Mesoscale modeling of smoke transport over the Southeast Asian Maritime Continent: coupling of smoke direct radiative effect below and above the low-level clouds

摘要

The online-coupled Weather Research and Forecasting model with Chemistry(WRF-Chem) is used to simulate the direct and semi-direct radiative impactsof smoke particles over the Southeast Asian Maritime Continent (MC,10° S–10° N, 90–150° E) during October 2006 whena significant El Niño event caused the highest biomass burning activitysince 1997. With the use of an OC (organic carbon) / BC (black carbon)ratio of 10 in the smoke emission inventory, the baseline simulation showsthat the clouds can reverse the negative smoke forcing in cloud-freeconditions to a positive value. The net absorption of the atmosphere islargely enhanced when smoke resides above a cloud. This led to a warmingeffect at the top of the atmosphere (TOA) with a domain and monthly averageforcing value of ~ 20 W m over the islands of Borneo andSumatra. Smoke-induced monthly average daytime heating (0.3 K) is largelyconfined above the low-level clouds, and results in a local convergence overthe smoke source region. This heating-induced convergence transports moresmoke particles above the planetary boundary layer height (PBLH), hencerendering a positive effect. This positive effect contrasts with a decreasein the cloud fraction resulting from the combined effects of smoke heatingwithin the cloud layer and the more stable boundary layer; the latter can beconsidered as a negative effect in which a decrease of the cloud fractionweakens the heating by smoke particles above the clouds. During thenighttime, the elevated smoke layer lying above the clouds in the daytime isdecoupled from the boundary layer, and the enhanced downdraft and shallowerboundary layer lead to the accumulation of smoke particles near the surface.Because of monthly smoke radiative extinction, the amount of solar input atthe surface is reduced by as much as 60 W m, which leads to adecrease in sensible heat, latent heat, 2 m air temperature, and PBLH by amaximum of 20 W m, 20 W m, 1 K, and 120 m, respectively.During daytime, the cloud changes over continents mostly occur over theislands of Sumatra and Borneo where the low-level cloud fraction decreasesmore than 10%. However, the change of local wind, including sea breeze,induced by the smoke direct radiative effect leads to more convergence overthe Karimata Strait and the south coastal area of Kalimantan during bothdaytime and nighttime; consequently, the cloud fraction there is increased upto 20%. The sensitivities with different OC / BC ratios show theimportance of the smoke single-scattering albedo for the smoke semi-directeffects. Lastly, a conceptual model is used to summarize the responses ofclouds, smoke, temperature, and water vapor fields to the coupling of smokedirect effect below and above clouds over the Southeast Asian MaritimeContinent.