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Cold Fronts Cold Air Is Dense and Tends to Sink

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Cold Fronts Cold Air Is Dense and Tends to Sink Synoptic meteorology Lecture 9 Sahraei Physics Department Razi University https://sci.razi.ac.ir/~sahraei Fronts The boundary between two different air masses is called a front It is a region of significant horizontal gradients in temperature or humidity (almost discontinuous) 2 Fronts When air masses meet is a front, the collision often causes storms and weather changes. A front may be 15 to 200 kilometers wide and extend as much as 10 kilometers up to the troposphere. The kind of front that develops depends on the characteristics of the air masses and how they move. Fronts The movement of fronts is responsible for much of the day-to-day variability in weather conditions. Northwest Europe receives many different air mass types, with frequent frontal passages – results in very variable weather. Commonly associated with: Moisture gradient, Temperature gradient, Wind shift 4 Frontolysis the weakening or dissipation of a front Decreased temperature contrast between two air masses Frontogenesis a formation, strengthening or generation of a front Increased contrast of temperature conditions between two air masses 5 A front is the transition zone between two air masses of different densities. Since density differences are most often caused by temperature differences, fronts usually separate air masses with contrasting temperatures. Often, they separate air masses with different humidities as well. Remember that air masses have both horizontal and vertical extent; consequently, the upward extension of a front is referred to as a frontal surface, or a frontal zone. Figure illustrates the vertical extent of two frontal zones — the polar front and the arctic front. The polar front boundary, which extends upward to over 5 km, separates warm, humid air to the south from cold polar air to the north. The arctic front, which separates cold air from extremely cold arctic air, is much more shallow than the polar front and only extends upward to an altitude of about one or two kilometers. 6 Classification of fronts Meteorologists classify fronts based on: a) the thermal and moisture characteristics of the airmasses b) the direction of movement of the airmasses c) whether the boundary between the airmasses is in contact with the ground (a surface front), or can only be found aloft (an “upper level front”). There are four types of fronts that will be described: 7 Cold Fronts Cold air is dense and tends to sink. Warm air is less dense and tends to rise. When a moving cold air mass runs into a slowly moving warm air mass, the denser cold air slides under the warmer air. Warm air can hold more water vapor than cool air. If there is a lot of water vapor in the warm air heavy rain or snow may fall. Cold fronts move quickly so they can cause weather changes. After a cold front passes, cool, dry air moves in. Cold Front - cold air is advancing & displacing wm air. Triangles point to where air is moving toward.(steep slope) Cold, dry stable polar air (cP) is replacing warm, moist, conditionally unstable subtropical air (mT) cold air warm air Movement of front On weather maps, cold fronts are indicated by blue lines with triangles pointing in the direction of frontal motion (towards warmer air) 9 10 MAP TAP 2002-2003 Weather Fronts 11 Steep vertical boundary due to surface friction slowing down the surface front Cold, dry stable polar air (cP) is replacing warm, moist, conditionally unstable subtropical air (mT) Has strong vertical ascent along the surface front Strong upper level westerlies push ice crystals far ahead of the front, creating Ci and Cs in advance of the front. Associated with cumulus & cumulonimbus clouds ahead of the front in the warm air, producing showers and thunderstorms 12 usually bring cooler weather, clearing skies, and a sharp change in wind direction On a surface pressure map, frontal location can be seen by “kinks” in the isobars, changes in wind direction from a southwesterly to a northwesterly wind, and decreases in temperature. 13 ~70 km 14 General weather characteristics of a cold front Variable Before passage After passage Temperature warm steady cooling Dew point high decreases steadily Pressure falling steadily steady rise Visibility fair to poor good Clouds Ci, Cs, Cb Cu Precipitation showers clearing 15 Cold Front Thunderstorm Development over Great Plains 16 17 The cold front between points B and C on the surface weather map represents a zone where cold, dry stable polar air is replacing warm, moist, conditionally unstable subtropical air. The front is drawn as a solid blue line with the triangles along the front showing its direction of movement. 18 How did the meteorologist know to draw the front at that location? A closer look at the front will give us the answer. The data plotted on the map represent the current weather at selected cities. The station model used to represent the data at each reporting station is a simplified one that shows temperature, dew point, present weather, cloud cover, sea level pressure, wind direction and speed. Fig. 11.15 19 Since surface winds normally blow across the isobars toward lower pressure, we find winds with a southerly component ahead of the front and winds with a northerly component behind it. The following criteria are used to locate a front on a surface weather map: 1. sharp temperature changes over a relatively short distance 2. changes in the air’s moisture content (as shown by marked changes in the dew point) 3. shifts in wind direction 4. pressure and pressure changes 5. clouds and precipitation patterns 20 Notice that light-to-moderate rain (color green) occurs over a wide area along the front, while the heavier precipitation (color yellow) tends to occur in a narrow band along the front itself. Thunderstorms (color red) do not occur everywhere, but only in certain areas along the front. A Doppler radar image showing precipitation patterns along a cold front similar to the cold front in pervious Fig. Green represents light-to-moderate precipitation; yellow represents heavier precipitation; and red the most likely areas for thunderstorms. 21 A vertical view of the weather across the cold front in Fig. 11.15 along the line X–X'. We can see from Fig. that, at the front, the cold, dense air wedges under the warm air, forcing the warm air upward, much like a snow shovel forces snow upward as the shovel glides through the snow. As the moist, conditionally unstable air rises, it condenses into a series of cumuliform clouds. 22 WARM FRONTS Transition zone between a retreating cold air mass and advancing warm air mass Designation – red semi-circles pointing in the direction where the warm air is advancing Warm and moist mT replacing dry cold mP 23 Occurs at the leading edge of an advancing warm, moist, subtropical air mass (mT) from the Gulf replacing a retreating cold, maritime, polar air mass from the North Atlantic (mP) warm air cool air movement of front Warm air behind the front overlies cold air 24 25 MAP TAP 2002-2003 Weather Fronts 26 Warmer, less-dense air rides up and over the colder, more-dense surface air Produces clouds and precipitation well in advance of the front Move slower than cold fronts Gradual warming and veering of the wind from the S to SW when the front passes. Convection depends on stability and humidity of the warm air behind of the front. If below freezing, snow or freezing rain may occur 27 WARM FRONTS Frontal changes are less abrupt than cold air frontal passages Long spells of cold weather do not come to a rapid end Weather at the warm front portion of a frontal system is more extensive than at the cold front portion. Cloud system & precipitation cover extensive areas. 28 WARM FRONTS Cold air lies as a wedge under warm air As warm front approaches depth of cold air decreases Advancing warm air overruns the retreating wedge Forces air to rise – creates frontal inversion Air expands and cools Extensive cloud on top of cold air 29 WARM FRONTS Warm fronts are slow About half that of average cold front Average slope of 1:300 More gentle than cold front 30 WARM FRONTS Weather associated with warm Fronts Weather patterns associated with warm fronts depend on: Moisture content of warm air mass precipitation Stability of the warm air mass uplift Degree of overrunning Uplift and precipitation 31 WARM FRONTS Weather associated with warm fronts If overrunning air is dry and stable, only high and middle clouds will form no precipitation If overrunning air is moist and unstable, heavy showers can develop as thunderstorms become embedded in the cloud mass Arrival of warm front produces wind shifts, warmer temperatures and overall improvement of weather conditions 32 Warm stable air Stable: clouds will be layered 33 Warm conditionally unstable air Unstable: Thunderstorms 34 warm front is associated with a broad cloud and precipitation shield that may extent hundred of kilometers ahead of the surface front although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of gentle ascent (stratiform clouds and light to moderate continuous rain). ~300 km ~500 km are usually preceded by cirrus first (1000 km ahead), then altostratus or altocumulus (500 km ahead), then stratus and possibly fog. behind the warm front, skies are relatively clear (but change gradually) 35 General weather characteristics of a warm front Variable Before passage Region of front After passage Temperature cool, slowly steady rise warmer warming Dew point steady rise steady increases, then steady Pressure usually falling levels off slight rise, followed by fall Visibility poor improving fair Clouds Ci, Cs, As, Ns, stratus Clearing with St, fog scattered Sc Precip light to moderate, drizzle or usually none can be SN or RA nothing 36 Comparing Warm and Cold Fronts Cold Front Warm Front -Brings cooler air -Both bring -Brings warmer air -Brings heavy showers masses of air -Brings slow drizzle rain -Can develop into a tornado -Bring --Could last for several days precipitation -Afterwards…becomes cool -Moves slower than cold front - can form and dry above land or -Moves in from south water -moves in & out quickly -comes from north Light rain, warm front 38.
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