Thursday, 10 July 2014: 2:00 PM
Essex Center/South (Westin Copley Place)
A systematic examination of the range of water and energy budget contributions made by different cloud systems requires the meaningful classification of such systems. The ingredients for doing so exist in oft-repeating patterns of co-variations of cloud vertical location and extinction that can be revealed by clustering analysis. Such an analysis ultimately leads to a few characteristic cloud regimes (CRs) or weather states that capture the main mixtures of cloud types encountered across the globe. The regimes can then be used as the basis for more detailed quantifications of cloud contribution to the average and extreme expressions of various components of the energy and water cycle. The foundational data around which this presentation is built are MODIS global cloud regimes derived from clustering analysis of 10 years (July 2002 to June 2012) of daily 1 degree MODIS (Terra and Aqua) joint histograms of cloud top pressure and cloud optical thickness. These regimes represent fundamental cloud mixtures with characteristic structures that relate to mesoscale meteorological conditions as well as global circulation features. We apply a straightforward and previously-tested compositing approach on Clouds and the Earth's Radiant Energy System (CERES) and Global Precipitation Climatology Project (GPCP) data of the same 1 degree spatial resolution as that of the MODIS regimes. The analysis reveals distinct radiative and hydrological signatures for the CRs. For example, even if some CRs have almost identical multi-year annual global means of one CRE component (SW or LW), they exhibit a diverse value for the other component. Moreover, their contribution to the total global CRE is quite dissimilar because of strong dependencies on regime frequency of occurrence. CR distinctiveness according to this criterion (resemblance of mean values not translating to similarity in contributions) carries to precipitation as well. We find that no CR exhibits positive net global TOA CRE, although two (comprising different cloud types) have near-zero values. This is broadly consistent with previous analysis and the long-known fact that clouds tend to radiatively cool the planet. A comparison between surface and TOA LW CRE reveals a group of regimes with an atmospheric radiative warming bent (made of regimes with large proportions of high clouds), another group with a cooling predisposition (regimes with many low clouds), and two regimes with a near-neutral effect. The three CRs that are the strongest precipitation producers, are overall responsible for about half the total global precipitation as recorded by GPCP. We show also that the precipitation histograms vary greatly by CR, and demonstrate unambiguously that the regimes with small precipitation means have very abrupt tails on the high end of precipitation strength, indicative of a virtually complete absence of intense precipitation events. We also illuminate the energetic imprint on the atmosphere of the combined radiative and precipitative behaviour of MODIS CRs: those that warm the atmosphere radiatively are also producers of the largest implicit latent heat warming On the other hand the CRs that cool the atmosphere radiatively, produce the smallest amounts of implicit latent heat warming. Cloud mixture decomposition by MODIS CR therefore reveals that clouds organize at global scales in such way as to induce energy flow tendencies from areas where certain CRs are more prominent to areas where other CRs, of very different character, prevail.
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