【作者】 苏婧；

【导师】 黄建平； 付强；

【作者基本信息】 兰州大学 ， 大气物理与大气环境， 2010， 博士

【摘要】 沙尘气溶胶对太阳辐射能量强烈的吸收作用,不仅会改变地表能量平衡,而且还会通过改变大气的辐射加热结构,加速大气低层水汽和云的蒸发,从而抑制和减弱局域降水。因此研究西北地区气溶胶的辐射强迫作用,对了解沙尘气溶胶对这一区域大气能量平衡和水份循环的影响,特别是对西北地区干旱的影响,有着重要意义。本论文利用CERES、MODIS和CALIPSO等卫星遥感资料及地面观测资料,借助于辐射传输模式,着重详细研究了沙尘气溶胶的直接辐射强迫效应,定量计算了沙尘的直接辐射强迫值及其对大气辐射加热率的影响;初步揭示了沙尘与云的相互作用对辐射能量平衡的影响,并在此基础上提出了分离沙尘气溶胶直接辐射强迫效应与间接／半直接综合辐射强迫效应的方法。西北沙尘气溶胶一直缺乏详细可靠的光学性质数据库,区域辐射强迫的计算存在着很大的不确定性。本论文首次利用2008年中美联合观测的张掖站数据反演得到的沙尘粒子的光学特性,特别是单次散射反照率,计算了西北地区的辐射强迫。利用辐射闭合试验验证了反演的沙尘粒子参数的可靠性,在试验成功的基础上计算了沙尘气溶胶在大气层顶和地表的日平均辐射强迫值。结果表明虽然沙尘气溶胶在大气层顶的辐射强迫值都接近于零,但是在地表却表现出很大的负的辐射强迫值,这种地表与大气层顶很大的差值说明,大气中的沙尘气溶胶有效阻挡了太阳光到达地表,增加了大气中太阳短波的吸收,这势必会影响大气的垂直温度分布,对大气层结的稳定和水分循环产生影响。气溶胶的垂直廓线对大气加热率和辐射强迫的的计算至关重要,但由于观测手段的限制,区域和全球尺度上的气溶胶垂直廓线的问题一直没有解决。本论文利用最新发射的CAPLISO卫星数据得到了塔克拉玛干地区沙尘气溶胶的垂直廓线,并输入Fu-liou模式,计算了沙尘辐射强迫值及加热率的垂直分布。结果表明由于沙尘的强吸收性,使得很大一部分太阳辐射被吸收留在了大气中,从而进一步加热大气。在强沙尘出现时,伴随着沙尘过程的加强发展,沙尘层大气的加热率可以从1K day-1,2Kday-1增加到3K day-1,甚至沙尘最强时可以达到5.5Kday-1。这项工作首次定量评估了在塔克拉玛干沙漠中强沙尘暴发生时,沙尘层对大气的加热作用。本文还通过分析4年的CERES卫星资料比较了纯云区(CLD)和被沙尘污染过的云区(COD)云微物理特性和辐射强迫的变化,再次证实了当云下有沙尘出现时,沙尘粒子对云光学特性有显著的改变；并发现沙尘的存在会明显减弱云在大气层顶的冷却效应,云冷却效应的减弱可认为是由于沙尘增暖作用抵消的结果。而沙尘在TOA处的这种增暖效应,不仅包括直接效应也包含了由云物理特性改变所引起的间接和半直接效应,但是无法只利用卫星资料将直接效应和其他两种效应分开。本文首次提出了一种将卫星资料分析结果与模式模拟结果相结合来分离直接与间接效应的新方法,来达到这一目的。利用此种方法估算的在TOA处沙尘短波瞬时直接辐射强迫值为22.7 Wm-2,间接与半直接辐射强迫和为82.2Wm-2,分别占总辐射效应的21.6%和78.4%。该方法的提出为定量研究云与气溶胶的相互作用提供了一个新的思路。更多还原

【Abstract】 Dust aerosols hava strong absorption of solar radiation. They not only modulate the surface energy, but also affect the heating rate of atmosphere which would enhence cloud evaporation, and weaken and suppress regional precipitation. The investigation of the dust aerosol radiative forcing is therefore important to understand the dust aerosol effects on the regional energy balance and hydrological cycle, over Northwestern China with the arid climate. In this study, we quantify the dust direct radiative forcing, and its influence on atmospheric radiative heating rate by using the CERES, MODIS and CALIPSO satellite data, ground-based observation and radiative transfer model. We also examine the radiation energy budget change due to the interaction of dust and cloud, where we propose a comprehensive approach to separate the dust aerosol direct radiative effect and indirect/semi-direct radiative effect.There are large uncertainties in estimation of regional radiative forcing of dust over northwestern China because of lacking detailed and reliable observations of dust optical properties. Herein we use the dust aerosol optical properties, retrieved from ground-based observations at Zhangye site during the 2008 China-US Joint Field Experiment, to calculate the dust aerosol radiative forcing. After verifying the reliability of the retrieved dust aeosol optical and microphysical properties though the radiation closure experiment, we calculate the daily mean radiative forcing of dust aerosol at both the top of atmosphere and surface. It is shown that dust aerosol has a strong negative forcing at the surface, but the magnitude of dust ARF at the TOA is very small. The large ARF difference between TOA and surface means that solar radiation is kept within the atmosphere. This would reduce the vertical temperature gradient, enhace the stability of the atmosphere of the atmosphere, and thus slow down hydrological-cycle.The vertical profile of dust aerosols is important to estimate the atmospheric heating rate, and is also important to calculate the dust ARF. However, it is difficult to obtain the aerosol vertical profiles on the regional and globle spatial scales untile the active lidar observation become available from CALIPSO satellite. In this study, the CALIPSO lidar measurements are used to derive the vertical distribution of dust aerosols. We use the Fu-Liou radiation model along with the input of dust aerosol vertical profile to estimate the dust radiative forcing and atmospheric radiative heating rate. It is found that dust aerosols can maintain a large portion of solar energy in the atmosphere because of the strong absorption, thereby heating the atmosphere. During an evolution of a dust storm case, the dust aerosols heat the atmosphere (daily mean) by up to 1,2, and 3Kday-1, respectively. The maximum daily mean radiative heating rate reaches 5.5Kday-1.This work quantitatively estimates the warm effect of dust layer during a dust storm in the Taklimakan desert for the first time.In our study, four years of CERES data are also used to quantify the differences of cloud microphysical properties and radiative forcing between CLD (pure cloud) and COD (cloud over dust). We shown that the analysis of satellite observations, dust aerosols do change cloud microphysical properties. It is shown that when dust exists under a cloud it has a warming effect at TOA, which combines direct, indirect and semi-direct effect. However, it is difficult, using only satellite observations, to separate the dust aerosol direct effect from those caused by altered cloud properties. We propose a new simple method that uses the satellite observations along with the radiation model calculations to separate these effects. It is shown that the averaged direct, and the combined indirect and semi-direct instantaneous shortwave radiative forcing are 22.7 and 82.2Wm-2, respectively, which correspond to 21.6 and 78.4% of the total RF value. This method provides a new way to quantitatively study the interaction between clouds and aerosols.更多还原