Summary of a 4-Year Fog Field Study in Northern Nanjing, Part 1: Fog Boundary Layer

Comprehensive fog field observations were conducted during the winters of 2006–2009 at the Nanjing University of Information Science and Technology to study the macro and micro-physical structures and the physical–chemical processes of dense fogs in the area. The observations included features of the fog boundary layer, characteristics of fog water, the particle spectrum, the chemical composition of atmospheric aerosols, radiation and heat components, turbulence, meteorological elements (air temperature, pressure, wind speed, wind direction), and environmental monitoring. The fogs observed were divided into four types: radiation fog, advection–radiation fog, advection fog, and precipitation fog, according to the mechanisms and primary factors of the fog processes. Fog boundary-layer structures of different types and their corresponding characteristics were then studied. Fog boundary-layer features, temperature structures, wind fields, and fog maintenance are discussed. The results show that radiation fog had remarkable diurnal variation and formed mostly at sunset or midnight, and lifted after sunrise or at noon, and that advection–radiation fog and advection fog were of very long duration. Extremely dense fogs occurred only in radiation-related cases. Inversion in radiation fog was short-lived, disappearing 1 or 2 hours after sunrise or at noon, faster than that in advection–radiation fog. When wind direction reversed from easterly to westerly or from southerly to northerly, the fog became an extremely dense fog. Low-level jet at times impeded fog development, whereas at other times it encouraged fog continuance. The deep inversion was merely an essential condition for a thick fog layer; sufficient vapor supply was advantageous to the formation and maintenance of a deep fog layer.     

A Review of some Recent Radiation Fog Prediction Studies and the Results of Integrating a Simple Numerical Model to Predict Radiation Fog over Brunei

— Several radiation fog studies with emphasis on numerical simulation and prediction are reviewed. One of the earliest attempts started with a given surface diurnal variation of temperature and water vapor, and concluded by forecasting the onset of saturation at various levels; thus fog, by examining the spread of temperature and moisture in the vertical. The one-dimensional (1-D) models are still popular. Some of the recent numerical simulations use more than 100 levels in the vertical and treat various kinds of vegetation, aerosols, and soils with moisture contents. Some also employ a mesoscale model in conjunction with a 1-D model to consider the advective effects. In the following a simple 1-D numerical model was used to predict the onset of fog at Brunei, based on a desktop computer and routine surface observations of dry bulb temperature (T), dewpoint temperature (T _d ), and wind speed at 1800 Local Time (LT). Optimism exists in improved predictions of fog and stratus as 1-D models incorporate many physical processes, and mesoscale models continue to improve in predicting advection and cloud cover.     

The Impact of Vertical Resolution in the Explicit Numerical Forecasting of Radiation Fog: A Case Study

Numerical experiments are performed with a comprehensive one-dimensional boundary layer/fog model to assess the impact of vertical resolution on explicit model forecasts of an observed fog layer. Two simulations were performed, one using a very high resolution and another with a vertical grid typical of current high-resolution mesoscale models. Both simulations were initialized with the same profiles, derived from observations from a fog field experiment. Significant differences in the onset and evolution of fog were found. The results obtained with the high-resolution simulation are in overall better agreement with available observations. The cooling rate before the appearance of fog is better represented, while the evolution of the liquid water content within the fog layer is more realistic. Fog formation is delayed in the low resolution simulation, and the water content in the fog layer shows large-amplitude oscillations. These results show that the numerical representation of key thermo-dynamical processes occurring in fog layers is significantly altered by the use of a grid with reduced vertical resolution.     

Use of a Sodar to Improve the Forecast of Fogs and Low Clouds on Airports

A sodar was deployed at Roissy–Charles de Gaulle airport near Paris, France, in 2008 with the aim of improving the forecast of low visibility conditions there. During the winter of 2008–2009, an experiment was conducted that showed that the sodar can effectively detect and locate the top of fog layers which is signaled by a strong peak of acoustic reflectivity. The peak is generated by turbulence activity in the inversion layer that contrasts sharply with the low reflectivity recorded in the fog layer below. A specific version of the 1D-forecast model deployed at Roissy for low visibility conditions (COBEL-ISBA) was developed in which fogs’ thicknesses are initialized by the sodar measurements rather than the information derived from the down-welling IR fluxes observed on the site. It was tested on data archived during the winters of 2008–2009 and 2009–2010 and compared to the version of the model presently operational. The results show a significant improvement—dissipation times of fogs are better predicted.     

Benchmarking the mesoscale variability in global ocean eddy-permitting numerical systems

The role of data assimilation procedures on representing ocean mesoscale variability is assessed by applying eddy statistics to a state-of-the-art global ocean reanalysis (C-GLORS), a free global ocean simulation (performed with the NEMO system) and an observation-based dataset (ARMOR3D) used as an independent benchmark. Numerical results are computed on a 1/4 ^° horizontal grid (ORCA025) and share the same resolution with ARMOR3D dataset. This “eddy-permitting” resolution is sufficient to allow ocean eddies to form. Further to assessing the eddy statistics from three different datasets, a global three-dimensional eddy detection system is implemented in order to bypass the need of regional-dependent definition of thresholds, typical of commonly adopted eddy detection algorithms. It thus provides full three-dimensional eddy statistics segmenting vertical profiles from local rotational velocities. This criterion is crucial for discerning real eddies from transient surface noise that inevitably affects any two-dimensional algorithm. Data assimilation enhances and corrects mesoscale variability on a wide range of features that cannot be well reproduced otherwise. The free simulation fairly reproduces eddies emerging from western boundary currents and deep baroclinic instabilities, while underestimates shallower vortexes that populate the full basin. The ocean reanalysis recovers most of the missing turbulence, shown by satellite products , that is not generated by the model itself and consistently projects surface variability deep into the water column. The comparison with the statistically reconstructed vertical profiles from ARMOR3D show that ocean data assimilation is able to embed variability into the model dynamics, constraining eddies with in situ and altimetry observation and generating them consistently with local environment.     

Modelling hydrodynamics in Yachiyo Lake using a non‐hydrostatic general circulation model with spatially and temporally varying meteorological conditions

In this study, a three‐dimensional (3D) non‐hydrostatic circulation model was applied to study the thermal structure, its evolution and water circulation of Yachiyo Lake in Hiroshima, Japan. The simulations were conducted for 1 month during July 2006. The meteorological forcing variables such as wind stress, surface atmospheric pressure and heat flux transfer through the lake surface were provided by an atmospheric mesoscale model run. The vertical mixing process of the lake was calculated using the Mellor‐Yamada turbulence model. The 1‐month numerical simulation revealed the wind‐induced currents of the lake, two gyres in the mid‐layer, and depth‐averaged monthly mean currents. Further numerical experiments studying the mechanism of the two gyres in the lake showed the important role of topography in gyre formation. The thermal structure of the lake and its evolution both in space and in time as predicted by the model showed very good agreement with the observed values and characteristics of Yachiyo Lake. The internal gravity waves, which are crucial for mixing in the stratified lake, are depicted by the vertical fluctuation of isotherms. Using the non‐dimensional gradient Richardson number, Yachiyo Lake was determined to be stable under strong stratification during the study period, and therefore very sensitive to wind stress. Copyright © 2009 John Wiley & Sons, Ltd.     

Integration of Local Observations into the One Dimensional Fog Model PAFOG

The numerical prediction of fog requires a very high vertical resolution of the atmosphere. Owing to a prohibitive computational effort of high resolution three dimensional models, operational fog forecast is usually done by means of one dimensional fog models. An important condition for a successful fog forecast with one dimensional models consists of the proper integration of observational data into the numerical simulations. The goal of the present study is to introduce new methods for the consideration of these data in the one dimensional radiation fog model PAFOG. First, it will be shown how PAFOG may be initialized with observed visibilities. Second, a nudging scheme will be presented for the inclusion of measured temperature and humidity profiles in the PAFOG simulations. The new features of PAFOG have been tested by comparing the model results with observations of the German Meteorological Service. A case study will be presented that reveals the importance of including local observations in the model calculations. Numerical results obtained with the modified PAFOG model show a distinct improvement of fog forecasts regarding the times of fog formation, dissipation as well as the vertical extent of the investigated fog events. However, model results also reveal that a further improvement of PAFOG might be possible if several empirical model parameters are optimized. This tuning can only be realized by comprehensive comparisons of model simulations with corresponding fog observations.     

Effects of spatial wind inhomogeneity and turbulence anisotropy on circulation in an elongated basin: A simplified analytical solution

The response of a closed basin to wind forcing has been studied extensively, but the role of turbulence anisotropy coupled with spatial wind inhomogeneity has never been considered explicitly. An analytical solution is presented for steady state hydrodynamics, considering the central part of a constant-depth, elongated basin with a laterally varying wind, and a homogeneous, yet anisotropic, eddy viscosity tensor. The solution is derived both for a single-layer and for a two-layer basin, which is representative of a stratified lake with a well-developed thermocline. Since the focus is on the short-term barotropic reaction to wind forcing, which determines the type of lake circulation, baroclinic effects are neglected as a first approximation. In this case, the development of planimetric (depth-averaged) circulation superimposed on circulation in the vertical plane can be determined as a function of wind lateral variation and a few dimensionless parameters. The relevance of such an analytical solution is twofold. Firstly, the knowledge of the prevailing circulation can help in the choice of the best type of numerical model (three-dimensional vs. two-dimensional, depth- or lateral-averaged). Secondly, it shows the importance of correct estimates of both vertical and horizontal eddy viscosity, whereas the latter is not usually considered as an important parameter in lake hydrodynamics modeling.     

A preliminary prediction analysis of radiation fog

Summary The system of physical equations describing temperature changes near the ground in fog-free air as well as in radiation fog is solved numerically. The variation of the exchange coefficient with height is taken into account using different models while time variations are still disregarded. Temperature changes due to latent heat effects are incorporated in this study. Moreover, the presence of radiative flux divergence is included in an approximate manner.The solution of the problem is presented in terms of graphs showing the development of temperature and water droplet profiles as function of time and height. Computed liquid water content as well as temperature profiles are in general agreement with observations while the vertical growth of fog usually proceeds too rapidly. Concrete suggestions are given of how to improve the model.     

Richardson number profiles in laboratory experiments applied to shallow seas

Abstract

A unified analysis is given of the critical conditions for the onset of stratification due to either a vertical or a horizontal buoyancy flux, with tidal or wind stirring.

The critical conditions for the onset of stratification with a horizontal buoyancy flux are found to be of the form of ratios of the tidal slope, or wind setup, to the equivalent surface slope due to the lateral density gradient. These ratios, which are easily determined from sea data, indicate that the profiles of critical flux Richardson Number, averaged over the stirring cycle, are similar to those inferred from the laboratory experiments of Hopfinger and Linden (1982) in which there is zero mean shear turbulence with a stabilising buoyancy flux, and also that the efficiency for the conversion of kinetic energy to potential energy for tidal stirring is similar to that for wind stirring.

The observed much greater efficiency for wind stirring, compared with tidal stirring with a vertical buoyancy flux, is also consistent with the existence of flux Richardson Number profiles in the sea similar to those occurring in the corresponding laboratory experiments. Using the solution of the turbulent kinetic energy equation for the water column, the relative importance of the production of turbulent kinetic energy, and its diffusion by turbulence are assessed, and the critical conditions for the onset of stratification with a vertical buoyancy flux are shown to reduce the classical Simpson—Hunter form.     

Quasi-periodic patterns coupling the Sun,solar wind and the Earth

The spectrum of velocity and magnetic fields in the solar wind is self-similar (power-law type) in the frequency range greater than >1/day indicating well-mixed turbulence. But it loses self-similarity for lower frequencies indicating the presence of large-scale patterns, which are intermittently generated inside the Sun and propagate from the Sun to the Earth.Here we discuss the spatia–temporal characteristics and origin of the 1.3-year quasi-periodic pattern found inside the Sun by helioseismic methods and detected in the solar wind. To identify and characterize this pattern on the Sun we use time series of solar magnetic Carrington maps generated at the Wilcox Solar Observatory and independent component data analysis. This analysis shows the latitudinal distribution of the pattern, its variable frequency and intermittent appearance.     

加利福尼亚海湾及其邻近海域亚中尺度现象的地震海洋学研究

亚中尺度过程连接了中尺度与小尺度过程, 在上层海洋物质垂向交换和能量传输方面起到重要的作用.在加利福尼亚湾及其邻近海域2002年采集的地震剖面上观测到了亚中尺度的涡丝, 锋面和涡旋结构.理论的涡丝垂向结构为一对偶极子次级环流, 由中间的高密度下沉流和两侧的低密度幅聚相向流构成, 密度结构呈人字形分布的多个相向倾斜密度锋.地震剖面上中尺度涡的亚中尺度涡丝由多组相对的叠瓦状倾斜反射体构成, 验证了涡丝内部密度锋的理论垂向结构.同时我们观察到亚中尺度涡旋的垂向结构及其涡丝结构, 其涡丝在地震剖面上呈现为近水平的括号状反射, 这种形态可能与旋涡星系的旋臂结构有一定相似性.随后计算了地震斜率谱和相对谱能量分布, 通过亚中尺度涡旋和其它区域在不同波数段的能量强度差异, 探讨了亚中尺度涡对能量耗散过程的影响.

     

风垂直切变对中尺度地形对流降水影响的研究

针对长江中下游中尺度地形特点以及暴雨过程发生发展期间风垂直切变的主要观测特征,设计了一系列中尺度地形的三维理想数值试验,分析了干大气地形流和重力波特征,探讨了条件不稳定湿大气地形对流降水的模态分布,在此基础上研究了圆形、直线风垂直切变和切变厚度对中尺度地形对流降水强度和模态分布的影响.结果发现:在 Fr≈1的干大气条件下,气流遇到地形后分支、绕流和爬升现象同时存在,地形激发的重力波在水平和垂直方向上传播,其在迎风坡、背风坡、地形上游和下游的振幅不同,并组织出不同强度的垂直上升运动.在Fr > 1的条件不稳定湿大气下,地形对流降水主要存在三种模态,即迎风坡和背风坡准静止对流降水以及地形下游移动性对流降水,地形对流降水的形成与重力波在低层组织的上升运动密切相关.风垂直切变对地形对流降水的强度和模态分布有重要作用,其中圆形风垂直切变(风随高度旋转)不仅影响地形下游对流降水系统的移动方向,而且影响迎风坡和背风坡山脚处对流降水中心的分布和强度;直线风垂直切变(风随高度无旋转)主要影响地形对流降水的移动速度和强度.风随高度自下而上顺(逆)时针旋转,地形对流系统向下游传播时向右(左)偏移.风垂直切变主要通过影响地形重力波的结构和传播以及对流系统的形成、移动方向和速度,来影响地形对流降水的模态分布,其中对流层中低层的风垂直切变对地形对流降水强度和模态分布有重要影响.     

Marine Layer Stratus Study

The intent of this study is to develop a better understanding of the behavior of late spring through early fall marine layer stratus and fog at Vandenberg Air Force Base, which accounts for a majority of aviation forecasting difficulties. The main objective was to use Leipper (1995) study as a starting point to evaluate synoptic and mesoscale processes involved, and identify specific meteorological parameters that affected the behavior of marine layer stratus and fog. After identifying those parameters, the study evaluates how well the various weather models forecast them. The main conclusion of this study is that weak upper-air dynamic features work with boundary layer motions to influence marine layer behavior. It highlights the importance of correctly forecasting the surface temperature by showing how it ties directly to the wind field. That wind field, modified by the local terrain, establishes the low-level convergence and divergence pattern and the resulting marine layer cloud thicknesses and visibilities.     

基于双金属球三维电场探空仪的一次雷暴云内电荷结构观测研究

本文自主研制性能稳定的双金属球三维电场探空仪,并结合气象探空仪等构建了雷暴电场-气象综合探空系统,实现了雷暴云内三维电场及温度、湿度的同步测量.2019年夏季对华北平原地区雷暴开展穿云观测,并结合地面大气电场、雷达回波、变分多普勒雷达分析系统(VDRAS)反演的动力场等资料进行综合研究,首次给出该地区雷暴云内的电场和电荷结构分布特征.对2019年8月7日发生的一次中尺度对流系统电场探空发现,在雷暴减弱阶段,其弱回波区内存在5个极性交替的电荷区：4.4~5.6 km之间的上部正电荷区(0℃附近)、3.6~4.4 km之间的中部负电荷区和1.0~3.6 km之间的下部正电荷区,此外在1 km下方有一个负极性电荷区,雷暴云顶附近5.7~6.9 km之间为一个弱负极性屏蔽电荷区.其中,中部负电荷区和下部正电荷区由多个不同强度、不同厚度的电荷层构成.此外,电场探空系统在中部负电荷区高度范围内经历的上升—下沉—再次上升的往返探空数据表明,雷暴云内动力环境复杂,电荷结构分布相似但又有所差异,反映了实际雷暴云内电荷分布的时空不均匀性和复杂性.

     

Fog interception by non‐vascular epiphytes in tropical montane cloud forests: dependencies on gauge type and meteorological conditions

Precipitation is the most fundamental input of water for terrestrial ecosystems. Most precipitation inputs are vertical, via rain, but can be horizontal, via wind‐driven rain and snow, or, in some ecosystems such as tropical montane cloud forests (TMCFs), via fog interception. Fog interception can be particularly important in ecosystems where fog is frequently present and there are seasonal periods of lower rainfall. Epiphytes in trees are a major ecological component of TMCFs and are particularly dependent on fog interception during periods of lower rainfall because they lack access to soil water. But assessing fog interception by epiphytes remains problematic because: (i) a variety of field or laboratory methods have been used, yet comparisons of interception by epiphytes versus interception by various types of fog gauge are lacking; (ii) previous studies have not accounted for potential interactions between meteorological factors. We compared fog interception by epiphytes with two kinds of commonly used fog gauges and developed relations between fog interception and meteorological variables by conducting laboratory experiments that manipulated key fog characteristics and from field measurements of fog interception by epiphytes. Fog interception measured on epiphytes was correlated with that measured from fog gauges but was more than an order of magnitude smaller than the actual measurements from fog gauges, highlighting a key measurement issue. Our laboratory measurements spanned a broad range of liquid water content (LWC) values for fog and indicate how fog interception is sensitive to an interaction between wind speed and LWC. Based on our results, considered in concert with those from other studies, we hypothesize that fog interception is constrained when LWC is low or high, and that fog interception increases with wind speed for intermediate values of LWC—a net result of deposition, impaction, and evaporation processes—until interception begins to decrease with further increases in wind speed. Copyright © 2007 John Wiley & Sons, Ltd.     

Characteristics of turbulence over the semi-fixed desert area north of Xinjiang,China

As the largest fixed and semi-fixed desert in China, the Gurbantünggüt Desert undergoes a long period of snow cover in the winter and the rapid growth of ephemeral plants in the spring, presenting obvious seasonal changes in the underlying desert surface type, which can lead to variation in the turbulence of the near-surface boundary layer turbulence over the desert. In this study, gradient tower data and eddy covariance data from 2017 were analysed to investigate the turbulence characteristics of the different surface boundary layers in the hinterland of the Gurbantünggüt Desert. The results indicate that stable atmospheric conditions in the desert occur exclusively during the early morning and at night in the desert, and the onset and duration of this stable state varies seasonally. Two regimes of intermittent turbulence occur during the night, a weak turbulent regime that occurs when the wind speed is less than the threshold and a strong turbulent regime when the wind speed exceeds the threshold, and different wind speed thresholds were observed at each level. These parameters follow a seasonal pattern of summer (July) > spring (April) > autumn (October) > winter (January) in terms of magnitude. The mean turbulence intensities of the along-wind, cross-wind and vertical wind are 0.5, 0.47 and 0.14, respectively, with I_u > I_v > I_w. The normalized standard deviation of the wind velocity components (σ_u, σ_v and σ_w) generally satisfies a 1/3 power-law relation. Our results show that the night-time turbulence regime classification for the Gurbantünggüt Desert strongly depends on meteorological and orographic features, and the intermittent turbulent events have the non-stationarity of the flow in common. The results can contribute to the study of land surface processes, climate change and desertification in inland arid desert areas.     

The Relation Between Humidity and Liquid Water Content in Fog: An Experimental Approach

Microphysical measurements of orographic fog were performed above a montane cloud forest in northeastern Taiwan (Chilan mountain site). The measured parameters include droplet size distribution (DSD), absolute humidity (AH), relative humidity (RH), air temperature, wind speed and direction, visibility, and solar short wave radiation. The scope of this work was to study the short term variations of DSD, temperature, and RH, with a temporal resolution of 3?Hz. The results show that orographic fog is randomly composed of various air volumes that are intrinsically rather homogeneous, but exhibit clear differences between each other with respect to their size, RH, LWC, and DSD. Three general types of air volumes have been identified via the recorded DSD. A statistical analysis of the characteristics of these volumes yielded large variabilities in persistence, RH, and LWC. Further, the data revealed an inverse relation between RH and LWC. In principle, this finding can be explained by the condensational growth theory for droplets containing soluble or insoluble material. Droplets with greater diameters can exist at lower ambient RH than smaller ones. However, condensational growth alone is not capable to explain the large observed differences in DSD and RH because the respective growth speeds are too slow to explain the observed phenomena. Other mechanisms play key roles as well. Possible processes leading to the large observed differences in RH and DSD include turbulence induced collision and coalescence, and heterogeneous mixing. More analyses including fog droplet chemistry and dynamic microphysical modeling are required to further study these processes. To our knowledge, this is the first experimental field observation of the anti-correlation between RH and LWC in fog.     

Assessment of the hydraulic slope flow approach using a mesoscale model

The simplified hydraulic two-layer model for a katabatic flow is analysed using the outputs from a high-resolution mesoscale simulation. A stably stratified night is simulated for the Duero basin, a complex terrain area located in the northern Spanish plateau, with large vertical and horizontal spatial resolution. Well-defined katabatic flows on the basin slopes are generated by the simulation, that are relatively stationary and quasi-bidimensional for some areas in the central part of the night. The bulk quantities used in the two-layer approach as well as the different terms in the equations are computed from the three-dimensional information provided by the mesoscale simulation. This method allows to inspect how well the simplified approach represents the katabatic flow generated by the mesoscale model. The study shows that the hydraulic model allows for a comprehensive analysis of the basic mechanisms of the slope flows but is not able to close the budget equations, since the residuals are large.