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Mweather pattern feedback loop6/9/2023 ![]() It's that downward motion that creates the eye of the storm, as shown in the visible satellite image of Hurricane Isabel from 1404Z on Septem(below).įor the record, the eye is a roughly circular, fair-weather zone at the center of a hurricane. Indeed, the predominant vertical motion over the center of a hurricane is downward. Strong tropical cyclones, on the other hand, don't have this "check and balance" over their centers. In other words, rising air actually works against the deepening of a mid-latitude cyclone it serves as a "check and balance" on the overall intensity of the system. But my point should now be clear: Rising air tends to make surface pressures higher, not lower. During the development stage of a mid-latitude cyclone, dominant weight-loss processes, such as net column divergence and warm advection near 200 mb overwhelmingly offset the tendency for air columns to gain weight from adiabatic and moist adiabatic cooling. Assuming a nearly hydrostatic atmosphere (in which the force of gravity is balanced by the upward pressure gradient force), this increase in mean column density serves to add column weight. In turn, cooling by forced ascent increases the mean density in the column of air that extends from the ground to the tropopause (low-level convergence and upper-level divergence are still at work). Recall that rising air cools via expansion, and once clouds and precipitation develop, can also yield evaporational cooling (assuming the atmosphere is not already at saturation). Rising air actually works against the overall reduction in surface pressure. You'll occasionally read or hear explanations that suggest that rising causes lower surface pressures, but that's just not true. Also note, however, that the divergence aloft along with low-level convergence drives upward motion over the center of the low. In a nutshell, the magnitude of the divergence aloft (which is greater than the magnitude of the convergence at lower altitudes) drives the intensity of the mid-latitude cyclone. This positive feedback loop continues uninterrupted until the late stages of occlusion, when the low moves back into the cold air (away from the baroclinic zone) and upper-level divergence over the low weakens (the low starts to "fill" - surface pressure rises). In turn, upper-air divergence increases over the center of the low, causing surface pressures to further decrease and surface winds to increase further. As winds around the cyclone increase, cold-air advection southwest of the low increases, causing 500-mb heights to fall and the 500-mb trough and vorticity maximum to strengthen. Decreasing surface pressures result in a stronger pressure gradient force, which causes faster winds. In short, divergence downwind of a 500-mb shortwave trough reduces the weight of air columns, forming an area of low pressure at the surface, around which winds rotate counterclockwise (in the Northern Hemisphere). This process requires the cyclone to develop in a region of strong horizontal temperature gradients (a baroclinic zone, or front) and under a region of strong upper-level divergence. With these observations in mind, a natural question might be, "Why do strong tropical cyclones often attain sea-level pressures that are notably lower than those associated with mid-latitude cyclones?" While both types of cyclones are low-pressure systems, the answer to that question can found by examining the differences in structure and strengthening mechanisms characteristic of each type of low-pressure system.įor starters, recall that mid-latitude cyclones undergo the process of self-development. For example, the Superstorm of 1993 (aka the "Storm of the Century"), had a central pressure of 963 mb at its peak. On the other hand, the sea-level pressure at the center of a mid-latitude cyclone rarely drops below 950 mb. Typhoon Tip (1979) had the all-time lowest at 870 mb, but other storms such as Hurricane Wilma (2005) and Super Typhoon Haiyan (2013) have had central pressures below 900 mb. For example, the most intense tropical cyclones can have sea-level pressures below 900 mb. Secondly, a tropical cyclone can attain a much greater intensity in terms of both sea-level pressure and wind speed (some even call hurricanes the "kings" of all low-pressure systems). Meanwhile, only about 80 tropical cyclones develop each year. ![]() Hundreds of them trek across the globe each year. But, why do powerful tropical cyclones more frequently steal national and international headlines, while mid-latitude cyclones rarely do? The first reason is likely that mid-latitude cyclones are more numerous. With the great potential for loss of life and property posed by tropical cyclones, they certainly garner great attention from weather forecasters and the public at large.
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