The climate of a natural region of small extent, for example, valley, forest, plantation, and park. Because of subtle differences in elevation and exposure, the climate may not be representative of th
e general climate of the region.
A cyclonically rotating vortex, around 2?10 km in diameter, in a convective storm. The vorticity associated with a mesocyclone is often on the order of 10-2 s-1 or greater. (It should be noted that a
mesocyclone is not just any cyclone on the mesoscale; it refers specifically to cyclones within convective storms.) Mesocyclones are frequently found in conjunction with updrafts in supercells. Tornadoes sometimes form in mesocyclones. Persistent mesocyclones that have significant vertical extent are detected by Doppler radar as mesocyclone signatures. Tornado warnings may be issued when a mesocyclone signature is detected.
The Doppler velocity pattern of a mesocyclone within a severe thunderstorm. In a storm-relative reference frame, the idealized signature is symmetric about the radar viewing direction with marked azim
uthal shear across the core region between peak Doppler velocity values of opposite sign. Typical signatures consist of Doppler velocity differences of 25? 75 m s-1 across core diameters of 2?8 km, with resulting azimuthal shear values of 5 ? 10-3 s-1 to 2 ? 10-2 s-1.
A mesoscale wind maximum. It typically may have an along-flow length scale of tens to hundreds of kilometers and a cross- flow length scale of 100 km. Mesojets differ from planetary-scale jets, which
can have length scales of several thousand kilometers, and synoptic-scale jets, which may have length scales of 1000? 2000 km and are commonly found in association with progressive synoptic-scale troughs and ridges. Larger mesojets may also sometimes be known as jet streaks. Mesojets can form adjacent to prominent orographic features in association with terrain-channeled flow. Mesojets are also seen in association with organized mesocale convective systems as typified by the evaporatively driven rear-inflow jet commonly found behind active squall lines lines. Mesojets may also be found in conjunction with prominent lower-tropospheric stable layers where the airflow can become decoupled from the planetary boundary layer, especially at night. An exceptionally well organized lower-tropospheric mesojet extending over hundreds of kilometers might be known as a low-level jet.
The top of the mesosphere and the base of the thermosphere. The mesopause is usually located at heights of 85?95 km, and is the site of the coldest temperatures in the atmosphere. Temperatures as low
as 100 K (- 173?C) have been measured at the mesopause by rockets. See atmospheric shell.