A Numerical Study of Sea-Fog Formation over Cold Sea Surface Using a One-Dimensional Turbulence Model Coupled with the Weather Research and Forecasting Model

The formation mechanism of a cold sea-fog case observed over the Yellow Sea near the western coastal area of the Korean Peninsula is investigated using numerical simulation with a one-dimensional turbulence model coupled with a three-dimensional regional model. The simulation was carried out using both Eulerian and Lagrangian approaches; both approaches produced sea fog in a manner consistent with observation. For the selected cold sea-fog case, the model results suggested the following: as warm and moist air flows over a cold sea surface, the lower part of the air column is modified by the turbulent exchange of heat and moisture and the diurnal variation in radiation. The modified boundary-layer structure represents a typical stable thermally internal boundary layer. Within the stable thermally internal boundary layer, the air temperature is decreased by radiative cooling and turbulent heat exchange but the moisture loss due to the downward vapour flux in the lowest part of the air column is compensated by moisture advection and therefore the dewpoint temperature does not decrease as rapidly as does the air temperature. Eventually water vapour saturation is achieved and the cold sea fog forms in the thermal internal boundary layer.     

华南沿海暖海雾过程中的湍流热量交换特征

湍流交换是海雾中的关键物理过程, 在海雾的热量和水汽平衡过程中起重要作用。本文根据2007年3月24～25日一次海雾的外场观测数据, 分析了海雾过程中近海面湍流热量交换特征; 并在区分风切变机械湍流与雾顶长波辐射冷却热力湍流的基础上, 着重分析了两种不同性质的湍流对海雾发展和维持的作用。结果表明: (1) 本次海雾是在西南低压和变性冷高压的控制下, 来自南海东部暖水区的空气平流在近岸冷海面上形成的暖海雾; 暖海雾中的湍流热量交换过程比冷海雾更为复杂; (2) 在暖海雾的形成和消散阶段, 风切变机械湍流的热量输送起主要作用; 而在发展和维持阶段, 既有风切变机械湍流的热量输送作用, 也有雾顶长波辐射冷却热力湍流的热量输送作用; (3) 风切变机械湍流向冷海面输送热量, 对近海面空气起到降温和增湿作用; 热力湍流同样向冷海面输送热量, 但对雾层起到增温和降湿作用; (4) 暖海雾中的湍流热量交换机制与雾层的非充分混合结构有密切关系。     

Convection model for stratus cloud over a warm water surface

A numerical model is developed to simulate convective stratus cloud formation over the sea. The model is based on quasi-steady state moist plumes advecting over an area of increasing sea-surface temperature with the sea warmer than the air, but it is also a good approximation for non-steady states whenever the model is matched to air-sea temperature differences. Combining the effects of upward transfer of heat and moisture fluxes as well as adiabatic cooling, stratus cloud forms and spreads downward in this field. The depth of the convective field, the sea surface temperature gradient, the liquid water content at cloud top, and the horizontal pressure gradient are the four controlling parameters for the convective field. Alternatively, the wind speed, air-sea temperature difference, and the mixing ratio of the air, derived variables in this treatment, can be taken, with the depth, as the basic parameters.The entity type of convective model used here has the advantage that it models the transport and modification of air parcels and hence provides a method for studying drop size development in stratus clouds. It also uses relationships derived from water tank experiments with plumes and tested in dry convection and so needs no parameters specific to each situation. Its most important feature, however, is that the mean motion of plumes, rather than turbulent diffusion, transports the moisture.The upward growth and erosion of a temperature step increase to produce an inversion can be attributed to liquid water present in haze drops or cloud drops, rather than to turbulent diffusion. Radiative transfer is not a necessary requirement, but may either enhance or slow down the process.     

Inversions,and fog,stratus and cumulus formation in warm air over cooler water

Two aspects of convection over oceans are discussed and the following conclusions are derived from theoretical considerations.

1. The air layer over the sea will usually convect even when the water surface is ten degrees or more colder than the initial air temperature.
2. An inversion at stratus cloud tops is created by the stratus, and is not a necessary preexisting condition. Such inversions persist after subsidence evaporates the cloud.
3. Radiation heat exchange does not play an essential role in stratus formation or maintenance, and can either heat or cool the cloud.
4. Dry air convection does not erode inversions at the top of the convecting layer. Examples of soundings are discussed.
5. Fogs are most likely to form at sea where the water is coolest, and need no radiation effects to initiate cooling, or a boost from patches of warmer water, to begin convection.
6. Both stratus cloud growth, and the evaporation of clouds by cloud top entrainment, readjust the vertical structure of the air to leave a constant wet-bulb potential temperature with height.

These conclusions are supported by, firstly, a convective model which has been developed and which shows that vapor-driven convection over the ocean will proceed with zero or negative heat fluxes, at rates which saturate the lowest layer of the atmosphere in a few hours to altitudes of many tens of meters. Secondly, the availability of condensed moisture at the top of the surface layer cools the warmer entrained overlying dry air parcels so that when they descend they are no warmer than the sea surface temperature, and this induces downward moving plumes. This occurs if the wet-bulb potential temperature of the overlying air is less than the sea surface temperature, even if it is ten degrees C, or more, warmer in actual temperature.     

一次黄海海雾的数据同化试验与形成机制研究

选取2012年4月14日的一次存在东西2片雾区的黄海春季海雾为研究对象,借助WRF（Weather Research and Forecasting）模式,采用循环3DVAR（3-Dimensional Variational）数据同化方案,考虑了湿度控制变量的背景误差协方差CV6,进行了AIRS（Atmospheric Infrared Sounder）卫星温度与湿度廓线数据的同化试验,并基于同化试验结果探讨了此次海雾的形成机制。同化试验结果表明：同化 AIRS 卫星温度与湿度廓线数据后,模式能成功再现海雾的形成过程,特别是东西2片雾区之间的晴空区的存在,这归功于AIRS数据的同化能够显著改善海上大气边界层的温湿结构、影响海雾的低层高压的范围与强度;机制分析揭示：东西2片雾均为典型的平流冷却雾,但二者厚薄和气团来源不同;海上高压控制黄海西岸陆地的暖空气入海,受低海温的冷却作用降温先形成逆温层,然后逆温层底部生成了较薄的西侧雾区;来自黄海中部的空气向东北移动至朝鲜半岛西部海域,高压下沉增温形成一个顶部较高的稳定层,从而生成较厚的东侧雾区;高压中心下沉区内,近海面风速小使得机械湍流弱,空气增温与海温暖舌共同作用下使得近海面气海温差小,海雾无法生成导致了晴空区的存在。     

Analysis of Coastal Fog from a Ship During the C-FOG Campaign

This work presents ship-based measurements of fog off St John’s, Newfoundland, on 13 September 2018 during the Coastal Fog field campaign. The measurements included cloud-particle spectra, cloud-base height and aerosol backscatter, radiation, turbulence, visibility, and sea-surface temperature. Radiosonde soundings were made at intervals of less than 2 h. Fog occurred in two episodes during the passage of an eastward-moving synoptic low-pressure system. The boundary-layer structure during the first fog episode consisted of three layers, separated by two saturated temperature inversions, and capped by a subsidence inversion. The lowest layer was fog, and the upper layers were cloud. The second fog episode consisted of one well-mixed fog layer capped by a subsidence inversion. Low wind speeds and stable stratification maintained low surface-layer turbulence during fog. Droplet size distributions had typical bimodal distributions. The visibility correlated with the droplet number concentration and liquid water content. The air temperature was higher than the sea-surface temperature for the first 30 min of the first fog episode but was lower than the sea for the remainder of all fog. The sensible heat flux was upward, from sea to air, for the first 62% of the first fog episode and then reversed to downward, from air to sea, for the remainder of the first fog episode and the second fog episode. The counter-gradient heat fluxes observed (i.e., opposite to what is expected from the instantaneous air–sea temperature difference) appear to be related to turbulence, entrainment, and stratification in the fog layer that overwhelmed the influence of the air–sea temperature difference. While the synoptic-scale dynamics preconditioned the area for fog formation, the final step of fog appearance in this case was nuanced by stratification–turbulence interactions, local advective processes, and microphysical environment.

     

黄渤海一次持续性大雾过程特征和成因分析

利用日本MTSAT1R卫星数据、常规地面和高空观测数据、NCEP FNL客观再分析资料和NEARGOOS（NorthEast Asian Regional Global Ocean Observing System）的海表温度（SST）数据,分析了2010年5月31日至6月5日发生在黄渤海及周边地区的一次持续性海雾天气的形成、维持、消散特征及其物理机制。结果表明：大雾形成前低层水汽非常充沛,入海变性冷高压的稳定维持为这次持续性海雾过程提供了有利的背景条件,海雾在夜间辐射冷却作用下形成;大雾期间黄渤海     

Impact of horizontal diffusion, radiation and cloudiness parameterization schemes on fog forecasting in valleys

Fog and low stratus forecasting experiments have been carried out with the numerical weather prediction model ALADIN on a case of long lasting fog. The model has been used with different radiation, cloud diagnosing and horizontal diffusion schemes, different representation of orography, increased vertical resolution and with or without prognostic condensates and turbulent kinetic energy (TKE). Some of the numerical set-ups are able to reproduce the fog (low stratus) field as seen in the satellite images as well as the measured 2m temperature and relative humidity diurnal cycles. The results show that cloud diagnosing schemes and overlap assumptions play a more important role than a more sophisticated radiation scheme, or introduction of prognostic cloud water, ice, rain, snow or TKE. More realistic orography representation and a more physical horizontal diffusion scheme significantly improve the modelled low stratus and 2m temperature in the areas with variable orography.     

Characteristics of the Boundary Layer Structure of Sea Fog on the Coast of Southern China

Using boundary layer data with regard to sea fog observed at the Science Experiment Base for Marine Meteorology at Bohe,Guangdong Province,the structure of the atmospheric boundary layer and the characteristics of the tops of the fog and the clouds were analyzed.In addition,the effects of advection,radiation,and turbulence during sea fog were also investigated.According to the stability definition of saturated,wet air,the gradient of the potential pseudo-equivalent temperature equal to zero was defined as the thermal turbulence interface.There is evidence to suggest that two layers of turbulence exist in sea fog.Thermal turbulence produced by long-wave radiation is prevalent above the thermal turbulence interface,whereas mechanical turbulence aroused by wind shear is predominant below the interface.The height of the thermal turbulence interface was observed between 180 m and 380 m.Three important factors are closely related to the development of the top of the sea fog:(1) the horizontal advection of the water vapor,(2) the long-wave radiation of the fog top,and(3) the movement of the vertical turbulence.Formation,development,and dissipation are the three possible phases of the evolution of the boundary-layer structure during the sea fog season.In addition,the thermal turbulence interface is the most significant turbulence interface during the formation and development periods;it is maintained after sea fog rises into the stratus layer.     

一次层积云发展过程对黑潮延伸体海洋锋强迫的响应研究——观测与机制分析

由于缺乏海上现场观测,对天气尺度扰动下,海表面温度锋 (海洋锋) 对海洋大气边界层 (MABL) 垂直结构和MABL内海洋性低云 (marine stratus) 的影响研究较少。2014年4月12日,中国海洋大学东方红2号科学考察船在黑潮延伸体海区的海洋锋附近捕捉到一次层积云的迅速发展。在比较稳定的天气形势下,由暖水侧向北穿越海洋锋时,云底和云顶高度升高,云区范围迅速扩大。本文利用多种大气-海洋联合观测数据,结合卫星观测和再分析资料,对此次层积云迅速发展的机理进行了综合分析。结果表明,在海上低压后部西北风控制下,在海洋锋的暖水侧 (下风方) 形成热通量大值中心和低压槽,有助于高空西风动量下传,进而又使得海气界面热通量增加,这种正反馈效应为MABL内混合层厚度加大和云底/顶高度在海洋锋的下风方升高提供有利背景条件。4月12日09:00~12:00(协调世界时),来自日本本州岛陆地的低空暖平流到达该热通量中心上空,暖平流与热通量中心的共同作用,导致该时段近海面暖中心强度异常增加,MABL中静力不稳定层加深和低压槽发展,综合作用的结果使得混合层厚度明显加深,云底高度升高,云区迅速发展。本研究有助于理解在复杂大气背景扰动下MABL对海洋强迫的响应机理。     

A numerical study of the marine stratocumulus cloud layer

A one-dimensional grid-level model including longwave radiative transfer and a level-4 second-order turbulent transfer closure which contains prognostic equations for turbulent quantities, is used to study the physics and dynamics of inversion-capped marine stratocumulus clouds.A set of numerical experiments had been performed to examined the role of sea surface temperature, large-scale vertical velocity, wind speed, and vertical wind shear in the formation and the structure of low-level clouds. For a given sea surface and geostrophic wind speed, stratocumulus clouds can grow higher with smaller large-scale subsidence as less dry air entrains into the cloud. Clouds grow higher with higher sea surface temperature for a given geostrophic wind speed and large-scale subsidence as a result of enhanced moist convection. In high wind speeds, the entire cloud deck is lifted up because of larger surface energy flux. In the budget studies of the turbulent kinetic energy (TKE), the buoyancy term is a major source term when the wind speed and the vertical shear are small across the inversion top. When the wind speed and the vertical wind shear across the inversion top become large, the mixed layer is decoupled into a cloud and a subcloud layer. In the TKE budget studies, the shear generation term becomes an important term in the budgets of the TKE and the variance of vertical velocity.     

A maritime boundary-layer model for the prediction of fog

A two-dimensional boundary-layer model is described. The model is designed to predict and study the effects of meteorological changes on the formation and dissipation of fog and stratus. Radiational heat loss along with the transport of static energy, moisture and momentum are treated. Cloud droplet distributions are parameterized using a gamma distribution from which radiative properties and droplet fall velocities are computed. Turbulent exchange coefficients are calculated using the Monin-Obukhov theory of similitude which accounts for variations in atmospheric stability. Although the boundary-layer depth depends only on turbulent intensity during stable atmospheric conditions, its growth during unstable conditions is determined from the capping inversion's intensity and the amount of turbulence generated at the surface.Several experiments are presented which demonstrate the effects of various meteorological parameters on the formation and duration of stratus and fog. Energy-budget analyses show the importance of each of the physical processes being modeled.Although not new, radiative transfer processes are shown to be extremely important in the transfer of heat from the boundary layer and in the process of fog formation. Fog formation location is highly sensitive to the moisture content upstream, whereas changes in wind speed had much less effect in the variance of fog location.Numerical experiments with other processes such as back radiation from the atmosphere, haze and cloud droplet population, are described and shown to have smaller effects.     

A cold air outbreak near Spitsbergen in springtime — Boundary-layer modification and cloud development

A moderate cold air outbreak from the Arctic ice over the warm West-Spitsbergen current on 15 and 16 May 1988 during the field experiment ARKTIS '88 is analysed using data from four aircraft and one research vessel.The downstream development of cloud coverage appears to depend sensitively on the moisture content above the inversion. The cloud amount determines the energy balance at the sea surface. Under daytime conditions and little cloud cover, energy is added to the ocean in spite of sensible and latent heat losses.The downstream temperature increase in the boundary layer is controlled by sensible heat flux and by longwave radiation cooling. The entrainment sensible heat flux is the dominating term in the region near the ice edge. The downstream moisture increase is controlled by surface evaporation. Condensation processes play no significant role.On 16 May 1988 cloud streets near the ice edge changed to closed cloud meanders in the downstream direction. The aspect ratio increased from 3 to around 10 over a distance of 200 km. In the cloud street region, the dynamical generation of turbulent kinetic energy due to wind shear at the tilted inversion was larger than the thermal generation.Cloud droplet concentration, mean droplet radius and liquid water content increased linearly with height. The maximum liquid water content was only 0.1 g/kg near the top of a 400 m thick closed cloud and clearly below the adiabatic value. The net longwave radiation flux decreased by 50 W/m² at cloud top and increased by 13 W/m² at cloud base.     

亲潮延伸体海区一次海雾过程的观测研究

西北太平洋是全球海雾最多的海域,但由于观测资料匮乏,对开阔大洋上海雾形成机理的个例研究很少。2019年9月12—14日,中国北极科考船“向阳红01号”在亲潮延伸体水域捕捉到一次海雾事件。主要利用船载观测数据,分析了海雾形成的物理过程。结果表明,这是一次温带气旋的暖锋和局地海洋锋(海面温度锋)共同影响下的海雾过程。伴随暖锋的偏南气流将暖湿空气向北输送,在亲潮延伸体区,海面空气增湿效应大于增温效应,导致相对湿度不断增加接近饱和。北上暖空气遇到较冷水域上空的冷空气团,向上爬升形成大范围锋面逆温;局地海洋锋强迫出大气边界层内的次级环流,其下沉支使该锋面逆温层底的高度进一步降低,有利于雾滴局限在近海面成雾,雾区出现于暖锋锋面过后局地海洋锋的冷水侧。这项研究明确了海雾形成过程中作为背景环流的大气暖锋与作为局地强迫项的海洋锋的贡献,可为海雾预报提供新的理论支撑。     

Ocean-atmosphere interactions and climate drift in a coupled general circulation model

 We have analysed numerical simulations performed with a global 3D coupled atmosphere-ocean model to focus on the role of atmospheric processes leading to sea surface temperature (SST) drift in the tropics. Negative SST errors occur coherently in space and time with large positive errors in latent heat and momentum fluxes at the tropical air-sea interface, as diagnosed from forced SST simulations. The warm pool in the western Pacific disappears after a few years of simulation. Strong SST gradients enforce regions of high precipitation that are thin and stationary north of the equator. We detail the implications for the ocean-atmosphere system of such upheaval in the deep convection location. A sensitivity experiment to empirically formulate air-sea drag coefficient shows that the rapid warm pool erosion is not sensitive to changes in the formulation of the surface drag coefficient over the oceans because the corresponding changes in turbulent heat fluxes and LW cooling approximately cancel one another. In the eastern Pacific, the improvement in SST is striking and caused by feedbacks between SST, surface turbulent fluxes and boundary layer cloud fraction, which decreases as SST warms. Received: 8 December 1998 / Accepted: 6 January 2000     

宁波北仑港区一次航道海雾地基多源信息观测特征分析

结合天气形势、高空风场、370m高塔、地面要素、激光云高仪、毫米波雷达等资料,对宁波北仑港区航道2018年2月15日海雾过程进行分析。结果表明,这是一次以锋前雾为主的海雾过程,在上暖下冷的锋面背景下,地面水汽充沛、风力温和,近地层逆温结构,都为海雾的形成和维持提供了有利条件。上下大气风向不一致,不利于垂直方向空气交换,有利于雾霾的产生。激光云高仪后向散射系数、颗粒物的消光系数及退偏振比,可以比较清晰地得出雾滴粒子的光学特性和过程演变趋势。随着海雾的生成、发展、消散,后向散射系数强度也呈增强、强、逐渐减弱的变化。雾天气现象下雾滴粒子小且轻,波动幅度比较大,某一高度粒子的傅里叶变换频率谱对应频率较高,高频所占比多数大于50%。     

上海一次连续大雾过程的成因分析

利用上海市遥测能见度数据、常规气象资料和T213、GFS分析资料以及GPS-PWV资料,分析了2008年1月5-11日发生在上海地区的一次连续大雾天气过程.分析表明当有天气尺度西风槽频繁活动、槽前暖湿气流不断向气层输入暖湿空气,如再遇特定暖水面或暖海面从近地面向气层输送暖湿气流时,在晴天和微风的早晨极易形成辐射雾;当东、黄海暖湿空气从近地面流向冷水面或冷陆面,如再遇江南低空偏南暖湿气流辐合聚积时,极易形成平流雾.雾区登陆距离与东风气流强弱有关,东风气流越强,雾区西进距离越长.在平流雾期间,0～300 m平均相对湿度的高湿区与雾区吻合.当绝对湿度小于3 g/kg时不会有大雾发生,绝对湿度在9～1 5 g/kg和GPS-PWV值在15 mm左右时,在合适的大气条件下容易形成大雾.     

2018年春节期间琼州海峡持续性大雾监测分析

2018年春节期间,琼州海峡持续6 d（2月15—20日）有雾,最低能见度不足200 m。主要从气象卫星监测上分析此次琼州海峡持续性海雾的时间演变特征,并从大气海洋条件分析形成持续性海雾的原因,得出如下结论：1）2018年2月华南东侧西太平洋海面温度较常年同期偏高,有3个暖海面温度距平中心,为持续性大雾提供暖湿气流;2）此次持续性大雾过程琼州海峡位于均压场,但西南低涡起着辅助的作用,致使海洋性的南风在边界层抵御了干燥的大陆性气团,利于维持边界层的湿度;3）低层流场为暖湿气流,925 hPa近地面湿层上空有一个干中心维持,“上干下湿”的湿度层结有利于海雾维持和向上发展形成一定厚度的海雾;4）低云增加和弱降水是使18日全天海雾得以维持的重要因素。     

Observation of a Self-Limiting,Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

High-resolution measurements of thermodynamic, microphysical, and turbulence properties inside a turbulent inversion layer above a marine stratocumulus cloud layer are presented. The measurements are performed with the helicopter-towed measurement payload Airborne Cloud Turbulence Observation System (ACTOS), which allows for sampling with low true air speeds and steep profiles through cloud top. Vertical profiles show that the turbulent inversion layer consists of clear air above the cloud top, with nearly linear profiles of potential temperature, horizontal wind speed, absolute humidity, and concentration of interstitial aerosol. The layer is turbulent, with an energy dissipation rate nearly the same as that in the lower cloud, suggesting that the two are actively coupled, but with significant anisotropic turbulence at the large scales within the turbulent inversion layer. The turbulent inversion layer is traversed six times and the layer thickness is observed to vary between 37 and 85 m, whereas the potential temperature and horizontal wind speed differences at the top and bottom of the layer remain essentially constant. The Richardson number therefore increases with increasing layer thickness, from approximately 0.2 to 0.7, suggesting that the layer develops to the point where shear production of turbulence is sufficiently weak to be balanced by buoyancy suppression. This picture is consistent with prior numerical simulations of the evolution of turbulence in localized stratified shear layers. It is observed that the large eddy scale is suppressed by buoyancy and is on the order of the Ozmidov scale, much less than the thickness of the turbulent inversion layer, such that direct mixing between the cloud top and the free troposphere is inhibited, and the entrainment velocity tends to decrease with increasing turbulent inversion-layer thickness. Qualitatively, the turbulent inversion layer likely grows through nibbling rather than engulfment.     

Numerical study of mesoscale cellular convection

Mesoscale cellular convections over the East China Sea during cold air outbreaks are simulated with a high-resolution numerical model. The model incorporates important physical processes involved in shallow convection, such as the exchange of heat and moisture between water and air; condensation; evaporation; and vertical turbulent transfer of heat, moisture, and momentum.The results show that open cells develop with aspect ratios as large as 14. The structure of the convection is examined in detail. The organized mesoscale circulation is responsible for breaking up the initial stratus cloud deck and enhancing turbulence in the upward-moving area (especially inside cumulus clouds). However, it is found that the heat flux contributed by MCC's themselves is much smaller than the eddy heat flux.