Aerosol-cloud interaction inferred from MODIS satellite data and global aerosol models

Abstract

Abstract. We have used the Modis satellite data and two global aerosol models to investigate relationships between aerosol optical depth (AOD) and cloud parameters that may be affected by the aerosol concentration. The relationships that are studied are mainly between AOD on the one hand and cloud cover, cloud liquid water path, and water vapour on the other. Additionally, cloud droplet effective radius, cloud optical depth, cloud top pressure and aerosol Ångström exponent, have been analysed in a few cases. In the Modis data we found as in earlier studies an enhancement in the cloud cover with increasing AOD. We find it likely that most of the strong increase in cloud cover with AOD, at least for AOD<0.2, is a result of aerosol-cloud interactions and prolonged cloud lifetime. Meteorology seems not to be a cause for the increase in cloud cover with AOD in this range. When water uptake of the aerosols is not taken into account in the models the modelled cloud cover actually decreases with AOD. Part of the relationship found in the Modis data for AOD>0.2 can be explained by larger water uptake close to clouds since relative humidity is higher in regions with higher cloud cover. The efficiency of the hygroscopic growth depends on aerosol type, hygroscopic nature of the aerosol, the relative humidity, and to some extent the cloud screening. By analysing the Ångström exponent we find that the hygroscopic growth of the aerosol is not likely to be a main contributor to the cloud cover increase with AOD. Since the largest increase in cloud cover with AOD is for low AOD (~0.2) and thus also for low cloud cover, cloud contamination is not likely to play a large role. However, interpretation of the complex relationships between AOD and cloud parameters should be made with great care and further work is clearly needed.

Analyzing signatures of aerosol‐cloud interactions from satellite retrievals and the GISS GCM to constrain the aerosol indirect effect

“…As CC increases, so does relative humidity in the clear regions adjacent to the clouds, resulting in an increase in AOT and an apparent correlation between AOT and CC. However Myhre et al [2007] in their analysis of the Angstrom exponent adjacent to clouds from MODIS, find that hygroscopic aerosol growth may not be a dominant factor to the cloud cover increase with AOT. Although Kaufman et al [2005c] did not find side scattering from clouds to be an issue when analyzing aerosolcloud effects, recent 3-D Monte Carlo simulations of sidescattering from clouds qualitatively capture both increases in AOT with CC and the spectral dependence in AOT with CC seen in the satellite retrievals [Wen et al, 2007].…”

“…[3] Satellite observations (such as those from the Moderate Resolution Imaging Spectroradiometer (MODIS)) can potentially decipher cloud responses to aerosol changes [Kaufman et al, 2005a] (hereinafter referred to as KF05) and thereby constrain model parameterizations of aerosolcloud interactions Chylek et al, 2006;Storelvmo et al, 2006;Myhre et al, 2007]. Such satellite based comparisons [Lohmann and Lesins, 2002; have been used to suggest that the AIE is closer to the smaller magnitude of the range of current predictions (>À1 W m À2 ).…”

Satellite observations of ship emission induced transitions from broken to closed cell marine stratocumulus over large areas

“…[3] The cloud cover effect is the focus of this study in which we demonstrate by observational means that aerosols can convert partially cloudy areas of MSC into fully cloudy areas. Previous studies have already shown a positive correlation between cloud cover and aerosol optical depth Kaufman et al, 2005;Loeb and Manalo-Smith, 2005;Matheson et al, 2006;Menon et al, 2008;Myhre et al, 2007;Rosenfeld et al, 2006;Sekiguchi et al, 2003], and further the dominance of the cloud cover and LWP effects over the Twomey effect [George and Wood, 2010;Kaufman et al, 2005;Lebsock et al, 2008;Sekiguchi et al, 2003]. However, it is debated whether the larger cloud cover is due to the aerosol effect, or rather due to other causes.…”

Aerosol effects on cloud cover as evidenced by ground‐based and space‐based observations at five rural sites in the United States

“…In particular, several satellite-based studies have documented the effect of aerosols on the thickness and horizontal extent of ship tracks [Ackerman et al, 1995;Christensen and Stephens, 2011]. Aerosol effects on clouds have been studied using models of varying length and time scales [Fan et al, 2013;Myhre et al, 2007;Quaas et al, 2010], yet many global models still do not accurately reproduce observed relationships [Quaas et al, 2009]. Due to the complex interaction between microphysical and dynamical effects and the fact that neither satellite observations nor global models can directly observe nor resolve the microphysical processes that contribute to the aerosol optical depth (τ)-cloud fraction (f c ) relationship, the cloud lifetime effect remains an area of active study.…”

“…Many observational and modeling studies have found a strong increase in f c with increasing τ, particularly at τ < 0.3 [e.g., Myhre et al, 2007]. Yet some studies indicate a reduction in f c for shallow cumulus clouds, particularly in regions with absorbing aerosol and at τ > 0.2-0.3 [Koren et al, 2008;Ten Hoeve et al, 2012].…”