Airflow patterns upwind of obstacles and their significance for echo dune formation: A field measurement of the effects of the windward slope angle

The velocities in front of five topographic obstacles with windward slope angles between 45A degrees and 80A degrees were measured using ultrasonic anemometers in the field at the southeastern margin of the Tengger Desert. The characteristics of the horizontal and vertical velocities indicated that the airflow pattern was significantly influenced by the obstacle's windward slope angle. The horizontal flow decelerated but the vertical flow accelerated upwind of the obstacles when the windward slope was less than 60A degrees, but a reversed flow formed in front of steeper obstacles. The size of the area occupied by the reversed horizontal component increased with increasing windward slope, but the region of reversed vertical velocity became narrower and taller. Airflow turbulence was strengthened as the wind approached the obstacle, and low-frequency high-energy velocity fluctuations were observed upwind of the obstacle, and could affect the entrainment and saltation processes of sand particles. Turbulence intensities for both the horizontal and vertical components of the airflow increased with increasing windward slope and with decreasing distance from the obstacle. The deceleration and downward movement of airflow upwind of the obstacle would result in supersaturation of the air-sand flow, leading to deposition of moving sand, and these deposited grains would become the material basis for the formation of echo dunes. The reversed airflow would both determine the initial position of the echo dune and provide the necessary motive power to subsequently shape the echo dune.