On the use of satellite remote sensing based approach for determining aerosol direct radiative effect over land: a case study over China

Abstract

Abstract. A satellite-based approach to derive the aerosol direct short wave (SW) radiative effect (ADRE) was studied in an environment with highly variable aerosol conditions over Eastern China from March to October 2009. The method is based on using coincident SW Top of the Atmosphere (TOA) fluxes from the Clouds and the Earth's Radiant Energy System (CERES) and aerosol optical depths (AODs) from the Moderate Resolution Imaging Sectroradiometer (MODIS). The estimate for instantaneous clear sky ADRE is obtained by establishing linear regression between CERES fluxes and MODIS AODs. Even though the approach has been used in a number of studies, less focus has been paid to the method itself. In this study the main goals were first to study the method in more detail as well as it's applicability over Eastern China, and second to derive a satellite-based estimate of ADRE over the study area. Before the linear fitting, CERES fluxes were normalized to a fixed solar zenith angle, Earth–Sun distance and atmospheric water vapour content to reduce the noise in the flux observations that was not related to aerosols. The satellite based clear sky estimates for median instantaneous and diurnally averaged ADRE over the study area were −8.8 W m−2, and −5.1 W m−2, respectively. Over heavily industrialized areas the cooling at TOA was two to more than three times the median value, and associated with high AODs (>0.5). Especially during the summer months positive ADREs were observed locally over dark surfaces. This was most probably a method artifact related to systematic change of aerosol type, subvisual cloud contamination or both. The key question in the satellite-based approach is the accuracy of the estimated aerosol-free TOA flux (F0,TOA). Comparison with simulated F0,TOA showed that both the satellite method and the model produced qualitatively similar spatial patterns, but absolute values differed. In 58% of the cases the satellite based F0,TOA was within ±10 W m−2 of the modeled value. Over bright surfaces the satellite-based method tend to produce lower F0,TOA than the model.

Aerosol Optical Properties and Associated Direct Radiative Forcing over the Yangtze River Basin during 2001–2015

“…In addition, "longitude and colatitude of CERES FOV at surface", "CERES solar zenith at surface" and "Clear area percentage at sub pixel resolution" are also read from the CERES_SSF_FM1_MODIS_Ed3A product for the qualifications in the following steps. [22], the correction of CERES upward flux due to the normalization of the solar zenith angle (SZA) varied from -15 to 15 Wm -2 in Middle-East China. Therefore, SZA is limited to less than 60 • for avoiding the oversized pixels.…”

“…In addition, the monthly and seasonal variations of IADRE over YRB are also associated with the periodic change of WVC (Figure 9). According to previous research [17,22], IADRE is estimated by the linear regression of CERES AOD and SWF in the condition of unrealistically low WVC. This may lead to an overestimation of IADRE due to the neglect of radiation attenuation caused by WVC [17].…”

“…To build a successful linear regression, those grid cells with small absolute values of correlation coefficients (<0.2) are also eliminated [22]. After that, the linear regression in each grid is built to estimate F clr , SWF = F clr + αAOD (4) where SWF is the upward SW flux, F clr is referred to the y intercept of the linear regression (i.e., the upward SW flux for AOD = 0), and α is the slope of the linear regression.…”

“…It triggers numerous studies for analyzing the aerosol optical and radiative properties in different regions of China. For instance, Sundström et al [22] estimated ADRE at TOA using a linear regression of MODIS AOD and CERES SWF in Middle-East China; Xia et al [14] analyzed the spatiotemporal distributions of aerosol optical and radiative properties at TOA and surface through ground-based data of 21 sites in China; Kang et al [15] and Yu et al [16] also reported the aerosol optical properties and ADREs based on RTM for Nanjing and Beijing, respectively. In general, previous studies in China mainly focused on the regions with intensive population and frequent industrial activities (short-term analyses), for example, Pearl River Delta (PRD), Yangtze River Delta (YRD) and Beijing-Tianjin-Hebei region.…”

“…For the model-simulated method, aerosol optical properties obtained from ground-based or remote-sensing observations have been used as input to the RTM to calculate ADRE at TOA and surface [11][12][13][14][15][16]. For the remote sensing method, one is to estimate ADRE by the linear relationship between AOD and concurrent upward SW flux at TOA obtained from satellite observations [17][18][19][20][21][22]; the other is to take Fclr as the minimum upward SW flux at TOA in clear sky during one month, but it is not suitable for the regions with high cloud coverage or aerosol loading such as the study region (YRB) [4]. In other words, the remote-sensing technique (linear fitting) is more suitable for estimating ADRE at regional and global scales.…”