|LANIGAN, DAVID - University Of Texas|
|ANDERSON, WILLIAM - University Of Texas|
Submitted to: Aeolian Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2016
Publication Date: 6/1/2016
Citation: Lanigan, D., Stout, J.E., Anderson, W. 2016. Atmospheric stability and diurnal patterns of aeolian saltation on the Llano Estacado. Aeolian Research. 21:131-137. doi:10.1016/j.aeolia.2016.04.001.
Interpretive Summary: Field research in West Texas has shown that wind erosion is more likely to occur during the day than at night. Other scientists working in China and Antarctica have confirmed this diurnal pattern. This paper proposes a theory to explain why this diurnal pattern exists. We show that the diurnal pattern of wind erosion activity results from a complex process that involves an interaction between solar heating, thermal instability, atmospheric turbulence and surface conditions. During the day, solar heating produces thermal instability, which enhances the convective mixing of high momentum winds from the upper levels of the atmosphere to the surface layer. Thus, in the afternoon, strong turbulent winds increase the likelihood that winds may intermittently exceed the critical threshold of the surface to produce bursts of blowing sand and dust.
Technical Abstract: Aeolian transport is driven by aerodynamic surface stress imposed by turbulent winds in the Earth’s atmospheric boundary layer (ABL). ABL regime is influenced by stratification, which can either enhance or suppress production of turbulence by shear associated with the vertical gradient of streamwise velocity. During the day and night, surface heat fluxes induce a negative (unstable) and positive (stable) vertical gradient of potential temperature, respectively, which modifies the role of buoyancy in turbulence production. During the brief morning and evening transition periods, the vertical gradient of potential temperature vanishes (neutral stratification). The Monin-Obukhov similarity theory describes how the vertical gradient of streamwise velocity varies with stratification. Simultaneous field measurement of wind speed and aeolian activity were obtained over a 218-day period on a bare, sandy surface on the high plains of the Llano Estacado of west Texas. Wind speed was measured at a height of 2 m with a propeller-type anemometer and aeolian activity was measured at the surface with a piezoelectric saltation sensor. We have used the wind speed measurements within the framework of the Monin-Obukhov similarity theory to estimate ‘typical’ shear velocity, u*, of the ABL as stratification is varied (characterized with the stability parameter). This approach results in a color flood contour of u* against time of day and stability parameter: the procedure demonstrates that aeolian activity is most likely to occur during the day, when buoyancy acts in conjunction with mechanical shear to increase u*.