Need for Better Connection between Small and Large Scales

Acquiring a better understanding of the interactions between meso- and microscale processes and the large-scale flow requires the provision of accurate boundary conditions for meso- and microscale studies. In recent years, this has been achieved using large-scale meteorological data from reanalysis datasets (e.g., Kistler et al. 2001; Uppala et al. 2005), which have proven extremely useful because they provide gridded data at regular temporal intervals everywhere in the atmosphere.

Current reanalyses place a high value on creating datasets that are homogeneous over a long period of time (40 years), hence preventing the use of the most advanced, but computationally expensive, models as well as limiting assimilation to data that are available over the whole reanalysis period, or at least relatively lengthy fractions of it. Aerosol processes, for instance, are not represented at all in current reanalyses; however, given the recent advances to our observing system, this should be possible. We argue that the "one-size-fits-all" approach is not optimal to address the cloud-climate problem. Indeed, we believe that focused reanalysis systems, which target key processes and assimilate new datasets, would be of major benefit to the observational community. These systems would not necessarily be global, but could instead be run over much smaller spatial and temporal domains at higher resolution. Regional reanalysis (e.g., the North American regional reanalysis) is a move in this direction. Incorporating physical process specialization with geographical specialization would also be a beneficial development. An assimilation system that focuses upon the lower troposphere, including the boundary layer, would greatly benefit our understanding of low cloud systems and their relationship with large-scale meteorological and aerosol forcings.

To optimize boundary conditions, the instrumental setup should be designed to connect the meso- to the microscale. This should rely on a combination of large-scale, though limited and discontinuous, sampling with instrumented aircraft and surface (either ground or ship) stations that provide continuous sampling of the atmosphere flowing above the site.

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