11/29/99

Air Masses and Fronts

Classification of Air Masses

Large bodies of air residing over a particular geographic region for a long period of time (several days to weeks) can acquire characteristics representative of that region. For example, air over a warm ocean will be warmed and water from the ocean surface will evaporate into this air. The result is a moist, warm air mass. Similarly, air over a large cold land surface far from any liquid water will become cold and dry. When these two kinds of air masses meet, we know that lifting can occur, and any water vapor may condense, forming clouds and rain.

Air masses are classified according to their temperature and moisture content. In the above examples, the moist ocean air woulf be referred to as a maritime ("small m") air mass, while the dry air over land would be a continental (small c) air mass. Since warmer temperatures are generally found at lower latitudes, where the insolation is greater, warm air from these regions gets the designation Tropical ("capital T"). Cold air from high latitudes is designated Polar ("capital P"). Extremely cold dry air gets the designation Arctic ("capital A").

Here in Tucson, our weather is dominated by warm, dry conditions that are representative of a cT (or continental Tropical) air mass. In summer, the cT air is replaced by mT (maritime Tropical) air from the Gulf of Mexico and the Gulf of California. The hot, humid conditions in July and August, and the associated thunderstorm activity, comes from the arrival of this mT air mass.

Fronts

The boundary between two different air masses is a front. It is most commonly defined in terms of changes in temperature (i.e. cold front, warm front), but temperature differences are not always the best way to identify a front. Abrupt changes in wind direction or humidity (evidenced by the dew point temperature) can also indicate a front. Watching the movement of fronts is a key part of weather forecasting. Review the fontal cyclone model first described in Danielson in Chapter 1 (p. 23-29) to see how cold and warm fronts move.

The different types of fronts can be summarized as follows:

• COLD FRONT: The boundary between an established warm air mass and an advancing cold air mass. The cold front's location is defined as the leading edge of the change to colder weather. Once the front passes, the temperature drops rapidly, as does the dew point temperature. Winds generally shift from the southwest to the northwest direction, in accordance with the passage of a low pressure system. In some cases, a cold air mass may advance from the east, rather than the west (see Figure 9.11. in Danielson). This is referred to as a backdoor cold front (these sometimes occur here in AZ, often in late winter or early spring). If there is little temperature difference between the established air mass and a slightly cooler advancing air mass, one looks to the difference in moisture content of the air masses to define the front. This is known as a dry line.
  
  
• WARM FRONT: Here a mass of warm (generally moist) air will advance on an established cooler air mass. An important difference between warm and cold fronts is in the vertical structure (see Figure 9.14 and discussion below). Warm air, being less dense than cold air, will tend to run up over the established cold, dense air mass at first. This overrunning is a lifing mechanism that can cause cloud formation and precipitation to occur in advance of the surface warm front. Compare this to the case of a cold front, which acts like a wedge to drive up warmer, less dense air ahead of it. Note also that the different types of lifting associated with a warm front and a cold front in Figure 9.14 means different type of clouds are associated with each front: Cumulus clouds at the cold frontal boundary; Stratus and cirrus-type clouds out ahead of the warm frontal boundary.
  
  
• STATIONARY FRONTS: As the name implies, you have a boundary between a colder air mass and a warmer air mass, but it doesn't move. Why? Most often, because the the flow of warm air alone is not enough to displace a well established cold, dense air mass. Instead, the warm air will overrun the cold air, setting up persistent cloudy, drizzly conditions more like that of a warm front. In certain cases, physical barriers such as mountain ranges may prevent air masses from moving, giving rise to a stationary front.
  
  
• OCCLUDED FRONTS: As we saw in Chapter 1, occluded fronts signal the dying stage of a frontal cyclone. Once an occlusion forms, the low pressure system that brought it about tends to weaken and dissipate. So what is an occlusion? This of a cold front extending in a line running southwest to northeast associated with a mature frontal cyclone (Figure 1.21b in Danielson), which also has a warm front extending out to the east ahead of the low. More often than not, the winds near the cold front are much stronger than near the warm front - the cold front can actually catch up to the warm front. When this occurs, and occluded front forms.

Vertical Structure of Fronts

Because of the different nature of warm and cold fronts, they bring about very different sets of weather conditions. These are summarized in Figure 9.14 of Danielson, which shows the vertical structure of both a cold and a warm front. In the case of a cold front, cumuliform clouds and precipitation are situated directly over the surface frontal boundary. This "line" of weather passes quickly, leaving cold, dry, stble conditions in its wake. In the case of a warm front, stratiform clouds form well ahead of the surface frontal boundary.

When an occlusion forms, the vertical structure of the frontal zones can get a little complicated (see Figure 9.17). If the cold air advancing behind the cold front is much colder than the air lying ahead of the warm front, the advancing cold air pushes under both the warm and cool air masses ahead of it, resulting in a cold occlusion. If, on the other hand, the air advancing behind the cold front is not as cold as the air already in place ahead of the warm front, both the warm air and the cool advancing air will overrun the established cold air mass, resulting in a warm occlusion.

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12/3/99

The General Circulation of the Atmosphere

A global composite water vapor image like the one shown above offers a view of the large scale patterns of clear skies, clouds, and winds over the entire planet, which together make up the general circulation of our atmosphere.

The general circulation is characterized by the following features:

1. Trade Winds: Just north of the equator, there exists a steady flow at the surface from northeast to southwest. Just south of the equator, there exists a steady flow at the surface from the southeast to the northwest. These wind patterns are present year-round.
   
   
2. Intertropical Convergence Zone (ITCZ): At the equator, the trade winds from the Northern and Southern Hemisphere converge. Where this occurs, rising motion takes place, accompanied by clouds and precipitation. This region of convergence is referred to as the ITCZ. The position of the ITCZ ranges between about 15N latitude and 5S latitude throughout the year, roughly following the latitude at which the sun is directly overhead throughout the year. This is due to the fact that the ITCZ, and the circulation of the tropical regions in general, is driven by a thermal circulation known as the Hadley Cell (see below). Because this is a region of strong convergence, there is no prevailing wind flow at the surface. Early sailors encountering this rainy region would have a hard time making their way out - and so this region was often called the "doldrums".
   
   
3. Subtropical High Pressure: Just north and south of the trade wind regions, between 20 and 35 degrees latitude in each hemisphere, lie regions of high surface pressure. These regions are generally very clear and very dry, with very calm wind conditions throughout the year. Land masses in this latitude region are often covered by vast deserts. Sailors encountering this region named it the "horse latitudes", because it was common to get caught in the calm, sunny conditions for weeks. As provisions ran scarce, livestock often became expendible. Use your imagination to figure out the rest.
   
   
4. Subtropical Jet Stream: At upper levels (say 200-300 millbars) in the subtropics there exists a strong zone of west-to-east air flow. The origin of this subtropical jet is the conservation of angular momentum in the upper branch of the Hadley Cell (see below).
   
   
5. Mid-Latitude Westerlies: Also called "prevailing westerlies" - you may have noticed by now that most of the weather across the United States (roughly halfway between the equator and the North Pole) moves from west to east. This is due to the prevailing westerly flow at mid-latitudes. This is a consequence of the north-south temperature gradient - i.e. temperatures are generally warmer to the south and colder to the north because of differences in insolation. Throughout the atmosphere, this temperature difference leads to differences in the thickness between constant pressure levels, which in turn establishes a pressure gradient (higher pressure to the south, lower pressure to the north). Air set in motion by this south-to-north pressure gradient will be deflected to the east (in the Northern Hemisphere) by the Coriolis force. The result is a prevailing west-to-east flow at mid-latitudes.
   
   
6. Polar Jet Stream: At the point where poleward moving tropical air masses (mT, cT) meet polar air masses (mP, cP) there will of course be a front making the boundary between the two air masses - the polar front. This front is a region of low pressure and rising motion, with easterly flow to the north of the front. The zone of strong westerly flow aloft associated with the strong horizontal temperature contrasts in the vicinty of the polar front is referred to as the polar jet.
   
   

Thermal Circulations: The Hadley Cell

The general circulation in the tropics is a thermally direct circulation, meaning it is driven directly by the heating of the sun. The location on the earth's surface that receives the most heating is where the sun is directly overhead. Because this location changes throughout the year (e.g. sun is overhead on the equator at equinox, overhead at 23N latitude at summer solstice, overhead at 23S latitude at winter solstice) this circulation will move along with it. The ITCZ coincides with the region of maximum heating. Moist tropical air heated by the sun rises, water vapor condenses, clouds form and precipitation falls. Air at the surface must flow into the ITCZ to replace the rising air, explaining the convergent flow of the trade winds. Eventually, the rising air over the ITCZ moves north and south, and begins to cool. As it cools it begins to sink. This region of sinking air over the subtropics (just north and south of the ITCZ) is the origin of the subtropical high pressure belt.