An Introduction of Wave Forecast System

In Thai Meteorological Department

Wattana Kanbua

Abstract

Thai Meteorological Department has provided daily wave analysis and 24-hour wave forecasting charts to the general public. The domain covers from longitudes 95E to 105E and from latitudes 5N to 15N, which encompasses the Gulf of Thailand, Andaman sea and South China Sea. The significant wave and wave spectral methods are the major tools to make wave analysis and forecast in the operation run. In this report, the modified scheme of significant wave method is to interpolate the growth, decay, propagation of wave energy and wave-wave interaction based on the wave forecasting relationships to the grid point and then to integrate with time to compute the next time of wave height and period. On the aspect of wave spectral method, the so called third generation of WAM model, it is used to make wave analysis and forecast in the routine operation forecasts. During the procedure to make wave forecast charts, they include automatic and manual operations. The automatic operation means that the products of model wave charts calculated from wind and wave models are automated. The final forecast charts are made based on the observed wave data and model wave charts through weather briefing and discussion.

Introduction

With the development in economics and the increase of the population, it goes the increasing deficiency in land mass resources and space exploitation. Thus this causes an increasing urgency in developing and exploiting the marine resources, and in navigating safely in seas or oceans. Considering these, Thai Meteorological Department (TMD) established the Marine Meteorological and Upper-air Observations Sub-division (MMUOS). This is a Sub-division under the Meteorological Observations Division, responsible for the monitoring of marine phenomena that is required for any maritime development.

The marine forecasts mean to forecast the next coming status, such as 24-hour or 48-hour on the sea surface conditions for a wide variety of physical variables. For instance, wave forecast aims on distributions of wave height contour at 00 GMT on a daily basis pass the forecast division. The TMD as the forecast division has provided daily wave analysis and 24-hour wave forecasting charts to the general public. The domain covers from longitudes 95E to 105E and from latitudes 5N to 15N, which encompasses the Gulf of Thailand, Andaman sea and South China. The main intent is to furnish users with sea level information according to their specific references. Basically, it provides information on navigational safety and precautions against sea disaster.

During the procedure to make wave forecast charts, it includes automatic and manual operations. The automatic operation means that the products of model wave charts calculated from wind and wave models via computer are automated. The final forecast charts are made based on the observed wave data and model wave charts through weather briefing and discussion.

There are two types of method involved in the wave forecasts. That is the significant wave and wave spectral methods. On the former, the wave height and period are not estimated from the graphic solution, known as Beaufort table, instead of making wave forecast by using wind speed. On the latter, the evolution of the spectrum is governed by the spectral energy balance equation. Based on the different physical concepts and calculated technique of wave growth, decay, and wave-wave interaction, the so-called third generation of WAM model, it is used to make wave analysis and forecast in the routine operation forecasts.

Introduction of Numerical Wave Models

1. The significant wave method

Sverdrup and Munk were the pioneers to develop the wave-forecast technique in terms of significant idea. After that, this method was extensively modified by Bretschneider who developed semi-empirical wave forecasting relationship using graphic solution to make forecast and sometimes called SMB (Sverdrup, Munk and Bretschneider) method. However the SMB method is mostly for local forecast. It is inefficient for two-dimension area by numerical techniques. Therefore, Peng (1991) proposed a scheme for computing the wave height and period in the grid point to make forecast for next time step over the ocean. The modified scheme is to interpolate the growth, decay, and propagation of wave energy based on the semi-empirical wave forecasting relationships to the grid point and then to integrate with time to compute the next time of wave height and period. The numerical processes contain four steps with initial value defined, wave growth and decay, wave propagation, and wave interpolated at grid point. Refer to Peng (1991) for details.

2. The second generation wind wave spectrum model

The numerical techniques of the second-generation wave model are based on the Golding (1983) and Chao (1993) proposed structure.

According to Hasselmann (1962), the evolution of the spectrum can apply the energy balance equation to demonstrate sea state condition. If the sea water depth changes are taken into account, the group velocity and propagating direction will change as time elapses during the evolution process of the wave energy spectrum. The equation can be written as

(1)

where S* _{in}* indicates the wave energy induced by the wind, S

3. Introduction of WAM

The WAM-model is a third generation wave model which solves the wave transport equation explicitly without any presumptions on the shape of the wave spectrum. It represents the physics of the wave evolution in accordance with our knowledge today for the full set of degrees of freedom of a 2d wave spectrum. The model runs for any given regional or global grid with a prescribed topographic dataset. The grid resolution can be arbitrary in space and time. The propagation can be done on a latitudinal - longitudinal or on a carthesian grid.The model outputs the significant wave height, mean wave direction and frequency, the swell wave height and mean direction, wind stress fields corrected by including the wave induced stress and the drag coefficient at each grid point at chosen output times, and also the 2d wave spectrum at chosen grid points and output times.

The model runs for deep and shallow water and includes depth and current refraction. The integration can be interrupted and restarted at arbitrary times. The source terms and the propagation are computed with different methods and time steps. The source term integration is done with an implicit integration scheme while the propagation scheme is a first order upwind flux scheme. The wind time step can be chosen arbitrarily.

Subgrid squares can be run in a nested mode.In a course grid run the spectra can be outputted at the boundaries of a subgrid . They can then be interpolated in space and time to the boundary points of the fine subgrid and the model can be rerun on the fine mesh grid.

Hasselmann in the Max-Planck Meteorological Institute, Hamburg, originally developed WAM. Then WAMDI group (The WAM Development and Implementation Group), headed by Hasselmann after years of researches and revision, announces the third generation of this model. The TMD now applies the fourth revised edition (Gunther et al., 1992). Within the WAM model, the basic equation is wave energy equation in relation to any specific spot on the sea surface, whose spectrum E (j ,l , f,q , t), where (j ,l ) are latitude and longitude of any specific spot on sea surface, is a wave field of two-dimensional frequency (f) and direction (q ).

(2)

where (Cj
, Cl
, Cq
) are the phase speeds of wave energy propagation on (j
, l
, q
) coordinates, S* _{in}* is the wave energy influx transferred to waves from winds, S

Future Outlook

The MMUOS under TMD, which is Sub-division and responsible about air-sea interaction serving the outside world, looks at observed data and forecast accuracy as important service parameters.

The TMD has co-operated with National Research Council of Thailand as SEAWATCH project. The objectives of this project are to develop a real time and continuously databases of marine environmental to serve fisheries, sea transportation, hydro-oceanography, meteorological services and others. Wave model is also being applied for interpreting and assimilating buoys wave data. These monitoring systems provide valuable information for making the wave forecast, especially during various phases of disaster mitigation

The monitoring systems produce essential data for researchers in understanding the physical phenomenon and in developing and operating forecasting models. So the second mission is to develop and upgrade the large scale and coastal models which can be useful and effective approach to the prediction of marine. That is important to the public safety and human well being, the national economy, and environmental management. Such data is also useful in the decision-making process, in issuing warning messages, and in studying coastal subsidence and erosion.

Reference

Golding, B., 1983: A wave prediction system for real time sea state forecasting. Quart. J. R. Met. Soc.109, 393-416.

Gunther, H., K. Hasselmann and P.A.E Janssen, 1992:Report NO.4, The WAM Model Cycle 4, Edited by Modellberatungsgruppe, Hamburg.

Hasselmann, K, 1962: On the nonlinear energy transfer in a gravity-wave spectrum. 1. General theory, JFM, vol. 12, 481-500.