Review 500 mb wind ; Precipitation Areas based on 500 mb Height Pattern

Review: 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 estimated 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, the wind at the 500 mb height level moves along the pattern of the height contours, without crossing them, such that lower 500 mb heights are toward the left of the wind direction and higher 500 mb heights are toward the right of the wind direction. Wind direction has been added to the figure below, which was linked on the previous reading page.

Arrows show the trajectory of the 500 mb wind for a flat 500 mb height pattern on the left and a wavy 500 mb pattern on the right.

Because the air temperature across the United States generally decreases in value as you move from south to north, the 500 mb height generally decreases as you move from south to north as shown in the figure above. This means that the 500 mb winds over the United States generally move from west to east, though the winds follow the wavy patterns that are commonly observed in the 500 mb height field. Keep in mind that this is the wind at the altitude where the air pressure is 500 mb, which is several thousand meters above the Earth's surface, and not the wind that would be felt on the ground.

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.

An example map for 500 mb winds is shown below. The 500 mb height pattern is shown with labeled height contours. Arrows have been drawn to indicate the wind trajectories at selected locations on the maps. The color shading on the map represents the wind speed at 500 mb as determined by the weather model. The important thing to note is that 500 mb wind speeds are highest where the contour lines are most closely spaced, while the 500 mb winds are slow where the contour lines are spaced further apart. See the color key for wind speeds below the maps. Areas with weak 500 mb winds speeds (less than 20 knots) are displayed in white. Notice that the height contours are spaced far apart in these areas. Notice also that the stronger winds happen where the height contour lines are spaced closer together. You are not expected to be able to determine actual wind speeds, just to point out areas where the winds are relatively strong and places where the winds are relatively weak. The contour interval on these maps is 6 decameters, rather than the 3 decameter intervals that were shown on the height anomaly map.

Map of the 500 mb height pattern for September 25, 2017 at 12Z. 500 mb wind trajectories are shown at various locations using bold arrows. The color shading indicates the 500 mb wind speed in knots (nautical miles per hour) with color key at the bottom. Generally, the closer the spacing of the contour lines, the faster the wind speed.

The wind trajectory arrows shown on the map above are parallel to the height contours and point in the direction such that lower 500 mb heights are to the left and higher 500 mb heights are to the right. Notice that the 500 mb air flow around closed highs turns clockwise, just as for ridges, and the 500 mb air flow around closed lows is counterclockwise, just as for troughs.

The pattern of wind flow is the perferable way to identify troughs and ridges on 500 mb height maps. 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. We are only going to discuss weather maps for the Northern Hemisphere and will not confuse the situation by covering differences between the Northern and Southern Hemisphere. In looking at these maps, you may notice that troughs and ridges come in a wide range of sizes. Some are quite small and may appear as a small wiggle along one of the contour lines, while others can include many contour lines and be almost as large as the entire continental United States. For this class you will only be expected to identify large and easy-to-find troughs and ridges.

The 500 mb winds 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. In fact where thunderstorms do form, they generally move as a unit in the direction of the winds near the 500 mb pressure level. Winter storms, which are characterized by a strong area of surface low pressure, also typically move in the direction of the 500 mb winds.

A visualization of the relationship between the 500 mb height pattern and the winds at high alitudes can often 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

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.

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. While the major regions of precipitation are most often found just beyond 500 mb troughs, it is fairly common for some precipitation to be happening underneath and around closed 500 mb lows.

The 500 mb height map for 12Z, September 25, 2017 is shown again below, but this time highlighting regions that are favorable and unfavorable for precipitation. The best chance for precipitation occurs just downwind from the center of 500 mb troughs. Another area of potential precipitation occurs near closed lows. Meteorogists often call this "wrap-around" precipitation since areas of precipitation are observed to move counterclockwise around the closed low. Typically rain or snow is not simultaneously happening over the entire region favorable for precipitation. Often small areas of precipitation form within the favored region. These small areas of precipitation move in the direction of the 500 mb winds (the "steering level" winds). Thus, it is common for locations in the favored areas for precipitation to get periods of rain or snow, rather than continuous precipitation over the entire favored region.

Based on the 500 mb height pattern, regions just downwind of ridges and near closed highs are least likely to have precipitation. These regions are indicated in the example map below. Precipitation is unlikely downstream of ridges or near closed highs because air is forced to sink vertically in those areas. Here is a link to Another example 500 mb height map showing precipitation areas.

500 mb map for 12Z, September 25 highlighting regions that are favorable and unfavorable for precipitation based on the location within the 500 mb height pattern. Click on the image to open a larger version.

This is somewhat simplistic and will not always give you all the details of where it is and is not raining. One problem 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 January 2013. Below the map is a weather analysis based on the map. This is an example of the type of 500 mb map analysis that you might be asked to perform on the module 3 exam.

According to the time stamp at the bottom of the map, this forecast map was 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 would be expected in Tucson based on this forecast, since the height on the map is about 100 meters above average. The average high temperature for mid-January is about 65°F, so expect significantly warmer than that based on this map. No rain would be expected in Tucson, since Tucson is downwind of a ridge.