Main Reason For Development of Surface Pressure Systems

(Note: aka Reading 4a for Metr 201 -- Metr 302 and 356 students can omit sections labeled for Metr 201 students)

Divergence- The motion that results in neighboring air parcels at a given level "spreading apart."

Rules:

Thus, if one can identify where divergence is occurring in the upper portion of the "business" portion of the atmosphere (the lowest 36000 feet or so, called the troposphere), one can pinpoint and anticipate the development of surface low pressure areas..

A. Development of upper level divergence because of heating and cooling patterns in adjacent columns of air: Thermal Pressure Systems

The question arises:  "what causes this upper tropospheric divergence?"  There are two ways, one of which is related to the heating and cooling patterns in adjacent columns of air.  For example, during the summer the surface of the hot continents* is the site of low pressure while the adjacent oceans have colder temperatures and high pressure areas over them. We can visualize a relatively hot column of air over the continent adjacent to cooler columns over the adjacent oceans.

*Note: or consider a heated air column at the equator and the chilled air columns at the poles--the same explanation works.

Rule: Heating An Unenclosed Gas Tends To Make It Expand (air columns get larger), whereas cooling an unenclosed gas tends to make it contract (air columns get smaller)

The same observation can be made over the oceans if relatively warm or hot ocean water is next to relatively cold water. This occurs, for example, in the eastern tropical Pacific and eastern tropical Atlantic, the locus for development of tropical low pressure areas.

We can trace the development of the continental low pressure areas in the summer and the development of the tropical cyclones initially to divergence aloft which is the ultimate result of heating of the air columns over the warmer surface (land or ocean). This in turn causes the development of upper tropospheric high pressure areas initially.  

Since according to our rule of thumb,  at a given elevation air tends to move from high pressure to low pressure, air diverges out of these high pressure areas aloft moving towards the low pressure areas in the colder air columns on either side, which lowers the pressure on the surface directly underneath the upper level high.  

The surface cyclones that are formed in this manner are called THERMAL (WARM CORE) LOWS.  Similar arguments can be used to explain THERMAL (COLD CORE) HIGHS which develop at the surface in the chilled air columns.

Chilled Air Column (65 F)
Heated Air Column (88 F)
Chilled Air Column (65 F)

Upper Level LOW

<--

Upper Level HIGH

-->

Upper Level LOW

36000 feet

Subsidence

Upward Motion

Subsidence

18000 feet

THERMAL HIGH

-->

THERMAL LOW

<--

THERMAL HIGH

Surface
Cross-section through differentially heated and cooled air columns

Examples of Thermal Pressure Systems

(a) Equatorial Lows and Polar Highs: Hemispheric Thermal Lows and Highs

The surface low pressure area linked to the main heating pattern of the globe (warmer equatorial region) is called the Equatorial Low. The high pressure area found on average at either pole linked to the average chilling patterns of the globe is known as the Polar High.

In reality, the air moving away aloft from the heated air column at the equator cools radiationally by the time it reaches around 30N/S, and sinks. Thus, there is a "miniature" version of the larger convection system one would expect from equator to poles, with the cold air columns locally found around 30N and 30S or so.

The high pressure systems at the base of these cold air columns that extends around the globe is known as the Subtropical High Pressure system. However, this pressure system is broken into somewhat elliptical "patterns" in the northern hemisphere because of the heating and cooling effects of the continents and the oceans.

(b) Summer Thermal Lows and Highs On The Continents and the Oceans

The complicating factor is that several sources of global and continental-scale heating and cooling occur simultaneously. The thermal pressures systems that occur due to the heating at the meteorological equator and chilling at the poles (called the Hadley Circulation) are added to the thermally-induced pressure changes in the middle latitudes due to the differential heating and cooling patterns of the continent and the oceans. This leads to the actual average surface pressure pattern in the Northern Hemisphere's summer, for example.

During the summer, the continents at a given latitude are much warmer than the oceans at the same latitude. Thus, using the reasoning above (check out the Table above), we would expect high pressure areas to be over the ocean and low pressure areas over the continent. The Pacific Ocean is the locale for a pressure system that develops in the manner discussed above which is called the Pacific High. The Atlantic Ocean is the locale for a pressure system that develops in the manner discussed above which is called the Bermuda High.  The low pressure area over the continent is known as the North American Thermal Low.

The summertime North American Thermal Low extends across the Great Basin, the Mojave Desert into the deserts of Mexico, regions which are extremely hot in between the Coast Range/Sierra Nevada and the Rockies. The portion of the North American Thermal Low that extends into California's deserts, and the Central Valley, is known locally as the California Thermal Low. The North American/California Thermal Low is a seasonal pressure system that develops because of differential heating and cooling.

Actually, the definition of monsoon is: a seasonally-reversing wind system that results from differential heating and cooling of the continents and the oceans. Since we know that wind results from pressure patterns, the thermal pressure systems described in this section cause the monsoon. The best example in the world is the Asian monsoon. But we also experience a monsoon in North America, as discussed in class.

(c) Tropical Cyclones

Tropical cyclones form when disturbances move over warm water and intensify because of the effects described above. However, tropical cyclones intensifying through all the stages to eventually become hurricanes are much more intense than can be anticipated by a consideration of the sea surface temperature patterns alone. Here is an example of a surface chart showing a hurricane and a 300 mb chart (at roughly 36000 feet) showing that a high pressure area is found aloft exactly over the hurricane. Sometimes the heating patterns associated with tropical cyclones are so profound that the upper level high is found only in the far upper portions of the middle atmosphere (at about 60000 feet or higher) and will not appear on the weather maps conventionally used by meteorologists.

Because surface low pressure areas that form in the manner described in this section have the warmest temperatures at their centers, they are called warm core lows. Here's a comparison of the sealevel pressures and average temperature of the surface to 18000 feet layer for Hurricane Katrina and an extratropical cyclone that affected the West Coast of the United States in January 2008. Note that Katrina has the warmest temperatures at its center, whereas the extratropical (wave) cyclone has an asymmetric temperature distribution.

It turns out that there is an additional and profound source of heating that occurs when tropical cyclones begin to form. In fact, this source of heating is many fold more important than the sea surface temperature patterns we have discussed in class. This sort of heating is related to the condensation of water vapor and will be discussed in class.

Advanced: For Metr 201 Students -- Does differential heating produce upper tropospheric divergence if the wind flow is nearly geostrophic?

Note that the model described above works if wind flow is not affected by Coriolis acceleration (is at right angles to the isobars). But differential heating as a way of producing upper tropospheric divergence does work at all latitudes, even where Coriolis accelerations are large.

Consider, for example, the heating produced by the North American continent during the summer. Let's say that the wind is in geostrophic balance over the continent around the upper tropospheric anticyclone at 300 mb in the early morning. There would be no divergence if the wind is in balance and flowing parallel to the height contours.

In the first hour after sunrise, the continent heats up, and the air columns get taller (have larger thickness) as a result. This strengthens the upper tropospheric antityclone and creates a larger pressure gradient than existed previously, which takes the air out of geostrophic balance. For a short time, the air flows outward from this high until it gets into geostrophic balance again...in short, divergence occurs in the upper troposphere. Pressures lower at the ground because of this.

If, in the next hour, heating continues, then the upper high strengthens again, taking the air out of geostrophic balance (creating larger lateral pressure gradients), resulting in continuing divergence. This continues during the day till around 4PM or so. As a result, the surface thermal low intensifies from morning to late afternoon, explaining the fact that all wind records for coastal stations in California show the greatest west wind speed to occur around 4PM on summer afternoons.

B. Development of upper level divergence because of nearly geostrophic wind flow patterns associated with the jet stream: Dynamic Pressure Systems

 Divergence aloft also can be associated with patterns aloft (generally associated with the jet stream and, thus, are best developed above 18000 feet). The primary way in which this occurs is summarized below. (Metr 201 Students should read this: There are other ways not discussed here that are beyond the level of Metr 302/356.)

If we can identify these areas, we can suspect that surface convergence and a disturbance will form at sealevel underneath them. These types of surface lows and highs are called generally "Dynamic Lows and Highs." Wave Cyclones are specific examples of dynamic lows, just as hurricanes are specific examples of thermal lows.

(Above) A schematic example of divergence and convergence patterns associated with troughs and ridges at jet stream levels and the resulting "rule of thumb." (Below) A real-life example from October 29, 2004