Abstract
Atmospheric aerosols play an important role in the formation of warm clouds by acting as efficient cloud condensation nuclei (CCN) and their interactions are believed to cool the Earth-Atmosphere system (‘first indirect effect or Twomey effect’) in a highly uncertain manner compared to the other forcing agents. Here we demonstrate using long-term (2003–2016) satellite observations (NASA’s A-train satellite constellations) over the northern Indian Ocean, that enhanced aerosol loading (due to anthropogenic emissions) can reverse the first indirect effect significantly. In contrast to Twomey effect, a statistically significant increase in cloud effective radius (CER, µm) is observed with respect to an increase in aerosol loading for clouds having low liquid water path (LWP < 75 g m−2) and drier cloud tops. Probable physical mechanisms for this effect are the intense competition for available water vapour due to higher concentrations of anthropogenic aerosols and entrainment of dry air on cloud tops. For such clouds, cloud water content showed a negative response to cloud droplet number concentrations and the estimated intrinsic radiative effect suggest a warming at the Top of the Atmosphere. Although uncertainties exist in quantifying aerosol-cloud interactions (ACI) using satellite observations, present study indicates the physical existence of anti-Twomey effect over the northern Indian Ocean during south Asian outflow.
Highlights
Atmospheric aerosols play an important role in the formation of warm clouds by acting as efficient cloud condensation nuclei (CCN) and their interactions are believed to cool the Earth-Atmosphere system (‘first indirect effect or Twomey effect’) in a highly uncertain manner compared to the other forcing agents
In case of warm clouds having a fixed liquid water path, an increase in aerosol (which acts like cloud condensation nuclei (CCN)) concentrations leads to a decrease in cloud effective radius (CER) and an enhancement in cloud albedo
Among the different cloud types, aerosol mediated changes in warm cloud properties are extensively studied, yet their estimated radiative effects are still highly u ncertain[10]. This high uncertainty stems from our inability to disentangle the effect of meteorology from the aerosol impact on cloud microphysics11. Theoretical[5] and observational studies[12] revealed that aerosol induced changes in macro and microphysical properties of warm clouds are determined by the competition between moistening of cloud layers by precipitation suppression and drying by enhanced entrainment of overlying air
Summary
Atmospheric aerosols play an important role in the formation of warm clouds by acting as efficient cloud condensation nuclei (CCN) and their interactions are believed to cool the Earth-Atmosphere system (‘first indirect effect or Twomey effect’) in a highly uncertain manner compared to the other forcing agents. Spatial distribution of long term (2003–2017) mean aerosol optical depth at 550 nm (AOD550, retrieved from Aqua-MODIS satellite) over the northern Indian Ocean during winter months shows high aerosol loading (AOD > 0.3) over the continental outflow regions (Fig. 1a).
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