Abstract
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.
Highlights
The effect of aerosol particles on the hydrological cycle remains one of the largest uncertainties in the understanding of Earth’s climate system
This mid-season peak in aerosol loading is indicative of a biomass burning peak that is largely determined by social behavior (Crutzen and Andreae, 1990)
It is improbable that these correlations are due to a 3-D cloud effect, which artificially increases aerosol optical depth (AOD) retrievals in the regions neighboring clouds (Wen et al, 2006). This effect has been suggested to be larger for greater cloud cover and aerosol loading, but our results indicate that the strongest positive correlation between cloud fraction (CF) and AOD occurs at lower values of AOD and cloud cover (Wen et al, 2006; Yu et al, 2007)
Summary
The effect of aerosol particles on the hydrological cycle remains one of the largest uncertainties in the understanding of Earth’s climate system. The study showed that the relative contributions of the microphysical and radiative effects are strongly tied to the initial CF prior to the influence of aerosols – the radiative absorption effect begins to dominate at lower values of AOD for lower initial cloud fractions This is due to the hypothesized aerosol absorption cloud fraction feedback: stabilization of the near-surface atmosphere due to aerosol absorption of radiation initially reduces cloudiness, which exposes more of the aerosol layer, further reducing cloudiness (Koren et al, 2008). We use CWV, which is shown to correlate with MODIS cloud parameters, to stratify the cloud data to ensure similar background moisture conditions exist along the range of AOD retrievals used The latter portion of the paper tests the assumption that CWV is not influenced by aerosol loading, which is assumed in the former portion of the paper, by analyzing effects of aerosols on CWV and clouds over different land surface types. This study investigates two primary questions: (1) how can physical aerosol-cloud relationships be extracted from the seasonal progression of CWV, CF, and AOD and (2) how can further differences in these aerosol-cloud relationships be extracted between forested and deforested land cover types?
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
