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This Week (5) Atmospheric Motions Reading chapter 4 of your text Study for Midterm on Friday in Discussion •Addressing our assumptions •Hadley Circulation, Coriolis Effect, Geostrophic Flow, •Deserts, Tropical Rainy/Dry seasons, Jet Streams •Surface Winds, Storms, Land/Ocean Contrasts •Cyclones and anticyclones, sea breeze, monsoons •Moist Convection and Clouds •Vertical distribution of water vapor—hydrologic cycle •Lapse Rate and the Greenhouse Effect •Types of Clouds and Roles in Climate Today—Addressing our assumptions •Isn’t this class about climate? •Energy-Imbalance: the incoming and outgoing energy flux is not the same everywhere on Earth •The atmosphere doesn’t just absorb outgoing radiation, it transports heat (and moisture) and absorbs incoming radiation •The atmosphere isn’t uniformly the same temperature A Brief Review Over the past 4 weeks, we have primarily learned about some of the physics which is important in the climate system. We used blackbody radiation physics to make predictions about the global average temperature and what major factors will influence that in the past and future. (This topic is at the heart of “global warming”) But, our definition of climate involved the “average of weather”. Weather is the response of the Earth system to energy imbalances, which we have so far ignored. Energy Imbalance Because of the curvature of the Earth, the equator receives a greater solar radiation flux than the poles I.e. more solar energy deposited per m2 per second at the equator than at higher latitudes Energy Imbalance 300 W/m2 EOUT (LW) EIN (SW) 100 W/m2 Higher latitudes emit more radiation back to space than they receive from the sun on average. The equator emits less radiation back to space than receives from sun Energy Imbalance Examining the energy imbalance tells us the tropics should be warmer than the higher latitudes. (recall the energy balance model) But it also says on average the northern latitudes should be getting colder and colder, and the tropics should be getting warmer and warmer. There must be other processes (besides radiation) which are helping maintain a closer energy balance at each latitude. The Atmosphere Moves The atmosphere (and oceans) move in response to the energy imbalance. This motion transports heat away from the equator to higher latitudes helping to minimize the imbalances. To simulate climate, we’ll need to simulate the weather. Questions 1. Where on Earth receives the largest daily average solar energy flux? 2. Why are seasons more intense in higher latitudes than in the tropics? 3. Give two forces that would cause air to move. Making Air Move Pressure Gradient Force Air/water will move from a region of high pressure to low pressure Fluid with horizontal pressure gradient Applies to horizontal (parallel to earth’s surface) and vertical (perpendicular to earth’s surface) Buoyant Forces Recall ideal gas law: density 1/T buoyancy: when pressure gradient force in vertical direction not equal to force due to gravity When an air parcel’s density is lower/higher than the surrounding air, it is “lighter”/”heavier” and will rise/sink convection Pressure Gradients Suggest Eq. To pole flow PGF The picture is roughly symmetric for the Southern Hemisphere (SH) General Circulation of the Atmosphere An early picture based on Hadley’s 1735 paper: Tropopause divergence subsidence Uplift (convection) subsidence convergence 90 45 0 -45 -90 Cold high lats Warm Tropics Cold high lats higher pressure Lower pressure higher pressure Intertropical Convergence Zone Convergence and uplift leads to saturation/condensation ITCZ moves around with season, and is not a continuous band around globe. Subsidence--Sinking Sinking air is compressed, therefore it warms, and becomes sub-saturated w.r.t. water vapor. In subsiding air, precipitation suppressed. The (original) Hadley Circulation (1735) COLD HOT COLD Explains: Intertropical Convergence Zone (ITCZ) Wet Tropics Dry Poles But it has some problems. Earth Rotates Hadley’s ideas were important and are still utilized today. To really explain the average wind directions and locations of general subsidence, the fact that the air motions occur on a rotating planet needs to be taken into account. 100 years after Hadley’s paper, Gustav de Coriolis, noted that moving air or water on the planet tends to be deflected from a straight path. This Week (5) Atmospheric Motions Reading chapter 4 of your text Study for Midterm on Friday in Discussion •Addressing our assumptions •Hadley Circulation, Coriolis Effect, Geostrophic Flow, •Deserts, Tropical Rainy/Dry seasons, Jet Streams •Surface Winds, Storms, Land/Ocean Contrasts •Cyclones and anticyclones, sea breeze, monsoons •Moist Convection and Clouds •Vertical distribution of water vapor—hydrologic cycle •Lapse Rate and the Greenhouse Effect •Types of Clouds and Roles in Climate IPCC In the News Monday, January 29, 2007 Last Update: 4:45 PM ET Scientists Gather to Finalize Climate Report By JAMES KANTER and ANDREW C. REVKIN 2:58 PM ET Today—General Circulation •Hadley Circulation + Coriolis Effect •A look at the major deserts •Wet and dry seasons in the tropics •Mid-latitude Circulation •Polar Front •Roaring 40s in SH •Geostrophic Flow: When PGF = Coriolis “Force” •Upper-level westerlies – jet stream Coriolis Effect Earth rotates from West to East (an Easterly direction) The equator is rotating with a larger velocity than higher latitudes because a larger circumference must be traveled in the same time period (1 day). coriolis effect movie Air (or any object) on Earth always has an easterly motion (relative to some fixed point in space like a distant star). Object from the equator and moving northward, its easterly motion is faster than the motion of the Earth at northern latitudes so it looks like it bends right. Coriolis Cartoon Trajectory seen by thrower Coriolis Effect Summary Coriolis effect applies only to moving objects (moving relative to Earth’s rotation) We analyzed motion in the N-S directions, but a similar effect occurs for objects traveling in E-W directions (the centrifugal force) Objects (air or water, e.g.) moving in the NH will curve to the right of their forward direction and to the left in SH. The strength of the coriolis effect decreases from the poles to the equator where it is zero. Questions 1. Suppose Earth were a cylinder. Where would there be a coriolis effect? A B C a) b) c) d) A only B only C only A, B, and C Coriolis Effect Modifies Idea of Hadley Circulation Modified Hadley Circulation * Horizontal motions convergence: divergence: Vertical motions convection: subsidence: coming together spreading apart rising air sinking air *not a fixed value Mean Surface Pressure Note the location of H’s for regions of High Pressure (near 30N or 30S?) Contours are lines of constant pressure (isobars). More closely spaced lines means a strong pressure gradient. Any hikers? World’s Deserts Not shown: Antarctica! World’s largest deserts tend to be located under subsiding branches of the Hadley cell –not only way for desert to exist. World’s Deserts Desert dust blows from Western Sahara and N. Morocco over Canary Islands. Dust from deserts can be an important source of nutrients to ocean and land biota (sometimes half a world away). Questions 1. What’s another mechanism for making/maintaining a desert? 2. The Amazon’s rainy season runs from Nov. – April. This period is Costa Rica’s dry season. Why? Seasonal Shift in Hadley Circulation ITCZ shifts N-S depending on season. Leads to wet and dry seasons in the tropics. This Week (5) Atmospheric Motions Reading chapter 4 of your text Study for Midterm on Friday in Discussion •Addressing our assumptions •Hadley Circulation, Coriolis Effect, Geostrophic Flow, •Deserts, Tropical Rainy/Dry seasons, Jet Streams •Surface Winds, Storms, Land/Ocean Contrasts •Cyclones and anticyclones, sea breeze, monsoons •Moist Convection and Clouds •Vertical distribution of water vapor—hydrologic cycle •Lapse Rate and the Greenhouse Effect •Types of Clouds and Roles in Climate Surface Winds and Pressure In the tropics air flows towards the ITCZ, impacted by Coriolis effect and friction. These are the “trades” Subsidence leads to surface highs and divergence both N and S subsidence subsidence Mid-latitude circulation Strong T gradients LP HP HP Northward-traveling warm air runs into southwardtraveling polar air. (A similar situation in SH – less land) Warm air rides up on top of colder air creating a front of uplift. The polar front zone is wavy in the horizontal A Jet Stream Is A Geostrophic Wind The PGF between 45 – 75 N generates strong flow towards pole. See the Coriolis Effect balances PGF so flow becomes westerly. The j indicates “location” of polar jet stream. Direction of flow is into the page j Polar-Front Jet in NH Blue arrows indicate wind speed (size) and direction at a pressure level of 300 mb. What does 300 mb mean in terms of altitude? Upper-level Flow tends to be Geostrophic The high-altitude branches of circulation cells feel little friction, and can be well organized flow. As air flows down pressure gradient, Coriolis Effect becomes more and more significant curving air mass to right of forward direction PGF Coriolis Flow direction Low Pressure Geostrophic Flow: When PGF balanced by Coriolis Effect High Pressure Upper-level Flow tends to be Geostrophic Centers of high or low pressure aloft induce clockwise and counterclockwise geostrophic flow around the centers W/out friction, flow becomes parallel to isobars. In NH flow counterclockwise around Lowcyclonic flow L Low Pressure Center In NH flow clockwise around Highanticyclonic flow H High Pressure Center Surface Flow Impacted by Friction Forces PGF Coriolis Friction Flow Directions Geostrophic Flow direction Actual Flow direction Low Pressure VG High Pressure Friction causes flow to move away from high pressure, but towards low pressure. Surface-level Flow affected by Friction Centers of high or low pressure at surface induce flow that spirals out or in, respectively. Convergence—Surface Storm L Low Pressure Center Divergence H High Pressure Center Tropical Cyclone—Hurricane Gordon Midlatitude Cyclones Land-Ocean Contrasts Heat capacity of water is 3 to 4 times greater than dry soil land T changes more than ocean for same E input Ocean can mix heat down into large thermal mass reservoir land heats/cools more rapidly than the ocean Land tends to have a higher albedo than ocean …but… Radiation can penetrate deeper into ocean before absorbed absorbed energy in ocean distributed over greater mass/depth Resulting T contrasts give rise to: diurnal sea breeze circulation, continentality in seasonal variability, and monsoonal circulation Continentality Seasonal variation in land temperatures much larger than over oceans. Feeds into circulation changes between seasons. Strongest continentality in mid-latitude NH, much weaker in mid-latitude SH due to less land. Seasonal variability in SH mid-latitude circulation is much less pronounced than in NH. Diurnal (Daily) Sea Breeze Day Night Monsoon Circulation The cause is the same as the diurnal sea breeze but on a larger, seasonal scale. This Week (5) Atmospheric Motions Reading chapter 4 of your text Study for Midterm on Friday in Discussion •Addressing our assumptions •Hadley Circulation, Coriolis Effect, Geostrophic Flow, •Deserts, Tropical Rainy/Dry seasons, Jet Streams •Surface Winds, Storms, Land/Ocean Contrasts •Cyclones and anticyclones, sea breeze, monsoons •Moist Convection and Clouds Today—Convection, Clouds, Climate •My mid-term evaluation •Vertical Heat transport—conduction and convection on firmer ground •Cementing the link between convection, lapse rate and greenhouse effect •Cloud Formation and Types •Clouds affect on climate Midterm Evaluation To help improve this course and to increase the amount you learn, please take a moment to answer the following questions. Remain anonymous (or not, up to you). 1. “This course is __________ than I thought it would be.” 2. Is the current mix of power point and writing on the board/overhead useful? 3. Does Prof. Thornton write legibly and speak clearly during lecture? 4. “If Prof. Thornton would just _________ during lecture, I would understand the material better.” Continentality-Find the Continents Contours show annual temperature range Moist Deep Convection—Where? divergence What is Buoyancy? Buoyancy refers to the density of an object in a fluid, relative to the density of that fluid. PGF Fluid (rf) PGF object (rob) Here “density” means density of substance : g of substance per cm3 of that substance. Fbuoyancy = PGF - Fg Fg Fg If density of object > density of fluid, force due to gravity will be a larger than the PGF, and object will sink. If density of object < density of fluid, PGF > Fg and it will rise. What is Buoyancy If the object and the fluid are the same gas, as they are in the atmosphere, the density becomes just proportional to 1/T. Consider air over a paved parking lot with a T = 303 K. If the surrounding air had T = 300 K, air over the parking lot would feel an acceleration upwards of 10 cm/s2. Typical convergence produces vertical velocities of ~ 0.1 cm/s (very slow compared to buoyant motion!). Convection is the result of buoyant forces Convection is the vertical transport of heat How do convection and buoyant forces get started? Conduction (sensible heat transfer) warms air over surface. A hypothetical situation (never really observed) ~ 2 km 0 km Dry adiabatic slope from expansion work T Slope greater than dry adiabatic lapse rate because of surface heating Air parcels become buoyant or are lifted mechanically to where they are buoyant. Rise following less steep lapse rateunstable vertical motion Moist Convection Heat eventually transferred to surrounding atmosphere Condensation Latent Heat Release More buoyant “Dry” convection Convection and the Energy Budget A large amount of energy is carried by convection Convection and Greenhouse Effect H2O vapor’s vertical distribution affects radiating temperature, and moist convection impacts lapse rate. New skin altitude with more GHGs Alt 255K “Skin” altitude T Convection/Advection/Water Cycle Advection: horizontal transport of energy or mass Warm Cloud Formation (similar for ice clouds but require much colder temperatures) 1. Start with air parcel (a volume of air with similar properties) containing water vapor and aerosol particles. 2. Lift parcel up • Heating at surface, thermals • Upslope wind forced by mountains • Converge of mass by large scale circulation 3. Parcel cools by rising, reaches saturation 4. Condensation occurs on aerosol particles => cloud forms More aerosol particles more cloud droplets No aerosol particles no cloud droplets Some Cloud Types Cumulus and cumulonimbus from space Cirrus-high, thin ice clouds Marine stratocumulus aviation contrails Clouds and Climate—a complex problem Surface Warming High thin Cirrus: Not so reflective, but absorb and emit at cold T Surface Cooling Low Warm Clouds: Absorb IR but emit like warm surface. Very reflective Activity •Think of a location other than Seattle where you have lived. Characterize the climate of that region. E.g. Was it rainy in the summer, dry in the winter, or vice versa? Was it humid, tropical with lush vegetation, or was it arid and dry? Always foggy at night and in the morning, but clear and sunny during the day? Come up with ideas as to why the climate might have been that way.