| 摘要: |
| 在实验室风浪槽中观测风浪和风速,研究粗糙度与波面特征量的关系,发现风浪谱宽度增加,粗糙度增大。在窄谱时,粗糙度随谱宽的增加变化不明显,当波陡降低,粗糙度降低;在宽谱时,当谱宽度增加,即使波陡降低,粗糙度仍可增大。这一结果表明,波陡不足以完全决定粗糙度。当风浪波龄增加,粗糙度呈下降趋势,但由于谱宽度对粗糙度的影响,当波龄增加,部分波浪可有较大的粗糙度。由于这一因素,在粗糙度与波龄关系的观测结果中,数据点的散落不完全由观测误差造成。 |
| 关键词: 粗糙度 谱宽度 波陡 |
| DOI： |
| 分类号: |
| 基金项目:国家自然科学基金资助项目,40106001号 |
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| DEPENDENCE OF SURFACE ROUGHNESS ON SPECTRAL WIDTH AND WAVE STEEPNESS |
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WEN Fan, SU Ze-Yu, Lü Hong-Min
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Laboratory of Physical Oceanography, Ocean University of China, Qingdao, 266003
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| Abstract: |
| The study of surface roughness is important in wave modeling and in remote sensing at sea. In the past four decades, extensive studies have been made on the roughness. On the basis of dimensional analysis Charnock (1955) suggested that the roughness depends on wind speed. Observations in recent years showed that the roughness decreases with wave age. More recently Taylor (2001) studied the dependence of the roughness on the state of the waves and found that the roughness increases with wave steepness. However, considerable scatter exists in some of Taylor’s results. This indicates that surface roughness may depend on some other factors in wind wave fields. In this paper we study the roughness by analyzing wind and wave data in an experiment made in wind wave channel.
The experiment was made in the wind wave channel in the Laboratory of Physical Oceanography in Ocean University of China. The wave surface displacements and the vertical wind profiles over the wave field were measured at ten fetches in the channel. Wind and wave data showed that the roughness decreases with wave age, which is in agreement with many results in current observations. However, it is found that for some waves the roughness increases with wave age. By studying the relation between roughness and wave steepness, it is found that the roughness increases with wave steepness. This is in agreement with Taylor’s result. However for some waves the roughness increases when wave steepness decreases. This indicates that wave steepness is not sufficient in determining the roughness. It is influenced by some other factors in wind wave fields.
By further analyzing the data it is found that the roughness increases with spectral width. For waves with broad spectrum the roughness is remarkably higher than waves with narrow spectrum. For narrow spectrum, the roughness decreases when wave steepness decreases. For broad spectrum the roughness may be high even for low wave steepness and high wave age. This indicates that spectral width is important in determining the roughness.
The question on which factor in wind wave field is important in influencing surface roughness has been extensively discussed in theory. It is believed that surface roughness is determined by multi-scale waves. Singlescale waves are not sufficient in determining the roughness. This implies that the shape of the spectrum (or variables that reflect the spectral shape, such as spectral width) is important in determining the roughness.
The low roughness in some waves in ocean is caused by swells. The observed high roughness in high wave age is often attributed to errors in the observations. From the above analysis we consider that the high roughness in some results may be due to the broad spectrum of the waves.
Vickers (1997) noticed the influence of spectral width on the roughness. He found that the variations of the coefficients in the relationship between roughness and wave age could be explained by the influence of the spectral width. In analyzing field and laboratory data, Toba found that the roughness increases with wave age. This is contrary to many results in current observations. Donelan (1993) pointed out some flaws in Toba’s analysis. However some results in Toba’s analysis may be correct. That is, although the roughness has a tendency of decrease when wave age increases, for waves with broad spectrum the roughness may be high even for high wave age. |
| Key words: Roughness, Spectral width, Wave steepness |
