University of Alaska
Fairbanks, AK 99775-7320
Cloud optical thickness is determined from ground-based measurements of broadband incoming solar irradiance using a radiation model in which the cloud optical depth is adjusted until computed irradiance agrees with the measured value. From spectral measurements it would be feasible to determine both optical thickness and mean drop size, which apart from cloud structure and morphology, are the most important climatic parameters of clouds. A radiative convective model is used to study the sensitivity of climate to cloud liquid water amount and cloud drop size. This is illustrated in Figure 21.1 which shows that for medium thick clouds a 10 % increase in drop size yields a surface warming of 1.5 ° C, which is the same as that due to a doubling of carbon dioxide. For thick clouds, a 5% decrease in drop size is sufficient to offset the warming due to doubling of carbon dioxide. A radiative transfer model for the coupled atmosphere/sea ice/ocean system is used to study the partitioning of radiative energy between the three strata, and the potential for testing such a model in terms of planned experiments in the Arctic is discussed.
The determination of cloud optical properties at the ground in the Arctic relies on the use of broadband surface albedo and solar irradiance measurements from the NOAA/CMDL station in Barrow, Alaska. The absorption of cloud drops increases with drop size, leading to decreased transmission, while the forward scattering increases, leading to enhanced transmission. The net effect is that the transmittance is insensitive to drop size. But the cloud optical depth can be determined by simply comparing the measured irradiance with radiative transfer computations in which the measured surface albedo is used. Then the cloud optical depth is adjusted until the computed irradiance agrees with the measured one. The seasonal variation in cloud optical thickness at Barrow, Alaska derived using this approach for the period April 1988 through August 1988 is shown in Figure 21.2. The potential for deriving optical depth from narrowband measurements and mean drop size from bispectral transmittance measurements is explored in terms of the channels available in the Multi-Filter Shadowband Radiometer (MFRSR) deployed in the ARM program. The optical depth can be reliably inferred from the 862 nm channel (which is less influenced by atmospheric aerosols than channels at shorter wavelengths), while the mean size could be determined from a combination of measurements in the 862 nm channel and a channel centered at 2.2 microns. The latter channel is currently not available, but would be a valuable addition to narrowband instruments such as the MFRSR.
A radiative convective model with accurate treatment of radiative transfer including clouds is used to study the climate sensitivity to changes in mean drop size and optical thickness. The cloud optical properties are parameterized in terms of cloud liquid water content and equivalent radius throughout the solar and infrared portion of the spectrum. It is found that the infrared properties of clouds are sensitive to cloud scattering, which implies that clouds should not be treated as black bodies in climate models (Figures 21.3 and 21.4).
A radiative transfer model for the coupled atmosphere/sea ice/ocean system is used to study the disposition of solar energy throughout the system. The effects of clouds, snow on ice, and sea ice properties and thickness are quantified. The potential for testing this model in terms of planned experiments in the Arctic in the near future are briefly discussed. These experiments include the North Slope of Alaska site to be established through DOE's Atmospheric Radiation Measurements (ARM) Program, the Surface Heat and Energy Budget of the Arctic Ocean (SHEBA) experiment led by NSF and ONR, as well as the FIRE Phase III experiment, which is an interagency experiment led by NASA. It is concluded that the opportunities for all three efforts to benefit from close collaboration are great, but that the challenges in experimental design are equally great. In spite of these challenges, even broader interagency and international participation would be helpful.
References
Leontieva, E. N., and K. Stamnes, Estimations of cloud optical properties from ground-based measurements of incoming solar radiation in the Arctic, J. Climate, 7, 566-578, 1994.
Hu, Y.-X., and K. Stamnes, An accurate parameterization of the radiative properties of water clouds suitable for use in climate models, J. Climate, 6, 728-742, 1993.
Hu, Y.-X., A Study of the Link between Cloud Microphysics and Climate Change, Ph.D. thesis, University of Alaska, 1994.
