Land cover controls on the diurnal cycle of smoke-land-atmosphere interactions

Tropical biomass burning emits large quantities of organic and black carbon particles into the atmosphere. The highly absorbing smoke can influence land-atmosphere interactions and clouds directly, semi-directly, and indirectly. We use the NCAR single-column community climate model (SCCM) to examine how the diurnal cycle of smoke direct and semi-direct effect on the land-atmosphere system in a typical burning season depends on land covers, namely forest and pasture. The radiative and hydrological distinctions, such as albedo, LAI, roughness length, and root distribution, determine the evolution of the atmospheric boundary layer (ABL) over forest and pasture differently. Over both forest and pasture, the absorption of solar radiation by ABL smoke can decrease the cloud-top radiative cooling and dissipate cloud, which is more pronounced in the afternoon. Such semi-direct effect is offset by the reduction of surface solar flux due to aerosol extinction, leading to a net surface cooling. The net cooling does not increase monotonously with smoke loading. The surface cooling and simultaneous atmospheric solar heating inhibit the sensible heat flux significantly, consequently reduce the Bowen ratio. The latent heat flux can be negative or positive changes, depending on competition of several environmental controls by leaf temperature, solar radiation, and specific humidity. The leaf temperature decreases in response to the reduced solar flux, while the air temperature may increase in the afternoon because of large atmospheric solar heating. These smoke-land-atmosphere interactions depend on land cover and soil moisture. The reduction of sensible heat flux is larger over a dry pasture than over a wet forest. The latent heat flux decreases over the wet forest but can increases in the afternoon over the dry pasture mainly due to the decreases of leaf temperature and stomatal resistance. The air temperature over the dry pasture decreases more in the morning and late afternoon due to greater reduction of sensible flux, but increases more in the early afternoon due to more atmospheric solar heating.