Aerosol and convective cloud interactions: Differentiating the roles of meteorology, aerosol and cloud-PBL coupling

Convective clouds are driven by meteorology, surface forcing, and aerosol, but differentiating them has been extremely challenging, especially by means of observation-based studies, as they all influence the aerosol-cloud-interaction (ACI). To resolve these fundamental problems, we have conducted systematic investigations concerning ACI by accounting for the cloud-PBL-coupling (CPC), as the latter plays a vital role in affecting atmospheric thermodynamics. Using comprehensive field observations made in China and the US, we gain insight into the response of the PBL process to aerosols, especially entrainment. We found that high aerosol loading can significantly suppress the entrainment rate, breaking the conventional linear relationship between sensible heat fluxes and entrainment fluxes. Related to aerosol vertical distributions, aerosol heating effects can alter vertical heat fluxes, leading to a strong interaction between aerosols and the entrainment process in the upper boundary layer. Such aerosol-entrainment coupling can inhibit boundary layer development and explains the greater sensitivity of observed entrainment rates to aerosols than traditional calculations.

While the fundamental foundation of the invigoration theories is generally sound, there have been controversial viewpoints regarding the causal relationship between any aerosol/CCN proxy variables and the indices of the cloud invigoration effect due to complicated processes that are likely intertwined. Among others, they include co-variability between aerosols, atmospheric thermodynamics as described as the convective available potential energy (CAPE), and clouds that may be coupled or decoupled with the planetary boundary layer (PBL) hosting the bulk of aerosols. We will try to sort out such complex relationships to better single out the effect of aerosol on convective clouds. To this end, we have analyzed ample observation data at high temporal resolution to allow for studying the effect in different phases of cloud development. It is found that CAPE is not a static external forcing factor but an internal variable subject to the influences of aerosol.