A novel method for the study of near-surface turbulence using 3-d hot-film anemometry: OTIHS

Under statically stable conditions in particular, and in the vicinity of near-surface roughness elements generally, a portion of the scales of turbulence relevant to calculating accurate fluxes become smaller than the pathlength of sonic anemometers. As a result, pathlength averaging occurs and estimates of the fluxes can be compromised or become otherwise questionable. In micrometeorology, other measurements of interest can also be similarly affected. When the winds are weak and the Richardson number achieves large values, turbulence becomes weak, smaller scale and quasi-two-dimensional (e.g. 'pancake eddies'); more importantly, significant portions of the turbulent flux may become immeasurable by current sonic anemometers. Thus, a satisfying understanding of these fluxes and balances can be hampered by existing technology. With regards to practical application, fundamental assumptions underlying surface layer theory and therefore numerical parameterizations based upon surface layer theory also breakdown. As a result, under these conditions, numerical weather prediction, large-eddy simulation, global climate simulation and worst-case air pollution and toxic substance calculations are subject to great uncertainty and possible failure.

We will describe an experimental technique that was developed in preparation for the extensive Terrain-induced Rotors Experiment (T-REX). Using in-situ calibration based on sonic anemometer readings, we measure the momentum flux on very small scales and at a very high rate – 2 kilohertz - using millimeter scale three-dimensional hot-film anemometers. As a result, we are able to calculate the momentum flux contributions from very small scales, assess the flux lost when using sonic anemometer techniques and, because we will be able to measure turbulence at the Taylor microscale (O[1]cm), estimate the dissipation with far greater fidelity. Advances in the understanding of the behaviour of the near-surface atmosphere under these conditions, including the thorny problem of globally intermittent turbulence, are expected.