- Slides: 42
COLD FRONT Cold front occurs when a colder air mass replaces a warmer one. At a cold front, the cold air is behind the warm air. Because the cold air is more dense, it pushes the warm air out of its way, forcing the warm air to rise into the atmosphere. As the warm air rises, it cools, the water vapour in the air condenses out and clouds start to form. The warm air is forced to rise rapidly due to strong undercutting by the cold air. This causes towering clouds to form and thunderstorms. Rain associated with cold fronts is usually heavy but short lived and only affects a small distance (about 50 to 70 km). The air behind a cold front is noticeably colder and drier than the air ahead of it. When the cold front passes through, temperatures can drop more than 15 o. C within the first hour
Classical and Split Fronts ‘Classical cold front’ appears as one band of mostly cold cloud tops ‘Split cold front’ appears as two adjacent bands with distinctly different cloud-top temperatures These different patterns provide important clues for locating the surface cold front, identifying the physical processes at work and determining the resulting distribution of surface weather
Development of Cold Frontal Cloud Bands Classical Front IR Image : 0302 UTC/ 14 Jan 89 Split Front IR Image : 0847 UTC/ 21 Jan 89
Development Sequence of Cold Frontal Cloud Bands Meteosat IR images for 22 February 1989 (a) 0000 UTC (b) 1100 UTC
Development Sequence of Cold Frontal Cloud Bands Meteosat IR / 22 Feb 89 (c) 2300 UTC NOAA VIS / 22 Feb 89 (d) 1307 UTC
Development Sequence of Cold Frontal Cloud Bands Little or no high cloud was associated with the front on the first day Changes noticed on the next day Appearance of Fibres of Ci Development of more extensive cloud reaching to Ci level Broadening of band Sharpening of the rear edge of the band The VIS image for nearly the same time as the image in Fig. (b) shows a solid band of bright cloud This combined with information from the IR sequence, shows the cloud is becoming deeper This evolution can be clearly seen on animated imagery
COLD FRONT : UPPER AIR ANALYSIS 300 h. Pa heights, isotachs, cloud and surface front at 0000 UTC on 23 Feb 89 Schematic of the direct solenoidal circulation
Cold Front Enhanced Meteosat imagery, showing a cold front moving across Europe, colour-coded with cloud-top temperatures The red and orange areas show the coldest cloud tops which may give the heaviest rain
COLD FRONT : UPPER AIR ANALYSIS The cloud band develops on the warm side of the jet stream The main forcing for vertical motion and the development of the frontal cloud bands result from an increase in temperature gradient along a parcel trajectory. Such bands are associated with a direct solenoidal circulation in which warm air is lifted and cold air descends. The vertical motions are coupled with the ageostrophic cross-components, which are directed from cold to warm air at low levels and warm to cold aloft. Very often a new jet maximum (or jet streak) develops as a result of the upper cross-component.
COLD FRONT SURFACE ANALYSIS & PRECIPITATION DISTRIBUTION At the surface, there is a sharp trough at the front with pronounced difluence in the flow to the rear, within which thermal fields at low levels become sharper with time. As the upper cloud band develops, areas of pptn are generated towards the warmer air side.
Model of a Cold front, Showing Air flows Relative to the System (a) from above; (b) from the side, along AB in (a). Large shaded arrows show warm, moist air ascending.
COLD FRONT : CONCEPTUAL MODEL Within a frontal zone, cloud bands relate directly to the principal airflow. These airflows determine the distribution of wx especially pptn at the surface: The cloud band forms as a result of warm, moist air ascending in a conveyor belt through a deep layer of the atmosphere The air in the warm conveyor belt(WCB) ascends towards the cold side of the front and is undercut by descending cold, dry air The cloud & rain lie on and to the rear of the surface cold front. At the front itself, the air may ascend rapidly through a layer 2 – 3 km deep, giving a narrow line of heavy pptn called line convection
COLD FRONT : CONCEPTUAL MODEL Ahead of this line is a low-level jet within the WCB. Behind the surface front, where the ascent is slower, the pptn is lighter. Relating surface wx to a position within the cloud band seen from imagery can be difficult because a canopy of high clouds usually masks the effects of the physical processes below. There is rarely a good correlation between the coldest tops in the IR images and the heaviest pptn.
COLD FRONT WEAKENING COLD FRONTS, WHERE THE WARM AIR IS DESCENDING RELATIVE TO THE COLD FRONTAL SURFACE, ARE CHARACTERIZED BY: MID- AND UPPER- LEVEL CLOUD THINNING. CLOUD TOP NARROWING. PPTN BECOMING LIGHTER & LESS WIDE SPREAD. SIGNIFICANT SPATIAL DIFFERENCES IN TEMPERATURE HUMIDITY AND VISIBILITY ARE POSSIBLE EVEN ACROSS A WEAK COLD FRONT, DEPENDING ON THE ORIGINS OF THE AIR ON EITHER SIDE OF IT. AS A GUIDE, IF THE FRONT IS MARKED BY A BAND OF Sc, ITS POSITION AT THE SURFACE IS TYPICALLY AROUND 50 KM INSIDE THE LEADING EDGE.
COLD FRONT SURFACE WX : LINE CONVECTION L L Rainfall distribution as shown by wx radar on 14 Aug 89 at 1330 UTC. LL is line convection. Rainfall rates shown are (mm br-1): dark blue 0. 3— 1, green 1— 4, yellow 4— 8, pink 8— 16, red 16— 32, light blue> 32.
COLD FRONT SURFACE WX : LINE CONVECTION There is often a narrow band of heavy pptn from the line convection, about 2 km wide, coincident with the surface front It is caused by rapid, near vertical ascent of air in the WCB. Line convection can give rise to different patterns on radar image. A narrow-band of pptn collocated with the surface cold front, may extend for hundreds of kilometers or may be broken into a series of elements separated by gaps. Line elements often move steadily for many hours and are sometimes identifiable for 12 hours or more.
COLD FRONT SURFACE WX : LINE CONVECTION The progress and intensity of line convection is apparently unaffected by small hills (upto about 1 km high), but if the height of a mountain is similar to that of the convection, the convection usually dissipates. Pptn from line convection will not be detected on radar at long ranges where the beam overshoots it. Peak surface pptn rates with line convection may be under estimated on radar if the data are smoothed at low resolution (e. g. 5 km).
COLD FRONT SURFACE WX : LINE CONVECTION Where line convection occurs, there are sharp changes of wind, temperature & pressure over a few kilometers, whereas between the elements changes are more gradual. Hail, thunder & tornadoes may occasionally occur in very active systems in this narrow-band when the convection is deeper than about 3 km, even though cloud tops may be as warm as – 20ºc. Line convection appears to decay or be absent along sections of fronts where waves are forming.
COLD FRONT SURFACE WX : PPTN AHEAD OF SURFACE FRONT Ahead of the surface front, light pptn often occurs, but there may be sometimes be heavier pptn caused by ascent where there is warm advection (WA) Surface pptn rates in this region may be underestimated by radar either because of the small droplets or because pptn over hills may be enhanced in the lowest 1 or 2 km below the radar beam.
COLD FRONT SURFACE WX : PPTN BEHIND THE SURFACE FRONT In the region of less rapid ascent behind the surface cold front, there are often embedded areas of heavier pptn, frequently in the form of band some tens of km wide, becoming lighter towards the rear as the pptn evaporates into drier air below. There is no rain towards the rear edge of the clouds. On radar, surface pptn rates behind the front are generally well represented. At extreme ranges the radar may overestimate surface pptn because of evaporation below the beam.
COLD FRONT SURFACE WX : SQUALL LINE DEVELOPMENT Sometimes deep convection may develop especially over continents in summer. It is triggered near the surface front in air made unstable by strong insolation. The deep convection may become organized into a squall line.
Warm Front warm fronts occur when a warm air mass approaches a colder one. the warmer air lifts up and over the colder air. warm fronts generally move more slowly than cold fronts, gently settling over the cold air and moving it out of the way. rain associated with warm fronts is less heavy but more extensive (300 to 400 km) than rain generated at cold fronts the air behind a warm front is warmer and more moist than the air ahead of it. warm fronts generate light rain or snow which can last from a few hours to several days. when a warm front passes through, the air becomes noticeably warmer and more humid than it was before The first signs of the warm front are the Ci, followed by the Cs, As, Ns and SC types of clouds. The Ciclouds can be up to 1000 km away from the position of the front at the ground. These clouds are all generally horizontal in structure because the front slopes gently into the air. Clouds associated with cold fronts are mostly vertical in structure because the front rises quickly
WARM FRONT The cloud & moisture patterns of warm fronts typically evolve according to the stages of cyclonic development. If there is little or no high clouds in the warm sector, the warm frontal position may be well defined by a band of cloud in the IR image. However, high cloud in the warm sector may merge with the frontal band, making the warm front difficult to locate.
WARM FRONT : THE CLOUD BAND CHARACTERISTICS The main characteristics on IR images of the cloud pattern associated with a warm frontal cloud band are: A single, broad band of middle and upper clouds, with the surface warm front positioned near the edge of the upper cloud closest to the warm air mass. A warm sector largely devoid of upper cloud
WARM FRONT L : Surface low centre WB : Warm frontal cloud band CW : Cloud developing in the warm sector AB : Line of the cross-section Meteosat IR image over western Europe at 1200 UTC on 09 Feb 87
Model Analyses with upper cloud areas (shaded) and fronts as in previous image 1000 mb heights 1000— 500 h. Pa thicknesses (every 4 gdm). 500 h. Pa heights (every 8 gdm); arrows show part of the jet axis Temperature advection (in °C/12 hours) between 1000 and 500 h. Pa;
WARM FRONT THE CLOUD BAND The relationships of cloud features to analysis are as follows: Cloudiness associated with the warm front can be found within the upper contour ridge and down stream of thermal ridge. There is a maximum of a WA within the cloud band ahead of the surface warm front. The northern edge of the cloud coincides with the jet axis. The frontal cloud is produced by an ascending WCB.
WARM FRONT Warm frontal cloud bands related to analysis Upper contours (e. g. at 500 h. Pa, solid lines) and cloud as seen on IR images (shaded). Arrows represent the jet axis and the dashed line is thermal ridge axis. The broad arrow represents the WCB.
Sequence of space-view Meteosat l. R images for 09 Feb 87 1500 UTC, (b) 2400 UTC at: (a)
CLOUD DEVELOPMENT IN WARM SECTOR W 1 produces the frontal cloud band wb. The main ascent within w 1 is close to and of the surface warm front. W 2 rises from lower levels in the equatorward parts of the warm sector to higher levels ahead of the warm front, overrunning w 1 there and producing cloud. The ascent within W 2 covers a larger horizontal distance and is more gradual than within W 1. Conceptual model of warm, moist air flows WI and W 2 which produce a cloud pattern of the type shown in next Image
CLOUD DEVELOPMENT IN WARM SECTOR Re-projected Meteosat IR image over western Europe at 0000 UTC on 10 Feb 87, the same time as previous Fig. (b), with surface fronts superimposed. WB is the warm frontal cloud band, CW is cloud in the warm sector
Warm Frontal Cloud Bands Related To Analysis The key relationship between warm sector & cloud area are as follows: The surface warm front is located within, or occasionally south of, the cloud area and near the warm side of the thickness gradient Cloudiness associated with the warm front can be found within the upper ridge and downstream of the thickness ridge axis There is a maximum of warm advection (WA) ahead of the surface warm front, but there may also be WA within the cloud in the warm sector, owing to the flow of warm air along conveyor belt W 2 When there is cloud in the warm sector, the surface warm front is difficult to locate using the features on imagery. For an active system, the distance between the cirrus edge and surface front is typically about 500 km but this will depend, for instance, on the slope of the front
Location of an Active Warm Front (a) The location of an active warm front as seen in IR imagery. Shading denotes cloud (b) Meteosat IR image at 1700 UTC on 26 February 1987 over western Europe. G is the cloud edge ahead of the surface warm front
WARM FRONT: PPTN DISTRIBUTION Sometimes bands are observed in radar images within a broad frontal pptn area. The rain bands, typically 50 km wide and few hundred km long ( apart by 50 km), are usually oriented at a small angle to the front. The rolls are embedded within a WCB and so move faster than the surface front. As the rolls move further ahead of the surface front, they rise , and eventually the associated pptn dies out since there is less moisture at higher levels and the pptn falls into a layer of dry air below the frontal zone.
WARM FRONT: PPTN DISTRIBUTION Rolls at higher levels may sometimes be inferred from striations in the upper clouds though the striations may have other causes. Investigations using radar suggest that the pptn bands are caused by conditional symmetric instability (csi) rolls.
Striations Visible geostationary satellite picture over southern North Island at 23 UTC 14 February.