NATURE GEOSCIENCE | VOL 8 | APRIL 2015 | www.nature.com/naturegeoscience 261 C louds stimulate the human spirit. Although they have been recognized for centuries as harbingers of weather, only in recent decades have scientists begun to appreciate the role of clouds in determining the general circulation of the atmosphere and its susceptibility to change.Forming mostly in the updrafts of the turbulent and chaotic airflow, clouds embody the complex and multiscale organization of the atmosphere into dynamical entities, or storms. These entities mediate the radiative transfer of energy, distribute precipitation and are often associated with extreme winds. It has long been recognized that the water and heat transfer that clouds mediate plays a fundamental role in tropical circulations, and there is increasing evidence that they also influence extratropical circulations 1 . Globally, the impact of clouds on Earth's radiation budget -and hence surface temperatures -also depends critically on how clouds interact with one another and with larger-scale circulations 2 . Far from being passive tracers of a turbulent atmosphere, clouds thus embody processes that can actively control circulation and climate (Box 1).For practical reasons, early endeavours to understand climate deployed a 'divide and conquer' strategy in which efforts to understand clouds and convective processes developed separately from efforts to understand larger-scale circulations. Over time, a gap developed between the subdisciplines. But technological progress and conceptual advances have tremendously increased our capacity to observe and simulate the climate system, such that it is now possible to study more readily how small-scale convective processes -that is, clouds -couple to large-scale circulations (Box 2). Much as a new accelerator allows physicists to explore the implication of the interactions among forces acting over different length scales, these new capabilities are transforming how atmospheric scientists think about the interplay of clouds and climate. This offers a great opportunity not only to close the gap between scientific communities, but Fundamental puzzles of climate science remain unsolved because of our limited understanding of how clouds, circulation and climate interact. One example is our inability to provide robust assessments of future global and regional climate changes. However, ongoing advances in our capacity to observe, simulate and conceptualize the climate system now make it possible to fill gaps in our knowledge. We argue that progress can be accelerated by focusing research on a handful of important scientific questions that have become tractable as a result of recent advances. We propose four such questions below; they involve understanding the role of cloud feedbacks and convective organization in climate, and the factors that control the position, the strength and the variability of the tropical rain belts and the extratropical storm tracks.also to answer some of the most pressing questions about the fate of our pl...

Atmospheric fronts in the lower troposphere often mark regions of significant weather and serve as an important conceptual tool that is frequently used and well understood by the public. This study uses an objective method to identify fronts in the ERA‐40 reanalysis and to compile a quantitative global climatology of their occurrence. The climatology confirms the mid‐latitude storm tracks as the regions of highest front frequency and identifies slow moving frontal boundaries in the subtropics corresponding to major climate features, such as the Mei‐Yu front. The climatology also shows fronts to be common features of the South Pacific convergence zone. In the northern hemisphere fronts shift poleward in summer. In the southern hemisphere this and other aspects of seasonal behavior are rather different.

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An analysis approach that uses the patterns of cloud property joint distributions at mesoscale (cloud type mixtures) from the International Satellite Cloud Climatology Project to identify distinct weather states of the tropical atmosphere is extended to the whole tropics covering the period 1983–2004. These patterns can be used as the basis for multi‐scale, multi‐variate compositing of other observations to understand how tropical cloud systems affect the atmospheric diabatic heating and interact with the large scale circulation. We illustrate how variations of the tropical climate on longer time scales can be described in terms of the changes in the frequency of occurrence of these weather states with their associated multi‐variate relationships.

[1] Atmospheric fronts are important for the day-to-day variability of weather in the midlatitudes, particularly during winter when extratropical storm-tracks are at their maximum intensity. Fronts are often associated with heavy rain, and strongly affect the local space-time distribution of rainfall. A recently developed objective front identification method that distinguishes between cold, warm and quasi-stationary fronts, is applied to reanalysis data and combined with a daily global gridded data set to investigate how precipitation around the globe is associated with atmospheric fronts. A large proportion (up to 90%) of rainfall in the major stormtrack regions is associated with fronts, particularly cold and warm fronts. Precipitation over the oceanic storm-tracks is mostly associated with cold fronts, while over the Northern Hemisphere continents precipitation is mainly associated with warm fronts. There are seasonal and regional variations in the proportion of precipitation associated with fronts.