Mesoscale modeling of smoke transport over the Southeast Asian Maritime Continent: coupling of smoke direct radiative effect below and above the low-level clouds
Abstract. The online-coupled Weather Research and Forecasting model with Chemistry (WRF-Chem) is used to simulate the direct and semi-direct radiative impacts of smoke particles over the Southeast Asian Maritime Continent (MC, 10° S–10° N, 90–150° E) during October 2006 when a significant El Niño event caused the highest biomass burning activity since 1997. With the use of an OC (organic carbon) / BC (black carbon) ratio of 10 in the smoke emission inventory, the baseline simulation shows that the clouds can reverse the negative smoke forcing in cloud-free conditions to a positive value. The net absorption of the atmosphere is largely enhanced when smoke resides above a cloud. This led to a warming effect at the top of the atmosphere (TOA) with a domain and monthly average forcing value of ~ 20 W m−2 over the islands of Borneo and Sumatra. Smoke-induced monthly average daytime heating (0.3 K) is largely confined above the low-level clouds, and results in a local convergence over the smoke source region. This heating-induced convergence transports more smoke particles above the planetary boundary layer height (PBLH), hence rendering a positive effect. This positive effect contrasts with a decrease in the cloud fraction resulting from the combined effects of smoke heating within the cloud layer and the more stable boundary layer; the latter can be considered as a negative effect in which a decrease of the cloud fraction weakens the heating by smoke particles above the clouds. During the nighttime, the elevated smoke layer lying above the clouds in the daytime is decoupled from the boundary layer, and the enhanced downdraft and shallower boundary layer lead to the accumulation of smoke particles near the surface. Because of monthly smoke radiative extinction, the amount of solar input at the surface is reduced by as much as 60 W m−2, which leads to a decrease in sensible heat, latent heat, 2 m air temperature, and PBLH by a maximum of 20 W m−2, 20 W m−2, 1 K, and 120 m, respectively. During daytime, the cloud changes over continents mostly occur over the islands of Sumatra and Borneo where the low-level cloud fraction decreases more than 10%. However, the change of local wind, including sea breeze, induced by the smoke direct radiative effect leads to more convergence over the Karimata Strait and the south coastal area of Kalimantan during both daytime and nighttime; consequently, the cloud fraction there is increased up to 20%. The sensitivities with different OC / BC ratios show the importance of the smoke single-scattering albedo for the smoke semi-direct effects. Lastly, a conceptual model is used to summarize the responses of clouds, smoke, temperature, and water vapor fields to the coupling of smoke direct effect below and above clouds over the Southeast Asian Maritime Continent.