Week of 9/26 Notes
Week of 9/26 Notes GEOG 1112
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This 9 page Class Notes was uploaded by Kaley Notetaker on Monday October 3, 2016. The Class Notes belongs to GEOG 1112 at University of Georgia taught by Dr. Elgene Box in Fall 2016. Since its upload, it has received 4 views. For similar materials see Intro to Weather and Climate in Geography at University of Georgia.
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Geography 9/26 Global Atmospheric Circulation (starting on p.56 of notebook) Driver: surface heating -> rising air -> lower surface pressure o Tropical convection: Hadley Cells Components of Global Circulation - Equatorial low pressure: the intertropical convergence zone (ITC) - Trade winds and intertropical convergence - Subtropical high pressure: Bermuda high - Westerly winds – and mid-latitude weather - Subpolar lows, polar highs, and the polar front Seasonal shift of wind/pressure belts: precipitation belts The Bermuda High Hot-dry west sides Upper-atmosphere circulation: Ridges: upward bulges Troughs: valley of low heights Jet streams Ocean circulation and currents - surface currents and gyres - warm and cold currents - upwelling - el Niño and southern oscillation - thermohaline circulation Hadley Cell – the driver of global circulation (driven itself by solar input) Idealized Global circulation: - ITC - Subtropical Highs - Trade winds - Westerlies ^ 4 main components! p.57* Circulation and Climate Types What factors control the local climate at the solstices? Precipitation (climate) zones – resulting from seasonal shift Dry Regions of the world – mainly at STHPs (plus continentality and rain shadows) - Deserts are on west side of mountains Precipitation ITC brings rain north of the equator (south is dry) [in July] ITC brings rain south of equator (north is dry) [in January] Westerlies - blowing W to E (even in troposphere – they are blowing faster there bc no friction) Rossby Waves are produces, they flow around the northern polar region These waves turn out to be our jet stream Idealized drivers of global circulation - 2 jet streams, at cell boundaries - tropopause lower toward pole - lift along polar front (induced by Ferrel cell) Two Jet Steams: - polar (mid-latitude) - subtropical o affects us in the south in el Niño years because the jet will pick up warm water and brings it to us through rain, making us have wetter winters Typical jet-stream positions in summer Winter: cold air mass expands around N pole Jet streams - shift southward - become faster (greater circumference) - faster in winter than in summer - in winter, polar jet stream acts as a boundary between cold and less cold air in winter - *knowing where the jet stream is tells you a lot about the weather - jet streams undulate. o Undulations cause N-S penetration of warmer and colder air The jet stream begins to undulate Rossby waves begin to form Waves are strongly developed. The cold air occupies troughs of low pressure When the waves are pinched off, they form cyclones of cold air There is a relationship between highs and lows at surface and position of jet stream above (“coupling effect”) - Bottleneck jet stream creates higher pressure and forces higher pressure down and high pressure in the surface (jet stream converges) - When jet stream diverges and there is lower pressure, low pressure is pushed down and causes low surface pressure - Oceanic currents follow atmospheric circulation Note the GYRE in each major ocean basin - Cold currents off west sides - Warm currents off east sides Cold ocean currents have greater affect on climate *Thermohaline Circulation p.220 - 230 Hadley Cell - Strong solar heating around the equator creates a zone of low pressure (since air is expanding upwards and diverging towards poles); this zone of low pressure at the equator is called the equatorial low or intertropical convergence zone (ITCZ) - At approximately 20-30 latitude, air in the Hadley cell sinks towards the surface to form subtropical highs – large bands of high surface pressure o This is why desert conditions are common in the subtropics; air warms adiabatically and cloud formation is suppressed o These areas typically have weak pressure gradients and minimal winds Leads to the term “horse latitudes” because legend has it that when travelers were stuck in this no-wind zone, they would throw the horse and cattle overboard into the sea - Formed by subtropical highs, equatorial lows, trade winds and upper-level westerly motions - Strongest in winter season when temperature gradients are strongest (bc produced thermally) - Accounts for movement and distribution of air over approx. half of the earth’s surface - Arises from differences in heating Ferrel Cell - Circulates air between subtropical highs and subpolar lows (areas of low pressure) - Westerlies are created by the air flowing poleward away from N. Hemisphere, where subtropical high undergo a substantial deflection to the right o Happens in S. Hemisphere as well; coriolis force deflects the air to the left, thus producing a zone of westerlies - Caused by the turning of the two adjacent cells Polar Cells - Surface air moves from polar highs to subpolar lows - Considered thermally direct circulations (like the Hadley Cells) - In both hemispheres, coriolis force turns the air to form a zone of polar easterlies in the atmosphere Pressure decreases more rapidly with altitude where the air is cold Temperature in the lower troposphere generally decreases from the subtropics to the polar regions Polar Front - Boundary between warm and cold air - Outside of the frontal zone, changes in temp. with latitude are gradual - Within the front, pressure surfaces slopes increase greatly because of abrupt horizontal change in temperature o This causes strong pressure gradient force, resulting in polar jet stream Jet stream is consequence of polar front (arising because of strong temperature gradient) Jet stream reinforces polar front Jet streams are highly turbulent & speeds vary considerably Difficult to pinpoint locations of jet streams Polar jet stream affects daily weather in mid-lattitudes Subtropical jet stream is another jet stream, near the equator, associated with the Hadley Cell 9/28 Air Masses and Fronts Air Masses: mP maritime polar cool and wet N. Pacific, N. Atlantic cP continental polar cool and dry Gulf of Mexico mT maritime tropical warm and wet central Canada (winter) cT continental tropical hot and dry SW deserts (summer) Air-mass modification: warmer surface -> instability Colder surface -> stability Ex: gulf and pacific effects, monsoonal air, lake-effect snow, etc. Fronts (front is a boundary between 2 air masses that have different properties; usually a warm & cold one) - Warm front: forced convection -> temperature inversion Stratus clouds (overcast) and gentle rain - Cold front: unstable atmosphere created Tall cumulo-nimbus clouds -> thunderstorms Mid-Latitude Cyclones: Occluded Fronts and Drylines p.62 in workbook! Air Mass: Vertically thick, horizontally large portion of the atmosphere with uniform properties of temperature and moisture content throughout - Characterized by 2 properties: o 1. General temp level Polar (cold) or Tropical (Warm) o 2. Relative moisture content Maritime (wet) or continental (dry) o This leads to FOUR main air-mass types: mP, mT, cP, cT - Generated by large source regions; source regions require 2 things: o 1. Fairly uniform surface (large plains, the ocean) o relatively little surface air movement (no wind!) Table 9-1 Air Mass Examples cP Monsoonal high pressure/polar outbreak “Siberian express” – cP air mass straight from eastern Siberia; moves quickly “Lake effect” snow over central Canada possibility of a “polar outbreak” southward winter-monsoon air is also cP air mP Oceanic air on windward west sides (e.g. Pacific Northwest, NW Europe) mP air from the Pacific Ocean dominates the northern US in winter onshore flow of mP air in winter makes NW Europe less cold mT oceanic air brought by monsoonal low “spring monsoon” in South and eastern US mT air comes from Gulf of Mexico (so gulf of mexico is a source region) dominates much of central and eastern US in spring and summer rainfall drops off with greater distance from the Gulf cT Hot desert air Hottest areas in Summer: cT air masses Air-Mass Modification As air masses move, they may be modified by moving over surfaces with different thermal properties: 1. Movement over a cooler surface (usually mT or cT air masses) results in cooling the air mass from the bottom a. This forms temp inversion (usually shallow) near surface and makes the air STABLE 2. Movement over a WARMER surface (mP or cP air masses) results in warming the air mass from the bottom, increases ELR a. Makes air UNSTABLE p.63 “Lake Effect” snow – modification of a cP air mass by a lake affects: wind and cP air come from NW Snow belts occur on the SE sides of the lakes Sea of Japan – another case of cP air modified by a water body cP air from cold-monsoonal Asia crosses the sea of japan and gets modified the now wet air hits the mountains of Japan and produces some of the world’s highest annual snowfall totals Fronts - Boundary between two air masses, one warmer and one colder o Requirements: o 1. Warmer and colder air masses o 2. Some motion that brings them into contact - If an air mass is moving, then the front is the leading edge of the “new” (moving) air mass Warm fronts: Warm air moving -> case of forced convection (warm air is lifted by the cold air wedge) -> temperature inversion (meaning it is a stable environment) Creates stratus clouds (overcast) and gentle rain Warm air is moving – overriding the cold air, which takes on the “Wedge” shape As the warm air is lifted, it cools adiabatically and nimbo-stratus clouds form, then allostratus, then cirrostratus, then cirrus o Halo around a moon indicated cirrus clouds, which means rain is probably coming in a day! Development of cold air wedge o Cold air is pushed further above the surface, where there is less friction Move slowly horizontally and produce slow, patchy rain Cold Front: Cold air mass is moving -> atmosphere is unstable (strong updraft) -> tall cumulo-nimbus clouds Thunderstorms, followed by rapid clearing and cold Cold air rises rapidly as instability is created; also rises because cold air is pushing it up combination of instability, free convection and forced convection) Cumulonimbus clouds form Faster horizontal movement Short, intense precipitation Cold Air after Cold front passes: - Colder and drier - Rapidly clearing skies - Windy - Cold air continues to pour in Other important fronts: Polar Front (arctic and Antarctic) - Between westerlies and erratic but often easterly winds in the polar regions – o Result: mid-latitude cyclones (cyclogenesis) Monsoonal fronts: - Between wet oceanic and drier continental air 9/30 MID-Latitude Cyclones *most common weather producer - Origins: o Dynamics around a low-pressure center o Jet streams and lows o Polar front and cylogenesis - Life cycle: o Fronts created (N-S Temperature gradient) o Mature cyclone o Occlusion - Weather with mid-latitude cyclones o Wind directions o Weather along fronts o Weather near center of low o Occlusion and associated weather - Movement of mid-latitude cyclones o Vorticity, upper-air waves and interaction with surface conditions o Flow patterns: zonal vs. meridional o Conveyor-belt model o Typical storm tracks: migration with upper air waves Flow around a low creates a warmer side and a colder side with - Advancing cold air (cold front) - Advancing warm air (warm front) Vorticity – tendency for rotation Positive vorticity = counter-clockwise Negative = clockwise Sheering – winds that go in the opposite direction close to each other - Generate turbulence Centrifugal life cycle - Waves develop along the front, air is drawn into the low - After a day or so, significant front develop (mid lat cyclone) - Maturing stage: fronts well developed, occlusion (means to close off) forming o Occlusion closes off warm air by lifting the warm air that is stuck between two cold air masses, warm air is pushed up - Life cycle: takes about a week, as the sythem is moving west-to- east across the US - P.66 Clouds and weather with a mid-latitude cyclone - Uplift in 3 places (forced convection) o Overrunning/overriding – when warm air is forced to ride up over cold air wedge -> stratus clouds and possible precipitation occur o Uplift by convergence at center of low and at outside of cyclone forming between cold and warm front - Free convection where air is unstable – warm air that is trapped creates large ELR and very unstable air that rises easily Cyclone examples: Springtime (cold air moves from north) Autumn (cold air moves from west) Cyclone sectors: Cold air ahead of warm front Warm sector between fronts Cold air behind cold front Occlusion: Cold air advances faster than warm air (because warm obstacles offer less resistance) May also “slide” along the warm front Lifts the warm air between the fronts - System now advances more slowly (new, more resistant obstacle: the cool air) - Drops more rain in one place Occlusion sequences generally happens while the system is crossing the US Cyclone Sequence Example (over 4 days in April): - Cyclone already mature (Day 1) - Occlusion forming over Canada, cold front through GA, notable clear skies behind the cold front (Day 2) - Cold air is blasting into the South, notable lower temperatures in South (Day 3) - Cyclone gone, next one coming (Day 4) Vorticity with a jet-stream wave: ability to induce rotation - Counterclockwise rotation through trough o Relative to surface - Clockwise rotation through ridge Cyclone and Anti-Cyclone with the Het Steam -surface LOW associated with divergence aloft -surface HIGH associated with convergence aloft - alternating LOW – HIGH – LOW – HIGH across US Storm Tracks: movement of the surface low (cyclone) and its fronts with the jet stream Complete Anatomy of a Mid-Latitude Cyclone: p.67 - What changes along the transects? - At each letter, what: o Sky conditions o Kind of clouds o Wind direction o Relative temperature (warmer or cooler) o Precipitation? A. cirrus clouds, blue sky, high stratus clouds, halo around the moon at night; cooler B. (ahead of warm front), stratus clouds because adiabatic cooling, probably raining a little bit; can’t say wind direction; (temp inversion?) C. sunny, some clouds but mostly clear; atmosphere might be unstable here so the clouds that form are probably cumulus clouds, not a lot of precipitation produced; wind is coming from the SouthWest D. cumulus clouds, thunderstorms, can’t say temperature because of precipitation, (this is the place in the system with the best potential to lift a lot of air quickly – cold air pushing up and instability that causes warm air to rise anyways); lots of thunderstorms here E. clear sky and sunny, cold and dry air, windy – wind is coming from North West, (windiest part of the system), no precipitation, cold air mass, (area between F AND G is the “LOW” - F. stratus clouds, lots of water available for precipitation, overcast, 10/3 1. Which side of a HIGH pressure cell is warmer? (East or West?) WEST because the air flows clockwise, wind is coming up from the South 2. Which side of a LOW pressure cell is warmer? East or West? EAST 3. Which sides of a LOW pressure cell normally receives more precipitation – east or west side? NORTH EAST 4. What kind of sky conditions can be expected… a. Near the center of a HIGH pressure cell? Clear b. Near center of LOW pressure cell? Cloudy/stratus clouds – stable atmosphere, not much surface heating 5. For each of the following locations relative to a mid-latitude cyclone, what kind of cloud and how much precipitation? Cloud Precipitation Center of low Stratus/overcast Significant amount Along/ahead of Stratus Varies, patchy, off & on warm front Along/ahead of cold Cumulus; Thunderstorms (short) front cumulonimbus Under an occlusion Stratus A lot of rain (long) 6. Which air mass types are involved with a WARM front? mT (maritime Tropical) air masses 7. What type of air masses are involved with a COLD front? CP cold dry air mass 8. IN a mid-latitude cyclone where is precipitation normally… a. The greatest (amount) Occlusion b. The most intense Cold front, thunderstorms c. The least Behind the cold front 9. Where is the wind normally strongest and from what direction? Behind the cold front; North West (also, in front of warm front) 10. How do the following change as a front approaches & passes a place? BEFORE AFTER BEFORE AFTER Warm Warm Cold Cold Sky Raining Clear Rapidly clear condition forming cumulus clouds; rainy wind NE S S NW direction Temperatu Cooler Warmer Warm Cooler re trend Precipitati Copious Stopped Short, quick Rapid on trend precipitation thunderstor clearing m