Simulations Of Planetary Boundary Layer Convection Over Different Underling Surfaces

Boundary layer convection plays a significant role in transport of heat, moisture, momentum and aerosols. Different underling surfaces affect not only the physical properties of the air but also the forms and developments of convection. Convective rolls have been shown to be common in the boundary layer. When the mesoscale forcings that often controls the deep convection are weak, convective rolls can provide a significant control on convective initiation. Variety terrains can change the flow within the boundary layer. The distribution of convection over an orographic area is determined by the depth and the width of a mountain. In extremely dry Saharan area, the depth of the boundary layer can reach upto 5km due to the very deep dry convection which is caused by the strong surface heat fluxes. Deep boundary layer convection also has important effects on the transport of dust. In this paper, a large eddy model (LEM) was used to simulate the necessary conditions of surface heat flux and wind for the formation and dissipation of boundary rolls, with comparing the differences of rolls and nonroll convection on initiation of deep convection. A high resolution boundary layer model (BLASIUS) was also utilized to investigate the characteristics of vertical velocity fields and thermal convection over complex terrain and effects of orography on them. The LEM model was applied over the Saharan desert to simulate the boundary structure in this area. The effects of surface flux anomalies on Saharan CBL, Saharan SRL and transport from the CBL into the SRL were investigated.Boundary layer convection can take variety forms. Convective rolls have been shown to be common in the boundary layer. Rolls play an important role in the deep convection initiation. Not only the interactions of rolls with the mesoscale convergence zones create preferred locations for convective development, but rolls themselves can initiate storms. In this paper we use the Met Office large eddy model (LEM) to explicitly resolve boundary layer rolls observed on 14 August 1995 during the Small Cumulus Microphysical Study (SCMS) field campaign. Comparing with the sounding and the aircraft data, average the simulations are 0.5K warmer and 0.2gkg^-1 drier than the observations, but the modeled and observed vertical velocity fields have similar range (-2 or -3 to 2.5ms^-1). And the observed boundary layer rolls were reproduced by the model. We also investigated how the rolls relate to the surface heat fluxes and the magnitude of the geostrophic wind shear. For the case modeled, rolls persisted for surface buoyancy fluxes less than 110Wm^-2 (because the limitation of the spin-up time, the lower value of surface heat fluxes for rolls persistence was not determined). Rolls formed for boundarylayer wind shears greater than 5×10^-3s^-1. Rolls started to decay when - Z_i, /L exceededa threshold, with a value between 5 and 45. The value was found to depend on a non-dimensional combination of the low-level wind shear, the height of the CBL, and theeddy velocity scale (e.g.(?)). Two dimensional correlation between w and a cosinefunction was performed to measure the linearity of the rolls (a threshold of 0.25 used as the criteria for linear convection).In order to investigate the importance of the rolls on the initiation of deep convection, simulated rolls convection was compared with nonroll simulation, which was identical except for the wind and wind shear used. In both the roll and nonroll cases the variability in convective inhibition (CIN) was dominated by the source air, rather than the lifting of the top of the boundary layer by the convection. Stronger moist updrafts existed in the nonroll convection, whereas roll convection gave a more symmetrical distribution of up and downdrafts, with stronger downdrafts than the nonroll case. The nonroll convection simulations have lower minimum values of CIN and clouds develop 15 min earlier this case. However, in the roll case clouds deepen slightly more rapid than in the nonroll case, so the cloud tops reach 2.7km at approximately the same time.In this paper, a series of idealised model simulations have been performed over a real hilly terrain located in north west region using a high resolution boundary layer model. The model used is Met Office boundary layer model called Boundary Layer Above Stationary, Inhomogeneous Uneven Surface (BLASIUS). Based on those simulations, the characteristics of vertical velocity fields and thermal convection over complex terrain are investigated and effects of orography on them are diagnosed. The simulated results show when the geostrophic wind is low (which is 5ms^-1 or 10ms^-1) (Froude number is less than 0.5), air flow tends to be blocked and upward motion can be forced by orography and convergence over windward slope. Under such circumstances deep convection is likely to be triggered. Due to the division of airflow over windward slope the vertical motion can also be induced in the lee because of convergence. The convergence in the lee of mountains is another important factor which may cause deep convective systems. When the gostrophic wind is high, air can climb over the mountain easily, and gravity waves can be induced at the downwind of mountains.We also analysed the characters of thermal convection over the complex terrain. The simulated results show that the rolls appeared around the mountains after 3 hours of model running. After 6 hours of the model run, weak rolls started to decay or combined with others, with the increase of rolls wavelength. And rolls tend to be strong with the lower static stability value. So the convection can develop to the higher altitude with small static stability value. The vertical updrafts were enhanced at where the convective updrafts combined with the upward motion of the gravity wave. Conversely, the vertical updrafts were inhibited when convective updrafts corresponded with the downward motion of the gravity wave. Not only the gravity wave moved the air to the upper altitude, but it altered the direction of the rolls when the direction of gravity wave was close to that of rolls. So we suggest that the gravity wave might affect the transmitting of the deep convection.The observation showed that a rocky area approximately 20km across, with an albedo of approximately 0.2 compared with 0.45 for the surrounding desert, was linked to a CBL temperature increase of approximately 2K. Such variations may significantly affect the vertical mixing of the SRL. This assume has to be confirmed by the numerical simulation due to the lack of the observed data in SRL. Using two cases based on observations from the GERBILS (GERB, Intercomparison of Longwave and Shortwave radiation) field campaign, large eddy model (LEM) simulations have been used to investigate the effects of surface flux anomalies on the growth of the summertime Saharan convective boundary layer (CBL) into the Saharan Residual layer (SRL) above, and transport from the CBL into the SRL.Modeled results show that hot surface anomalies (increase the surface flux over the warm patch) generated updraughts and convergence in the CBL that increased transport from the CBL into the SRL. The induced subsidence in regions away from the anomalies inhibited growth of the CBL there. Under stronger surface flux anomalies and lower ambient wind conditions, if the domain averaged surface flux kept constant this led to a shallower, cooler CBL, while if fluxes outside the anomaly were kept fixed this gave a warmer, shallower CBL.In order to further understand the effects of surface anomalies on the transport between CBL and SRL, a passive tracer with a fixed value of 100 was added below the 200m model level in all simulations. Vertical distributions of the horizontally-averaged passivetracer concentrations at different values of (?) or MD show that not only do strongersurface flux anomalies with lighter winds tend to decrease the CBL depth (using balanced or unbalanced surface fluxes), but they also enhance the vertical transport from the CBL into the SRL. The simulations also show that the uplift of tracer was enhanced at the west of the warm patch because the convergence increase the wind at the west of the patch, with the ambient wind increased. When the ambient wind speed is low (< 15ms^-1), anomaly tend to increase tracer uplift due to anomalies lead to locally increase wind speeds.