Large Eddy Simulation Of The Distribution And Evolution Of Snow Over Complex Terrain

Snow is one of the most dynamic natural elements on the Earth's surface,which is the material source of the polar ice sheet and mountain glaciers.The variations in snow distribution in time and space profoundly affect the hydrological cycle,ecological system and climate evolution as well as other natural processes.Snow over complex terrain always displays the characteristics of nonuniform distribution and rapid evolution.Within a small time scale,the inhomogeneity of falling snow deposition over complex terrains and the drifting snow process are the two main processes that lead to the nonuniform spatial distribution and time evolution of snow.For the falling snow deposition over complex terrain,the non-uniform falling snow deposition contributed by the effect of turbulent wind on the trajectories of snow particles has not be considered effectively.For the drifting snow process,rarely researches have concentrated on the drifting snow process and its transportation features in the turbulent boundary layer,especially the quantitative prediction drifting snow process over complex terrain need to be further studied,this is indispensable for us to understand the drifting snow process in the field and the snow distribution over complex terrain.Therefore,based on the non-hydrostatic and compressible large eddy simulation model(Advanced Regional Prediction System shows that large eddy simulation model),through improving the accuracy of spatial discretization and introducing the Lagrangian dynamic sub-grid model,the model can simulate the turbulent boundary layer and flow characteristics of boundary layer over complex terrain correctly.At the same time,introduce the Lagrangian particle tracking method to track the trajectories of snow grains,and explicit consider the coupling effect between snow grains with sublimation effect and flow field,and established a three-dimensional wind-snow two-phase flow model in the turbulent boundary layer that considered the phase change process of solid particles.Mainly has carried out the following work:First of all,we established a falling snow deposition model based on particle tracking.The flow characteristics are reproduced by the large eddy simulation model.And the Lagrangian particle tracking method is used for the first time to track the movement process of snow grains in the turbulent boundary layer and accesses to the falling snow deposition around the terrain directly.The influences of different factors on the uneven settlement over terrains are discussed by changing the shape and distribution type of terrains as well as wind conditions.It is found that the snow particles conduct complex movements under the influence of turbulence.Under the same conditions,the influence of turbulence on the trajectories of snow grains decreases with the increase of particle size;the falling snow deposition over terrain strongly depends on the wind speed and the shape of the terrain,under the same snowfall intensity,the total deposit amount on the terrain increases with the increasing terrain height and wind speed;the preferential deposition is directly related to wind speed,that is snow trends to accumulate on the leeward slope under small wind speed,and may drift to the windward slope with the increase of wind speed.Secondly,the drifting snow process is performed based on the two-phase model that considered the inter-coupling effect among snow particle,wind field,humidity and temperature fields.With the full development of turbulent boundary layer,calculate the three-dimensional trajectories of snow particles by Lagrangian particle tracking model and consider the sublimation effect and mid-air collision mechanism,and the band structure of blown snow that is consistent with the natural phenomenon is reproduced,known as snow streamers.Simulation results show that snow streamers are a kind of self-organization phenomenon of particle concentration generated by the suction effect of high-speed rotating vortexes;Turbulent wind field results in uneven spatial entrainment of snow grains on the bed,however,this process does not affect the formation of the snow streamer,but the grain-bed interactions largely determine the shape of snow streamers.In general,the snow streamer display a characteristic length and width of about 0.5 m and 0.16 m,respectively,and they are not change with the wind speed;compared with the typical sand streamer,snow streamer is little narrower and the occurrence of nested streamers and other complex streamer patterns needs much higher wind speed.Thirdly,the drifting snow model on a slope surface is established through theoretical analysis.The relationship between snow transport intensity and friction velocity is acquired at first through simulating the drifting snow processes in the turbulent boundary layer under various wind speeds.In view of most existing snow distribution patterns only evaluate the intensity of the drifting snow with the drfting snow model on flat terrain,based on theoretical analysis,the relationship between the critical threshold wind speed of snow grains on the slope surface and that of on the flat surface is established.Then the correction is introduced into the model of probability of blowing snow occurrence and the frequency of drifting snow under different slope angles is obtained.At the same time,the effect of terrain on the snow transport rate is introduced into the snow transport flux formula of flat surface to correct the wind blowing snow process on the slope bed.Thus a drifting snow model that can be used to calculate the snow redistribution over complex terrain caused by wind blowing snow is established.Respect to the flat surface,the start-up of snow particles on windward slope is more difficult and results in a smaller snow transport flux,while the leeward is just opposite.Under the same slope angle,the impact of terrain on the drifting snow process on leeward slope is much larger than that of windward slope.Finally,a snow distribution predict pattern is developed through combining the drifting snow model over complex terrain and the falling snow deposition model together.Snow particles are still tracked by Lagrangian particle tracking method,which can get the deposited location of snow grains directly.During this process,the snow redistribution caused by wind blowing snow process is calculated based on the parameterized scheme,and further obtain the spatial distribution and time evolution process of snow over complex terrain.At the same time,the reacting force of snow particles on the wind field is considered in this model and it is found that the coupling effect between wind field and snow grains is very important to accurately predict snow distribution because the wind blowing snow process is largely sensitive to the local wind speed.Snow deposition under different wind speed show that the snow sedimentary characteristics over complex terrains is directly related to the wind speed and the local terrain under the mutual influence of actual complex terrain,it is difficult to find a unified snow deposition pattern to describe the deposition law of falling snow over very complex terrains,and the method of particle tracking is certainty suitable for this situation and can obtain the falling snow deposition directly.All in all,this article uses the compressible fluid large eddy simulation method and dynamic sub-grid turbulence closure model to obtain the atmospheric boundary layer flow over complex terrain,track the movement process of snow grains in the flow field with Lagrangian particle tracking method,combine the drifting snow model of complex terrain,and consider the coupling effect between the snow grains and the wind field,establishes a snow distribution prediction model in the turbulent boundary layer over complex terrains.Compared with the existing snow distribution prediction models,this model can directly get the snow sedimentary characteristics over complex terrains by tracking the typical trajectories of snow grains,which improve the spatial accuracy of model largely.At the same time,the influence of slope angle on the drifting snow process is considered effectively,which improves the prediction accuracy of the model for complex terrains.