SWAN模型用户手册：第三代海洋气象模式指南

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"SWAN用户手册是第三代海浪模式SWAN的官方文档，由代尔夫特理工大学环境流体力学系的SWAN团队编写。该手册提供了关于SWAN模式的详细指南，适用于海洋气象学领域的研究人员和从业者。" 在手册中，SWAN (Simulating Waves Nearshore) 模型被介绍为一个复杂的计算程序，用于模拟近岸海浪的生成、发展和变化。SWAN模型被广泛应用于全球和区域的海洋预报系统中，对于理解海洋动力学、海岸工程设计以及灾害预警等方面具有重要意义。 1. **介绍** 手册的开头部分对SWAN模型进行了基本的介绍，解释了其目的和应用范围，同时也可能涉及模型的历史和发展。 2. **一般定义和注意事项** 这一部分详细阐述了模型的基本概念，如海浪的生成机制、能量平衡等，并可能提到了使用模型时的一些限制条件和注意事项。 3. **内部场景、限制器、缺点和编码错误** SWAN模型的内部工作流程可能包括多个场景设置，如风暴事件或不同天气条件下的海浪模拟。这部分还讨论了模型中的限制器，用于防止数值不稳定，以及已知的模型缺陷和编程错误。 4. **与其他模型的关系** SWAN与WAM（Wave Action Model）和WAVEWATCH III等其他海浪模型的关系在此处被比较和解释。这可能包括它们在算法、适用范围和精度上的异同。 5. **单位和坐标系统** 这一部分详细介绍了SWAN模型中使用的物理量单位，以及模型所采用的地理坐标系统，这对于正确理解和应用模型结果至关重要。 6. **网格选择、时间窗口和边界/初始/初猜条件** 这是SWAN模型设置的重要部分，讨论了如何选择合适的网格分辨率，确定模拟的时间跨度，以及设定初始条件、边界条件和猜测值，这些都是影响模拟准确性的关键因素。 7. **内容概览** 除了上述主题，手册可能还包含更多章节，如模型的输入数据格式、输出结果的解析、模型参数调整、案例研究和结果验证等。通过这份用户手册，用户可以深入理解SWAN模型的运作原理，掌握如何正确配置和运行模型，从而进行有效的海浪模拟分析。此外，手册中提供的链接指向了SWAN团队的联系方式和在线资源，便于用户获取最新的支持和更新。

8 Chapter 2

below). Directions and spherical coordinates are in degrees (

) and not in radians.

For the output of wave energy the user can choose between variance (m

) or energy (spa-

tial) density (Joule/m

, i.e. energy per unit sea surface) and the equivalents in case of

energy transport (m

/s or W/m, i.e. energy transport per unit length) and spectral energy

density (m

/Hz/Degr or Js/m

/rad, i.e. energy per unit frequency and direction per unit

sea surface area). The wave−induced stress components (obtained as spatial derivatives of

wave-induced radiation stress) are always expressed in N/m

even if the wave energy is in

terms of variance. Note that the energy density is also in Joule/m

in the case of spherical

coordinates.

SWAN operates either in a Cartesian coordinate system or in a spherical coordinate sys-

tem, i.e. in a ﬂat plane or on a spherical Earth. In the Cartesian system, all geographic

locations and orientations in SWAN, e.g. for the bottom grid or for output p oints, are

deﬁned in one common Cartesian coordinate system with origin (0,0) by deﬁnition. This

geographic origin may be chosen totally arbitrarily by the user

. In the spherical system,

all geographic locations and orientations in SWAN, e.g. for the bottom grid or for output

points, are deﬁned in geographic longitude and latitude. Both coordinate systems are des-

ignated in this manual as the problem coordinate system

In the input and output of SWAN the direction of wind and waves

are deﬁned according

to either

• the Cartesian convention

, i.e. the direction to where the vector points, measured

counterclockwise from the positive x−axis of this system (in degrees) or

• a nautical convention

(there are more such conventions), i.e. the direction where the

wind or the waves come from

, measured clockwise from geographic North.

All other directions

, such as orientation of grids, are according to the Cartesian convention!

For regular grids, i.e. uniform and rectangular, Figure 4.1 (in Section 4.5) shows how the

locations of the various grids are determined with respect to the problem coordinates. All

grid points of curvi-linear and unstructured grids are relative to the problem coordinate

system.

2.6 Choice of grids, time windows and bou ndary /

initial / ﬁrst guess conditions

2.6.1 Introduction

Several types of grids and time window(s) need to be deﬁned : (a) spectral grid, (b) spatial

(geographic) grids and time window(s) in case of nonstationary computations.

The spectral grid that need to be deﬁned by the user is a computational

spectral grid on

General deﬁnitions and remarks 9

which SWAN performs the computations.

SWAN has the option to make computations that can be nested in (coarse) SWAN, WAM

or WAVEWATCH III. In such cases, the spectral grid need not be equal to the spectral

grid in the coarse SWAN, WAM or WAVEWATCH III run.

The spatial grids that need to be deﬁned by the user are (if required):

• a computational

spatial grid on which SWAN performs th e computations,

• one (or more) spatial input grid(s) for the bottom, current ﬁeld, water level, bottom

friction and wind (each input grid may diﬀer from the others) and

• one (or more) spatial output

grid(s) on which the user requires output of SWAN.

The wind and bottom friction do not require a grid if they are uniform over the area of

interest.

For one-dimensional situations, i.e. ∂/∂y ≡ 0, SWAN can be run in 1D mode.

If a uniform, rectangular computational spatial grid is chosen in SWAN, then all input and

output grids mu st be uniform and rectangular too, but they may all be diﬀerent from each

other.

If a cur vi-linear computational spatial grid is chosen in SWAN, then each input grid should

be either uniform, rectangular or identical to the used curvi-linear grid or staggered with

respect to the curvi-linear computational grid.

If an unstructured computational spatial grid is chosen in SWAN, then each

input grid

should be either uniform, rectangular or identical to the used unstructured grid.

SWAN has the option to make computations that are nested in (coarse) SWAN, WAM or

WAVEWATCH III. In such runs, SWAN interpolates the spatial boundary of the SWAN,

WAM or WAVEWATCH III grid to the (user provided) grid of SWAN (that needs to

(nearly) coincide along the grid lines of WAM or WAVEWATCH III or the output nest

grid boundaries of SWAN). S ince, the computational grids of WAM and WAVEWATCH

III are in spherical coordinates, it is recommended to use spherical coordinates in a nested

SWAN when nesting in WAM or WAVEWATCH III.

SWAN using an unstructured mesh

may be nested in SWAN employing a regular grid and

vice versa. However, SWAN using an unstructured grid cannot be nested in WAM or

WAVEWATCH III.

Nesting from a 2D mod el to a 1D model is possible although is should not be done by

using the commands NGRID and NEST. Instead, deﬁne the boundary point of the 1D mo del

as an output point (using command POINTS) and write the spectra for that point using

the command SPECout. In the 1D model, this spectra is used as boundary condition using

10 Chapter 2

the BOUNDSPEC command.

Similarly, the wind ﬁelds may be available in diﬀerent time windows than the current and

water level ﬁelds and the computations may need to be carried out at other times than

these input ﬁelds. For these reasons SWAN operates with diﬀerent time windows with

diﬀerent time steps (each may have a diﬀerent start and end time and time step):

• one computational time window

in which SWAN performs the computations,

• one (or more) input time window(s)

in which the bottom, current ﬁeld, water level,

bottom friction and wind ﬁeld (if present) are given by the user (each input window

may diﬀer form the others) and

• one (or more) output time window(s)

in which the user requires output of SWAN.

In case of nesting, SWAN searches the boundary conditions in the relevant output ﬁle of

the previous SWAN, WAM or WAVEWATCH III runs to take the boundary conditions

at the start time of the nested run. It will not take the initial condition (i.e. over the

entire computational grid) for the n ested run from the previous SWAN, WAM or WAVE-

WATCH III run.

During the computation SWAN obtains bottom, current, water level, wind and bottom

friction information by tri-linear interpolation from the given input grid(s) and time

window(s). The output is in turn obtained in SWAN by bi-linear interpolation in space

from the computational grid; there is no interpolation in time, the output time is shifted

to the nearest computational time level. Interpolation errors can be reduced by taking the

grids and windows as much as equal to one another as possible (preferably identical). It

is recommended to choose output times such that they coincide with computational time

levels.

2.6.2 Input grid(s) and time window(s)

The b athymetry, current, water level, bottom friction and wind (if spatially variable) need

to be provided to SWAN on so-called input grids. It is best to make an input grid so large

that it completely covers the computational grid.

In the region outside the input grid SWAN assumes that the bottom level, the water level

and b ottom friction are identical to those at the nearest boundary of the input grid (lateral

shift of that bound ary). In the regions not covered by this lateral shift (i.e. in the outside

quadrants of the corners of the input grid), a constant ﬁeld equal to the value at the nearest

corner point of the input grid is taken. For the current and wind velocity, SWAN takes 0

m/s for points outside the input grid.

In SWAN, the bathymetry, current, water level, wind and bottom friction may be time

varying. In that case they need to be provided to SWAN in so-called input time windows

General deﬁnitions and remarks 11

(they need not be identical with the computational, output or other input windows). It is

best to make an input window larger than the computational time window. SWAN assumes

zero values at times before the earliest begin time of the input parameters (which may be

the begin time of any input parameter such as wind). SWAN assumes constant values

(the last values) at times after the end time of each input parameter. The input windows

should start early enough so that the initial state of SWAN has propagated through the

computational area before reliable output of SWAN is expected.

One should choose the spatial resolution for the input grids such that relevant spatial de-

tails in the bathymetry, currents, bottom friction and wind are properly resolved. Special

care is required in cases with sharp and shallow ridges (sand bars, shoals) in the sea bot-

tom and extremely steep bottom slopes. Very inaccurate bathymetry can result in very

inaccurate refraction computations the results of which can propagate into areas where

refraction as such is not signiﬁcant (the results may appear to be unstable). For instance,

waves skirting an island that is poorly resolved may propagate beyond the island with

entirely wrong directions. In such a case it may even be better to deactivate the refraction

computations (if refraction is irrelevant for the problem at hand e.g. because the refracted

waves will run into the coast anyway and one is not interested in that part of the coast).

In such cases the ridges are vitally important to obtain good SWAN results (at sea the

waves are ’clipped’ by depth-induced breaking over the ridges which must

therefore rep-

resented in SWAN computation). This requires not only that these ridges should be well

represented on the input grid but also after interpolation on the computational grid. This

can be achieved by choosing the grid lines of the input grid along the ridges (even if this

may require some slight ”shifting” of the ridges) and choosing the computational grid to

be identical to the input grid (otherwise the ridge may be ”lost” in the interpolation from

the bottom input grid to the computational grid).

Finally, one should use a time step that is small enough that time variations in the

bathymetry, current, water level, wind and bottom friction are well resolved.

2.6.3 Computational grids and boundary / initial / ﬁrst guess

conditions

The computational spatial grid must be deﬁned by the user. The orientation (direction)

can be chosen arbitrarily.

The bou ndaries of the computational spatial grid in SWAN are either land or water. In the

case of land there is no problem: the land does not generate waves and in SWAN it absorbs

all incoming wave energy. But in the case of a water boundary there may be a problem.

Often no wave conditions are known along such a boundary and SWAN then assumes

that no waves enter the area and that waves can leave the area freely. These assumptions

obviously contain errors which propagate into the model. These boundaries must therefore

be chosen suﬃciently far away from the area where reliable computations are needed so

12 Chapter 2

that they do not aﬀect the computational results there. This is best established by varying

the location of these boundaries and inspect the eﬀect on the results. Sometimes the waves

at these boundaries can

be estimated with a certain degree of reliability. This is the case if

(a) results of another mo del run are available (nested computations) or, (b) observations

are available. If model results are available along the boundaries of the computational

spatial grid, they are usually from a coarser resolution than the computational spatial grid

under consideration. This implies that this coarseness of the boundary propagates into

the computational grid. The problem is therefore essentially the same as if no waves are

assumed along the boundary except that now the error may be more acceptable (or the

boundaries are permitted to be closer to the area of interest). If observations are available,

they can be used as input at the bou ndaries. However, this usually covers only part of the

boundaries so that the rest of the boundaries suﬀer from the same error as above.

A special case occurs near the coast. Here it is often possible to identify an up-wave

boundary (with proper wave information) and two lateral boundaries (with no or partial

wave information). The aﬀected areas with errors are typically regions with the apex

at the corners of the water boundary with wave information, spreading towards shore at

an angle of 30

to 45

for wind sea conditions to either side of the imposed mean wave

direction (less for swell conditions; the angle is essentially the one-sided width of the

directional distribution of wave energy). For propagation of short crested waves (wind sea

condtions) an example is given in Figure 2.1. For this reason the lateral boundaries should

be suﬃciently far away from the area of interest to avoid the propagation of this error

into this area. Such problems do not occur if the lateral boundaries contain proper wave

information over their entire length e.g. obtained from a previous SWAN computation or

if the lateral boundaries are coast.

When output

is requested along a boundary of the computational grid, it may occur that

this output diﬀers from the boundary conditions that are imposed by the user. The reason

is that SWAN accepts only the user-imposed incoming

wave comp onents and that it re-

places the user-imposed outgoing

wave components with computed out going components

(propagating to the boundary from the interior region). This is only the case for struc-

tured grids (both regular and curvi-linear ones). The user is informed by means of

a warning in the output when the computed signiﬁcant wave height diﬀers more than 10%,

say (10% is default), from the user-imposed signiﬁcant wave h eight (command BOUND...).

The actual value of this diﬀerence can be set by t he user (see the SET command). Note

that this warning will not apply in the case of unstructured grids.

If the computational grid extends outside the input grid, the reader is referred to Sec-

tion 2.6 to ﬁnd the assumptions of SWAN on depth, current, water level, wind, bottom

friction in the non-overlapping area.

The spatial resolution of the computational grid should be suﬃcient to resolve relevant

details of the wave ﬁeld. Usually a good choice is to take the resolution of the compu-

tational grid approximately equal to that of the bottom or current grid. If necessary, an

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