Asian dust storms are one of the significant events which influence the environment, climate, as well as land and ocean ecology in East Asian, and at least western Pacific. Understanding these dust storms, about their spatial and temporal variabilities and the characteristics of mineral dust aerosols is of first priority. Owing to limited observation facilities and coarse spatial resolution, so far, our knowledge about Asian dust storms is still very limited, in particular mesoscale structure and mineral dust aerosol's optical depth spectra in severe situation when a lot of instruments are ineffective. In present paper, based on surface meteorological observation relatively high spatial and temporal resolution (about 100 km resolution and 1 hr), we investigated the mesoscale variability of spring dust storms with case study and statistical investigation. Also, base on our solar spectral observation over Beijing, the optical depth spectra of mineral dust aerosols are given with case study and statistics. Comparisons are also made with other related observations in eastern Asia. These results will be useful for estimation of atmospheric dust loading and their transport over eastern part of Northern China.
Infrared remote sensing of clouds is one of the main subjects in the field of atmospheric remote sensing from satellites. As its characteristics of intense infrared radiation and absorption the heights of cloud top can generally be derived from the brightness temperature obtained from satellite infrared cloud images and then a series of study about cloud morphologic microphysics and dynamics can be conducted. On the same principle the heights of cloud base can also be determined by sky infrared brightness temperature observations on the ground. Some preliminary studies were made in the past but have not been extended. The reason is that the range of zenith angle is very wide for ground-based remote sensing of cloud base heights. From near horizontal ground up to zenith at different zenith angle the background infrared brightness temperature of clear sky atmosphere is very different it is the function of the atmospheric temperature and humidity profiles as well as the zenith angle. Hence the question must be considered is how to obtain the correct information about the heights of cloud base from sky background brightness temperatures in large variational range. In addition the contributions of aerosols in near ground atmosphere to sky brightness temperatures observed on the ground cannot be neglected. In order to correctly derive cloud base heights from sky brightness temperatures the effect of aerosol layer must be studied seriously. Using radiative transfer equation to study th sensitivities of thermal infrared brightness temperature values observed on the ground to different aerosols and different cloud base heights is an important basic work. In this paper systematic study on the variations of sky infrared brightness temperatures caused by different kinds of clouds under the different kinds of the different intensities (represented by different ground visibilities) of aerosols using MODTRAN 4.0 model is presented. In this study the feasibility of cloud base height remote sensing by ground-based sky infrared brightness temperature observation and the quantitative effects of aerosol layer on it can be found. From the calculated results by model the following can be concluded:
1. For low and middle clouds, the sky thermal infrared brightness temperature observed on the ground is very sensitive to the variations of cloud base height, therefore as the "clear" sky brightness temperature is known it can be used to retrieve and remote sensing of cloud base height.
2. The influence of aerosols (visibilities and aerosol types) above the ground on the sky background brightness temperatures is considerable, it must be corrected. Because the aerosol's influence on brightness temperatures with different zenith angles is fairly clear, it can be corrected very well.
The quasi-continuous measurements of direct solar and sky scattered spectrum over Beijing have been conducted since September 1998 with an automatic moderate resolution solar spectroradiometer (MORSAS) developed by the Institute of Atmospheric Physics, Chinese Academy of Sciences. The measurements of direct solar spectrum under cloud free conditions, including clear and turbid skies, were used to derive atmospheric aerosol optical thickness. During the period of Asian-Pacific Regional Aerosol Characterization Experiment (ACE-Asia) in spring 2001, a spectral radiometer CIMEL CE-318 in AERONET (owned by US NASA/GSFC) was operated at the same time and the same place with MORSAS. Comparing the observed results from the two instruments, they are consistent with each other. In this paper we present the variation of atmospheric aerosol optical thickness and Angstrom exponent which symbolizes the width of particle spectrum in Beijing during recent three years. In contrast with the middle of 1990’s, the atmospheric aerosol optical thickness in autumn and winter in recent years decreased slightly, indicating that Beijing air quality has been improved by certain local environmental management, whereas in spring in the latest 2 years the atmospheric aerosol optical thickness has evidently increased as the increase of the dust weather events, and the Angstrom exponent tends to be lower. This reveals that the ratio of larger particles especially the large dust-sand particles in the air has increased. So it is very important to enhance dust source management for those upstream and local arid and semi-arid regions.
Atmospheric aerosols play important roles in climate radiative forcing as well as environmental issues. In addition to microphysical and optical parameters, its vertical distribution is also important for both surface visibility, transport and related boundary layer stratification. Satellite-borne instruments, such as MODIS, AVHRR have monitored global aerosol distribution, but ground-based optical remote sensing is still of significance for validation and more detailed observation, such as diurnal variation and vertical distribution. In this paper, numerical simulations are conducted for clear atmosphere with various vertical structures of atmospheric aerosols. The sun direct observation is used to derive aerosol optical depth. In sun-zenith principal plane, pairs of directional scattered radiances symmetrically to solar direction (i.e. scattering angles are same but one is near zenith and the other near horizon) may be used to derive aerosol’s vertical distribution.
Atmospheric aerosol enhance greatly the atmospheric scattering which decrease the signal to noise ratio of the satellite images. It is therefore important to correctly estimate and remove the effects of the atmospheric aerosols on signals from the underlying surfaces. Depth of the absorption band of oxygen molecules at 760 nm wavelength will be changed by multiple scattering. The new generation of space-borne spectrometers under development provides sufficient spectral resolution to detect this effect. This paper discuses the possibility to distinguish the type of aerosols and retrieve aerosol optical depth from the variation of the features of the A-band at 760 nm. It is shown that this effect can be used to classify aerosols when aerosol optical depth is very large or for cases over typical land surfaces. For the visibility ranging from 5 to 20 km. Aerosol optical depth can be retrieved accurately over the ocean even if the type of aerosols is unknown. Therefore, this method can provide valuable information to current atmospheric correction and spaceborne aerosol-retrieval schemes.
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