INVESTIGATION ON SPATIAL REPRESENTATIVENESS OF POINT MEASUREMENTS

IMPLICATION FOR IMBALANCE PROBLEM

Abstract

The spatial representativeness of turbulent fluxes on the basis of point measurements has been investigated through a theoretical approach and numerical experiments using Large Eddy Simulation (LES) Model. These investigations have been done for the daytime atmospheric boundary layers heated uniformly over the flat surface with no synoptic vertical motion under several geostrophic winds from 1 to 4 (ms^-1). The following results were obtained;
(1) The spatial representativeness of turbulent fluxes based on the time-series analysis are shown to be problematic: Their horizontally averaged value has a negative imbalance irrespective of the density of measurements points, and their spatial variability increases, in accordance with averaging time, or under lower horizontal wind.
(2) The imbalance intensities are closely related to turbulent organized structure (TOS). The points over the updraft regions are likely to have positive imbalance due to highly activated thermal plumes. The temporal and spatial change of TOS pattern cause low-frequency trends in time-series data, which can account for the large horizontal scatter of flux estimates.
(3) High-pass filter excluding the temperature trend is theoretically useful to improve the representativeness of point measurements, although the filter choice actually needs a great care not to damage the turbulent components.