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Free keywords:
CONVECTIVE BOUNDARY-LAYER; ATMOSPHERIC SURFACE-LAYER; LAGRANGIAN
COHERENT STRUCTURES; PROPER ORTHOGONAL DECOMPOSITION; TRANSILIENT
TURBULENCE THEORY; NUMERICAL-SIMULATION; EVAPORATIVE FRACTION;
PROBABILITY DENSITY; DIURNAL BEHAVIOR; WALL TURBULENCEMeteorology & Atmospheric Sciences; Atmosphere; Turbulence; Boundary layer; Surface layer; Surface fluxes;
Large eddy simulations;
Abstract:
According to Townsend's hypothesis, so-called wall-attached eddies are the main contributors to turbulent transport in the atmospheric surface layer (ASL). This is also one of the main assumptions of Monin-Obukhov similarity theory (MOST). However, previous evidence seems to indicate that outer-scale eddies can impact the ASL, resulting in deviations from the classic MOST scaling. We conduct large-eddy simulations and direct numerical simulations of a dry convective boundary layer to investigate the impact of coherent structures on the ASL. A height-dependent passive tracer enables coherent structure detection and conditional analysis based on updrafts and subsidence. The MOST similarity functions computed from the simulation results indicate a larger deviation of the momentum similarity function phi(m) from classical scaling relationships compared to the temperature similarity function phi(h). The conditional-averaged phi(m) for updrafts and subsidence are similar, indicating strong interactions between the inner and outer layers. However, phi(h) conditioned on subsidence follows the mixed-layer scaling, while its updraft counterpart is well predicted by MOST. Updrafts are the dominant contributors to the transport of momentum and temperature. Subsidence, which comprises eddies that originate from the outer layer, contributes increasingly to the transport of temperature with increasing instability. However, u of different signs are distributed symmetrically in subsidence unlike the predominantly negative as instability increases. Thus, the spatial patterns of uw differ compared to w in regions of subsidence. These results depict the mechanisms for departure from the MOST scaling, which is related to the stronger role of subsidence.
