Mesoscale modeling of Central American smoke transport to the United States: 2. Smoke radiative impact on regional surface energy budget and boundary layer evolution

[1] During 20 April to 21 May 2003, large amounts of smoke aerosols from Central American Biomass Burning (CABB) fires were transported to southeastern United States. Using a coupled aerosol, radiation, and meteorology model built upon the heritage of the Regional Atmospheric Modeling System ( RAMS) with new capabilities called the Assimilation and Radiation Online Modeling of Aerosols ( AROMA), this paper, the second of a two-part series, investigates smoke radiative impact on the regional surface energy budget, temperature and relevant boundary layer processes. Comparisons with limited ground-based observations and MODIS aerosol optical thickness (AOT) showed that model consistently simulated the smoke AOT and smoke radiative impacts on the 2 m air temperature (2mT) and downward shortwave irradiance (DSWI). Over 30 days the 24-hour mean smoke AOT was 0.18 ( at 0.55 mu m) near the smoke source region (Yucatan Peninsula and southern Mexico), and 0.09 in downwind region ( e. g., southern Texas), both showing a diurnal variation of 24%. Maximum AOT occurred during late afternoon and minimum during early morning in smoke source region. The smoke radiative effects were dominant mostly during the daytime and resulted in the decrease of DSWI, sensible heat and latent heat by 22.5 W m(-2), 6.2 W m(-2), and 6.2 W m(-2), respectively, near the source region, in contrast to 15.8 Wm(-2), 4.7 Wm(-2), and 7.9 Wm(-2), respectively, in downwind regions. Both maximum and minimum 2mT were decreased, and the overall diurnal temperature range (DTR) was reduced by 0.31 degrees C and 0.26 degrees C in the smoke source and downwind regions, respectively. The smoke absorption of solar radiation increased the lapse rate by 0.1 - 0.5 K/day in the planetary boundary layer (PBL), thus warming the air over the ocean surface. However, over the land surface where the coupling between the lower PBL and the cooler land surface is strong, such warming only occurred in the upper PBL and is amendable to the diurnal variation of smoke emission. The simulation numerically verifies the smoke self-trapping feedback mechanism proposed by Robock ( 1988), where the increase of the atmospheric stability in the PBL caused by the smoke radiative effects further traps more smoke aerosols in the lower PBL. Such feedbacks, when coupled with favorable synoptic systems, may have important implications for air quality modeling and hydrological processes.