Closed Low A low pressure area with a distinct center of cyclonic circulation which can be completely encircled by one or more isobars or height contour lines. The term usually is used to distinguish a low pressure area aloft from a low-pressure trough. Closed lows aloft typically are partially or completely detached from the main westerly current, and thus move relatively slowly (see Cutoff Low).
A closed low has several characteristics. Closed lows tend to be deep pressure systems with one or more height contours encircling them on the analysis charts at all levels in the troposphere. A closed low is occluded and vertically stacked. Because the low is closed, it does not generate much PVA and NVA. The vort max will be near the center of rotation. It is difficult to get a windflow that produces vorticity advection because the vorticity isopleths for the most part are parallel to the height contours. The weather associated with closed lows tends to be cloudy with showers in the vicinity of the low's center. Closed lows are often slow movers but can move quickly if they are embedded within the jet stream. Cloudy and rainy weather can persist with the slow movers. Thermal advection tends to be weak with closed lows since they are occluded. A special case of the closed low is the cut-off low. A cut-off low moves slowly since the jet stream winds aloft are displaced away from the low and are thus not able to move it along.
closed low—A low that may be completely encircled by an isobar or contour line. (This means an isobar or contour line of any value, not necessarily restricted to those arbitrarily chosen for the analysis of the chart.) Strictly, all lows are closed. However, in weather-map analysis terminology, this designation is used commonly in two respects: 1) on surface charts, to distinguish a low from a trough, especially as a low develops within the trough; and 2) on upper-level charts, to accentuate the fact that the circulation is closed, especially at levels and over latitudes where such an occurrence is unusual. The definition of closed high is analogous.
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