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81.
E. García-Bustamante J. F. González-Rouco J. Navarro E. Xoplaki P. A. Jiménez J. P. Montávez 《Climate Dynamics》2012,38(1-2):141-160
The variability and predictability of the surface wind field at the regional scale is explored over a complex terrain region in the northeastern Iberian Peninsula by means of a downscaling technique based on Canonical Correlation Analysis. More than a decade of observations (1992–2005) allows for calibrating and validating a statistical method that elicits the main associations between the large scale atmospheric circulation over the North Atlantic and Mediterranean areas and the regional wind field. In an initial step the downscaling model is designed by selecting parameter values from practise. To a large extent, the variability of the wind at monthly timescales is found to be governed by the large scale circulation modulated by the particular orographic features of the area. The sensitivity of the downscaling methodology to the selection of the model parameter values is explored, in a second step, by performing a systematic sampling of the parameters space, avoiding a heuristic selection. This provides a metric for the uncertainty associated with the various possible model configurations. The uncertainties associated with the model configuration are considerably dependent on the spatial variability of the wind. While the sampling of the parameters space in the model set up moderately impact estimations during the calibration period, the regional wind variability is very sensitive to the parameters selection at longer timescales. This fact illustrates that downscaling exercises based on a single configuration of parameters should be interpreted with extreme caution. The downscaling model is used to extend the estimations several centuries to the past using long datasets of sea level pressure, thereby illustrating the large temporal variability of the regional wind field from interannual to multicentennial timescales. The analysis does not evidence long term trends throughout the twentieth century, however anomalous episodes of high/low wind speeds are identified. 相似文献
82.
Summary ?During recent years, numerous studies have examined the Buenos Aires urban climate, but the relationship between large-scale
weather conditions and the Buenos Aires urban heat island (UHI) intensity has not been studied. The goal of this paper is
to apply an objective synoptic climatological method to identify homogeneous air masses or weather types affecting Buenos
Aires during winter, and to relate the results to the UHI intensity. A K-means clustering method was used to define six different
air masses considering the 03:00, 09:00, 15:00 and 21:00 LT surface observations of dry bulb temperature, dew point, cloud
cover, atmospheric pressure and wind direction and velocity at Ezeiza, the most rural meteorological station of the Buenos
Aires metropolitan area (Fig. 1). Results show that the mean UHI intensity is at its maximum (2.8 °C) a few hours before sunrise
when conditions are dominated by cold air masses associated with cold-core anticyclones, weak winds and low cloud cover. Inverse
heat islands are found during the afternoon for all air masses indicating that surface processes are not dominant at that
time. The relatively infrequent and warmest air mass is the only one that presents a mean negative urban-rural temperature
difference (−0.1 °C) during the afternoon with the smallest diurnal cycle of the UHI intensity probably due to the prevailing
high humidity and cloudy sky conditions. The paper provides an insight into the Buenos Aires urban–rural temperature difference
under a variety of winter weather types and results could be useful to improve local daily temperature forecasts for the metropolitan
area of Buenos Aires on the basis of the routine forecasts of weather types.
Received October 24, 2001; revised June 12, 2002; accepted October 10, 2002 相似文献
83.
84.
C. Serra G. Fernández Mills M. C. Periago X. Lana 《Theoretical and Applied Climatology》1998,59(1-2):29-49
Summary Catalonian daily precipitation patterns were obtained by means of S-mode Principal Component Analysis (PCA) applied to subsets
built up according to synoptic surface airflow. Daily data from November to April 1990–1994 from 45 raingauge sites in Catalonia
without missing data were used. Eight circulation classes for days in this period were considered. A Precipitation Activity
Index (PAI) was calculated for each station and class, leading to the determination of the highest activity regions for each
subset. In addition, we determined which circulation type produced most precipitation at each station of the network. Separate
PCAs were performed for each of the eight classes considered and the first four Varimax rotated solutions were interpreted
for each circulation type. The physical meaning of the rest of the factors were omitted as they were associated with local
behaviour. Finally, some concluding remarks on the cyclogenic properties of the Western Mediterranean Basin and the diversity
of the extracted patterns are presented and supported by the authors experience in forecasting in Catalonia.
Received February, 11, 1997 Revised May 23, 1997 相似文献
85.
Heat waves and dry spells are analyzed (i) at eightstations in south Moravia (Czech Republic), (ii) inthe control ECHAM3 GCM run at the gridpoint closest tothe study area, and (iii) in the ECHAM3 GCM run fordoubled CO2 concentrations (scenario A) at thesame gridpoint (heat waves only). The GCM outputs arevalidated both against individual station data andareally representative values. In the control run, theheat waves are too long, appear later in the year,peak at higher temperatures and their numbers areunder- (over-) estimated in June and July (in August).The simulated dry spells are too long, and the annualcycle of their occurrence is distorted.Mid-tropospheric circulation, and heat waves and dryspells are linked much less tightly in the controlclimate than in the observed. Since mid-troposphericcirculation is simulated fairly successfully, wesuggest the hypothesis that either the air-masstransformation and local processes are too strong inthe model or the simulated advection is too weak. Inthe scenario A climate, the heat waves become a commonphenomenon: warming of 4.5 °C in summer(difference between scenario A and control climates)induces a five-fold increase in the frequency oftropical days and an immense enhancement of extremityof heat waves. The results of the study underline theneed for (i) a proper validation of the GCM outputbefore a climate impact study is conducted and (ii)translation of large-scale information from GCMs intolocal scales using downscaling and stochasticmodelling techniques in order to reduce GCMs' biases. 相似文献
86.
气象卫星云图的多分辨小波分解及人工神经网络降水估计研究 总被引:7,自引:0,他引:7
采用多分辨小波分析对卫星图象进行预处理 ,在保留其特征信息的同时 ,减小了数据量 ,改善了神经网络训练过程的收敛性能 ,提高了处理速度。采用这一方法根据 GOES- 8的红外亮温图象和气象雷达资料对巴西圣保罗州中部的降水量估计进行了试验 ,取得了良好的效果。 相似文献
87.
General purpose Computational Fluid Dynamics (CFD) solvers are frequently used in small-scale urban pollution dispersion simulations
without a large extent of ver- tical flow. Vertical flow, however, plays an important role in the formation of local breezes,
such as urban heat island induced breezes that have great significance in the ventilation of large cities. The effects of
atmospheric stratification, anelasticity and Coriolis force must be taken into account in such simulations. We introduce a
general method for adapting pressure based CFD solvers to atmospheric flow simulations in order to take advantage of their
high flexibility in geometrical modelling and meshing. Compressibility and thermal stratification effects are taken into account
by utilizing a novel system of transformations of the field variables and by adding consequential source terms to the model
equations of incompressible flow. Phenomena involving mesoscale to microscale coupled effects can be analyzed without model
nesting, applying only local grid refinement of an arbitrary level. Elements of the method are validated against an analytical
solution, results of a reference calculation, and a laboratory scale urban heat island circulation experiment. The new approach
can be applied with benefits to several areas of application. Inclusion of the moisture transport phenomena and the surface
energy balance are important further steps towards the practical application of the method. 相似文献
88.
R. García-Herrera D. Barriopedro E. Hernández D. Paredes J. F. Correoso L. Prieto 《Meteorology and Atmospheric Physics》2005,90(3-4):225-243
Summary This paper characterizes Mesoscale Convective Systems (MCSs) during 2001 over Iberia and the Balearic Islands and their meteorological
settings. Enhanced infrared Meteosat imagery has been used to detect their occurrence over the Western Mediterranean region
between June and December 2001 according to satellite-defined criteria based on the MCS physical characteristics.
Twelve MCSs have been identified. The results show that the occurrence of 2001 MCSs is limited to the August–October period,
with September being the most active period. They tend to develop during the late afternoon or early night, with preferred
eastern Iberian coast locations and eastward migrations. A cloud shield area of 50.000 km2 is rarely exceeded. When our results are compared with previous studies, it is possible to assert that though 2001 MCS activity
was moderate, the convective season was substantially less prolonged than usual, with shorter MCS life cycles and higher average
speeds. The average MCS precipitation rate was 3.3 mm·h−1 but a wide range of values varying from scarce precipitation to intense events of 130 mm·24 h−1 (6 September) were collected. The results suggest that, during 2001, MCS rainfall was the principal source of precipitation
in the Mediterranean region during the convective season, but its impact varied according to the location.
Synoptic analysis based on NCEP/NCAR reanalysis show that several common precursors could be identified over the Western Mediterranean
Sea when the 2001 MCSs occurred: a low-level tongue of moist air and precipitable water (PW) exceeding 25 mm through the southern
portion of the Western Mediterranean area, low-level zonal warm advection over 2 °C·24 h−1 towards eastern Iberia, a modest 1000–850 hPa equivalent potential temperature (θe) difference over 20 °C located close to the eastern Iberian coast, a mid level trough (sometimes a cut-off low) over Northern
Africa or Southern Spain and high levels geostrophic vorticity advection exceeding 12·10−10 s−2 over eastern Iberia and Northern Africa. Finally, the results suggest that synoptic, orographic and a warm-air advection
were the most relevant forcing mechanisms during 2001. 相似文献
89.
The surface renewal (SR) method was applied for the first time to measurements of air temperature over four Amazonian forest sites and different seasons in order to obtain estimates of buoyancy heat flux. The required calibration of this method against eddy covariance resulted in a value for a specific parameter that is close to the range reported in other studies, contributing to the generalization of the SR method to different kinds of canopies. The comparison with fluxes obtained using the eddy covariance technique revealed a good match between the two methods for different sites, heights and seasons. Sites with high levels of non-stationarity in the signals of temperature and wind speed presented higher scatter in the regression with fluxes from eddy covariance. For a particular site with previously reported influence of low-frequency motions, the regression was only satisfactory, i.e., slope parameter close to unity and small offset, when oscillations with periods longer than $\approx $ ≈ 13 min were filtered out. The SR method has a great potential due to the simplicity of the instrumentation required. However, care should be taken when measuring under the influence of mesoscale motions, which can lead to high levels of non-stationarity, compromising the fundamental concepts of the SR theory. 相似文献
90.