BOX S.1

The Meaning of Mesoscale

The term mesoscale derives from the Greek meso, which translates approximately to intermediate in English. In meteorology, this term refers to weather phenomena occurring at horizontal sizes that range from the size of a small city to that of an average Midwestern state (e.g., Iowa). The Glossary of Meteorology (Glickman, 2000) defines mesoscale as:

Pertaining to atmospheric phenomena having horizontal scales ranging from a few to several hundred kilometers, including thunderstorms, squall lines, fronts, precipitation bands in tropical and extratropical cyclones, and topographically generated weather systems such as mountain waves and sea and land breezes.

The Glossary notes that from a physical or dynamical perspective, the horizontal extent of mesoscale features ends just short of where the Earth’s rotation exerts a significant influence on air motions. Beyond that are macro- (“large”) scale features, including synoptic features. Synoptic Meteorology, whose name derives from the Greek sunoptikos, meaning “seen together,” includes the commonly understood low and high pressure systems often shown on weather maps by broadcast meteorologists. Synoptic low and high pressure systems usually come in pairs (“seen together”), and their evolution governs general regional and national weather patterns on the time scale of a few days (e.g., low pressure/stormy days followed by high pressure/fair days). However, mesoscale features embedded within larger synoptic-scale systems, including individual thunderstorms, rainbands, and frontal passages, often provide the high-impact weather at a particular location.

In mesoscale features, vertical air motions can be intense and vary significantly over short horizontal distances, resulting in strong fluctuations in the temperature, moisture, momentum, and chemical species concentrations observed at any given location. These are some of the quantities associated with the weather “sensed” by humans where they live. In general, the vertical variations of these quantities in the ambient atmosphere (i.e., the vertical gradients) are relatively large near the surface of the Earth. Thus, large vertical motions near the surface of the Earth can be very effective at redistributing temperature, moisture, momentum, and chemical constituents. It is this interplay of vertical air motion with sharp vertical gradients in these quantities that results in high-impact weather and air quality events at the mesoscale. Observations of these conditions are the key to improving predictions of high-impact events, because the sophisticated computer models that provide such predictions are inherently limited by the quality and quantity of observations, which serve as a starting point for the calculations.

Standard weather observations typically resolve larger-scale features that enable a computer model to provide skillful predictions of those features while also producing events that are mesoscale in scope. However, because they lack observations that resolve more of the antecedent mesoscale structure, models are limited in their ability to predict specific high-impact mesoscale events. Therefore, a more effective meteorological and chemical weather observing system must include nationwide observations that are faster in time, more densely spaced horizontally, and are designed to capture the detailed vertical structure of the lower atmosphere.



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