Abstract
Abstract. We conduct several sets of simulations with a version of NASA's Goddard Earth Observing System, version 5, (GEOS-5) Atmospheric Global Climate Model (AGCM) equipped with a two-moment cloud microphysical scheme to understand the role of biomass burning aerosol (BBA) emissions in Southeast Asia (SEA) in the pre-monsoon period of February–May. Our experiments are designed so that both direct and indirect aerosol effects can be evaluated. For climatologically prescribed monthly sea surface temperatures, we conduct sets of model integrations with and without biomass burning emissions in the area of peak burning activity, and with direct aerosol radiative effects either active or inactive. Taking appropriate differences between AGCM experiment sets, we find that BBA affects liquid clouds in statistically significantly ways, increasing cloud droplet number concentrations, decreasing droplet effective radii (i.e., a classic aerosol indirect effect), and locally suppressing precipitation due to a deceleration of the autoconversion process, with the latter effect apparently also leading to cloud condensate increases. Geographical re-arrangements of precipitation patterns, with precipitation increases downwind of aerosol sources are also seen, most likely because of advection of weakly precipitating cloud fields. Somewhat unexpectedly, the change in cloud radiative effect (cloud forcing) at surface is in the direction of lesser cooling because of decreases in cloud fraction. Overall, however, because of direct radiative effect contributions, aerosols exert a net negative forcing at both the top of the atmosphere and, perhaps most importantly, the surface, where decreased evaporation triggers feedbacks that further reduce precipitation. Invoking the approximation that direct and indirect aerosol effects are additive, we estimate that the overall precipitation reduction is about 40% due to the direct effects of absorbing aerosols, which stabilize the atmosphere and reduce surface latent heat fluxes via cooler land surface temperatures. Further refinements of our two-moment cloud microphysics scheme are needed for a more complete examination of the role of aerosol–convection interactions in the seasonal development of the SEA monsoon.
Highlights
Use of fossil fuels for ever-growing energy demands, in developing countries, has led to increased concentrations of aerosol-laden combustion by-products, especially in the planetary boundary layer (PBL) (Roelofs, 2013). Moorthy et al (2013) estimate that aerosols over India have been increasing at the rate of 2–4 % per year over the last three decades, resulting in doubled aerosol optical depth (AOD) in India’s lower atmosphere
When Biomass burning (BB) is high in FMA over inland areas of Southeast Asia (SEA) compared to normal days, anomalously high AOD appears over the northern part of SEA up to the coast of southern China
This is a classic manifestation of aerosol indirect effect whereby increased Biomass burning aerosol (BBA) reduces the size of cloud droplets by increasing cloud condensation nuclei (CCN) number concentration
Summary
Use of fossil fuels for ever-growing energy demands, in developing countries, has led to increased concentrations of aerosol-laden combustion by-products, especially in the planetary boundary layer (PBL) (Roelofs, 2013). Moorthy et al (2013) estimate that aerosols over India have been increasing at the rate of 2–4 % per year over the last three decades, resulting in doubled aerosol optical depth (AOD) in India’s lower atmosphere. Constrained model simulations are one plausible way to better distinguish between the roles of direct and indirect effects and their interactive influences that depend on circulation, cloud types, and aerosoldependent cloud microphysics While in principle these effects can be properly simulated only with a coupled ocean– atmosphere model, as a first step we use an AGCM with prescribed monthly SSTs and with aerosol emission anomalies prescribed from the Quick Fire Emissions Dataset (QFED) data set This way we can isolate the influence of BBA over land by comparative assessments of circulation and rainfall changes in neighboring regions In this endeavor, we perform a comprehensive model simulation study with the physically interactive aerosol–cloud– radiation treatment of McRAS-AC as implemented in the.
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