Longshore drift estimation using fluorescent tracers: New insights from an experiment at Comporta Beach, Portugal

A fluorescent sand-tracer experiment was performed at Comporta Beach (Portugal) with the aim of acquiring longshore sediment transport data on a reflective beach, the optimization of field and laboratory tracer procedures and the improvement of the conceptual model used to support tracer data interpretation.

The field experiment was performed on a mesotidal reflective beach face in low energetic conditions (significant wave height between 0.4 and 0.5 m). Two different colour tracers (orange and blue) were injected at low tide and sampled in the two subsequent low tides using a high resolution 3D grid extending 450 m alongshore and 30 m cross-shore. Marked sand was detected using an automatic digital image processing system developed in the scope of the present experiment.

Results for the two colour tracers show a remarkable coherence, with high recovery rates attesting data validity. Sand tracer displayed a high advection velocity, but with distinct vertical distribution patterns in the two tides: in the first tide there was a clear decrease in tracer advection velocity with depth while in the second tide, the tracer exhibited an almost uniform vertical velocity distribution. This differing behaviour suggests that, in the first tide, the tracer had not reached equilibrium within the transport system, pointing to a considerable time lag between injection and complete mixing. This issue has important implications for the interpretation of tracer data, indicating that short term tracer experiments tend to overestimate transport rates. In this work, therefore, longshore estimates were based on tracer results obtained during the second tide.

The estimated total longshore transport rate at Comporta Beach was 2 × 10^− 3 m³/s, more than four times larger than predicted using standard empirical longshore formulas. This discrepancy, which results from the unusually large active moving layer observed during the experiment, confirms the idea that most common longshore transport equations under-estimate total sediment transport in plunging/surging waves.     

赤道西太平洋大气边界层的诊断分析

利用１９９３年ＥＮＳＯ事件爆发（４月）前酝酿时期“热带大洋与全球大气－海洋耦合响应试验”强化观测阶段“向阳红五号”科学考察船１５５°Ｅ，２°Ｓ定点海洋气象和高空大气探测资料，分析了赤道西太平洋大气边界层特征。结果表明：赤道西太平洋上空辐合对流区边界层内也有等温或逆温层存在，大气边界层物理参数变化与大尺度海－气变异有密切关系，赤道太平洋沃克环流加强、东移，边界层上部逆温层出现概率明显增大，边界层内高湿层湿度减小。另外，边界层内实测风速、风向廓线随高度的变化基本上不服从Ｅｋｍａｎ规律，但由其平均，分量合成得到的平均风随高度的变化却基本符合Ｅｋｍａｎ规律。     

Study on the characteristics of the marine boundary layer in the Equatorial Pacific

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“暖池”表层对大气局地强迫的响应特征

利用湍流动能垂直混合模式和ＴＯＧＡ－ＣＯＡＲＥ加强观测期的观测资料,对“暖池”上混合层的垂直混合过程进行数值试验和数值模拟,分析表层温度、盐度的变化特征。结果表明：ＴＫＥ模式可以较好地模拟混层,尤其是表层温度、盐度对大气局地强迫的响应;太阳辐射是热源,感热、潜热通量等会造成“暖池”上混合层的温度降低,“暖池”对大气释放热量;降水有利于“淡水盖”形成和维持,从则使层结稳定,ＳＳＴ升高。但在气温低于海     

Optical water type discrimination and tuning remote sensing band-ratio algorithms: Application to retrieval of chlorophyll and Kd(490) in the Irish and Celtic Seas

This paper assesses the feasibility of applying remote sensing algorithms based on blue/green reflectance ratios to Case 2 waters. Two algorithms from the SeaDAS (NASA) image processing package, OC4v4 for surface chlorophyll concentration, Chl, and K(490) for the attenuation coefficient for downward irradiance at 490 nm, K_d(490), were investigated using an extensive set of observations from the Irish and Celtic Seas. In situ data from a profiling radiometer were used as inputs for the algorithms to avoid uncertainties in atmospheric correction procedures, and direct measurements of Chl and K_d490 and were used for validation purposes. The standard versions of the algorithms performed poorly: OC4v4 generally overestimated Chl (with a very low coefficient of determination), and K(490) progressively underestimated K_d490 for values greater than 0.3 m⁻¹. A two-step procedure for level 2 product generation was therefore devised in which the numerical coefficients of OC4v4 and K(490) were tuned for the two optical water types known to occur most frequently in this region (McKee, D., Cunningham, A., 2006. Identification and characterisation of two optical water types in the Irish Sea from in situ inherent optical properties and seawater constituents. Estuarine, Coastal and Shelf Science 68, 305–316) by statistical regression on a data set of 102 stations from the Irish and Celtic Seas. The water types were distinguished by the magnitude of their normalised water leaving radiance signals at 665 nm, nL_w(665), and appropriate versions of the tuned algorithms applied to each water type. When this procedure was tested on an independent data set of 19 stations from the Bristol Channel, Chl values were recovered with an RMS error of 0.36 mg m⁻³ and K_d(490) values with an RMS error of 0.095 m⁻¹. The identification of water types from water-leaving radiance signals, and the application of band-ratio algorithms tuned for specific water types, may therefore provide a simple means of improving the quality of remote sensing products in optically complex shelf seas.     

The concentration and distribution of dimethyl sulfide in the marine atmospheric boundary layer near the equator

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底部边界层颗粒态物质迁移特征的放射性核素分析方法

利用颗粒态放射性核素携带的颗粒物历经过程信息,我们提出用泥沙扩散方程和颗粒态放射性核素扩散方程联解底部边界层颗粒态物质迁移参数的方法。分析实例的样品取自荷兰Waden Sea南部Balgzand潮滩(砂坪)和Mok湾潮滩(泥坪)的两个站位(BG1和Mok2)。示踪核素为~(234)Th,~(210)Pb和~(137)Cs,其放射性比度由r能谱测出。     

Steady Response of a Ventilated Thermocline to Enhanced Ekman Pumping

The steady response of the ventilated thermocline to an increase in Ekman pumping is investigated, focusing on the effect of the mixed layer depth distribution on the subsurface density anomaly. We consider only the subtropical gyre, and the mixed layer is assumed to be deep in the northwest and shallow elsewhere with a narrow transition zone separating the deep and shallow mixed layer regions. At the intersection of this narrow transition zone and the outcrop line, low potential vorticity fluid is subducted into and ventilates the thermocline. In such a situation, an enhancement of the Ekman pumping confined to the northern subtropical gyre leads to pronounced subsurface cold anomalies in the southern subtropics, which is free of anomalous forcing. These density anomalies are much greater than those that occur when either the mixed layer depth is zonally uniform or the Ekman pumping is enhanced in the whole subtropical gyre. They are caused by anomalous changes in the trajectory of the low potential vorticity fluid in response to anomalous Sverdrup flow. This revised version was published online in July 2006 with corrections to the Cover Date.