Quasi-stationary State of Convective Boundary Layer over Heterogeneous Flat Surfaces

Abstract:

Large-eddy simulations are carried out for the adjustment of convective boundary layer (CBL) over heterogeneous surfaces.Simulations are confined for zero mean-wind and flat terrain.Surface heat-flux is set varying in x-direction with its average to be 0.15K¡¤ms^-1 and varying magnitude 0.05K¡¤ms^-1.Wavelengths or scales of surface heterogeneity are less than 20km.Simulation results show that CBLs do reach a quasi-stationary state correspondent to specified surfaces,after a long enough evolution.In this transition process and after that,horizontal statistics,i.e.mean potential temperature and vertical heat-flux,show profiles little differences to those over homogeneous flat surface.Main effect of surface heterogeneity is to increase kinetic energy in CBL and the increase concentrates in the direction of surface heterogeneity.The time of CBL spent to reach its first peak of mean kinetic energy,t_c,and dimensionless kinetic energy of CBL in its stationary state, E_eq,could be two scales for the adjustment process.Results show that, qualitatively,the larger scale of surface heterogeneity,the higher value of dimensionless kinetic energy E_eqªªcould be reached,meanwhile the longer transition time t_c is needed.

Key words:

convective boundary layer; quasi-stationary; heterogeneous; large-eddy simulation

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