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2006年12月24—27日大范围大雾过程数值模拟
A Numerical Study on the Long-lasting Wide Spread Dense Fog Event During December 24—27, 2006
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摘要: 利用美国国家大气研究中心(NCAR)和宾夕法尼亚州立大学(PSU)联合研制的第5代中尺度气象模式系统MM5对2006年12月24—27日江苏及其周边地区出现的一次罕见持续性大雾进行数值模拟和诊断分析, 同时对影响大雾过程的辐射条件进行敏感性试验。结果表明:形成持续性大雾的主要原因是大气层结稳定, 水汽充沛, 同时, 地面和大气的长波辐射冷却是雾形成和发展的最重要因素; 而日出后太阳短波辐射加热和热量湍流输送是辐射雾消散的主要原因。在大雾发展和维持期间, 雾区近地层基本上为弱的水汽辐合区; 在大雾减弱和消散期间, 雾区大部分为弱的水汽辐散区。大范围的下沉辐散运动有利于中低层大气增温, 与近地层的辐射降温相配合, 加上近地层弱冷平流作用, 使低层大气降温, 有助于逆温形成, 而深厚逆温层的存在, 对雾区的长时间维持起着决定性作用。Abstract: Fog presents a severe hazard in areas of intense traffic, such as airports and highways. Moreover, because air pollutants are not easy to be dispersed in foggy days, it is also harmful to human health and agricultural production. Fog prediction is essential to public safety and has a high economic value. To precisely predict foggy weather, it is essential to first understand the mechanisms responsible for fog formation and maintenance.The NCAR/PSU MM5 V37 is used to simulate and diagnose a dense fog event in Jiangsu Province and the surrounding areas during December 24—27, 2006. In the control simulation, the Gayno Seaman parameterization of the boundary layer is used. The deep convection parameterization of Grell is adopted, and the explicit treatment of warm rain is employed. To account for the important role that longwave and shortwane radiation play in fog formation, the cloud radiative scheme is employed. The center of the simulation area is set in Nanjing(32.03°N, 118.46°E). Two computational grids with horizontal grid distances of 30 and 10 km and domain sizes of 60×60 and 76×76 grid points are considered respectively in a two-way nesting method. High resolution in the boundary layer(another 9 levels joins below 200 m)is provided by thirty-two vertical levels, stretched monotonically from the surface to 100 hPa. Sensitivity experiments are performed to under stand the role of radiation. In the sensitivity tests, a series of simulations are performed with different radiative schemes, while keeping the others physic schemes fixed. The results show that longwave radiative cooling from the Earth's surface and the atmosphere are the most important factors for the formation and development of this fog event. Meanwhile, the steady atmospheric stratification and plenty moisture supply also play an important role. The fog dissipation is influenced mostly by the shortwave radiative heating and turbulent heat transfer after sunrise. During the development and maintenance stages of the fog, the surface layer is basically weak convergence region of water vapor, while in the weakening and dissipating period, the majority of the fog area is weak divergence region of water vapor. It indicates that the wide area of divergence descending accompanied with surface cooling and cold advection are the favorable conditions for the formation of a temperature inverse layer near the ground which is responsible for the maintenance of this prolonged fog event.
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Key words:
- fog;
- liquid water content;
- radiation cooling
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图 1 2006年12月24—27日10 m高处的液态水含量模拟结果(单位: g·kg-1)
(a)24日20:00,(b)25日01:00,(c)25日11:00,(d)25日22:00,(e)26日14:00,(f)27日14:00
Fig. 1 Liquid water content section of simulated(unit:g·kg-1)at 10 m
(a)20:00 Dec 24, 2006,(b)01:00 Dec 25, 2006,(c)11:00 Dec 25, 2006,(d)22:00 Dec 25, 2006,(e)14:00 Dec 26, 2006,(f)14:00 Dec 27, 2006
图 3 0.97σ层水汽通量散度(单位:10-7 g·cm-2·hPa-1·s-1)
(a)2006年12月25日02:00,(b)2006年12月26日14:00
Fig. 3 The simulated 0.97σ divengenee of moisture flux(unit:10-7 g·cm-2·hPa-1·s-1)at 02:00 Dec 25, 2006(a)and 14:00 Dec 26, 2006(b)
图 4 2006年12月在32°N, 118.5°E散度场(单位:10-4 s-1)(a), 涡度场(单位: 10-4 s-1)(b), 垂直速度(单位: m·s-1)(c)模拟结果时间-高度剖面图
Fig. 4 The time-height cross section of simulated divergence(unit:10-4 s-1)(a), vorticity(unit:10-4 s-1)(b) and vertical velocity(unit:m·s-1)(c)at 32°N, 118.5°E in Dec, 2006
图 5 2006年12月25日20:00温度平流沿32°N的高度-纬向剖面图(单位: K·s-1)
Fig. 5 The zone-height cross section for simulated temperature horizontal advection(unit:K·s-1) along 32°N at 20:00 Dec 25, 2006
图 6 2006年12月24—27日长波辐射降温率沿32°N的高度-纬向剖面图(单位: K·h-1)
(a)24日20:00,(b)25日05:00,(c)26日14:00,(d)27日08:00
Fig. 6 The zone-height cross section for simulated radiative tendency(unit:K·d-1)along 32°N at 08:00 Dec 24, 2006(a), 05:00 Dec 25, 2006(b), 14:00 Dec 26, 2006(c)and 08:00 Dec 27, 2006(d)
图 7 不同辐射方案模拟2006年12月27日08:00的雾中液态水含量分布图(单位:g·kg-1)
(a)云辐射方案,(b)RR TM方案
Fig. 7 Comparisons of the liquid water content's distribution between cloud scheme(a)and RRTM scheme(b)(unit:g·kg-1)
表 1 南京信息工程大学观测得到的能见度与数值模拟液水含量
Table 1 The visibility observed in NUIST and the liquid water content simulated
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