Seasonal climatology of cloud at resolute (75°N)

Abstract

An analysis of surface observations of cloud characteristics from a selected high‐latitude station, Resolute (N.W.T., Canada), is described. The data set has a high temporal resolution (1 h) and is extensive although here we report only on an analysis of 10 years: 1970–1979, inclusive. The pattern of cloudiness variations is more complex than previous studies of Arctic data have suggested. The predominant reported low‐level cloud type is stratocumulus, not stratus. The generally overcast summer conditions are found to extend over a considerable period when cloud cover is composed of several layers, often of types other than stratus. This observation of multilayering suggests that middle and high cloud layers may occur more frequently than they can be observed. Clearly a more accurate awareness of hidden cloud is important for improving solutions of the planetary radiation budget. The surface radiation budget is found to be strongly affected by the development of the summertime overcast but also exhibits features caused by the mid‐summer break‐up of the lower cloud layer.     

Low Ozone Episodes at Amphitrite Point Marine Boundary Layer Observatory,British Columbia,Canada

At Amphitrite Point, ozone (O₃) mixing ratios are observed to drop steadily to 5–15?ppb over a period of 12 hours or less with a frequency approaching one event per week (with highest frequencies occurring in summer and fall). Analysis of 47 such O₃ depletion events reveals that low O₃ episodes are a predominantly nocturnal phenomenon associated with anticyclonic conditions characterized by light onshore or alongshore winds and an absence of fog and mist. Back-trajectories show air carried to the Amphitrite Point Observatory (APO) during depletion events remains in the marine boundary layer and is not brought to the surface from aloft. There is no strong correlation with other “criteria” pollutants (CO, NO_x, SO₂, PM_2.5) that might be indicative of a mechanism for O₃ destruction linked to human, terrestrial, or marine pollutant sources. However, CO₂ mixing ratios are observed to increase, coincident with O₃ depletion. Together, these results point to a natural marine boundary layer phenomenon in which O₃ destruction dominates O₃ production and/or replenishment by vertical mixing. While there are several candidate mechanisms, the conditions for O₃ depletion (and CO₂ buildup) to occur are set by meteorology and, in particular, development of a stable marine boundary layer in which vertical mixing is suppressed. Support for this interpretation is provided by simultaneous increases in CO₂ in the stable marine boundary that are indicative of an important role played by marine biogenic processes (respiration). Future research should be directed at elucidating the chemical mechanisms responsible for O₃ destruction in the coastal zone, which means that there would be a need for a much broader range of measurements at APO (including halogenated species) as well as offshore measurements of both chemical and marine boundary layer meteorological variables.     

Laboratory simulations of coastally trapped waves with rotation, topography and stratification

We describe observations of the generation and propagation of coastally trapped waves in the laboratory and their comparison with theory, over a range of values of several experimental parameters. The topography and stratification used consisted of a sloping continental shelf and vertical continental slope with three-layer stratification that could be approximated by an extended version of the Gill and Clarke model [Gill, A.E., Clarke, A., 1974. Wind-induced upwelling, coastal currents and sea level changes. Deep Sea Res. 21, 325–345]. The latter was modified to accommodate a central mixed layer, curved geometry, and friction on the shelf. This configuration represents coastal geometry with large Burger number. The experiments were successful in realizing coastally trapped waves that were consistent with the theoretical expectations. However, the waves propagated more slowly, and for narrow shelves were damped more rapidly than predicted by the theory. The first was attributed to: (i) the effect of stratification on fluid on the shelf, reducing the topographic Rossby wave effect; (ii) the parameterization of the viscosity. The second difference was attributed to the mechanism of generation: the paddle used did not always generate sinusoidal waves, and the subsequent dispersion resulted in a net loss of amplitude.     

Analysis of dust events in 2008 and 2009 using the lidar network, surface observations and the CFORS model

The Asian dust events in 2008 (May 24–June 4 in 2008) and in 2009 (March 12–25, October 13–26, and December 15–28 in 2009) were analyzed with the lidar network observations, surface observations in China, Korea, Japan, and Mongolia, and with the chemical transport model CFORS. Transport of Asian dust and mixing of dust with air pollution aerosols were studied. The event of May 24 to June 4 in 2008 was a significant event unusually late in the spring dust season. The dust event of March 12–25, 2009 was an interesting example of elevated dust layer, and transport of dust from the elevated dust layer to the ground by the boundary layer activity was observed with the lidars and surface observations in Japan. The concentration of air pollution aerosols was relatively high during the dust event, and the results suggest that vertical structure as well as transport path is important for the mixing of dust and air pollution aerosols. The dust events in October and December 2009 were examples of dust events in autumn and winter. The online mode CFORS reproduced the observation data generally well, except for the event of May 24 to June 4 in 2008. The results of the fourdimensional variational assimilation of the lidar network data reproduced the dust concentration in Korea and Japan reasonably in that event.     

Propagation of coastal‐trapped waves under an ice cover in Hudson Bay*

Abstract

Three arrays of current‐meter moorings were deployed under landfast sea ice in southeast Hudson Bay for eight weeks in spring 1986. Spectral analysis shows low‐frequency signals with periods of 3 to 11 days. These signals are interpreted as being due to coastal‐trapped waves propagating cyclonically in Hudson Bay; their theoretical dispersion relations and corresponding modal structures are presented for winter stratification and are compared with observations. At a period of 3 days both the modified external Kelvin wave and higher mode continental shelf waves may be important in describing the observed low‐frequency variability, whereas at a period of 10 days the Kelvin wave appears to be the dominant mode. The generation mechanisms for these coastal trapped waves are also investigated. Two sources have been studied: the longshore atmospheric pressure gradient and the average atmospheric pressure over the ice cover in Hudson Bay. Coherence and phase analyses performed with time series of longshore current and atmospheric forcing data reveal that both the average atmospheric pressure and the longshore atmospheric pressure gradient are important in explaining the observed low‐frequency variability, without indicating which one is the most important.     

Evolution of a Southeast Australian Coastally Trapped Disturbance

Summary  The southeast Australian coastally trapped disturbance (CTD) of 9–11 November 1982 that was previously studied by Holland and Leslie is re-visited. Additional observational data not considered by these authors and a numerical simulation using the Colorado State University Regional Atmospheric Modeling system (RAMS) are used for this purpose. Following initiation of the event on the south coast, mesoscale ridging propagated along the east coast to just north of Brisbane. Associated with the arrival of the event were a marked increase in surface pressure, drop in temperature and a shift and strengthening of the wind. While the simulation does not appear to capture the details of the boundary layer as well as one would like, it does represent the main features of the event, including the speed of propagation along the coast, reasonably well. Similar to the observed, the model event shows gravity current-like characteristics. The significance of topographic variability (e.g., large gap in the coastal mountains at the Hunter Valley) is considered. It is suggested that the topography and ambient stratification in southeastern Australia are less favourable for CTD occurrence than those in southern Africa and western North America where these systems have been extensively studied. Consequently, when CTD do occur in southeastern Australia, the less pronounced topography and weaker stratification may enable local effects to mask the CTD signal to some extent, thereby posing challenges in observation and forecasting. Received March 2, 1999     

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.     

Fine-Scale Characteristics of Temperature, Wind, and Turbulence in the Lower Atmosphere (0–1,300 m) Over the South Peruvian Coast

We report results of preliminary high-resolution in situ atmospheric measurements through the boundary layer and lower atmosphere over the southern coast of Perú. This region of the coast is of particular interest because it lies adjacent to the northern coastal edge of the sub-tropical south-eastern Pacific, a very large area of ocean having a persistent stratus deck located just below the marine boundary layer (MBL) inversion. Typically, the boundary layer in this region during winter is topped by a quasi-permanent, well-defined, and very large temperature gradient. The data presented herein examine fine-scale details of the coastal atmosphere at a point where the edge of this MBL extends over the coastline as a result of persistent onshore flow. Atmospheric data were gathered using a recently-developed in-house constructed, GPS-controlled, micro-autonomous-vehicle aircraft (the DataHawk). Measured quantities include high-resolution profiles of temperature, wind, and turbulence structure from the surface to 1,300 m.     

Sensitivity of Offshore Surface Fluxes and Sea Breezes to the Spatial Distribution of Sea-Surface Temperature

A series of numerical sensitivity experiments is performed to quantify the impact of sea-surface temperature (SST) distribution on offshore surface fluxes and simulated sea-breeze dynamics. The SST simulations of two mid-latitude sea-breeze events over coastal New England are performed using a spatially-uniform SST, as well as spatially-varying SST datasets of 32- and 1-km horizontal resolutions. Offshore surface heat and buoyancy fluxes vary in response to the SST distribution. Local sea-breeze circulations are relatively insensitive, with minimal differences in vertical structure and propagation speed among the experiments. The largest thermal perturbations are confined to the lowest 10% of the sea-breeze column due to the relatively high stability of the mid-Atlantic marine atmospheric boundary layer (ABL) suppressing vertical mixing, resulting in the depth of the marine layer remaining unchanged. Minimal impacts on the column-averaged virtual potential temperature and sea-breeze depth translates to small changes in sea-breeze propagation speed. This indicates that the use of datasets with a fine-scale SST may not produce more accurate sea-breeze simulations in highly stable marine ABL regimes, though may prove more beneficial in less stable sub-tropical environments.     

The Influence of Thermal Effects on the Wind Speed Profile of the Coastal Marine Boundary Layer

The wind speed profile in a coastal marine environment is investigated with observations from the measurement program Rødsand, where meteorological data are collected with a 50 m high mast in the Danish Baltic Sea, about 11 km from the coast. When compared with the standard Monin—Obukhov theory the measured wind speed increase between 10 m and 50 m height is found to be systematically larger than predicted for stable and near-neutral conditions. The data indicate that the deviation is smaller for short (10–20 km) distances to the coast than for larger (>30 km) distances. The theory of the planetary boundary layer with an inversion lid offers a qualitative explanation for these findings. When warm air is advected over colder water, a capping inversion typically develops. The air below is constantly cooled by the water and gradually develops into a well-mixed layer with near-neutral stratification. Typical examples as well as scatter plots of the data are consistent with this explanation. The deviation of measured and predicted wind speed profiles is shown to be correlated with the estimated height and strength of the inversion layer.     

The contribution of moisture to heat stress in a period of global warming: the case of the Mediterranean

This study deals with potential changes in the relative humidity associated with global warming and their implications on heat stress along the coastal region of the Mediterranean in the summer season. It is based on the assumption that the regional warming will enhance the lower-level stability due to the thermal inertia of the sea with respect to its overlying air. The enhanced stability implies more effective trapping of the near surface moisture, and as a result—further increase of the relative humidity. The marine boundary layer over the Mediterranean is modeled. The central feature of the model is the marine inversion capping the marine moist air, which intensity is positively correlated with the stability. Simple calculations indicate that if the temperature increases, while the stability remains unchanged, the near-surface relative humidity would not be affected. But, an increase in the stability would result in an increase in the near-surface relative humidity. This prediction is validated through observed trends of the respective fields, using the NCEP/NCAR reanalysis data and soundings from the eastern Mediterranean. The results are consistent in indicating an increase in the near-surface temperature, the lower-level stability and the relative humidity over the eastern part of the Mediterranean, but not in its western part. The results for the eastern Mediterranean support the expectation for an aggravation of heat stress beyond that imparted by the temperature rise.     

A study on the transition mechanism of a stratus cloud into a warm sea fog using a single column model PAFOG coupled with WRF

The transition mechanism of stratus cloud into warm sea fog over the Yellow Sea near the western coastal area of the Korean Peninsula is investigated using numerical simulation with a 1D turbulence model, PAFOG, coupled with a 3D regional model, WRF. The coupled model system was run in the two approaches, Eulerian and Lagrangian. For the selected warm sea fog case, the model results in the Eulerian approach showed that the bottom of the stratus cloud was lowered by cooling of the air just below the cloud base by turbulent heat loss. The Lagrangian approach showed the lowering of the stratus cloud top, owing to the evaporation of cloud droplets in this region by the entrainment of warm and dry air above the cloud top. The sensitivity test to SST indicated that the timing of water vapor saturation just below the cloud base depended on the magnitude of the turbulent heat flux from the sea surface. The subsidence rate was found to be important: when the subsidence rate was set to be half of the prescribed value, neither the lowering of the stratus cloud top nor the bottom occurred and the model could not produce a fog.     

Tidal circulation and buoyancy effects in the St. Lawrence Estuary

Abstract

The action of tides on density‐driven circulation, internal gravity waves, and mixing was investigated in the St. Lawrence Estuary between Rimouski and Québec City. Time‐varying fields of water level, currents and density were computed under typical summer conditions using a three‐dimensional hydrostatic coastal ocean model that incorporates a second order turbulence closure submodel. These results are compared with current meter records and other observations. The model and the observations reveal buoyancy effects produced by tidal forcing. The semi‐diurnal tide raises the isopycnals over the sills at the head of the Laurentian Trough and English Bank, producing internal tides radiating seaward. Relatively dense intermediate waters rise from below 75‐m depth to the near surface over the sills, setting up gravity currents on the inner slopes. Internal hydraulic controls develop over the outer sills; during flood, surface flow separation occurs at the entrances of the Saguenay Fjord and the upper estuary west of Ilet Rouge Bank. Early during ebb flow (restratification), the surface layer deepens to encompass the tops of the sills. As the ebb current intensifies, the model predicts the formation of seaward internal jumps over the outer sills, which were confirmed from acoustic reflection observations. As the internal Froude number increases further, flow separation migrates up to sill height. As a result of these transitions, internal bores emanate from the head region one to two hours before low water. We find that the mixing of oceanic and surface waters near the sills is driven by the vertical shear produced during ebb in the channel south of Ilet Rouge, the shear produced in the bottom gravity flood currents, and, to a lesser extent, the processes over the sills.     

层积云覆盖的海洋边界层云详细微物理过程的数值模拟

文中建立了一个含显式分档的云微物理模式和辐射传输模式的一维 3阶湍流闭合模式 ,该模式可用于研究海洋边界层云中气溶胶和云的相互作用过程 ,同时提出了一种新的动力模式和微物理模式耦合方法 ,该方法可使动力模式中液态水相关项可以直接由微物理模式变量计算得到。作为模式的初步应用模拟了 2 0 0 1年APEX/ACE Asia在西太平洋上所观测到的一个个例。模拟结果和观测资料比较表明该模式基本上模拟出层积云覆盖的海洋边界层的基本结构     

A numerical investigation of a moderate coastal storm with intense precipitation

Abstract

In this study, the development of a moderate coastal storm with intense precipitation that occurred during 12–14 February 1993 is examined using a high‐resolution version of the Canadian Regional Finite‐element (RFE) model with more realistic physical representations. It is shown that the improved RFE model predicts well the coastal cyclogenesis events and also the distribution and intensity of heavy mixed precipitation (rain and snow) associated with the storm. It is found that the cyclogenesis takes place in response to the low‐level inshore advection of high‐θ_e air from the maritime boundary layer, and the approach of a mid‐level shortwave trough with a warm pool above that is previously associated with a decaying cyclone upstream. More rapid deepening of the cyclone ensues as intense precipitation falls along the warm and cold fronts near the cyclone centre.

Diagnosis of the control and sensitivity simulations reveals that the low‐level inshore warm advection and the propagation of the stratospheric warm pool contribute more significantly to the surface pressure falls during the incipient stage, whereas the mid‐level shortwave trough plays an important role in the cyclogenesis at later stages. Overall, latent heat release accounts for about 50% of the cyclone's total deepening, in agreement with the presence of a moderate baroclinic environment and the generation of intense precipitation.

The diabatic and kinematic structures near the rain‐snow boundary are examined to gain insight into the influence of melting snow on the cyclogenesis. It is shown that the improved RFE model reproduces well the rain‐snow boundary structures as previously observed. Moreover, a thermally indirect circulation (perturbation) can be seen in the vicinity of the rain‐snow boundary. It is found, however, that melting of snow tends to produce a weak negative or negligible impact on the cyclogenesis, as opposed to previous hypotheses.     

Airborne radar and in-situ observations of a shallow stratus with drizzle

Observations were made of a shallow stratus of upslope origin using an aircraft equipped with insitu probes and with a vertically-pointing radar of 3-mm wavelength. A cloud layer of 300 m thickness was found below the inversion; an additional layer of 100 m thickness was located within the inversion. The coldest temperature within the cloud was -2°C and the cloud contained no ice particles. Drizzle drops up to 180 Am were present in both cloud layers.The observations reveal precipitation and air motion structures of approximately 1 km horizontal dimensions. The origin of this organization appears to be weak convection. In addition, mixing played an important role in forming the cloud droplet and drizzle drop size distributions.     

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。     

利用CloudSat/CALIPSO卫星资料分析北半球污染排放区云型频率分布特征北大核心

本文利用A-Train卫星队列中的Cloudsat卫星所提供的二级云分类产品资料(结合了CALIPSO卫星气溶胶激光雷达)2B-CLDCLASS-LIDAR,选取2007年3月至2017年2月的样本数据进行统计分析,研究北半球主要的气溶胶排放源区(中国东部,美国东部和欧洲西部)不同云型出现频率的分布特征。结果表明,在以单层云出现的8种云类中卷云,层积云和积云的发生频率总和高达50%~70%,其次为高层云、高积云和雨层云,而深对流云和层云这两种云仅占10%以下。各类云的发生频率的空间分布可看出卷云和层积云的发生频率可高达90%以上,高层云的发生频率在70%~80%左右,高积云和积云的发生频率则接近70%以上,深对流云和层云的发生频率则在20%以下。其中,卷云、深对流云和积云主要出现在低纬度的海洋上;高层云和高积云主要出现在中低纬工业发达的陆地上;层积云、层云和雨层云主要出现在中高纬地区,其中层积云和层云出现在海洋上居多,雨层云出现频次的海陆差别不大。不同云型在不同的季节出现频次差异明显,在夏季出现较多的云型以卷云、深对流云,积云和层云为主;在冬季则是高层云、层积云和雨层云这样稳定型的云型占据主导,同时还发现卷云和层积云发生频率的月变化相反,而高层云和雨层云发生频率的月变化相似。     

Divergence of turbulent fluxes in the surface layer: case of a coastal city

This study quantifies the processes that take place in the layer between the mean building height and the measurement level of an energy balance micrometeorological tower located in the dense old core of a coastal European city. The contributions of storage, vertical advection, horizontal advection and radiative divergence for heat are evaluated with the available measurements and with a three-dimensional, high-resolution meteorological simulation that had been evaluated against observations. The study focused on a summer period characterized by sea-breeze flows that affect the city. In this specific configuration, it appears that the horizontal advection is the dominant term. During the afternoon when the sea breeze is well established, correction of the sensible heat flux with horizontal heat advection increases the measured sensible heat flux up to 100 W m⁻². For latent heat flux, the horizontal moisture advection converted to equivalent latent heat flux suggests a decrease of 50 W m⁻². The simulation reproduces well the temporal evolution and magnitude of these terms.     

Differences in Meteorological Conditions between Days with Persistent and Non-Persistent Pollution in Beijing,China

We compared the regional synoptic patterns and local meteorological conditions during persistent and non-persistent pollution events in Beijing using US NCEP–Department of Energy reanalysis outputs and observations from meteorological stations. The analysis focused on the impacts of high-frequency (period < 90 days) variations in meteorological conditions on persistent pollution events (those lasting for at least 3 days). Persistent pollution events tended to occur in association with slow-moving weather systems producing stagnant weather conditions, whereas rapidly moving weather systems caused a dramatic change in the local weather conditions so that the pollution event was short-lived. Although Beijing was under the influence of anomalous southerly winds in all four seasons during pollution events, notable differences were identified in the regional patterns of sea-level pressure and local anomalies in relative humidity among persistent pollution events in different seasons. A region of lower pressure was present to the north of Beijing in spring, fall, and winter, whereas regions of lower and higher pressures were observed northwest and southeast of Beijing, respectively, in summer. The relative humidity near Beijing was higher in fall and winter, but lower in spring and summer. These differences may explain the seasonal dependence of the relationship between air pollution and the local meteorological variables. Our analysis showed that the temperature inversion in the lower troposphere played an important part in the occurrence of air pollution under stagnant weather conditions. Some results from this study are based on a limited number of events and thus require validation using more data.