GEOG 102 - Module 8&9 and Reading Notes Week 7 PDF

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Summary

Lecture and Reading Notes - Module 8/9. This content covers topics such as clouds and precipitation, particularly atmospheric lifting mechanisms....

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Clouds and Precipitation Atmospheric Lifting Mechanisms When an air mass is lifted, it cools adiabatically (by expansion). When the cooling reaches the dew-point temp., moisture in the saturated air can condense, forming clouds and perhaps precipitation. Four principal lifting mechanisms operate in the atmosphere: • Convergent lifting results when air flows toward an area of low pressure. • Convectional lifting happens when air is stimulated by local surface heating. • Orographic lifting occurs when air is forced over a barrier such as a mountain range. • Frontal lifting occurs as air is displaced upward along the leading edges of contrasting air masses. In each situation, the air is forced to rise. If the air has enough humidity, and the air cools adiabatically to the dew point, cloud will form. If the air is dry, no matter how much vertical uplift there is, it may not cool to the dew point. In this case there is no cloud formation. In contrast, the air may be very humid but there is not enough vertical motion to cool it to the dew point, in which case, there will be no cloud formation. It is only when the conditions are such that dew point is reached that there will be cloud formation.

Convergent Lifting Occurs!when air moves horizontally over a smooth surface, such as a calm lake surface,!and then blows over a rougher surface, such as the land. The increase in friction over the rough surface slows the wind down and the air then “converges” or piles up along the shoreline. The only place for this accumulation of air to go is up, and therefore it rises. This can happen in any instance of an increase in surface friction, such as when wind blows from a large expanse of agricultural field onto a rougher forested region.

Convectional Lifting Is the term for!rising air resulting from heating at the surface. This is a common cause of cloud formation on a hot summer day over agricultural country. On the prairies, there are large expanses of fields covered in grain adjacent to fields without vegetation that may be fallow. There is a tremendous difference in surface heating between the two fields because of the difference in reflectivity or albedo of the dark soil and lighter vegetation. Air rises over the dark fields and this leads to local convection, which may cause clouds to form over these fields.

Orographic Lifting Occurs when air is forcibly lifted upslope as it is pushed against a mountain (Figure!8.3c). The lifting air cools adiabatically. Stable air forced upward in this manner may produce stratiform clouds, whereas unstable or conditionally unstable air usually forms a line of cumulus and cumulonimbus clouds. An orographic barrier enhances convectional activity and causes additional lifting during the passage of weather fronts and cyclonic systems, thereby extracting more moisture from passing air masses and resulting in!orographic precipitation. Example: When the wind approaches the Rockies from the Pacific Ocean, the clouds are saturated with moisture

and when they are forced to rise over the mountains, they quickly cool to the dew point and precipitation is formed. This is why the western coast of Canada has such lush vegetation, and cloudy weather!!On the downwind side of the orographic obstruction, the air is descending and the opposite happens. The air is descending so it is now!warming!at the dry adiabatic lapse rate. With the increasing temperature, all of the moisture in the cloud is now evaporating and the cloud dissipates; there is no precipitation and this lee side of the mountain is a rain shadow. *More on Chinook winds in 8.2 Lesson and text pp. 211.

Frontal Lifting This is the uplift that occurs at fronts which are the main drivers of weather in the mid-latitude regions. A front is a place of atmospheric discontinuity, a narrow zone forming a line of conflict between two air masses of different temperature, pressure, humidity, wind direction and speed, and cloud development. The leading edge of a cold air mass is a!cold front, whereas the leading edge of a warm air mass is a!warm front. Fronts are boundaries between the cold air masses that typically originate in the Arctic and the warm air masses that originate towards the equator. They move back and forth over the landscape under control of the jet stream, which is a river of air flowing around the globe above us in mid-latitudes. The movement of the jet stream “steers” the fronts and the associated low and high pressure cells. The daily analysis of the jet stream is therefore an important part of weather forecasting. The slope for the warm front is much more gentle, being between 1 and 1.5 degrees.In both cases, warm air is being pushed up over the cold air so that it reaches the dew point and cloud forms from condensation. The difference in the slope means that there is a difference in cloud type. The steep slope of the cold front induces more severe vertical motion and the clouds are of greater height and there is more vertical motion within the cloud. Therefore, the intensity of the precipitation is more extreme than for the warm front. The shallower slope of the warm front means less intense vertical motion and more shallow clouds. But the lower slope means that the front extends over a larger area (note the horizontal distance of 1000 km for the warm front versus 400 km for the cold front) and therefore the clouds are more widely spread or dispersed. Cold Front The steep face of an advancing cold air mass reflects the ground-hugging nature of cold air, caused by its greater density and more uniform characteristics compared to the warmer air mass it displaces (Figure!8.8a). Warm, moist air!in advance of the cold front lifts upward abruptly and experiences the same adiabatic rates of cooling and factors of stability or instability that pertain to all lifting air parcels. Warm Front Warm air masses can be carried by the jet stream into regions with colder air, such as when an airflow called the “Pineapple Express” carries warm, moist air from Hawai‘i and the Pacific to the Pacific coast of North America. The leading edge of an advancing warm air mass is unable to displace cooler, passive air, which is denser along the surface. Instead, the warm air tends to push the cooler, underlying air into a characteristic wedge shape, with the warmer air sliding up over the cooler air. Thus, in the cooler-air region, a temperature inversion is present, sometimes causing poor air drainage and stagnation.

Clouds and Fog

The visual aspect of clouds indicates a great deal about the nature of the uplift that causes them and therefore is an excellent indicator of what type of weather is occurring and will occur. Basically, clouds can be classified based upon the altitude in which they form and the visual structure. Principle cloud types and special cloud forms according to form and altitude (low, middle, high, and vertically developed), see right.

Cloud Types Clouds that are formed with vertical force tend to have bulbous structures and are named as a type of!cumulus!which is derived from the Latin for “heap or pile” cloud. The bulbous structures are individual parcels of air (the “balloons” discussed in Module 7) that have risen through the lifting condensation level, condensed, are unstable, and continue to rise through the cloud until you see them as they increase the cloud’s volume. As these features represent very unstable conditions, they are often found at cold fronts with their steep slope of 2º, over heated surfaces where there is strong convection, and along steep upslope sides of mountains. If there is precipitation from cumulus clouds, it is termed a!cumulonimbus cloud:!nimbus!is Latin for a “dark cloud.” If there is more gentle vertical motion extended over a large area, such as is found with a warm front, the cloud has less vertical development but more extensive horizontal development; it looks more like a blanket and is therefore called a!stratus cloud. It comes from the Latin!sternere!which means “strewn,” which seems appropriate for the form. These clouds are often found with the gentle slopes of the warm fronts or gentle topographic slopes. A sub category is determined according to the height of the clouds, which includes high, middle, and law clouds: Cirrus!are high clouds. Cirrus refers to clouds composed of ice crystals as opposed to water droplets and therefore are clouds above the freezing level. In many locations in mid-latitudes, this will be above 7000 m altitude. As they are composed of crystals - they have the distinctive form of small structures with little cohesiveness that you see in the photograph of Figure 7.17 (c). There are many subcategories but some are called!mare’s tails!because of the appearance of being flung across the sky. Others are stratus variants or cumulus variants (cirrostratus!and!cirrocumulus), depending upon the dynamics of the air motion at high altitudes. As cirrus clouds are high altitude, they are often the first visual cue of a warm front coming into the region. Note that fronts most often come in from the west with the westerly flow of the Jet Stream. With airflow from the west, you will see first the cirrus cloud, then the lower cirrostratus, then the middle layer of altostratus and stratus of the lower level. After a period of fair weather, if the sky starts to become cloudy with cirrus cloud, you can be very certain that a warm front will come within 12 to 24 hours. Middle layer clouds have the designation “alto” such as altostratus or altocumulus. Lower clouds are just cumulus or stratus, and if they start to precipitate we have!nimbostratus!or!cumulonimbus!formations. In Figure 7.17 (g) you see!lenticular!clouds which are specific to topographic obstructions. The smooth surfaces

are caused by the streams of air rising above the ridge with moisture condensing, and the curving down on the lee side with the moisture evaporating as the air warms adiabatically. Other unique formations of include the morning glory found in Australia. Human activity can induce cloud as well. In mid-latitudes, water droplets need condensation nuclei to form around and these can be dust particles, salt crystals from sea spray, and pollutants, amongst others. Satellite images show ship tracks created from the exhaust particulates.! Exhaust from jet planes produces contrails from these particulates that can expand to create expanses of cirrus cloud. During the shutdown of air traffic after 9/11, there was a measurable reduction in cloud cover over North America.

Weather Patterns The warm and cold fronts of the midlatitudes bring us the local weather changes in precipitation, temperature, and humidity as we alternate between cold air masses and warm air masses. Typically, they move into an area from the west. How does this happen? To understand this, we have to examine the Jet Stream, which is a river of air flowing around the globe in midlatitudes. We will see that the Jet Stream “steers” the surface fronts and this is a fundamental aspect of weather forecasting. Four forces determine both speed and direction of winds. The first of these is Earth’s!gravitational force, which exerts a virtually uniform pressure on the atmosphere over all of Earth. Gravity compresses the atmosphere, with the density decreasing as altitude increases. The gravitational force counteracts the outward centrifugal force acting on Earth’s spinning surface and atmosphere. (Centrifugal force is the apparent force drawing a rotating body away from the centre of rotation; it is equal and opposite to the centripetal, or “centre-seeking,” force.) Without gravity, there would be no atmospheric pressure—or atmosphere, for that matter. The other forces affecting winds are the pressure gradient force, Coriolis force, and friction force. All of these forces operate on moving air and ocean currents at Earth’s surface and influence global wind-circulation patterns. Three physical forces interact to produce wind patterns at the surface and in the upper atmosphere: pressure gradient force, Coriolis force (which counters the pressure gradient force, producing a geostrophic wind flow in the upper atmosphere), and fictional force (which, combined with the other two forces, produces characteristic surface winds. Figure!6.8b!illustrates the combined effect of the pressure gradient force and the Coriolis force on air currents in the upper atmosphere, above about 1000 m. Together, they produce winds that do not flow directly from high to low but flow around the pressure areas, remaining parallel to the isobars. Such winds are!geostrophic winds!and are characteristic of upper tropospheric circulation. (The suffix!strophic!means “to turn.”) Near the surface, friction prevents the equilibrium between the pressure gradient and Coriolis forces that results in geostrophic wind flows in the upper atmosphere

(Figure!6.8c). Because surface friction decreases wind speed, it reduces the effect of the Coriolis force and causes winds to move across isobars at an angle. Thus, wind flows around pressure centres form enclosed areas called!pressure systems, or!pressure cells, as illustrated in!Figure!6.8c.

High and Low-Pressure Systems In the Northern Hemisphere, surface winds spiral out from a!high-pressure area!in a clockwise direction, forming an!anticyclone, and spiral into a!low-pressure area!in a counterclockwise direction, forming a!cyclone!(Figure!6.8). In the Southern Hemisphere these circulation patterns are reversed, with winds flowing counterclockwise out of anticyclonic high-pressure cells and clockwise into cyclonic low-pressure cells. Anticyclones and cyclones have vertical air movement in addition to these horizontal patterns. As air moves away from the centres of an anticyclone, it is replaced by descending, or subsiding (sinking), air. These high-pressure systems are typically characterised by clear skies. As surface air flows toward the centres of a cyclone, it converges and moves upward. These rising motions promote the formation of cloudy and stormy weather. Often these cells have elongated shapes and are called low-pressure “troughs” or high-pressure “ridges”. Surface winds spiralling clockwise out of the high-pressure area toward the low pressure, where winds spiral counterclockwise into the low. The pattern of high and low pressures on Earth in generalized belts in each hemisphere produces the distribution of specific wind systems. These primary pressure regions are the!equatorial low, the weak!polar highs!(at both the North and the South Poles), the!subtropical highs, and the!subpolar lows. All along the equator, winds converge into the equatorial low, creating the!intertropical convergence zone!(ITCZ). Air rises in this zone and descends in the subtropics in each hemisphere. The winds returning to the ITCZ from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere produce the!trade winds. The subtropical high-pressure cells on Earth are generally between 20° and 35° in each hemisphere. In the Northern Hemisphere, they include the!Bermuda High,!Azores High, and!Pacific High. Winds flowing out of the subtropics to higher latitudes produce the!westerlies!in each hemisphere. In January, two low-pressure cells known as the!Aleutian Low!and!Icelandic Low!dominate the North Pacific and Atlantic, respectively. The region of contrast between cold polar air and the warmer air toward the equator is the!polar front. The weak and variable!polar easterlies!diverge from the high-pressure cells at each pole, the stronger of which is the!Antarctic High.

Upper-Air Circulation A!constant isobaric surface—a surface that varies in altitude from place to place according to where a given air pressure, such as 500 mb, occurs—is useful for visualizing geostrophic wind patterns in the middle and upper troposphere. The variations in altitude of this surface show the ridges and troughs around high- and low-pressure systems. Areas of converging upper-air winds sustain surface highs, and areas of diverging upper-air winds sustain surface lows. Vast wave motions in the upper-air westerlies are known as!Rossby waves. Prominent streams of high-speed westerly winds in the upper-level troposphere are the!jet streams. Depending on their latitudinal position in either

hemisphere, they are termed the!polar jet stream!or the!subtropical jet stream.

Monsoons and Local Winds Intense, seasonally shifting wind systems occur in the tropics over Southeast Asia, Indonesia, India, northern Australia, equatorial Africa, and southern Arizona. These winds are associated with an annual cycle of returning precipitation with the summer Sun and named using the Arabic word for season,!mausim, or!monsoon.!The location and size of the Asian landmass and its proximity to the seasonally shifting ITCZ over the Indian Ocean drive the monsoons of southern and eastern Asia. The difference in the heating characteristics of land and water surfaces creates!land and sea breezes. Temperature differences during the day and evening between valleys and mountain summits cause!mountain and valley breezes.!Katabatic winds, or gravity drainage winds, are of larger regional scale and are usually stronger than valley and mountain breezes under certain conditions. An elevated plateau or highland is essential, where layers of air at the surface cool, become denser, and flow downslope.

Midlatitude Cyclonic Systems A!midlatitude cyclone,!or!wave cyclone,!is a vast low-pressure system that migrates across a continent, pulling air masses into conflict along fronts. These systems are guided by the jet streams of the upper troposphere along seasonally shifting!storm tracks.!Cyclogenesis, the birth of the low-pressure circulation, can occur off the west coast of North America, along the polar front, along the lee slopes of the Rockies, in the Gulf of Mexico, and along the East Coast. A midlatitude cyclone can be thought of as having a life cycle of birth, maturity, old age, and dissolution; or open, occluded, and dissolving stages. An!occluded front!is produced when a cold front overtakes a warm front in the maturing cyclone. Sometimes a!stationary front!develops between conflicting air masses, where airflow is parallel to the front on both sides.

The Jet Stream This illustrates the difference in the net radiation for the planet by latitude (the net radiation is the imbalance between the net received solar irradiance and the net longwave emission from the Earth). There is a surplus in the tropics and subtropics and a deficit poleward of about 35ºN and S. Energy always flows from source to sink, so there must be some mechanism to transport it from the surplus of the tropics to the deficit of the poles. That mechanism is the circulation of the atmosphere (the winds) and the currents of the ocean. About half of the surplus is transported by the atmosphere and the balance by the ocean. How is the atmosphere circulation driven? First, note the right of Figure 6.1b which provides a cross section of the winds from the equator to the pole. At the heated equator, along the “thermal equator,” the air rises and because of the high humidity and cooling to the dew point, there is extensive precipitation, which you can see in Figure 6.2. When it rises to approximately 15 km, it hits a temperature inversion between the lower troposphere and the overlying stratosphere called the tropopause and cannot rise further (because the vertical motion becomes stable), so it branches horizontally, both to the north and to the south. As it moves towards the poles, it cools with the lower net radiation in these regions and eventually sinks at 30ºN and S. As the air is subsiding, there will not be any

precipitation and this is the region of the great deserts of the world, such as the Sahara, the outback of Australia, the Namibian desert of southern Africa, and the Mojave of the United States. With subsiding air, there is an accumulation of the weight of air at the surface, and this creates the major semipermanent high pressure cells of the globe. Some of this subsiding air will slide north and some will slide south, back towards the equator. The surface winds towards the eq...