Taylors hypothesis and its impact on flux measurements in a forest clearcut

TL;DR

This study investigates the validity of Taylor's hypothesis in heterogeneous forest clearcut environments using DTS and EC data, revealing scale-dependent convective speeds that impact turbulence measurements and flux estimates.

Contribution

It provides the first detailed analysis of Taylor's hypothesis applicability over heterogeneous forest clearcuts, highlighting the effects of heterogeneity on turbulence measurements.

Findings

Convective speeds follow a power-law with wavenumber, differing from homogeneous flow.

Heterogeneity influences the relationship between convective speeds and mean wind.

A critical frequency limit is identified, affecting flux estimates beyond this point.

Abstract

Taylors hypothesis is the backbone to convert observations done over time to spatial information of the flow while carrying out turbulence measurements on a micrometeorological tower. To address its validity over a highly heterogeneous forest clearcut surface, we utilize an extensive Distributed Temperature Sensing (DTS) and Eddy Covariance (EC) datasets. The DTS measured space-time correlation curves of temperature fluctuations are used to compute the bulk convective speeds of temperature structures in buoyant conditions at a height of 3.1 m above the clearing. These convective speeds are compared with the mean wind speed and turbulent intensities of streamwise velocities obtained from the EC system at the middle of the clearcut. Depending on if is parallel or perpendicular to the forest edge, the relationships between and are significantly different. However, irrespective of the wind direction, the convective speeds of temperature structures at inertial subrange scales behave in a power-law fashion with increasing wavenumbers. The exponent of this power-law differs from a homogeneous atmospheric surface layer flow, thereby pointing towards the effects of heterogeneity. The scale-dependent convective speeds non-linearly transform the temporal frequencies to streamwise wavenumbers, which, eventually, impacts the properties of turbulence (co) spectra. More importantly, this non-linear transformation yields a critical frequency limit, beyond which the eddy length scales derived from frequencies are smaller than the physical dimension of the sonic anemometers, and therefore, cannot be faithfully resolved. This critical limit questions the EC flux estimates beyond this frequency.