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Weather Systems Chapter 7 Atmospheric Moisture and Precipitation Some patterns of wind circulation occur regularly and give rise to recurring weather conditions. For example, travelling lowpressure centres often bi bringclouds l d and d precipitation; i it ti outbreaks tb k off polar l air i typically give cold, clear weather in winter. These types of recurring circulation patterns and the conditions associated with them are called weather systems. Air Masses An air mass is a large body of air with fairly uniform temperature and moisture characteristics. Air masses can be several thousand kilometres across and extend to the top p of the troposphere. p p Each air mass is characterized by a distinctive combination of conditions at the surface, particularly temperature, environmental temperature lapse rate, and humidity. These properties are acquired in source regions over which the air moves slowly or stagnates. 1 Air Masses Five types of air masses are distinguished with respect to latitude. These are referred to as arctic (A), antarctic (AA), polar (P), tropical (T), and equatorial (E) For the type of underlying surface, surface two subdivisions are used: maritime (m) and continental (c). Six important air mass types can be identified from these descriptive labels: cA, cP, cT, mP, mE, mT. 2 Air Masses North American Air Masses Continental polar (cP) and continental arctic (cA) air masses are the dominant types affecting North America. Maritime polar air masses originate over the North Pacific and Bering Strait, in the region of the persistent Aleutian low pressure centre. Air Masses North American Air Masses Maritime tropical air masses from the Gulf of Mexico most commonly affect the central and eastern United States; mT air is the main opponent of cA and cP over North America. The properties of air masses are modified as they move away from their source regions (degree of modification depends on their speed and also the general routes they follow). Air Masses North American Air Masses An air mass usually has a well-defined boundary between itself and a neighbouring air mass; these transitional zones are termed fronts. In North America, the boundary between polar and tropical air masses produces the well-defined polar front that is located below the axis of the polar front jet stream. 3 Poleward Transport of Heat and Moisture Because solar energy does not warm the Earth’s surface uniformly, the atmosphere and oceans exhibit a complex circulation pattern of air and water flows, which acts to redistribute absorbed solar radiation more evenly by way of various convection loops (poleward heat transport). Lower Latitudes: through thermally direct Hadley Cells. Mid, High Latitudes: through Rossby Wave mechanism. 4 Travelling Cyclones and Anticyclones Air masses are set in motion by the pressure gradients (between regions of high and low pressure) created by uneven heating or cooling of the atmosphere. Then driven along by the general global wind system and by regional pressure gradients associated with centres of high and low pressure. Travelling Cyclones and Anticyclones Because pressure varies regionally over great distances, air masses are moved far their source regions. This type of movement is a characteristic feature of the midlatitude atmosphere atmosphere, where travelling cyclones and travelling anticyclones bring changing weather conditions as they pass across a region. Travelling Cyclones and Anticyclones In a travelling cyclonic system, convergence and upward motion cause air to rise and be cooled adiabatically. If the air is moist, condensation or sublimation can occur; this may lead to cyclonic precipitation precipitation. When pressure gradients are steep and the air is laden with water vapour, strong winds and heavy rain or snow can accompany them (producing a cyclonic storm). 5 Travelling Cyclones and Anticyclones Travelling cyclones fall into three types distinguished primarily by their intensity and geographic location: 1) Midlatitudes, arctic and Antarctic - wave cyclones or depressions 2) Tropical and subtropical – tropical cyclones (which can intensify to hurricanes and typhoons) 3) tornado 6 Travelling Cyclones and Anticyclones Wave Cyclone In mid- and high latitudes, the dominant form of weather system is the wave cyclone, a large mass of inward-spiralling air that repeatedly forms forms, intensifies intensifies, and dissolves along the polar front front. A situation favourable to the formation of a wave cyclone occurs when two large anticyclones lie on either side of the polar front. The boundary between the two high-pressure cells is necessarily a region of lower pressure and so forms a low-pressure trough. Travelling Cyclones and Anticyclones Wave Cyclone A warm front is where warm air is moving over a region of colder air. The cold air mass remains in contact with the ground because it is denser, so consequently the moving mass of warm air is forced to rise over it. The gradient on a warm front is generally about 1:300. 7 Travelling Cyclones and Anticyclones Wave Cyclone A cold front develops when a cold air mass advances into the zone occupied by the warm air mass. Because the colder air mass is denser, it remains in contact with the ground. The gradient of a cold front is about 1:50, so it is much steeper than a warm front. In this case, the front represents the leading edge of the approaching cold air. Travelling Cyclones and Anticyclones Wave Cyclone Cold fronts normally move across a region at a faster rate than warm fronts, and where they are in close proximity, a cold ld ffrontt can eventually t ll overtake t k a warm ffront. t This produces the occluded stage of the wave cyclone. The colder air of the faster-moving cold front remains next to the ground, forcing the warm air ahead of it to rise along the occluded front. 8 Travelling Cyclones and Anticyclones Wave Cyclone Two types of occluded fronts: A warm occlusion develops when the leading mass of cold air is colder and denser than the trailing mass of cold air (the trailing cold air mass will rise up the warm front and lift the warm air off of the surface). In a cold occlusion, the trailing mass of cold air is denser than the cold air ahead of the warm front (the warm sector air is lifted off of the surface as the trailing cold air catches up with and pushes under the leading mass of cold air). 9 Travelling Cyclones and Anticyclones Cyclone Tracks and Cyclone Families Wave cyclones tend to form in certain areas and follow similar routes until they dissolve. In the northern hemisphere, p , wave cyclones y are heavilyy concentrated in the neighbourhood of the Aleutian and Icelandic Lows. Three or four commonly form in succession, then travel as a chain across the North Pacific and North Atlantic oceans The general eastward movement of wave cyclones is largely controlled by the paths of the Rossby waves and their relationship to the polar front jet streams. The curving paths of Rossby waves is a prime factor in wave cyclone development. 10 Tropical and Equatorial Weather Systems Tropical and equatorial zone weather systems show some basic differences from those of the midlatitudes. Upper-air winds are often weak, so air mass movement is slow and gradual. Tropical and equatorial air masses tend to be warm and moist moist, and consequently clearly defined fronts and large, intense wave cyclones do not develop. On the other hand, strong convectional activity occurs because of the high moisture content of low-latitude maritime air masses. With these conditions, even slight convergence and uplift can be enough to trigger abundant precipitation. 11 Tropical and Equatorial Weather Systems Easterly Waves and Weak Equatorial Lows One of the simplest forms of tropical weather systems is an easterly wave, a slowly moving, convectively active trough of low pressure that is driven westward by the trade winds. The easterly waves that cross the Atlantic Ocean are initiated by instability over northwestern Africa and are associated with the low latitude easterly jet. Another related weather system is the weak equatorial low, a disturbance that forms near the centre of the equatorial trough. Moist equatorial air masses converge on the centre of the low, causing numerous convectional storms and heavy rainfall. Tropical and Equatorial Weather Systems Polar Outbreaks Another distinctive feature of low-latitude weather is the occasional penetration of powerful tongues of cold polar air from the midlatitudes into very low latitudes. The leading edge of these polar outbreaks is marked by squalls along the cold front, and is followed by unusually cool, clear weather with strong, steady Winds (best developed in the Americas). 12 Tropical and Equatorial Weather Systems Tropical Cyclones Tropical weather disturbances range from weak tropical lows to powerful and destructive tropical cyclones (“a storm of tropical origin of small diameter with a minimum surface pressure below 95 kPa, with greater than 125 kph, p and accompanied p by y torrential violent winds g rain.”) A tropical cyclone is known variously as a hurricane in the western hemisphere, a typhoon in the western Pacific, and a cyclone in the Indian Ocean. These storms revolve around a central “eye,” which is about 30 to 50 km in diameter and characterized by light winds and clear skies. 13 Tropical and Equatorial Weather Systems Tropical Cyclones Tropical cyclones develop over oceans at 7° to 15° N and S latitudes. The poleward limit is determined by the need for sea-surface temperatures that are high enough to ensure sufficient evaporation and release of latent heat to maintain the storm (sea-surface temperatures of over 27°C). Although sea-surface temperatures exceed 27°C near the equator, at latitudes below 7° the Coriolis effect is too weak to initiate the storm’s circular motion. 14 Tropical and Equatorial Weather Systems Tropical Cyclones A characteristic feature of the tropical cyclone is its central eye, in which clear skies and calm winds prevail. The eye is a cloud-free vortex produced by the intense spiralling of the storm, and usually develops when sustained wind speeds exceed 120 kph. 15 Tropical and Equatorial Weather Systems Tropical Cyclones The Saffir-Simpson scale rates the intensity of tropical cyclones. The rating is based on the central pressure of the storm, mean wind speed, and height of accompanying storm surge, which can raise sea level several metres above normal. Category 1 storms are weak, while Category 5 storms are devastating. Tropical and Equatorial Weather Systems Tropical Cyclones Weather forecasters tracking tropical cyclones give them names (alternate male and female names in an p sequence, q , with the exception p of ones alphabetical beginning with Q, U, X, Y, and Z). Each region has its own set of names; in North America one set is used for hurricanes in the Atlantic, and another for typhoons of the Pacific. 16 Tropical and Equatorial Weather Systems Impacts of Tropical Cyclones Tropical cyclones can be tremendously destructive. Islands and coasts feel the full force of the high winds and flooding as tropical cyclones move onshore. Densely populated low-lying areas are particularly vulnerable to tropical cyclones Deadliest tropical cyclone on record struck Bangladesh on November 13, 1970, with at least 500,000 deaths reported. In terms of cost, Hurricane Katrina, in August 2005, caused an estimated US$200 billion in damage. 17 Tornado A tornado is a small but intense cyclonic vortex in which air spirals at tremendous speed. Tornadoes generally occur in the midlatitudes between about 30° and 50°. Nowhere are they as frequent, or as violent, as in the United States, where 1,000 or more strike annually. In Canada, about 80 tornadoes are reported each year (tornado season in Canada typically extends from May to September, with most occurring in June or July). Tornado Tornadoes form most often in association with intense supercell thunderstorms, with the first stage of tornadogenesis indicated by the development of a mesocyclone due to rotation in the updraft core induced by strong wind shear. Initial dust whirl stage: a tornado usually takes the form of a swirling i li d dustt cloud l d th thatt extends t d ffrom th the ground d tto a short, h t pendant funnel cloud hanging below the base of the mesocyclone cloud mass. Funnel clouds form by condensation of water vapour when the air is cooled adiabatically through expansion as it is taken aloft in the low-pressure vortex. Tornado The organizing stage: often marked by the rapid lowering of the base of the thunderstorm as a rotating wall cloud develops beneath the mesocyclone (many tornadoes never develop beyond the organizing stage, but simply lose energy and disappear). The mature stage: in which the vortex becomes nearly vertical and is darkened with dust and debris (the tornado is generally in the order of 100 to 500 m in diameter with wind speeds ranging from 250 to 350 kph). 18 19 Tornado Development in the United States Tornado In 1971, the Fujita (F) scale was devised to assess the intensity of North American tornadoes on the basis of the damage they caused (F1 to F5). The Fujita scale has been criticized since it is biased toward the worst damage, even though no consideration is given for building b ildi ttype or method th d off construction t ti ((resulting lti iin overestimated wind speeds in some cases). As result, in Canada and the United States, the F-scale was replaced in 2007 by the Enhanced Fujita (EF) scale. (incorporates 28 damage indicators, including different types of buildings and trees for which varying degrees of damage are assessed). 20 A Look Ahead Annual cycles of temperature and precipitation in most regions are quite predictable given the changes in wind patterns, air mass flows, and weather systems that occur with the seasons. This creates spatially variable weather patterns and provides the basis for climate and climate classification (Chapters 8 to 10). 21