| Using sounding data observed from Dunhuang,Gansu Province during the intensive period of a land-atmosphere interaction field experiment over the arid region of North-west China from May to June 2000 as initial field for the Large Eddy Model(LEM),two types of experiments with whether it windless or not are performed to investigate the characteristics of turbulent structure of convective cold pool and the effects of cold pool on dust uplifting and vertical transport by setting cold source in the model.Furthermore,a series of numerical sensitivity tests are performed to study the effects of intensity and scale of the cold source,environmental vertical wind shear(Hereinafter referred to as wind shear)on the structure of convective cold pool,the velocity of leading edge of cold pool and the dust uplifted rate,etc.The following conclusions are drawn.(1)Using temperature profiles at 16:00 on June 3 as initial field for the Large Eddy Model(LEM),characteristics of turbulent structure of convective cold pool and effects of turbulence of the cold pool?s head and tail on dust uplifting and vertical transport are investigated.Moreover,sensitivity tests with varying cooling rate and the size of cold source in the model center are performed to study the effects of intensity and scale of the cold source on the turbulent structure of convective cold pool,the velocity of leading edge of cold pool and the dust uplifted rate,etc.The results show that the simulated structural characteristics of the convective cold pool are consistent with observations.The head of the cold pool is a large turbulent vortex.There is weaker and smaller turbulence in the tail due to effects of blocking of downdraft from head's vortex and the vertical wind shear.Turbulence in the tail becomes weaker if it is far from the head.As the convective cold pool propagting outward,the intensity and height of the head vortex of the cold pool decrease,and the height of the turbulent vortex in the tail increases slightly.Dust uplift potential(DUP)caused by the head vortex is much larger than that of the tail.Dust is largely trapped in the well-mixed cold-pool density current except that only a small fraction of dust diffuses outside the cold pool.The absolute concentration of dust in the head is about twice as much as that in the tail and the transmission height of dust in the head is about three times that of the tail.Dust in the tail is confined to the lower height(about 500 m).With the cold pool diverging outward,the height of dust transport and the absolute concentration of dust in the head of the cold pool decrease,and the total dust uplift potential at the head and tail of the cold pool also decreases.Simulated results by changing cooling rate and size of cold source in the LEM present that intensity and moving speed of leading edge of convective cold pool increase with increasing cooling rate and size.It leads to more dust is uplifted from the surface and more dust is transported to the higher altitude due to the enhanced turbulence,but cold pool becomes less long-lived because of the stronger entrainment of the ambient air with increasing cooling rate and size.(2)Using temperature and wind profiles at 16:00 on June 3 as initial field for the Large Eddy Model(LEM),characteristics of turbulent structure of convective cold pool in wind field are simulated and effects of turbulence of the cold pool on dust uplifting and vertical transport are investigated.Further,sensitivity tests with varying wind shear in the model are applied to study the effects of wind shear on the turbulent structure of convective cold pool and the dust uplifted rate,etc.The results show that the head and tail of the convective cold pool can also be clearly distinguished in the ambient wind field,and the vertical transport form of dust caused by turbulent vortex of head and tail of the cold pool is similar to that of the cold pool in the static wind field.However,the cold pool in the environmental wind field is no longer symmetrical in the horizontal direction.During the the cold pool spreading out,the velocity of leading edge of the cold pool along the downwind is the fastest,followed by cold pool?s in static wind field and the upwind is the slowest,while the velocity of leading edge of the cold pool along the upwind decreases fastest with time,followed by the static wind field?s,and the downwind is the smallest.The turbulent vortex of the head of the cold pool in the downwind is higher and stronger than that of the cold pool in the static wind field,the intensity of the head vortex of the cold pool in the upwind direction is approximately equal to that of the cold pool?s head in the static wind field,but heigher than that of in the static wind field.The head vortex in the upwind is slightly inclined towards the downwind.The tail area is wider along the downwind and the intensity and height of the tail?s turbulence are slightly higher than those in the static wind field.Because of the strong mixing and entrainment of the ambient air,the temperature of internal cold pool increases faster.The cold pool spreads slowly in the upwind results to a smaller tail area,and a large amount of cold air accumulates in a limited range,which makes the tail temperature of the cold pool lower than those of the downwind and the static wind field.The turbulent vortex of tail is weaker than that of the downwind.The dust uplift caused by the cold pool in the downwind is the largest,followed by the cold pool?s in the static wind field,and minimum in the upwind,which is more obvious in the late stage of the cold pool.In the downwind,on the one hand,the ambient flow accelerates the diffusion of the cold air inside the cold pool in the horizontal and vertical,and on the other hand,making the horizontal wind speed of cold pool near the ground decrease not too fast with time,so as to the dust lifting potential and the total absolute concentration of the dust column,while the upwind is opposite.The influence of ambient wind field on the inner of cold pool becomes greater with time increasing.(3)The vertical wind shear test shows that in the downwind,with the wind shear increasing,the maximum horizontal wind speed and the velocity of leading edge of the cold pool increases,and the temperature of the internal cold pool increases quickly as well.These phenomena is opposite in the upwind,and the velocity of leading edge of the cold pool decreases faster with the wind shear increasing.With the increase of wind shear,the head vortex intensity of the cold pool does not change obviously,and the height of the turbulence vortex increases obviously.But may duo to strong turbulence mixing and entrainment between the cold pool and ambient atmosphere,the height of the head of the cold pool decreases more rapidly with the simulation time increase.In the early stages of cold pool spreading out,the smaller of wind shear(Ug<15 m/s),the stronger is secondary surge vortex of the tail,but when wind shear is larger(Ug<15 m/s),secondary surge turbulence vortex of the tail is weaker and lower.The tail turbulent vortex arrangement is more organized.With the increase of simulation time,the intensity and height of secondary surge vortex of the tail increase with the increase of wind shear,and the microstructure of tail vortex arrangement becomes better.While in the upwind: with the increase of wind shear,the height of the head?s tubrulent vortex of the cold pool do not change obviously and the intensity is weakening,but the height of the head?s turbulent vortex decreases more quickly with time increasing,and the head is inclined to the downwind.The intensity of secondary surge vortex in the tail do not changes obviously as well,and the bigger the wind shear is,the more obvious the secondary surge vortex tilts downwind.The vertical transport of dust caused by the head and tail vortex of the cold pool has the same characteristics as the turbulent vortex.The head of the cold pool in the upwind is inclined to the downwind and the height of the dust transport in the tail secondary surge decreases with the increase of wind shear.The wind shear increase,the dust uplift potential caused by the head and tail of cold pool increases in the downwind,but opposite to the upwind.The dust uplift potential of the cold pool in all wind field tests is greater than that of the cold pool in the static wind field and the dust uplift potential in the downwind is greater than that in the upwind. |
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