How does the Environment Modulate Aerosol Impacts on Tropical Sea Breeze Convective Systems?

Abstract. This study investigates how the enhanced loading of microphysically and radiatively active aerosol particles impacts tropical sea breeze convection and whether these aerosol impacts are modulated by the multitudinous environments that support these cloud systems. To achieve these goals, we have performed two large numerical model ensembles, each comprised of 130 idealised simulations that represent different initial conditions typical of tropical sea breeze environments. The two ensembles are identical with the exception of the fact that one ensemble is initialised with relatively low aerosol loading or pristine conditions, while the other is initialised with higher aerosol loading or polluted conditions. Six atmospheric and four surface parameters are simultaneously perturbed for the 130 initial conditions. Analysis of the ten-dimensional parameter simulations was facilitated by the use of a statistical emulator and multivariate sensitivity techniques.

Comparisons of the clean and polluted ensembles demonstrate that aerosol direct effects reduce the incoming shortwave radiation reaching the surface, as well as the outgoing longwave radiation, within the polluted ensemble. This results in weaker surface fluxes, a reduced ocean-land thermal gradient, and a weaker sea breeze circulation. Consequently, irrespective of the different initial environmental conditions, increasing aerosol concentration decreases the three ingredients necessary for moist convection: moisture, instability, and lift. As reduced surface fluxes and instability inhibit the convective boundary layer development, updraft velocities of the daytime cumulus convection developing ahead of the sea breeze front are robustly reduced in the polluted environments. Furthermore, the variance-based sensitivity analysis reveals that the soil saturation fraction is the most important environmental factor contributing to the updraft velocity variance of this daytime cumulus mode, but that it becomes a less important contributor with enhanced aerosol loading.

It is also demonstrated that enhanced aerosol loading results in a weakening of the convection initiated along the sea breeze front. This suppression is particularly robust when the sea breeze-initiated convection is shallower, and hence restricted to warm rain processes. However, when the sea breeze-initiated convection is deep and includes mixed-phase processes, both the sign and magnitude of the convective updraft responses to increased aerosol loading are modulated by the environment. The less favourable convective environment arising from aerosol direct effects also restricts the development of sea breeze-initiated deep convection. While precipitation is ubiquitously suppressed with enhanced aerosol loading, the magnitude of this suppression remains a function of the initial environment. Altogether, our results highlight the importance of evaluating aerosol impacts on convection systems under the wide range of environments supporting such convective development.