Coriolis Effect (see book chapter 2)
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However, there is a wrong, or at least misleading, statement: The narrator says several times that points near the equator spin faster than points near the poles, and that the 'spinning' momentum (angular momentum??) is preserved. This is INCORRECT! Every point on the solid Earth spins with the same spin rate (revolution speed) around Earth's axis, no matter where on Earth this point is. But the distance travelled is greater for points near the equator, hence the VELOCITY (= distance/time) is greater near the equator than near the poles. This excess velocity is what makes objects deflect when the move between lines of latitudes.
Adiabatic Changes
adiabatic cooling: air moves from high air pressure to low air pressure
- example in the air column: air moves from high surface pressure to low pressure aloft
- when air rises, it expands and cools at 6-10oC/km
- cooler air can hold less water vapor so the relative humidity increases
- condensation takes place when the rel. humidity reaches 100%; latent heat is released during condensation, so this is no longer an adiabatic process
- rising air causes a low surface pressure relative to the surroundings
- examples: rising air at the equator, rising air in a developing storm system
- example in the air column: air moves from low pressure aloft to high surface pressure
- when air sinks, it contracts and warms
- warm air can hold more water vapor so the relative humidity decreases
- evaporation leads to the take-up of more water vapor and latent heat; this is then no longer an adiabatic process
- sinking air causes a high surface pressure relative to the surroundings
- examples: sinking air over subtropical latitudes, sinking air in stable air masses
Low- and High Pressure Systems, Cyclonic and Anti-Cyclonic Air Flow
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- Earth's spin deflects moving objects -> Coriolis forces make the movement to become a spiral movement
- use right-hand rule to determine wind direction in high-/low-pressure system: make a fist with thumb sticking out; thumb: direction of air flow; fingers then point in direction of winds
- low-pressure system (cyclones): air rises at center; winds blow inward and counterclockwise in northern hemisphere (clockwise in southern hemisphere)
- high-pressure system (anti-cyclones): air sinks at center; winds blow outward and clockwise (counterclockwise in southern hemisphere)
Example for a High Pressure System affecting local climate: Santa Ana Weather
- caused by high pressure system in the Southwest (e.g. Nevada)
- typically in fall and early winter
- lasts a few days
- winds blow clockwise, so winds in San Diego come from northeasterly directions
- in mountains, extremely strong winds (30km/h or 20mph, with gusts up to 80 km/h or 50 mph)
- very dry, with relative humidity below 20%, sometimes reaching into the single digits
- effect in San Diego county enhanced by orographic lifting and additional loss of moisture as air has to pass Laguna Mountains
- people may get dehydrated; sweat evaporates quickly and there is no feeling of sweating
- EXTREME FIRE RISK! Low humidity and high temperatures dry out potential fuel (e.g. dead wood) and gusty winds can whip fires out of control
- also see special Santa Ana page
Example for changes of Low Pressure Systems and Global Climate Change: El Niño and La Niña
- (Spanish for "Christ Child") fishermen in Peru named the El Niño phenomenon after a time near Christmas when they sometimes catch less fish than normal.
- the main modern, satellite-based observation is that during an El Niño, equatorial waters in the East Pacific ocean are up to 5°C warmer than normal, whereas during a La Niña, surface waters are colder than normal
- for more details on observations see lecture on climate change (Lecture 22)
- the exact cause not entirely understood but phenomenon is linked to weakening trade winds and shifts in the low pressure system in the western Pacific Ocean
- Normally, a major equatorial low-pressure cell in the western Pacific and high-pressure over East Pacific create a large east-west convection cell, causing deep water to ascend in the east Pacific bringing up nutrients.
- During El Niño conditions, the low-pressure area moves eastward, causing high rainfall, and a high-pressure cell to develop in the western Pacific. West-east convection cell inhibits nutrient-rich water to ascend from depth in the east Pacific.
- El Niño oscillations also cover parts in the Southern Pacific, affecting weather patterns in Australia in a similar way as Indonesia; hence the term ENSO is used for El Niño southern oscillation.
- ENSO events have a periodicity of 4-7 years but the strength of events varies. A possible cause is the interaction with other oscillations (e.g. the Pacific Decadal Oscillation with a periodicity of 20-30 years.)
- La Niña events are between El Niño events and have the opposite effect compared to El Niño events (the textbook phrases this a little differently).
Observations:
Mechanisms:
Global Air Circulation and Air Pressure At the Surface
- equatorial areas receive more insolation than higher latitudes
- excess heat transported toward poles
- air rises (adiabatically) at the equator (leaving a low pressure system at the surface); adiabatically rising air cools; can hold less moisture, water vapor condenses and eventually rains down -> rain forest
- air sinks (adiabatically) near the poles (creating a high pressure system at the surface); adiabatically sinking air warms; can hold more moisture -> extremely dry climate (Antarctica is driest place on Earth)
- Earth's spin deflects moving air (see Lecture 15). This leads to additional low- and high-pressure belts at the surface:
- high pressure at 30o latitude: air sinks and heats adiabatically; captures moisture, therefore it rains more rarely -> deserts
- low pressure near 60o adiabatically rising air cools; can hold less moisture, water vapor condenses and eventually rains down
Global Air Circulation and the Coriolis Effect
- An idealized non-rotation Earth (i.e. if it would not revolve once a day) has a large convection cell going from the equator to the poles.
- Earth's spin deflects moving air (Coriolis effect). Earth's spin adds west-east component to motion. When air travels to poles or equator, it now experiences additional forces than make air sink/rise earlier along way to the pole/equator.
Coriolis forces break convection cell into three sub-cells
- polar: from +/-60° to poles
- Ferrel: from +/-30° to +/-60°
- Hadley: from equator to +/- 30°
Prevailing Winds
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NB: The trade winds received their name because historical trading ships used these winds to get from Europe to the Americas. The horse latitudes got their name because early trading ships got stuck in windless belts and crew disposed of horses for easier journey. |
Intertropical Convergence Zone
The ITCZ and Monsoons: An example of large-scale, seasonal changes
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major reversal in wind direction that causes a shift from very dry to very wet season (southern Asia);
- winter: stable high pressure cell over central Asia due to cold, dry air -> ITCZ gets pushed out into Indian Ocean; no rain in India
- summer: central Asia heats up dramatically creating pronounced low-pressure area -> ITCZ moves north allowing moisture rich marine air to approach and bring rain the India.
- a similar process occurs in the sub-Sahara Sahel Zone
The Jet Streams
- high-altitude winds near the top of the troposphere
- basic mechanism starts with air rising at equator, raising the troposphere relative to the poles (the latter having greater air pressure)
- air then flows north; become high-altitude Westerlies because of Coriolis effect (deflected to the east but coming from the west)
- wind speeds particularly high where pressure gradients are steep; strong winds at 200-400km/h; these are the jet streams
- 2 special places where pressure gradient is particularly steep: over the polar front (convergence zone between Polar and Ferrel cells) and over the horse latitudes (divergence zone between Ferrel and Hadley cells)
- Jet streams control weather. Their path depends on the season and it tends to be farther south in the winter.
- jet streams undulate around high- and low-pressure systems, so their path depends on the large-scale arrangement of air masses
San Diego's Marine Layer and June Gloom
- usually occurring during spring, when land heats up faster than ocean
- condition especially strong along cold currents that are found near the eastern rims of oceans (e.g. Cold California Current, Humboldt Current off Peru, Canary Current off Northwestern Africa, Benguela Current off Namibia)
- air above ocean is cooled above ocean surface by cold ocean current
- water vapor condenses -> fog
- in second half of day, air heated above land pushes out the sea, above cold air over ocean
- cold air and fog can't ascend because it is cooler than layer above -> inversion layer
- onshore flows during night time brings fog toward coast
- fog often does not burn off until afternoon, when inversion layer is broken down by strong sun
- in areas with high pollution, smog can develop
- the condition is more likely to occur during La Niño years when sea surface temperatures are lower check out SIO's page on the June Gloom
The Pineapple Express and the Jet Stream - A warm, soggy Storm
In late fall and winter, the U.S. West Coast sometimes receives particularly wet storms that bring relatively warm rain. Pushed by the subtropical jet stream (the Pineapple Express) warm, moist air from around the Hawaiian Islands pushes east toward the West Coast (Hawaii is the land of pineapples, which is how the Pineapple Express got its name). This moisture can be drawn into storm systems that approach the West Coast (typically from the north). The wikipedia website cites a southern branch/loop of the unusually far south-reaching polar jet stream as the Pineapple Express but sometimes the jet stream can actually split in two branches. The band of moist air driven by the jet stream is also called atmospheric river.
Remarkable recent Pineapple Express Storms:- 2014 (Dec): Northern California. a powerful storm brought snow, wind and flood watches. A blizzard watch was in effect for the northern Sierra Nevada for the first time since 2008. Power outage to more than 50,000 people. On 12 Dec, a rare tornado touched down in Los Angeles.
- 2010 (15-22 Dec): Southern California. a storm brought 61 cm (2 ft) of rain the San Gabriel Mountains and over 4 m (13 ft) of snow to the Sierra Nevada. Costal and hillside areas in Southern California experienced mudsldies and major flooding.
- 2006 (first week of Nov): Pacific Northwest (Oregon/Washington). A "worst-in-a-decade" storm brought record rainfall, flooding rivers, washing out roads and killing 4. The flooding forced the closing of Mt. Rainier National Park for the first time since 1980, when it was closed after the eruption of Mt. St. Helens.
- 2006 (Oct): Alaska. An unusually intense rainstorm hit south-central Alaska
- 2005 (7-11 Jan): Southern California. The Pineapple Express storm was the biggest to hit the area since the 1998 El Niño winter. The storm caused mud slides and flooding in the normally dry Morongo Valley. It also triggered a catastrophic landslide in La Conchita, killing 10 people.
- El Niño winter of 1998: first week of February. Wide-spread flooding and landslides in Southern California; mudslides in Tijuana. Record runoffs from Mt. Shasta in Northern California to Fresno in the south. Levees overtopped or stressed to the maximum by flows higher than their design capacity. El Nino arrived just two months after the Army Corps completed the levee reconstruction effort.
- 1997 (26 Dec 1996 - 3 Jan 1997): caused hundreds of landslides in Northern California and produced catastrophic flooding throughout Northern and Central California. Over 100,000 people had to flee their homes, mud slides closed road, property damages amounted to more than $1.5 billion (including crop losses) and 8 people lost their lives. Yosemite National Park sustained over $170 Mio in damages due to flooding and was forced to close for more than 2 months. (1)
- 1952 (second week of Jan): A series of Pineapple Express events swept into Northern California, causing wide-spread flooding in the Bay Area.
- 1862 (8 Nov 1861 - Jan 1862): U.S. West Coast. Worst flooding in recorded history of California, Oregon and Nevada. Preceded by weeks of continuous rains (and snows at higher elevations). Record amounts of rain 9-12 Jan that caused flooding from the Columbia River down to San Diego.
Sometimes, the Pineapple Express feds a storm fueled by an Arctic low, which can lead to catastrophic snow-melt flooding. When warm, tropical rain falls on frozen ground covered by snow.
- 1964 (18 Dec - 7 Jan): Christmas flood - Pacific Northwest. 19 fatalities. Preceded by atypical cold spell and heavy snow. The Pineapple Express led to drastic temperature increases of 17-22°C (30-40°F). Considered a 100-year flood. Worst flood in recorded history for coastal Northern California rivers. The Villamette River in Oregon was also affected. The flood heavily damaged or completely devastated more than 10 towns.
- 1996 (late Jan - mid Feb): Willamette Valley Flood - Pacific Northwest. A series of floods caused by storms in January and February. Oregon's largest floods in terms of fatalities (8) and monetary damage in the 1990ies. Winter had produced abnormally high rainfall, followed in late January by heavy snow and a 6-10-day deep freeze. The new snow melted during the subsequent Pineapple Express.
Weather Links
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Government Sites:
- National Weather Service
- National Weather Service (Climate Prediction Center)
- National Weather Service (Storm Prediction Center)
- San Diego weather at NWS
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Commercial Sites:
- AccuWeather
- The Weather Channel