Lecture 3: Weather/Disturbance Transient and Eddy Transient: deviations from time mean
Transients and Eddies Time Mean Climate Roles Mid-Latitude Cyclones Eddy: deviations from zonal mean Tropical Hurricanes Zonal Mean Mid-Ocean Eddies
Why transients/eddies matter to zonal and time means? (From Weather & Climate)
ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
Flux Components Transient/Eddy Flux temperature motion
(1)(2) (3)
Three components contribute to the zonal- and time-mean transport:
(1) Mean Meridional Circulation (such as the three-cell circulation)
(2) Stationary planetary Waves (such as the wavenumber 1-3 eddies in the Northern Hemisphere).
(3) Transient Eddies (such as the weather systems = midlatitude cyclones [V*T*] > 0 Î On zonal average, eddies transport heat northward. and anticyclones). Î Eddies contribute to zonal-mean heat transport ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
1 Climate Roles of Eddies Poleward Flux of Temperature
Transient eddy fluxes Stationary and transient eddies are important to dominant the meridional flux the poleward fluxes of temperature, moisture, of temperature except in the energy, and angular momentum. Northern Hemisphere during winter, when stationary eddies contribute up to half of the flux.
The low-level maximum in the troposphere is associated with the structure of growing mid-latitude cyclones (I.e., weather systems). (From Global Physical Climatology) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
Poleward Flux of Moisture Poleward Flux of Momentum
Both the mean meridional In the tropical easterlies, eastward circulation and eddies transport angular momentum is transferred water and play an important from Earth to the atmosphere via role in determining the nature frictional forces and mountain of the hydrological cycle. torque. Moisture convergence in the This westerly angular momentum tropics is dominated by the is transported upward and then transport provided by the mean polarward into the Hadley Cell. meridional circulation. Eddies then transport angular momentum polarward and The subtropcs serves as source downward into mid-latitude regions for water vapor. westerlies. Eddies remove water from the In the mid-latitude, the westerly (From Global Physical Climatology) tropics and supply it to middle momentum is returned to the Earth. and high latitudes. ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
2 Stationary Planetary Waves Transient Eddies
Mid-latitude cyclone and anticyclone are the January Height of 500-mb Pressure Surface Stationary: These waves do not move around much and are fixed in certain major transient eddies that play an important geographic locations. role in meridional transports of heat, momentum, and moisture. Planetary: These waves have large These mid-latitude weather systems grow wavelengths, one the order of several from the baroclinic instability associated thousands of kilometers. with the strong north-south temperature gradients in mid-latitudes. Wave: Their structures vary in the zonal direction. Mid-latitude cyclones have typical spatial scales of wavenumbers 5-6 and have typical Stationary planetary waves are forced by time scale of 7-10 days. large-scale mountains (such as Himalaya Mid-latitude cyclones are marked by well- (From Weather & Climate) and Rocky mountain ranges) and heat defined fronts separating the warm air mass contrasts between continents and oceans. from the south and the cold air mass from the north. (Very different from tropical Stationary planetary waves are stronger hurricanes, which do not have frontal in winter than in summer. features).
(From Global Physical Climatology) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
Air Masses Fronts
North American air masses and • The wettest air is called maritime air, Fronts separate air masses air mass source regions while the driest is called continental. and bring about changes in • Air deemed, from warmest to temperature and humidity coolest, tropical, polar, or arctic as one air mass is replaced • Once formed, air masses migrate by another. within the general circulation. There are four general • Upon movement, air masses types of fronts associated displace residual air over locations with mid-latitude cyclones thus changing temperature and with the name reflective of humidity characteristics. the advancing air mass. • Further, the air masses themselves moderate from surface influences.
(from Weather & Climate) ESS200A (from Weather & Climate) ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
3 Cold and Warm Fronts Cold Fronts
Cold fronts form when cold air displaces warm air. Mid-Latitude Cyclone Indicative of heavy precipitation events, rainfall or snow, combined with rapid temperature drops. t n ro Extreme precipitation stems from rapid vertical lifting associated with f ld the steep cold front boundary profile. co Because cold air is dense, it spills across the surface producing a steeply inclined leading edge. wa rm Warm moist air ahead of the front is forced aloft with great vertical fro nt displacement. This accounts for large vertical cumulonimbus clouds and heavy precipitation. Such sharp transitions between the colder, drier air behind the front (From Weather & Climate) and the warmer, moisture air ahead of the front, can be easily detected on satellite images and radar composites.
ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
Cold Fronts Warm Fronts
(from Weather & Climate) (from Weather & Climate) Created when warm air displaces colder air. Above: The vertical displacement of air along Even though the warmer air advances, it is displaced aloft. a cold front boundary This overrunning process places large amounts of warm, moist air over cooler, drier air along extensive spatial areas. Right: The sharp cold front boundary is evident Shallow horizontal stratus clouds dominate and bring light precipitation on both satellite pictures and radar composites to affected regions Frontal fogs may occur as falling raindrops evaporate in the colder air
ESS200A near the surface. ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
4 Occluded Fronts Stationary Fronts When two fronts meet, the warm air mass between them is displaced aloft resulting in When two unlike air masses remain side by side, with an occluded front. neither encroaching upon the other, a stationary front This typically occurs when a exists. cold front closes on, and meets a warm front as it circulates Fronts may slowly migrate and warmer air is displaced about the low pressure center above colder. of a mid-latitude cyclone. Fronts are zones of transition rather than sharp Cold air now occupies the boundaries. surface completely around the low while warmer air is displaced aloft.
ESS200A ESS200A (from Weather & Climate) Prof. Jin-Yi Yu Prof. Jin-Yi Yu
Life Cycle of Mid-Latitude Cyclone Polar Front Theory
Bjerknes, the founder of the Bergen school of meteorology, developed polar front theory to describe interactions between unlike air masses and related aspects of the mid- Cyclogenesis latitude cyclone. Mature Cyclone Occlusion
ESS200A (from Weather & Climate) ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
5 Steering of Mid-latitude Cyclones Rossby Wave (from Weather & Climate)
The movement of surface systems can be predicted by the 500 mb pattern. (from Weather & Climate) The surface systems move in about the same direction as the 500 mb flow, at about 1/2 the speed. ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
Tropical Hurricane
The hurricane is characterized by a strong thermally direct circulation with the rising of warm air near the center of the storm and the sinking of cooler air outside. The warm core of the hurricane serves as a reservoir of potential energy, which is continuously being converted into kinetic (from Weather & Climate) energy by the thermally direct circulation.
ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
6 Hurricane Characteristics Conditions Necessary for Hurricane Formation
Hurricanes form only over deep water layers with surface • Hurricanes, the most powerful of all storms, have sustained winds of o 120 km/hr (74 mph). temperatures in excess of 27 C. • Although of lesser intensity than tornadoes, the much larger size and Energy is derived from latent heat release and associated longer life span makes hurricanes much more devastating. evaporation of water • Average diameters are approximately 600 km (350 mi) and central Poleward of about 20o, water temperatures are usually below this pressure averages about 950 mb but may be as low as 870 mb. threshold • Most energy attained by hurricanes stems from latent heat release in Hurricanes are most frequent in late summer and early autumn the cloud formation process. during high SST times • Hurricanes occur where warm waters abound and during the times of highest SSTs. Coriolis force is an important contributor, and as such, hurricanes do not form equatorward of 5o. • For the N.H., August and September are the most active months.
ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu
ESS200A Prof. Jin-Yi Yu
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