Microphysical and radiative effects of aerosols on warm clouds during the Amazon biomass burning season as observed by MODIS: impacts of water vapor and land cover

Abstract

Aerosol, cloud, water vapor, and temperature profile data from the Moderate Resolution Imaging Spectroradiometer (MODIS) are utilized to examine the impact of aerosols on clouds during the Amazonian biomass burning season in Rondônia, Brazil. It is found that increasing background column water vapor (CWV) throughout this transition season between the Amazon dry and wet seasons likely exerts a strong effect on cloud properties. As a result, proper analysis of aerosol-cloud relationships requires that data be stratified by CWV to account better for the influence of background meteorological variation. Many previous studies of aerosol-cloud interactions over Amazonia have ignored the systematic changes to meteorological factors during the transition season, leading to possible misinterpretation of their results. Cloud fraction (CF) is shown to increase or remain constant with aerosol optical depth (AOD), depending on the value of CWV, whereas the relationship between cloud optical depth (COD) and AOD is quite different. COD increases with AOD until AOD ~ 0.3, which is assumed to be due to the first indirect (microphysical) effect. At higher values of AOD, COD is found to decrease with increasing AOD, which may be due to: (1) the inhibition of cloud development by absorbing aerosols (radiative effect/semi-direct effect) and/or (2) a possible retrieval artifact in which the measured reflectance in the visible is less than expected from a cloud top either from the darkening of clouds through the addition of carbonaceous biomass burning aerosols within or above clouds or subpixel dark surface contamination in the measured cloud reflectance. If (1) is a contributing mechanism, as we suspect, then an empirically-derived increasing function between cloud drop number and aerosol concentration, assumed in a majority of global climate models, is inaccurate since these models do not include treatment of aerosol absorption in and around clouds. The relationship between aerosols and both CWV and clouds over varying land surface types is also analyzed. The study finds that the difference in CWV between forested and deforested land is not correlated with aerosol loading, supporting the assumption that temporal variation of CWV is primarily a function of the larger-scale meteorology. However, a difference in the response of CF to increasing AOD is observed between forested and deforested land. This suggests that dissimilarities between other meteorological factors, such as atmospheric stability, may have an impact on aerosol-cloud correlations between different land cover types

Global cloud condensation nuclei influenced by carbonaceous combustion aerosol

“…For the Amazonian biomass burning season Tan Hoeve et al (2011) showed that at lower values of AOD (<0.3) cloud optical depth and AOD were positively correlated whereas at higher values of AOD they were negatively correlated. The authors postulated that positive correlations at low AOD may be due to the first aerosol indirect effect whereas negative correlations at high AOD may be due to inhibition of cloud development by absorbing aerosols or potentially due to satellite retrieval artifacts.…”

“…Remote sensing studies have also been used to explore the relationship between cloud (cloud fraction and cloud optical depth) and aerosol (aerosol optical depth) properties (Koren et al, 2004;Kaufman and Koren, 2006;Ten Hoeve et al, 2011). For the Amazonian biomass burning season Tan Hoeve et al (2011) showed that at lower values of AOD (<0.3) cloud optical depth and AOD were positively correlated whereas at higher values of AOD they were negatively correlated.…”

Analysis of aerosol effects on warm clouds over the Yangtze River Delta from multi-sensor satellite observations

“…However, positive correlations between CDR and aerosol optical depth (AOD) have also been found in some study areas, from both observations and models, especially over land (Feingold et al, 2001;Grandey and Stier, 2010;Yuan et al, 2008). Different behaviours of CDR as a function of AOD for different AOD regimes (low or high) have been observed by, for example, Tang et al (2014) and Wang et al (2015). Feingold et al (2001) concluded that there are three kinds of CDR responses to aerosol enhancement: the CDR decreases with increasing aerosol loading followed by (1) a saturation of the value of CDR in response to high AOD, (2) a decrease in the CDR with further increasing AOD due to suppression of cloud water vapour supersaturation caused by abundant large particles, or (3) an increase in CDR with further increases in AOD due to an intense competition for vapour which evaporates the smallest droplets.…”

Spatio-Temporal Distribution of Aerosol and Cloud Properties over Sindh Using MODIS Satellite Data and a HYSPLIT Model

“…Physically, aerosol absorption of solar radiation can evaporate or thin cloud optically (Hoeve et al, 2011), and that the decrease of CF with AOD. Thus absorption effects suppress the formation and growth of clouds.…”