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991.
The ability of an atmospheric general circulation model to reproduce fundamental features of the wintertime extratropical Southern Hemisphere (SH) circulation is evaluated with emphasis on the daily variability of the SH mean flow and the mean flow-transient perturbations interaction. Two 10-year simulations using a new version of the LMDZ GCM with a stretched grid scheme centered at 45 °S and forced by climatological SST are performed: a high (144Ꮡ) and low (64Ꭹ) horizontal resolution runs. The performance of both simulations was determined by comparing several simulated fields (zonal wind, temperature, kinetic energy, transient eddy momentum and heat fluxes, Eliassen-Palm fluxes, Eady growth rate and baroclinic conversion term) against the European Centre for Medium Range Weather Forecast reanalyses (ERA). High and low-resolution simulations are similar in many respects; in particular, both experiments reproduce the main patterns of the southern extratropical large-scale circulation satisfactorily. Increasing resolution does not improve universally some spurious aspects of the low resolution simulation (e.g. the cold bias in the high polar troposphere, the debilitated subtropical jet, the low baroclinic conversion rate). Those aspects present little sensitivity to the model resolution. The interaction between transient eddies and zonal mean flow are examined. The low-resolution experiment is able to qualitatively represent the acceleration/deceleration of the mean flow by transient perturbations, south/north of 30 °S with an accuracy similar to that of the high-resolution experiment. Although both experiments represent the baroclinic structure of the mean flow satisfactorily, the model underestimates some transient properties due to the underestimation of the baroclinic conversion term in middle latitudes. Such misrepresentation does not improve with increasing resolution and is related to the relatively weak meridional temperature gradient and the inadequate geographical distribution of the eddy heat fluxes. In particular, the eddy kinetic energy is always underestimated. Eddy kinetic energy does not improve convincingly with increasing resolution, suggesting that the adequate representation of the storm tracks is highly influenced by the physical parametrizations. 相似文献
992.
Summary The uncertainty in aerosol size distributions is a main source of errors in aerosol optical thickness determined from satellite
measurements. To reduce the errors resulting from the uncertainty in aerosol size distributions, we have performed sensitivity
analyses. It is found the errors resulting from the uncertainty in aerosol size distribution can be considerably reduced by
using the Junge power law to approximate the aerosol size distribution in an actual atmosphere, if the exponent value is determined
at the same time. An iterative algorithm is then developed for the simultaneous determination of aerosol optical thickness
and the exponent of the Junge power law over ocean areas from the upwelling radiances measured in AVHRR visible and near infrared
channels. A number of numerical experiments are carried out to investigate the validity of the Junge power law approximation
by assuming the aerosol size distributions in an actual atmosphere are bimodal with different mode parameters, and by using
the actual aerosol size distributions determined at several places by Kaufman et al. (1994). The results show that the errors
in determined aerosol optical thickness resulting from the Junge power law approach are significantly reduced. The iterative
algorithm is investigated further by comparing the aerosol optical thickness deduced from satellite measurement with that
observed by a sun photometer.
Received October 10, 2001 Revised December 28, 2001 相似文献
993.
Summary ?For the LITFASS-98 experiment, from June 1 until June 30, 1998, the spatially resolved insolation at surface could be computed
from NOAA-14 AVHRR data applying the modular analysis scheme SESAT (Strahlungs- und Energiebilanzen aus Satellitendaten). The satellite inferred insolation for this period shows for clear-sky regions a good agreement with surface
based observations with a rms error of 76 Wm−2. For cloudy conditions the insolation is overestimated with respect to ground based observations, with a rms error between
83 and 118 Wm−2, depending on the cloud optical thickness. This overestimation can be explained by the surface heterogeneity, leading to
underestimated cloud optical thickness, and also by a fixed relative humidity below clouds (55%, dry atmosphere) and a fixed
horizontal visibility (50 km, clear atmosphere). A detailed study of comparable scales in space and time, considering the
different observation geometries and sampling intervals, shows that a 30 min ground based observation can be compared with
a 8 × 8 km2 mean by the satellite data.
Received July 12, 2001; revised April 29, 2002; accepted June 7, 2002 相似文献
994.
A. S. Isaev G. N. Korovin S. A. Bartalev D. V. Ershov A. Janetos E. S. Kasischke H. H. Shugart N. H. F. French B. E. Orlick T. L. Murphy 《Climatic change》2002,55(1-2):235-249
Russian boreal forests are subject to frequent wildfires. The resulting combustion of large amounts of biomass not only transforms forest vegetation, but it also creates significant carbon emissions that total, according to some authors, from 35–94 Mt C per year. These carbon emissions from forest fires should be considered an important part of the forest ecosystem carbon balance and a significant influence on atmospheric trace gases. In this paper we discuss a new method to assess forest fire damage. This method is based on using multi-spectral high-resolution satellite images, large-scale aerial photography, and declassified images obtained from the space-borne national security systems. A normalized difference vegetation index (NDVI) difference image was produced from pre- and post-fire satellite images from SPOT/HRVIR and RESURS-O/MSU-E images. A close relationship was found between values of the NDVI difference image and forest damage level. High-resolution satellite data and large-scale aerial-photos were used to calibrate the NDVI-derived forest damage map. The method was used for mapping of forest fire extent and damage and for estimating carbon emissions from burned forest areas. 相似文献
995.
C. P. Jacovides C. Theophilou F. S. Tymvios S. Pashiardes 《Theoretical and Applied Climatology》2002,72(3-4):259-263
Summary ?A statistical analysis of wind speed and direction data for five selected meteorological stations at the Cyprus coast, is
carried out in this study. Daily, monthly and annual variations of wind speed are established. The Weibull distribution statistics
of the sites are also determined. In addition, an attempt is made to find the sea-breeze circulation effects in the same wind
data. The wind statistics obtained are expected to serve as a validation test for wind energy applications, mainly along the
southeastern coastline.
Received April 5, 2001; revised February 13, 2002; accepted March 3, 2002 相似文献
996.
997.
M. J. Costa M. Cervino E. Cattani F. Torricella V. Levizzani A. M. Silva S. Melani 《Meteorology and Atmospheric Physics》2002,81(3-4):289-298
Summary ?Retrievals of atmospheric aerosol optical thickness are highly dependent on the choice of the class describing the aerosol
properties leading to significant errors while using classes available in the literature. High spectral resolution measurements
from GOME (Global Ozone Monitoring Experiment) between the ultraviolet and the near infrared can be used for an accurate characterization
of the aerosol optical properties. The radiometer MVIRI (METEOSAT Visible and Infrared Imager) on board the geostationary
satellite METEOSAT, while being equipped only with broadband VIS channel, ensures an adequate half-hourly monitoring of the
atmospheric conditions over a large portion of the Earth. The present algorithm is based on a combination of data from both
sensors for the retrieval of the aerosol optical thickness at the reference wavelength of 0.55 μm (AOT). A case of a desert
dust outbreak from the African continent over the Atlantic Ocean is examined. AOT values obtained using a priori fixed classes
taken from the literature are compared with those retrieved with this algorithm using the GOME-derived classes. Systematic
differences of the order of a few tenths on average are found which remain significant also after considering the measurement
errors. This represents a novelty introduced by the synergetic use of both sensors.
Received March 13, 2002 相似文献
998.
A. Ankilov A. Baklanov M. Colhoun K. -H. Enderle J. Gras Yu. Julanov D. Kaller A. Lindner A. A. Lushnikov R. Mavliev F. McGovern T. C. O'Connor J. Podzimek O. Preining G. P. Reischl R. Rudolf G. J. Sem W. W. Szymanski A. E. Vrtala P. E. Wagner W. Winklmayr V. Zagaynov 《Atmospheric Research》2002,62(3-4)
During an international workshop at the Institute for Experimental Physics of the University of Vienna, Austria, which was coordinated within the Committee on Nucleation and Atmospheric Aerosols (IAMAS-IUGG), 10 instruments for aerosol number concentration measurement were studied, covering a wide range of methods based on various different measuring principles. In order to investigate the detection limits of the instruments considered with respect to particle size, simultaneous number concentration measurements were performed for monodispersed aerosols with particle sizes ranging from 1.5 to 50 nm diameter and various compositions.The instruments considered show quite different response characteristics, apparently related to the different vapors used in the various counters to enlarge the particles to an optically detectable size. A strong dependence of the 50% cutoff diameter on the particle composition in correlation with the type of vapor used in the specific instrument was found. An enhanced detection efficiency for ultrafine hygroscopic sodium chloride aerosols was observed with water operated systems, an analogous trend was found for n-butanol operated systems with nonhygroscopic silver and tungsten oxide particles. 相似文献
999.
F. J. Molina G. Verzeletti G. Magonette Ph. Buchet V. Renda M. Geradin A. Parducci M. Mezzi A. Pacchiarotti L. Federici S. Mascelloni 《地震工程与结构动力学》2002,31(8):1563-1582
A pseudodynamic testing procedure has been applied by which the seismic response of a base‐isolated building is obtained by using as specimen the isolators, while the superstructure is numerically simulated. The procedure also takes advantage of the continuous pseudodynamic testing capabilities of the ELSA laboratory, which increase the accuracy of the results and reduce the strain‐rate effect of the rubber bearings. A simple proportional correction of the measured forces compensates the remaining strain‐rate effect due to the unrealistic speed of the test. The correction factor is obtained by means of a characterizing test on the specific rubber isolators. The developed method has been successfully applied to the prediction of the seismic response of a base‐isolated four‐storey building submitted to several specified accelerograms. The results for those earthquakes as well as the effects of some changes of the parameters of the system are discussed. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
1000.
Most current methods of design for concrete structures under earthquake loads rely on highly idealized ‘equivalent’ static representations of the seismic loads and linear‐elastic methods of structural analysis. With the continuing development of non‐linear methods of dynamic analysis for the overload behaviour and collapse of complete concrete structures, a more direct and more accurate design procedure becomes possible which considers conditions at system collapse. This paper describes an evaluation procedure that uses non‐linear dynamic collapse–load analysis together with global safety coefficients. A back‐calibration procedure for evaluating the global safety coefficients is also described. The aim of this paper is to open up discussion of alternative methods of design with improved accuracy which are necessary to move towards a direct collapse–load method of design. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献