TY - JOUR

T1 - The Departure from Mixed-Layer Similarity During the Afternoon Decay of Turbulence in the Free-Convective Boundary Layer

T2 - Results from Large-Eddy Simulations

AU - Elguernaoui, Omar

AU - Reuder, Joachim

AU - Li, Dan

AU - Maronga, Björn

AU - Paskyabi, Mostafa Bakhoday

AU - Wolf, Tobias

AU - Esau, Igor

N1 - Funding Information: The first author is grateful to Siegfried Raasch for inspiring discussions about the atmospheric boundary-layer, turbulence, and large-eddy simulation. The first author is grateful to Elie Bou-Zeid for invaluable input and insights about the concepts of equilibrium and quasi-equilibrium. We thank Bert Holtslag for his comments which helped improving the initial version of the manuscript. We thank Marie Lothon and Peter Sullivan for sharing with us their data. We thank Margaret LeMone and two anonymous reviewers for the constructive comments and insightful questions, which helped to improve the manuscript. The third author acknowledges support from U.S. National Science Foundation (NSF) under the Award number AGS-1853354 and also support from the Alexander von Humboldt Foundation. The computations were performed on resources provided by Sigma2—the National Infrastructure for High Performance Computing and Data Storage in Norway, Grant No NN9506k. Funding Information: The first author is grateful to Siegfried Raasch for inspiring discussions about the atmospheric boundary-layer, turbulence, and large-eddy simulation. The first author is grateful to Elie Bou-Zeid for invaluable input and insights about the concepts of equilibrium and quasi-equilibrium. We thank Bert Holtslag for his comments which helped improving the initial version of the manuscript. We thank Marie Lothon and Peter Sullivan for sharing with us their data. We thank Margaret LeMone and two anonymous reviewers for the constructive comments and insightful questions, which helped to improve the manuscript. The third author acknowledges support from U.S. National Science Foundation (NSF) under the Award number AGS-1853354 and also support from the Alexander von Humboldt Foundation. The computations were performed on resources provided by Sigma2—the National Infrastructure for High Performance Computing and Data Storage in Norway, Grant No NN9506k.

PY - 2023/8

Y1 - 2023/8

N2 - This study analyses the departure of the velocity-variances profiles from their quasi-steady state described by the mixed-layer similarity, using large-eddy simulations with different prescribed shapes and time scales of the surface kinematic heat flux decay. Within the descriptive frames where the time is tracked solely by the forcing time scale (either constant or time-dependent) describing the surface heat flux decay, we find that the normalized velocity-variances profiles from different runs do not collapse while they depart from mixed-layer similarity. As the mixed-layer similarity relies on the assumption that the free-convective boundary layer is in a quasi-equilibrium, we consider the ratios of the forcing time scales to the convective eddy-turnover time scale. We find that the normalized velocity-variances profiles collapse in the only case where the ratio (r~) of the time-dependent forcing time scale to the convective eddy-turnover time scale is used for tracking the time, supporting the independence of the departure from the characteristics of the surface heat flux decay. As a consequence of this result, the knowledge of r~ is sufficient to predict the departure of the velocity variances from their quasi-steady state, irrespective of the shape of the surface heat flux decay. This study highlights the importance of considering both the time-dependent forcing time scale and the convective eddy-turnover time scale for evaluating the response of the free-convective boundary layer to the surface heat flux decay.

AB - This study analyses the departure of the velocity-variances profiles from their quasi-steady state described by the mixed-layer similarity, using large-eddy simulations with different prescribed shapes and time scales of the surface kinematic heat flux decay. Within the descriptive frames where the time is tracked solely by the forcing time scale (either constant or time-dependent) describing the surface heat flux decay, we find that the normalized velocity-variances profiles from different runs do not collapse while they depart from mixed-layer similarity. As the mixed-layer similarity relies on the assumption that the free-convective boundary layer is in a quasi-equilibrium, we consider the ratios of the forcing time scales to the convective eddy-turnover time scale. We find that the normalized velocity-variances profiles collapse in the only case where the ratio (r~) of the time-dependent forcing time scale to the convective eddy-turnover time scale is used for tracking the time, supporting the independence of the departure from the characteristics of the surface heat flux decay. As a consequence of this result, the knowledge of r~ is sufficient to predict the departure of the velocity variances from their quasi-steady state, irrespective of the shape of the surface heat flux decay. This study highlights the importance of considering both the time-dependent forcing time scale and the convective eddy-turnover time scale for evaluating the response of the free-convective boundary layer to the surface heat flux decay.

KW - Afternoon transition

KW - Free-convective boundary layer

KW - Large-Eddy simulation

KW - Mixed-layer similarity

KW - Quasi-equilibrium assumption

KW - Time scales

UR - http://www.scopus.com/inward/record.url?scp=85163030075&partnerID=8YFLogxK

U2 - 10.1007/s10546-023-00812-2

DO - 10.1007/s10546-023-00812-2

M3 - Article

AN - SCOPUS:85163030075

VL - 188

SP - 259

EP - 284

JO - Boundary-Layer Meteorology

JF - Boundary-Layer Meteorology

SN - 0006-8314

IS - 2

ER -