Mesometeorology

(meteorology) That portion of the science of meteorology concerned with the study of atmospheric phenomena on a scale larger than that of micrometeorology, but smaller than the cyclonic scale.

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, but not always, less than 1 day. Unlike the larger weather systems on synoptic scales (the scales resolved by current weather reporting station networks) which typically produce significant changes over periods of days, most mesoscale phenomena have interdiurnal periods (less than 1 day), and consequently their changes are often more startling. In addition to time and space criteria for defining mesoscale, dynamical considerations can be used.

For observing mesoscale phenomena over midlatitude land masses, the average spacing between atmospheric sounding stations is about 180–360 mi (300–600 km) and soundings are taken twice each day. Consequently, only the largest mesoscale phenomena, with wavelengths greater than about 600 mi (1000 km), are routinely observed (resolved) by this network. Information with higher time-and-space resolution is available from aircraft observations and networks of radar stations, profilers, and satellite imagery. The profiler is a ground-based hybrid observing system of vertically pointing radar and microwave radiometry. The remote-sensing platforms often show that mesoscale weather systems are distinct components of larger synoptic-scale cyclones (low-pressure systems) and anticyclones (high-pressure systems).

Although the satellite imagery and radar data provide extensive areal coverage and clearly reveal the presence of the mesoscale systems, they do not provide measurements of certain atmospheric parameters (such as temperature, moisture, and pressure) in a form in which mesoscale structures and circulations can be readily quantified and understood. Thus, while mesoscale phenomena can be “observed,” they cannot be studied and predicted (in the conventional manner) as easily as synoptic-scale systems. See also Meteorological satellites; Radar meteorology; Weather forecasting and prediction.

Mesoanalysis is the analysis of meteorological data in a manner that reveals the presence and characteristics of mesoscale phenomena. Because sounding stations are so widely spaced, only the largest of mesoscale systems can be resolved by the free-air (sounding) data. On the other hand, the density of surface observing stations is often satisfactory for identifying mesoscale features or circulations. Probably the most common application of mesoanalysis is for forecasting convective (thunderstorm) weather systems.

Modern weather-forecasting techniques rely heavily upon predictions made from computers. These predictions, commonly called numerical model forecasts, require as input the three-dimensional initial state of the atmosphere. Normally, this initial condition is produced from the previous forecast and the most recent observations from the network of atmospheric soundings. For many mesoscale phenomena, it has been possible to develop relationships between the large-scale (synoptic) environment and the occurrence of particular types of mesoscale events. By using these relationships, the prediction of the synoptic-scale environment by the numerical models is then used to infer the likelihood of specific mesoscale events. Using satellite, radar, and conventional surface observations, the onset of an event is readily detected and appropriate adjustments to local forecasts are implemented. These adjustments usually come in the form of very short-term forecasts, commonly called nowcasts, and typically are valid for only about 3 h. However, depending upon the particular mesoscale phenomena, longer-term (3–12 h) forecasts sometimes are possible. See also Meteorology; Storm detection.

The study of middle-scale meteorological phenomena; between small features, like cumulus clouds, and large features, like anticyclones. Meso-scale features are up to 100 km across and last for less than a day, for example, the polar low which, in a British winter, brings an hour or two of falling pressure followed by some three hours of snow.