Assessment of shipboard measurements and space-based estimates of the surface radiation budget for the northeastern Pacific

Clouds and the Earth's Radiant Energy System (CERES) uses a suite of instruments on the Terra and Aqua satellites combined with analyzed weather data and information on surface conditions to estimate surface radiative fluxes. These estimates are compared with measurements of the surface radiation budget collected with the RV Wecoma's radiometers for cruises in the Northeastern Pacific. To assess the shipboard measurements, the radiometer observations were analyzed to identify cloud-free conditions. Cloud-free conditions were characterized by long periods ~2 - 4 hours of relatively stable values of the normalized downward shortwave radiative flux. The normalized flux is the flux divided by the cosine of the solar zenith angle and is thus a measure of the broadband transmittance of the cloud-free atmosphere. Under cloud-free conditions, variability of the normalized shortwave flux was less than 1 % for 15-minute periods of 1-minute observations. The longwave flux was likewise steady, but the longwave flux was less sensitive to the presence of broken clouds than was the normalized shortwave flux. Observations for cloud-free conditions were compared with those calculated using broadband radiative transfer models. Profiles of temperature and humidity for these calculations were obtained from analyzed meteorological fields associated with the radiative flux measurements. In addition, for summertime conditions in the Northeastern Pacific, a 0.55-micron aerosol optical depth of 0.1 reduces the noontime normalized shortwave flux by 15 Watts per meter squared, or by approximately 1.5%. Aerosol optical depths inferred from the comparisons typically fell within the range of 0.1-0.3, and once the optical depth was set, modeled and observed values of the normalized downward shortwave flux agreed to within 1-3%. The downward longwave flux under cloud-free conditions was within 1-2%. These fluxes were within the accuracies claimed for the radiometers. For all sky conditions, the absolute magnitude of the mean difference between the shipboard measurements and the CERES CRS estimates of the normalized downward shortwave flux was 2% with an RMS difference of 5%. The absolute magnitude of the mean difference for the downward longwave flux at the surface was less than 2% with an RMS difference of order 3%.