAM model The model output was tested using data derived from the Topex altimeter as well as in-situ wave height data taken from the Bach Ho Oil Rig in the East Sea Wind data from Bach Ho was also utilized to test the accuracy of the QuikSCAT satellite data prior to using
it for the WAM model
The foundation of the WAM model is based on the wave spectrum1; it is expressed as the energy conservation equation
where E is the energy density spectrum, E =
E(f, ,,,t) with respect to frequency f and direction
as a function of latitude , longitude and time t, .
(Cg E) is the divergence of energy flux Sin, Sds, Snl is the source represented as a superposition of the wind input, white capping dissipation and non-linear transfer respectively
Model runs in this study have been made for the area 00N-300N and 1000E-1300E, the spectral grid was chosen as 25 frequencies and 12 directions, the computation grid was chosen as 0.250 x 0.250, the time step for model integration is set to 20 min and the model output is stored at every six hours
The wind data input for the WAM models was derived from QuikSCAT level 3 The Level 3 data were obtained from the Direction Interval Retrieval with Threshold Nudging (DIRTH), wind vector solutions contained in the QuikSCAT and are being provided on
an approximately 0.25° x 0.25° global grid The Scatterometer sensor does not cover the entire study region in one day Therefore gaps in the data must be
Trang 3interpolated A bi-cubic interpolation scheme was
applied to generate wind data for each model grid point
Wind field from QuikSCAT after filling data gaps is
showed below
Fig 1: Wind vector from QuikSCAT during the typhoon
Ling Ling
The bathymetry data for the model was derived
from the National Geographic Data Centre (NGDC,
www.ngdc.noaa.gov) It is a worldwide bathymetry
data set with 5 min x 5 min resolution The model
was initialized with cold start, which means that it has
been assumed that no waves are present at the first time
step of the runs Due to the cold start runs, the model
takes some time to get stabilized Hence, the derived
wave parameters may not be quite accurate at the initial
period
Fig.2: Significant wave height of WAM model (12Z Dec 14,
2001)
WAM model output includes wind speed and
direction, friction velocity, significant wave height,
wave directions, wave period, 2D wave spectrum, swell
height, swell period, swell direction and swell
spectrum
3 RESULTS AND DISCUSSIONS
In this study, various experiments with the wave
model were conducted Comparisons of model derived
parameters were made with the available in-situ data of Bach Ho Oil Rig and with the TOPEX altimeter derived wave parameters As wind is major input parameter in deriving the wave model, an initial comparison was made between the input wind data from QuikSCAT and the available in-situ data
3.1 Comparison of QuikSCAT and In-situ wind speeds
The Bach Ho Oil Rig data is available for all months from January to December 2001 and is collected four times a day The correlation coefficient (R) between wind speed from QuikSCAT and oil rig data for all the months of 2001 is presented in table 1
Table 1 The correlation coefficient between wind
speed from QuikSCAT and oil rig data, 2001
Mont
Mont
The results indicate a sufficiently high correlation coefficient between wind speed derived from QuikSCAT and the oil rig data The highest correlation occurs in the months of the winter monsoon, the lowest correlation is in the summer monsoon The reason for this may relate to the observed limit of QuikSCAT (Vmax = 30m/s) Tropical cyclones were very active in the summer monsoon of 2001 (13/15 tropical cyclones) with high wind speeds, especially in July where four typhoons and one tropical cyclone occurred on EAST SEA QuikSCAT was unable to observe these high wind speeds, thus resulting in less accurate data during the summer monsoonal months
3.2 Comparison Hs from Model and TOPEX
The significant wave height provided by Topex-altimeter is characterized by very high accuracy data and is ideal for checking the model output For comparison of Topex along track data for significant wave heights with the model, Topex data were averaged over the model area within the model grid size Spatial interval was kept at ±0.250 from the model grid point, the time interval was chosen at ±3 hrs and maximum number of points available in a grid is approximately 3
Significant wave height data obtained with the WAM model was compared with the Topex-altimeter observations WAM model runs were made for the area from 00N-1000E and 300N-1300E This area was assumed to represent a closed sea In real situations waves may travel inside the model area from very long
Trang 4distances that may influence the wave hindcast near
spatial boundaries Hence, the model hindcast near the
spatial boundary may have errors To avoid this effect,
the area for comparisons of the model was smaller than
the total extent of the model area In summer, the area
for comparison was chosen to be 100N-250N, 1000
E-1200E because monsoonal winds are south westerly and
hence model output maybe not correct for regions
below 100N Similarly, in the winter monsoon, the area
chosen for comparison was 50N-200N, 1000E-1150E
because winds are north easterly hence model output
were discarded for regions north of 200N or east of
1150E
Model runs for the whole year were made from
cold start It was seen from earlier experiments that
wave model needs approximately 3 days spin-up time
hence first three days hindcast of wave model were
discarded for the comparison with in-situ and Topex
altimeter data The results of comparisons are shown in
Table 2, Fig.3 below
The correlation coefficient between Hs from the
model and TOPEX was significantly high, ranging
from 0.84 to 0.95; the highest correlation is in the
winter monsoon This comparison for all months of
2001 proves that the WAM model is suitable to
hindcast/forecast wave height over the East Sea Hence
we can be sure that the wind data used in this study to
input into the model is highly accurate
Table 2 The correlation coefficient
between Hs from Model and TOPEX
over EAST SEA, 2001
y = 1.2211x - 0.075
R = 0.934
0 1 2 3 4 5 6
Topex altimeter w ave height (m)
Fig 3: Comparison Hs of WAM model and Topex (data of
July, 2001)
3.3 Comparison of WAM model results with in-situ data
Wave data from Bach Ho Oil Rig is observed four times a day, giving more than one hundred samples for model comparison per month Wave data of Bach Ho includes significant wave height and wave period Results of the comparisons are presented in Table 3 and Table 4 The correlation coefficient between wave height of WAM and Bach Ho data is significantly high,
in the range of 0.82 to 0.94 However for wave period the correlation was less (R from 0.68 to 0.74)
Table 3 The correlation coefficient between Hs by
model and oil rig, 2001
Table 4 The correlation coefficient between wave period by model and oil rig, 2001
In addition to the scatter plot, time variation of model derived wave height and in-situ data is also displayed (Fig 4) Although the time series for comparison is short and there is only one station to
Trang 5access, the results confirm a very high correlation
between the WAM model and in-situ data
0
1
2
3
4
5
6
7
8
9
4 6 8 10 12 14 16 18 20 22 24 26 28 30
Time (Date) December 2001
Observed Hs
WAM Hs
Fig.4: Comparison significant wave height of model and oil
rig (data of December, 2001)
4 CONCLUSIONS
The comparisons between wave model derived
parameters, the Bach Ho Oil Rig data and the
Topex-altimeter data confirm that the WAM model is ideal for
hindcast of significant wave height in the East Sea
Comparisons of model predicted significant wave
height and in-situ observations are well in agreement
The model is unable to reproduce small scale variations
observed in significant wave height This is due to the
frequency of wind input was once a day If more
frequent wind data is available then it may be possible
to observe small scale variations in the model wave
field
The model is able to reproduce large variations in
significant wave height observed by Topex altimeter
for almost all months of 2001, although the correlation
in the summer monsoon was less This is due to the
characteristic of wind field in the summer (the active
cyclone season), and the ability of QuikSCAT to be
able to observe high speed and highly variable winds
This may also be the reason for the low correlation
coefficients between wind speed of QuikSCAT and
in-situ data and between Hs of the model and the in-in-situ
data for the summer monsoon
The analysis of WAM model in this study is
limited For increased accuracy, wind data input for the
model needs to be increased Model derived
parameters can be validated using in-situ and remote
sensing data products as detail presentation above Also
analysis across a greater time scale (several years) will
enable the confirmation of the ability of wave models
to forecast wave dynamics in the East Sea region in any
weather conditions
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