Increase of cloud droplet size with aerosol optical depth: a likely new effect of aerosols on climate

Since the Twomey effect was proposed in 1977, it has been demonstrated that cloud particle size decreases with increasing aerosol loading. Using the National Aeronautics and Space Administration's (NASA's) Moderate Resolution Imaging Spectroradiometer (MODIS) products, an opposite trend is found, together with a general finding that cloud particle size may increase or decrease with aerosol loading depending on the water vapor supply, the cloud regime and atmospheric circulation. Convective clouds formed in moist regions show an overwhelmingly positive dependence, as opposed to the negative dependence for stratiform clouds in water-limited regions. The slope of the dependence is driven primarily by the water vapor amount, which explains 70% of the variance. To check if the positive relation is a real effect or an artifact, various potential artifacts that could lead to a false correlation were identified and investigated. The potential artifacts include water vapor swelling, cloud three-dimensional (3-D) effects, partially cloudy effect, atmospheric dynamics and surface influence. None seems to completely explain the observed new phenomenon, although a certain degree of influence exists for some of the factors. Intercomparison study using two different methods also shows the remote sensing artifacts do not harm the conclusion. A likely mechanism is hypothesized concerning with droplet coalescence growth. If the phenomenon is real, it may help resolve some puzzles surrounding the aerosol indirect effect (AIE). Consideration of this phenomenon in climate models could lower the estimation of aerosol indirect forcing and result in a better agreement with the estimations constrained by the global temperature records. The effect may also help reconcile controversial findings concerning whether pollutants suppress or enhance precipitation, i.e. suppression in dry regions and enhancement in humid regions.