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851.
Two cases of on-ice and off-ice air flow characterizing the opposite weather situations over the ice-edge zone in the northern Baltic Sea are analysed on the basis of aircraft observations, and modelled using atwo-dimensional mesoscale model. The stable boundary layer (SBL) during theon-ice flow exhibited little thermal modification, but a low-level jet (LLJ) was generated at the 250-m high top of the SBL. In the model, the LLJ was associated with inertial oscillations in space, while the baroclinicity explained the shape of the wind profile well above the SBL. Although the observed LLJ was most pronounced over the ice, the modelling suggests that it was not generated by the ice edge but by the coastline some 400 km upwind of the ice edge, where a much more drastic change in the thermal stratification and surface roughness took place. The generation, maintenance, and strength of the LLJ were very sensitive to the parameterization of turbulent mixing in the SBL. In the case of the off-ice flow, the modification of the air mass and the development of a convective boundary layer (CBL) both over the ice and open sea were reasonably well modelled. Sensitivity runs suggested that it was essential to take into account the effects of subgrid-scale leads, a forest in the archipelago (which was crossed by the air flow), and water vapour condensationinto ice crystals. The heat flux from leads was particularly important for the heatbudget of the CBL, and the observed growth of the CBL was partly due to theeffective mixing over the rough and relatively warm forest. 相似文献
852.
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
853.
Summary ?Microclimatological data obtained during a field experiment in the nongrowing winter period were used to study the microclimatologically
stable night conditions of a 200 × 150 m miscanthus (Miscanthus cv. giganteus) stand and compared to open field conditions. The microclimatological pattern within the miscanthus canopy
was characterized by long-wave radiative cooling of the plant stand and by an established temperature inversion within the
canopy at calm nights. The results show that there are significant differences in air temperature and energy balance components
between the open field and the miscanthus field during calm and clear nights. In general, net radiation difference during
the cold and calm nights was relatively constant and about 20 W m−2 less negative in miscanthus (because of lower surface temperatures) than at the open field. Air temperature differences also
remained fairly constant and were up to 3 °C lower than at the open field (at the height of 1 m). Through thermal inversion
cold air accumulated in the lower parts of the canopy as shown by the vertical air temperature profiles. They showed a greater
amplitude within the diurnal cycle in the miscanthus stand than in the open field. Through the onset of wind, temperature
profiles changed rapidly and differences diminished. Vertical katabatic air drainage into the canopy layers was estimated
indirectly by using the energy balance approach. It was calculated from the significant energy balance closure gap and showed
a mean air exchange rate of up to 22 m3 m−2 h−1, related to a stand volume of 1 m2 area and 4 m height, during the mostly calm and clear nights, depending on the canopy net radiation and turbulent heat exchange
forced by slight wind spells. Quantitative uncertainties in calculated cold air drainage which are introduced by the measurement
method and certain assumptions in the calculations, were considered in a sensitivity analysis. In spite of these uncertainties
evidence of katabatic cold air flow is given.
Received July 29, 1999; revised June 11, 2001; accepted March 14, 2002 相似文献
854.
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
855.
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
856.
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
857.
Anders Moberg Hans Bergström Josefin Ruiz Krigsman Ola Svanered 《Climatic change》2002,53(1-3):171-212
Daily meteorological observations have been made at the old astronomical observatory in Stockholm since 1754. Complete daily mean series of air temperature and sea level pressure are reconstructed from the observational data for 1756–1998. The temperature and pressure series arereconstructed and homogenized with the aid of metadata, statistical tests and comparisons with data from other stations. Comparisons with independently reconstructed daily series for nearby Uppsala (1722–1998) show that the quality of thedaily Stockholm data is good, although the reliability is lower before the mid-19th century. The daily temperature data show that the colder winter mean temperatures of the late 18th to early 19th centuries were connected with a particularly high frequency of very cold winter days. The warmer summers of the same period are more connected with a general shift of the temperature distribution towards higher temperatures than in the late 20th century. 相似文献
858.
859.
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
860.
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