Mesometeorology

Article By:

Fritsch, J. Michael Department of Meteorology, College of Earth and Mineral Sciences, Pennsylvania State University, University Park, Pennsylvania.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.416690

That portion of meteorology comprising the knowledge of intermediate-scale atmospheric phenomena, that is, in the size range of approximately 1–1200 mi (2–2000 km) and with time periods typically less than 1 day. In addition to these time and space criteria for defining mesoscale, dynamical considerations can be used. For example, a dynamical definition requires that both background rotation and ageostrophic advection be present to a significant degree. These requirements are based upon the relative magnitudes of the two major forces, pressure gradient and Coriolis, that govern movement of the atmosphere. To the extent that these forces are not in balance, air is accelerated. When the forces are of comparable magnitude, accelerations are relatively small and the atmosphere is considered to be in geostrophic equilibrium. The wind, under these so-called balanced conditions, is called the geostrophic wind. For weather systems larger than mesoscale, the atmosphere is predominantly geostrophic. However, for mesoscale systems, geostrophic balance is not routinely present and accelerations can be quite large. Thus, the wind departs from geostrophic balance, and the departure is termed the ageostrophic wind. The term total wind can then be thought of as the sum of the geostrophic and ageostrophic components, and each component can advect (move, carry, transport, and so on) various properties or constituents of the atmosphere. Therefore, one of the clearly definable dynamic differences between large and mesoscale systems is that ageostrophic advection is negligible for large-scale circulations, while for mesoscale circulations it is not. A convenient way of measuring this condition is through the nondimensional Rossby number (R). This number is the ratio of the net acceleration of the horizontal wind by all the forces to the acceleration produced by the Coriolis force alone. For mesoscale motions, R is on the order of 1, as shown below.

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