Weakening of tropical sea breeze convective systems through interactions of aerosol, radiation, and soil moisture

Abstract

Abstract. This study investigates how the enhanced loading of microphysically and radiatively active aerosol particles impacts tropical sea breeze convective systems and whether these impacts are modulated by the many environments that support these cloud systems. Comparisons of two 130-member pristine and polluted ensembles demonstrate that aerosol direct effects reduce the surface incoming shortwave radiation and the surface outgoing longwave radiation. Changes in the ensemble median values of the surface latent heat flux, the mixed layer depth, the mixed layer convective available potential energy, the maximum inland sea breeze extent, and the sea breeze frontal lift suggest that enhanced aerosol loading generally creates a less favorable environment for sea breeze convective systems. However, the sign and magnitude of these aerosol-induced changes are occasionally modulated by the surface, wind, and low-level thermodynamic conditions. 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 polluted environments across the environments tested. Statistical emulators and variance-based sensitivity analyses reveal that the soil saturation fraction is the most important environmental factor contributing to the updraft velocity variance of this daytime cumulus convection, but that it becomes a less important contributor with enhanced aerosol loading. It is also demonstrated that increased aerosol loading generally results in a weakening of the sea-breeze-initiated convection. This suppression is particularly robust when the sea-breeze-initiated convection is shallower and, hence, restricted to warm rain processes. While the less favorable convective environment arising from aerosol direct effects also restricts the development of sea-breeze-initiated deep convection in some cases, the response does appear to be environmentally modulated, with some cases producing stronger convective updrafts in more polluted environments. Sea breeze precipitation is ubiquitously suppressed with enhanced aerosol loading across all of the environments tested; however, the magnitude of this suppression remains a function of the initial environment. Altogether, our results highlight the importance of evaluating both direct and indirect aerosol effects on convective systems under the wide range of convective environments.

Deforestation-Driven Increases in Shallow Clouds are Greatest in Drier, Low-Aerosol Regions of Southeast Asia

“…Areas in the low aerosol category have AODs below ~0.2, while those in the high aerosol category have AODs above ~0.4 (Figure S2b). A higher concentration of aerosol particles (higher AOD) would tend to result in less solar radiation reaching the surface, thus reducing the importance of surface perturbations on the overall circulation (Park & van den Heever, 2022). Unlike with PWAT, we find that the response of cloud fraction to deforestation is not strongly modulated by AOD.…”

“…The presence of aerosol particles absorbing and/or scattering radiation also alters the amount of radiation reaching the surface, and thus the partitioning of the surface energy budget (Jiang & Feingold, 2006;. This can either dampen or strengthen cloud responses to surface perturbations depending on the aerosol loading (Grant & van den Heever, 2014;Park & van den Heever, 2022). In addition, aerosol particles can also interact with cloud microphysics, leading to either synergistic or competing impacts in cloud properties relative to surface perturbations alone (Tao et al, 2012).…”

Deforestation‐Driven Increases in Shallow Clouds Are Greatest in Drier, Low‐Aerosol Regions of Southeast Asia

“…Areas in the low aerosol category have AODs below ∼0.2, while those in the high aerosol category have AODs above ∼0.4 (Figures S4b and S4d in Supporting Information S1). A higher AOD would result in less solar radiation reaching the surface, thus reducing the importance of surface perturbations on the overall circulation (Park & van den Heever, 2022).…”

“…The presence of aerosol particles absorbing and/or scattering radiation also alters the amount of radiation reaching the surface, and thus the partitioning of the surface energy budget (Jiang & Feingold, 2006; Leung & van den Heever, 2023). This can either dampen or strengthen cloud responses to surface perturbations depending on the aerosol loading (Grant & van den Heever, 2014; Park & van den Heever, 2022). In addition, aerosol particles can also interact with cloud microphysics, leading to synergistic or competing impacts in cloud properties relative to surface perturbations alone (Tao et al., 2012).…”