Evaluating a Constrained Cloud Response to Varying Atmospheric Climate and Chemical States

University Of Utah, Salt Lake City UT

Investigators

Abstract

Clouds are studied intensively due to their potential contribution to climate change, as changes in cloud amount or properties due to greenhouse warming could either offset or intensify the warming. The study of cloud change caused by climate change and its subsequent contribution to climate change has been productive but not conclusive, and future climate projections show a wide spread in cloud changes and their subsequent effects. Given the large uncertainty as to how clouds will change it would be helpful if there were some bulk cloud properties which can be counted on to not change, or in other words cloud properties which are invariant under climate change. In earlier work the Principal Investigators (PIs) of this award argued that the cloud size distribution could be just such an invariant, where cloud size distribution means the average number of clouds that occur for a particular cloud size, as measured by cloud area or cloud perimeter. They developed a theory for the cloud size distribution using thermodynamic arguments in which the cloud edge plays a special role as a surface of neutral energy through which ambient air of lower energy mixes with cloud air of higher energy, thereby dissipating the energy generated by solar heating (see AGS-2022941). The theory predicts that the number of clouds of a given size is inversely proportional to a power of the size, where alpha and beta are the powers for size measured by area and perimeter, respectively. The theory also depends on D, the fractal dimension of clouds, making for three geometrical parameters, as well as the square root of the atmospheric stability. Cloud data from satellites confirms the power law relationships but gives values of alpha and beta which differ slightly from theory, in particular the theory predicts beta=1 but cloud data shows beta=1.26. Work performed under this award seeks to further develop the theory and determine the extent to which it lives up to the promise of invariance to climate state. One issue to be addressed is the cause of the discrepancy between observed and predicted parameter values, in particular whether it is an artifact of satellite viewing angle or a real physical effect. The invariance of the three parameters is tested under a variety of climatic conditions including the warm pool region of the western tropical Pacific and the cooler eastern tropiical Pacific, and the contrasting conditions of low and middle latitudes. A further issue to be addressed is the extent to which the invariance is affected by atmospheric composition, as chemical compounds like nitrous oxide can affect cloud properties by generating aerosols. The project also seeks to derive minimal models which correctly predict all three cloud parameters and also account for general characteristics of cloud shape and spacing. The work is of societal as well as scientific interest given concerns over climate change and the importance of clouds for determining the amount of warming produced by a given increase in greenhouse gas concentrations. The work also builds international research capacity as the PIs collaborate with colleagues at the University of Lille who are developing satellite data products based on geostationary satellite observations. The project also supports a graduate student and a postdoctoral research fellow, thereby providing for the future workforce in this research area. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.