地表非均匀加热影响对流边界层湍流特征的大涡模拟研究

Large-eddy simulation on the turbulence characteristics in the convective boundary layer driven by heterogeneous surface heating

本文运用大涡模拟方法研究了地表非均匀加热对边界层湍流特征的影响.共进行五个算例的数值模拟,其中一个算例为地表均匀加热,四个非均匀算例的地表加热方式均采用"马赛克"分布,非均匀尺度分别为1.2km、2.5km、5.0km和10.0km,其平均地表热通量与地表均匀加热算例相同.模拟结果表明:地表非均匀加热激发出有组织湍流涡旋,非均匀尺度越大越有利于有组织湍流涡旋的维持;当非均匀尺度足够大时,边界层湍流能量明显增大,增大的原因在于水平湍流运动的显著增强;湍流温度方差随着非均匀尺度增大而增大,但该效应在边界层发展过程中逐渐减弱;地表非均匀加热造成热量垂直输送的水平分布不均匀,当边界层高度小于非均匀尺度时不均匀性贯穿整个边界层,当边界层高度大于非均匀尺度时这种不均匀性只存在于边界层的下部;非均匀尺度为1.2km的算例结果表明,地表非均匀加热在混合层中造成的扰动影响的水平范围不超过非均匀尺度的4倍.

Large-eddy simulations are employed to investigate the influence of heterogeneous surface heating on the turbulence characteristics in the convective boundary layer(CBL).There are five simulated cases.One is driven by homogeneous surface heating,while the other four are driven by heterogeneous surface heating.For the heterogeneous cases,the patterns of surface heating are arranged as mosaic-like distributions with two different surface heat fluxes in the neighbor patches,and the heterogeneity scale in four different cases is 1.2 km,2.5 km,5.0 km,and 10.0 km,respectively.The mean surface heat fluxes are same as that in the homogeneous case.The results show that,heterogeneous surface heating can stimulate organized turbulent eddies,and the larger the heterogeneity scale is,the longer the organized turbulent eddies can sustain.The largest volume-averaged turbulent kinetic energy(TKE) is produced in the case with the largest heterogeneity scale,while the TKEs in the other heterogeneous cases are close to those in the homogeneous case.This result indicates that the TKE is not enhanced unless the scale of the heterogeneous surface heating is large enough.The reason for the enhancement of TKE is that the horizontal turbulent velocity variance is significantly enlarged.The potential temperature variance is enhanced more significantly by a larger surface heterogeneity scale,but this effect diminishes with the increasing CBL height.The horizontally heterogeneous distribution of heat flux is caused by heterogeneous surface heating.When the CBL height is larger than the heterogeneity scale,the heterogeneous distribution exists in the whole CBL,while when the CBL height is smaller than the heterogeneity scale,the heterogeneous distribution can only exist in the lower part of the CBL.The results form the case with the minimum surface heterogeneity scale show that the heat flux averaged in the area with the width of 4 times of the heterogeneity scale is close to that averaged in the domain.