平坦非均匀地面对流边界层的准定常态

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

用大涡模拟方法研究对流边界层大气对非均匀地面条件的适应性。研究限于无风、平坦地形条件 ,地面热通量在一个水平方向作非均匀变化 ;平均地面热通量为 0 .15K·ms- 1、非均匀性变化幅度为 0 .0 5K·ms- 1;地面非均匀性的尺度在 2 0km之内。模拟结果表明 ,在充分长的时间后 ,边界层达到一种适合于地面条件的准定常态。这种准定常态和向其过渡的过程中 ,水平平均的边界层廓线性质 ,如平均位温廓线、垂直热通量廓线等几乎与平坦均匀地面的结果相同 ,或差异极小。地面非均匀性的主要作用是使边界层动能增大 ,并以地面非均匀性变化方向的动能增加为主。边界层动能以一种振荡衰减的形式向其准定常态过渡。可以引入边界层平均动能达到其第 1个峰值所需的时间tc 和准定常态无因次动能 Eeq为这一过渡过程的 2个特征量。此 2个特征量与地面非均匀性尺度的定性关系是 ,非均匀性尺度越大 ,过渡时间tc 越长 ,达到的准定常态无因次动能 Eeq也越大。

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 0.15?K·ms -1 and varying magnitude 0.05?K·ms -1 .Wavelengths or scales of surface heterogeneity are less than 20?km.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, 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 eq could be reached,meanwhile the longer transition time t \-c is needed.