Observation And Numerical Simulation Research On Atmospheric Boundary Layer Over Inhomogeneous Underlying Surface

In the summer of2005and the spring of2006, flux measurements were twice taken in urban Nanjing (Municipal Party School) and suburban Nanjing (Pukou area). Heat flux, latent heat flux, carbon dioxide flux as well as friction velocity were obtained applying the eddy-covariance (EC) technique. In order to eliminate the impact of complex terrain, a planar-fit (PF) method for tilt correction was adopted. A thorough analysis of the PF method indicated that PF coefficients are closely related to wind direction. Thus, wind directions must be taken into consideration when processing data. To be specific, winds from all directions were divided into several sectors and PF method was applied to each of them in order to generate a fitted plane for each sector. This method was named sector planar fit (SPF) as distinguished from the general planar fit (GPF) which does not consider wind sectors. The differences of corrected fluxes by the two methods (GPF/SPF) for the two seasons and two locations were mainly considered. It was clearly revealed that both urban and suburban flux results share a consistent trend in spring and summer; geographically, in urban areas, the corrected fluxes using SPF and GPF show obvious differences. Differences are much smaller in suburban areas. Moreover, the vertical velocity (w) was corrected using the two methods and it was found that w also exhibits significant differences. Finally, according to the probability distribution of corrected vertical wind velocity by the two methods, it was concluded that distribution of corrected vertical velocities by SPF is closer to normal than GPF.After observing the complicated underlying surface and near-surface layer of urban area, it has been found out necessary to apply numerical simulation technique to further investigate the features of the convective boundary layer (CBL) affected by inhomogeneous underlying surface. The mesoscale model WRF (Weather Research and Forecasting) and its large eddy simulation (LES) subroutine was chosen to study the characteristics of the CBL driven by inhomogeneous surface heating. Different "Chessboard-like" patterns with patches of surface heat fluxes at two fixed values were designed. The inhomogeneity scale in two schemes are5kilometers and2.5kilometers. Together with homogeneous underlying surface, there are total three schemes (scheme A, B, C). After comparing the results of inhomogeneity and homogeneity schemes, it turns out there are not significant effects on domain-averaged parameters in CBL. To be specific, there are no obvious influences on profiles of domain-averaged potential temperature and heat flux. Yet, different inhomogeneous surface heating schemes cause different turbulence features. The scheme with the largest inhomogeneity scale produces the largest turbulent kinetic energy (TKE) while the TKE of the rest inhomogeneity scheme is very close in value with that of homogeneous surface, which indicates TKE is enhanced only when the scale of inhomogeneity is large enough. The profiles of parameters’variance are under the effects of inhomogeneity scale:In the mixed layer, the potential temperature variance with the largest inhomogeneity scale is larger than the rest two. But as time goes by, with the CBL height increasing, the potential temperature variance profile of each case tends to be close to each other, which means the impact of inhomogeneity scale has been decreased. It is thus indicated that turbulent eddy structure changes with the CBL development. As for the turbulent velocity variance, maximum inhomogeneity scale case has the largest horizontal velocity variance and the smallest vertical velocity variance. This trend continues throughout the simulation time. Considering the results of TKE, the largest inhomogeneity scale triggers a stronger local circulation. The local circulation contributes to the horizontal motion and thus causes the largest horizontal velocity variance and the smallest vertical velocity variance. Circulations caused by thermal inhomogeneity can maintain well in the early development of the CBL when turbulent eddies are under the control of the surface inhomogeneity scale. At some point, however, circulations fail to maintain. The CBL continues to develop driven by free convection, and its turbulent eddies are mainly left with inherent turbulent eddies, ie. large eddies.