Chapter 10 Meteorology

Daniel defoe

1661-1731, first to propose storms generally track from west to east in middle latitudes

Benjamin franklin

1706-1790, first american to discover storms usually move in an easterly/nor easterly direction, 1743 concluded that wind direction wasn’t an indication of storm’s direction of movement

Air mass

huge expanse of air covering thousands of square kilometers, generally uniform horizontally in temperature and humidity, properties depend upon type of surface over which it develops (source region)

Continental polar (cP)

cold and dry

Maritime polar (mP)

cold and humid

Continental tropical (cT)

warm and dry

Maritime tropical (cP)

warm and humid

Arctic (A)

cold

Air mass modification

air mass properties modify as they move with the exchange of heat/moisture with surface over which they move and radiational heating/cooling, adiabatic heating/cooling with large-scale vertical motion

Air mass modification over different areas

winter with cP air mass travels from Canada into lower 48 states with S state temperatures not dropping much below freezing point, sun warms snow-free ground and warmer ground heats the bottom of the air mass, destabilizing it and triggering convection currents that distribute heat vertically, cP air mass traveling over snow covered ground experience less modification with incoming solar radiation reflecting not absorbing, tropical air masses modify less than polar air masses, significant change with orographic lifting

Tropical air masses

air mass often warmer than the ground it travels over, hence the bottom cools and air mass stabilizes, when a tropical air mass moves over a warmer surface it becomes even warmer

Air mass modification with orographic uplifting

mP air mass sweeping inland from pacific ocean, rising air cools adiabatically, condensation/deposition occurs and triggers precipitation on windward slopes, descent on leeward side leads to adiabatic warming and cloud dissipation, air mass emerges considerably milder and drier

Front

narrow zone of transition between air masses that differ in density and usually because of temperature contrasts, where contrasting air masses meet, slope of front influences the types of clouds that form

Front conditions

humidity causes density differences, front transition zone may be 100+ km on a weather map it’s drawn as a line along the warm edge of the zone, associated with a trough in sea-level pressure pattern and corresponds to wind shift and converging winds, colder and denser air forces the warmer less dense air to rise, induces cooling and often clouds/precipitation

Stationary front

exhibits little/no forward movement, slopes from earth’s surface toward denser air, lies in a trough in pressure pattern, wind changes direction abruptly across the front, broad region of clouds and precipitation (overrunning)

Example of stationary front

shallow pool of polar air surges S over plains and leading edge is too shallow to cross the mountains, milder air remains in the great basin W of the rockies

Other ways stationary fronts can develop?

pre existing front aligns with upper-level flow pattern, along a boundary in the surface temperature pattern

Warm front

warm/less dense ari advances while cold/more dense air retreats, characteristics similar to stationary

Warm front approaching

clouds thicken and become lower in altitude and cirrus, cirrostratus, altostratus, nimbostratus, stratus clouds and initial cirrus may be more than 1000 km ahead of the front, just ahead of front and steady precipitation gives way to drizzle and sometimes frontal fog and develops when rain falling through the shallow layer of cool air at the ground saturation evaporates and increases water vapor concentration to saturation, if advancing warm air is unstable and more vigorous uplift occurs with thunderstorms embedded in overrunning zone

Cold front

colder/more dense air displaces warmer/less dense air

Cold front conditions

over north america in winter and temperature contrast along a cold front greater than across a warm/stationary front, in summer temperatures on either side of the front may be essentially the same and humidity differences cause density contrast, slope on a cold front is much steeper than a warm front and uplift confined to a narrow area near the cold front’s leading edge and if warm air is unstable and thunderstorms may form and a squall line develops with the band of intense thunderstorms that forms either at the front/as much as 300 km ahead of front, if the warm is stable nimbostratus and altostratus may form, generally trails S/SW from center of an extratropical cyclone, back-door cold fronts move S along E side of appalachian mountains

Occluded fronts

typically form late in a cyclone’s life cycle as it moves into colder air and faster moving cold front catches up with the warm front, 2 types distinguished by the temperature contrast between air behind the cold front and air ahead of the warm front

Cold occlusion

air behind cold front colder than cool air ahead of warm front, like a cold front at the surface but with less air mass contrast

Warm occlusion

air behind cold front isn’t as cold as the air ahead of the warm front, like a warm front at the surface

Fronts summary

characterized by movement of cold air mass, defined by differences in temperature/humidity, wind shift, convergence, and a trough

Cloud and precipitation development

significant density contrast between air masses, adequate supply of water vapor

Frontogenesis

front forms, grow stronger

Frontolysis

front weakens

Norwegian cyclone model

conceptual model developed around WWI, still closely approximates current understanding, developing cyclone needs upper-air support

Cyclogenesis

birth of a cyclone, occurs along polar front directly under area of strong horizontal divergence in upper troposphere W of the low center and polar front pushes SE as a cold front, E of the low and polar front advances N as a warm front, air pressure at the bottom of the air column falls, horizontal air pressure gradient develops and cyclonic circulation begins, westerlies aloft steer and support the cyclone as it progresses through its life cycle

Wave cyclone

central pressure continues to drop winds strengthen due to increased pressure gradient and upper-level trough deepens while remaining W of the surface low center, warm sector becomes better defined, fronts form pronounced wave pattern with comma cloud is seen in satellite images, extensive stratiform cloudiness appears N of the warm front, cyclone moves E/NE at 40-55 km/hr

Beginning of occlusion

faster moving cold front advances on the warm front, warm sector area diminishes occluded front begins to form, upper level pattern shows closed circulation directly over the surface low (vertically stacked), dry slot, cyclone moves slower at 30 km/hr

Dry slot

separates the cold front cloud band from comma cloud

Bent-back occlusion

surface low becomes detached from the westerly steering flow and occluded front drawn around low center, warm sector is detached from cyclone center, triple point, eventually the cyclone weakens, cycloysis

Triple point

favors development of a secondary cyclone

Conveyor belt model

depicts circulation in mature cyclone with 3 broad interacting air streams that transport air (3 belts), 3D cyclone model combines horizontal and vertical air motions

What are the 3 belts?

warm and humid, cold, dry

Warm and humid conveyor belt

originates in the cyclone warm sector, ascends slightly flowing N in the warm sector at low levels and ascends more rapidly over the warm front, helps explain the broad region of clouds/precipitation N of the warm front

Cold conveyor belt

originates N of the warm front, ascends as it progresses toward W, forms comma cloud and produces precip, turns clockwise at upper levels and follows S westerly/westerly flow aloft

Dry conveyor belt

originates in upper troposphere and lower stratosphere upstream of the upper-level trough, one branch descends S behind the cold front, other branch forms the dry slot and separates the head and tail of the comma cloud

Northwest of low

lowest temp, strong winds, stratiform clouds, non-convective precipitation

Southwest sector

sinking air and mostly clear

Southeast of low

mildest temps, partly cloudy, scattered showers

Northeast of low

extensive overrunning zone

Principal cyclone tracks

depend on direction and strength of upper-level westerlies, converge toward the NE, cyclone centers move in same direction about half speed of 500 mb winds, appearing to originate E of the Rocky Mountains form over the pacific ocean near alaska and traveling over the mountains and cyclone loses its identity and reforms over the great plains, noreasters intensify off the NC coast then track NE, cyclones forming in S yield more precip and greater amount of mT air, winter with polar front and jet stream shift S so cyclogenesis is more frequent in US

Extratropical cyclones example

track A puts Chicago on the warm side with passage of the warm and cold fronts, track B puts Chicago on the cold side with no frontal passage

Winter storm

extratropical cyclone produces frozen/freezing precip, requires cold air and moisture supply and uplift mechanisms with a major storm requires warm and humid air brought N, storm moving NE produces heaviest snow to N and W of low center

Blizzard

severe storm characterized by high winds and reduced visibility due to falling/blowing snow, requires sustained winds of 35 mph greater and visibility less than 0.25 mi

Cold-core cyclone

lowest temperatures occur above low pressure center, isobaric surfaces dip downward toward center, depth of low increases with altitude, cyclonic circulation prevails throughout the troposphere with most intense at high altitudes, thickness

Thickness

lowest at the low center, produces the classic isobaric pattern

Non-occluded cold-core cyclone

lowest temp NW of cyclone center and highest to the SE, low center aloft displaced to cold side of the storm and upper level low lags behind surface low and system tilts with altitude

Warm-core cyclone

stationary, no fronts, associated with fair weather, forms over a broad expanse of arid/semi arid land with response to intense solar heating of the ground and hot surface heats the overlying air and lowers the density of the air column enough for a low to form, usually very shallow and weakens rapidly with altitude, anticyclone aloft overlies thermal low at middle and upper troposphere

Anticyclones

subsiding air and diverging surface winds favor formation of a uniform air mass, no fronts, and generally fair skies

Cold-core anticyclone

dome of cP/A air labeled polar/arctic high, formed from extreme radiational cooling and often over snow-covered land and clockwise circulation weakens with altitude and frequently reverses direction aloft, usually a cold trough overlying it, exerts the highest pressure in winter, extremely stable with inversion in lower km and strong subsidence and adiabatic heating above inversion, interact with circulation of extratropical cyclone and help maintain and strengthen temperature contrast along cyclone’s cold front with clearing skies and sharply lower temperature usually follow winter stroms

Example of cold-core anticyclone

siberian high

Warm-core anticyclone

forms S of the polar front and consists of extensive areas of subsiding warm and dry air, strengthens with altitude, greater mass of air over the anticyclone center (related to a higher tropopause) responsible for the high surface pressure, cold-core anticyclone can become warm-core as it modifies

Example of warm-core anticyclone

bermuda-azores high

Anticyclone weather

surface winds blowing in clockwise and outward direction (N hemisphere) with inducing subsidence over a broad area, arctic highs produce the lowest temperatures of winter, stalled warm anticyclone can lead to drought and excessive summer heat, weak horizontal air pressure gradient near the center and leads to intense nighttime radiational cooling, ahead of an anticyclone and may be strong NW winds with polar/arctic air may being heavy lake-effect snows to lee side of the lakes in summer with the most noticeable effect a lowering humidity

Sea/lake breeze

under exposure to the same intensity of solar radiation with land surface warms more than water surface, higher pressure over water and cool freeze sweeps inland, shallow circulation has maximum strength in mid-afternoon, uplift may lead to thunderstorms

Land breeze

by late evening with winds blow offshore due to a reversal in the heat differential between land and water, obtains maximum strength around sunrise but is weaker than a sea breeze

Valley breeze

bare valley walls absorb solar radiation and heat the surrounding air, cooler and denser air over the valley sinks and air adjacent to the valley walls blows upslope, cumulus clouds may form near summit, develops between late morning and sunset

Mountain breeze

bare valley walls chilled by radiational cooling and cools the surrounding air, cold and dense air near the valley walls sinks and gusty breeze blows downslope, fog/low stratus clouds may form in the valley

Chinook winds

warm and dry downsloping wind with usually being W/SW and develops when air descending the leeward slopes of a mountain range is adiabatically compressed, strong winds cause stable air in the lower troposphere to ascend the windward slopes, leeward side and stability causes it to descend to original altitude and larger scale circulation causes further descent, distinct mountain-wave clouds can form, onset of air chinook with air temperatures can tense of degrees due to compressional warming, winds may reach destructive speeds especially along foothills of the front range of the rocky mountains

Example of chinook winds

boulder CO

Santa ana

hot and dry wind blowing from the NE to E and impacts S CA from october to march, winds originates in great basin and high mojave desert and air is cool and dry, gravity initiates downslope flow and helped by a strong pressure gradient and adiabatic compression produces hot and dry winds

Katabatic wind

shallow layer of cold and dense air flowing downhill by gravity, originates in winter over extensive snow-covered plateau/highland, weak and speeds less than 10 km/hr, mistral and bora

Mistral

descends from alps down the rhone river valley of france into the gulf of lyons along the mediterranean

Bora

originates in high plateau region of croatia and cascades onto coastal plain along the adriatic sea

Desert winds

absorbed radiation goes into sensible heating with no water with vegetation/evapotranspiration, hot surfaces generate superadiabatic lapse rates in the lowest levels of the atmosphere with great instability with vigorous convection and gusty surface winds and few clouds

Dust devil

whirling mass of dust-laden air formed by localized hot spot, air is heated and rises rapidly and cooler surface winds converge on the hot spot, horizontal wind shear causes the column of rising hot air to spin about vertical axis, dust and debris collected and made visible to altitudes topping 3000 ft, may have winds as higher than 45 mph, strong electrical fields can develop and airds in lifting of dust off ground

Haboob

dust storms generated by strong thunderstorm downdrafts with rain from thunderstorm evaporates by cooling counteracts warming by compression, thunderstorm downdraft exists cloud base, hits ground as cool gusty air, lifts dust off ground creating a huge ominous black cloud

Heat burst

convective rain showers fall from clouds with a high bast and relatively dry air below, more common over great plains where cloud bases tend to be higher

Temperature in heat burst

rise from the 70s and 80s to 100 to 105 degrees fahreinheit

Conclusion

air masses and fronts and cyclones and anticyclones interact and linked to the planetary-scale circulation, synoptic-scale weather systems play an important role in poleward heat transport, planetary and synoptic scale patterns set boundary conditions for any smaller scale circulation system with synoptic scale winds may reinforce mesoscale winds and may overwhelm mesoscale winds with regional winds in same direction as sea breeze and northerly arctic winds sweep along E rockies eliminating possibility of chinook winds