TY - JOUR

T1 - Heterogeneity-Induced Heat-Flux Patterns in the Convective Boundary Layer

T2 - Can they be Detected from Observations and is There a Blending Height? A Large-Eddy Simulation Study for the LITFASS-2003 Experiment

AU - Sühring, Matthias

AU - Raasch, Siegfried

N1 - Funding Information: Acknowledgments This study was supported by the German Research Foundation (DFG) under grant RA 617/21-1. All simulations were performed on the IBM Power6 at The German Climate Computing Center (DKRZ), Hamburg. We appreciate the two anonymous reviewers for their constructive and valuable comments that helped to improve the manuscript.

PY - 2013/8

Y1 - 2013/8

N2 - An understanding of how the convective boundary layer (CBL) is mixed under heterogeneous surface forcing is crucial for the interpretation of area-averaged turbulence measurements. To determine the height and degree to which a complex heterogeneous surface affects the CBL, large-eddy simulations (LES) for two days of the LITFASS-2003 experiment representing two different wind regimes were undertaken. Spatially-lagged correlation analysis revealed the turbulent heat fluxes to be dependent on the prescribed surface flux pattern throughout the entire CBL including the entrainment layer. These findings prompted the question of whether signals induced by surface heterogeneity can be measured by airborne systems. To examine this question, an ensemble of virtual flights was conducted using LES, according to Helipod flight measurements made during LITFASS-2003. The resulting ensemble-averaged heat fluxes indicated a clear dependence on the underlying surface up to the top of the CBL. However, a large scatter between the flux measurements in different ensemble runs was observed, which was the result of insufficient sampling of the largest turbulent eddies. The random and systematic errors based on the integral length scale did not indicate such a large scatter. For the given flight leg lengths, at least 10-15 statistically independent flight measurements were necessary to give a significant estimate of heterogeneity-induced signals in the CBL. The need for ensemble averaging suggests that the observed blending of heterogeneity-induced signals in the CBL can be partly attributed to insufficient averaging.

AB - An understanding of how the convective boundary layer (CBL) is mixed under heterogeneous surface forcing is crucial for the interpretation of area-averaged turbulence measurements. To determine the height and degree to which a complex heterogeneous surface affects the CBL, large-eddy simulations (LES) for two days of the LITFASS-2003 experiment representing two different wind regimes were undertaken. Spatially-lagged correlation analysis revealed the turbulent heat fluxes to be dependent on the prescribed surface flux pattern throughout the entire CBL including the entrainment layer. These findings prompted the question of whether signals induced by surface heterogeneity can be measured by airborne systems. To examine this question, an ensemble of virtual flights was conducted using LES, according to Helipod flight measurements made during LITFASS-2003. The resulting ensemble-averaged heat fluxes indicated a clear dependence on the underlying surface up to the top of the CBL. However, a large scatter between the flux measurements in different ensemble runs was observed, which was the result of insufficient sampling of the largest turbulent eddies. The random and systematic errors based on the integral length scale did not indicate such a large scatter. For the given flight leg lengths, at least 10-15 statistically independent flight measurements were necessary to give a significant estimate of heterogeneity-induced signals in the CBL. The need for ensemble averaging suggests that the observed blending of heterogeneity-induced signals in the CBL can be partly attributed to insufficient averaging.

KW - Aircraft observation

KW - Blending height

KW - Convective boundary layer

KW - Large-eddy simulation

KW - LITFASS-2003

UR - http://www.scopus.com/inward/record.url?scp=84880507296&partnerID=8YFLogxK

U2 - 10.1007/s10546-013-9822-1

DO - 10.1007/s10546-013-9822-1

M3 - Article

AN - SCOPUS:84880507296

VL - 148

SP - 309

EP - 331

JO - Boundary-Layer Meteorology

JF - Boundary-Layer Meteorology

SN - 0006-8314

IS - 2

ER -