12. Weather Patterns. Reading Assignment: A&B: Ch. 9, Air Masses

Transcription

1 1 Reading Assignment: A&B: Ch. 9, Air Masses Extremely large body of air whose T and moisture (q) are fairly similar in any horizontal direction at any given altitude Very large > 1600 km across Extend across a range of latitudes Small differences in comparison to the rapid rates f experienced across boundaries (fronts) Source regions region where air masses originate determine the characteristics of the air mass associated with anticyclones (H) o very weak winds - new air sink o tend to stay in region so pick up the characteristic T and q Moisture status m Maritime c continental Temperature status A Arctic T Tropical P Polar E Equatorial

2 2 letter code to describe the moisture and Temperature status code Definition Conditions Source region mp Maritime Polar moist cool air North Pacific cp Continental dry cold North Canada & (ca) Polar (Arctic) (extremly cold) Siberia mt Maritime Tropical moist - warm Atlantic, S. Pacific, Gulf of Mexico Air Mass Modification Pressure differences cause the air to move out of the source region As air moves into a new region it is modified Air masses Modify the weather of the area it traverses Are modified by the surface Example: If a cp or ca air moves over the ocean in winter what will happen to the: Moisture at the surface: -- Temperature at the surface: -- What will happen to the stability? ---Unstable as airmass is heated from below--- Given that stability: what will happen to the T and Moisture distribution increased mixing--moist and warmer (cool) What will happen to the air mass class? changed from c to m

3 Two general classes: a) Air moves over a colder surface air warmer increases stability b) Air moves a warmer surface instability 3 If becomes more stable: characteristics will remain the same for a longer period of time If becomes more unstable: characteristics through the air mass change rapidly because of the mixing 2. FRONTS Transition zone between the air masses. Air masses have different T different densities Fronts between contrasting T and often different humidity Manually use the station model to see the locations where there is a contrast. Front located between these places ca--f---cp---f warmer air mass

4 Passage of a Polar front (continental Polar air mass) 4

5 5 Air masses have horizontal & vertical extent frontal surface between them 4 Types of fronts

6 a) WARM Front (boundary of advancing warm air mass) Surface position of a front moves so that warm air occupies area that was formerly occupied by cooler air Average speed: ½ that of cold front (~5 m s -1 = 18 km h -1 ) due to smaller pressure gradient Warmer less dense air rides up and over the colder more dense surface air overrunning Average slope: 1:200 (lifting over extensive horizontal distances) Warm air ascend the retreating wedge of cold air (similar to orographic lift) Cools adiabatically Produces clouds and light continuous precipitation Sequence of clouds: cirrus stratiform 6 Cirrus: 1 st sign of warm front high clouds form far in advance of the surface expression of the front

7 overrunning warm air ascended high up the wedge of cold air 7 Aircraft contrails Another indicator of warm front approaching On a clear day they can persist for several hours Relative warm moist air ascending overhead Slow rate of advance and very gradual slope Light to moderate precipitation over a large area for an extended period If the overrunning air is inherently unstable and front is rather sharp cumulonimbus clouds & thunderstorms Passage of Warm front Gradual increase in T most apparent when there is a large T contrast exist between air masses

8 8 b) COLD Front (advancing wedge of cold air) Cold air actively advances into a region occupied by warmer air Friction tends to slow surface position compared to the air aloft Cold front steepens as it moves Normally 2 x steeper than warm front (1:100) Average speed 35 km h -1 (0-50 km h -1 ) More violent weather Stronger uplift Displacement of air along a Cold front often rapid enough to permit the release of latent heat Significant Increase air buoyancy Sudden down pours Vigorous gusts (mature cumulonimbus) Clouds frequently develop Produces the same amount of lifting as a Warm front but over a shorted distance Greater Intensity of precipitation

9 Shorter duration of precipitation Sometimes preceded by altocumulus clouds Towering clouds can be seen in the distance Near the front dark band of clouds Significant T drop Wind shift Pressure decrease at vicinity of front Goes up afterwards 9

10 c) STATIONARY Front (nonmoving boundaries) 10 Occasionally flow on both sides of a front are almost parallel to the position of the front Surface position of the front does not move May get overrunning (inclined surface over cold air) d) OCCLUDED front (cutting off warm air mass from the surface..merging of two cold air masses ) An active cold front overtakes a warm front Advancing cold air wedges the warm upwards new front emerges small temp. difference between air masses Precipitation warm air forced aloft

11 11 Two types Cold type of occluded front eg east of the Rockies Air behind the Cold front is colder than the air ahead of the warm front Warm type of occluded front e.g. along Pacific coast milder mp invades cp Air behind the Cold front (mp) advancing from over the ocean on to land is milder than the very cold cp air mass it meets up with at the occluded front.

12 12 3. Wave Cyclones or Mid-latitude cyclones Large Weather systems Travel great distances Affect wide areas (week or more) Precipitation----sometimes severe weather Forms and moves along a front Circulation of winds about the cyclone tend to produce a wavelike deformation on the front Develop along a polar front Polar front - semi-continuous global boundary separating cold polar air from warm subtropical air

13 13 Cyclogenesis (formation mid latitude cyclone)--stages Stationary front Air flow parallel to front in opposite directions wave like kink frontal wave develops along boundary cold air pushes south and displaces warm air north (ccw rotation in NH)

14 14 Cyclogenesis Intensifies Low pressure deepens and distinct cold and warm front emerge -Uplift -Cloud formation -Distinct frontal characteristics Steered by winds aloft move E or NE Occlusion occurs

15 Common, but not exclusive 15 -Interruption in the isobar pattern -Abrupt transition of wind direction -SE SW across warm front -SW NW across cold front Warm Front always located ahead of cold front