Understanding Upper Level Lows and Disturbances

In several of our products and messages you may hear us refer to “upper level disturbances” as a variable in determining our weather over the next few days.   An upper level disturbance is a feature in the upper levels of the atmosphere that is usually responsible for inclement weather such as clouds and precipitation.

These disturbances include longwave and shortwave troughs, as well as upper level lows. All of these components play a large role in providing a spark or inhibiting precipitation in an area depending on their location in relation to surface systems, such as surface low pressures or fronts that you may be more familiar with.

Upper Level Low

The first key to understanding these upper level systems is to start with the upper level low. This system is similar to its surface counterpart largely because air flow is counterclockwise (in the northern hemisphere) around the low, and it is associated with poor weather conditions. The map below shows the heights at which the pressure is 500mb (millibars). 500mb means that roughly half of the air is above the recorded height, and half is below. Purple and darker blues correspond to colder temperatures in the map. The low in the map below is located in western Quebec.

Now it is likely that you may have heard the phrase that cold air sinks and hot air rises. You may experience this in the summer at your house when the lower levels of the house are cooler than the upper floors. The same holds true in the atmosphere. In colder conditions, air sinks, and, as a result, more air will be closer to the surface than if the conditions were warmer. Since more air is compacted towards the surface, the point where pressure is 500mb will be at a lower height than if conditions were warmer. As a result, in the above map, the low is centered in the coldest temperatures.

Think about the below example to clear things up. Imagine you have 8 red particles and a column like container. Now in colder conditions, the red particles will “sink” more towards the bottom (left). So the point where half is above and half is below will be much lower than in warmer conditions where “air rises”. As a result, the upper air low is more a measure of heights rather than low pressure like at the surface.

Long Wave Trough

Now forming around these lows are features called longwave troughs. For informational purposes, the features around upper level highs are called ridges.  Below is a 500mb chart from July 22, 2009. The upper level low and high have been marked in light blue as well as a few of the lines of constant height (heights get numerically smaller closer to the low). The area where these lines curve around the low is the longwave trough. You may notice an even more defined longwave trough in the first chart where the lines of constant height tightly hug the low centered north of New York State. The flow of air around these longwave troughs can create, what we call, vorticity. Vorticity is a complex concept but basically is a measure of spin in the atmosphere. Troughs can generate a lot of vorticity and when this occurs, it can help to deepen and strengthen low pressures on the surface. More information on vorticity is easily available on the web.

Upper Low and Surface Low Connection

In order for the upper level low (below, top left L) and longwave trough to help strengthen a surface based low pressure (below, right L), the upper level low must be oriented to the northwest of the surface low. This way, the upper level low helps to pump in warm air from the south, diverts more air away from where the surface low is oriented (less air means lower pressure and a stronger low), and the transferred vorticity gives the surface low more spin.  There are more factors that go into determining the strength of a surface low, but this orientation can help create a strong mid-latitude cyclone.

Shortwave Trough

The last piece of the upper air puzzle is the shortwave trough. The shortwave trough is an upper air feature that is tougher to detect on a map as it is a lot less defined than a longwave trough and moves almost twice as fast.  The below picture illustrates a shortwave on a map. The chart shows that shortwaves often form along longwaves and their presence is denoted by a kink or series of kinks as highlighted below. The particular formation of shortwaves involves more complex science but can be easily researched on the web.

However, shortwaves do provide a lifting mechanism and can spark precipitation and thunderstorms. A prime example would be in a capped environment. Many may be familiar with how a cap inhibits the formation of thunderstorms under warm and humid conditions. However, it is the arrival of a shortwave trough in the upper atmosphere that provides the extra boost for these thunderstorms to start forming and to initiate thunderstorm formation. Predicting the arrival of these waves can be challenging and their timing can determine the threat level for strong and severe thunderstorms in a particular situation.

Cut-off Low

One last interesting thing to note: When the lines of constant height on the 500mb chart encircle an upper level low (see below), it is likely that the low can become a cut-off low. This can happen when a system becomes stacked. This means the surface low is exactly beneath the upper level low, thus it is “stacked”. When this happens, the surface system begins to weaken as a lot of the upper level ingredients begin to shift eastward. When this occurs, the low moves extremely slowly and independently of the predominant weather pattern. This can be responsible for days of dreary and damp weather, as has occurred a couple times so far in the summer of 2009. These cut off lows slowly weaken and usually do not produce strong or severe thunderstorms.

The contribution of upper level systems in determining the formation and strength of weather systems on the surface is extremely important. As a result, meteorologists need to look at both in order to forecast accurately and predict the weather on a daily basis. It is the position of these systems that helps us to look for potentially dangerous weather days in advance and helps us to provide the most accurate predictions possible for a future weather event.

Andrew Winters-NWS Milwaukee/Sullivan Student Volunteer

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