The objectives of the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE) are to directly measure clear and cloudy sky shortwave atmospheric absorption and to quantify any absorption found in excess of model predictions. We undertake detailed model comparisons to near-infrared and total solar flux time series observed by surface and airborne radiometric instruments during the ARESE campaign. Model clear-sky absorption biases generally fall within the range of uncertainty generated in sample size, and assumptions of aerosol properties and surface albedo. Direct measurements by stacked aircraft on the overcast day of October 30, 1995, confirm the detection of enhanced cloud shortwave absorption during ARESE. The detection is substantiated by, and consistent with, three independent measures of cloudy sky absorption estimated in previous studies: cloud forcing ratio, insolation forcing ratio, and albedo/transmission slope. A significant portion of the enhanced absorption occurs at visible wavelengths. Collocated measures of liquid water path (LWP) suggest the magnitude of the enhanced absorption increases with LWP.

As part of the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE), we have obtained and analyzed measurements made from collocated aircraft of the absorption of solar radiation within the atmospheric column between the two aircraft. The measurements were taken during October 1995 at the ARM site in Oklahoma. Relative to a theoretical radiative transfer model, we find no evidence for excess solar absorption in the clear sky atmosphere and significant evidence for its existence in the cloudy atmosphere. This excess cloud solar absorption appears to occur in both visible (0.224-0.68 micron) and near-infrared (0.68-3.30 micron) spectral regions, although not at 0.5 microns for the visible contribution, and it is shown to be true absorption rather than an artifact of sampling errors caused by measuring three-dimensional clouds.

Atmospheric Radiation Measurements Enhanced Shortwave Experiment (ARESE) was conducted to study the magnitude and spectral characteristics of the absorption of solar radiation by the clear and cloudy atmosphere. Three aircraft platforms. a Grob Egrett, a NASA ER-2, and a Twin Otter, were used during ARESE in conjunction with the Atmospheric Radiation Measurements (ARM) central and extended facilities in north central Oklahoma. The aircraft were coordinated to simultaneously measure solar irradiances in the total spectral broadband (0.224-3.91 micron), near-infrared broadband (0.678-3.3 micron), and in seven narrow bandpass (approximately 10 nm width) channels centered at 0.500, 0.862, 1.064, 1.249, 1.501, 1.651, and 1.750 microns. Instrument calibration issues are discussed in some detail, in particular radiometric power, angular, and spectral responses. The data discussed in this paper are available at the ARM ARESE data archive via anonymous FTP to ftp.arm.gov.

Measurements of downward surface solar radiation (global radiation) and albedo taken during the Central Equatorial Pacific Experiment (CEPEX) are used to obtain baseline estimates for two quantities concerning the radiation budget of the tropical oceans: 1) surface absorption of solar radiation in the central equatorial Pacific under cloud-free conditions, and 2) the corresponding absorption by the atmosphere. These values are then compared to two state-of-the-art radiative transfer models to determine if the models are accurately partitioning solar absorption between the atmosphere and the ocean.

The paper develops an independent approach to obtain a clear-sky signal from 10-s resolution surface pyranometer data that is in excellent agreement with upper envelope methods. Over a diurnal average, the ocean absorbs 70.9% +/- 1.3% of the solar radiation incident the top of the atmosphere (TOA). The data, measured from ship and low-flying aircraft platforms, also yield the zenith angle dependence of the surface absorption. The clear-sky data are representative of dry regions east of the date line during March 1993.

Likewise, a combination of tropopause albedo measurements from the ER-2 aircraft and Earth Radiation Budget Experiment (ERBE) clear-sky TOA albedos are used to find the absorption of solar radiation by the atmosphere (integrated from the surface to the TOA). Clear-sky TOA albedo is computed from the ER-2 tropopause measurements using a radiative transfer model and measurements of stratospheric aerosol and ozone. The computed TOA albedos agree with ERBE at about 6% for overhead sun. The diurnal average fractional atmospheric column absorption is 20.0% +/- 1.6%.

Two multispectral radiation models agree to within 5 W/m2 of the observed daily average clear-sky oceanic solar absorption when the atmospheric profile is constrained by measurements and the observed TOA albedo is used as a boundary condition.

Airborne radiometric measurements were used to determine tropospheric profiles of the clear sky greenhouse effect. At sea surface temperatures (SSTs) larger than 300 Kelvin, the clear sky water vapor greenhouse effect was found to increase with SST at a rate of 13 to 15 watts per square meter per Kelvin. Satellite measurements of infrared radiances and SSTs indicate that almost 52 percent of the tropical oceans between 20 N and 20 S are affected during all seasons. Current general circulation models suggest that the increase in the clear sky water vapor greenhouse effect with SST may have climatic effects on a planetary scale.