Switching cloud cover and dynamical regimes from open to closed Benard cells in response to the suppression of precipitation by aerosols

[摘要] The dynamic structure of the weakly sheared atmospheric marine boundarylayer (MBL) supports three distinct states of cloud cover, which areassociated with the concentrations of cloud condensation nuclei (CCN)aerosols in the MBL: (i) CCN rich MBL with closed Benard cellular convectionthat forms nearly full cloud cover; (ii) CCN depleted MBL with open cellularconvection that forms <40% cloud cover; and, (iii) CCN starved MBLwhere clouds cannot form due to insufficient CCN, with near zero cloudcover. Here we propose a mechanism for the transition between these threestates that involves the aerosol impacts on precipitation and the feedbackson the dynamics of the clouds and on the aerosols deposition. By suppressingprecipitation aerosols can reverse the direction of the airflow, convertingthe cloud structure from open to closed cells and more than doubling thecloud cover. The three states possess positive feedbacks for selfmaintenance, so that small changes of the conditions can lead to bifurcationof the MBL cloud regime. The transition between the closed and open cellsoccur at near pristine background level of aerosols, creating a largesensitivity of cloud radiative forcing to very small changes in aerosols atthe MBL. The third state of super clean air can occur as the more efficientprecipitation in cleaner air deposits the aerosols ever faster in a runawaypositive feedback process. The proposed mechanism suggests that very smallchanges in the aerosols input to the MBL can have large impacts on theoceanic cloud cover and likely in turn on the global temperature, in waysthat are not yet accounted for in the climate models.

[学科分类] 大气科学