Study On Sea Ice Geometric Parameters And Parameterization Of Drag Coefficient Based On Image Analysis

Based on the sea ice in-situ investigation images obtained during the 2nd Chinese National Arctic Research Expedition and the 19th, 21st Chinese National Antarctic Research Expedition, combing with laboratory physical modelling experiments, image analysis technologies are used in the present dissertation to provide a deep discussion on image acquisition, image processing, parameter analysis and parameterization method buildup related with sea ice dynamic processes.As the beginning of this thesis, the important meanings of sea ice to global climate change and ocean engineering in polar and subpolar sea regions are explained firstly. Then, it is pointed out that sea ice numerical modelling, as a powerful tool of sea ice research, is limited by the precision of parameters within models instead of itself so far. It is meaningful to study the important parameters of sea ice dynamics by means of the rapidly developing digital imaging techniques so as to provide better input parameters needed in numerical models. Moreover, the research conditions and developing trends of the sea ice parameters and parameterization are also summarized there.Techniques of image acquisition and image processing used in this thesis are discussed in the second chapter. The image acquisition techniques used in sea ice in-situ investigation and laboratory physical model experiment are firstly introduced, and then the image processing techniques for sea ice aerial photographing that needed in later chapters are studied. Different image segmentation algorithms are compared and their advantages and disadvantages are pointed out as well as their own applicabilities. Furthermore, a kind of floe separation algorithm based on mathematical morphology is proposed to improve the efficiency of floe identification in sea ice image analysis, and such method is proved to be more automatic and exact through many applications.Some important parameters of sea ice dynamics are then discussed in the follow parts. The third chapter is focus on sea ice thickness and concentration. Based on sea ice images captured by ship-based CCD camera and aerial photographing during the 2nd Chinese National Arctic Research Expedition, the methods of obtaining ice thickness and concentration from such images are firstly introduced, and then it is found that ice/snow thickness and ice concentration increase with the increasing latitude in summer Arctic after data analysis. Moreover, ice concentration data obtained at different scales are compared and the result from satellite remote sensing is found to be obviously higher than others at middle values. Besides, to a systemic method of obtaining ice thickness and concentration simultaneously by using two ship-based CCD cameras, the corresponding algorithm modifing the image distortion is also provided to obtain more veracious ice concentrations.The objective of the fourth chapter is floe size distribution. Through analyzing aerial images of sea ice at different latitudes in the 2nd Chinese National Arctic Research Expedition, an obvious variation of morphology of Arctic ice floes with increasing latitude is obtained, and after mean caliper diameter of ice floe is used as characteristic size to analyze floe size distribution, it is found that cumulative probabilities of floe size agree with a power-law function, and distribution dimension is generally in the range of 1.05～1.27 and slightly increases with increasing latitude. While in the analysis of sea ice images captured in the marginal ice zone of summer Prydz Bay during 21st Chinese National Antarctic Research Expedition, obvious variations of floe size and shape parameters with the distance into the marginal ice zone are also observed, and the mainly reason of which is the wave-ice interaction there. But in floe size distribution analysis, large deviations of cumulative distribution of floe size to the ideal power law induced by both the truncation effect of sampling and the considerable thermodynamic effect on ice floes in summer Prydz Bay occur, and so that an upper-trtmcated power-law function and a Weibull function are used in curve fitting and four calculated parameters of alternative functions are confirmed to be important descriptors of the evolution of floe size distribution in the marginal ice zone. Moreover, the iceberg size and drift data obtained during the 19th Chinese National Antarctic Research Expedition is analyzed, and the result shows that icebergs of different size have different size distribution characteristics owing to their different dynamic and thermodynamic experiences. The Weibull function is suitable only for smaller iceberg but size distribution of larger iceberg agrees better with the power-law function. Combining with synchronously collected wind, current velocity and iceberg drift velocity, a kinematic relationship is developed as a multiple linear regression function and the result shows that the contribution of wind to iceberg drift is just half of that of current.In the last chapter, ice-water drag coefficient and corresponding parameterization method is studied. After a general review of measuring methods of drag coefficient (including eddy correlation method, profile method, momentum method), the limitations of such methods are explained and a parameterization method relating ice-water drag coefficient with ice morphological parameters is proposed as a good solution. Then a model to distinguish skin drag coefficient and form drag coefficient is established based on a floe drag experiment by using a wave-current tank in laboratory, and the quantitative relationship between skin/form drag coefficient and floe geometric parameters is established as a result. Afterwards, considering the total ice-water drag comprise of skin friction, form drag induced by ice edge and form drag induced by ice ridge, a quantitative relationship between ice-water drag coefficient and. ice/ridge geometric parameters is established, and from computational analysis it is found that the unmonotonous variations of drag coefficient with ice concentration that frequently observed in sea ice field investigation mostly result from the influence of form drag from ice edge.