Bài giảng khí hậu học chương 8

G304 – Physical Meteorology and Climatology Chapter Air masses and fronts By Vu Thanh Hang, Department of Meteorology, HUS 8.1 Formation of air masses • The areas where air masses form are called source regions • An area must be quite large, many tens of thousands of square kilometers • Horizontal teperature gradient is ~5-7oC/1000km and relative humudity gradient is ~2-3%/1000km • These vertical differences in temperature affect the stability of the atmosphere Æ affects precipitation • Air masses are not permanently confined to their source regions, they are able to migrate to regions marked by less extreme weather conditions 8.1 Formation of air masses (cont.) • Based on moisture content, air masses can be considered either continental (dry) or maritime (moist) • According to their temperature, they are either tropical (warm), polar (cold), or arctic (extremely cold) • A small letter c or m indicates the moisture conditions, followed by a capital letter T, P, or A to represent temperature 8.1 Formation of air masses (cont.) Continental polar (cP) air masses form over large, high-latitude land masses In addition to having very low temperatures, winter cP air masses are extremely dry; summer cP air masses warmer and more humid than in winter Continental arctic (cA) air is colder than continental polar and separated by a transition zone similar to the polar front called the arctic front 8.1 Formation of air masses (cont.) Maritime polar (mP) air masses are similar to continental polar air masses but are more moderate in both temperature and dryness Maritime polar air forms over the North Pacific as cP air moves out from the interior of Asia Maritime polar air also affects much of the East Coast with the circulation of air around mid-latitude cyclones after they pass over a region The resultant winds are the famous northeasters (above) that can bring cold winds and heavy snowfall 8.1 Formation of air masses (cont.) • Continental tropical (cT) air forms during the summer over hot, low-latitude areas These air masses are extremely hot and dry, and often cloud-free • Maritime tropical (mT) air masses develop over warm tropical waters They are warm, moist, and unstable near the surface, which are ideal conditions for the development of clouds and precipitation 8.1 Formation of air masses (cont.) 8.2 Fronts • Fronts are boundaries that separate air masses with differing temperature and other charactersistics • A cold front occurs when a wedge of cold air advances toward the warm air ahead of it • A warm front represents the boundary of a warm air mass moving toward a cold one • A stationary front differs in that neither air mass has recently undergone substantial movement • Occluded fronts appear at the surface as the boundary between two polar air masses, with a colder polar air mass usually advancing on a slightly warmer air mass 8.2 Fronts (cont.) In a typical mid-latitude cyclone, cold and warm fronts separated by a wedge of warm air meet at the center of low pressure Cold air dominates the larger segment on the north side of the system 8.2 Fronts (cont.) Cold fronts typically move more rapidly and in a slightly different direction from the warm air ahead of them This causes convergence ahead of the front and the uplift of the warm air that can lead to cumuliform cloud development and precipitation 8.2 Fronts (cont.) • Five features to determine cold front positions: - Significant temperature differences between adjacent regions - Dew point differences - Bands of cloud cover and precipitation - Narrow zones where wind direction changes - Boundaries separating regions where the atmospheric pressure changes over the 3-hour period 8.2 Fronts (cont.) - Warm fronts have gentler sloping surfaces (1:200) - Surface friction decreases with distance from the ground, as indicated by the longer wind vectors away from the surface (a) - This causes the surface of the front to become less steep through time (b) 8.2 Fronts (cont.) Warm fronts separate advancing masses of warm air from the colder air ahead As with cold fronts, the differing densities of the two air masses discourage mixing, so the warm air flows upward along the boundary This process is called overrunning, which leads to extensive cloud cover along the gently sloping surface of cold air 8.2 Fronts (cont.) • For identifying warm front positions: - look for zone where warmer air advances toward cooler air - dew points typically increase behind the position of the warm front - winds commonly shift from southwesterly ahead of the front to southeasterly behind it - cloud cover and precipitation bands are common - the zone ahead of the warm front gererally undergoes decreasing air pressures while the area immediately behind the front typically has stable air pressure 8.2 Fronts (cont.) • Nonmoving boundaries are called stationary fronts • Although they not move as rapidly as cold or warm fronts, they are identical to them in terms of the relationship between their air masses • The frontal surface is inclined, sloping over the cold air 8.2 Fronts (cont.) - The most complex type of front is an occluded front - When the cold front meets the warm front ahead of it, that segment becomes occluded - The warm air does not disappear, but gets lifted upward, away from the surface - The occluded front becomes longer as more of the cold front converges with the warm front 8.2 Fronts (cont.) - Eventually, the cold front completely overtakes the warm front, Æ the entire system is occluded - In this occlusion, the air behind the original cold front was colder than that ahead of the warm front - This is an example of a cold-type occlusion 8.2 Fronts (cont.) The boundaries separating humid air from dry air are called drylines and are favored locations for thunderstorm development The dryline above (the dashed line) separates low humidity to the west while to the east humidity is higher as indicated by the dew point temperatures [...].. .8. 2 Fronts (cont.) • Five features to determine cold front positions: - Significant temperature differences between adjacent regions - Dew point differences - Bands of cloud cover and precipitation - Narrow zones where wind direction changes - Boundaries separating regions where the atmospheric pressure changes over the 3-hour period 8. 2 Fronts (cont.) - Warm fronts... less steep through time (b) 8. 2 Fronts (cont.) Warm fronts separate advancing masses of warm air from the colder air ahead As with cold fronts, the differing densities of the two air masses discourage mixing, so the warm air flows upward along the boundary This process is called overrunning, which leads to extensive cloud cover along the gently sloping surface of cold air 8. 2 Fronts (cont.) • For identifying... immediately behind the front typically has stable air pressure 8. 2 Fronts (cont.) • Nonmoving boundaries are called stationary fronts • Although they do not move as rapidly as cold or warm fronts, they are identical to them in terms of the relationship between their air masses • The frontal surface is inclined, sloping over the cold air 8. 2 Fronts (cont.) - The most complex type of front is an occluded... becomes longer as more of the cold front converges with the warm front 8. 2 Fronts (cont.) - Eventually, the cold front completely overtakes the warm front, Æ the entire system is occluded - In this occlusion, the air behind the original cold front was colder than that ahead of the warm front - This is an example of a cold-type occlusion 8. 2 Fronts (cont.) The boundaries separating humid air from dry air