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- Author or Editor: S. Twomey x
- Journal of Applied Meteorology and Climatology x
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Abstract
Combustion Processes that Produce greenhouse gases also increase cloud condensation nuclei (CCN) concentrations, which in turn increase cloud droplet concentrations and thereby cloud albedo. A calculation of cloud susceptibility, defined in this work as the increase in albedo resulting from the addition of one cloud droplet per cubic centimeter (as cloud liquid water content remains constant), is made through the satellite remote sensing of cloud droplet radius and optical thickness. The remote technique uses spectral channels of the Advanced Very High Resolution Radiometer (AVHRR) instrument on board NOAA polar-orbiting satellites. Radiative transfer calculations of reflectance and effective surface and cloud emissivities are made for applicable sun and satellite viewing angles, including azimuth, at various radii and optical thickness for each AVHRR channel. Emission in channel 3 (at 3.75 µm) is removed to give the reflected solar component. These calculations are used to infer the radius and optical thickness that best match the satellite measurements. An approximation for the effect of the atmosphere on the signal received by the AVHRR is included in the analysis. Marine stratus clouds. as well as being important modifiers of climate, are cleaner than continental clouds and so likely to be of higher susceptibility. Analysis of several stratus scenes, including some containing ship tracks supports this expectation. The retrieved range of susceptibilities for all marine stratus clouds studied varied by about two orders of magnitude. This variation implies that climate studies that include possible marine stratus albedo modification from anthropogenic CCN are incomplete without accounting for existing susceptibilities.
Abstract
Combustion Processes that Produce greenhouse gases also increase cloud condensation nuclei (CCN) concentrations, which in turn increase cloud droplet concentrations and thereby cloud albedo. A calculation of cloud susceptibility, defined in this work as the increase in albedo resulting from the addition of one cloud droplet per cubic centimeter (as cloud liquid water content remains constant), is made through the satellite remote sensing of cloud droplet radius and optical thickness. The remote technique uses spectral channels of the Advanced Very High Resolution Radiometer (AVHRR) instrument on board NOAA polar-orbiting satellites. Radiative transfer calculations of reflectance and effective surface and cloud emissivities are made for applicable sun and satellite viewing angles, including azimuth, at various radii and optical thickness for each AVHRR channel. Emission in channel 3 (at 3.75 µm) is removed to give the reflected solar component. These calculations are used to infer the radius and optical thickness that best match the satellite measurements. An approximation for the effect of the atmosphere on the signal received by the AVHRR is included in the analysis. Marine stratus clouds. as well as being important modifiers of climate, are cleaner than continental clouds and so likely to be of higher susceptibility. Analysis of several stratus scenes, including some containing ship tracks supports this expectation. The retrieved range of susceptibilities for all marine stratus clouds studied varied by about two orders of magnitude. This variation implies that climate studies that include possible marine stratus albedo modification from anthropogenic CCN are incomplete without accounting for existing susceptibilities.
Abstract
Using actual rainfall records for selected areas, one can by computer simulation apply hypothetical seeding effects and then compare the results obtained and inferences drawn by individual “experimenters” who have superimposed the effects of seeding on the natural variations of rainfall in space and in time. In this way it is possible to infer which kinds of experimental design are preferable and the duration of experiment likely to give meaningful results in a given area for some prescribed increase by seeding. Some results for three different Australian areas are given. These show a marked superiority of the crossover design as compared with target-control or single-area experiments. They also suggest that in more favorable areas a meaningful result could be obtained with a four- to eight-year experiment when the average increase due to seeding was 10%, but in the less favorable (arid) areas even a 16-year experiment would not be long enough. An average 20% increase would, however, be detectable with confidence even in the least favorable area but it would require an experimental period of about 16 years.
Abstract
Using actual rainfall records for selected areas, one can by computer simulation apply hypothetical seeding effects and then compare the results obtained and inferences drawn by individual “experimenters” who have superimposed the effects of seeding on the natural variations of rainfall in space and in time. In this way it is possible to infer which kinds of experimental design are preferable and the duration of experiment likely to give meaningful results in a given area for some prescribed increase by seeding. Some results for three different Australian areas are given. These show a marked superiority of the crossover design as compared with target-control or single-area experiments. They also suggest that in more favorable areas a meaningful result could be obtained with a four- to eight-year experiment when the average increase due to seeding was 10%, but in the less favorable (arid) areas even a 16-year experiment would not be long enough. An average 20% increase would, however, be detectable with confidence even in the least favorable area but it would require an experimental period of about 16 years.
