Downward longwave radiative fluxes in an urban tropical atmosphere

In order to improve the available knowledge about longwave radiative transfer under urban tropical conditions, measurements of atmospheric irradiance reaching the ground at São Paulo (23.5 S, 46.6 W) for cloudless periods are compared with respective radiative transfer computations. Measurements are performed with two artificially-ventilated pyrgeometers, which were previously calibrated according to different procedures (basically, taking in account the influence of instrument temperatures, or not). Computations are performed through an updated version of a line-by-line radiative transfer model, by employing temperature and humidity profiles provided by meteorological soundings near to the pyrgeometers site.

Atmospheric irradiance measurements may be underestimated (up to 12 W/m2 or 5 %) if the influence of instrument temperatures is neglected, even with the artificial ventilation. Preliminary comparisons show that the agreement between measurements and computations increase (a) when a spectral range as broader as possible is taken into account, (b) when the lower atmosphere becomes cooler, and (c) when the integrated water vapor content in lower atmosphere becomes smaller.

Such an investigation is being conducted in the scope of a large research program devoted to the reciprocal influences between air pollution and meteorology at São Paulo. A better understanding of longwave radiative transfer in urban atmospheres could contribute to numerical forecasts of the lower troposphere cooling during the night, and therefore to estimates of atmospheric dispersion conditions.