Variability in the correlation between satellite-derived liquid cloud droplet effective radius and aerosol index over the northern Pacific Ocean

The relationship between aerosol index (AI) and cloud effective radius (CER) was examined over five sea regions in the northern Pacific using daily Moderate Resolution Imaging Spectroradiometer data from 2002–2014. The results show that there tends to be a negative relationship between AI and CER at lower AI, becoming positive at higher AI, suggesting the Twomey effect held in low aerosol environment. Over the entire AI range, the correlation between AI and CER was significantly positive over the two marginal seas (the Bohai–Yellow Sea and the Sea of Japan), and it was significantly negative over the three open oceans (the North Pacific subtropical gyre, the western and eastern subarctic North Pacific) for all seasons, except that the correlations in summer were significantly positive over the two subarctic North Pacific regions. A series of statistical analyses showed that the AI–CER relationship (the regression slope) is significantly correlated with relative humidity (RH) and precipitable water vapour (PWV); on average, PWV accounts for 60% of the variance of the slope for the two marginal seas and higher for the three open oceans (79%). The slope can change from negative to positive at high PWV levels, suggesting that water vapour plays an important role in the variability of the slope. Aerosol hygroscopic growth and the growth of CER in humid condition may counteract the Twomey effect at high AI, particularly over the two marginal seas. The error high contribution of the low liquid cloud fraction (LCF) to AI–CER relationship may partially account for the positive slope over the two marginal seas, while the low LCF has a negligible impact on the three open oceans. Additionally, over the three open oceans, stable thermodynamic state may prevent the effect of aerosol on cloud at high AI.