engleski

Determination and characteristics of nonstationarity in the surface layer during the T-REX experiment

Stationarity of turbulence statistics is a fundamental assumption in the theory of homogeneous turbulence. A lack of stationarity introduces uncertainty in the applicability of statistical turbulence theory in surface-layer parameterizations. It is therefore important to determine the degree of nonstationarity in turbulence time series. Many approaches are developed over the years to determine nonstationarity but so far, there has been no systematic investigation of the differences and similarities between the approaches. In this paper, we contrast several approaches often used in investigation of surface layer turbulence, including a statistical test to determine trends (Bendat and Piersol, 1986), a determination of the Mahrt's (1998) nonstationarity ratio and a determination of the differences between 5 - and 30 minute variances (Foken and Wichura, 1996). We apply these approaches to time series of wind and temperature collected during the Terrain-induced Rotor EXperiment in Owens Valley during spring 2006. Measurements were made on three 30m towers with 6 levels of ultrasonic anemometers. Our results show that the degree of nonstationarity varies considerably with the used approach. We argue that absence of trend is not a sufficient condition for stationarity in time series of variances because in many cases such time series still show a behavior indicative of intermittent turbulence, both for stable and unstable conditions. When the trend test is combined with an additional condition that the intermittency level is below a critical value, this approach resembles the Foken and Wichura method. Mahrt's method mostly corresponds with the trend test. We show examples of the degree of nonstationarity for individual days and a climatology for the entire duration of the experiment. We also relate the degree of nonstationarity to stability and flow conditions and discuss implications of our results for the test of surface layer similarity functions in complex terrain.

TREX experiment; Turbulence; Nonstationarity

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