Simulation Study Of Effects Of Surface Heterogeneities On Vertical Velocity Field In The Boundary Layer

In this paper, a series of idealised model simulations have been performed over a mesoscale flat domain using a high-resolution boundary-layer model (BLASIUS). Based on these simulations, effects of heterogeneity with different vegetation surfaces on boundary-layer thermal convection are investigated, and implications of surface inhomogeneity on initiation and adjusting of deep convection are discussed. In the paper, orographic effects on the vertical velocity field in boundary layer and its evolution are also simulated and the properties of orographic gravity waves are discussed. Main conclusions can be summarized as follows:In order to examine the overall features of the thermal convection over flat inhomogeneous surfaces, two different vegetations (bare soil and shrub) are chose and arranged into two different patterns (three strips-type and chessboard-type) in these idealise simulations. We analyzed the effects of the various vegetation distributions in different background wind conditions. It is found that the effect of surface heterogeneity is still significant when the geostrophic wind speed is 10 m s-1, which is different from the finding in previous studies which pointed out that the effect of surface heterogeneity is not significant when the wind speed is larger than 5 m s-1. However, when the geostrophic wind direction is perpendicular to the strip surfaces, higher winds tend to weaken the coherent circulations caused by the surface heterogeneities. The vertical winds generated by the mesoscale circulations associated with the surface heterogeneities are on the order of 0.5 cm s-1. When the geostrophic wind direction is parallel to the alignment of the strip surfaces, the mesoscale pattern in horizontal velocity is especially pronounced, with significant fluctuations at the interfaces between two different surface patches. The heights at which the heterogeneity effects on potential temperature and winds vanish are well above the convective boundary layer top and reach at least 3.3 Zi under light winds, but it is depends on the wind speed, wind directions as well as the orientation of surface heterogeneities.For a 3-strip surface pattern, when the geostrophic wind direction is perpendicular to the alignment of the surface heterogeneities, deep convection tends to be triggered at the interface between the bare soil and shrub surfaces. The interaction between westerly background winds and the secondary circulation which is caused by surface heterogeneity can trigger the initiate deep convection at the eastern, downwind interface under favourable conditions. According to analysis of Convective Available Potential Energy (CAPE) and water vapor field we can find that the value of CAPE changes continually with the development of thermal convection. Under a light background wind, when the wind is perpendicular to the strip surface covered with different vegetations, the maximum CAPE and Lifting Condensation Level (LCL) occurs near the interface of two land surfaces. The larger CAPE provides necessary energy for the deep convective activities at this area. Under high background wind, differences in CAPE and LCL, specific humidity over the bare soil and shrub surfaces are highly depending on background wind direction and strip of inhomogeneous surfaces. When the geostrophic wind direction is perpendicular to the strip surface, the differences decreased with wind speed due to strong advection.In this paper, we also simulate the orographic effects on the vertical velocity field in the boundary layer. The simulations show that the flow pattern over hills closely related to background wind speeds, the scale of hills as well as static stability of atmosphere.When the wind speed is higher, the flow can flow over hills and a strong subsidence area in the lee of the hill can be noted. Meanwhile, orographic gravity waves form over hills and propagate upward and dissipate with height. When the background wind speed is above 6 m s-1, the orographic gravity waves can propagate to a high altitude. If the scale of the hill and the static stability of atmosphere are keep unchanged, the amplitude and wavelength of the orographic gravity waves will increase with the wind speed. When background winds are light, the airflow is hard to flow over hills and the signal of the orographic gravity wave is not obvious. At the night time, the forced lift of airflow by a hill triggers the orographic gravity waves with their amplitude increasing with the increasing of wind speed and stability with time until the surface thermals begin to develop.The interaction between gravity waves and the convective activities in the boundary layer tends to enhance upward motions at the peak of waves and depress upward motions at the trough or waves. This effect of the gravity waves on thermal convection can extend to the downstream hills. During the day time, when thermal convective activities are become stronger, the stable layer near the surface is destroyed and orographic gravity wave in low-level of the atmosphere disappeares.