The heating of the ocean surface by longwave radiation from convective clouds has been estimated using measurements from the Central Equatorial Pacific Experiment (CEPEX). The ratio of the surface longwave cloud forcing to the cloud radiative forcing on the total atmospheric column is parameterized by the f factor. The f factor is a measure of the partitioning of the cloud radiative effect between the surface and the troposphere. Estimates of the f factor have been obtained by combining simultaneous observations from ship, aircraft, and satellite instruments. The cloud forcing near the ocean surface is determined from radiometers on board the National Oceanic and Atmospheric Administration P-3 aircraft and the R/V John Vickers. The longwave cloud forcing at the top of the atmosphere has been estimated from data obtained from the Japanese Geostationary Meteorological Satellite GMS 4. A new method for estimating longwave fluxes from satellite narrowband radiances is described. The method is based upon calibrating the satellite radiances against narrowband and broadband infrared measurements from the high-altitude NASA ER-2 aircraft. The average value of f derived from the surface and satellite observations of convective clouds is 0.15 +/- 0.02. The area-mean top-of-atmosphere longwave forcing by convective clouds in the region 10 S - 10 N, 160 E - 160 W is 40 W/m2 during CEPEX. These results indicate that the surface longwave forcing by convective clouds was appoximately 5 W/m2 in the central equatorial Pacific and that this forcing is the smallest radiative component of the surface energy budget.

In the fall of 1995 the Atmospheric Radiation Measurement Enhanced Shortwave Experiment (ARESE) was conducted to address fundamental questions about the amount of absorption of solar radiation occurring in clear and cloudy skies. Irradiances measured from instruments flown on stacked aircraft as part of this experiment are compared to modeled irradiances for several representative days. As a basic check of the data and the model, the downwelling irradiances at 13 km are examined and found to be in good agreement, with both model and observations showing day-to-day variations as well as shorter timescale variations due to changing atmospheric conditions. Evidence of cloud-induced enhanced absorption is seen from an analysis of column reflectance and absorptance as determined from simultaneous measurements made from the stacked aircraft. When compared to model results, profiles of irradiance versus altitude as measured by one aircraft indicate cloud-induced enhanced absorption as well.

Data sets acquired during the Atmospheric Radiation Measurement Enhanced Shortwave Experiment (ARESE) using simultaneous measurements from five independent platforms (GOES 8 geostationary satellite, ER-2, Egrett and Twin Otter aircraft, and surface) are analyzed and compared. A consistent data set can be built for selected days during ARESE on the basis of the observations from these platforms. The GOES 8 albedoes agree with the ER-2, Egrett, and Twin Otter measured instantaneous albedos within 0.013 +/- 0.016, 0.018 +/- 0.032, and 0.006 +/- 0.011, respectively. It is found that for heavy overcast conditions the aircraft measurements yield an absorptance of 0.32 +/- 0.03 for the layer between the aircraft (0.5-13 km), while the GOES 8 albedo versus surface transmittance analysis gives an absorptance of 0.33 +/- 0.04 for the total atmosphere (surface to top). The absorptance of solar radiation estimated by model calculations for overcast conditions varies between 0.16 and 0.24, depending on the model used and on cloud and aerosol implementation. These results are in general agreement with recent findings for cloudy skies, but here a data set that brings together independent simultaneous observations (satellite, surface, and aircraft) is used. Previous ARESE results are reexamined in light of the new findings, and it is concluded that the overcast absorptance in the 0.224-0.68 micron spectral region ranges between 0.04 +/- 0.06 and 0.08 +/- 0.06, depending on the particular case analyzed. No evidence of excess clear-sky absorption beyond model and experimental errors is found.

Pyranometers are reliable, economical radiometers commonly used to measure solar irradiances at the surface in a long term, monitoring mode. This paper presents a discussion of the response of these instruments to varying environmental conditions, including the magnitude and variability of the irradiance being measured. It is found that different conditions, commonly occuring in field experiments, affect the thermal balance and temperature gradients within the instrument in a variety of ways. Such effect results in variable offset systematic errors whose origin and magnitude is investigated in laboratory and field experiments. It is shown that these offset errors are proportional to the difference between the fourth power of the dome and detector temperatures, following closely the Stefan-Boltzman radiation law. Results of field experiments are presented for daytime and nighttime operation over a variety of atmospheric conditions ranging from clear to heavy overcast and rain. All measurements took place from May through October 1998 in La Jolla, California at the Scripps Institution of Oceanography. Laboratory experiments are used to quantify the magnitude of the thermal offset errors under controlled conditions and to calibrate them as a function of thermal gradients between the dome and detector. The quality of the data resulting from pyranometer measurements can be improved in a significant way by proper knowledge of the thermal parameters affecting the operation of the thermopile system. To that end, a data correction algorithm that requires an extensive thermal calibration procedure and a simple modification of the instrument is proposed. Such an algorithm needs to be applied to the power calibration procedure as well to the retrieval of data acquired during normal field operations. The experimental results presented in this paper could potentially affect analyses based on surface insolation.