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. Try to look for closed and cut-off lows at 500 mbs and adjacent pressure levels when examining the model products such as UNISYS. Every once in a while a very good one will develop: http://weather.unisys.com/nam/500.php |