The Atmospheric Mesoscale

Introduction It is fitting to begin this exploration of mesoscale-convective processes with a definition of the atmospheric mesoscale . Likely, the reader has at least a vague idea of atmospheric phenomena that are normally categorized as mesoscale. Thunderstorms and the dryline are common examples. What the reader might not yet appreciate, however, is that devising an objective and quantitative basis for such categorization is nontrivial. Indeed, even the more basic practice of separating the atmosphere into discrete intervals can be difficult to rationalize universally, because the atmosphere is, in fact, continuous in time and space in its properties. Consider the atmospheric measurements represented in Figure 1.1. These have been analyzed to reveal a frequency spectrum of zonal atmospheric kinetic energy. Although the spectrum is continuous, it does exhibit a number of distinct peaks. Conceivably (and arguably), the intervals centered about the peaks represent atmospheric scales. The relatively narrow peak at a frequency of 10 0 (/day) is compelling here, because it indicates the existence of energetic eddies with a diurnal cycle. Dry and moist convective motions that grow and decay with the daily cycle of solar insolation are the presumed manifestations of such eddies, and would fall generally within the atmospheric mesoscale.