Critical scientific issues Weather phenomena, as well as climate, are made up of diverse processes involving interactions between the atmosphere, the ocean, the biosphere, and the land surfaces, including snow and ice. These interactions occur over a range of spatial and temporal scales. Processes such as cloud formation, strong vertical motions, and rapid heat exchanges occur over spatial scales typically less than 100 km, and time scales of minutes to hours. These are the scales in which tornadoes, violent thunderstorms, flash floods, downslope wind storms, lake-effect snow squalls, downbursts, and wind shears take place. Although progress has been made in the scientific understanding of the structure and dynamics of mesoscale precipitation systems and of the scale-interactive processes that lead to their initiation and modulation, there are many areas where additional study is required.
Vast gaps remain in our ability to simulate many of the mesoscale processes in numerical models.
It will also depend on their ability to represent properly the interactions between mesoscale systems and cloud microphysical processes involving latent heat release. The effect that precipitation processes have on diffusion, transport, and scavenging of chemical species important to public safety and air quality also demands intensive research. Similarly, the importance of surface characteristics to the development of mesoscale precipitation systems underscores the need for adequate information concerning the spatial and temporal patterns of evapotranspiration and solar radiation, important terms in the surface energy balance. Many of the issues just discussed are equally important at the interannual and longer time scales. Clouds are the most crucial factor causing uncertainties in model estimates of global warming.