COMPREHENSIVE ANALYSIS OF THE MACRO- AND MICRO-PHYSICAL CHARACTERISTICS OF DENSE FOG IN THE AREA SOUTH OF THE NANLING MOUNTAINS

Using the composite field observational data collected in the area south of the Nanling Mts. and numerical modeling, the seasonal features of dense fog and visibility, fog drop spectrum and physical concept of fog forming have been analyzed. The occurring frequency of low visibility（≤200 m） is very high with a mean of 24.7%, a maximum of 41.8% from the end of autumn to winter and next spring. The fog processes that occur in the area south of the Nanling Mts. in spring and winter result from the interactions of complicated micro-physical processes, the local terrain, water vapor transportation and the influencing weather system. The fog processes are arisen from advection or windward slope, which is much different from the radiation fog. Cooling condensation due to the air lifted by the local mountain plays an important role in fog formation. Windward slope of the mountain is favorable to the fog formation. Dense fog can occur at lower altitudes in the windward slope of mountain, resulting in the lower visibility. The fog is mainly of small-drop spectrum with smaller number-density than that of urban fog, and its drop spectrum has descending trend in the section of smaller diameter. The inverse relationship between fog water content and visibility is the best among several relationships of micro-variables. In addition to micro-physical processes of fog body itself, the motion of irregular climbing and crossing over hillside while the fog body is being transported by the wind are also important reasons for the fluctuation of micro-physical parameters such as fog water content.     

高速公路团雾分级预警研究

团雾和高速公路团雾的定义内涵外延尚未统一,没有标准定义。为有效预防高速公路团雾天气灾害,减少因高速公路团雾所造成的交通事故,有必要规范高速公路团雾分级预警标准。基于高速公路团雾的水平能见度和水平尺度等监测数据,借鉴部分学者专家的意见和建议,确定团雾和高速公路团雾的定义。分析高速公路团雾的特征、成因、时空分布及团雾的危害,统计分析高速公路团雾多发路段的交通事故数据资料与AWMS气象监测数据资料,确立高速公路团雾与车辆交通安全性关系。依据同车道前后车辆的相对停车视距和安全停车距离等分析,评估车辆在不同水平能见度数值条件下,行驶在团雾区域内外车辆发生交通事故的危险性大小,遵照相关法律、条例、标准及其规范,按水平能见度数值范围和覆盖路面长度划分高速公路团雾预警等级。研究结果表明:(1)高速公路团雾的水平尺度一般小于5 km,水平能见度通常小于200 m。(2)高速公路团雾具有多发性,时空分布不均,多发生于当日22时次日08时。(3)高速公路团雾的事故率高,造成的交通事故比较严重。(4)可以根据团雾特征和潜在危害性确定团雾和高速公路团雾定义的内涵外延,给出术语定义。(5)可根据高速公路团雾的水平能见度、交通事故、停车视距等分析,划分预警级别,规范监测和预报预警服务,减少交通事故的发生。     

南京雾-霾天气个例湍流运动特征的对比研究

利用超声风速仪、能见度仪等探测资料,采用Fortran、Matlab、Origin等软件处理数据并绘图分析,对比了南京郊区2013年12月4日一次霾过程、2013年12月7—9日一次雾过程和2013年12月3日一个晴天的湍流运动特征。结果表明,霾天和晴天平均水平风速、平均动能、湍流动能、湍流强度、摩擦速度、动量通量和热量通量都有明显的日变化,而这些参量在雾天没有明显的日变化,不同天气湍流参量大小有差异;霾、雾、晴天近中性层结下,u、v、w三个方向风速归一化标准差近似为常数,霾天分别为3.15、2.72、1.17,雾天分别为3.11、2.45、1.25,晴天分别为3.40、3.45、1.50;不稳定条件下霾、雾、晴天风速归一化标准差和湍流动能归一化标准差符合1/3幂次律,稳定条件下霾、雾、晴天无因次湍流动能均满足1/3幂次律;不稳定条件下霾、雾、晴天温度和湿度归一化标准差满足-1/3幂次律,稳定条件下霾、雾、晴天温度归一化标准差符合-2/3幂次律;雾天归一化u、v、w谱与Kansas谱吻合度比霾天和晴天高,霾天u、v、w谱峰值频率约为0.01 Hz,雾天u谱峰值频率约为0.004 Hz,v、w谱峰值频率约为0.1 Hz,晴天u、v、w谱峰值频率约为0.01 Hz。得出结论:雾天的边界层结构与霾天和晴天有所不同;不稳定条件下霾、雾、晴天风速、湍流动能、温度、湿度归一化标准差均符合莫宁-奥布霍夫相似理论;雾天归一化u、v、w谱比霾天和晴天更加符合莫宁-奥布霍夫局地各向同性理论,且霾天和晴天以机械湍流为主,雾天既有机械湍流也有热力湍流。     

On the Evolution and Structure of a Radiation Fog Event in Nanjing

An extremely dense radiation fog event during 10-11 December 2007 was studied to understand its macro-micro-physics in relation to dynamic and thermodynamic structures of the boundary layer, as well as its structural evolution in conjunction with the air-surface exchange of heat and water vapor. The findings are as follows. The extreme radiation fog process was divisible into formation, development, mature, and dissipation phases, depending on microstructure and visibility. This fog event was marked by rapid evolution that occurred after sunrise, when enhanced surface evaporation and cold air intrusion led to a three order of magnitude increase in liquid water content (LWC) in just 20 minutes. The maximum droplet diameter (MDD) increased four-fold during the same period. The fog structure was two-layered, with the top of both the surface-layer and upper-layer components characterized by strong temperature and humidity inversions, and low-level jets existed in the boundary layer above each fog layer. Turbulence intensity, turbulent kinetic energy, and friction velocity differed remarkably from phase to phase: these features increased gradually before the fog formation and decreased during the development phase; during the mature and dissipation phases these characteristics increased and then decreased again. In the development and mature stages, the mean kinetic energy of the lower-level winds decreased pronouncedly, both in the horizontal and vertical directions.     

延庆-张家口地区复杂地形冬季山谷风特征分析

基于2016年12月—2017年2月和2017年12月—2018年2月两年冬季的近地面自动气象站逐时观测数据以及张家口探空数据分析延庆-张家口一带（包括张家口崇礼、赤城、海坨、小五台山区,延怀、怀涿、洋河、蔚县盆地以及北京延庆、昌平、怀柔部分平原地区）复杂地形的风场精细化时、空分布特征,揭示不同复杂地形下局地风场的时、空变化规律,加深对复杂地形动力、热力作用对近地面风场影响的认识,为冬季山区风场预报以及复杂地形数值模式改进提供参考。结果表明:晴朗小风天风持续性作为矢量平均风速和标量平均风速的比值,可以作为研究风场变化规律的重要参数。根据风持续性的日变化特征,可以将研究区域内所有站点分为10种类型,分别代表不同局地地形特征的影响,风持续与风向变化的相关也很强。研究区域主要有3种类型的地形风:斜坡风、峡谷风以及较大尺度的山区平原风。不同地形特征下的风场、风持续性存在明显不同的日变化特征,山风和谷风相互转化的时间也不同,山区最早,盆地次之,平原区最晚;山风时段持续时间较谷风时段长,风速小;晴朗小风天实测风反映了实际风场的特征,而排除环境背景风场,弱化地形动力作用后整个冬季的局地风作为理论山谷风,更能反映热力作用下的山谷风特征。      

Analysis of the wind field and heat budget in an alpine lake basin during summertime fair weather conditions

Summary ?Observational data collected in the Lake Tekapo hydro catchment of the Southern Alps in New Zealand are used to analyse the wind and temperature fields in the alpine lake basin during summertime fair weather conditions. Measurements from surface stations, pilot balloon and tethersonde soundings, Doppler sodar and an instrumented light aircraft provide evidence of multi-scale interacting wind systems, ranging from microscale slope winds to mesoscale coast-to-basin flows. Thermal forcing of the winds occurred due to differential heating as a consequence of orography and heterogeneous surface features, which is quantified by heat budget and pressure field analysis. The daytime vertical temperature structure was characterised by distinct layering. Features of particular interest are the formation of thermal internal boundary layers due to the lake-land discontinuity and the development of elevated mixed layers. The latter were generated by advective heating from the basin and valley sidewalls by slope winds and by a superimposed valley wind blowing from the basin over Lake Tekapo and up the tributary Godley Valley. Daytime heating in the basin and its tributary valleys caused the development of a strong horizontal temperature gradient between the basin atmosphere and that over the surrounding landscape, and hence the development of a mesoscale heat low over the basin. After noon, air from outside the basin started flowing over mountain saddles into the basin causing cooling in the lowest layers, whereas at ridge top height the horizontal air temperature gradient between inside and outside the basin continued to increase. In the early evening, a more massive intrusion of cold air caused rapid cooling and a transition to a rather uniform slightly stable stratification up to about 2000 m agl. The onset time of this rapid cooling varied about 1–2 h between observation sites and was probably triggered by the decay of up-slope winds inside the basin, which previously countered the intrusion of air over the surrounding ridges. The intrusion of air from outside the basin continued until about mid-night, when a northerly mountain wind from the Godley Valley became dominant. The results illustrate the extreme complexity that can be caused by the operation of thermal forcing processes at a wide range of spatial scales. Received June 25, 2001; Revised December 21, 2001     

Temperature And Wind Velocity Oscillations Along a Gentle Slope During Sea-Breeze Events

The flow structure on a gentle slope at Vallon dOl in the northern suburbs of Marseille in southern France has been documented by means of surface wind and temperature measurements collected from 7 June to 14 July 2001 during the ESCOMPTE experiment. The analysis of the time series reveals temperature and wind speed oscillations during several nights (about 60--90 min oscillation period) and several days (about 120–180 min oscillation period) during the whole observing period. Oscillating katabatic winds have been reported in the literature from theoretical, experimental and numerical studies. In the present study, the dynamics of the observed oscillating katabatic winds are in good agreement with the theory.In contrast to katabatic winds, no daytime observations of oscillating anabatic upslope flows have ever been published to our knowledge, probably because of temperature inversion break-up that inhibits upslope winds. The present paper shows that cold air advection by a sea breeze generates a mesoscale horizontal temperature gradient, and hence baroclinicity in the atmosphere, which then allows low-frequency oscillations, similar to a katabatic flow. An expression for the oscillation period is derived that accounts for the contribution of the sea-breeze induced mesoscale horizontal temperature gradient. The theoretical prediction of the oscillation period is compared to the measurements, and good agreement is found. The statistical analysis of the wind flow at Vallon dOl shows a dominant north-easterly to easterly flow pattern for nighttime oscillations and a dominant south-westerly flow pattern for daytime oscillations. These results are consistent with published numerical simulation results that show that the air drains off the mountain along the maximum slope direction, which in the studied case is oriented south-west to north-east.     

Analysis and high-resolution modeling of a dense sea fog event over the Yellow Sea

A ubiquitous feature of the Yellow Sea (YS) is the frequent occurrence of the sea fog in spring and summer season. An extremely dense sea fog event was observed around the Shandong Peninsula in the morning of 11 April 2004. This fog patch, with a spatial scale of several hundreds kilometers and lasted about 20 h, reduced the horizontal visibility to be less than 20 m in some locations, and caused a series of traffic collisions and 12 injuries on the coastal stretch of a major highway. In this paper, almost all available observational data, including Geostationary Operational Environmental Satellite (GOES)-9 visible satellite imagery, objectively reanalyzed data of final run analysis (FNL) issued by the National Center for Environmental Prediction (NCEP) and the sounding data of Qingdao and Dalian, as well as the latest 4.4 version of Regional Atmospheric Modeling System (RAMS) model, were employed to investigate this sea fog case. Its evolutionary process and the environmental conditions that led to the fog formation were examined by using GOES-9 visible satellite imagery and sounding observations. In order to better understand the fog formation mechanism, a high-resolution RAMS modeling of 4 km × 4 km was designed. The modeling was initialized and validated by FNL data. A 30-h modeling that started from 18 UTC 10 April 2004 reproduced the main characteristics of this fog event. The simulated lower horizontal visibility area agreed reasonably well with the sea fog region identified from the satellite imagery. Advection cooling effect seemed to play a significant role for the fog formation.     

Mountain–valley precipitation differences in the northern Alps: an exemplary high-resolution modeling study

This paper investigates the dependence on environmental conditions of altitudinal precipitation differences in the northern Alps, based on high-resolution numerical simulations with the MM5 model for a selected region in the Bavarian Alps (Zugspitze mountain and surrounding valley stations). Three exemplary precipitation events representing climatological regimes with different orographic enhancement characteristics are selected. After validating the MM5 precipitation fields against the available surface observations, the model results are used to analyse the interactions of atmospheric dynamics and cloud microphysics with the local orography. The first two cases (19–22 March 1997, 05–09 February 1999) are characterized by a strong northwesterly or northerly flow, associated with large precipitation differences between the mountain and the surrounding valley stations. For these cases, the model results indicate a dominance of the classical seeder–feeder mechanism, with strong orographic lifting generating dense orographic clouds over each individual mountain ridge, which in turn intensify precipitation. The related surface precipitation maxima can be found near the mountain peaks or somewhat in the lee due to hydrometeor drifting. The third case (05–07 December 1992) represents conditions with relatively small (i.e. below climatological average) precipitation differences between the Zugspitze and the surrounding valley stations. For this event, the model results indicate that relatively weak ambient winds at and below Alpine crest level (700 hPa) were primarily responsible for the lack of substantial precipitation enhancement. Precipitation was nevertheless moderately intense because of strong frontal lifting at higher levels. In all three cases, the agreement between simulated and observed precipitation patterns is so high that there is good reason to expect that mountain–valley precipitation differences will be quantitatively predictable for nonconvective events once a sufficiently high model resolution is computationally affordable.     

An observational and modeling study of a sea fog event over the Yellow Sea on 1 August 2003

A dense sea fog episode that occurred near the coastal city of Qingdao in the Shandong Peninsula of China on 1 August 2003 is investigated by using all of the available observational data and high-resolution modeling results from the Regional Atmospheric Modeling System (RAMS). This fog event reduced the horizontal visibility to be less than 60 m in some locations and caused several traffic accidents locally. In this paper, all of the available observational data, including visible satellite imagery of Geostationary Operational Environmental Satellite (GOES)-9 and MODerate-resolution Imaging Spectroradiometer (MODIS), objectively reanalyzed Final Analysis (FNL) data issued by the National Centers for Environmental Prediction (NCEP), sounding data at the Qingdao and Dalian stations, and the latest 4.4 version of the RAMS model, were employed to study this sea fog case. We begin with the analyses of the environmental conditions of the sea fog event, including the large-scale conditions, the difference between T _2m (air temperature at 2 m altitude) and sea surface temperature (SST), and the atmospheric sounding profiles of the two stations. The characteristics of this sea fog event was documented by using visible satellite imagery of GOES-9 and MODIS. In order to better understand the fog formation mechanism, a high-resolution RAMS model of dimensions 4 km × 4 km was designed, which was initialized and validated by FNL data. A 54-h modeling period that started from 18 UTC 31 July 2003 reproduced the main characteristics of this sea fog event. The simulated lower visibility area agreed well with the sea fog area identified from the satellite imagery. It is shown that advection cooling effect plays a significant role in the fog formation.     

Characteristics of Thermally-Induced Near-Surface Flows over An Enclosed Crater： Observations of the Meteor Crater Experiment（METCRAX）

Meteorological measurements of the Meteor Crater Experiment（METCRAX） were used to analyze the characteristics of the thermally-induced flows over an enclosed crater（Arizona, USA）. Despite its relatively small size, this crater acts as an effective wind shelter. Daytime upslope winds and nocturnal downslope flows are regularly observed on its west/east sidewalls. The times of diurnal wind-direction shift（i.e., around sunrise and sunset） were slightly mismatched between the sites based on the east and west sidewalls, owing to the crater＇s shadowing effects. Under conditions of relatively weak synoptic forcing, the nocturnal downslope flows prevail throughout the night, despite quite a low wind velocity near the surface. ？     

A THREE-DIMENSIONAL NUMERICAL STUDY ON DENSE FOG OVER MOUNTAINOUS AREAS

Using the non-hydrostatic meso-scale model MM5v3, dense fog that occurred from March 7 to March 8, 2001 over the Mts. Nanling area was studied. With integrated field experiments and observations, the occurrence, development and lift mechanism of fog were analyzed. The results indicate that before the coming of stratiform clouds, southerly warm and wet air ascended along mountainside and cooling condensation formed mountain fog. Then fog was formed by the stratiform on cloud-contacting mountaintop. A front inversion layer accelerated the development and extended the duration of the lower cloud and fog. The intensity, occurrence time, mass content and the variation of temperature and relative humidity of the fog agreed with those of the observation. It showed that the meso-scale model has the potential to forecast mountain fog.     

厦门能见度的准周期性变化

利用厦门机场2000～2002三年白天（08～20时）逐时能见度及相应的相对湿度、风资料，通过统计方法和小波分析，发现厦门能见度有显著的日变化、季节变化、季节内和准两周等不同尺度的变化，并且分析了这几种变化的基本特征。在每天有记录的时刻08～20时，能见度日变化明显，在清晨、傍晚最低，午后最高；能见度存在季节变化，在一年中3～5月能见度最低，这正对应着厦门的雾季，厦门地区的能见度与当地的雾关系密切；能见度还存在显著的季节内（30～60d）变化和准双周（10～20d）变化，这与东亚季风区内的大气振荡有关。     

浙江中部一次大雾过程分析与预报

利用NCEP再分析格点资料、常规观测资料、数值预报产品以及卫星云图对2009年4月6日早晨浙江中部一次大范围大雾天气过程进行诊断分析。结果表明：高空槽后暖脊的大气环流背景有利于地面长波辐射冷却和中层增温,易形成逆温,为大雾生成提供了有利条件,而边界层逆温层或等温层的形成与维持对雾的发展持续具有重要作用;凝结核、低层湍流混合是山区能见度较城镇大的主要原因;交叉温度可用来预示整个潜在雾层的湿度状态,温度与交叉温度差值可作为了解大雾生成、发展、消散过程的一项指标;模式产品能提供雾形成的环境条件;红外云图、可见光云图有助于预测雾的形成、发展、消散过程。     

Convective initiation by topographically induced convergence forcing over the Dabie Mountains on 24 June 2010

The initiation of convective cells in the late morning of 24 June 2010 along the eastward extending ridge of the Dabie Mountains in the Anhui region,China,is studied through numerical simulations that include local data assimilation.A primary convergence line is found over the ridge of the Dabie Mountains,and along the ridge line several locally enhanced convergence centers preferentially initiate convection.Three processes responsible for creating the overall convergence pattern are identified.First,thermally-driven upslope winds induce convergence zones over the main mountain peaks along the ridge,which are shifted slightly downwind in location by the moderate low-level easterly flow found on the north side of a Mei-yu front.Second,flows around the main mountain peaks along the ridge create further convergence on the lee side of the peaks.Third,upslope winds develop along the roughly north–south oriented valleys on both sides of the ridge due to thermal and dynamic channeling effects,and create additional convergence between the peaks along the ridge.The superposition of the above convergence features creates the primary convergence line along the ridge line of the Dabie Mountains.Locally enhanced convergence centers on the primary line cause the initiation of the first convection cells along the ridge.These conclusions are supported by two sensitivity experiments in which the environmental wind(dynamic forcing) or radiative and land surface thermal forcing are removed,respectively.Overall,the thermal forcing effects are stronger than dynamic forcing given the relatively weak environmental flow.     

南京冬季浓雾的演变特征及爆发性增强研究

2007年12月18—19日,南京地区出现了一次持续20h的浓雾过程,其中能见度低于50m的强浓雾几乎占到整个雾过程的1／3。利用同期在南京市北郊的外场观测数据,结合NCEP再分析资料,分析了该次雾的演变过程、微物理结构及边界层特征,探讨了地面雾爆发性增强的成因。结果表明：本次雾在西南平流的增湿作用下触发生成;日出后,平流输送和地表蒸发提供了充足水汽来源,贴地层逆温因高空下沉增温而向上抬升且稳定存在,因此大雾得以维持;整个雾过程中雾滴数浓度、平均直径、含水量随时间的变化趋势基本一致,平均谱曲线均呈指数下降分布,雾滴集中在小滴端;两次地面雾爆发性增强均发生在夜间,其特征为各微物理参量明显增大,滴谱上抬拓宽;爆发性增强的原因是地表气温陡降、贴地层逆温增强及可充当雾滴凝结核的气溶胶大粒子数增多。     

Dynamical controls on the diurnal cycle of temperature in complex topography

We examine the climatological diurnal cycle of surface air temperature in a 6 km resolution atmospheric simulation of Southern California from 1995 to the present. We find its amplitude and phase both have significant geographical structure. This is most likely due to diurnally-varying flows back and forth across the coastline and elevation isolines resulting from the large daily warming and cooling over land. Because the region’s atmosphere is generally stably stratified, these flow patterns result in air of lower (higher) potential temperature being advected upslope (downslope) during daytime (nighttime). This suppresses the temperature diurnal cycle amplitude at mountaintops where diurnal flows converge (diverge) during the day (night). The nighttime land breeze also advects air of higher potential temperature downslope toward the coast. This raises minimum temperatures in land areas adjacent to the coast in a manner analogous to the daytime suppression of maximum temperature by the cool sea breeze in these same areas. Because stratification is greater in the coastal zone than in the desert interior, these thermal effects of the diurnal winds are not uniform, generating spatial structures in the phase and shape of the temperature diurnal cycle as well as its amplitude. We confirm that the simulated characteristics of the temperature diurnal cycle as well as those of the associated diurnal winds are also found in a network of 30 observation stations in the region. This gives confidence in the simulation’s realism and our study’s findings. Diurnal flows are probably mainly responsible for the geographical structures in the temperature diurnal cycle in other regions of significant topography and surface heterogeneity, their importance depending partly on the degree of atmospheric stratification.     

基于多源数据的北疆一次持续性浓雾天气过程的演变特征

利用常规高空地面、机场跑道自动观测系统(AWOS)、微波辐射计及FY4A新一代静止气象卫星等资料对2019年12月9～13日发生于北疆沿天山一带的一次持续性浓雾天气进行观测特征及演变分析，结果表明：（1）此次大雾天气过程是发生在500 hPa高空脊区控制，低层不断有暖平流东伸，地面位于蒙古冷高压后部均压场的大尺度环流背景下。（2）大雾发生前，地面明显升温有利于地表融雪、水汽蒸发，这为浓雾的形成和维持提供有利的水汽条件。浓雾维持期间，地面风速维持1 m.s-1左右的弱风场，温度露点差≤2℃，空气接近饱和，准噶尔盆地低洼地形均为浓雾维持提供有力环境条件。浓雾消散期间，风速增大，急剧降温，快速增湿，有利于雾滴凝结为米雪，使得浓雾消散。（3）Brunt-Vaisala（布伦特-维萨拉）指数能较好的反映浓雾期间边界层稳定度，并能提炼出相关稳定度阈值。浓雾期间相对湿度≥85%高度层主要集中在100米以下的贴地层，持续深厚的湿度层为浓雾形成和持续提供较好水汽条件，大雾期间强逆温层顶主要维持在600 m高度，当逆温层顶高度抬升时，有利于雾滴粒子、水汽粒子向上扩散，能见度好转。（4）FY4A卫星的多通道可见光及红外通道差图像能较好的监视白天及夜间大雾的形成、维持及生消变化，对于业务中短时临近预报有较好的帮助。     

华北平原一次持续性大雾过程的动力和热力特征

利用台站加密观测资料和NCEP/NCAR再分析资料对2004年11月29日—12月3日华北平原一次罕见的持续性大雾天气过程进行了研究, 通过对本次过程动力和热力特征的深入分析, 揭示了其成因和维持机制。结果表明:对流层中低层暖性高压脊及地面变性冷高压的稳定维持为持续性大雾过程提供了良好的背景条件; 地表净辐射引起的近地层冷却是大雾的触发和加强机制; 中低空下沉气流的存在有助于近地层的弱风条件和稳定层结的建立; 低层暖平流的输入和边界层的浅层抬升有利于大雾的长时间维持; 伴随负温度平流南下的偏北风的爆发是使大雾消散的动力因子。     

辐射雾雾滴谱拓宽的微物理过程和宏观条件

为研究南京雾的物理化学过程,2006～2007年冬季,在南京信息工程大学(原南京气象学院)院内进行了雾的外场综合观测实验,共获得27次雾过程资料,其中有22次过程进行了全过程不间断的雾滴谱和湍流观测.根据雾滴谱分布特征可以将辐射雾分为宽谱辐射雾(都为强浓雾,最大直径大于40μm)和窄谱辐射雾(多为浓雾,最大直径小于22...