On the vertical structure of Mediterranean explosive cyclones

An attempt is made to explore the vertical structure of the surface explosive cyclones in the Mediterranean on a climatological basis during the cold period of the year in order to get a better insight in the interaction between the upper and lower levels responsible for the genesis and evolvement of the phenomenon. The vertical profile of the explosive cyclones was examined with the aid of the vertical tracing software of the University of Melbourne Cyclone Tracking Algorithm, using the 1?×?1° spatial resolution of ERA-40 reanalysis data. It was found that about 57?% of the track steps of surface explosive cyclones extend up to 500?hPa. The north-westward tilting of the surface cyclones with height during the stage of explosive cyclogenesis, with a mean distance of 350?km between mean sea and 500?hPa levels, confirms the importance of baroclinicity. About 45?% of the surface explosive cyclones reached their maximum depth before their 500?hPa counterparts, implying the role of surface processes.     

On the bogusing of tropical cyclones in numerical models: The influence of vertical structure

Summary In this study, idealised conditions are used to study the influence of vertical structure of the bogus vortex on its motion in numerical models by comparing the resultant forecast tracks. Two vortices were used: one has a cyclonic circulation throughout the troposphere and the other has an upper tropospheric anticyclone. Both vortices have the same structure in the middle and lower troposphere. The two vortices were inserted into four different environmental flows on a beta-plane: (a) a resting atmosphere; (b) a uniform flow; (c) a horozontal shear flow and (d) a vertical shear flow. The results show that the forecast tracks are very sensitive to the vertical structure of the bogus vortex, especially when the environmental flow is very weak, or is westerly and has a cyclonic horizontal shear. However, this sensitivity is reduced in moderate vertical shear. This motion sensitivity is found to arise from the vertical coupling mechanism by which the upper-and lower-level circulations interact with each other when a horizontal displacement occurs between them.The vertical structure of the bogus vortex can also affect the intensity of the model cyclone, depending on the configuration of the environmental flow. In general, the bogus vortex without an upper-level anticyclone will intensify quicker and will develop more intense than the one with an upper-level anticyclone. The vertical coupling mechanism can result in different asymmetric rainfall pattern in cyclone core region depending on the vertical structure of the bogus vortex. The asymmetric divergent flow associated with these convective asymmetries may in turn further influence the vortex motion. It is suggested that care needs to be taken in determining the vertical structure of the bogus vortex in numerical models.With 14 Figures     

陆地上爆发性温带气旋的暖锋后弯结构分析

尽管关于温带气旋发展和演变的观点不尽相同,但目前普遍被接受的两种模型是：挪威气旋模型、Shapiro和Keyser模型。以FY-2E卫星云图为基础,先给出8个温带气旋过程实例,然后结合常规高空、地面观测及NCEP的1°×1°再分析场等资料,通过个例分析,对暖锋后弯气旋发生发展的环流背景、结构及成因进行分析。结果表明：(1)卫星云图显示东亚陆地上温带气旋存在T-bone结构和暖锋后弯的事实。(2)温带气旋发生在500 hPa东亚大陆中高纬两脊一槽的背景下,槽加深及下游脊的发展有利于气旋的发展,与经典温带气旋发生发展的环流背景类似。(3) 2012年5月11—13日个例分析表明蒙古气旋中存在锋面波动、锋面断裂、T-bone结构和暖锋后弯、暖核被隔离现象;暖核可从地面向上伸展到600 hPa。(4)在地面气旋初生和发展阶段,地面气旋中心西侧高低层正相对涡度区呈后倾结构;当高低层正涡度区几乎垂直重合时,地面气旋停止发展;气旋中心西侧对流层中低层的锋区一直存在。(5)当高低层涡度平流差值为正、300 hPa正涡度平流引起的辐散叠加到对流层中低层锋区之上,地面气旋才会生成和发展。逐渐增强的暖平流从气旋中心的东部和北部向气旋的西部和西南部输送,从而形成了卫星云图上的T-bone结构和暖锋后弯现象。

     

On the synoptics and dynamics of orographically induced cyclones in the Mediterranean

Summary In the first part of the present paper all cases of orographically induced cyclogeneses in the Western Mediterranean (40 cases) during the whole 13 months of the ALPEX Observing Period (AOP: 1 September 1981–30 September 1982) are investigated statistically to find similarities between them. It can be shown that essentially there exist two different types of such cyclogeneses: the Vorderseiten-type (southwesterly upper-level flow) and the Überströmungs-type (northwesterly upper-level flow). Both types are accompanied by low level blocking of cold air by the Alps. The second part of the paper is concerned with the understanding of the explosive development of an orographically induced cyclone in the Western Mediterranean by means of vorticity dynamics on isentropic surfaces. Budget equations for the shear and curvature vorticity allow us to find out the reasons for the increase of curvature vorticity, locally as well as individually. Such growth rates are caused by corresponding convergences and by transformation from shear into curvature vorticity, respectively. Case studies of some ALPEX-SOP cyclogeneses illustrate the vorticity-dynamical considerations.

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Zur Synoptik und Dynamik orographisch induzierter Zyklogenesen im Mittelmeerraum

Zusammenfassung In der vorliegenden Studie werden zunächst alle orographisch induzierten Zyklogenesen im westlichen Mittelmeerraum (40 Fälle) während der gesamten 13 monatigen ALPEX-Beobachtungsperiode (AOP: 1. September 1981–30. September 1982) einer statistischen Untersuchung unterzogen, um Gemeinsamkeiten herauszufinden. Dabei zeigte es sich, daß im wesentlichen zwei Typen von derartigen Zyklogenesen auftreten: der Vorderseiten-Typus und der Überströmungs-Typus. In beiden Fällen kommt es dabei in den unteren Luftschichten zur Blockierung der Kaltluft durch die Alpen. Im weiteren wird dann die vielfach explosionsartige Entwicklung der orographisch induzierten Zyklogenese im westlichen Mittelmeer mit Hilfe der Vorticitydynamik auf isentropen Flächen zu verstehen versucht. Die Aufspaltung der Bilanzgleichung für die Vorticity in eine Bilanzgleichung für die Scherungs- und in eine Bilanzgleichung für die Krümmungsvorticity, erlaubt das genaue Wachstumsstudium der Krümmungsvorticity sowohl lokal als auch individuell. Derartige Wachstumsraten kommen durch entsprechende Konvergenzen bzw. durch die Umwandlung von Scherungs- in Krümmungsvorticity zustande Mehrere Fallstudien von ALPEX-SOP veranschaulichen diese wirbeldynamischen Überlegungen.

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With 9 Figures     

Mesoscale structure of tropical cyclones in the surface wind field

The mesoscale structure of tropical cyclones in the northwest Pacific in 2002–2007 at different stages of evolution (from the genesis to the maximum intensity) is studied using the QuikSCAT satellite data and JRA-25 reanalysis data. It is demonstrated that the genesis of the tropical cyclone was preceded by the formation of the stable disturbance that was observed in the vorticity field on the average 47 hours before the first report. The variability is noted of the mesoscale structure of the cyclone during the process of its formation and evolution: the increase in the intensity of mesovortices, the decrease in their number as a result of the merging, the narrowing of the area occupied by them, and localization of this area near the center at the stage of maximum development. It is shown that the relationship between the mean intensity of mesovortices at the initial disturbance and the tropical cyclone intensity is close to linear and has high correlation coefficients.     

Climatic characteristics of Mediterranean cyclones

Based on the data for the period of 1948?C2004 using the objective methods of automatic identification of cyclones, the main areas of Mediterranean cyclogenesis are revealed, the main regularities of their trajectories are described, and the peculiarities of interseasonal variability of characteristics of the genesis and movement of Mediterranean cyclones are determined. Special attention is paid to the cyclones coming to the European part of Russia and to the territory of CIS countries, i.e., to so-called southern cyclones. The areas of genesis and trajectories of southern cyclones are classified. A trend of the total number of Mediterranean cyclones and of the number of southern cyclones is analyzed and their synchronous relationships with the low-frequency variability of atmospheric circulation are estimated using the indices of major teleconnection systems. It is demonstrated that the significant feedbacks exist between the number of southern cyclones and the NAO and EA2 teleconnection indices, as well as the index of North Asian system.     

Mediterranean warm-core cyclones in a warmer world

Regional climate model projections over the Mediterranean region are analysed for the presence of intense, warm-core lows that share some of the characteristics of tropical cyclones. The results indicate that the number of such systems decreases in a warmer world, particularly in winter. Comparison of the simulated numbers to changes in relevant climate diagnostics suggests that numbers decrease due to an increasingly hostile environment for storm formation, combined with a general poleward shift in the incidence of wintertime lows over western Europe.     

Analysis of conveyor belts in winter Mediterranean cyclones

The relevance of the midlatitude conveyor belt model to Mediterranean cyclones (MCs) is examined using data from two winters. Eight MCs, which exhibit typical midlatitude cyclone structure, were scrutinized and their conveyor belts were examined. The analysis was based on satellite imagery, isentropic wind maps, vertical cross-sections of potential and equivalent potential temperatures, and air back-trajectories. The conveyor belts found in the studied MCs were similar to the common features of midlatitude cyclones, except for three aspects. First, the warm conveyor belt was not associated with massive organized cloudiness in five of the eight cyclones since it consisted of dry air originated from the Saharan desert. Second, the anticyclonic branch of the cold conveyor belt was not found in half of the MCs. Third, the dry air intrusion originated north of the cyclone and extended southward around it, unlike its common midlatitudinal northwest–southeast orientation. This is consistent with the relatively small baroclinic vertical-westward tilt of the cyclones analyzed.     

Cyclogenesis in the Mediterranean region

A statistical study is made of cyclones originating in the Mediterranean region in the months October through May from 1929 to 1939 and 1945 to 1952. The following features are summarized: synoptic type, points of origin, monthly and geographic frequencies, relation of displacement to topography and deepening, 24-hr deepening, isobar direction at 3 km at time and place of origin, relation between 3-km isobar direction and cyclone trajectory, and relation between vertical temperature distribution and central pressure.

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Zusammenfassung Es wird eine statistische Untersuchung der Zyklonen durchgeführt, die sich während der Perioden 1929 bis 1939 und 1945 bis 1952 (jeweils in den Monaten Oktober bis Mai) im Mittelmeergebiet gebildet haben. Dabei wird über folgende Gesichtspunkte zusammenfassend berichtet: Synoptische Wetterlage, zyklogenetische Ursprungsstellen, monatliche und geographische Häufigkeitsverteilung Beziehung zwischen Verlagerung einerseits und topographischen Einflüssen und Vertiefung anderseits, Depressionsvertiefung innerhalb 24 Stunden, Isobarenrichtung in 3 km Höhe im Zeitpunkt und Ort der Zyklonenentstehung, Beziehung zwischen der Isobarenrichtung in 3 km und der Zyklonenbahn, Beziehung zwischen der vertikalen Temperaturverteilung und dem Luftdruck im Zyklonenzentrum.

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Résumé Il s'agit d'une étude statistique des dépressions formées en Méditerranée pendant les périodes 1929–1939 et 1945–1952, d'octobre à mai. L'auteur considère les points de vue suivants: situation météorologique générale, régions cyclogénétiques, distribution des fréquences par mois et par régions, relation entre le déplacement et les influences topographiques et le creusement, creusement en24 heures, direction des isobares à 3 km d'altitude au moment et au lieu de formation, relation entre cette même direction et la trajectoire dépressionnaire, relation entre la répartition verticale de la température et la pression au centre de la dépression.

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With 20 Figures.

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Contribution to a research project sponsored by the Geophysics Research Directorate of the Air Force Cambridge Research Center under Contract No. AF 19 (122)-466.     

On the vertical profile of surface radiative fluxes in southwest Germany

Summary Vertical profile of surface radiative fluxes in an area of heterogeneous terrain in south-west Germany is presented. Main data sets utilized for the study were recorded during the REgio KLIma Projekt (REKLIP). Supporting observational data were provided by the German weather service and German geophysical consultant service. Elevation of the study sites ranged from 212 m a.s.l. to 1489 m a.s.l. From May to September, monthly mean albedo was generally low at the study sites, ranging from 19% to 24%. For the other months, monthly mean albedo lie between 22% and 25% at the lowland site but extended between 27% and 71% at the highly elevated mountain site. Following the altitudinal increase in surface albedo, net radiative flux and radiation efficiency declined with elevation at an annual mean of 1.15 Wm⁻²/100 m and 0.008/100 m respectively. Absorbed shortwave radiation and effective terrestrial radiation showed mean decline of 1.54 Wm⁻²/100 m and 0.34 Wm⁻²/100 m, respectively, with the mean sky-to-earth radiation deficit amounting to about 52 Wm⁻² for the lowland site and 73 Wm⁻² for the highest elevated site. Some empirical models which express shortwave and longwave radiative fluxes in terms of meteorological variables have been validated for the lowland and mountain sites. Monthly mean daily total estimates of solar radiation obtained from ?ngst?m-Prescott relation were quite consistent with observed values. Parameterisation of downward atmospheric radiation under all sky condition was achieved by extending Brutsaert clear sky atmospheric model. Relationship between outgoing longwave radiation and screen temperature at the study sites was best described by an exponential function unlike the linear relationship proposed by Monteith and Unsworth. Net radiative flux for the lowland and mountain sites has been expressed in terms of absorbed shortwave radiation, cloud amount and screen temperature. Received March 5, 2001 Revised October 29, 2001     

Assessing characteristics of Mediterranean explosive cyclones for different data resolution

A comparison of two objective climatologies of explosive cyclones in the Mediterranean region is performed. The results are derived from two different mean sea-level pressure reanalysis data resolutions, but from the same assimilation model, in order to quantify the pure impact of higher resolution on the identification and characteristics of explosive cyclones, when the assimilation model is the same. The explosive cyclones were identified with the aid of the Melbourne University automatic cyclone finding and tracking scheme over a 40-year period, using the 6-hourly analyses of ERA-40 mean sea-level pressure (MSLP) on: (a) 2.5?×?2.5 and (b) 1?×?1 latitude–longitude grid. The comparison of the two datasets revealed the significant role of the increase in spatial resolution of MSLP data on the identification and tracking process, and the number of the explosive cyclones in the high-resolution dataset is almost four times greater than the respective one in the lower resolution dataset. However, the comparison of explosive cyclone characteristics, including spatial and temporal variations of explosive deepening, revealed differences in the geographical distribution of the location of the maximum explosive deepening and average explosive cyclone Laplacian of the central pressure. These differences are due to the identification in the higher resolution set of smaller scale and secondary explosives along the strongly baroclinic northern Mediterranean boundaries, south of the Alps and the Pyrenees. Explosive deepening appears a bias to the daytime period from 12 to 18 Coordinated Universal Time (UTC) for both datasets, which is more prominent in the LR dataset. Statistically significant difference of pressure tendency between the two datasets appear for the daytime period from 06 to 12 UTC, accounting for better representation of orographic forcing in the HR dataset.     

Analyzing the genesis, intensity, and tracks of western Mediterranean cyclones

Midlatitude cyclones are analyzed on a selected region covering most of southern Europe and the western part of the Mediterranean Sea (35–50°N, 10°W–25°E). On the basis of mean sea level pressure fields of the ECMWF (European Centre for Medium-range Weather Forecast) Reanalysis Dataset (ERA-40), detailed evaluation of Mediterranean cyclones is accomplished for the period between 1957 and 2002 on a 1° horizontal resolution grid. Cyclone centers are identified and their paths are tracked with a 6-h time step (using 00 UTC, 06 UTC, 12 UTC, and 18 UTC). Decadal, annual, and seasonal statistical analysis of cyclone tracks includes the study of the genesis, frequency, and activity of the Mediterranean cyclones as well as the variability of cyclone tracks. The results suggest that the cyclone frequency in the western Mediterranean region increased in summer and autumn, and decreased in winter and spring. A special belt-shaped area is identified, which plays a special role in cyclogenesis, and also, the cyclone tracks often remain within this belt. An overall decreasing trend is detected in winter and spring in the entire Mediterranean belt, while cyclone frequency increased in autumn. The largest positive and negative trend coefficients are identified in summer.     

Consistency of observed near surface temperature trends with climate change projections over the Mediterranean region

We examine the possibility that anthropogenic forcing (Greenhouse gases and Sulfate aerosols, GS) is a plausible explanation for the observed near-surface temperature trends over the Mediterranean area. For this purpose, we compare annual and seasonal observed trends in near-surface temperature over the period from 1980 to 2009 with the response to GS forcing estimated from 23 models derived from CMIP3 database. We find that there is less than a 5% chance that natural (internal) variability is responsible for the observed annual and seasonal area-mean warming except in winter. Using additionally two pattern similarity statistics, pattern correlation and regression, we find that the large-scale component (spatial-mean) of the GS signal is detectable (at 2.5% level) in all seasons except in winter. In contrast, we fail to detect the small-scale component (spatial anomalies about the mean) of GS signal in observed trend patterns. Further, we find that the recent trends are significantly (at 2.5% level) consistent with all the 23 GS patterns, except in summer and spring, when 9 and 5 models respectively underestimate the observed warming. Thus, we conclude that GS forcing is a plausible explanation for the observed warming in the Mediterranean region. Consistency of observed trends with climate change projections indicates that present trends may be understood of what will come more so in the future, allowing for a better communication of the societal challenges to meet in the future.     

The vertical structure of concentration fluctuation statistics in plumes dispersing in the atmospheric surface layer

This paper describes a study of the vertical structure of concentration fluctuations in a neutrally buoyant plume from an elevated point source in slightly convective to moderately stable meteorological conditions at ranges of between 12.5 and 100 m for a range of source heights between 1 and 5 m. Observations were made of concentration fluctuations in a dispersing plume using a vertical array of sixteen very fast-response photoionization detectors placed at heights between 0.5 and 16 m. Vertical profiles of a number of concentration statistics were extracted, namely, mean concentration, fluctuation intensity, intermittency factor, peak-to-mean concentration ratio, mean dissipation rate of concentration variance, and various concentration time and length scales of dominant motions in the plume (e.g., integral macro-scale, in-plume mid-scale and Taylor micro-scale). The profiles revealed a similarity to corresponding crosswind profiles for a fully elevated plume, but showed greater and greater departure from the latter shapes once the plume had grown in the vertical so that its lower dege began to interact progressively more strongly with the ground. The evolution of the concentration probability density function at a fixed range, but with decreasing height from the ground, is similar to that obtained at a fixed height but with increasing distance from the source. Concentration power spectra obtained at different heights all had an extensive inertial-convective subrange spanning at least two decades in frequency, but spectra measured near the ground had a greater proportion of the total concentration variance in the lower frequencies (energetic subrange), with a correspondingly smaller proportion in the higher frequencies (inertial-convective subrange). It is believed that these effects result from the increased mean shear near the surface, and blocking by the surface. The effect of enhanced shear-induced molecular diffusion on concentration fluctuations is examined.     

On the formation and development of tropical cyclones

A method of diagnostic calculation of the maximal wind speed in the tropical cyclone, its radius, and central pressure is proposed taking into account large-scale air motions in a low-gradient baric field between subtropical anticyclones. The results of such calculations are considered. A conclusion is made about sufficient accuracy of calculations of the parameters within the tropical zone using only dynamic factors.     

Modeling the evolution of the family of Mediterranean cyclones using the regional model of the atmosphere

Due to the complex orography and the presence of the moisture-saturated air, the Mediterranean region is characterized by the increased baroclinic and convective instability, that leads to the sudden cyclogenesis and the formation of dangerous weather phenomena. The results are given of the investigation of formation mechanisms of Mediterranean cyclones, peculiarities of stages of their evolution and dynamical processes, which occur throughout the atmosphere, using the regional numerical ETA model of the atmosphere by the example of individual cases of the cyclogenesis over the Mediterranean Sea. It is revealed that the cold Arctic air outbreak (the intrusion of the cold Arctic air) to the south of the Western Europe, leading to the formation of the areas of the baroclinic instability and the increased moisture content of the air in the area of the vortex origin, favors the cyclogenesis. The use of the vertical coordinate η in the model enabled to compute more precisely the vertical wind speed, therefore, the influence of the orography on the moisture content and precipitation increase became pronounced. The transformation of the structure of meteorological fields in the course of the development of vortexes is considered. The computation of the helicity is made, and it is shown that this characteristic can be one of the earliest predictors of cyclogenesis.