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Dynamic Cooling of the Atmosphere

This will be a somewhat difficult explanation but I make my attempt.

Definition

Dynamic cooling refers to cooling of the atmosphere caused by water phase changes that remove heat from the atmosphere. Cooling also results from the expansion of air associated with mid to upper Tropospheric divergence and the vertical motion induced by that divergence.

Water phase changes

Water phase changes that cool the air can be the sublimation of snow and other frozen precipitation into dry air, the evaporation of rain into dry air, and the melting of snow and other frozen precipitation. All three of these processes involve water changing from a lower energy state to a higher energy state by removing heat from the air. Cooling due to water phase changes is particularly important in early and late season snow scenarios when the vertical profile of the atmosphere is cold enough to support snow except at the surface or within several hundred to several thousand feet of the surface. Elevated temperature layers above freezing, for example between 850 mb and 700 mb, that are preventing snow from reaching the surface, can also be cooled by the same processes. Precipitation could start off a freezing rain then change over to snow once the layer cools to around 0oC.

A late-season snowstorm in Duluth, MN on May 8 to 9, 2019 is a good example of how sublimation, evaporation, and melting can dramatically cool the air in the bottom several thousand feet of the atmosphere. Winds off the cool water of Lake Superior apparently helped mitigate heating at the immediate surface that occurred during the day from the strength of the early May sun.

Click HERE to see a summary of the storm.

Mid to upper Tropospheric divergence

Divergence in the mid to upper Troposphere causes expansion and hence cooling of the air in the layer of divergence. The air lower in altitude also expands as some of it accelerates upward to compensate for the loss of pressure. The result can be a deepening column of cooling air that eventually cools enough to change rain to snow. Could also have a situation where temperatures in the clouds producing precipitation are below freezing but not cold enough for ice forming nuclei to activate. The cooling could drop temperatures to below -8 oC, a temperature range where the number of active ice-forming nuclei dramatically increases. Divergence in the mid to upper Troposphere can be produced by a variety of wind structures including a jet streak, diffluent flow, or a shear axis.

Jet streaks

A jet streak is the core of strong winds embedded in a larger region of wind flow. The left-front quadrant exit region and right-rear quadrant entrance region of jet streaks are favorable areas for divergence. Two jet streaks can couple to enhance the divergence. In the coupled jet streak structure, the left-front quadrant of a jet streak to the southwest can come into phase with or close proximity to the right-rear quadrant of a jet streak to the northeast. An example of a coupled jet streak is seen in the two 250 mb wind upper air analysis images below. Winds at 250 mb are often good for depicting the structure of jet streams. A large spring storm was spreading snow from the central High Plains to Minnesota and northern Wisconsin. Mixed precipitation, rain, and thunderstorms were occurring to the south. The first image is valid at 7:00 PM CDT, April 13, 2018. The second image is valid at 7:00 AM CDT, April 14, 2018.

250 mb, 7:00 PM CDT, April 13, 2018 (00:00 UTC, April 14)
Storm Prediction Center upper air map archive


250 mb, 7:00 AM CDT, April 14, 2018 (12:00 UTC, April 14)
Storm Prediction Center upper air map archive

On the first image, the right-rear portion of a strong jet streak core is located over the western Great Lakes and even more so in adjacent southern Canada. The smaller core of another jet streak is located over north-central Texas. The second image shows the jet streak over Texas intensifying and moving northeast to eastern Kansas, western Missouri, and southwest Iowa. The jet streak to the north remains about the same place but intensifies and expands to cover central and northern Minnesota. Winds between the two jet cores also increase in response to the merging of the two wind features.

Divergence due to diffluent wind flow

Diffluent flow, the spreading out of the wind, often produces divergence but is not a guarantee. Sometimes the air in a diffluent wind zone slows down enough to allow upstream winds to replace enough air to prevent more from moving out than moves in. The northeast quadrant of an upper-level low pressure system is a favorable area for diffluence and divergence to occur. The following two images are from the same storm from April 13 to 14, 2018. The first image is an upper air analysis at 250 mb at 7:00 PM CDT, April 13, 2018. The second image is a radar mosaic valid at the same time.

250 mb, 7:00 PM CDT, April 13, 2018 (00:00 UTC, April 14)
Storm Prediction Center upper air map archive


Radar Mosaic, 7:00 PM CDT, April 13, 2018 (00:00 UTC, April 14)
National Centers for Environmental Information

The 250 mb image shows a broad area of wind barbs spreading out from each other over the central and northern Plains and middle to upper Mississippi Valley. Some splitting of the winds can also be seen as far south as north-central and northeast Texas. Notice that the diffluent wind pattern is particularly pronounced over Nebraska and South Dakota. The diffluence matches well with the arc of precipitation from western Nebraska to eastern South Dakota that is shown on the radar mosaic. The rising and cooling air below a region of divergence associated with the diffluent winds would at least enhance the precipitation. Other atmospheric processes could also be aiding in the production of precipitation.

Shear axis

A shear axis can also produce divergent wind flow. In the region of shear, adjacent winds of different speeds or directions pull the air apart. A shear axis can result from the variation of wind direction and speed within an upper level trough, especially one with a closed circulation. A shear axis can also be produced by the interaction of two weather systems. For example, two upper level troughs can exist with one to the northwest of the other. The southwest winds on the southeast side of the trough to the northwest will oppose the north winds on the northwest side of the upper level trough to the southeast.