Organization of Cumulus Convection over (sub)tropical oceans

Abstract

Climate on Earth is changing rapidly. In order to take the right measures it is important to know with which magnitude and rate the warming of the climate continues in the future. However, climate models currently predict different scenarios. The differing in response of shallow cumulus clouds to a warming climate explains the largest part of the spread of climate sensitivity in these models. It is known that clustered clouds have a different climate feedback than randomly distributed clouds and that we can expect more clustered cloud fields in a warmer climate. It is however unknown how these clouds respond and what the exact difference in climate feedback is. Future research must provide us with more insights and eventually a better understanding of the response of shallow cumulus clouds to warming. However, in order to investigate this an objective measure of the degree of cloud organization is necessary, something that is currently lacking. In this thesis a dimensionless combined organization measure is introduced that can be applied to a large range of cloud field organizations on different field sizes. This is a combined measure, as it makes use of the existing organization index I_org and the size of clouds and clear sky areas in the cloud field. A different approach on calculating I_org was introduced taking into account the sizes of the clouds, resulting in an useful and more realistic values from this parameter. The results of the combined organization measure were compared to a visual inspection of 557 cloud fields with dimensions of 10x10 degree, which showed promising results. Sub-fields of 5x5 degree and 2.5x2.5 degree were also analysed, which showed that the combined organization measure could often be applied on smaller scales as well but yielded some problems when areas contained either no or little clouds or some very large ones. It was concluded that the newly developed method is an improvement of the already existing method, providing scientists with a better and more reliable index to quantify the degree of cloud organization.