| Using the observed data of temperature,water vapor,pressure and wind field during the period of the experiment “Theoretical study and observations of water energy exchange and planetary boundary on inhomogeneous underlying surface in oasis system ”(JTEXP)and a high resolution model over the complex terrain(BLASIUS,Boundary Layer Above Stationary,Inhomogeneous Uneven Surface),a series of idealized numerical simulations are performed to investigate the effects of vegetation type,Average surface temperature difference between the desert and the oasis at the initial time of numerical test(hereinafter referred to as surface temperature difference),buoyancy frequencies(Brunt – V?is?l? frequency),topography and background wind velocities on the oasis effect.The following conclusions are drawn.(1)Using the GPS sounding data at 10:00 on August 3 in the Jinta experiment as the initial field of the sensitivity test of vegetation types in the BLASIUS model,the effects of five typical vegetation types on the oasis effect are investigated.The simulation results indicate that the latent heat flux on crops is larger than that of other vegetation types,but the sensible heat flux is smaller and the oasis effect is stronger.When the vegetation type is shrub and perennial ground cover,the latent heat flux is smaller,the sensible heat flux is larger,and the oasis effect is weaker and the influence time is short.At the same time,when the intensity of "cold island" and "wet island" is strong,the influence range can be extended to the desert area around the oasis.The tests of changing the average surface temperature difference between the desert and the oasis at the initial moment of the simulation are performed.The simulated results present that he difference in vertical direction of "cold island" in oasis occurs below 500 m above the ground level,while the difference caused by the "wet island" in the vertical direction occurs below 250 m above the ground level.Simulated results by changing the surface temperature difference show that,the performance of "wet island" on the three-dimensional structure changes,and the influence on the “humid island effect” is about an hour later than that on the “cold island effect”.When the surface temperature difference increases from 1 k to 6 k,the width of the "cold island" on the oasis increases and the height of the "wet island" decreases by about 70 m.In the local circulation driven by temperature difference between the desert and the oasis,the stable stratification is dominant over the oasis and is controlled by the downdraft.The ideal numerical test results indicate that the vertical velocity increases over the desert area above the height of 1.5 km,and with more updrafts over the oasis with increasing the buoyancy frequency.(2)The effects of terrain configuration and background wind speed on the oasis effect are investigated by performing numerical sensitivity test.The observation data from the Jinta experiment are also used as the initial field.The three topographic configurations are designed in this paper.The duration of "cold" and "wet" phenomena on the slopes in the simulated region is less than that on the oasis,and the influence height of "cold" and "wet" phenomena in the valley is higher than that in the oasis.In the case that the background wind is a static wind field and the terrain is flat,the oasis-desert circulation is symmetrical about the oasis,and the airflow over the desert near the oasis has a higher vertical velocity than other areas.Under the condition of topography,the oasis-desert circulation becomes asymmetrical with increasing the initial background wind speed,and the stronger upwelling flow is located at the low level of the oasis in the upper part of the wind field.At the same time,as the wind speed increases,the height of the secondary circulation center in the upstream of the wind field decreases,while the height of the secondary circulation center in the downstream of the wind field increases. |
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