Improved Quadrant Analysis for Large-Scale Events Detection in Turbulent Transport

Abstract

Quadrant analysis has been widely used to investigate the turbulent characteristics in the atmospheric boundary layer (ABL). Although quadrant analysis can identify turbulent structures that contribute significantly to turbulent fluxes, the approach to the hyperbolic hole and its parameter, referred to as hole size, remains uncertain and varies among different studies. This study discusses an improved quadrant analysis with an objective definition of the hole size for the isolation of large coherent structures from small-scale background fluctuations. Eddy covariance data collected 50 m above the grass canopy were used to analyze and evaluate the proposed method. This improved quadrant analysis ensures that the detected large coherent eddies play a dominant role in transporting fluxes, occupying 10% of the total time, with mean flux contributions ranging from 62% to 95% for momentum and 35–104% for scalars. The separated background small-scale eddies are isotropically characterized by a comparable time duration and flux contributions in each quadrant. It is observed that under an unstable atmosphere, large-scale ejections are more active than sweeps, while under stable conditions, they are equally important. Furthermore, mechanical-driven transport under near-neutral conditions only enhances ejection and sweep motions of momentum. In contrast, the buoyancy-driven scenarios under unstable conditions enhance the large-scale activities of sensible heat alone.

Linkage between surface energy balance non‐closure and horizontal asymmetric turbulent transport

“…The observation site, as a part of the Guangdong Provincial Observation and Research Station, is located in a subtropical suburban environment (23.20°N, 113.49°E, 30 m above sea level). The measurement, conducted during an intensive observation period from September 5 to October 31 in 2018, is a part of the long‐term land–atmosphere exchange observation project in the Pearl River Estuary area (Lan et al ., 2022; Wang et al ., 2022). The experimental site is surrounded by a diversity of land types (e.g., rural roads in the east, forested hills in the west, sparse short buildings to the north, and a small parking lot to the south) that provide a complex underlying surface condition (Figure 1a).…”

“…, where 𝜎 w and 𝜎 T are the standard deviations of vertical velocity and temperature respectively. Detailed information about how flux contribution is calculated by quadrant analysis can be found in the Supporting Information and prior studies (Lan et al, 2022;Wang et al, 2022). Taking the partial derivative of Equation ( 9) with respect to x and taking the absolute value for both sides establishes the link between the turbulent transport asymmetric and horizontal inhomogeneity of w ′ T ′ :…”