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1.
Changes in atmospheric circulation over Europe detected by objective and subjective methods 总被引:1,自引:0,他引:1
Summary Changes in atmospheric circulation over Europe since 1958 were examined using both objective (modes of low-frequency variability
and objective classification of circulation types) and subjective (Hess-Brezowsky classification of weather types) methods.
The analysis was performed with an emphasis on the differences between the winter (DJF) and summer (JJA) seasons, and between
objectively and subjectively based results. Majority of the most important changes in atmospheric circulation are same or
similar for the objective and subjective methods: they include the strengthening of the zonal flow in winter since the 1960s
to the early 1990s; the increase (decrease) in frequency of anticyclonic (cyclonic) types in winter from the late 1960s to
the early 1990s, with a subsequent decline (rise); and the sharp increase in the persistence (measured by the mean residence
time) of all groups of circulation types in winter around 1990 and of anticyclonic types in summer during the 1990s. Differences
between the findings obtained using the objective and subjective methods may result from the intrinsically different approach
to the classification (e.g. the Hess-Brezowsky weather types have a typical duration of at least 3 days while objective types
typically last 1–3 days). Generally, changes in atmospheric circulation which have taken place since the 1960s were more pronounced
in winter than in summer. The most conspicuous change seems to be the considerable increase in the persistence of circulation
types during the 1990s, which may be also reflected in the increase in the occurrence of climatic extremes observed in Europe
during recent years. 相似文献
2.
Dongliang Yuan 《Theoretical and Applied Climatology》2009,96(1-2):3-15
This study is motivated by an interest in obtaining a new automated classification scheme of daily circulation types suitable for use throughout Europe. The classification scheme is performed on 500 hPa geopotential height anomalies (NCEP Reanalysis data, 2.5°×2.5°). Nine grid points represent the study area. Five anticyclonic types (Anw, Ane, A, Asw and Ase) and seven cyclonic types (C, Cnnw, Cwnw, Cwsw, Cssw, Cse, Cne) are defined. Each of the circulation types has a distinct underlying synoptic pattern that produces the expected type and direction of flow over the study area. The classification scheme is applied to three different case studies in the Mediterranean Basin: Greece, Cyprus and central Italy. The precipitation percentage of the cyclonic type and the mean seasonal correlation coefficients for all circulation types are the two criteria used to evaluate the performance of the classification scheme. The ability of the HadAM3P general circulation model to reproduce the mean pattern and frequency of circulation types at the 500 hPa level in comparison to the NCEP dataset for the period 1960–1990 is also evaluated. The percentage of rainfall that corresponds to the cyclonic circulation types is greater than 85% for the three study regions. Furthermore, the correlation coefficients for the three classifications are very encouraging, for nearly all days of the study period. Compared to observations, the GCM is able to capture the mean patterns but not able to replicate exactly the observed variability of the circulation types over the three study regions. 相似文献
3.
Summary Circulation types were identified by means of zonal and meridional indices calculated separately over ten different regions of 20° × 20° over the Mediterranean and Europe. Seasonal and annual rainfall totals in four stations Lisbon, Luqa (Malta), Athens and Jerusalem, were compared with circulation types for the period 1873–1991. Correlation coefficients of circulation indices with precipitation, for each station in each season were calculated and mapped.An oscillation in the meridional index during the winter and the spring, between the western and eastern Mediterranean, was detected.Time series analysis of the circulation indices demonstrates a significant reduction in zonality and an increase in meridionality mainly in spring and in summer, over most of the study area.With 9 Figures 相似文献
4.
Summary Monthly rainfall conditions in Israel were determined, using data from 12 stations, during 30 years (1961–90). The definition
of a month to be dry, normal or wet, was done using standardized rainfall totals. Pressure departures for each of the three rainfall categories for each month
of the rainy season, were calculated and mapped. Correlation between rainfall totals at each of the 12 stations and monthly
mean sea level pressure at 72 grid points in the area delimited by the 20° W and 50° E meridians and the 20° N and 60° N parallels,
was performed. For each month, 12 correlation maps were prepared (one of each station). Similar maps were averaged together
to form coherent rainfall regions. At the beginning of the rainy season (October) the rainfall in Israel is sporadic and spotty
without a distinctable coherent region. At the end of the rainy season (April) the rainfall is more widespread, forming a
large coherent region covering most of the country.
Dry rainfall conditions in Israel, were found to be characterized by positive pressure departures in the eastern Mediterranean
and over Israel and/or by easterly or southerly circulation over the eastern Mediterranean. Wet rainfall conditions in Israel,
were found to be characterized by negative pressure departures in the eastern Mediterranean and over Israel and/or by westerly
or northerly circulation over the eastern Mediterranean. Moreover, in many cases dry conditions in Israel, were associated
with below normal pressure conditions over central or western Europe, while wet conditions in Israel, with above normal conditions
over the same region, thus, reflecting the so-called Mediterranean Oscillation. Finally, normal rainfall conditions are characterized by very slight to negligible pressure departures over the entire Mediterranean
and Europe.
Received November 18, 1997 Revised March 3, 1998 相似文献
5.
The spatio-temporal variability in summer rainfall within eastern China is identified based on empirical orthogonal function
(EOF) analysis of daily rain-gauge precipitation data for the period 1979–2003. Spatial coherence of rainfall is found in
the Yangtze Basin, and a wavelet transform is applied to the corresponding principal component to capture the intraseasonal
oscillation (ISO) of Yangtze rainfall. The ensemble mean wavelet spectrum, representing statistically significant intraseasonal
variability, shows a predominant oscillation in summer Yangtze rainfall with a period of 20–50 days; a 10–20-day oscillation
is pronounced during June and July. This finding suggests that the 20–50-day oscillation is a major agent in regulating summer
Yangtze rainfall. Composite analyses reveal that the 20–50-day oscillation of summer Yangtze rainfall arises in response to
intraseasonal variations in the western North Pacific subtropical high (WNPSH), which in turn is modulated by a Rossby wave-like
coupled circulation–convection system that propagates northward and northwestward from the equatorial western Pacific. When
an anomalous cyclone associated with this Rossby wave-like system reaches the South China Sea (SCS) and Philippine Sea, the
WNPSH retreats northeastward due to a reduction in local pressure. Under these conditions, strong monsoonal southwesterlies
blow mainly toward the SCS–Philippine Sea, while dry conditions form in the Yangtze Basin, with a pronounced divergent flow
pattern. In contrast, the movement of an anomalous anticyclone over the SCS–Philippine Sea results in the southwestward extension
of the WNPSH; consequently, the tropical monsoonal southwesterlies veer to the northeast over the SCS and then converge toward
the Yangtze Basin, producing wet conditions. Therefore, the 20–50-day oscillation of Yangtze rainfall is also manifest as
a seesaw pattern in convective anomalies between the Yangtze Basin and the SCS–Philippine Sea. A considerable zonal shift
in the WNPSH is associated with extreme dry (wet) episodes in the Yangtze Basin, with an abrupt eastward (westward) shift
in the WNPSH generally leading the extreme negative (positive) Yangtze rainfall anomaly by a 3/8-period of the 20–50-day oscillation.
This finding may have implications for improving extended-range weather forecasting in the Yangtze Basin. 相似文献
6.
AMIP model simulations of the east China (5–50°N; 105–122°E) monsoon system are analyzed to study coherent relationships
between rainfall and wind annual cycle biases. A comparison with observed interannual variability patterns is carried out
to identify the physical processes that explain the biases. The analyses show that poleward displacement of the simulated
east Asian jet stream causes the ascending branch of the jet-induced transverse circulation to move north and, as a consequence,
produces negative (positive) rainfall biases occur in central (northeast) China. The model simulations show decreased southwesterly
flow and ITCZ rainfall over the South China Sea when weaker (versus observations) summer Hadley and Walker circulations are
present. This results from diminished model tropical disturbance activity, and highlights the importance of air-sea interactions.
In addition, during October–January, intensified model low-level easterlies enhance moisture transport and produce positive
local rainfall biases over central and northeast China. Biases in the east China monsoon system are concurrently reflected
in the planetary circulation. Enhanced northeast China rainfall results from increased surface pressure over the North Pacific
and an amplified zonal pressure gradient along the east China coast. This bias pattern is associated with differences in model
representations of topography. On the other hand, the South China Sea experiences an extensive elongated meridional rainfall
bias dipole structure that straddles the equator. This is accompanied by a baroclinic vertical pattern over the tropics as
well as a barotropic wave train that extends from Australia to the Antarctic, where the teleconnection is likely a direct
atmospheric response to tropical convective heating.
Received: 20 June 2000 / Accepted: 17 September 2000 相似文献
7.
Anatomy of the synoptic conditions occurring over southern Greece during the second half of the 20th century. Part I. Winter and summer 总被引:1,自引:0,他引:1
P. A. Kassomenos 《Theoretical and Applied Climatology》2003,75(1-2):65-77
Summary ?In this study a methodology for grouping seasonal circulation types occurring over an area is introduced. This procedure
combines the surface air mass characteristics affecting the area with the synoptic conditions prevailing over it. Factor Analysis
and Cluster Analysis are used to derive the circulation types, based on surface meteorological data and surface pressure grid
data. The methods are applied to Athens, Greece, using data over the period 1954–1999 for winter (December, January, February)
and summer (June, July, August) seasons. The daily circulation types are analyzed at surface level and their temporal evolution
is examined via transition matrices. 315 grid points are used covering the area between 25° N to 60° N and 10° W to 40° E.
This analysis derives 8 circulation types for the winter and 4 for the summer.
A reduction in cyclonic activity and an increase in anticyclonic activity in the Central Mediterranean are detected in the
late 1980s and early 1990s during the winter period. During summer the etesian winds and the local flows are dominant over
Athens.
Received February 20, 2002; accepted January 9, 2003
Published online May 26, 2003 相似文献
8.
G. Spellman 《Theoretical and Applied Climatology》2000,66(3-4):229-239
Summary A large number of atmospheric circulation classification techniques have been developed in the investigation of synoptic
controls on regional rainfall. Often the rationale is to aid efforts to downscale GCM output for the purpose of producing
more confident climate change impact scenarios. Discrete weather typing techniques, although proven to be successful do not
capture weather type intensity and within-type variability can often be high. In this study an objective indexing method,
developed for Egypt and the British Isles area is applied to the Iberian peninsula. Air flow index values are then used as
predictor variables in simple linear regression models to estimate monthly mean grid point rainfall amounts. Separate models
are evaluated for the winter and summer halves of the year and also for surface and mid-tropospheric flow (500 hPa). The models
are evaluated and compared indicating that the index values provide good estimation of rainfall but variability in performance
between season and site is noted.
Received February 10, 2000 相似文献
9.
夏季长江淮河流域异常降水事件环流差异及机理研究 总被引:10,自引:5,他引:5
长江、淮河同处东亚中纬度,天气过程的大尺度环流背景相似,大量相关研究基本是把江淮流域天气气候事件作为一个整体研究,然而对长江、淮河流域夏季降水的时空变化进行分析发现,长江、淮河流域夏季异常降水事件有各自不同的年际、年代际变化特征,但环流差异及成因并不十分清楚。本文根据中国台站降水资料及NCEP/NCAR再分析资料,利用物理量诊断和现代统计学等方法,重点分析长江、淮河流域梅雨期降水异常事件发生时南北半球大气环流内部动力过程的差异及成因。研究指出:长江(淮河)流域梅雨期降水异常偏多年500 hPa位势高度场亚洲中高纬度环流呈现为南北向(东西向)的波列与东亚中高纬鄂霍茨克海阻塞频次增多(减少)以及200 hPa高度场上东亚副热带高空西风急流强度加强(减弱)、稳定(移动)有关;长江(淮河)流域梅雨期降水异常偏多年主要水汽来源与南半球澳大利亚高压、马斯克林高压位置偏东(西)造成西太平洋150°E~180°(阿拉伯海50°E~60°E)地区越赤道气流加强有关。长江(淮河)流域梅雨期异常降水事件大气环流内部动力过程最显著的差异表现为:东亚副热带高空西风急流加强(减弱)以及南半球澳大利亚高压、马斯克林高压位置偏东(西)。 相似文献
10.
A weather-type catalogue based on the Jenkinson and Collison method was developed for an area in south-west Russia for the period 1961–2010. Gridded sea level pressure data was obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. The resulting catalogue was analysed for frequency of individual types and groups of weather types to characterise long-term atmospheric circulation in this region. Overall, the most frequent type is anticyclonic (A) (23.3 %) followed by cyclonic (C) (11.9 %); however, there are some key seasonal patterns with westerly circulation being significantly more common in winter than summer. The utility of this synoptic classification is evaluated by modelling daily rainfall amounts. A low level of error is found using a simple model based on the prevailing weather type. Finally, characteristics of the circulation classification are compared to those for the original JC British Isles catalogue and a much more equal distribution of flow types is seen in the former classification. 相似文献
11.
Summary An attempt has been made to relate a large-scale airflow classification to the local weather at two stations in Estonia.
To describe the features of the general circulation over Europe, the large-scale circulation patterns (Grosswetterlagen) of the German Weather Service have been chosen. Daily precipitation and temperature deviations from the monthly mean in
P?rnu and Tartu for 1961–1993 have been used. It can be said that the weather in Estonia shows a good relationship with the
general circulation types (Zirkulationsformen) that are defined by means of the Grosswetterlagen. Zonal circulation brings to Estonia wet weather that in winter is warmer than average and in summer cooler than average.
Mixed circulation is associated with warmer winters and average summers. Meridional circulation brings cold winters and variable
weather in summer. To explain the large dispersion of values of meteorological elements within any one circulation type, a
more detailed analysis of the relationships between circulation patterns and meteorological situation in Estonia is needed.
Received June 17, 1998 Revised January 13, 1999 相似文献
12.
全球变暖背景下亚非典型干旱区降水变化及其与水汽输送的关系研究 总被引:2,自引:0,他引:2
分析比较了中蒙(35°N~50°N,75°E~105°E)、中亚(28°N~50°N,50°E~67°E)和北非(15°N~32°N,17°W~32°E)三个典型干旱区水汽输送特征的异同,及其1961~2010年间的降水时空变化,分析了水汽来源和输送变化及其可能原因。结果显示,由于受不同的气候系统影响,中蒙、北非和中亚干旱区的降水在年内变化上有着显著不同。中蒙和北非干旱区降水呈现夏季风降水的特征;而中亚干旱区降水则为更多受到冬季风的影响。1961~2010年,随着全球气温上升,中蒙干旱区冬季纬向水汽输送增加而经向输送减少,总水汽输送增加;中亚干旱区冬季纬向输送减少而经向增加,总水汽输送减少;北非干旱区冬季纬向输送增加而经向输送减少,总水汽输送增加。夏季中蒙和北非干旱区经向、纬向输送均减小,中亚干旱区夏季纬向输送减少而经向减少,总输送增加。相应的,中蒙干旱区年、冬季和夏季降水分别以4.2、1.3和1.0 mm/10 a的趋势增加;而中亚干旱区冬季(1.2 mm/10 a)和夏季(0.1 mm/10 a)降水增加,年降水则呈减少趋势(-0.8 mm/10 a);北非干旱区年降水和夏季降水分别以0.5 mm/10 a和0.1 mm/10 a的速率增加。冬季中蒙干旱区主要水汽来源是水汽经向输送,而中亚干旱区水汽主要为纬向输送,经纬向水汽均为净输出是北非干旱区降水极少的主要原因,平均总水汽输送量约为-9.48×104 kg/s。冬季低纬度和高纬度环流通过定常波影响干旱区冬季降水。中蒙和中亚干旱区冬季降水主要受西太平洋到印度洋由南向北的波列影响,北非干旱区冬季降水主要和北大西洋上空由北到南的波列相联系。各干旱区的降水对海温变化有着不同的响应:中蒙干旱区冬季降水与冬季太平洋西海岸和印度洋海温呈显著正相关,夏季与海温相关不显著;中亚干旱区与地中海和阿拉伯海温相关,且与阿拉伯海温为正相关。 相似文献
13.
Winter weather regimes over the Mediterranean region: their role for the regional climate and projected changes in the twenty-first century 总被引:2,自引:0,他引:2
M. Rojas L. Z. Li M. Kanakidou N. Hatzianastassiou G. Seze H. Le Treut 《Climate Dynamics》2013,41(3-4):551-571
The winter time weather variability over the Mediterranean is studied in relation to the prevailing weather regimes (WRs) over the region. Using daily geopotential heights at 700 hPa from the ECMWF ERA40 Reanalysis Project and Cluster Analysis, four WRs are identified, in increasing order of frequency of occurrence, as cyclonic (22.0 %), zonal (24.8 %), meridional (25.2 %) and anticyclonic (28.0 %). The surface climate, cloud distribution and radiation patterns associated with these winter WRs are deduced from satellite (ISCCP) and other observational (E-OBS, ERA40) datasets. The LMDz atmosphere–ocean regional climate model is able to simulate successfully the same four Mediterranean weather regimes and reproduce the associated surface and atmospheric conditions for the present climate (1961–1990). Both observational- and LMDz-based computations show that the four Mediterranean weather regimes control the region’s weather and climate conditions during winter, exhibiting significant differences between them as for temperature, precipitation, cloudiness and radiation distributions within the region. Projections (2021–2050) of the winter Mediterranean weather and climate are obtained using the LMDz model and analysed in relation to the simulated changes in the four WRs. According to the SRES A1B emission scenario, a significant warming (between 2 and 4 °C) is projected to occur in the region, along with a precipitation decrease by 10–20 % in southern Europe, Mediterranean Sea and North Africa, against a 10 % precipitation increase in northern European areas. The projected changes in temperature and precipitation in the Mediterranean are explained by the model-predicted changes in the frequency of occurrence as well as in the intra-seasonal variability of the regional weather regimes. The anticyclonic configuration is projected to become more recurrent, contributing to the decreased precipitation over most of the basin, while the cyclonic and zonal ones become more sporadic, resulting in more days with below normal precipitation over most of the basin, and on the eastern part of the region, respectively. The changes in frequency and intra-seasonal variability highlights the usefulness of dynamics versus statistical downscaling techniques for climate change studies. 相似文献
14.
Atmospheric circulation patterns in southern Chile (42° 30′ S) were studied in order to determine and analyse the most characteristic synoptic types and their recent trends, as well as to gain an understanding of how they are associated with low-frequency variability patterns. According to the Jenkinson and Collison (J&C) classification method, a 16-point grid of sea-level pressure data was employed. The findings reveal that some synoptic types show statistically significant trends with a 95% confidence level, positively for anticyclonic westerly hybrids (AW) and advective types for third and fourth quadrant wind flows (W, NW, and N) and negatively for SW and cyclonic hybrids (CS and CSW). A model has been constructed of the linear regression of some weather types with teleconnections that most affect Chile: the undetermined types (U), AW were associated with El Niño or the warm phase of the Pacific Decadal Oscillation (PDO), whereas the cyclonic northerly and cyclonic northeasterly types (CN and CNE) were associated with La Niña or cool phase of the PDO. The weather types associated with Antarctic Oscillation (AAO) in its positive phase are anticyclonic northerly and northeasterly and northerly advection types, while in its negative phase are cyclonic southwesterly and advection types. 相似文献
15.
Regional anomalies of the surface climate over Europe are defined by a simultanous EOF-analysis of the normalized monthly mean sea level pressure, temperature and precipitation fields of 100 winters (December–February, 1887–1986) at 40 stations. The monthly amplitudes of the first EOF (about 25% of the total variance) are used as an index for the monthly winter climate anomaly. They characterize a high (low) pressure cell over central Europe associated with a positive (negative) temperature and precipitation anomaly over northern (central-southern) Europe as indicated by a northward (southward) shift of the tail end of the cross-Atlantic cyclone track. These patterns resemble the phenomenological anticyclonic (cyclonic) Grosswetter classification and the European blocking (enhanced zonal flow) regime. The second EOF is of similar magnitude and gives latitudinal corrections to these two basic flow regimes. The joint probability distribution of both amplitudes shows a weak bimodality mainly associated with the first EOF. Further insight into the underlying physical processes of the climate anomaly patterns in Europe is obtained from the extended Eliassen-Palm flux diagnostics of the barotropic transient eddy-mean flow interaction (Hoskins et al. 1983) and the stationary wave propagation (Plumb 1985). The diagnostics confined to the barotropic components and applied to the regression and the composite anomaly fields of the transient and stationary eddy flows of the 500 hPa geopotential (1946–87, north of 20°N) leads to the following results: (1) The bandpass filtered transient eddy variances of the 500 hPa geopotential show a shift of the cross-Atlantic storm track: In high (low) pressure situations over Europe the cross-Atlantic storm track intensity is enhanced (reduced) and its tail end is shifted northward (remains zonal); the North Pacific storm track extends further (less) eastward and thus closer to the west coast of North America. (2) The extreme high pressure system over Europe tends to be supported by an anomalous transient eddy forcing of the mean flow stream-function: it enhances the zonal wind to its north and generates anticyclonic vorticity about 10° upstream from its center. In the low pressure composite the anomalous cyclonic vorticity is generated reducing the zonal flow to its north. (3) The occurrence (lack) of a strong eastward stationary wave activity flux over the Atlantic is associated with the high (low) pressure situations over Europe. Finally, a positive feedback is conjectured between the stationary wavetrain modifying the tail end of the cross-Atlantic storm track and the transient eddies intensifying this anomaly. 相似文献
16.
Variations in the Temperature Regime Across the Mediterranean During the Last Century and their Relationship with Circulation Indices 总被引:2,自引:2,他引:0
Summary Circulation types were identified by means of zonal and meridional indices calculated separately over ten different regions
of 20°×20° over the Mediterranean and Europe. Seasonal temperature trends in 22 grid boxes of 5°×5° covering the entire Mediterranean,
and at six stations Lisbon, Madrid, Florence, Luqa (Malta), Athens and Jerusalem, were calculated.
A warming trend in the period 1873–1989 was detected. The warming is more evident in the western Mediterranean with an average
rate of about 0.4 [°C/100 yr], than in the eastern Mediterranean with an increase of only 0.2 [°C/100 yr]. A cooling trend
in autumn in the eastern Mediterranean with an average rate of −0.5 [°C/100 yr] was detected and attributed to an increase
in northerly meridional circulation in that region.
Warming trends at Lisbon, Madrid, Florence, Athens and Jerusalem, were more important than the trends in the grid boxes containing
these stations. This rapid warming was attributed to urban effects. No such effects were found in Luqa due to its location
and the lack of urban effects there.
Temperatures at Luqa, Athens and Jerusalem are highly positively correlated. Likewise, temperatures at Lisbon and Madrid.
Temperatures at Florence are either correlated with Madrid or with Luqa. Negative or no correlations were found between Lisbon
or Madrid with Athens or Jerusalem, except during the winter. This was attributed to the fact that favourable circulation
for high temperatures in the eastern stations was opposite to the favourable circulation for high temperatures in the western
stations and vice versa.
Finally, the above reinforces the concept of a Mediterranean Oscillation between the western and eastern basins.
Received November 14, 1997 Revised June 2, 1998 相似文献
17.
Atmospheric circulation anomalies over the Ural Mountains are crucial indicators of the anomalous downstream weather and climate over East Asia. Here, we provide a new perspective on the mechanism of Ural circulation anomalies. We use a simple theoretical model to determine that the relationship between the solar forcing and three Ural circulation patterns, namely, neutral type, trough anomaly and ridge anomaly, is a nonlinear relationship following the supercritical pitchfork bifurcation theory. The theory predicts that when the total solar irradiance (TSI) is below a critical value, trough and ridge anomalies represent duplex equilibria and are equally likely to occur at the same TSI. Based on 180 winter months record, we have estimated the bidimensional probability density of TSI and the monthly mean geopotential height at 500 hPa or zonal wind at 850 hPa over the Ural Mountains. Results show that Sc = 1360.9 W m−2 is a critical value of TSI, the neutral type pattern is the single circulation regime when TSI > Sc, whereas trough and ridge anomaly patterns are duplex circulation regimes when TSI < Sc. Besides, when TSI < Sc, during the same TSI range, trough and ridge anomaly events occur at nearly the same frequencies. These results generally agree with the theoretical model. We demonstrate that trough and ridge anomalies, as duplex equilibria, result from the large-scale zonal flow interacting with the Ural Mountains. Low TSI tends to strengthen the large-scale zonal flow over the Ural Mountains, hence inducing either a trough anomaly or ridge anomaly. 相似文献
18.
Detailed spatiotemporal structures for the submonthly-scale (7–25 days) intraseasonal oscillation (ISO) in summer monsoon rainfall and atmospheric circulation were investigated in South Asia using high-quality rainfall and reanalysis datasets. The Meghalaya–Bangladesh–coast of the western Myanmar (MBWM) region is the predominant area of submonthly-scale ISO in the Asian monsoon regions. The distinct rainfall ISO is caused by a remarkable alternation of low-level zonal wind between westerly and easterly flows around the Gangetic Plain on the same timescales. In the active ISO phase of the MBWM, a strong low-level westerly/southwesterly flows around the plain and a center of cyclonic vorticity appears over Bangladesh. Hence, a local southerly flows toward the Meghalaya Plateau and there is strong southwesterly flow towards the coast along southeastern Bangladesh and western Myanmar, resulting in an increase in orographic rainfall. Rainfall also increases over the lowland area of the MBWM due to the low-level convergence in the boundary layer under the strong cyclonic circulation. The submonthly-scale low-level wind fluctuation around the MBWM is caused by a westward moving n = 1 equatorial Rossby (ER) wave. When the anticyclonic (cyclonic) anomaly related to the ER wave approaches the Bay of Bengal from the western Pacific, humid westerly/southwesterly (easterly/southeasterly) flows enhance around the Gangetic Plain on the northern fringe of the anticyclone (cyclone) and in turn promote (reduce) rainfall in the MBWM. Simultaneously, robust circulation signals are observed over the mid-latitudes. In the active phase, cyclonic anomalies appear over and around the TP, having barotropic vertical structure and also contributing to the enhancement of low-level westerly flow around the Gangetic Plain. In the upper troposphere, an anticyclonic anomaly is also observed upstream of the cyclonic anomaly over the TP, having wavetrain structure. The mid-latitude circulation around the TP likely helps to induce the distinct ISO there in conjunction with the equatorial waves. Thus, the distinct ISO in the MBWM is strongly enhanced locally (~500 km) by the terrain features, although the atmospheric circulation causing the ISO has a horizontal scale of ~6,000 km or more, extending across the whole Asian monsoon system from the tropics to mid-latitudes. 相似文献
19.
A multi-model ensemble approach for assessment of climate change impact on surface winds in France 总被引:2,自引:1,他引:1
Statistical downscaling of 14 coupled atmosphere-ocean general circulation models (AOGCM) is presented to assess potential
changes of the 10 m wind speeds in France. First, a statistical downscaling method is introduced to estimate daily mean 10 m
wind speed at specific sites using general circulation model output. Daily 850 hPa wind field has been selected as the large
scale circulation predictor. The method is based on a classification of the daily wind fields into a few synoptic weather
types and multiple linear regressions. Years are divided into an extended winter season from October to March and an extended
summer season from April to September, and the procedure is conducted separately for each season. ERA40 reanalysis and observed
station data have been used to build and validate the downscaling algorithm over France for the period 1974–2002. The method
is then applied to 14 AOGCMs of the coupled model intercomparison project phase 3 (CMIP3) multi-model dataset. Three time
periods are focused on: a historical period (1971–2000) from the climate of the twentieth century experiment and two climate
projection periods (2046–2065 and 2081–2100) from the IPCC SRES A1B experiment. Evolution of the 10 m wind speed in France
and associated uncertainties are discussed. Significant changes are depicted, in particular a decrease of the wind speed in
the Mediterranean area. Sources of those changes are investigated by quantifying the effects of changes in the weather type
occurrences, and modifications of the distribution of the days within the weather types. 相似文献
20.
本文利用常规气象资料、NCEP (1°×1°)再分析资料和环监站实时监测数据,分析了2017年12月四川盆地南部一次持续性雾霾的气象特征和增强机制。结果表明:(1)中高纬平直纬向环流、低层弱偏南偏东气流、地面均压场组成的静稳天气长时间维持为雾霾天气的形成和维持提供了有利的环流背景条件。(2)上干下湿、逆温明显的大气稳定层结,连续无降水、近地层高湿和较弱的风场提供有利的气象要素条件。(3)低层弱冷平流、上层暖平流促进稳定层结长时间维持和近地层水汽凝结作用加强,为雾霾天气的增强和持续起到了重要作用。(4)冷空气、降水,水平和垂直扩散能力增强是空气污染物浓度降低的重要条件。 相似文献