| The stability of atmospheric stratification, the thickness of mixed layer, micrometeoro-logical characteristics of the surface layer, mountain-valley breeze, structure of boundary layer and dynamic parameters of near surface layer in heavy pollution day were studied. These studies were based on observations of atmospheric boundary layer in Urumqi, and provide a reference for structure and parameterization scheme of boundary layer in Urumqi. There are many conclusions:1. There were significantly seasonal and diurnal variation of the frequency distribution of atmospheric stability, the height of boundary layer and thickness of the mixed layer in the urban, suburbs and rural of Urumqi.The B, D, F class stability of the whole year occured most frequency in urban, suburban and rural. The most frequency of steady state was in northern rural, but the most frequency of unsteady state was in urban. The proportion of unstable was the maximum (88.3-96.3% of all day) during daytime in summer, but minimum (51.3-60%) in winter. The proportion of stable during night was the maximum in autumn (81.4-84.5%) and minimum (71.1-75.8%) in summer. It was consistent for the proportion of neutral category in each season. The frequency of the stability and instability was easily changed in the period after about 2 hours of the sunrise and the sunset, it caused appeared neutral state more easily. The phase of diurnal variation under neutral and stable were same in spring, summer and winter, but it was high during daytime and low during night under the neutral stratification condition in autumn. The monthly average mixed layer thickness was thick in summer, thin winter. The maximum thickness during day time of summer was 1569-1772 m, and was 526-1156 m in winter. The peak thickness of mixed layer was late about 2-3 h than the development of sensible heat flux in urban, and late about 1-2 h in suburb. The thickness of mixed layer decreased with the altitude in winter in Urumqi.2. The wind speed significantly decreased with the height when the relative height of boundary layer under 800-900 m. The wind speed increased from 1 m/s to 6.2-7.7 m/s in winter under 800-900 m, but it increased slowly above this height in Urumqi. The wind speed was small below 300 m in winter that it was corresponded to the temperature inversion. The wind speed increased maximum with height in near below 100 m of surface layer in the southern rural, and increased about 1 m/s to larger than 4 m/s, and the wind speed increased slowly with the height, it is about 1-2 m/s in winter. The air flow was dragged stronger by the buildings when it more closer to the ground, at the same time the difference of wind speed greater between urban, suburbs and rural. The wind speed of urban near the ground was significantly lower than in suburb and rural, and about lower 5-32% in spring, about 8-30% in summer, about 15-37% in autumn, about 14-48% in winter. The characteristic of mountain-valley wind was obviously, and most notably in summer, but it was not obviously in winter, it was partial N valley wind during the daytime, and was the partial S mountain wind at night. The valley wind converted to mountain wind at 08:00-10:00 and 19:00-21:00, and the minimum wind speed during this period. So we simulated how the terrain affected on mountain-valley wind in Urumqi by using the WRF model. The mountain-valley wind can reach the 950 hPa height.3. The inversion layer was strong (average 1050-1150 m) in winter in Urumqi. This was due to a high pressure system in high-altitude and ground in winter, and the snow cover of surface was stable.There always had temperature inversion phenomenon under 100 m near surface at night. It was most notably in the northern rural but not obviously in urban, and the temperature inversion could reached 1.7-4.4℃/100 m. The temperature of urban was higher than suburbs at night in spring, autumn and winter. The difference temperature was small during daytime and large between urban, suburbs and rural, and was larger when it closer to the ground. The maximum temperature difference up to 3.0℃ of urban and southern rural in autumn, and this difference up to 2.7℃ in urban and northern rural in winter. The probability of humidity inversion was between 35%(at 17:00 in July) and 100%(at 5:00 in January), and the maximum moisture inversion height was above 1500 m, and the maximum moisture inversion was larger than 2.5 g/kg/100 m in summer and autumn. There were multi moisture inversion layers, more or less, under 100 m in near surface layer. The strong moisture inversion was at low level of near surface layer, and the maximum moisture inversion could reach 3.1-8.1 g/kg/100 m in northern rural. Always, the moisture advection flow and a large amount of water vapor were intercepted in the inversion layer, and the accumulation at the top of the inversion layer formed, all these caused the moisture inversion.4. The aerodynamic roughness length z0m was about 1.43 m and it was greater than the suburb and rural in Urumqi. It had a good relationship between the average z0m, Footprint scales, and the roughness elements which from upwind directions. The change of friction velocity u* with z0m in urban was lower than in suburb and rural. The value of z0m was rapidly decreased when wind speed was in 1-2 m/s in urban, suburb and rural, especially in southern rural. The value of z0m was increased with u* when u* was over 0.5-0.6 m/s. The average frictional velocity was 0.37 m/s in southern rural, it was 0.28 m/s in urban, and was 0.23 m/s in the closer northern suburb and northern rural. It was large in spring and summer, but small in winter. The maximum friction velocity u* was about 0.5-0.75 m/s in spring, and was about 0.23-0.56 m/s in winter. The average of turbulent kinetic energy TKE was 1.38 m2/s2 in the southern rural and 0.7m2/s2 in the urban,0.6 m2/s2 in the near northern suburb and northern rural. It was large in spring and summer but small in winter. The maximum was 1.79-3.39 m2/s2 in spring and 0.33-2.82 m2/s2 in winter. The maximum friction velocity, turbulent kinetic energy and turbulence intensity of January were in the southern rural. It was always rendering the law Iu≈Iv>Iw. The value of Iu and Iv was gathering at 0.1-0.4 in suburb and rural, gathering at 0.1-0.2 in urban and 0.1-0.3 in northern suburbs. The Iw was gathering at 0.01-0.1. The turbulence intensity of three directions was 0.14-0.33 in the southern rural,0.16-0.33 in urban,0.13-0.33 in northern suburbs and 0.16-0.38 in northern rural. The value of u* and TKE during day time were maximum in southern rural, it showed that vertical turbulent diffusion was the strongest of each season in the southern rural.5. The dynamic parameters and structure of boundary layer were significantly different during heavy pollution, there was a critical value for turbulence intensity (Iw) impact on the temperature lapse rate of the boundary layer. The middle and upper height of the boundary was controlled by strong east or southeast wind, the wind speed of near surface layer was less than 1 m/s in low-lying areas (urban and northern rural). At the same time, there were significantly temperature inversions and the specific humidity exceeded 2 g/kg, it’s an important factor for the accumulation of pollutant. It severe disturbances for small-scale where the altitude higher (southern rural), and the disturbances decreasing with terrain decreased. The turbulence motive difference had increased between southern rural and urban during heavy pollution days. The value of friction velocity u*, turbulence kinetic energy TKE and momentum flux in urban area were 64%,85% and 84% less than in the south rural, and were 39-64%,73% and 100-200% larger in north suburb and rural. Under heavy pollution weather, the value of friction velocity u* and turbulence kinetic energy TKE in urban were 35.7% and 62.9% less than average, and were 43.5% and 75% smaller in the north suburb, and were 52.2% and 75% smaller in north rural, and were 35.1% and 30% larger in the south suburb. The temperature inversion disappeared when the turbulence intensity (Iw) was higher than the critical value, which different between heavy pollution days and non-polluting day. The temperature inversion more significantly when the turbulence intensity (Iw) was lower than the critical value and with a weaker disturbance. |
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