Downward solar global irradiance at the surface in São Paulo city—The climatological effects of aerosol and clouds

• Published in: Journal of geophysical research. Atmospheres 2017-01, Vol.122 (1), p.391-404
• Main Authors: Yamasoe, M. A., Rosário, N. M. E., Barros, K. M.
• Format: Article
• Language: English
• Subjects: aerosol; Aerosols; Aircraft components; Atmospheric aerosols; Attenuation; Biomass; Biomass burning; Burning; Climate; Climatology; Cloud height; Cloud modification; Clouds; Combustion; Dry season; Drying; Fires; Geophysics; Irradiance; Irradiation; Low clouds; Mathematical analysis; Megacities; Meteorological satellites; Meteorology; Ozone; Pollution effects; Radiation; radiative effect; Radiative transfer; River basins; Smoke; Solar irradiance; Solar radiation; Spring; Spring (season); Transmittance; Transport; urban air pollution; Variability; Water pollution; Water vapor; Water vapour; Winter
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1002/2016JD025585

We analyzed the variability of downward solar irradiance reaching the surface at São Paulo city, Brazil, and estimated the climatological aerosol and cloud radiative effects. Eleven years of irradiance were analyzed, from 2005 to 2015. To distinguish the aerosol from the cloud effect, the radiative transfer code LibRadtran was used to calculate downward solar irradiance. Two runs were performed, one considering only ozone and water vapor daily variability, with AOD set to zero and the second allowing the three variables to change, according to mean climatological values. The difference of the 24 h mean irradiance calculated with and without aerosol resulted in the shortwave aerosol direct radiative effect, while the difference between the measured and calculated, including the aerosol, represented the cloud effect. Results showed that, climatologically, clouds can be 4 times more effective than aerosols. The cloud shortwave radiative effect presented a maximum reduction of about −170 W m−2 in January and a minimum in July, of −37 W m−2. The aerosol direct radiative effect was maximum in spring, when the transport of smoke from the Amazon and central parts of South America is frequent toward São Paulo. Around mid‐September, the 24 h radiative effect due to aerosol only was estimated to be −50 W m−2. Throughout the rest of the year, the mean aerosol effect was around −20 W m−2 and was attributed to local urban sources. The effect of the cloud fraction on the cloud modification factor, defined as the ratio of all‐sky irradiation to cloudless sky irradiation, showed dependence on the cloud height. Low clouds presented the highest impact while the presence of high clouds only almost did not affect solar transmittance, even in overcast conditions. Plain Language Summary In this manuscript, we estimated the mean climatological aerosol and cloud solar radiative effect at the surface for the megacity of Sao Paulo. Clouds can be more than three times effective in attenuating solar radiation in this city, particularly at low levels. The aerosol effect is enhanced when smoke from biomass burning in the Amazon basin and central Brazil is transported towards Sao Paulo, during the dry season. Key Points For the first time, climatological aerosol and clouds SW radiative effects are estimated for the São Paulo megacity The SW effect of smoke from distant fires surpasses the local pollution effect and is equivalent to the cloud radiative effect in winter Cloud radiative eff