Presentation on theme: "EXTRATROPICAL CYCLONES (Winter Storms and Blizzards)"— Presentation transcript:
EXTRATROPICAL CYCLONES (Winter Storms and Blizzards)
WINTER STORMS: CYCLONES, ANTICYCLONES, FRONTS Outside the tropics, the winds are organized into large systems called low and high pressure areas, that produce much of our weather. Highs and lows are typically about 1000 miles in diameter and travel from west to east at about 25 miles per hour. Earth's rotation gives them the form of giant wind spirals. Air spirals counterclockwise in toward lows (cyclones) and then is forced to rise. Therefore, lows bring mostly cloudy, wet weather. Highs (anticyclones) have the opposite circulation. Air spirals clockwise and outward from the center. Then air from above sinks to fill “the void”. Highs bring mostly clear weather but if sunshine heats the ground enough thunderstorms are possible. As winds spiral into low pressure areas, large regions of polar and of tropical air called air masses are brought close together. Temperature contrasts become concentrated in long narrow zones or fronts, which are basically boundaries that separate air masses. It is important to identify the fronts and predict their motion because much of the major weather changes and stormy weather outside the tropics forms along fronts. The next slide describes the different air masses. Fronts are diagnosed in several slides after that. Then 2-D and 3-D illustrations of a low pressure area with winds, air masses (cold or warm), fronts and clouds. WEB Sources: NOAA Summaries of Major Winter Storms http://www.hpc.ncep.noaa.gov/winter_storm_summaries/winter_storm_summaries.shtml Weather Archive – You make the maps http://vortex.plymouth.edu/u-make.html
Air Masses Air masses are large regions of air with reasonably uniform properties of temperature and humidity that tend to form in high pressure areas. There are four air masses, Continental Polar, cP. This cold, dry air mass forms in polar regions when the surface (often snow covered) cools by radiating heat to space. The air also loses moisture by deposition of frost on the ground. Continental Tropical, cT. This hot, dry air mass forms over the subtropical deserts, where air from the tropics sinks and the sun superheats the dry ground. Maritime Tropical, mT. This humid, warm air mass forms over tropical and subtropical oceans, acquiring moisture from the sea below or from abundant tropical showers and heat from the warm tropical surface. Maritime Polar, mP. This chilly, humid air mass forms over cold polar waters or when cP air is charged with moisture from rain or snow under warm frontal surfaces. At least 3 different air masses converge in most extratropical cyclones, - cP from the northwest, mT from the south and mP from the northeast. The cold air masses form a large dome thousands of miles wide and 5 - 10 km high. The air masses converge in extratropical cyclones. When the warm air encounters the cold air dome at the surface it forms fronts. The lighter warm air then either rises abruptly to form thunderstorms or glides over the dome of denser cold air to produce widespread clouds and precipitation.
Winds near the Earth’s surface spiral counterclockwise and inward around low pressure areas in the North Hemisphere.
Fronts are depicted with thick lines that are studded with triangles (for cold fronts) and with semicircles for warm fronts that protrude in the direction the front moves. For a stationary front alternating triangles and semicircles extend in opposite directions. For an occluded front alternating triangles and semicircles protrude in the direction of motion. When a cold front passes by, T can drop more than 10 F in an hour. In the typical cold front (below left) at two times 12 hours apart, the cold air (shaded) blows from the ___ and the warm air blows from the __. Temperature changes tend to be more gradual at warm fronts. In the typical warm front (below right), the cold air is on the ___ side and blows from the ___, while the warm air is on the ____ side and blows from the ___.
Indicators for Finding Fronts Fronts are located in low pressure troughs Fronts separate regions with different wind directions
Indicators for Finding Fronts Fronts separate regions with different weather. A line of thunderstorms often is parallel to the cold front Fronts often lie at the tropical side of regions of large temperature gradients.
Simple Model of Surface Low Pressure Area Warm Sector Hazy, Hot and Humid Cold, Dry Sector Clear, Crisp and Cold Stormy Sector Dismal, Damp, Drenching
http://rst.gsfc.nasa.gov/Sect14/Sect14_1c.html Cold Front – Cold Air AdvancesWarm Front – Warm Air Advances Model Extratropical Cyclone (Low) with Cold and Warm Fronts In the drawing of the low to the left, the clouds NE of the warm front should be much wider and continuous, as in the warm front drawing above.
An intense low gave Minnesota and Wisconsin up to a foot of snow while thunderstorms raged along the cold front (blue line) that separates warm and cold air masses.
Infrared satellite images are temperature maps. Cold regions are usually clouds with high tops. The warm clouds west of the cold front over Iowa and Missouri are cumulus and stratocumulus.
Jet Stream Cirrus Cumulus cloud streets Cold Front with Cb’s A higher resolution view of the same storm by the MODIS satellite shows the cold front, individual cumulus in rows parallel to the wind and cirrus of the jet stream further west.
Fog An idealized view of the clouds of the Extratropical Cyclone. Numbers indicate the likely Locations of the Paintings
H H HH H H HH L L L L Ci Cs As Ns Ci Cs As Ns NYC DCA Weather Sequences in the Moving Cyclone If you know the pattern of clouds and weather then the sequence at any place follows naturally by moving the storm because whenever a pattern in space moves it becomes a sequence in time.
Structure and Weather of the Extratropical Cyclone The classic extratropical cyclone is about 1000 km across and extends from the ground to the tropopause. It consists of three sectors separated by two fronts. 1. The wet sector, north and east of the center and the warm front. 2. The warm sector, southeast of the center. 3. The cold, dry sector, west of the center and northwest of the cold front. Some storms have one or two additional sectors that are described later, namely, 4. The warm, dry sector, between the dry line and the cold front. 5. The dry slot, wedged between the cold, dry sector and the wet sector. Each sector is filled by a giant conveyer belt of moving air. The warm conveyer belt fills the warm sector. It consists of maritime Tropical (mT) air from the tropics that turns eastward as it overruns the dome of polar air. It then produces the wide area of clouds and precipitation in the wet sector. Poleward of the warm front, a cold, damp conveyer belt from the east slithers under the warm conveyer belt. Precipitation falling from the warm conveyer belt above soaks this air and transforms it to maritime Polar (mP). The polar conveyer belt consists of cold, dry continental Polar (cP) air from the northwest. It sinks as it wraps around the western side of the low to produce mostly crystal clear skies.
The stormy sector contains the storm’s Dank, Dreary, and Drenching (DDD) weather. The air is cold and skies are overcast because the warm conveyer belt rises aloft. The stratiform cloud cover includes a wide area of continuous precipitation, sometimes with heavy snow and blizzard conditions. The form of precipitation in the stormy sector depends on the soundings and cross sections. Snow almost always forms high in the clouds, but the entire sounding must remain below freezing temperatures for the snow to reach the ground. This usually happens far poleward of the warm front. Rain occurs where a layer of air near the ground is warm and thick enough to melt snow. This usually happens near the surface warm front. Most freezing rain and ice pellets are produced when the air above the warm frontal surface is above freezing but the air near the ground is below freezing. This occurs in storms with strong warm fronts and is usually sandwiched in a narrow zone between snow and rain. Wrap Around: As storms intensify, the cold, damp conveyer belt spirals upward as it wraps around the center. This extends the stormy sector west of the surface low center and gives the storm a cold core. Precipitation here becomes more showery. At the western edge of the cloud shield the polar conveyer belt undercuts the other conveyer belts and lifts the cloud shield to altostratus. Tilted shreds of low scud clouds, that form as the cP acquires moisture from the wet ground or the lingering precipitation, race across the sky. The scud give the sky an ironically ominous appearance, but it is usually a sign of imminent clearing. Clearing may be delayed for a few hours during the peak of the day, if the sun can provide enough heat to launch new cumulus or stratocumulus that may produce showers or flurries. But almost invariably, these clear out by sunset.
Cross Sections and Sequence of Precipitation RS RSZR - IP
Lake Effect Snow Enormous snow totals both from individual storms and for the entire winter occur on the downwind side (usually to the SE) of the Great Lakes. Some individual storms have produced more than 60” of snow. The record for a single event is 141” from 3-12 February 2007, which is shown on the first slide of the Presentation. The Lakes serve as sources of heat and moisture for the snow. When cold air from northern Canada passes over the Lakes it is heated and charged with vapor. Within a few miles cloud lines form (as when cold air pours over the Atlantic Ocean and the Gulf of Mexico), and when the air is lifted as it passes onto the land downwind from the lakes it drops much of this vapor as snow if it is still cold enough. A forecast rule is that the air at 850 hPa must be at least 13 C colder than the lake, provided the lake is not iced over. The greater the temperature difference the greater the potential intensity of the snow. Since the coldest air typically comes from the NW, the Lake Effect Snows most often occur on the SE shores. Also, since the typical snow from extratropical snowstorms occur with NE winds (Nor’easters), Lake Effect snows often occur after the main storm has passed, and when skies are normally clear.
The prong of extra snow in West Virginia occurs because the air is force to rise over the Appalachians. The snow maximum east of Lake Ontario occurs where the air is force to rise over the Adirondacks.
MODIS 04 FEB 2007 Image during the Record Lake Effect Storm. Extremely cold air poured over the Great Lakes, after months of abnormally warm weather kept the Lakes warm and open. Cloud rows trace out the winds as they cross the Great Lakes
Some winter storms produce truly unBEARable weather. In addition to snow, there can be ice, which is rain that freezes on contact with the ground. This is called black ice and it is the most dangerous, paralyzing form of winter precipitation. It creates a world without friction. Earlier slides and lectures illustrate the vertical temperature profiles of winter storms that produce freezing rain, ice pellets, and snow. http://severInewx.atmos.uiuc.ed u/06/online.6.1.html http://severInewx.atmos.uiuc.ed u/06/online.6.1.html
The Strange Tilt of Weather Systems Outside the tropics, it is often observed that highs and lows and troughs and ridges are not vertically aligned but tilt upward to the West. Recall the rules: 1: Lows and troughs always tilt upward toward the coldest air. 2: Highs and ridges always tilt upward toward the warmest air. 3. Systems with symmetrical Temperature patterns do not tilt with height. The westward tilt with height of troughs and ridges outside the tropics is due to the fact that the west side is the cold side of most extratropical cyclones and the warm side of most extratropical high pressure areas. As a result of this slope, Jet Stream winds typically blow from the SW directly above Lows and from the NW directly above highs. This in turn, moves Lows from SW to NE and Highs from NW to SE, as you can see in the next slide. L L COLDCOLD WARMWARM
Extratropical Cyclones and Anticyclones and Waves in the Jet Stream The Jet Stream is fastest directly above fronts since they have the largest horizontal temperature gradients. Thus, Extratropical Cyclones form beneath the Jet Stream and are swept along it much as a railroad moves along tracks. As the extratropical cyclones evolve, they modify both fronts and the jet stream. Thus the cyclone is much like a train that modifies its own tracks. L H
Link between Surface Features and Jet Stream Aloft Convergence Aloft Forces Air Down East of Ridge Divergence Aloft Forces Air Up East of Trough Wind Fast in Ridge Slow in Trough
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