On the relationship between climatic variables and pressure systems over Saudi Arabia in the winter season

The synoptic circulation over Saudi Arabia is complicated and frequently governed by the effect of large-scale pressure systems. In this work, we used NCEP–NCAR global data to illustrate the relationship between climatic variables and the main pressure systems that affect the weather and climate of Saudi Arabia, and also to investigate the influence of these pressure systems on surface air temperature(SAT) and rainfall over the region in the winter season. It was found that there are two primary patterns of pressure that influence the weather and climate of Saudi Arabia. The first occurs in cases of a strengthening Subtropical High(Sub H), a weakening Siberian High(Sib H), a deepening of the Icelandic Low(Ice L), or a weakening of the Sudanese Low(Sud L). During this pattern, the Sub H combines with the Sib H and an obvious increase of sea level pressure(SLP) occurs over southern European, the Mediterranean, North Africa, and the Middle East. This belt of high pressure prevents interaction between midlatitude and extratropical systems, which leads to a decrease in the SAT,relative humidity(RH) and rainfall over Saudi Arabia. The second pattern occurs in association with a weakening of the Sub H, a strengthening of the Sib H, a weakening of the Ice L, or a deepening of the Sud L. The pattern arising in this case leads to an interaction between two different air masses: the first(cold moist) air mass is associated with the Mediterranean depression travelling from west to east, while the second(warm moist) air mass is associated with the northward oscillation of the Sud L and its inverted V-shape trough. The interaction between these two air masses increases the SAT, RH and the probability of rainfall over Saudi Arabia, especially over the northwest and northeast regions.     

Atmospheric GCM response to an idealized anomaly of the Mediterranean sea surface temperature

The atmospheric response to an idealized 2 K cooling of the Mediterranean Sea is studied in a general circulation model (GCM). In the downstream region, from the eastern Mediterranean basin to the whole Asian continent (following the sub-tropical jet-stream), a baroclinic response is produced with high pressure near the surface and low pressure in the upper layers. It is the direct response to the Mediterranean cooling and it needs only a few days to be established. Teleconnections with strong zonal characteristics are found following the sub-tropical jet-stream to the North Pacific and North America, propagating further across the North Atlantic towards Northern Europe. Two remarkable remote features are the deepening of the Aleutian Low in the North Pacific and the weakening of the Icelandic Low in the North Atlantic. These two teleconnections form and grow in several days in the North Pacific and several tens of days in the North Atlantic. Both have a quasi-barotropic vertical structure. It is believed that they are the consequence of complex interactions between the mean flow and the transient eddies in the atmosphere.     

The role of dynamic and diabatic processes in the generation of cut-off lows over Northwest Africa

Summary The present observational study addresses the role of dynamic and diabatic processes leading to the generation of four deep upper-level troughs/cut-offs, involved in two extreme precipitation episodes over West Africa during the cool season. The elongated potential vorticity (PV) streamers associated with the observed troughs form as a result of an equatorward transport of high-PV air downstream of a large ridge over the central North Atlantic. Strong deformation along the eastern side of the ridge leads to a thinning of the PV streamers. In some situations the tips of the streamers break up and form distinct and long-lived stationary cut-offs near West Africa, in particular if the presence of another PV ridge downstream allows a complete isolation from the midlatitude westerlies. In other situations a prior anticyclonic wave-breaking event over Europe leads to an advection of high-PV towards the Iberian Peninsula that merges with the streamer and impedes a complete cut-off. The observations presented here suggest that the rapid amplification of the PV ridges over the North Atlantic and thus the subsequent streamer formation are related to upstream latent heating through non-conservative diabatic reduction of upper-level PV and through the strong divergent outflow near the tropopause that support large negative isentropic PV advection. The intense latent heat release is promoted by cyclo- and frontogenesis, and the transport of warm, moist air by a low-level jet ahead of the surface cold front (often called a warm conveyor belt; WCB). Diabatic PV tendencies are highest where the WCB rises over the surface warm front to the northeast or east of the cyclone centre. In most cases the distinct heating occurs in connection with a strong upper-level jet and a rapid deepening of the involved surface cyclone. More quantitative dynamical and statistical studies of the suggested relation are needed to better understand the relative contributions of single factors to the large and synoptic scale evolution that leads to PV streamers/cut-offs near West Africa.     

Role of Extratropical Cyclones in the Recently Observed Increase in Poleward Moisture Transport into the Arctic Ocean

Poleward atmospheric moisture transport(AMT) into the Arctic Ocean can change atmospheric moisture or water vapor content and cause cloud formation and redistribution, which may change downward longwave radiation and, in turn, surface energy budgets, air temperatures, and sea-ice production and melt. In this study, we found a consistently enhanced poleward AMT across 60?N since 1959 based on the NCAR–NCEP reanalysis. Regional analysis demonstrates that the poleward AMT predominantly occurs over the North Atlantic and North Pacific regions, contributing about 57% and 32%, respectively, to the total transport. To improve our understanding of the driving force for this enhanced poleward AMT, we explored the role that extratropical cyclone activity may play. Climatologically, about 207 extratropical cyclones move across 60?N into the Arctic Ocean each year, among which about 66(32% of the total) and 47(23%) originate from the North Atlantic and North Pacific Ocean, respectively. When analyzing the linear trends of the time series constructed by using a 20-year running window, we found a positive correlation of 0.70 between poleward yearly AMT and the integrated cyclone activity index(measurement of cyclone intensity, number, and duration). This shows the consistent multidecadal changes between these two parameters and may suggest cyclone activity plays a driving role in the enhanced poleward AMT. Furthermore, a composite analysis indicates that intensification and poleward extension of the Icelandic low and accompanying strengthened cyclone activity play an important role in enhancing poleward AMT over the North Atlantic region.     

Global climate change and variability and its influence on Alpine climate — concepts and observations

Summary The paper discusses annual to decadal climate variability and change in the European Alps by utilizing the procedure of synoptic downscaling, i.e. it investigates the influence of global to continental scale synoptic structures and processes on the regional climate of the Alps. The European Alps lie to the southeast and under the right exit zone of the southwest-northeast oriented axis of the polar front jet over the North Atlantic ocean, in a transition zone between the Azores high and Icelandic low, between oceanic and continental and between Mediterranean and North Atlantic climates. Together with complex topographically induced phenomena like lee cyclogenesis, orographic precipitation, strong downslope winds and thermotopographical circulation systems, this transitional position makes climate studies in the Alps even more interesting. Only a minor correlation can be observed between global climate variability and Alpine climate. In contrast, the Alpine climate is strongly related to processes over the North Atlantic ocean and its sea ice system (e.g. it has a high correlation with the North Atlantic Oscillation and the dynamics and position of the Icelandic low), an area with a rather low climate prediction potential.Since the early 1970's (or just after the Great Salinity Anomaly in the North Atlantic Ocean) the intensification of the wintertime westerly jet over the North Atlantic area led to a noticeable northwest-southeast mass transport in the exit area of the jet over Central Europe, leading to pressure and temperature rises and an increase in the amount of precipitation. There is a question over whether this phenomenon is a consequence of natural climate variability or the beginning of an anthropogenic climate change.With 16 Figures     

冬季北太平洋爆发性气旋的天气气候特征

本文利用1975—1984年12—2月北太平洋爆发性温带气旋资料以及500 hPa天气图，概括出气旋爆发式加强前12小时五种环流型，即北美高压型（NAH）、东太平洋高压型（EPH）、中太平洋高压型（MPH）、白令海阻高型（BBH）、太平洋低压型（PL）。以NAH型和EPH型出现的机会最多，稳定时间最长，并且在这两种天气形势下气旋最易发展。另外本文还分析了气旋爆发后的路径趋势，指出在形势稳定期，气旋爆发后的路径亦稳定；在形势调整期，气旋后期路径随太平洋长波系统的演变而变化。     

Kinetic energy budget and moisture flux convergence analysis during interaction between two cyclonic systems: Case study

An analysis of the kinetic energy budget during a case of interaction between middle latitude and extratropical cyclones has been made in this work. Horizontal flux convergence constitutes a major energy sink. Generation of kinetic energy via cross-contour flow is a persistent source throughout the growth and decay periods. Dissipation of kinetic energy from subgrid to grid scales is an important source during the pre-storm period; it acts as a sink during the growth and decay periods. The major contribution to kinetic energy comes from a persistent upper tropospheric jet stream activity throughout the period of the cyclone development. The characteristics of moisture-flux components (divergent and rotational) along with precipitable water content for different tropospheric layers throughout the life cycle of our cyclone are also studied in this work. It is found that most of required humidity for our cyclone are initiated from Arabian Sea and then to some extent are reinforced over Gulf of Aden and east of central Africa and then by passing over Red sea enter to the south and south east of Mediterranean Sea. The rotational component of the moisture transport brings moisture from two regions; the first which is considered the main region is the Indian Ocean, Arabian Sea, Gulf of Aden and north east of Sudan. The second source region is the Atlantic and Mediterranean Sea. In the middle troposphere, the primary moisture source is found over central Africa, which in turn is traced to the Atlantic Ocean, the Indian Ocean, and the Arabian Sea. The upper-level moisture fluxes are weak and play a minor role over the area of interaction between two cyclones.     

On the vertical structure of composite surface cyclones in the Mediterranean region

Summary The average pressure distribution at mean sea level and the vertical structure of synoptic scale surface cyclones (with central pressure less than 1000 hPa) that occur in the Mediterranean region is studied for a 40 year period (1958–1997) on a seasonal and daily basis. The cyclonic occurrences are studied in three regions of enhanced cyclonic activity: gulf of Genoa, Southern Italy and Cyprus. The cyclones are identified with the aid of an objective method based on grid point values, available every 6 hours. The analysis revealed different characteristics of the cyclones that occur in the three regions, reflecting the different mechanisms that are responsible for their occurrence in each region. For the Genoa region the cyclone pressure minimum is located over the gulf, associated with orographic forcing, while surface dynamics occur further south. Over Southern Italy, the pressure minimum covers a wide area, whilst the surface dynamics are found to act in the same region, becoming more important in winter and spring. The pressure minimum of cyclones over Cyprus is located over the land during winter and spring and is influenced by surface dynamics and orography. Received November 7, 2000 Revised July 14, 2001     

Development of an eye-wall like structure in a tropical cyclone model simulation

Structural changes during the intensification of a tropical storm into a hurricane in a numerical simulation are examined. A 10 layer primitive equation model that employs a horizontal grid spacing of 20 km over 4400 × 4400 km area is integrated. An elongated band in vertical motion over the storm area intensifies slowly during the first few hours. In the upper troposphere high pressures arise due to condensational heating. Between 8–12 h strong outflow winds develop in the upper troposphere due to the increased pressure gradients. Strong divergence occurs in the outflow wind region, and a large increase in the vertical motion, condensational heating and intensification rate of the storm ensues. Between 12–24 h the elongated band of the storm stage transforms into an eye-wall like structure, and the tropical storm intensifies into a hurricane. Regions with negative moist potential vorticity appear in the high troposphere. Widening of area of condensation and slanting of the convergence area occurs with height in the high level negative moist potential vorticity regions. Results suggest that the formation of anvil clouds in some cases may be due to the development of slantwise convection on the outer periphery of a hurricane's eye-wall.     

Quantifying the Contribution of Track Changes to Interannual Variations of North Atlantic Intense Hurricanes

Previous studies have linked interannual variability of tropical cyclone(TC)intensity in the North Atlantic basin(NA)to Sahelian rainfall,vertical shear of the environmental flow,and relative sea surface temperature(SST).In this study,the contribution of TC track changes to the interannual variations of intense hurricane activity in the North Atlantic basin is evaluated through numerical experiments.It is found that that observed interannual variations of the frequency of intense hurricanes during the period 1958–2017 are dynamically consistent with changes in the large-scale ocean/atmosphere environment.Track changes can account for~50%of the interannual variability of intense hurricanes,while no significant difference is found for individual environmental parameters between active and inactive years.The only significant difference between active and inactive years is in the duration of TC intensification in the region east of 60°W.The duration increase is not due to the slow-down of TC translation.In active years,a southeastward shift of the formation location in the region east of 60°W causes TCs to take a westward prevailing track,which allows TCs to have a longer opportunity for intensification.On the other hand,most TCs in inactive years take a recurving track,leading to a shorter duration of intensification.This study suggests that the influence of track changes should be considered to understand the basin-wide intensity changes in the North Atlantic basin on the interannual time scale.     

2018年一次温带气旋入海爆发性增强时期的集合预报对比分析

基于实况观测资料、欧洲中期天气预报中心(European Centre for Medium-Range Forecast, ECMWF)0.5°（纬度）×0.5°（经度）水平分辨率的再分析数据和集合预报数据,对2018年2月一次入海爆发性气旋在黄海南部的爆发性增强时期的动力和热力因子进行了对比分析。根据气旋路径、强度和海面风的检验结果挑选出两组集合成员——好成员组和坏成员组。通过组间对比分析得到如下主要 结论 1）在气旋入海之后爆发性增强时,500 hPa高空槽和850 hPa中低层低涡迅速加强,同时低层和高层的西南急流均明显加大,中高层系统快速增强,上述因子均为气旋出现爆发性发展提供有利条件。2）气旋入海之后上升运动快速增强,这加剧了低层辐合与高层辐散,有利于地面降压,促使地面气旋的爆发性发展。水汽在中低层辐合后随气流上升发生凝结并释放潜热,这加强了高层辐散、低层辐合以及上升运动,促使气旋进一步爆发性发展。与此同时,对流层顶的高值位涡下传增强,低层大气斜压性受气旋上空冷暖平流的增强而增大,导致垂直稳定度减小,地面气旋性涡度增强,也有利于气旋爆发性发展。最终此次气旋快速增强并达到中等爆发性气旋的强度。3）虽然集合预报两组成员的平均场均比分析场弱,但是好成员组抓住了气旋上空中高层天气系统的快速增强过程,以及垂直运动、温度平流、水汽条件、位涡等预报因子和物理量的快速增强过程,其预报效果在气旋强度和路径等方面均显著优于坏成员组。     

Projected future changes in tropical cyclone-related wave climate in the North Atlantic

Tropical cyclones are a major hazard for numerous countries surrounding the tropical-to-subtropical North Atlantic sub-basin including the Caribbean Sea and Gulf of Mexico. Their intense winds, which can exceed 300 km h⁻¹, can cause serious damage, particularly along coastlines where the combined action of waves, currents and low atmospheric pressure leads to storm surge and coastal flooding. This work presents future projections of North Atlantic tropical cyclone-related wave climate. A new configuration of the ARPEGE-Climat global atmospheric model on a stretched grid reaching ~ 14 km resolution to the north-east of the eastern Caribbean is able to reproduce the distribution of tropical cyclone winds, including Category 5 hurricanes. Historical (1984–2013, 5 members) and future (2051–2080, 5 members) simulations with the IPCC RCP8.5 scenario are used to drive the MFWAM (Météo-France Wave Action Model) spectral wave model over the Atlantic basin during the hurricane season. An intermediate 50-km resolution grid is used to propagate mid-latitude swells into a higher 10-km resolution grid over the tropical cyclone main development region. Wave model performance is evaluated over the historical period with the ERA5 reanalysis and satellite altimetry data. Future projections exhibit a modest but widespread reduction in seasonal mean wave heights in response to weakening subtropical anticyclone, yet marked increases in tropical cyclone-related wind sea and extreme wave heights within a large region extending from the African coasts to the North American continent.

     

The Role of diabatic heating, torques and stabilities in forcing the radial- vertical circulation within cyclones part iii: case study of lee-side cyclones

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1998年7月河套气旋强烈发展时的暴雨过程分析

采用位涡理论对1998年7月4—7日的一次河套气旋强烈发展中的暴雨过程进行分析。结果表明:此次夏季河套气旋的强烈发展是在高层正位涡平流和低层暖平流的共同作用下产生的。高空双急流结构产生的强烈辐散加强了低层辐合，有利于气旋的加强。强降水出现在河套气旋强烈发展过程中，是由高层冷空气与季风涌带来的西南暖湿气流辐合而引起的大尺度降水过程。在这次气旋强烈发展过程中，对流层低层到中上层均出现强的上升气流，使得南方深厚的暖湿空气不断随西南风流入暴雨区上空。暴雨发生时，华北地区处于地面Ω型的θ_se高能舌之中，其上空500 hPa存在一个由大尺度动力强迫形成的东北—西南向的非地转湿 Q 矢量辐合带，对流云带与 Q 矢量辐合中心有非常好的对应关系。     

Sensitivity of last glacial maximum climate to sea ice conditions in the Nordic Seas

Published reconstructions of last glacial maximum (LGM) sea surface temperatures and sea ice extent differ significantly. We here test the sensitivity of simulated North Atlantic climates to two different reconstructions by using these reconstructions as boundary conditions for model experiments. An atmospheric general circulation model has been used to perform two simulations of the (LGM) and a modern-day control simulation. Standard (CLIMAP) reconstructions of sea ice and sea surface temperatures have been used for the first simulation, and a set of new reconstructions in the Nordic Seas/Northern Atlantic have been used for the second experiment. The new reconstruction is based on 158 core samples, and represents ice-free conditions during summer in the Nordic Seas, with accordingly warmer sea surface temperatures and less extensive sea ice during winter as well. The simulated glacial climate is globally 5.7 K colder than modern day, with the largest changes at mid and high latitudes. Due to more intense Hadley circulation, the precipitation at lower latitudes has increased in the simulations of the LGM. Relative to the simulation with the standard CLIMAP reconstructions, reduction of the sea ice in the North Atlantic gives positive local responses in temperature, precipitation and reduction of the sea level pressure. Only very weak signatures of the wintertime Icelandic Low occur when the standard CLIMAP sea surface temperature reconstruction is used as the lower boundary condition in LGM. With reduced sea ice conditions in the Nordic Seas, the Icelandic Low becomes more intense and closer to its present structure. This indicates that thermal forcing is an important factor in determining the strength and position of the Icelandic Low. The Arctic Oscillation is the most dominant large scale variability feature on the Northern Hemisphere in modern day winter climate. In the simulation of the LGM with extensive sea ice this pattern is significantly changed and represents no systematic large scale variability over the North Atlantic. Reduction of the North Atlantic sea ice extent leads to stronger variability in monthly mean sea level pressure in winter. The synoptic variability appears at a lower level in the simulation when standard reconstructions of the sea surface in the LGM are used. A closer inspection of storm tracks in this model experiment shows that that the synoptic lows follow a narrow band along the ice edge during winter. The trajectories of synoptic lows are not constrained to the sea ice edge to the same degree when the sea ice extent is reduced. Seasonally open waters in the Nordic Seas in the new reconstruction apparently act as a moisture source, consistent with the current understanding of the rapid growth of the Fennoscandian and Barents Ice Sheets, during the LGM. The signal from the intensified thermal forcing in the North Atlantic in Boreal winter is carried zonally by upper tropospheric waves, and thus generates non-local responses to the changed sea ice cover.     

On the change of the tropical cyclone structure during its passage over islands

The impact is studied of small land areas on the configuration and structure of the tropical cyclone as well as on the variations of different characteristics of hurricanes (wind field, kinetic energy, and vorticity) during their passage over islands. The results of computations based on the regional numerical atmospheric ETA model for the hurricanes of the Caribbean Sea and typhoons of the Northwestern Pacific revealed that the disturbance of the symmetric circulation in the vortex accompanied by significant kinetic energy losses takes place when crossing the archipelagos or separate islands. It is demonstrated that the vortex intensity depends not on the energy loss due to the underlying surface roughness only but on the heat flux from it as well. The kinetic energy generation in the hurricane sharply decreases as a result of the decrease in the pressure gradient over the land that is caused, in turn, by the tropical cyclone moving away from the oceanic heat source. At the recurring appearance of the cyclone over the warm ocean waters, its deepening and intensification recommence.     

The influence of sea-surface temperatures on Eastern North Pacific tropical cyclone activity

The influence of sea-surface temperatures on six measures of tropical cyclone activity in the Eastern North Pacific is examined using historical sea-surface temperature and tropical cyclone data spanning from 1971 to 2002. Relationships are evaluated using methods of trend analysis, extreme year analysis, and bivariate correlation. Results suggest that in order to understand the climatological factors affecting topical cyclone activity in the Eastern North Pacific, the main development region must be divided into two sub-regions of development to the east and west of 112°W longitude. Increasing trends of sea-surface temperature are not accompanied by increasing trends in tropical cyclone activity. In the western development region, sea-surface temperatures are significantly correlated with all measures of tropical cyclone activity during extreme years. In this region, sea-surface temperatures are on average below the threshold for tropical cyclone development. In the Eastern development region, the only significant correlation with sea-surface temperatures is for the more intense measures of hurricane activity. In this region, sea-surface temperatures are on average above the threshold for cyclone formation. This leads to the hypothesis that the proximity to the cyclone formation temperature threshold in the WDR enhances the sensitivity of tropical cyclone activity to SSTs. This may have application to other tropical cyclone basins such as the North Atlantic.     

Winter mean temperature variability in Turkey associated with the North Atlantic Oscillation

Changes and variability in seasonal average mean and monthly mean winter (DJF) air temperature series at 70 stations of Turkey and the circulation types at 500-hPa geopotential height level were investigated to explain atmospheric controls of temperature variations during the extreme (weak and strong) phases and normal (negative and positive) phases of the North Atlantic Oscillation (i.e., Ponta Delgada–Reykjavik and the Gibraltar–Reykjavik) indices. During the positive phases of the North Atlantic Oscillation indices (NAOIs), northeasterly circulation increased, and thus spatially coherent and significant cold signals dominate over the majority of Turkey. This pattern is closely linked to anomalously low 500-hPa heights over the region of the Icelandic Low, and anomalously high geopotential heights over the regions of the Azores High, the western Mediterranean basin and the Europe, in general including the Balkans and northwest Turkey. Contrarily, during the negative phases of the NAOIs, prevailing westerly winds that originate from the subtropical northeast Atlantic increase, and thus spatially coherent and significant warm signals over the Anatolian peninsula appear. This pattern is closely linked to the increased cyclonic activity and associated increased westerly and southwesterly circulation causing warm maritime air advection over the Mediterranean basin toward Turkey.     

Hurricanes Ivan, Jeanne, Karl (2004) and mid-latitude trough interactions

Summary Singular vectors (SVs) constructed from the adjoint model of the U.S. Naval Operational Global Atmosphere Prediction System (NOGAPS) for three Atlantic hurricanes in 2004, Ivan, Jeanne and Karl, are examined to understand interactions between them and a mid-latitude trough system. By optimizing the perturbation energy localized in a small region centered at the 48-hour projected position of a tropical cyclone, the initial time singular vector represents the sensitive region to the final state within the specified region for a specified optimization period. For hurricane Ivan, the SV analysis reveals the merging of a shortwave mid-latitude trough and Ivan to form a new trough system. This new trough system impacted the evolution of hurricane Jeanne in subsequent time through the upstream flow of the trough that moved toward Jeanne. This is consistent with previous studies on the sensitivity of tropical cyclone prediction using SV diagnostics. The SV associated with Jeanne at later stage shows that Jeanne influenced the third hurricane Karl through the trough system as Karl went through extratropical transition and became part of the trough. This effect is magnified when the SV is computed using the moist adjoint system containing large scale precipitation. Detailed diagnostics of the SVs for individual components at different levels show that the sensitivity associated with the trough is very similar for those optimized for Jeanne and Karl, respectively, thus providing evidence that the mid-latitude trough impacted Jeanne and Karl at the same time. This study demonstrates the capability of singular vector in diagnosing complicated interactions among a mid-latitude trough and three co-existing tropical cyclones.     

我国北方地区一次沙尘暴天气特征分析

使用美国大气中心6h一次的NCEP再分析格点资料(1.0°×1.0°)对2002年3月18—22日发生在我国北方的大范围沙尘暴天气成因进行分析。从环流形势、物理量诊断、高空急流等方面进行研究分析,结果表明:蒙古气旋是这次沙尘暴天气的主要影响系统,这次气旋发生发展在斜压区,气旋的发展阶段温度平流作用明显。沙尘暴过程主要是由气旋冷锋及锋后地面大风触发的,地面大风的形成与气旋发展、锋后冷平流及高空急流动量下传有关。