Review 500 mb wind ; Inferring precipitation areas based on 500 mb height patterns

Wind direction and speed at 500 mb

This section is mostly a review of what was covered at the beginning of the semester. The wind at 500 mb can be easily determined from the height pattern. The wind blows parallel to the height contours with lower heights to the left of the wind direction. In other words, if you are moving with the 500 mb level wind at your back, lower 500 mb heights will be toward your left and higher 500 mb heights will be toward your right. Some examples are provided below. During the winter months across the United States, this means that the 500 mb winds across generally blow from west toward east, but follow the wavy pattern of the height contours. This is because the air temperature generally gets colder as one move toward the north and 500 mb heights generally get lower. Keep in mind that this is the wind several thousand meters above the Earth's surface and not the wind that would be felt on the ground.

Here is something new to consider. The pattern of wind flow is the perferable way to identify troughs and ridges in the pattern. In general troughs and ridges do not have to be oriented along a north-south axis. The best way to identify troughs and ridges is to visualize the 2-Dimensional wind trajectory based on the 500 mb height pattern. The wind trajectory traces the motion of air. With troughs (and closed lows) the air flow makes a counterclockwise turn and with ridges (and closed highs) the air flow makes a clockwise turn (in the Northern Hemisphere).

The wind speed is faster where the height lines are closer together, and slower where the height contours are spaced further apart. If this helps, you can think of the 500 mb height lines as channels through which the air moves. The wind follows the wavy pattern of the lines, and gets squeezed together where the lines get closer together, resulting in faster winds. This is just how water would flow in a hypothetical channel. A few examples are included below.

In the first example for 500 mb winds the lines with arrows represent the trajectory of the air flow at various places within the 500 mb height pattern. Regions of strong and weak wind speeds based on the spacing of the height contours are marked on the map. In the second example for 500 mb winds arrows are again used to mark wind trajectories. It is also emphasized that the flow around closed highs is clockwise and the flow around closed lows is counterclockwise. A visualization of the relationship between the 500 mb height pattern and the winds at high alitudes can be seen by looking at a movie of the 500 mb height pattern superimposed over a satellite image that is sensitive to the motions of clouds and water vapor at high altitudes. The weather data available from the UA Department of Atmospheric Sciences produces a movie showing the evolutions of the 500 mb height pattern over the last 24 hours (one frame per hour) superimposed on what is known as a satellite "water vapor image." The water vapor image is able to "see" or "sense" the presence of clouds and relatively high water vapor concentrations at high altitudes. Although, it is most sensitive to clouds and water vapor at altitudes higher than the 500 mb height (lower air pressure), the winds at those altitudes are often similar to those at 500 mb. When you look at the loop, notice that the features in the water vapor image generally move parallel to the 500 mb height pattern with lower heights to the left of the wind direction. It can be a bit tricky because the 500 mb height pattern is changing with time along with the winds. Link to 500 mb movie for the last 24 hours

The 500 mb winds can have a large influence on the motion of smaller-scale weather features, such as the movement of surface low pressure areas, the movement of hurricanes, and the movement of individual thunderstorm cells. In this context, smaller-scale means small compared to the size of the larger scale trough and ridge pattern. For these reasons, the winds at the altitude of the 500 mb pressure level are often referred to as the "steering level" winds.

Estimating likely areas for precipitation from 500 mb height patterns

Clouds and precipitation are most likely to be occurring just downwind (or downstream) from the location of 500 mb troughs. Following the 500 mb wind flow, this is the region just after the wind has gone through the trough and starts heading toward the next ridge (see example map below). The reason for this is that rising air motion is forced in this part of the flow pattern. Rising motion means that air moves vertically upward. At this point, you are only expected to be able to identify regions favored for rising air motion based on the 500 mb height pattern, not to understand or be able to explain why rising motion happens in those regions. Clouds and precipitation will develop where air rises (as long as there is sufficient water vapor). Conversely, sinking air motion is forced over areas downstream of ridges. Clouds do not develop where air is sinking, or moving vertically downward. Fair weather is most likely in these areas. By looking at the height patterns on a 500 mb map, you should be able to distinguish where clouds and precipitation are most likely and where fair weather is most likely.

Example 500 mb map indicating where precipitation is most and least likely based on the pattern of the height contours.

Combining the precipitation guideline in the last paragraph with the definition of a troughs and closed lows given in the section above, consider the following more general statement: Northern Hemisphere 500 mb troughs can be identified as regions where the air flow makes a counterclockwise turn. Where the 500 mb air flow transitions from a counterclockwise curve to a straighter (less curved) flow (typically just after the air has gone through a trough) is the region where rising air motion happens, which favors the development of clouds and precipitation. Note that the 500 mb air flow is counterclockwise around closed lows. Thus, it is common for there to be areas of precipitation that "wrap around" the counterclockwise circulation associated with closed lows. Conversely, northern hemisphere ridges can be identified as regions where the air flow makes a clockwise turn. Where the 500 mb flow transitions from a clockwise turn to a straighter flow (typically after the air has gone through a ridge) is a region where sinking air motion happens, which favors fair weather and the lack of clouds.

This is somewhat simplistic and will not always give you all the details of where it is and is not raining. One problem mentioned above is that the 500 mb height map does not contain any information about the amount of moisture or water vapor that is present in the atmosphere. Therefore, even a strong trough, which forces strong upward air motions, may not produce any precipitation (or even clouds) if there is not enough water vapor in the air. In fact it is fairly common here in the desert to have troughs move through without much in the way of clouds or rain. Since this is not strictly a weather class, we do not have time to cover details like this. The use of 500 mb maps allows you to make a quick (and often decent) assessment of the large-scale temperature and precipitation patterns.

An example of a trough and closed low that did not produce precipitation until sufficient moisture was drawn into the system happened in January 2013 with the passage of a trough and closed low over Tucson. The problem as too often happens in the desert southwest, is that there was not enough moisture in the air to produce much in the way of clouds or precipitation. Link to 500 mb pattern at 15Z, 01/07/13 with satellite water vapor overlaid Notice that lack of clouds in the region just downstream of the trough and to the east of the closed low. The time on the image is 15Z, Monday, January 7, 2013, Local time 8 AM. By the way, if you were asked to analyze the weather based on the 500 mb height pattern shown, I would expect you to predict a good chance of precipitation happening in Tucson. The closed low slowly moved eastward and by Wednesday at 22Z, Local time 3 PM, it is centered just south of the Texas Big Bend area. Link to 500 mb pattern at 22Z on 01/09/13 with satellite water vapor overlaid. Now that low level water vapor mainly from the Gulf of Mexico is brought into play, widespread precipitation is happening. There are thunderstorms and heavy rain in eastern Texas and western Louisiana. Precipitation has wrapped around the counterclockwise circulation surrounding the closed low and heavy rain is also falling in western Texas.

An Example from January 2013

The map below is a forecasted 500 mb height pattern obtained from the University of Wyoming's weather model page in January 2013. Below the map is a weather analysis based on the map.

According to the time stamp at the bottom of the map. This is a 216 hour forecast (9 days into the future) valid for the time 00Z, Thursday, January 17. The local Tucson time is 7 hours earlier, which is 5 PM, Wednesday, January 16. The main features affecting the continental United States is a closed high and ridge centered near the west coast and a large trough extending roughly along the Mississippi River. Expect warmer than average temperatures near the ridge and colder than average temperatures near the trough. Just downstream of the trough (east of the trough) expect a chance of precipition. Using the marked points on the map to give you an idea of what you should recognize ... The most above average temperature is expected at point a, which is under the ridge, while the most below average temperature is expected at point c, which is near the center of the trough. The 500 mb wind direction at point b is from the northwest (northwest toward southeast), and the 500 mb wind direction at point d is from the southwest (southwest toward northeast). The best chance of precipitation will be at point d, which is located just downstream of the trough. Tucson is marked by point T. The forecasted 500 mb height for Tucson is about 5780 meters. If you look at the climatological or average 500 mb height map for January, you will see that the average 500 mb height over Tucson during January is 5680 meters. Thus, well above average temperature can be expected based on this forecast. The average high temperature for mid-January is about 65°F, so expect significantly warmer than that.