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1.
Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses 总被引:1,自引:0,他引:1
NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa
and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional
wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly
wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the
African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude
17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from
about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical
Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day
waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics.
The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength
is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar
to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large
modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The
zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation
north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly
waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective
bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are
associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves
are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day
wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly
wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained
by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the
jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component.
The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over
West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us
to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and
ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR
reanalyses, especially for the 3–5-day regime over the Atlantic.
Received: 28 May 1998 / Accepted: 2 May 1999 相似文献
2.
E. A. Afiesimama 《Theoretical and Applied Climatology》2007,90(1-2):103-111
Summary The paper examines the annual cycle of the mid-tropospheric easterly jet (MTJ) over West Africa against the background of
many reviews indicating different locations and characteristics of the jet and considering it as a summer feature. NCEP–NCAR
reanalysis zonal wind datasets for the period 1971–2000 and upper air datasets over the region are used. The results exhibit
realistic spatial structure of the easterly jet. The long-term mean of the datasets suggests that the jet over West Africa
is not only a summer feature but can also be found in winter with the same order of magnitude in the wind velocity at the
core. The jet axis is located at about lat. 2° N close to the Guinean Coast in winter and at lat. 14° N in summer. The meridional
oscillation of the jet suggests that as it advances northward, it maintains an altitude of 700 hPa in winter and transits
in mid-spring to 650 hPa and reaches 600 hPa in summer. In the retreat, it displaces to 650 hPa at the end of September rather
sharply to reach 700 hPa in October. The jet’s core has been observed to have a northeast–southwest orientation from season
to season, covering a longitude of 29° from its southernmost to the northernmost positions. 相似文献
3.
The evolution and structure of a tropical island sea/land-breeze system, northern Australia 总被引:1,自引:0,他引:1
Summary
This paper is concerned with sea/land-breeze systems over relatively flat tropical islands to the north of continental Australia.
The purpose of this study is to contribute to the relatively small body of knowledge on tropical island sea/land-breeze systems
in this region and to highlight their particular characteristics. The evolution and structure of coastal circulations over
the Tiwi Islands, northern Australia are examined using observations made during the Maritime Continent Thunderstorm Experiment
(MCTEX), November/December 1995. During the transition period between dry and wet (monsoon) seasons, strong diurnal surface
heating dominates the local meteorology. Thermally modified pressure differences across the coastline are seen to control
the timing, direction and intensity of local winds. The evolution and structure of the resulting circulations appear to be
affected greatest by tropospheric stability and friction, while the Coriolis force, synoptic winds and topography are of much
less importance in this case. Consequently, even small differences in surface properties seem to produce strong and well defined
local wind circulations. The depth of the sea breeze averaged 1200 m, while the land breeze was considerably shallower (290 m).
Return flows were evident in both circulations, although better defined in land breeze cases. Day to day variation in vertical
structure was considerable and appeared to be controlled by stability in the lower troposphere. Spatial patterns of surface
temperature, pressure and wind show formation of an island heat low by day and a cool high pressure centre at night, resulting
in island scale convergence and divergence, respectively.
Received February 27, 2000/Revised October 16, 2000 相似文献
4.
An investigation into high Reynolds number turbulent flow over a ridge top in New Zealand is described based on high-resolution
in-situ measurements, using ultrasonic anemometers for two separate locations on the same ridge with differing upwind terrain
complexity. Twelve 5-h periods during neutrally stratified and weakly stable atmospheric conditions with strong wind speeds
were sampled at 20 Hz. Large (and small) turbulent length scales were recorded for both vertical and longitudinal velocity
components in the range of 7–23 m (0.7–3.3 m) for the vertical direction and 628–1111 m (10.5–14.5 m) for the longitudinal
direction. Large-scale eddy sizes scaled to the WRF (Weather Research and Forecasting) numerical model simulated boundary-layer
thickness for both sites, while small-scale turbulent features were a function of the complexity of the upwind terrain. Evidence
of a multi-scale turbulent structure was obtained at the more complex terrain site, while an assessment of the three-dimensional
isotropy assumption in the inertial subrange of the spectrum showed anisotropic turbulence at the less complex site and evidence
of isotropic turbulence at the more complex site, with a spectral ratio convergence deviating from the 4/3 or unity values
suggested by previous theory and practice. Existing neutral spectral models can represent locations along the ridge top with
simple upwind complexity, especially for the vertical wind spectra, but sites with more orographic complexity and strong vertical
wind speeds are often poorly represented using these models. Measured spectra for the two sites exhibited no significant diurnal
variation and very similar large-scale and small-scale turbulent length scales for each site, but the turbulence energy measured
by the variances revealed a strong diurnal difference. 相似文献
5.
Mesoscale aspects of the Urban Heat Island around New York City 总被引:15,自引:1,他引:15
S. D. Gedzelman S. Austin R. Cermak N. Stefano S. Partridge S. Quesenberry D. A. Robinson 《Theoretical and Applied Climatology》2003,75(1-2):29-42
Summary ?A mesoscale analysis of the Urban Heat Island (UHI) of New York City (NYC) is performed using a mesoscale network of weather
stations. In all seasons the UHI switches on rapidly in late afternoon and shuts down even more rapidly shortly after dawn.
It averages about 4 °C in summer and autumn and 3 °C in winter and spring. It is largest on nights with clear skies, low relative
humidity through much of the troposphere, and weak northwest winds, when it may exceed 8 °C. The synoptic meteorological situation
associated with the largest UHI occurs roughly two to three nights after cold front passages.
During spring and summer, sea breezes commonly reduce and delay the UHI and displace it about 10 km to the west. Backdoor
cold fronts, which occur most frequently in spring and early summer, reduce or even reverse the UHI, as cold air from the
water to the northeast keeps NYC colder than the western suburbs. Cases documenting the sensitivity and rapidity of changes
of the UHI to changes in parameters such as cloud cover, ceiling, and wind speed and direction are presented.
Received August 16, 2001; revised October 6, 2002; accepted November 20, 2002
Published online March 17, 2003 相似文献
6.
T. Klein G. Heinemann D. H. Bromwich J. J. Cassano K. M. Hines 《Meteorology and Atmospheric Physics》2001,78(1-2):115-132
Summary
Simulations of the katabatic wind system over the Greenland ice sheet for the two months April and May 1997 were performed
using the Norwegian Limited Area Model (NORLAM) with a horizontal resolution of 25 km. The model results are intercompared
and validated against observational data from automatic weather stations (AWS), global atmospheric analyses and instrumented
aircraft observations of individual cases during that period. The NORLAM is able to simulate the synoptic developments and
daily cycle of the katabatic wind system realistically. For most of the cases covered by aircraft observations, the model
results agree very well with the measured developments and structures of the katabatic wind system in the lowest 400 m. Despite
NORLAM’s general ability of reproducing the four-dimensional structure of the katabatic wind, problems occur in cases, when
the synoptic background is not well captured by the analyses used as initial and boundary conditions for the model runs or
where NORLAM fails to correctly predict the synoptic development. The katabatic wind intensity in the stable boundary layer
is underestimated by the model in cases when the simulated synoptic forcing is too weak. An additional problem becomes obvious
in cases when the model simulates clouds in contrast to the observations or when the simulated clouds are too thick compared
to the observed cloud cover. In these cases, the excessive cloud amount prevents development of the katabatic wind in the
model.
Received September 22, 2000/Revised March 16, 2001 相似文献
7.
Summary The University of New South Wales (UNSW) High Resolution numerical weather prediction model (HIRES) is run routinely, on
a daily basis, at a horizontal resolution of 25 km. The output is made available to the New South Wales (NSW) regional office
of the Australian Bureau of Meteorology in Sydney. HIRES has been used to forecast mean wind direction and speed at a height
of 12 metres for a number of events in the past. The opportunity was used in December 1997 to run the model for the annual
Sydney to Hobart yacht race area.
For the 1997 Sydney to Hobart yacht race the model was run at 25 km horizontal resolution and the output was made available
to all competitors on the morning of the race, namely December 26th. It was also decided by the authors to expand the verification
to include all observations available both from land and sea within the model domain an well as those available from a moving
single point at sea, namely a yacht.
After the event, the model was run once at the increased resolution of 10 km, out to 5 days ahead. Both model runs were subjected
to detailed verification by one of the authors (KLB) who participated in the race aboard the maxi-yacht Nicorette and who carried out a pre-arranged observational program during the race. Surface synoptic weather maps prepared in the NSW
office of the Bureau of Meteorology were also consulted in order to extend the verification scheme. The model predicted winds
were verified on a six-hourly basis utilising instrumentation on the yacht as well as surface observations plotted in standard
World Meterological Organisation (WMO) format on surface synoptic weather maps. The yacht carried wind sensors situated on
top of the mast at a height of 30 metres above the water. The authors were most interested in the accuracy of the wind velocity
forecast by the model.
It is important to note that forecasting for points over the ocean at widely separated time intervals represents a very difficult
challenge. The verifications reveal that the model gave overall forecast guidance of very good to excellent quality and was
particularly accurate early in the race, when a Southerly Buster event occurred during the evening of the first day. However,
a caveat should be added that this forecast, despite its remarkable accuracy, is not a claim that accurate very high resolution
regional prediction is a solved problem. Rather, it is a demonstration that in particular instances current models are now
capable of achieving high levels of skill a number of days ahead.
Received September 28, 1999 Revised November 25, 1999 相似文献
8.
Summary Dubbed Ice Storm ’98, an extreme weather event characterized by two synoptic systems in succession dropped about 70–100 mm
(in terms of water equivalent) of freezing precipitation over southeastern Ontario, southwestern Quebec and northeastern New
York during a 6-day period from January 5 to 10 in 1998. Individually, the two synoptic systems were not dramatically more
extreme in freezing precipitation than other major freezing rain events (4 since 1961) which occurred in the past over the
affected area. Some regions in the target area, however, were impacted more by the second system. Based on an analysis of
the 500 hPa vorticity field during the ’98 event, we suggest that the 1997/98 El Ni?o had a role in creating a flow environment
conducive to the rapid formation of the second synoptic system. In contrast, other major freezing rain events in the last
30 years involved only one synoptic system per event lasting no more than 3 days, and producing 20–50 mm of precipitation.
We have also found that, 3 out of 4 past major freezing rain events since 1958 were associated with the positive phase of
the North Atlantic Oscillation (NAO). Consistent with this usual past association between the NAO and a major freezing rain
event, Ice Storm ’98 also occurred when the phase of the NAO was positive. Analysis of these 3 past and the ’98 events also
indicates an apparent connection between the positive phase of the NAO and the northern Quebec high pressure system, which
is an essential synoptic feature of a major freezing rain occurrence over the southcentral region of Canada. As measured by
their respective indices, the maximum positive NAO state leads the maximum northern Quebec high by about 2 days (5 days in
the ’98 event). There is some suggestive evidence to indicate that the persistence of the northern Quebec high pressure system
is connected to the persistence of the positive phase of the NAO.
Received January 17, 2000 相似文献
9.
Summary The Australian east coast is subject to a range of synoptic/mesoscale weather systems ranging from low-pressure troughs, small
explosive low-pressure systems near the Australian east coast, to synoptic scale maritime low-pressure systems, and both tropical
and extra-tropical cyclones. These systems vary dynamically in intensity and structure across a spectrum that includes both
weak and intense cold-cored systems, to warm-cored tropical cyclones and “hybrid” systems. A preliminary 10 year climatology,
from 1992 to 2001, is presented that concentrates on those cyclones in open waters that threaten life and property as a result
of heavy coastal rain or the combined effects of wind, sea state and swell, including both long fetch and storm surges. There
are 28 systems in the 10-year climatology. In terms of their development, the most numerous were the 50% of storms that occurred
as systems within an easterly trough, followed by 21% that occurred as circulations on decaying Tasman Sea cold fronts.
In addition, a numerical weather prediction (NWP) modelling case study of the ‘hybrid’ sub-tropical cyclone system of March
2001 was carried out using archived real-time data. Operational numerical weather prediction (NWP) model forecasts of this
system, which was not named in accordance with Australian classification policy, were all poor except for the US MRF model,
which provided marginally useful guidance. None of the operational global and regional models available to forecasters predicted
an intense cyclone, and only the US MRF model predicted landfall. A high-resolution forecast using all available data produced
significantly improved predictions over the operational models up to four days before landfall. 相似文献
10.
W. R. Skinner M. D. Flannigan B. J. Stocks D. L. Martell B. M. Wotton J. B. Todd J. A. Mason K. A. Logan E. M. Bosch 《Theoretical and Applied Climatology》2002,71(3-4):157-169
Summary
In Canada, the average annual area of burned forest has increased from around 1 million ha in the 1970’s to over 2.5 million
ha in the 1990’s. A previous study has identified the link between anomalous mid-tropospheric circulation at 500 hPa over
northern North America and wildland fire severity activity in various large regions of Canada over the entire May to August
fire season. In this study, a northern North American study region of the hemispheric gridded 5° latitude by 10° longitude
500 hPa dataset is identified and analysed from 1959 to 1996 for a sequence of six monthly periods through the fire season,
beginning in April and ending in September. Synoptic types, or modes of upper air behavior, are determined objectively by
the eigenvector method employing K-means cluster analysis. Monthly burned areas from the Canadian Large Fire Database (LFDB)
for the same period, 1959 to 1996, are analysed in conjunction with the classified monthly 500 hPa synoptic types. Relationships
between common monthly patterns of anomalous upper flow and spatial patterns of large fire occurrence are examined at the
ecozone level. Average occurrence of a monthly synoptic type associated with very large area burned is approximately 18% of
the years from 1959 to 1996. The largest areas burned during the main fire (May to August) season occur in the western Boreal
and Taiga ecozones – the Taiga Plains, Taiga Shield, Boreal West Shield and Boreal Plains. Monthly burned areas are also analysed
temporally in conjunction with a calculated monthly zonal index (Zim) for two separate areas defined to cover western and eastern Canada. In both western and eastern Canada, high area burned
is associated with synoptic types with mid-tropospheric ridging in the proximity of the affected region and low Zim with weak westerlies and strong meridional flow over western Canada.
Received April 3, 2001 Revised July 13, 2001 相似文献
11.
本文采用观测和NCEP分析场资料对2012年7月21日发生在北京地区的特大暴雨过程的天气形势、水汽来源和中尺度对流系统的特征进行了研究。结果如下:“7.21”北京暴雨过程是高低空与中低纬系统共同配合的结果,暴雨发生在“东高西低”的环流形势下,低涡、切变线、低槽冷锋和低空急流为此次过程的主要影响天气系统;孟加拉湾至西太平洋地区热带辐合带(ITCZ)活跃,其中热带气旋的活动有利于水汽向东亚大陆输送,此次暴雨过程中华北地区的水汽源地包括孟加拉湾和我国东部的渤海、黄海等,低层的水汽主要来自东部,中层的水汽主要来自孟加拉湾;北京的强降雨有两段,第1段降雨虽然发生在冷锋前,但有明显冷空气的侵入,并与地形和东风的作用有关,第2段降雨对流的组织和增强与冷锋强迫相关。在有利环境下,中尺度对流系统频繁发生发展,持续时间长,且稳定少动是此次特大暴雨形成的重要原因。 相似文献
12.
Kenneth E. Kunkel 《Boundary-Layer Meteorology》1982,23(4):473-487
Acoustic sounder measurements of the temperature structure parameter were obtained at the edge of an escarpment which is part of a ridge of mountains. These measurements indicate that in mountainous terrain, the daytime two-dimensional field of thermal turbulence is strongly affected by relative sun-slope orientation and wind direction out to ranges of at least 200–300 m. For the geometry of this site, westerly flow results in a field which tends to decrease rapidly to the west in the morning with a much less rapid decrease in the afternoon. At night, easterly flow results in significantly higher thermal turbulence compared to that obtained during westerly flow.These measurements show an increase in thermal turbulence at horizontal ranges of 100–200 m to the west of the escarpment during early afternoon on days with deep mixed layers. It is conjectured that this is due to the mountain upslope wind. 相似文献
13.
W.-L. Cheng J.-L. Pai B.-J. Tsuang C.-L. Chen 《Meteorology and Atmospheric Physics》2001,78(1-2):11-21
Summary
Recent studies have shown that regional orography and synoptic patterns greatly affect ozone concentrations. The orography
of certain locations is unfavorable for pollutant dispersion, and specific synoptic patterns in these locations lead to serious
air pollution episodes. Frequent ozone episodes (defined as hourly ozone ≥ 120 ppbv) in west-central Taiwan during recent
years have generated much concern. High ozone days (defined as hourly ozone ≥ 80 ppbv) are also occurring more frequently.
High ozone days occur mainly during autumn, but there has been no clear linear relationship demonstrated between any single
meteorological variable and pollutant concentration. In this study, statistical data from 1997–1999 has shown that high ozone
levels occur with two types of synoptic patterns. The first type consists of a continental anticyclone emanating from mainland
China which is swept towards Taiwan. The second is a tropical low pressure system moving northwards closer to Taiwan.
West-central Taiwan is located in the lee of the Central Ranges (altitude of 2500 ∼ 3500 m). These synoptic patterns are unfavorable
for pollutant dispersion and cause high ozone days.
Received October 14, 2000/Revised January 5, 2001 相似文献
14.
The report presents the results of a wind-tunnel study of the flow of the natural wind over complex terrain. A 1:4000 undistorted scale model of Gebbies Pass in the South Island of New Zealand was prepared and tested in the boundary-layer wind tunnel in the Department of Mechanical Engineering at the University of Canterbury.Three forms of construction, viz., terraced, contoured and roughness-added, were compared. Velocity and turbulence profiles, Reynolds stresses and spectra were measured, and correlation of results between different types of construction was calculated. The terraced form was much simpler to construct but was found to be unsatisfactory. The correlation between the contoured and roughness-added models was as high as 0.94, although the roughness-added model made a significant difference to the results in the lower 20%; of the boundary layer. The results of these tests will be compared with results from the field in a future report. 相似文献
15.
M. Haeger-Eugensson 《Theoretical and Applied Climatology》1999,64(1-2):69-82
Summary The winter wind regime of G?teborg, located on the West coast of Sweden, is composed of three different wind systems besides
the ambient wind; a nocturnal low level jet (NLLJ), a winter land breeze (WLB) and an urban heat island circulation (UHIC).
An inversion divides the air column into two layers, one between 10 – 50 m and one between 50 – 100 m. The UHIC is located
in the lower layer, the WLB in the top layer and the NLLJ above the top layer. The intensity of the interacting processes
depends on the stability of each layer as calculated from the bulk Richardson number (BRilow and BRihigh) using continuous data collected during four years (1991 – 94) from two sites (one within and one outside the urban area)
and sampled at three levels. In the evening the WLB develops from the ground level and increases in height until after midnight.
At about the same time an UHIC develops in the urban area, below the WLB and causing an uplift of the latter. However, at
both sites the WLB does not exceed the 100 m level. At this time BRi in both layers are below one resulting in continuous
coupling between the WLB, the UHIC layers and the regional wind. Consequently, the exchange of momentum is still effective
between all layers and this is highlighted by a change in the wind direction and a regulation of wind-speed to more constant
levels. When BRihigh≥1, the layers become frictionally decoupled, as indicated by a return in the wind direction in the top level to the regional
wind, and an acceleration of the top wind. The top level then becomes incorporated in to a nocturnal low-level jet (NLLJ)
system. The normally acknowledged development of the NLLJ, with a start around sunset, is in this case delayed for several
hours at the top level. The reason for this is that there are meso-scale/local wind systems present in layers beneath the
jet causing an interaction between the layers. In the morning, when the layers are again coupled the top layer wind is once
more influenced by the WLB and therefore changes direction and speed. The local and meso-scale wind systems thus delay the
current nocturnal wind development.
Received August 24, 1998 Revised March 17, 1999 相似文献
16.
The horizontal and vertical structure of the 3–5-day and 6–9-day easterly waves over West Africa and tropical Atlantic are
investigated. NCEP/NCAR reanalyses are used for the period 1979–1995 to produce a 17-year climatology of both 3–5-day and
6–9-day easterly waves. Composite patterns of convection, wind, temperature and vertical velocity are analysed with respect
to the following: the modulation by 3–5-day and 6–9-day wave regimes; the contrasts between the ITCZ (5°N–10°N) and the Sahelo-Saharan
band (15°N–20°N); the difference between land and ocean, and seasonal variations. Similarities and differences in the characteristics
of the two wave regimes are identified.
Received: 18 August 1999 / Accepted: 14 March 2001 相似文献
17.
Summary A methodology developed for automatic classification of Objective Synoptic Processes (OSP) and its application to the study
of the mesoscale atmospheric circulatory patterns associated with them is described. The classification was based on the analysis
of the evolution of surface pressure and geopotencial height at 500 hPa during three days. An iterative procedure results
in an objective grouping of the main configurations describing different large-scale situations. This routine has been applied
over an appropriate domain covering the Iberian Peninsula to obtain OSPs in the region for the two month period July–August,
using daily synoptic maps for years 1990 to 1999 (a total of 7304 surface and 500 hPa synoptic maps have been used in the
analysis). Finally, for a characteristic day for two of the OSPs obtained, a mesoscale meteorological model (TAPM) has been
run at high resolution for the region of Catalonia, Northeastern Spain, in order to describe the local atmospheric circulatory
patterns associated with a given large-scale situation. Results show that the complex orography modifies the large-scale forcings
resulting in wind fields with a very important horizontal variability, significant daily cycle, and specific local features
related to orographic elements, which the model was able to incorporate due to the highly-resolved orography used. 相似文献
18.
Summary Continental wind storms are common along the Mediterranean coast. Along the northern coast they are mostly cold, similar
to the Bora or the Mistral, and along the southern coast they are mostly warm, e.g., the Ghibli or the Shirocco. At the eastern
Mediterranean basin and the Levant region, these storms are intermittently warm and cold during the same season and often
even during the same event. Quasi-stationary systems, as well as moving disturbances, are the cause of such wind storms. Accordingly,
the resulting weather conditions may be extremely converse due to the characteristics of the advected airmass. Specific regions
in Israel, sensitive to easterly storms, are influenced by these wind storms for about 10% of the year (e.g., the westerly
slopes of the mountains and valleys with west-east orientation). The frequency, however, of widespread storms covering the
entire region is only approximately 1.4% of the entire year. These wind storms are therefore classified in the present study
according to their climatological and synoptic characteristics; indicating that the dominant synoptic situation is the Red-Sea
trough and the warm advections. These storms appear only from October-May and are most frequent during the cold season. The
diurnal course is characterized by a strengthening in the morning hours and a weakening at noon and in the afternoon hours,
due to the opposing effect of the westerly sea breeze, suppressing the easterly winds and the effect of katabatic winds. Nevertheless,
synoptic conditions may contribute to this tendency as well. Accordingly, a significant increase in the frequencies of easterly
storms, in relation to distance from the seashore has been identified. Although most of the stormy days are with westerly
winds, the easterly wind storms has vast environmental implications, creating damage especially to agriculture and occasionally
also to property and life; coastal flooding, potential air pollution, intensifying of forest fires and occasionally dust and
sand storms.
Received September 9, 1996 Revised March 6, 1997 相似文献
19.
N. G. Prezerakos 《Theoretical and Applied Climatology》1998,60(1-4):63-76
Summary In this paper an attempt is made to detect prolonged (of more than 24 hours duration) temperature inversions in the planetary
boundary layer over Athens, to study their main characteristics and to find out the synoptic situations with which the inversions
are associated. Given the close relationship between the synoptic-scale atmospheric circulation and the occurrence, maintainance
and decay of temperature inversions, a simultaneous three category classification of presented inversions and their respective
synoptic situations is presented. The classification relies mainly on the similarities and differences in the formation and
the maintenance of prolonged temperature inversions. To provide a record of the structure of the lower troposphere and the
synoptic conditions favourable to the formation of inversions, mean ascents of temperature and dew-point temperature and mean
wind profiles for the years 1980–1994 were calculated for each category into which a total of 297 cases fell. The main element
of this structure which strongly affects the pollution of the lower troposphere is the prolonged temperature inversion. Also,
for each category, mean 500 and 850 hPa heights and temperature charts, 500 hPa height anomaly charts, mean sea level (MSL)
pressure charts and MSL pressure anomaly charts were drawn.
Received March 11, 1997 Revised October 6, 1997 相似文献
20.
A. P. Sturman 《Meteorology and Atmospheric Physics》1992,47(2-4):229-236
Summary The Southerly Change Experiment (SOUCHEX) was conducted to examine the influence of the New Zealand Southern Alps on the structure and evolution of cold fronts, locally called southerly changes, as they travel up the east coast. The extensive data obtained by the augmented surface weather station network is used to examine in detail the mesoscale wind field associated with the events observed during the experiment. A comparison of the wind fields observed during the different events illustrates the influence of local dynamic and thermal factors. In particular, lee trough-induced northeasterlies and thermally developed diurnal wind systems are seen to interact with the wind field created by the passage of the front over the Southern Alps.It is apparent that the wind field associated with southerly changes responds to a variety of factors as the cold fronts propagate northwards. For example, there is a tendency for the flow to turn onshore producing a southeast wind during daytime over the Canterbury Plains south of Banks Peninsula probably due to diabatic heating of the mountains and plains. This onshore flow is in direct opposition to pre-frontal foehn northwesterly flow which often continues in the mountain regions and aloft after the front has moved up the coast. The interaction of these air masses over Canterbury creates difficulties for local forecasting. Also, the nocturnal passage of a southerly change is often difficult to detect in surface anemograph traces because of the decoupling of the boundary layer air from that above, producing low level drainage flow over the Canterbury Plains. The overall effect is to create a complex mesoscale wind field resulting from interaction of cold fronts with regional orographic and thermal influences.With 8 Figures 相似文献