Diagnosis of cloud amount increase from an analogue model of a "warming -world

Abstract

"Warming world" analogue model diagnosis of total cloud amount trends is reviewed. Using cloud amount records for the continental U. S. A. (published), Canada, including parts of the Arctic (in preparation), Europe (published) and the Indian sub-continent (new results), cloudiness changes have been analyzed in the context of the analogue model which compares records of two contrasting twenty year periods. Cloud amount is found to increase over practically the entire U. S. A., Canada, most of the Indian sub-continent, and parts of Europe in all seasons. These results have been derived for a wide range of climates and considerably strengthen the more tentative findings of Henderson-Sellers (l986a, b) and MeGuffie and Henderson-Sellers (1987) that total cloud amount increases in a warming world. On the other hand, the record of total cloud amount since the 1900s has suffered from changes in observing and reporting practice from differing emphasis on observer training and from time sampling biases. These aspects of the record are considered here in detail. Moreover it must be recognized that the historical record reviewed here is land-based only, contained within the northern hemisphere and excludes many areas especially the tropics and equatorial regions. The results achieved so far could indicate that the current real-world transient experiment in which CO₂ and temperatures are increasing includes a negative feedback on increasing temperatures due to increasing cloud amount. However the very restricted area considered also means that the apparent trend may be much less than global. Specifically results are not inconsistent with numerical model predictions of storm tracks shifted poleward in doubled CO₂ experiments. At the least, the predictions of cloud changes made by numerical models could be re-examined in the light of the results described here.