本科气象双语标准课件第二章

1、Introduction to the synoptic charts,Weather map- the most valuable tool the meteorologist uses to forecast the weather. By looking at weather maps from different heights in the atmosphere, a meteorologist can make a three-dimensional picture of what is happening in the atmosphere. The weather map wa

2、s not developed until the telegraph was working. Back in the 1870s, the first weather map came to be.,Projections of Base Map,Projection-transforming three dimensional space onto a two dimensional map . This process distorts at least one of these cartographic properties: shape, area, distance, direc

3、tion and often more. Projection types: Planar Cylindrical Conic,Polar Stereographic,A planar perspective projection where the south pole is viewed from the north pole, vise versa. The Pole is at the center of the map with lines of longitude as straight lines radiating from the map center. Lines of l

4、atitude are presented as a series of concentric circles centered on the Pole. Good for mapping a hemisphere, continent at medium scale.,Universal Transverse Mercator,Features are projected from the center of the globe onto a cylinder placed over the globe and touching it at the equator. The unwrappe

5、d cylinder forms the map. Lines of latitude and longitude are straight lines and intersect at right angles. The true heading between two points is shown as a straight line. Often used for the tropical areas.,Lambert Conformal Conic,A conic projection usually based on two standard parallels. Represen

6、t the pole as a single point. Often for middle latitudes.,syn means the same or together noptic means visible; hence, seen together. Synoptic weather analysis requires the simultaneous observation of the weather at many widely located sites using standardized instruments and techniques. By internati

7、onal agreement all meteorological observations are taken at the same time according to UTC or Z time, generally 0000, 0600, 1200,1800.,Headings: type, area, radio station, synoptic time, forecast valid time.,Surface station models,Isobaric analysis,The packing of the isobars reveals how rapidly the

8、pressure varies with distance in the horizontal direction. A tighter packing indicates a much more rapid horizontal variation of air pressure.,to show the features of the horizontal pressure field at MSL , High and low pressure centers the isobar spacing is at 4 mb intervals, centered upon 1000 mb,A

9、 front is defined as the transition zone between air masses having dissimilar thermal and moisture properties. Fronts are classified according to their movement.,Understanding the Code,GALE 大风，风力8-9级 STORM 暴风，10-11级 HURCN Hurricane，飓风，风力12级 DVLPG Developing 发展 DSIPT Dissipate 消亡STNRY Stationary 静止 W

10、KNG Weakening 减弱 RPDLY Rapidly快速 FRMG Forming 形成MOVG Moving移动 INLD Inland 登陆 DCRS Decrease 减弱 INCRS Increase 增强 INTSFY Intensify 加强 COMB Combined 组合（通常指双低压） MERGE 合并 NEW 新生 ABSORBED 吸收（通常指两个高（低）压合并成一个） DOWNGRADED （热带气旋）降格 UPGRADED （热带气旋）升格,W GW SW TW FOGW TD TS STS T,Air Mass,A large body of air of

11、relatively similar temperature ,humidity and stability characteristics covering thousands of square kilometers. Classified according to the characteristics of their source region or area of formation. Appears and travels as a recognized entity. Thus it is distinguished from the surroundings and when

12、 it moves, it retains its basic original characteristics.,Air mass source regions,Simplified classification of Air Mass,Cold air mass a cold air mass is colder than the underlying surface. Air masses can usually be identified by the type of clouds within them. Cold air masses usually show cumuliform

13、 clouds, whereas warm air masses contain stratiform clouds.,Movement and Modification,When an air mass departs from its sources, its structure begins to change, due in thermal modification-heating or cooling from below dynamic modification-uplift, subsidence, convergence, turbulence addition or subt

14、raction of moisture The air mass modifies the weather of the region characteristics into which it moves.,North American Air Masses,several possibilities are commonly found associated with North America: maritime polar (mP) continental polar (cP) maritime tropical (mT) continental tropical (cT) conti

15、nental arctic (A).,Air mass characteristics can differ tremendously,Front,Frequently, two air masses, especially in the middle latitudes, develop a sharp boundary or interface, where the temperature difference between them becomes intensified and is called a frontal zone or a front. The boundary bet

16、ween the warm and cold air masses always slopes upwards over the cold air. This is due to the fact that cold air is much denser than warm air. The sloping of warm air over the cold air leads to a forced uplifting (frontal lifting) of the warm air if one air mass is moving toward the other. In turn,

17、this uplifting causes condensation to occur and the possibility of precipitation along the frontal boundary.,Characteristics of fronts,Across the front Change of Temperature Change of Moisture characteristic Change of Wind Direction Change in direction of Pressure Gradient Characteristic Precipitati

18、on Patterns,Frontal element - Temperature,Frontal surface,Frontal surface,Frontal slope,Cold air mass,Warm air mass,Warm air mass,Inversion layer, accumulation of moisture here,Frontal zone,Surface position of front,Frontal Slope,The slope of a front is basically of the steepness of the frontal surf

19、ace, using a vertical dimension and a horizontal dimension. A slope of 1:50 (1 mile vertically for every 50 miles horizontally) would be considered a steep slope, and a slope of 1:300 a gradual slope. The frontal slope therefore depends on the latitude of the front the wind speed and the temperature

20、 difference between the air masses. Because cold air tends to under run warm air, the steeper the slope, the more intense the lifting and vertical motion of the warm air and, therefore, the more intense the weather.,Front element- pressure,Fronts are associated with troughs of low pressure. On both

21、sides of a front the pressure is higher than at the front. How the pressure changes with the passage of a front is of prime importance when determining frontal passage and future movement.,Frontal element - wind,An essential characteristic of a frontal zone is a wind discontinuity through the zone.

22、A front is a Wind Shift Line and that wind shifts in a cyclonic direction.,If you stand with your back against the wind in advance of the front, the wind will shift clockwise (NH)/counterclockwise (SH) as the front pass.,Warm front,Warm front,An advancing warm subtropical, moist air mass replaces a

23、retreating cold, dry polar air mass. A solid red line with half-circles. The position of the half-circles shows the direction of frontal movement. move about 10 to 20 knots in a NE direction. The slope is 1/100-1/200.,Warm front,Because of gradual frontal lifting. Ci, Cs, As (Ac) clouds are found we

24、ll in advance of the front. About 600 km ahead of the front, Ns clouds occur. These clouds produce precipitation. Finally, a few hundred km behind the front scattered St clouds are common. Fog is common in the cold air ahead of a warm front. When the overrunning air is moist and unstable, Cu and Cb

25、clouds are frequently imbedded in the Ns and As clouds. In such cases, thunderstorms occur along with continuous precipitation.,Weather conditions associated with a warm front,Cold front,Cold Front,An advancing cold, dry stable air mass displaces a warm, moist unstable subtropical air mass. On a wea

26、ther map, the cold front is drawn as a solid blue line with triangles. The position of the triangles shows the direction of frontal movement. Cold fronts move between 10 to 25 knots in a SE to Ely direction. The formation of clouds and precipitation at the frontal zone is caused by frontal lifting.,

27、Slow-moving cold front (active cold front),The slow-moving cold front is an Active front because it has widespread frontal cloudiness and precipitation at and behind the front caused by actual frontal lifting. The type of precipitation observed is also dependent upon the stability and moisture condi

28、tions of the air masses.,On upper air charts, slow-moving cold fronts are characterized by a packing of isotherms behind them. The more closely packed the isotherms and the more nearly they parallel the fronts, the stronger is the front.,Slow-moving cold front (active cold front),The average slope o

29、f the front is approximately 1:100 miles. When the warm air mass is stable, a rather broad zone of As and Ns clouds accompany the front and extend several hundred miles behind the front. If the warm air is unstable, thunderstorms and Cb clouds may develop and may stretch for some 50 miles behind the

30、 surface front. The pressure tendency- falls prior to frontal passage and then weak rises behind. usually moves at an average speed between 10 and 15 knots.,The fast-moving cold front is a very steep front that has warm air near the surface being forced vigorously upward. At high levels, the warm ai

31、r is descending downward along the frontal surface. This front has a slope of 1:40 to 1:80 miles and usually moves rapidly-at an average speed of 25 to 30 knots. As a result of these factors, there is a relatively narrow but often violent band of weather. Pressure tendencies fall ahead of the front

32、with sudden and strong rises after frontal passage.,Fast-moving cold front (inactive cold front),Fast-moving cold front (inactive cold front),Cb clouds are observed along and just ahead of the surface front. St, Ns, and As may extend ahead of the front in advance of the Cb and may extend as much as

33、150 miles ahead of the front. If the warm air is moist and unstable, a line of thunderstorms frequently develops along this front and may develop into a squall line. When the warm air is stable, an overcast layer of As clouds and rain may extend over a large area ahead of the front.,If the warm air

34、is very dry, little or no cloudiness is associated with the front.,Weather conditions associated with a cold front (fast-moving),Cold front weather,Cold fronts that move very rapidly have very steep slopes in the lower levels and narrow bands of clouds that are predominant along or just ahead of the

35、 front. Slower moving cold fronts have less steep slopes, and their cloud systems may extend far to the rear of the surface position of the fronts. Both fast-moving and slow-moving cold fronts may be associated with either stability or instability and either moist or dry air masses. In general, the

36、temperature and humidity decrease, the pressure rises, and in the NH the wind shifts (usually from south-west to northwest) with the passage of a cold front.,Occluded fronts,Occluded fronts are produced when a fast moving cold front catches and overtakes a slower moving warm front.,Note that in the

37、occlusion process the invading mild moist air that was found behind the warm front has been lifted into the upper troposphere.,Occluded fronts,A cold type occluded front occurs when the air behind the front is colder than the air ahead of the front.,When the air behind the cold front is warmer than

38、the air ahead of the front a warm type occluded front is produced.,The weather associated with occlusions has the characteristics of both warm and cold fronts.,Stationary fronts,A quasi-stationary front, or stationary front as it is often called, is a front along which one air mass is not appreciabl

39、y replacing another air mass. The slope is 1/150-1/300. A front moving less than 5 knots is usually classified as a stationary front. There is frictional inflow of warm air toward a stationary front causing a slow up glide of air on the frontal surface. As the air is lifted to and beyond saturation,

40、 clouds form in the warm air above the front. One of the most annoying characteristics of a stationary front is that it may greatly hamper and delay air operations by persisting in the area for several days, e.g., baiu.,Fronts in satellites images,Fronts in satellites images,Anticyclone,Every ocean

41、basin has an area of high pressure sitting over it. Highs over land are transient since land heats up and cools down daily and seasonally. High pressure can bring warm or cold temperatures depending where they form, but they always bring clear weather. The centers of high-pressure systems are calm,

42、however, a band of dependable wind near a highs perimeter.,Satellite imagery of highs,High-pressure centers have few upper level clouds and scattered puffy cumulus clouds at the surface. Consistent cloud banding exists along a highs perimeter and this is where wind is found. Perimeter clouds align t

43、hemselves with wind flow and are called cloud streets. Where distinct and well-defined individual clouds are seen there is certain to be consistent surface winds.,Cold continental high,Pivotal areas 70E-90E, 45N-65N,Sources:1- west of Novaja Zemla (新地岛) most frequent and strongest 2- east of Novaja

44、Zemla rare but strong 3- south of the Iceland frequent but relatively weak,Path of a cold air burst,Consequent weather when the cold air moves,Strong Nly wind, severely temperature decrease Before a cold air- exceptional warm and moist, Sly weak wind Perimeter, near the cold front- cold frontal clou

45、ds, rain and snow, strong Nly wind, force 7-8 (11), 24-48 hours, heavy seas Center of the high- calm, clear, radiation fog Rearward- Sly wind, damp, warm frontal type weather The most severe weather occurs 3 hours after the passing of the cold front 3 days cold and 4 days warm strong NE wind in the

46、Taiwan strait Unstable weather over the Kuroshio current,Cold wave,大范围的强烈的冷空气活动，称为寒潮。中央气象台规定：长江中下游及其以北地区48小时内降温10以上，长江中下游最低气温Tmin4，陆上三个大区有5级以上大风，渤海、黄海、东海先后有7级以上大风，作为寒潮警报标准。如果上述区域48小时内降温达14，其余同上时，则作为强寒潮警报标准。,The North Wall phenomenon,During the winter months (January thru March), the waters between t

47、he east coast of the U.S. and the Gulf Stream are at their coldest, yet water in the Gulf Stream can be as warm as 75F. When cold arctic air moves over the Gulf Stream, a dramatic temperature contrast occurs. Arctic air approaching from the north or northwest may be as cold as -6C (21F). Over the Gu

48、lf Stream, where sea surface temperatures are around 24C (75F), a 30C (54F) temperature difference occurs.,The North Wall phenomenon,Temperature gradients of this magnitude will cause sudden warming of the air in the lower layers, and create a condition of extreme instability. Actual surface wind sp

49、eeds in these cases may exceed forecast winds by as much as twice the predicted value.,General guidelines for forecasting North Wall weather elements,When a cold air mass moves over the Gulf Stream and there is a 20F (or greater) differential between sea surface and air temperature, expect North Wal

50、l conditions. When wind is perpendicular to the Gulf Stream, forecast wind 33% above normal. When wind direction opposes the flow of the Gulf Stream, forecast wind 95% of boundary layer + 20kts. For either wind regime, forecast seas 6 to 8 feet higher than would normally be expected.,North wall effe

51、ct advisory,1. THIS OTSR NORTH WALL ADVISORY AMENDS REF A TO REFLECT LATER ONSET OF CONDITIONS DESCRIBED BELOW AND IS NOW EFFECTIVE COMMENCING 260000ZJAN07 AND EXTENDING THRU 270600ZJAN07 FOR AREA DESCRIBED BELOW: WIND/SEAS FORECAST OVER THIS AREA 35 TO 55 GUSTS 75 KNOTS/12 TO 16 FEET WITH MAX OF 20

52、 TO 26 FEET IN THE VICINITY AND EAST OF THE GULF STREAM.,2. BY 25/1200Z A TROUGH OF LOW PRESSURE TRACKS SOUTHEAST ACROSS THE GREAT LAKES INTO THE ATLANTIC COASTAL WATERS. AS THIS TROUGH MOVES OFFSHORE IT DEVELOPS INTO A LOW PRESSURE SYSTEM AND TRACKS EAST INTO THE WESTERN ATLANTIC WHILE INTENSIFYING

53、. STRONG HIGH PRESSURE OVER THE PLAINS STATES WILL MOVE EAST-SOUTHEAST AND INTERACT WITH THE OFFSHORE LOW AND ASSOCIATED TROUGH TO PRODUCE A STRONG NORTHWEST FLOW OFF THE ATLANTIC SEABOARD, ADVECTING ARCTIC AIR (30-40F) OVER THE RELATIVELY WARMER WATERS (70-75F) OF THE GULFSTREAM. THE VIOLENT INTERA

54、CTION OF THIS COLD AIR WITH THE WARM GULF STREAM WATERS WILL PRODUCE STRONG GALE FORCE WINDS AND HIGH SEAS ACROSS PORTIONS OF THE VACAPES AND CHERRY POINT OPAREAS. SEAS WILL REMAIN SIGNIFICANTLY LOWER IN THE FETCH LIMITED COASTAL WATERS TO THE WEST AND NORTHWEST OF THE GULF STREAM. 3. OTSR ADVISES A

55、LL UNITS WHOSE LIMITS ARE EXCEEDED BY CONDITIONS DESCRIBED PARA 1, SEEK SAFE HAVEN IN PORT OR AVOID THIS WARNING AREA UNTIL CONDITIONS ABATE TO ACCEPTABLE LEVELS.,North wall effect advisory,Subtropical high,Weather of Subtropical high,North of a subtropical high. Any mT air that moves northward beco

56、mes cooled over the cool ocean surface. A stratus overcast may form, and drizzle may fall. Farther north, low ceilings (usually below 1,000 feet) may reach the surface, producing fog. This mT air brings most of the summer fogginess to northern seas and coasts. It brings the greatest fogginess in the

57、 Atlantic where it blows from the warm Gulf Stream over the cold Labrador current (near Newfoundland), and in the Pacific where it blows from the warm Kuroshio current over the cold Oyashio current (near the Kamchatka peninsula).,Weather of Subtropical high,East of a subtropical high. Along the Cali

58、fornia coast, and along the Atlantic coast of North Africa, the mT air blows from the west and the northwest. This air tends to remain stable for the following reasons: a. It is coming from the northern, cooler portion of the source region; b. Its surface layers remain cool because it moves over col

59、d ocean currents; c. Its upper portions warm adiabatically because of subsidence.,Weather of Subtropical high,South of a subtropical high. Where the mT air moves southward or southwestward (as trade winds), its lower layers are warmed by the tropical ocean surface. This produces scattered cumulus. N

60、ear the equator, after absorbing much moisture and being heated, this air may develop cumulonimbus. West of a subtropical high. This mT air blows from the east and the southeast. Over the ocean near the Philippines (and near Florida and the West Indies), this trade wind brings good weatherclear or s