Research Of The Atmospheric Boundary Layer Features Over Water-Land Mixed Underlying Surface In Southern Xinjiang

Through a series of sensitivity tests, the simulating ability of WRF model (Weather Research and Forecasting) in south Xinjiang is confirmed. Then using the best combination of parameterization schemes, atmospheric boundary characteristics of a special water-land composite underlaying surface in Bayangol Mongol Autonomous Prefecture South Xinjiang are simulated. The features of local air circulation, cold wet island effect and thermal forcing, caused by a large water in extreme arid region, are mainly analyzed, the mechanism of forming, disappearing, development and changing of this atmospheric boundary characteristics on water-land composite underlaying surface is also discussed.First of all, the performance of WRF model is inspected through simulating of a rainstorm in south Xinjiang in August. A series of sensitivity experiments are done using different microphysics, cumulus parameterization schemes and horizontal resolutions of the model. Simulating effects about rainfall, temperature and relative humidity are emphatically analyzed through simulations using 36 combinations of different microphysics and cumulus schemes of the model. The simulation ability of WRF is fully confirmed in extremely complicated topography in arid desert inland.And then, base on above basis, using reasonable options and Dahaizi reservoir near 34 Regiment in Bayangol Mongol Autonomous Prefecture as the target area, atmospheric boundary characteristics of this non-uniform underlaying surface are simulated. The physical images of"cold-wet island effect", characteristics of circulation and thermal radiation fields above this water-land composite underlaying surface are given. Influences of the existence of a large water body on local temperature, water vapor, circulation and thermal radiation fields are emphatically discussed, also, rules of heat and vapor distribution, atmosphere system interaction and energy exchanging were preliminarily revealed about water and desert, certain theoretical basis about researching and conservation of the arid area water body and its surrounding ecosystems is proposed.The main conclusions of this paper are:1. There is hardly no difference using different microphysics and cumulus schemes when simulating the distribution features, development and changing characters of temperature and relative humidity. But the simulating of precipitation are dissimilar using different microphysics and cumulus schemes. The Betts-Miller-Janjic scheme (BMJ) simulating effects is the best, which can meticulously reflect the rainfall area, strength, tendency and terrain effects; the simulating effect of Kain-Fritsch(new Eta), a simple warm cloud scheme, is not so accuracy as BMJ; ensemble schemes of Grell-Devenyi and New Grell have the worst simulating effects. Microphysics, if chosing appropriately, can play a role of compensation.2. High horizontal resolution can better reflect the terrain and underlaying surface influence, can improve simulating effects in places where terrain drastic changes, can distinguish discontinuity caused by terrain. Precipitation center is more compact when using high horizontal resolution, and also the rainfall area, tendency and total rainfall are more close to the measured results. In addition, the resolution of initial input data affect the simulate effect while using high horizontal resolution.3. On a windless sunny summer day in arid region, the thermal differences between water and desert can cause a weak local water-land atmosphere circulation, then produce a"cold-wet island effect"above water body, which does not limited by water area size. Compared with humid region,"cold-wet island effect"lasts longer, but affects a lower vertical area, the space distribution of this"cold-wet island effect"is affected by wind field, which often cause the deforming of the space distribution. The existence of cold-wet island has a favorable effect on the nearby vegetational community, the cold-wet island can reduce temperature and maintain humidity in the day, maintain temperature and moisturizing at night. 4. The cold island effect has a horizontal extent of a size of the water body, and a vital extent high about 150 meters. At night and before dawn, atmospheric vertical layer structure is stable, the rich water vapor in atmosphere close to the ground is pegged there and separate out on the surface of soil and vegetable, this water is a great supply to the arid region plant communities. A temperature inversion"hat"is formed at moon, the vapour below is locked near water surface and can not be evaporated, protecting the very limited water from being rapidly deplete in the extremely drought and high temperature weather conditions.5. The duration of wet island is longer than cold island, the closer to water body the more moisture is there. Specific humidity above water body does not change much but has a low ebb above desert in a day. The distribution curves of atmosphere moisture content and temperature are roughly equal in atmosphere surface layers, cold area and high temperature area have the same extending tendency. The phenomenon of wet inversion above the ground in the day and near the ground at night is clear in humid region, but can not be distinguished in arid region.6. Relatively low wind speed helps the formation of"cold-wet island", and continually low wind speed not only cause the formation of"cold-wet island"but also create favorable conditions for its development and strengthen, in turn, the existence of cold-wet island and temperature inversion prevent horizon wind field from further development. There is a horizontal wind field convergence above water body in the day and a divergence at night. The moisture above water body is deliver to the nearby desert and its vegetation through low layer atmosphere advection, this moisture is condensed on the surface of the desert at night and evaporated to the low atmosphere layer in the day, when it is carried back to water surface through advection. This water circulation mechanism increases air humidity above water surface and limiting water evaporation at the same time, making a effective protection and a full use of arid limited water. 7. Heat differences caused by sensible heat and latent heat between water and desert are the primary cause of water clod-wet island. Land receiving incident radiation is affected by groove face and gradient of the ground, water receiving incident radiation is affected by moisture content above water surface. Moisture content near water surface absorbs some of the visible light bands and infrared bands in incident radiation, this lowers water temperature and lessens water-atmosphere vapor pressure, and consequently restrains water evaporation. Judged from sensible heat and latent heat, moisture fountainhead is to some extent a sensible heat converge.