Air masses, fronts and cyclones

Transcription

1 Air masses, fronts and cyclones G Aims Air masses, fronts and cyclones To explain the seasonal changes in weather patterns in the midlatitudes To outline the sequence of stages in the life cycle of a migrating midlatitude wave cyclone and their related weather patterns Objectives To discuss the basis of air mass classification and to differentiate among the various major air mass types. To list two criteria that an air mass source region must meet. To identify the various air masses that regularly form over or invade North America and discuss the typical weather conditions associated with these air masses. To describe the processes that contribute to air mass modification, identifying those conditions that determine the degree of air mass modification. To list at least two criteria involved with the analysis of a front on a surface weather map. To identify the conditions needed for frontogenesis and frontolysis. To compare and contrast warm fronts, cold fronts, stationary and occluded fronts in terms of their structure and associated weather. To describe the weather sequences (i.e., changes in wind direction, pressure tendency, cloud type and coverage, precipitation and temperature) when the warm or cold side of a wave cyclone passes an observer. To locate the position and forecast the movement of an extratropical storm by interpreting the sequence of wind direction, pressure change, temperature change and cloud/precipitation type at a station. To identify the relative size and characteristics of an extratropical cyclone. To locate and describe the general temperature-field, the wind regime and the cloud and precipitation pattern associated with a model extratropical cyclone. To outline the sequence of stages in the life cycle of a migrating midlatitude wave cyclone. GY

2 Air masses, fronts and cyclones Outline Introduction Patterns of the 500-mb flow An extratropical cyclone Waves in the Westerlies Polar jet as part of the Westerlies Wavy flow of the Westerlies Contribution to the latitudinal heat transport Impact on mid-latitude weather Air Masses Definition of an air mass Source regions of air masses Fronts Definition and types synoptic scale fronts Structure of a frontal surfaces Weather related with fronts Temporal evolution of fronts Synoptic weather charts Surface weather elements The synoptic surface chart Life cycle of extratropical wave cyclones Development along a stationary front Cyclogenesis Occlusion Idealized weather of wave cyclones Time Sequences of weather events Clouds and precipitation, wind shift Single point forecasting Upper level structure Upper level charts Flow and divergence and convergence aloft GY

3 Air masses, fronts and cyclones Bullets Introduction Upper air flow typically meandering Large scale weather systems Implies North -South air transport Part of the latitudinal heat transport Waves in the Westerlies Polar jet integral part of the Westerlies - just the fastest part Westerlies flow in paths with long wave lengths, such that three to six cycles fit around the globe - so called Rossby Waves - after C.G. Rossby who first explained these waves. Shorter waves occur in the middle and upper troposphere, being associated with cyclones at the surface traveling at rates up to 15 o per day. By alternating flow patterns between flat flow and strongly wavy flow the Westerlies contribute to the heat transport to the poles (cf. the latitudinal heat budget) and mixing of air between the mid latitudes and the poles During phases of strong meandering of the flow (large amplitudes of the waves) cold air is transported into the mid latitudes and the South-North temperature gradient is increased - cyclonic activity and more violent weather Dish-pan experiments suggests that it is mainly the latitudinal temperature contrast which generates the wavy circumpolar circulation patterns. Air Masses Air mass - large body of air characterized by homogeneous physical properties (in particular similar temperature and moisture throughout) at any given altitude Movement of air masses will bring the conditions form the source region elsewhere Air-mass weather - generated by the moving in of an air mass Source regions - large physically uniform, allow for periods of longer stagnation Typically regions dominated by stationary (or slow moving) anticyclones with their extensive areas of calms or light winds Regions dominated by cyclones are not likely to produce air masses due to convergence Source regions for North America not found in the mid latitudes Classification depends on the latitude of the source region (P -polar, A - arctic, T - tropical, E - equatorial) and the area of origin - land or ocean (m -maritime, c -continental) ma and ce usually do not form, since the arctic is ice covered (air is dry) and the equator mainly ocean dominated (air wet anyway) Air mass modification due to underlying surface, mixing from above and lifting processes Modification processes imply changes in the stability and often transport of heat and moisture to higher levels in the atmosphere Fronts Fronts - transition zone between air masses of different densities km wide Confluence line between two different air masses is called the front Region of strong thermal contrast on the cold air side is called frontal zone or baroclinic zone Fronts are labeled by the air mass which replaces the other one Warm air in fronts above colder air - overrunning by warm air. Frontal surface always slopes in direction of the cold side with increase in height GY1

4 Air masses, fronts and cyclones Frontal zone lies always below the frontal surface with the warm air mass above it. Cold fronts do sometimes overhang. Frontal zone trapped in the wedge below the frontal surface and can not move relative to the frontal surface and the direction and speed of the movement of the front is determined by the winds in the frontal zone. For analyses of sequences of synoptic charts it is important to have consistency between the frontal movement and the wind field Warm Fronts - warmer air replaces colder air at about 25 km/h - gradually sloped, about 1:200 Overrunning is slow and slope is gradual thus light to moderate precipitation over a large area and an extended period Cold fronts - about 35 km/h - steeper sloped - about 1:100. Thus more violent weather - the lifting is faster and can cause convection. Thus more precipitation in a shorter period of time Behind cold front subsidence and cool air masses, thus clearing of the sky Occluded fronts - a cold front catches up with a warm front or it may rather be that it forms as a new front what the surface low separates itself from the junction of their warm and cold fronts. Cold type and warm type occluded fronts Warm type - front aloft (and so precipitation) precedes the arrival of the surface front - only warm front extends to the ground. Passage of warm type occlusions decreases stability. Cold type - front aloft lags behind surface front - only cold front extends to the ground. Passage of cold type occlusions increases stability Idealization! Synoptic weather charts Delineation of air masses Location of fronts and determination of types Sea level pressure lines Movement of pressure centers Life cycle of extratropical wave cyclones Cyclone develops along a front and proceeds through a generally predictable life cycle - hours to days - as suggested by the Bergen school around Vilhelm and Jacob Bjerknes around Initial development of the low pressure center along a stationary front on the crest of a wavelike undulation in the shape of the front - cold polar Easterlies meet warm midlatitude Westerlies, small irregularities lead to shearing and wave formation (several 100 km long) The wave shape results into invasion of warm air into the crest of the wave and cold air moving equatorward behind the crest - readjustment in pressure to nearly circular isobars and low pressure at the center at the apex of the wave Establishment of a cyclonic circulation and end of the period of rapid development (cyclogenesis) Begin of occlusion as the low pressure center propagates toward cold air and deepens with an occluded front connecting the low center to the junction of the warm and cold fronts. The cyclone enters maturity (maximum intensity) when it enters this stage. Warm air is displaced aloft during occlusion and the pressure gradient between cold and warm air masses weakens eventually leading to a dissolution of the cyclonic movement and the storm GY

5 Air masses, fronts and cyclones Idealized weather of wave cyclones Typically a cyclone needs 2-4 days to pass and on the northern hemisphere they tend to move eastward. The passing of a cyclone is accompanied by a typical sequence of weather patterns as envisaged in the classical Norwegian model. First high clouds on the horizon - about 1000 km in front of the approaching warm front. As the warm front approaches the could deck is coming lower and deepens. After h after the first sighting of clouds light precipitation starts due to the overriding with warm air. The precipitation is widespread and fairly uniform and reaches its peak intensity just prior to the passage of the warm front. After passage of the warm front the weather is in general warm, with southerly winds and clear skies. However, depending on ground temperatures and heating fog or isolated thunderstorms are possible. The cold front brings another well -organized cloud system. Typically a wall of clouds due to the fast lifting produces heavy precipitation. After passage the wind tends to shift from southeast to northwest and there is a pronounced drop in temperature. The weather is typically much brighter, but in marine polar air convective clouds can locally produce heavy precipitation. Upper level structure Little cyclonic activity occurs while the upper level flow aloft is relatively straight, because in this case there is not much North-South transport of cold or warm air. Meandering and formation of ridges and troughs of the upper air stream typically is accompanied by surface cyclonic activity, as in this case cold air is transported southward behind a cold front and warm air northward pushing the warm front north-eastwards - on the Northern Hemisphere. It is observed that winds in the middle troposphere ( mb) tend to act as steering flow for features on the surface. Because the warmer air slopes into the colder air over cold and warm fronts, cold fronts aloft lag the surface cold front, whereas warm fronts aloft precede the warm front at the surface. Links BBC Weather British Met Office US National Weather Service home page: Unisys weather page NOAA links: Western Regional climate center: University of Illinois Online Guides: Meteorology: Air masses and fronts: Midlatitude cyclones: GY1

6 Geopotential height on the 500-mb surface The distribution of geopotential height on the 500-mb surface at 00 GCT 20 November Labels on contours represent geopotential height, in tens of meters. The letters H and L denote centers of high and low geopotential height respectively.

7 A vigorous low pressure system A vigorous area of low pressure is producing strong winds, heavy rain and thunderstorms in the Northeast and Mid Atlantic United Staates. Further to the west, in the colder air, snow is falling in Ohio. Behind the cold fronts associated with this storm, temperatures are some 20 deg.f colder

8 Idealized westerly flow at the 500 mb level Source: Lutgens, F.K. and E.J. Tarbuck, The Atmosphere

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10 Cyclic changes occuring in upper-level air flow in the Westerlies Source: Lutgens, F.K. and E.J. Tarbuck, The Atmosphere

11 and Model Extratropical Wave Cyclones

12 A cold air mass over the U.S.A.

13 Air masses influencing the climate in North America and their source regions Source: Ward's Natural Science Establishment, Inc. after Lutgens, F.K. and E.J. Tarbuck, The Atmosphere

14 Air mass weather in North America Source: Lutgens, F.K. and E.J. Tarbuck, The Atmosphere

15 Fronts Fronts are boundary areas that separate contrasting air masses Fronts are named by the advancing air mass Fronts act nearly as walls between air masses

16 Simplified weather map and idealized structure of a middle-latitude cyclone Source: Lutgens, F.K. and E.J. Tarbuck, The Atmosphere

17 The Warm Front Transition zone where a warm air mass is replacing a colder air mass Warm fronts generally move from SW to NE and the air behind the front is warmer and moister than the one in front Simple model of a cyclone with a warm front extending to the East Warm fronts are gently sloping above cold air being retarded by friction - lifting slow - light to moderate rain over large area for extended periods

18 The Cold Front Transition zone where a cold air mass is replacing a warmer air mass Simple model of a cyclone with a cold front extending to the south Cold fronts steeper and faster than warm fronts - the warm air is lifted much faster - more violent weather

19 Fronts Warm front Stationary front Cold front Occluded fronts Cold type Warm type Occluded fronts and temperature isolines

20 Development of an occluded front Source: Lutgens, F.K. and E.J. Tarbuck, The Atmosphere

21 Life cycle of a wave cyclone Source: Lutgens, F.K. and E.J. Tarbuck, The Atmosphere

22 Weather patterns associated with a mid-latitude cyclone Source: Lutgens, F.K. and E.J. Tarbuck, The Atmosphere

23 Distribution of geopotential height on the 500-mb surface at 12-h intervals The distribution of geopotential height on the 500-mb surface at 12-h intervals beginning at 00 GCT 19 November 1964 in (a) and ending at 12 GCT 20 Novemver 1964 in (d). Contours are labeled in tens of meters.

24 Support of cyclones and anticyclones by divergence and convergence aloft Source: Lutgens, F.K. and E.J. Tarbuck, The Atmosphere

25 Air masses, fronts and cyclones Air masses and source regions Summary Cold front - cold air advances on cool/warm air Warm front - warm air advances on cooler air Weather at fronts Polar front as region generating mid latitude cyclones Cyclone life cycle - generation, fronts, occlusion, dissipation Weather within a cyclone Role of upper air flow Next: The global water cycle Reading: Smithson, P., K. Addison, and K. Atkinson, Chap 5, Chap. 14, Briggs, D., P. Smithson, K. Addison, and K. Atkinson, Chap. 7, Chap. 14, pp Gk