Aerosol indirect effect on warm clouds over South-East Atlantic, from co-located MODIS and CALIPSO observations

Abstract

In this study, we provide a comprehensive analysis of aerosol interaction with warm boundary layer clouds, over South-East Atlantic. We use MODIS retrievals to derive statistical relationships between aerosol concentration and cloud properties, together with co-located CALIPSO estimates of cloud and aerosol layer altitudes. The latter are used to differentiate between cases of mixed and interacting cloud-aerosol layers from cases where the aerosol is located well-above the cloud top. This strategy allows, to a certain extent, to isolate real aerosol-induced effect from meteorology. <br><br> Similar to previous studies, statistics clearly show that aerosol affects cloud microphysics, decreasing the Cloud Droplet Radius (CDR). The same data indicate a concomitant strong decrease in cloud Liquid Water Path (LWP), in evident contrast with the hypothesis of aerosol inhibition of precipitation (Albrecht, 1989). Because of this water loss, probably due to the entrainment of dry air at cloud top, Cloud Optical Thickness (COT) is found to be almost insensitive to changes in aerosol concentration. The analysis of MODIS-CALIPSO coincidences also evidenced an aerosol enhancement of low cloud cover. Surprising, the Cloud Fraction (CLF) response to aerosol invigoration is much stronger when (absorbing) particles are located above cloud top, than in cases of physical interaction, This result suggests a relevant aerosol radiative effect on low cloud occurrence. Heating the atmosphere above the inversion, absorbing particles above cloud top may decrease the vertical temperature gradient, increase the low tropospheric stability and provide favorable conditions for low cloud formation. <br><br> We also focus on the impact of anthropogenic aerosols on precipitation, through the statistical analysis of CDR-COT co-variations. A COT value of 10 is found to be the threshold beyond which precipitation mostly forms, in both clean and polluted environments. For larger COT, polluted clouds showed evidence of precipitation suppression. Results suggest the presence of two competing mechanisms governing LWP response to aerosol invigoration: a drying effect due to aerosol enhanced entrainment of dry air at cloud top (predominant for optically thin clouds) and a moistening effect due to aerosol inhibition of precipitation (predominant for optically thick clouds)

Mapping the Twilight Zone—What We Are Missing between Clouds and Aerosols

“…Striking patterns are the reflective properties and the global distribution, especially in regions of high aerosol loadings, e.g., the South-East Atlantic in August (Figure 5j), when biomass burning occurs [2]. In February, there is a northward shift of the region at the west coast of West-Africa indicated by high mean reflectances (Figure 5i) with the relocation of the biomass burning region and the increasing Saharan dust winds [2]. A possible explanation could be that these pixels include hydrated aerosols, but also discarded aerosol pixels from the aerosol-clear interpolation, described in Section 2, which is supported by the high occurrence frequency near aerosol pixels (Figure 8f).…”

“…By comparing these months, the seasonal differences in aerosol and cloud dynamics are considered, which are, induced by the shift of the inner-tropical convergence zone (ITCZ), seasonal differences in biomass burning, or dust aerosols [2,[21][22][23]. Figure 4 shows the global distribution of the occurrence frequency of all classes, as the ratio of amount of the pixels of each class and all pixels, for February and August respectively.…”

“…Magnitude and mechanisms of these radiative effects and their impact on the global climate system are still highly uncertain, including direct radiative effects of aerosols, as well as their various interactions with clouds, believed to alter cloud reflectivity, lifetime and precipitation susceptibility [1][2][3][4]. While global-scale observation-based studies of aerosol-cloud interactions commonly focus on regions with fairly homogeneous aerosol or cloud, large transition regions exist.…”

Effects of Wintertime Polluted Aerosol on Clouds over the Yangtze River Delta: Case Study

“…With regard to the ROI-B, ROI-C and ROI-D (CF > 40%), high-level AODs are not always associated with small CTPs, suggesting that aerosol-cloud interaction do not lead to the variations of CTPs. A possible reason is that aerosols influence the horizontal extension of clouds rather than the vertical distribution (Costantino et al, 2013).…”

“…There exists a functional relationship between COT, CDR, LWP (Costantino et al, 2013) and the variations of COT and CDR are sensitive to LWP. Therefore, in order to understand AOD-CDR, cloud height and LWP are controlled to evaluate their potential impacts on clouds of different height-type by calculating correlation coefficients of cloud parameters (e.g., CDR, LWP, COT) (Table S1).…”