GEOL/METR/OCN 405:
Planetary Climate Change
(Spring 2018)
Notes on
Lab Activity #8
Dr. Dave Dempsey
Dept. of Earth & Climate Sci.,
SFSU

Global Patterns of Temperature, Pressure, Wind, and Clouds

  1. Global Temperature Patterns in the Lower Troposphere

    Prominent features of the lower-troposphere pattern (but well above the direct influence of the earth's surface locally, with a few exceptions) include:

    1. Temperature is generally warmest at low latitudes and decreases toward the poles.
    2. There is a relatively narrow zone in midlatitudes across which the temperature varies rapidly, from warm on the equatorial side to cold on the polar side. This is a zone of large temperature gradient. It is called the polar front.
    3. The polar front is not a straight east-west feature—it has "wobbles" in it. These wobbles form alternating "tongues" of relatively cold air "protruding" toward the equator and relatively warm air "protruding" toward the poles, in the midlatitudes of both hemispheres.

    How these features change over period of several days:

    1. The alternating tongues of warm and cold air at midlatitudes migrate eastward in both hemispheres. At any fixed point at these latitudes, it would be alternatively warmer and colder and warmer again, relatively speaking, on time scales of a few days to a week.

    How September and February patterns differ:

    1. Northern Hemisphere high latitudes become colder by February and warmer by September.
    2. Temperature gradient across the polar front is noticeably larger in February then September in N. Hem.
    3. In both hemispheres, the polar front shifts southward by February and northward by September. (Equivalently, by February the polar front shifts toward the equator in the N. Hemisphere but away from equator in S. Hemisphere; the opposite is true in September.) The sun strikes the earth most directly at points that shift northward by late June and southward by late December (the solstices); the temperature pattern tracks the sun, with a lag of several months.

    This has significant implications for variability in temperature experienced over several days to a week in the lower to middle midlatitudes (including California)—by summer there is relatively little variability because the polar front has shifted poleward, and the eastward-propagating waves or "wobbles" in the polar front, defined by alternating "tongues" of relatively warmer and colder air, no longer pass over California; by winter the polar front has shifted equatorward again and there is more variability in the lower and middle midlatitudes as the alternating tongues of warm and cold air pass by.

  2. Global Pressure Patterns in the Upper Troposphere

    Pressure in the atmosphere decreases rapidly with increasing altitude (i.e., upward). However, at any particular altitude, pressure will vary slightly from place to place—some places will have slightly higher pressure, some slightly lower. The resulting, relatively small horizontal pressure differences are responsible for creating winds, so we care about them—they're important.

    1. The pressure pattern in upper troposphere (say, 6-7 miles or 8-11 km up) is very similar to the temperature pattern in the lower troposphere. In particular, we find relatively higher pressure aloft above warmer air (particularly obvious at low latitudes) and relatively lower pressure aloft above colder air (particularly noticeable at high latitudes). ("Aloft" here means "significantly far up into the troposphere—say, half way up or farther.)
    2. There are large pressure gradients aloft above large temperature gradients in the lower troposphere (that is, above—at higher altitude than but the same latitude as—the polar front at midlatitudes).
    3. The zone of large pressure gradient, which lies (mostly) at midlatitudes above the polar front (which is, by definition, a zone of large temperature gradient), has "wobbles" in it, and the wobbles migrate eastward in both hemispheres, just as wobbles in the the polar front do.

  3. Pressure differences between places (horizontally) push air into motion (that is, create winds). Larger pressure gradients imply a bigger push and should produce faster winds. Let's see if that's true...

  4. Global Patterns of Winds in the Upper Troposphere

    1. We observe a narrow belt of fast winds, blowing eastward, in the upper troposphere at (mostly) midlatitudes in both hemispheres. These features are called jet streams.
    2. In winter the jet stream is stronger, wider, and closer to the equator; in summer it is weaker, narrower, and farther from the equator.
    3. The jet stream exhibits "wobbles", just as the polar front in the lower troposphere and the pressure pattern aloft do.
    4. These wobbles migrate eastward, in both hemispheres (though much more slowly than the winds themselves blow through the wobble pattern).

  5. Global Sea-Level Pressure Patterns

    1. In the higher midlatitudes and lower high latitudes in each hemisphere (particularly clearly in the Southern Hemisphere) there is a series of relatively low sea-level pressure areas encircling the globe. (Each adjacent pair of sea-level low pressure areas is separated by a much subtler region of relatively higher sea-level pressure.)
    2. In the subtropics there is a series of relatively high sea-level pressure areas around the globe in each hemisphere (clearer in the Southern Hemisphere, at least in September).
    3. In the tropics the sea-level pressure is lower than in subtropics, though not very dramatically so.
    4. Over Antarctica there is relatively high sea-level pressure. (We would see relatively high pressure over Arctic much of the time, too, if we had more maps to look at, especially in winter.)

    Variability over several days:

    1. Little change in tropics, subtropics, and polar areas.
    2. The sea-level lows and weaker, interspersed sea-level higher pressure areas in the midlatitudes and lower high latitudes migrate eastward in both hemispheres.

    Connections with previous maps are in some respects hard to see, but here's what we should see if we looked carefully:

    1. Midlatitude sea-level lows, and weaker, interspersed sea-level highs, form along the polar front, beneath the jet stream aloft, and move along with the waves/wobbles in both of them (eastward).
    2. No really obvious connections between sea-level pressure and other quantities at other latitudes—the connections must be more subtle or complex.


  6. Global Cloud Patterns

    Most infrared satellite images show the intensity of infrared radiation emitted by features of the earth, at a wavelength in the part of the atmospheric absorption spectrum called the "atmospheric window", where water vapor, carbon dioxide, and other greenhouse gases (but not clouds) don't absorb and hence don't emit radiation. Clouds do, though. Hence, images recording emission intensity at this wavelength show clouds, the ocean surface, land surface, and snow/ice surfaces, but not greenhouse gases.

    In the middle and high latitudes especially, for clouds to produce precipitation they must consist at least partly of a mixture of ice particles and water droplets. (Unfortunately we don't have time to discuss why.) For this to happen the clouds have to be sufficiently cold, which means they have to be sufficiently high (recall that temperature decreases rapidly with altitude in the troposphere, where most clouds form) and deep. Deep clouds with high tops will have cold tops, which won't emit as much longwave infrared radiation as clouds with lower, warmer tops (Stefan-Boltzmann relation).

    Infrared satellite images are typically created so that greater intensities of longwave IR radiation (and hence warmer features) are shown in darker shades of gray and lower intensities (and hence colder features) are shown in lighter shades of gray (and sometimes in color, to highlight them). With this convention, the tops of deep and hence high-topped and hence cold-topped clouds show up as relatively light-shaded (or colored) against a darker (lower, warmer) background, which makes clouds capable of producing precipitation easier to see. In contrast, lower clouds, which have tops that aren't much colder than surrounding ocean or land surfaces, typically appear as darker shades of gray that are hard to distinguish from surrounding land and ocean surfaces. Hence, infrared satellite images are good for spotting storms (thunderstorms, hurricanes, and midlatitude cyclones), which often contain clouds capable of producing precipitation.

    (High, cold) cloud features visible on global (composite) infrared satellite images:

    1. In the tropics there is a band of clouds encircling most of the globe. The band is narrower and better defined over the central and eastern Pacific Ocean and to some extent over the Atlantic Ocean, and broader and patchier over the western Pacific Ocean , eastern Indian Ocean, and South American and Africa. (These clouds are mostly disorganized, sometimes loosely clustered thunderstorms that last less than a day each but are constantly popping up there. The band is a persistent feature of much of the tropics most of the year and is a product of the convergence of surface winds into the tropics from the subtropics along a zone called the Intertropical Convergence Zone, or ITCZ.)
    2. At midlatitudes and lower high latitudes in both hemispheres (especially obvious in the Southern Hemisphere), there is a sequence of narrow, often slightly arced strips of cloud that gradually migrate eastward. (These arced strips of cloud are associated with a type of storm that we call midlatitude cyclones.)
    3. In the subtropics there is little cloudiness (or at least high cloudiness).
    4. Over continents we can see a "wave" of darker shade of gray (warmer temperatures) propagating across the globe from east to west (right to left). By checking the time of day when the land surface appears darkest, we see that this wave essentially tracks the sun (lagging a little behind it) over the course of each day. This feature isn't visible over the oceans, which, we deduce, must not change temperature very much from day to night.

  7. Regional Patterns of Sea-Level Pressure, Winds, and Clouds

    1. Surface winds blow clockwise around high-pressure centers, counter-clockwise around low-pressure centers. [In the Southern Hemisphere the pattern turns out to be the opposite.] This pattern can be understood as a consequence of the combined effects of the net force due to pressure differences (trying to push air from higher toward lower pressure) and the effect of the rotation of the earth (the Coriolis effect), which tries to deflect moving objects (including air and water), to the right of their direction of motion in the N. Hem. and to the left of their direction of motion in the S. Hemisphere.
    2. The association is not simple, but generally speaking, the long, often arced (curved) strips of cloud with high, cold tops seem to originate near low pressure centers and extend out from them in an arc or even a spiral.
    3. The low-pressure, wind, and cloud patterns seem to grow and decay, while migrating more or less eastward. Sometimes they migrate both eastward and poleward instead of simply eastward. These coordinated patterns define what we call midlatitude cyclonic storms (or midlatitude cyclones, sometimes also called frontal cyclones because of one of their features, a tightening of the polar front—along which these storms form—into cold fronts and warm fronts).


  8. Heat Transport in Midlatitude Cyclones

To summarize, broadly speaking, based on what we've learned so far, we can make the following generalized sequence of dynamical connections:

  1. Because the earth is a sphere and it's surface curves, and given the orientation of the earth's axis of rotation relative to the position of the sun, the sun strikes the lower latitudes more directly than high latitudes—that is, the sun differentially heats the earth.

  2. This differential heating produces a temperature difference between the lower and higher latitudes. The temperature varies most rapidly (that is, the temperature gradient is largest) across a relatively narrow zone at (mostly) midlatitudes that we call the polar front.

  3. The pressure aloft (that is, in the middle and upper troposphere) is controlled by the temperature in the lower troposphere. (We didn't have time to discuss the reasons for this.) In particular, where lower-tropospheric temperatures are relatively high, the pressure aloft is relatively high. Where the lower-tropospheric temperatures are relatively low, the pressure aloft is relatively low. As a result, pressures aloft are relatively high at low latitudes and relatively low at high latitudes. They vary the most rapidly across a relatively narrow zone at (mostly) midlatitudes, which lies above (that is, at higher altitude than, but at the same latitudes as) the polar front.

  4. Pressure differences aloft (created by temperature differences below) push air aloft into motion, creating winds aloft. The larger the pressure gradient, the harder the push on the air and the faster the winds tend to become. The rotation of the earth modifies these air motions (just as it modifies the motions of water in the oceans and, indeed, the motion of any object in motion on the planet, except at the equator), and in the midlatitudes the net effect is to produce a relatively narrow band of fast, eastward-moving air above the polar front. This is the jet stream. (There is a jet stream in both hemispheres.)

  5. Once winds are created aloft, these winds will converge (think "blow together") in some places and diverge (think "blow away from each other") in others, which requires that air sink to lower altitudes or rise up from lower altitudes, respectively, in such regions. Where air rises, clouds tend to form; where air sinks, clouds don't form and if they existed already they dissipate. Also, below places where air converges aloft, the pressure at the earth's surface tends to increase, creating areas of high sea-level pressure; where air diverges aloft, the surface pressure tends to decrease, creating areas of low sea-level pressure.

  6. Areas of low and high sea-level pressure imply the existence of pressure differences between places in the lower troposphere, which push air into motion and creates winds in the lower troposphere and at the earth's surface. The rotation of the earth modifies these motions, and the net result is that air tends to spiral into low-pressure areas near the earth's surface (counter clockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere), converge in the low, and hence be forced up out of the low. Clouds typically form in rising air, so clouds tend to form in low-pressure areas. The opposite typically occurs in sea-level high pressure areas; air tends to spiral out of them (clockwise in the Northern Hemisphere, counter clockwise in the Southern Hemisphere), diverging out of them and forcing air to descend into them to replace the diverging air. These areas tend to be relatively cloud-free.

  7. The winds in the lower troposphere created by pressure differences in the lower troposphere carry heat around with them. This rearranges the temperature pattern created by differential heating (in particular, creating the "wobbles" in the polar front at midlatitudes), which modifies the pressure pattern aloft (see #3 above), which modifies the winds aloft (see #4 above), which modifies the patterns of convergence and divergence of winds aloft (see #5 above), which modifies both the patterns of rising and sinking motion in the troposphere below and the pattern of high and low sea-level pressure (see #6 above), which modifies the lower tropospheric winds pattern.

    This dynamic, interactive system, all forced by differential heating by the sun, manifests itself over short time periods in what we call "weather" and, characterized over time, as "climate". Its net effect on the earth's energy budget is to transport heat from middle to higher latitudes, reducing the temperature difference that would otherwise exist between high and low latitudes and keeping both the higher latitudes more habitable (by keeping it warmer than it would otherwise be) and the middle latitudes more habitable (by producing precipitation in midlatitude cyclones and transporting heat away that is transported there by ocean currents). Because surface winds also drive ocean currents, which by transporting heat from low to middle latitudes, also reduce the temperature difference that would otherwise exist between high and low latitudes, we can say more generally that the net effect of this dynamic system is to transfer heat from lower to higher latitudes, partly counteracting the effect of differential heating by the sun, which drives the atmospheric and (less directly) oceanic circulations in the first place. This effect—in which one action (differential heating by the sun to produce a temperature gradient between low and high latitudes) produces another action (winds in midlatitude cyclones and in Hadley cells at low latitudes, and ocean currents at low and midlatitudes) that acts (transports head from low latitudes to high latitudes) to reduce the effect of the first action (reducing the low-to high latitude temperature gradient), is an example of what we call negative feedback. We'll talk more about what positive and negative feedback are later in the course.

    Anything that changes this system will produce a change in climate.

    (We then summarized the contributions of atmospheric and oceanic circulations to the earth's separate energy budgets at low, middle, and high latitudes.)

Hadley Cells, the ITCZ, and the Subtropical Deserts

This is an addendum to this lab activity, not discussed in class. It's enlightening to examine a long-term, zonal average wind pattern in a vertical, pole-to-pole cross section through the atmosphere (with the vertical scale dramatically distorted to make things visible). The schematic diagram below shows some of the features of the long-term average wind and surface-pressure patterns, plus some characteristic features of "snap shots" of the atmosphere like the ones we examined above. (Long-term average features include the circulating "cells" shown around the outer edges of the diagram, plus surface winds in the tropics; snap-shot features that aren't as clear in long-term averages are shown at the surface in middle and high latitudes).

Global Wind Patterns
Features of the long-term, zonal average winds, plus
"snap shot" features of the surface wind and pressure
patterns on the earth.
(From Ocean Circulations, 2nd Ed., Open University.)

One feature of of the long-term average pattern of winds and surface pressure is a pair of oppositely-circulating "cells" lying on either side of the equator (roughly) and extending to the subtropics. These are called Hadley cells (not labeled in the figure above).

At the earth's surface, the winds within each Hadley cell on either side of the equator (roughly) blow equatorward and converge in a zone near the equator called the Intertropical Convergence Zone (ITCZ) (see figure). (The ITCZ is actually located a little north of the equator, thanks to the greater amount of land area in the Northern Hemisphere.) Converging air rises at the ITCZ; the rising air cools as the pressure on it decreases; water vapor in the rising, cooling air condenses to form clouds; and the clouds produce precipitation in a narrow strip along the ITCZ (see figure).

Aloft, the air then moves away from the ITCZ and hence divererges, which produces the lower pressure at the surface in the tropics, which in turn draws air toward the equator that produced the ITCZ (see figure). The diverging air is pushed poleward by the pressure difference between low and high latitudes aloft, which in turn was created by the temperature difference between low and high latitudes created by differential heating by the sun.

The air poleward-moving air in the Hadley cells aloft subsequently converges with air in the subtropics aloft. This convergence aloft raises the surface pressure in the subtropics. The converging air aloft descends, warming (as the pressure on it increases and it becomes compressed) and preventing any clouds from forming. This (in the subtropics) is where most of the earth's deserts are located. Once the descending air reaches the surface, it diverges again, some heading back to the ITCZ and some heading poleward into the midlatitudes (see figure).

Two other pairs of counter-rotating cells emerge in the long-term, zonal averaging of winds (see figure). One pair of cells appear in the midlatitudes and another at high latitudes, but these are located where weather patterns are much more active and variable, so unlike the Hadley cells, they are not typically visible at any particular moment in time. The ascending branches of these cells are located in the upper midlatitudes/lower high latitudes, where upward motions in midlatitude cyclones tend to be the most intense (on the average) and hence where cloudiness and preciptiation tend to occur more frequently. (In reality, the clouds and precipitation are features of the storms and are not present all the time; they do show up in the long-term average, of course, but so do the clear, dry periods between storms. As a result, the average albedo and precipitation at these latitudes is not as great as it is along the ITCZ, which is much more persistent and dominates the long-term average in the tropics.)


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