Collaborative Research: Small-Scale Variability in Aeolian Sand Transport

该项目将研究自然环境中风沙和其他松散物质运动的小尺度变化的特征和驱动因素，特别关注与跃变飘带的组织和动力学相关的沙运输的水平变化。该项目建立在两个概念上：(1)如果风吹得足够快，穿过含有松散或松散沙粒的表面，一些沙粒将被气流夹带并顺风输送；(2)风吹得越快，产生的输送速率就越大。这两个基本概念是指导假设，支撑了用于理解和预测风沙输送速率和特定风沙地貌演化的机械数学模型。在地球和火星的干旱地区，风是主要的地表过程；地球和火星的大部分陆地表面都是由风产生或风改造的地貌。风沙主要通过跳跃过程移动，通过收缩（盘）、增生（沙丘）和磨损（沙塘）形成了许多地貌。在许多陆地环境中，风沙是一种严重的自然灾害，但在其他环境中，它是一种需要管理的资源。因此，了解跃变的特征和控制是理解和管理这种环境的基础。本项目重点研究自然环境中风沙跃迁的小尺度变化特征和驱动因素，特别关注跃迁飘带的组织和平流。研究人员将在巴西杰里科阿卡拉进行一套全面的实地实验，以收集风沙的点和面测量数据，并对风系统进行详细测量，以确定飘带的物理尺寸、速度、飘带内外的跳跃强度，以及风和风沙的不稳定性和空间不均匀性之间的关系。由于信风的一致性，巴西东北海岸的位置是理想的研究类型。研究人员还将检查“上层结构”，这是在湍流边界层中发现的米到公里大小的运动，以确定上层结构是否导致了飘带的形成。该项目将量化以小时和米为尺度的运输变异性，并将指导未来在进行运输点测量的情况下的实验设计。研究结果将为今后研究的评价和对过去研究的再评价提供必要的信息。该项目应促进对风和跃变相互作用的基本理解，特别是在将风中的相干湍流结构（上层结构）与跃变结构模拟物（飘带）联系起来的背景下。由于运输可变性通常被认为是改善运输率预测的障碍，因此更好地再现原型条件的模型将通过向环境管理者和规划者提供高质量的预测，提高规划和减轻风沙对环境影响的能力。项目结果将提供一个更好的基础，将目前的风成状态外推到在未来不同气候条件下的预期情况和对地外环境的假设情况。

This project will examine the characteristics and drivers of small-scale variability of wind-driven movement of sand and other unconsolidated materials in natural environments, with special attention given to the horizontal variability in sand transport associated with the organization and dynamics of saltation streamers. This project builds on two concepts: (1) if the wind blows fast enough across a surface comprising unconsolidated or poorly consolidated sand grains, some grains will be entrained by the flow and transported downwind, and (2) the faster the wind blows, the greater the resulting transport rate will be. These two basic concepts are the guiding assumptions that underpin the mechanistic, mathematical models used to comprehend and predict the rates of aeolian (wind) sand transport and evolution of particular aeolian landforms. In the arid regions of Earth and on Mars, wind is the dominant surficial process; wind-generated or wind-modified landforms cover much of the land surface of Earth and Mars. Blowing sand, moving mainly through the process of saltation, is responsible for many of those landforms via deflation (pans), accretion (dunes), and abrasion (yardangs). In many terrestrial environments, blowing sand is a critical natural hazard, but in others, it is a resource to be managed. Understanding the characteristics and controls on saltation therefore is fundamental to understanding and managing such environments. This project focuses on the characteristics and drivers of small-scale variability of aeolian saltation in natural environments, with special attention given to the organization and advection of saltation streamers. The investigators will conduct a comprehensive set of field experiments in Jericoacoara, Brazil, in order to gather point and areal measurements of wind-blown sand as well as detailed measurements of the wind system in order to characterize the physical dimensions of streamers, their velocity, the intensities of saltation within and without the streamers, and the relationship between unsteadiness and spatial non-uniformity in the wind and the blowing sand. The location on the northeast coast of Brazil is ideal for type of research because of the consistency of the tradewinds that blow onshore there. The investigators will also examine 'superstructures,' which are meter- to kilometer-sized motions found within the turbulent boundary layer, in order to determine whether superstructures cause the formation of streamers.This project will quantify the transport variability over scales of hours and meters and should guide future experimental design in instances where point measures of transport will be made. The results should provide necessary information for appraisal of future studies and the reevaluation of past studies. This project should advance basic understanding of the interactions of wind and saltation, especially in the context of linking coherent turbulent structures (superstructures) in the wind with the saltation structure analog (streamers). Because transport variability is commonly described as a hurdle to improving transport rate predictions, models that better reproduce prototype conditions will enhance the ability to plan for and to alleviate the environmental impacts of blowing sand by providing high-quality predictions to environmental managers and planners. Project results will provide a better foundation for the extrapolation of current aeolian regimes to those anticipated under alternative future climates and to those posited for extraterrestrial environments.