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Register a new userWind-induced drift of objects at sea: the leeway field method
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Added by
{\\O}yvind Breivik PhD
Publication date
2011
fields
Physics
and research's language is
English
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A method for conducting leeway field experiments to establish the drift properties of small objects (0.1-25 m) is described. The objective is to define a standardized and unambiguous procedure for condensing the drift properties down to a set of coefficients that may be incorporated into existing stochastic trajectory forecast models for drifting objects of concern to search and rescue operations and other activities involving vessels lost at sea such as containers with hazardous material. An operational definition of the slip or wind and wave-induced motion of a drifting object relative to the ambient current is proposed. This definition taken together with a strict adherence to 10 m wind speed allows us to refer unambiguously to the leeway of a drifting object. We recommend that all objects if possible be studied using what we term the direct method, where the objects leeway is studied directly using an attached current meter. We divide drifting objects into four categories, depending on their size. For the smaller objects (less than 0.5 m), an indirect method of measuring the objects motion relative to the ambient current must be used. For larger objects, direct measurement of the motion through the near-surface water masses is strongly recommended. Larger objects are categorized according to the ability to attach current meters and wind monitoring systems to them. The leeway field method proposed here is illustrated with results from field work where three objects were studied in their distress configuration; a 1:3.3 sized model of a 40-ft Shipping container, a World War II mine and a 220 l (55-gallon) oil drum.
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The Arctic sea ice represents an important energy reservoir for the climate of the northern hemisphere. The shrinking of the polar ice in the past decades decreases the stored energy and raises serious concerns about future climate changes.[1-4] Model calculations of the present authors [5,6] suggest that half of the global warming during the past fifty years is directly related to the retreat of the sea ice, while the cause is not well understood, e.g. the role of surface pollution [7-10]. We have analysed the reported annual melting and freezing data of the northern sea ice in the years 1979 to 2018 [11] to gain some insight. Two features can be deduced from our simple model: (i) recent results [12,13] are confirmed that approximately 60 % of the loss of sea ice stems from energy transport to the arctic region. (ii) We find evidence that the remaining part of the ice retreat originates from an increasing surface absorption of solar radiation, obviously due to the rising surface pollution of the sea ice. While the phenomenon was previously considered by several authors in a qualitative way, our analysis contributes semi-quantitative information on the situation. We estimate that the relevant fall-out of light absorbing aerosols onto the sea ice increased by 17 +/- 5 % during the past fifty years. A deposition of additional 3 +/- 1 % of solar radiation in the melting region results that accounts for the ice retreat. Recalling the important role of the ice loss for the terrestrial climate,[3,5,9] the precipitation of air pollution in the Arctic seems to be an important factor for the global warming.
The importance of snow cover and ice extent in the Northern Hemisphere was recognized by various authors leading to a positive feedback of surface reflectivity on climate. In fact, the retreat of Arctic sea ice is accompanied by enhanced solar input in the Arctic region, i.e. a decrease of the terrestrial albedo. We have studied this effect for the past six decades and estimate the corresponding global warming in the northern hemisphere. A simple 1-dimensional model is used that includes the simultaneous increase of the greenhouse gases. Our results indicate that the latter directly cause a temperature rise of only 0.2 K in 1955 to 2015, while a notably larger effect 0.7 +/- 0.2 K is found for the loss of Arctic sea ice in the same time. These numbers comprise most of the reported mean temperature rise of 1.2 +/- 0.2 K of the northern hemisphere. The origin of the sea-ice retreat is discussed, e.g. internal variability or feedback by the CO2 concentration increase. Our data also suggest a delayed response of the global surface temperature rise to the loss of sea ice with a time constant of approximately 10 to 20 years.
The growing concentrations of the greenhouse gases CO2, CH4 and N2O (GHG) in the atmosphere are often considered as the dominant cause for the global warming during the past decades. The reported temperature data however do not display a simple correlation with the concentration changes since 1880 so that other reasons are to be considered to contribute notably. An important feature in this context is the shrinking of the polar ice caps observed in recent years. We have studied the direct effect of the loss of global sea ice since 1955 on the mean global temperature estimating the corresponding decrease of the terrestrial albedo. Using a simple 1-dimensional model the global warming of the surface is computed that is generated by the increase of GHG and the albedo change. A modest effect by the GHG of 0.08 K is calculated for the period 1880 to 1955 with a further increase by 0.18K for 1955 to 2015. A larger contribution of 0.55 +/-0.05 K is estimated for the melting of polar sea ice (MSI) in the latter period, i.e. it notably exceeds that of the GHG and may be compared with the observed global temperature rise of 1.0 +/- 0.1 K during the past 60 years. Our data also suggest a delayed response of the mean global temperature to the loss of sea ice with a time constant of approximately 20 years. The validity of the theoretical model and the interrelation between GHG-warming and MSI-effect are discussed.
The importance of the sea ice retreat in the polar regions for the global warming and the role of ice-albedo feedback was recognized by various authors [1,2]. Similar to a recent study of the phenomenon in the Arctic [3] we present a semi-quantitative estimate of the mechanism for the Southern Hemisphere (SH). Using a simple model, we estimate the contribution of ice-albedo feedback to the mean temperature increase in the SH to be 0.5 +/- 0.1 K in the years 1955 to 2015, while from the simultaneous growth of the greenhouse gases (GHG) we derive a direct warming of only 0.2 +/- 0.05 K in the same period. These numbers are in nice accordance with the reported mean temperature rise of 0.75 +/- 0.1 K of the SH in 2015 since 1955 (and relative to 1880). Our data also confirm previously noticed correlations between the annual fluctuations of solar intensity and El Nino observations on the one hand and the annual variability of the SH surface temperature on the other hand. Our calculations indicate a slowing down of the temperature increase during the past few years that is likely to persist. Assuming a continuation of the present trends for the southern sea ice and GHG concentration we predict the further temperature rise to decrease by 33 % in 2015 to 2025 as compared to the previous decade.
Ocean hazardous spills and search and rescue incidents are more prevalent as maritime activities increase across all sectors of society. However, emergency response time remains a factor due to a lack of information to accurately forecast the location of small objects. Existing drifting characterization techniques are limited to objects whose drifting properties are not affected by on-board wind and surface current sensors. To address this challenge, we study the application of multirotor unmanned aerial systems (UAS), and embedded navigation technology, for on-demand wind velocity and surface flow measurements to characterize drifting properties of small objects. An off-the-shelf quadrotor was used to measure wind velocity at 10 m above surface level near a drifting object. We also leveraged UAS-grade attitude and heading reference systems and GPS antennas to build water-proof tracking modules that record the position and orientation, as well of translational and rotational velocities, of objects drifting in water. The quadrotor and water-proof tracking modules were deployed during field experiments conducted in lake and ocean environments to characterize the leeway parameters of manikins simulating a person in water. Leeway parameters were found to be an order of magnitude within previous estimates derived using conventional wind and surface current observations. We also determined that multirotor UAS and water-proof tracking modules can provide accurate and high-resolution ambient information that is critical to understand how changes in orientation affect the downwind displacement and jibing characteristics of small objects floating in water. These findings support further development and application of multirotor UAS technology for leeway characterization and understanding the effect of an objects downwind-relative orientation on its drifting characteristics.
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