Cloud Optical Depth and Cloud Solar Radiative Effects Computed at São Paulo, Brazil

At Metropolitan Area of São Paulo, effective cloud optical depth (ECOD) every 1 min of cirrus and low clouds was retrieved from total transmittance (T) at 415 nm channel of a Multi-filter Rotating Shadow band Radiometer (MFRSR) in the time period from June of 2012 up to September 2017. ECOD is computed from lookup tables designed for every cloud type whose inputs are: T, cosine of solar zenith angle (csza), season defining the aerosol vertical profile, and aerosol optical depth (AOD) at 415 nm. A 1D Radiative Transfer Model (RTM), the LibRadtran, was employed and cloud property inputs were selected from synergy products of CALIPSO-CloudSat. Cloud types were defined by the coincidence of three criteria: visual observation, the ratio of spectral diffuse irradiance between 870 nm and 415 nm channels and direct solar radiation blocked by clouds. Besides, from September 2016 visual observations by a sky-camera were included. ECOD values have shown a seasonal variability, with the smallest values for low clouds in winter with median of 21.2 and maximum in spring with 28.6. For cirrus clouds the minimum was observed in autumn with median of 2.21 and maximum in spring with 3.11. In addition, for low clouds (cirrus) near sunrise the ECOD values were smaller with median of 18.9 (1.94) compared to afternoon, with 28.3 (3.21). Values of ECOD for cirrus clouds can reach 10 in few cases. Cloud effects on solar radiation at surface were computed by the normalized shortwave cloud radiative effect (NCRE) where surface albedo is neglected and it is less sensitive to csza. Solar global irradiance at surface under cloudy conditions were measured by pyranometer and clear sky irradiance was computed by the RTM. Cloud radiative effects depend on conditions of solar disk, cloud type and cloud cover. When solar disk is blocked by clouds, low cloud and total overcast presented a median of NCRE of -0.72, but when the sky is not total overcast the median was -0.68. For mid and high clouds the medians of NCRE were -0.57 and -0.33, respectively. An enhancement effect was observed when the solar disk was clear for low (0.1), mid (0.12) and high clouds (0.07), with extreme values for low clouds (above 0.4). This enhancement effect can last about ten minutes.