Weather unit 3

Atmospheric stability and effect of friction

(a) When the air is stable and the terrain fairly smooth, vertical mixing is at a minimum, and the effect of surface friction only extends upward a relatively short distance above the surface. (b) When the air is unstable and the terrain rough, vertical mixing is at a maximum, and the effect of surface friction extends upward through a much greater depth of atmosphere. Vertical mixing increases near-surface wind speed in the region of frictional influenceUsually surface winds are measured at 10m above the surface

Cloud to ground lightning

2 types positive and negative: Positive- Base of the cloud is positive and the ground is the negative return. Associated with supercell. Negative- Invisible, negatively charged stepped leader is present from the cloud (seen as a return strike), lightning strike, positive return. Makes up 90% of all cloud to ground lightning.

Gust fronts

A cold front with strong and gusty winds Straight line winds with high winds behind a gust front, VERY POWERFUL. Warm air rising along a gust front could create a shelf cloud or a roll cloud Multiple gust fronts may produce an outflow boundary

Lightning

A discharge of electricity that usually occurs in mature t storms May occur within clouds, between clouds , from clouds to the surrounding air, or from a cloud to the ground. 20% of all lightning is from cloud to ground. Lightning heats the air to 30,000 degrees celsius, or 54,000 degrees F, this is five times hotter than the surface of the sun!

Lake breeze

A lake breeze often develops on the shoreline of lake Michigan in spring and summer when synoptic-scale winds are relatively weak and surface waters are cooler than adjacent land surface • A shallow mesoscale high develops over the lake and surface winds diverge from the high toward the shoreline. A lake breeze front may then form following convective clouds and precipitation on the leading edge of the frontSinking air develops where surface winds move offshore, speed up, and diverge. Rising air develops as surface winds move onshore, slow down, and converge.

Heat island circulation

A local circulation between relatively warm urban areas and its relatively less warm surroundings• It develops when synoptic-scale winds are weak and transports air pollutants, mainly aerosols, in a dust dome over a city. The pollutants may become as much as a thousand times more concentrated over an urban area than in the air over the rural area. If local winds strengthen to more than about 15 km/hr (9 mi/hr), the dust dome elongates downwind to form a dust plume which spreads the urban pollutants to the rural areas The factors contribute the temperature contrast between the urban and rural areas: i) high concentration of heat sources in urban areas such as industry, furnaces, cars, ii) high rate of construction in urban building materials, and iii) low rate of evapotranspiration in urban areas

Mountain and valley breeze

A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day.

Mid latitude cyclones life cycle

A mid-latitude cyclone's life cycle typically last THREE days. However, when conditions are favorable, the cyclone can progress from its birth to maximum intensity in 24 hr or less. A rapid intensification of a cyclone whose pressure drops by at least 24 mb in 24 hr is named BOMB. Only a few cyclones satisfy the condition for the bomb, which is usually observed when a cyclone moves off the east coast of North America or Asia over warm ocean currents of Gulf Stream or Kuroshio. Under unfavorable conditions, the cyclone never progress to a fully occluded system, instead spending its lifespan as a weak cyclone rippling along a stationary front.

Air masses

A very large body of air (could be a million square kilometer in size) with similar properties of temperature and humidity in any horizontal direction • Because of equator to pole temperature gradient air mass with different temperature and humidity move around between latitudes • According to the origin of the air it can be classified in various types of air masses • To have a uniform temperature and moisture, air mass must stay in its Source Region for several days or weeks.

Stationary front

A nearly stationary narrow zone of transition between contrasting air masses • A stationary front is drawn as an alternating red and blue line on weather maps. - Red semicircles point toward colder air. -Blue triangles point toward warmer air. • Surface winds blow parallel to the front, but in opposite directions on the two side of the front. Upper-level winds blow parallel to the front. •Stationary front is often associated with a wide region of clouds and rain or snow on the cold side of the front. •As warm humid air flows upward over the cooler air mass, along the frontal surface, cools through adiabatic expansion which results in condensation and precipitation.

Tornado alley

A region from central Texas to Nebraska. Central Oklahoma has the highest annual incidence of tornadoes. Local tornado maxima occur in central Illinois, Indiana, and southern Mississippi.

Monsoon winds

A regional circulation characterized by the seasonal reversal of prevailing winds such that winds blow from land to sea in winter and vice versa in summer over eastern and southern Asia Monsoon winds have sufficiently long trajectories and persist long enough to be influenced by the Coriolis effect such that surface winds are deflected to the right in Northern Hemisphere and to the left in the S. Hemisphere• Monsoon winds occur in South and Southeast Asia, Southwest US, and Australia

Water spout

A rotating column of air over a large body of water. It is usually less energetic, smaller, and short-lived than a tornado. It is associated with a cumulus congestus or cumulonimbus cloud.

Downburst

A severe, localized downdraft of a t storm

Tornadoes

A small mass of air that whirls rapidly about a nearly vertical axis around a small area of intense low pressure. It is made visible by clouds and by dust and debris sucked into the system. Tornadoes can have variety of shapes ranging from cylindrical funnels to long, slender roselike pendants. A funnel cloud is a tornado whose circulation has not yet reached the ground.

Cold fronts

A transitionary boundary between cold, dry polar and warm, moist unstable subtropical air. Drawn as a solid blue line, with triangles showing the direction of movement. A cold frontal passage is associated with a drop in temperature and humidity. The temperature drop is often much larger during winter time. The slope of a cold front is steeper than the slope of a warm front. A typical flow aloft across the front is from the cold to the warm side.

Tropical Easterly Wave

A tropical wave (also called an easterly wave) as shown by the bending of streamlines - lines that show wind flow patterns. (The heavy dashed line is the axis of the trough.) The wave moves slowly westward, bringing fair weather on its western side and showers on its eastern side.

Air masses

Air Masses A very large body of air (could be a million square kilometer in size) with similar properties of temperature and humidity in any horizontal direction • Because of equator to pole temperature gradient air mass with different temperature and humidity move around between latitudes • According to the origin of the air it can be classified in various types of air masses • To have a uniform temperature and moisture, air mass must stay in its Source Region for several days or weeks.

Mature stage

An extremely steep horizontal air pressure gradient between the tornado center and outer edge that is responsible for a tornado intensity and damage potential. Air pressure drops about 10% over a distance of only 100meters. The Coriolis effect is present but the system is so small that its influence is negligible. Thus, winds in a tornado may rotate either Clockwise, or Counterclockwise. Tend to rotate counter clockwise in the northern hemisphere and clockwise in the Southern Hemisphere. In violent tornadoes >180 knots, small whirs called suction vortices rotate themselves causing damage.

Generation and Advection of Vorticity

An increase in absolute vorticity is related to upper level convergence A decrease in absolute vorticity is related to upper level divergence Advection is horizontal transfer of property- vorticity is advected by horizontal winds. Forecasters review 200mb, 500mb, and surface maps to examine pressure, convergence, vorticity, and advection Relative vorticity results from curvature + shear Absolute vorticity = Earth's spin + relative vorticity Earth's vorticity is always positive!

Anti-Cyclone

Anticyclone: a weather system characterized by relatively high surface pressure compared with the surrounding air; surface winds blow clockwise in the Northern Hemisphere (counter-clockwise in the Southern Hemisphere) and outward that is associated with divergence, and therefore sinking motion, and a fair weather. Cold-core anticyclone: a shallow weather system that coincides with the dome of continental polar or arctic air. They are responsible for the frigid temperatures over the continental US in winter. They are shallow systems in which the clockwise circulation weaken with altitude and often reverses. Warm-core anticyclones: a weather system characterized by strengthening high pressure center with altitude. The thickness between pressure surfaces is highest at the center of the high. The semi-perminent subtropical anticyclones, such as Bermuda-Azores high, are examples of warm-core anticyclones which are accompanied by subsiding, warm, dry air.

Polar front theory

As the mid latitude cyclone progresses through its life cycle, it is supported and steered by the upper-level circulation toward the east and northeast. The storm typically begins as a wave along the polar front and deepens as the surface air pressures continue to drop. As a result, winds strengthen and frontal weather develops. The storm finally occludes as the faster moving cold front "catches up" with the slower moving warm front and the upper- level low center becomes vertically stacked over the surface low center.

Baroclinic atmosphere

Baroclinic atmosphere: Density depends on the pressure and temperature. Baroclinic instability is a necessary condition for the development of a mid-latitude cyclone in which a meridional (north-south) temperature gradient, a strong vertical wind shear, temperature advection, and divergence aloft occur.

Barotrophic atmosphere

Barotrophic atmosphere: density depends only on the pressure.

Thunder and thunderstorms

Booming sound wave caused by the extreme heating of air and its expansion due to lightning which is a discharge of electricity within or among clouds, and between clouds and the ground. T storms are weather systems that produce lightning and thunder along with rain, hail, and intense winds. They are typically mesoscale systems but can range to diff scales. Mesoscale has- Alpha: >200 to <2000km Beta: 20 to 200km Gamma: 2 to 200km Most T storms develop within warm, humid, and usually conditionally unstable maritime air as a consequence of uplift along fronts, on mountain slopes, via convergence of surface winds, or through intense solar heating of the earths surface.

Mesoscale convective vortices

Can form from bow echoes May linger for days and serve as a starting point for additional t storms

Cumulus stage

Characterized by updraft throughout the entire cell as cumulus cell grows vertically. There is no thunder or lightning or precipitation yet!

Multi-cell and severe storms

Contain a number of cells in differing stages with overshooting convective cloud tops in the stratosphere. Forms in a region of wind shear of horizontal winds Lasts for a long time Often produces: Shelf/roll clouds outflow boundaries downbursts microbursts heat bursts squall lines

Downslope winds

Chinook wind: the downslope flow of compressionally warmed and dry air. It usually originates over areas where surface temperatures often climb abruptly tens of degrees in response to compressional warming Foehn: a chinook wind that is drawn down the Alpine valleys of Austria and Germany Zonda: a chinook wind that is drawn down the Andes mountain valleys of Argentina Santa Ana wind: a hot and dry chinook-type wind that blows downward from the desert plateaus of Utah and Nevada toward coastal southern California • It typically develops in autumn and winter when a strong high pressure center centered over Great Basin• Santa Ana winds sometimes gusts to 130 to 145 km/hr (80 to 90 mi/hr) and almost always cause some property damage, also forest and brush fires Katabatic wind: the downslope flow of cold and dry air under the influence of gravity. It usually originates over extensive snow- covered plateaus or other highlands• Most katabatic winds are weak, usually under 10 km/hr (6.2 mi/hr), however, in some places where steeper slopes are present and/or katabatic winds are channeled by narrow valleys, the katabatic winds are sometimes accelerated to potentially destructive speeds such as gusts of 100 km/ hr (62 mi/hr) observed along the edge of the massive Greenland and Antarctic ice sheets Mistral: a katabatic wind that flows from the Alps down the Rhone River Valley of France to the Mediterranean coast Bora: a katabatic wind that originates in Serbia and flows onto the coastal plane of the Adriatic Sea. Bora can produce gusts of 50 to 100 km/hr (31 to 62 mi/hr) Columbia Gorge wind: a katabatic wind that originates Columbia plateau and flows westward through the Columbia River Gorge as a strong, gusty, and sometimes violent wind

Supercell storm types

Classic- produces heavy rain, large hail, high surface winds, and tornadoes HP (High Precipitation)- dominated by heavy precipitation, strong downdrafts and large hail LP (Low precipitation)- produces little rain, but can produce some hail, and has a corkscrew shaped pattern.

Organized Convection Theory

Cold air above an organized mass of tropical thunderstorms generates conditionally unstable air and large cumulonimbus clouds. • The release of latent heat warms the upper troposphere, creating an area of high pressure. Upper-level winds move outward away from the high. This movement, coupled with the warming of the air layer, causes surface pressures to drop. • As air near the surface moves toward the lower pressure, it converges, rises, and fuels more thunderstorms. Soon a chain reaction develops, and a hurricane forms.

Cold-core anticyclones

Cold-core anticyclone: a shallow weather system that coincides with the dome of continental polar or arctic air. They are responsible for the frigid temperatures over the continental US in winter. They are shallow systems in which the clockwise circulation weaken with altitude and often reverses.

Mid Latitude Cyclones

Cyclone: A weather system characterized by relatively low surface pressure compared with the surrounding air; surface winds blow counter-clockwise in the Northern Hemisphere (clockwise in the Southern Hemisphere) and inward that is associated with convergence, and therefore rising motion, cloudiness, and precipitation. The formation, strengthening, or regeneration of a cyclone is called CYCLOGENESIS while the weakening or dissipation of a cyclone is called CYCLOLYSIS. The former is associated with deepening while the latter is associated with filling of a low pressure center. The original description of the structure and life cycle of a midlatitude low- pressure system, first proposed during World War I by researchers at the Norwegian School of Meteorology at Bergen. This conceptual model is therefore referred to as the Norwegian cyclone model.

Wave cyclone

Cyclonic circulation causes a meridional wave to form along the front, during the mature stage of a cyclone

Desert winds

Desert winds: the regional gusty surface winds associated with vigorous convection (absolutely unstable) depending upon the intensity of the solar radiation such that the wind speed and gustiness usually peak in the early afternoon and during the warmest months Sand and dust storms are produced by the larger scale winds of thunderstorms or migrating cyclones in deserts Sand storms differ from dust storms based on particles size range: Sand consists of large particles (0.06 to 2.0 mm in diameter), while dust consists of very small particles (less than 0.06 mm in diameter)

Divergence

Divergence: air parcels coming together or going apart in given area Divergence aloft causes an increase in the cyclonic vorticity of surface cyclones that results in cyclogenesis and upward air movement

Dissipating stage

Dominated by downdrafts throughout the convective system often bringing light to moderate precipitation.

Stages of tornadoes

Dust whirl stage Organizing stage Mature stage Shrinking stage Decay stage Minor tornadoes may only evolve up to the organizing stage, some may skip stages and go directly to the decaying stage.

Cyclone energy sources

Energy for the cyclone is derived from 1) conversion of potential to kinetic energy when cold air sinks, 2) at surface low, convergence of air from surrounding areas also supply kinetic energy, 3) latent heating of condensation

Hurricane structure

Eye of a hurricane: the center of a hurricane characterized by clear skies, subsiding air, and light winds (< 25 km/hr). It generally ranges from 20 to 65 km across, shrinking in diameter as the hurricane intensifies and wind strengthen. At a hurricane's typical forward speed, the eye of a hurricane passes through a given location up to an hour. Eye wall: a circle of cumulonimbus clouds surrounding the eye of a mature hurricane. It is associated with heavy precipitation and strong winds. The most dangerous and destructive part of a hurricane is near the eye on the side where winds blow on the same direction as the storm's forward motion. Rain Bands: include strong winds, associated tornadoes, heavy rains, and storm surge.

Types of Lightning

Forked: caused by various dart leader paths Ribbon: forms inside an ionized wind channel Bead: appears to break up like a string of beads Ball: floats in the air like a luminous sphere Sheet: lightning inside the cloud; clouds appear like a luminous sheet Heat: lightning that is seen but not heard Dry: occurs in storms that do not produce rain

Supercell structure

General features- Mesocyclones: a rotating air column on the south side of the storm, usually 5-10km across, updraft creates a rain free zone Wall cloud: a rotating cloud descending from the base of the storm

local wind names

Haboob: a dust storm caused by the downdraft of a desert thunderstorm. It may be more than 100 km wide and may reach altitudes of several kilometers. It is most common in Sudan (North Africa), but also occur in the southwest US. A haboob severely restrict the visibility. Buran, a strong cold wind that blows over Russia and Central Asia.Purga, a buran accompanied by blowing snow. Pampero, a cold wind blowing from south to Argentina, Uruguay, and into the Amazon basin. Burga, a cold northeasterly wind in Alaska usually accompanied by snow. Bise, a cold north or northeasterly wind that blows over southern France. Papagayo, a cold northeasterly wind along Pacific coast of Nicaragua and Guatemala. Tehuantepecer, a strong northlerly or northwesterly wind funneled through the gap between Mexico and Guatemala. Levanter, a mild, humid, often rainy east or northeast wind that blows over the southern Spain. Harmattan, a dry, dusty, but mild wind from northeast or east that originates over the coll Sahara in winter and blows over the west coast of Africa. Simoom, a strong, dry, and dusty wind that blows from Sahara into Mediterranean. If it is from Morocco to Atlantic Ocean, it is named as Leste. If it is from Algeria to Spain, it is called Leveche . If it is blowing over Eygpt, it is called Khamsin, while over Israel, it is called Sharav.

Hook Vs. Bow echo

Hook: A distinct radar pattern that often indicates the presence of a severe t storm and potential tornado. Typically appears on the south side of a severe t storm cell in the presence of a mesocyclone. Bow: A line of t storms on a radar screen that appears in the shape of a bow. Bow echoes are often associated with damaging straight line winds and small tornadoes.

Hurricane Development/Formation

Hurricanes ALWAYS form over water! • Hurricanes (typhoons and tropical cyclones) develops in a uniform mass of very warm and humid air without associating a front or frontal weather. Necessary conditions for hurricane formations are: warm sea surface temperatures (greater than 26.5° C or 80° F), low level convergence, high moisture in the lower troposphere and deep moist layer extending up to middle and upper troposphere, weak vertical wind shear. • A ʻtriggerʼ is required to start low-level convergence resulting in surface low pressure -- to start a hurricane. For example, an easterly wave or thunder storm cluster forming within ITCZ can provide such a trigger. • Coriolis force is necessary for cyclonic circulation, but at and near the equator it is close to zero. Therefore, hurricanes from between 5°-20° latitudes and not at or close to the equator. • Hurricanes form during summer months. Over the Atlantic Ocean, official hurricane season is considered to span June to November. Hurricane Formation and Development • The energy for hurricanes come from the direct transfer of sensible and latent heating from the warm ocean surface. This energy is converted to kinetic energy during stormʼs lifecycle. • Typically, a hurricane starts as a tropical disturbance, in which a low-pressure center is triggered by within ITCZ or by an easterly wave. Thunderstorm cluster form, which in moist and conditionally unstable atmosphere release a large amount of latent heating in the atmospheric column. Rising motion and warming due to to latent heating result in high pressure in the upper troposphere, which result in divergence at upper level and further lowering of surface pressure. • This causes more low level convergence from surrounding atmosphere, and cyclonic circulation sets up. Under right conditions, the tropical disturbance grows into 1) tropical depression, 2) tropical storm, and 3) hurricane.

Vertical Structure for Mid-latitude Cyclone Development

If lows and highs aloft were always directly above lows and highs at the surface, the surface systems would quickly dissipate. An idealized vertical structure of cyclones and anticyclones. The low pressure centers (L) are dynamical lows and not thermal lows

Supercell thunderstorms

Intense, long lasting t storms with a single violently rotating updraft. Can generate an updraft that may exceed 90 knots and may produce damaging surface winds as well as tornadoes Can extend 60,000ft above the surface with a width of 25 miles May produce large hail too

Mesoscale convective complexes

Large, organized convective weather systems comprised of a number of individual thunderstorms Often large enough to cover an area in excess of 100,000 square kilometers. Tend to form in summer in regions where upper level winds are weaker

Scales of motion

Microscale: The smallest scale of atmospheric motions (~ a few meters). Mesoscale: The scale of meteorological phenomena that range in size from a few km to about 100 km. It includes local winds, thunderstorms, and tornadoes. Synoptic scale: The typical weather map scale that shows features such as high- and low-pressure areas and fronts over a distance spanning ~1000 km. Global or planetary scale: winds usually have ~5000 km of scale. Combined global and synoptic scale winds are sometimes referred to as macroscale winds.

T storm distribution

More than 50,000 t storms occur every day on earth, and over 18 million per year! The occurrence of storms over equatorial landmasses is much less frequent at around every 3 days Storms are prevalent over water along the intertropical convergence zone (ITCZ). Less prevalent in polar and desert regions.

Squall lines

Multi cell t storm that is often severe in strength, forming a line of t storms Can extend hundreds of kilometers along a cold front and have extensive lifetimes Pre frontal squall lines form in warm air 100-300km ahead of a cold front Most severe form of squall line in mid latitudes Ordinary squall lines have short life spans

Most tornadoes occur in...

No where sees more tornado activity than the U.S. We average 700-1100 per year. 2011 was a record year when we experienced 1691 tornadoes!

Occluded front

Occlusions are created when a cold front over takes a warm front. •An occluded front is drawn as a purple line with alternating cold-front triangles and warm-front half circles. When air behind the advancing cold front IS colder than the air ahead, it is called cold-type occlusion. When air behind the advancing cold front IS NOT as cold as the air ahead of the warm front, it is called warm-type occlusion • The cold-type occlusion is often observed in northern US and southern Canada, while the warm-type occlusion is less common, but often observed in the northerly portions of western coasts, such as in Pacific northwest.

Ordinary T storms

Often form along zones where surface winds converge Stages of Development: Cumulus/Growth stage Mature stage Dissipating stage

Prime Tornado conditions

Peak months for tornadoes are: April at 13% May at 22% June at 21% This is due to the relative instability of the lower atmosphere, and ideal synoptic weather conditions. Slightly above 50% develop during the warmest hours of the day from 10am to 6pm.

Radar observations

Radar Observations • A network of more than 150 dual polarized Doppler radars, called Weather Surveillance Radar - 88 Doppler (WSR-88D) operates across the United States. • The WSR-8DD was built by NEXRAD (next generation weather radar), a joint effort of the National Weather Service (NWS), US Air Force, and FAA to provide data for civil, military, and aviation needs. • Wind profiler: a vertically pointed Doppler radar that measures the wind speed and direction at 65 different altitudes up to 16.25 km at its location. • A network of 35 wind profilers are currently operated in 18 Midwestern states, and Alaska by NOAA

Scales of motions and winds

Scales of Motion: Winds have scales of motion ranging from a few meters to ~5000 km and from a few seconds to a week or more. Winds of many scales: microscale, mesoscale, synoptic scale, and global scale resulting from a combination of a variety of forces (defined later). Local winds: Winds that tend to blow over a relatively small area; often due to regional effects, such as mountain barriers, large bodies of water, local pressure differences, and other influences. Local features have to be known as they influence wind speed and direction. Prevailing wind: The wind direction most frequently observed during a given period.

Sea and land breeze

Sea Breeze: A relatively cool mesoscale surface winds directed from sea toward land in response to differential heating between sea and land. It develops during the day reaching its maximum strength by mid-afternoon. The inland extent of the breeze varies from only a few hundred meters to many tens of kilometers. Land breeze: a relatively cool mesoscale surface winds directed from land toward sea in response to differential heating between sea and land. It develops during the night reaching its maximum strength just before the sunrise but tends to be weaker than a sea breeze. Sea and land breezes are shallow systems, generally confined to the lowest kilometer of the troposphere and develop by a localized horizontal pressure gradient in the absence or weak synoptic-scale winds. *At the surface, sea breeze blows from the water onto the land during day*Land breeze blows from the land out over the water during night

Electrification of clouds

Soft hail falls through a region of supercooled liquid droplets and ice causing the transfer of charges from warmer hail to frozen supercooled water. Regions of separate charge exist within tiny cloud droplets and larger precipitation particles.

Lightning and Thunder facts

Sounds of thunder travel at 330 meters per second/1100 feet per second! Thats 5 sec/mile! The sound of thunder takes longer to reach your ears that it takes for lightning to reach your eyes. You see lightning and then hear the thunder. If one sees lightning and hears thunder 15 sec later, the storm is 3 miles away.

4 types of fronts

Stationary Cold Warm Occluded

Dust devils

Swirling mass of dust caused by intense solar heating of dry surface areas. They are microscale systems, usually less than 1 m in diameter that typically last less than 1 minute. However, they occasionally exceed 100 m in diameter lasting for 20 minutes or longer. Such dust devils may be visible to altitudes topping at 900 m, but the rising air column may reach 4500 m. Dust devils can cause substantial damage when surface winds exceed 70 km/ hr (45 mi/hr).

Elman spiral and upwelling

The Elman Spiral: Winds move the water, and the Coriolis force deflects the water to the right (Northern Hemisphere). • Below the surface each successive layer of water moves more slowly and is deflected to the right of the layer above. • The average transport of surface water in the Ekman layer is at right angles to the prevailing winds •Near the coast, as water drifts away from the coast It is replaced by cold water by a process known as upwelling

Tropical Cyclone and Hurricane Naming

The World Meteorological Organization (WMO) maintains a rotating list of cyclone names appropriate for each ocean basin • The U.S. National Hurricane Center (NHC) compiles names of hurricanes in the Atlantic and Caribbean regions • Cyclone names are given alphabetically, alternating female and male names • Some names are retired if the storms were greatly destructive • Six years of names are decided and are then rotated • First 21 characters of alphabets are used - Q, U, X, Y and Z are not used

Warm fronts

The leading edge of advancing warm , moist subtropical air. Usually separates advancing mT air from the Gulf of Mexico from retreating cold mP air of the North Atlantic. Drawn as a solid red line. Direction indicated by half circles which point toward cold air. Overrunning: warm, less dense, air rides up over colder, more dense air.

Front facts:

The location of fronts is determined by the pressure tendency, and temperature and dew point changes. Fronts may develop lines of active thunderstorms.

Mesocyclones

The mesocyclone circulation begins in the mid-troposphere followed by upward and downward motion. During the mature stage of a tornado, mesocyclone stretches vertically and shrinks horizontally as the spinning intensifies and extends downward to the cloud base, known as tornado cyclone. • The intensity of a mesocyclone can vary on that F-scale from weak to devastating. • Most of the North American tornadoes are linked to thunderstorms associated with midlatitude cyclones. Most of the others are the product of convective instability triggered by hurricanes. Tornadoes often develop on the northeast sector of a hurricane, after the system has curved toward the north and northeast. Most hurricanes that strike the southeastern United States are accompanied by tornadoes.

Mature stage

The most intense stage with a well developed convective cell growing vertically to the upper troposphere. Heavy precipitation, thunder and lightning, and wind gusts are present. Updrafts and downdrafts occur in this stage.

Comma cloud

The pattern of cloudiness associated with a wave cyclone is called comma cloud. The head of the comma stretches from the low center to the northwest and its tail follows along the cold front.

Tornado intensity scale

Tornadoes are classified in six intensity scale, called Enhanced F-scale following Professor T. Fujita of the University of Chicago, based on the estimated wind speed from property damage. • The American Meteorological Society (AMS) reports that 79% of all tornadoes are weak, 20% are strong, and only about 1% are violent in a typical year. [Scale Wind speed (mph) - Damage] -F0 65-85 Light -F1 86-110 Moderate -F2 111-135 Considerable -F3 136-165 Severe -F4 166-200 Devastating -F5 > 200 Incredible -tree branches broken, trees snapped, large trees uprooted, cars overturned, frame houses destroyed, not much survives an F5!

Tropical Cyclones

Tropical Cyclones • The tropics cover global band between 23.5°S to 23.5° N latitudes. Sun is usually high in the sky compared to middle and higher latitudes. • Surface heating and higher moisture content in the atmosphere favor cloud and thunderstorm development in the tropics. The tropical thunder storms more commonly form in clusters. • Seasonal weather in the tropics is not characterized by big temperature differences, but rather by rainfall pattern, controlled by a number of factors such as Inter-tropical Convergence Zone (ITCZ), land-ocean distribution, and orography. • Tropical winds are easterly (trade winds). In the easterly flow, tropical easterly waves form, with wavelength ~2500 Km and westward speed of ~5-10 m/s. Occasionally, an easterly wave grows into a cyclonic storm or Hurricane.

Micro and Mesoscale winds

Turbulence plays a major role. Turbulence is any irregular or disturbed flow in the atmosphere that produces gusts and eddies. Turbulence is caused by viscosity Viscosity: friction of fluid flowMolecular viscosity: due to motion of gas moleculesEddy viscosity: produced by turbulent eddies; results from surface roughness Eddy: A small volume of air (or any fluid) that lasts or a few minutes and behaves differently from the larger flow in which it existsEddies of various sizes result from mechanical or thermal turbulence and are found in the atmospheric boundary layer Mechanical turbulence: Turbulent eddy motions caused by obstructions, such as trees, buildings, mountains, and so on. Also produced by strong winds Thermal turbulence: Turbulent vertical motions that result from surface heating and the subsequent increase in instability, and rising and sinking of air Planetary boundary layer:- Influenced by friction- Top is near 1000 m (3300 ft)- Height is increased by surface heating and instabilityFrictional drag decreases as we move away from the Earth's surface

Tornado specs

Typical tornado on the ground has a diameter between 100 and 600 meters with winds less than 182km/hour or 113mph and a lifetime of only 1-3min. An intense tornado can exceed a diameter of 1600 meters with winds up to 513km/hour or 318mph and a lifetime of over 2 hrs. They move in circles with their average speed being around 55km/hr or 34mph but up to 240km/hr or 149mph. Most travel with a severe thunderstorm supercell about 90% of the time from the southwest to the northeast

T storm lifespan

Typically lasts from 30min to an hour.

Vorticity

Vorticity is a measure of the spin of small air parcels Positive vorticity: cyclonic Negative vorticity: anticyclonic

Warm-core anticyclones

Warm-core anticyclones: a weather system characterized by strengthening high pressure center with altitude. The thickness between pressure surfaces is highest at the center of the high. The semi-perminent subtropical anticyclones, such as Bermuda-Azores high, are examples of warm-core anticyclones which are accompanied by subsiding, warm, dry air.

Winds and ocean currents

Winds blowing over oceans cause surface water to drift Due to the Coriolis force, water moves at an angle between 20° and 45° of the wind direction Warm current carries warm water from the equator to the poles on the eastern edge of continents (e.g. the Gulf Stream) and the western side of continents carries cool currents flowing from the poles toward the equator.

Doppler radar

a conventional radar that can detect the detailed motion of precipitation toward and away from the radar. As precipitation particles move away from or toward the radar unit, the frequency of the radar signal shifts slightly between emission and reception. The Doppler effect was named after Johann Christian Doppler and is calibrated in terms of motion of the precipitation. Multiple doppler units provide a 3D image of air circulation. Dopplers can detect wind shears, gust fronts, mesocyclone, and can provide an advanced warning of about 20 min of severe weather compared to 2 min from conventional radar.

Microburst

a downburst can produce winds > 100knots/115mph! The leading edge can develop into a gust front Can blow down trees and cause structural damage Very dangerous for aircrafts

Polar low

a low pressure system that develops over polar water behind (or poleward side of) a polar front. Its diameter is 500 km or even less, making it smaller than the mid-latitude cyclone. Polar low has a hurricane like center (eye), comma shape cloud band, and relatively warmer core with strong winds and heavy showery precipitation (snow). It forms in winter. During this time, the sun is low on the horizon or absent, and therefore the air next to the snow- and ice-covered ground cools rapidly and become frigid cold. If this air meets with relatively warm air above warm ocean current, an arctic front forms.

bow echo

a squall line pushed outward bu strong winds which creates a bow on radar.

cold-core cyclone

a weather system characterized by deepening low pressure center with altitude. The thickness between pressure surfaces, which is directly proportional to the layer temperature, is lowest at the center of the low. The low pressure center aloft is located left of that at the surface.

warm-core cyclone

a weather system characterized by the thermal lows that are stationary, have no fronts and are associated with very hot, dry air. The shallow near-surface cyclonic circulation weakens and then reverses with altitude. Hurricanes are a warm-core cyclone

Derechoes

straight line winds gusting to more than 58mph for at least 250miles

Heat bursts

sudden warm downbursts thought to originate high up toward the back edge of thunderstorm anvils Warmed by compressional heating as the air plunges to the surface.

Warm sector

the region between cold and warm front where temperatures are relatively higher than behind the cold front as well as the ahead of the warm front in the presence of an occlusion.

Weather fronts

• A narrow zone of transition between air masses of contrasting density, that is, air masses of different temperatures and humidity • The extension of a front is called a frontal surface or a frontal zone • The formation, strengthening (stronger density gradient), or regeneration of a front is called frontogenesis, while the weakening (weaker density gradient) or dissipation of a front is called frontolysis • Precipitation associated with the fronts tend to increase or diminish in intensity as the front strengthens or weakens. • Fronts slope back from the Earth's surface toward colder (denser )air.

Air Masses: Classification

• Air masses are usually classified by temperature and humidity. And grouped according to their source regions • c denotes continental source region • m is maritime source region • P for polar • T for tropical • A for arctic • There are five air masses according to their source: i) maritime polar (mP) ii) continental polar (cP) iii) maritime tropical (mT) iv) continental tropical (cT) v) continental arctic (cA) is distinguished from cP by its bitter cold air mass.

Hurricanes

• An intense storm of tropical origin, with a low pressure center and cyclonic, sustained winds exceeding 64 knots (~30 m/s, 74 mile/hr), which brings heavy rain, damaging winds, and surge over ocean and coastal regions. • Hurricanes form mostly over tropical north Atlantic, north Pacific (called typhoons), and Indian Oceans (called tropical cyclones). • Air pressure is distributed symmetrically about the hurricane center and a hurricane has about one-third the diameter of an average extratropical cyclone. The central pressure is lower and the horizontal air pressure gradient is much steeper in a hurricane than in an extratropical cyclone. • A mature hurricane is a warm-core cyclone and therefore weakens with altitude, especially above 3 km such that an anticyclonic flow usually exists at altitudes of about 15 km above the hurricane.

Hurricane intensity

• Based on range of central air pressure and wind speed, and potential for storm surge and the property damage, a hurricane is rated 1 (weak) to 5 (very intense) on the Saffir-Simpson scale. (Picture in notes on phone) • TROPICAL STORM is a tropical cyclone having wind speeds of 63 to 119 km/hr (39 to 74 mi/hr), while TROPICAL DEPRESSION is the developing stage of a tropical storm or a hurricane with wind speeds of 37 to 65 km/hr (23 to 40 mi/h).

cT Air Mass

• Found in the southwestern United States • Hot, dry, and unstable at low levels and Stable air aloft • Leads to clear skies and no rain conditions. • Can be responsible for droughts in the Great Plains

Hurricane features

• Highest winds on the eastern side of storm (wind + speed of storm) • Storm surge on north side of storm (tide) • Coastal flooding • River flooding • Hurricane spawned tornadoes

mT Air Mass

• In Winters mP over the western US brings warm, moist air to the West Coast that produces heavy precipitation. Referred to as "the Pineapple Express" Brings a lot of moisture that is referred to as atmospheric river • Over the eastern US it often bring heat waves, rains, and low clouds

Hurricane decay

• Once a hurricane makes landfall, it loses its warm-water energy source and experiences greater surface roughness. Hence, its circulation weakens rapidly so that most wind damage is confined to the coast. Heavy rains, on the other hand, often continue well inland and may cause severe flooding. • Another way a hurricane looses energy is by infrared cooling from cloud-tops.

cP and cA Air Masses

• Originate over the Arctic, Canada, and Alaska. • Cold, dry air bring high pressure at the surface over the US • May trap pollutants and affect visibility. • To the west of Rockies, mountains can act as barrier to cold air. Cities east of mountains are often warmer. Sometimes if air rises over the mountains it can come down to valley, get compressed and warm • When cP air travels over warm Great Lakes or Atlantic water, in addition to temperature, its humidity also modifies leading low-level cloudiness and fog. • If the on shore winds are present over Great Lakes, a localized snow-storm forms on downwind side of the lake, known as lake-effect snow. Such storms are common in late autumn and early winter before the lakes become frozen. • In summer, as the polar regions are snow-free and less cold, cP and cA air mass quality changes. • The snow-melt and its evaporation add moisture to the air mass. • They bring relief from hot temperatures over the central and eastern US. • As the cP and cA air masses reach the Gulf of Mexico and Atlantic Ocean, the air mass further modifies, results in clouds and showers.

mP Air Mass over the Western US

• Originate over the Pacific Oceans • Pacific coast air is cool, moist, and unstable. Air is warmer at the surface than aloft. • Water vapor produces rain and snow and cumulus clouds over the ocean. • As it is forced up the windward slopes of coastal mountain ranges, and undergo adiabatic cooling, leading development of clouds and precipitation. • As the mP air descends the leeward slopes, clouds dissipate as a result of adiabatic warming. A similar process is repeated when mP air passes up and over the Rockies and it becomes considerably milder and drier that its original air.

mP Air Mass over the Eastern US

• Originates in the North Atlantic polar region and are usually colder than their Pacific counterpart as they travel less distance over ocean. • Atlantic air brings cold, moist air, Rains, heavy snow, and may result in coastal flooding • Though the upper level air flow is usually westerly, polar north-easterlies sometimes swing the air southward bringing mP air mass to mid-Atlantic .

Source Regions

• Region where air masses originate is called its Source Regions • Source Regions are dominated by high surface pressure and light winds (e.g., ice- and snow-covered arctic plains and subtropical oceans in summer) • To have a uniform temperature and moisture, air mass must stay in its Source Region for several days or weeks.

Weather around a cyclone

• The environmental conditions around a cyclone differ substantially from one location to another. For example, air temperatures are lowest to the northwest of a cyclone center where strong and gusty northwest winds advect cP and A air mass air southward and eastward. • A tendency to clear sky is evident to the west of the center where precipitation tapers off to showers as the cyclone moves toward northeast, while the leading of cold air located south of the cyclone center is accompanied by a narrow band of showers, and embedded thunderstorms. • To the southwest, skies are generally clear associated with sinking motion, while skies are partly cloudy with possible convective showers during the afternoon to the southeast of the center where south and southwest winds advect milder mT air northward from over Gulf of Mexico. • To the north and northeast, a zone of extensive overrunning as mT air surges over a wedge of cool air maintained by east and northeast winds at the surface. Skies are cloudy, and precipitation is steady and substantial to the northeast.

Tornado formation

• The most intense tornadoes usually appear on the rain-free rear portions of a severe thunderstorms. Typically occurs in the conditionally unstable, warm and humid air between the surface and 800 mb followed by a shallow inversion that acts a cap on the moist layer below. • A cold, dry and conditionally stable air existing above the inversion produces convective instability, favorable c for the tornado formation. • A funnel cloud may appear from Wall cloud that extends beneath a supercell thunderstorm. • A tornadic circulation evolves from an interaction between the updraft in the thunderstorm and the larger scale horizontal wind. • The horizontal wind exhibits a strong vertical shear in both direction and speed as wind speed increases height. • The shear in the horizontal wind initiates the cyclonic rotation in the updraft. The rising, spinning column of air of 10 to 20 km in diameter is known as mesocyclone. It represents the organizing stage of a tornado and is evolved as one or two into a tornado.