04Precipitation

04Precipitation - 10/09/2007 ESM 203: Atmospheric...

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Unformatted text preview: 10/09/2007 ESM 203: Atmospheric circulation and precipitation Jeff Dozier and Thomas Dunne Fall 2007 Effects of spatially variable heating on a uniform, non-rotating Earth nonHeating at equator and cooling near poles would cause a single convection cell in the atmosphere if Earth were covered with a uniform surface and if Earth did not rotate Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 3 Circulation driven by radiation imbalance with latitude Movement across Earth's rotating surface A substance that moves across Earth's rotating surface moves from a place where the planet is rotating at one h th l t i t ti t speed to a position where it is rotating at a different speed (measured in m/s, not in radians/s or deg/s) Ignoring friction, if an air parcel moves directly north in N hemisphere, it begins with a faster W E velocity than does the place it is heading for. Also, as it moves "inward" relative to Earth's axis of rotation the air's rotation speeds up because its angular momentum (mr, where is angular velocity) must remain constant, like a spinning skater. 2 excess Absorbed solar radiation deficit Emitted infrared radiation From the point of view of an observer on the surface, the air appears to move to the right 0 30 latitude 60 90 Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 4 ESM 203: Atmospheric circulation and precipitation 1 10/09/2007 Coriolis force Displacement of the air parcel is to the right in N hemisphere appears to be subject to a [Coriolis] force that increases with increasing latitude and air speed Using same reasoning, imagine the effect on air moving south (toward faster rotating surface) in the hemisphere i th N h i h In the S hemisphere, the sense of the force is reversed (i.e. to left) whether moving north or south Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 5 Effect of latitude, Coriolis force on a sphere N true direction of Coriolis force Equator latitude the horizontal (only important) component, proportional to sin(latitude) 7 Coriolis force on a rotating disk Definitions: velocity of rotation v = r, = angular velocity (rad sec-1) r = radius of rotation angular momentum = vr = r2 r r0 Coriolis force and cooling of air raised at equator disrupt the simple circulation of a non-rotating Earth nonAir rising at Equator moves N and increasingly to E while cooling (densifying) By time it has reached ~30N, some of it sinks and flows back along surface to S (and therefore W) and to N (E) Remainder of air aloft continues towards pole, where it sinks and flows S (W) meeting the NEflowing air at the surface Reverse in S hemisphere Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 6 8 Conserving angular momentum with change r=r0-r requires r02 = (+)r2 = (+v/r)r2, where v is relative velocity caused by r from r0 to r Solving, v = (r02/r - r) ESM 203: Atmospheric circulation and precipitation 2 10/09/2007 Schematic zonal circulation is complicated by unmixed boundaries between cold and warm air, creating `fronts' Force balances, geostrophic Gradients of pressure (the pressure-gradient force) drive air flow Geostrophic: balance between pressure gradient and Coriolis forces where friction is negligible (aloft) geostrophic wind blows parallel to isobars Geostrophic + friction at surface, wind slowed by friction, friction is not parallel to isobars but still moves to right of direction of pressure-gradient force 9 11 Simplified zonal pattern of surface winds Where winds converge, air must rise, and thus pressure is lowered Where winds diverge, they must be supplied by sinking air, and the pressure must be relatively high Geostrophic winds in upper atmosphere flow around the highhighand low-pressure lowcells Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 10 Duxbury, A.C. & Duxbury, A. B. (1989) An Introduction to the World's Oceans 12 ESM 203: Atmospheric circulation and precipitation 3 10/09/2007 Surface pressure, January and July From Columbia University We have derived the following characteristics of the atmosphere It has mass, and therefore the mass above any point creates a pressure on the air below It contains water vapor (~1% by mass), which January 1000 mb height affects it radiation absorbing properties ff t its di ti b bi ti can change state to liquid or solid It has an average temperature profile resulting from the processes controlling its radiation balance; the ambient atmospheric lapse rate It moves as a result of pressure gradients caused by temperature g y p gradients that are caused by unequal net radiation July 1000 mb height Air masses with different temperatures and therefore densities don't mix efficiently, but maintain `fronts' between themselves 13 15 Forces on air parcel in atmosphere Gravity: g (force per unit volume is density of air gravitational acceleration) air has mass, so the mass above any point creates pressure on the air below Variation of atmospheric temperature with elevation reflects absorption of radiation emitted from surface and absorbed by atmospheric gases < 0,1m absorbed by N2, O2, N, O Pressure gradient force: spatial variability in air pressure, from high to low, =P/distance Coriolis force: right in northern hemisphere, left in southern, magnitude depends on latitude (zero at equator) and wind speed Friction: small except near surface Centrifugal force: where winds are turning rapidly such as in a rapidly, hurricane Wind speed and direction balance all these forces < 0.2m absorbed by O2 O3 absorbs < 0.31 m and ~8 m > 0.31m (~ 50% of incident solar ) warms surface, which radiates and warms atmosphere 14 Graedel, T.E. and Crutzen, P.J. (1995) Atmosphere, Climate and Change 16 ESM 203: Atmospheric circulation and precipitation 4 10/09/2007 The radiant and convective energy transfers cause changes in the temperature and vapor content of the atmosphere Thus, they cause differences in sa aturation vapor pressure, mb Saturation vapor pressure and condensation 45 40 35 30 25 20 15 10 5 0 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 temperature, C water t ice water vapor content p air temperature the capacity of air to hold water in the vapor form which depends on its temperature The atmosphere condenses part of its water vapor content as clouds and precipitation When the vapor holding capacity is equaled by the water vapor content, the vapor condenses to liquid water droplets (clouds, fog) If droplets coalesce to form drops large enough to fall through the atmosphere, precipitation occurs -- in the form of rain, snow, and fog drip from vegetation 17 Initial state of p a parcel of air Dew point temperature 19 Capacity of air to retain water in the vapor state Amount (number of moles per unit volume) of water vapor in the air is conveniently expressed in terms of its absolute contribution to the total pressure (usually in millibars = 100Pa) Sea-level pressure is about 1 bar = 1000 mb = 105 Pa 1 Pa = 1 Nm2 At T < 0C, saturation vapor pressure with respect 0 to ice is lower than for water, so water vapor would deposit on ice crystals before water droplets 10 water vapor satu uration vapor pressure, mb water ice 1 Initial state of a parcel of air When water vapor is allowed to equilibrate with a flat water surface, its vapor pressure is called the saturation vapor pressure -- a function of the temperature water Dew point temperature 0.1 -40 -35 -30 -25 -20 temperature, C 18 20 -15 -10 -5 0 ESM 203: Atmospheric circulation and precipitation 5 10/09/2007 Effect of a curved surface on saturation vapor pressure (e*): why condensation nuclei are necessary for rapid droplet production Driver of precipitation Precipitation mechanism is thus to cool air below its dew point, forming clouds in the presence of condensation nuclei: water droplets or ice crystals aerosols such as salt crystals and dust (subject to perturbation by humans) Vapor Vapor Droplets coalesce and, when large enough, fall to ground. So ... how do we cool the air? Water or Ice Water or Ice Water or Ice Flat Convex, e*>e*(flat) Concave, e*<e*(flat) Convection from underlying surface Mixing with colder air Both of these produce condensed water droplets but are not efficient enough to produce continuous heavy rain or snow Raising the air cools it rapidly enough to condense significant amounts of water vapor 21 23 Effect of aerosol condensation nuclei is to intensify the rate of drop production Precipitation results from cooling of air to its dewpoint temperature in the presence of condensation nuclei Rising air encounters lower pressure, pressure so it expands Expansion requires that the air do work (expend energy) against the surrounding air Energy expenditure cools the air In Earth's atmosphere, rising air cools by 1C/100m, the dry dr adiabatic lapse rate adiabatic means without the introduction of heat from external sources 5 dry di b ti d adiabatic 4 3 2 1 0 -10 0 ambient ht, km 10 T, C 20 24 22 ESM 203: Atmospheric circulation and precipitation 6 10/09/2007 Variation of atmospheric temperature with elevation reflects absorption of radiation emitted from surface and absorbed by atmospheric gases Temperature profile at any particular time and place may deviate dramatically from global average Particular rate of decrease is called the ambient atmospheric lapse rate Averages 0.65C/100m, but varies tremendously, can even be positive (inversion). Variations driven by recent history of mixing, conduction, and radiation Graedel, T.E. and Crutzen, P.J. (1995) Atmosphere, Climate and Change 25 Atmospheric stability Air's stability depends on the relationship between ambient atmospheric and adiabatic lapse rates If the ambient lapse rate is lower (more negative) than the dry ALR, the raised air is cooler (denser) than its surroundings Only way to rise is to be pushed up by some external agent (like a pressurepressure gradient force pushing air over a mountain range) 5 dry di b ti d adiabatic 4 3 2 1 0 -10 0 ambient Most stable is a temperature inversion ht, km 10 T, C 20 27 Lapse rates in rising air Atmospheric Instability If the ambient lapse rate is greater (less negative) than the dry ALR, the raised air is increasingly warmer (less dense) than its surroundings, and continues to rises Bucks a small plane around in clear air Dry adiabatic lapse rate dT g (gravity) =- dz c p (specific heat of air at constant pressure) =- 9.8 ms -2 -0.01 K m-1 ( - 10 K km-1) 1005 J kg-1 K -1 5 Dry adiabatic 4 3 2 1 Ambient Recall J = Nm and N = kg m s-2 But if water vapor condenses from the air during the cooling, latent heat is released, it warms the ascending air Wet (saturated) adiabatic lapse rate = Dry ALR + heat added by condensing water, -0.4 to 0.9 C /100 m 26 Elevation, km 0 -10 0 10 20 TC 28 ESM 203: Atmospheric circulation and precipitation 7 10/09/2007 Conditional instability Air forced to rise cools along the dry ALR and is g y cooler than surroundings (stable) Eventually cools to its dew point, releasing latent heat and then cools at the wet ALR At a certain height, the air height becomes warmer than its surroundings and thereafter rises unstably 5 Dry Wet adiabatic 4 3 2 1 Ambient Mechanisms that cool air to generate precipitation 2: Orographic lifting Pressure-gradient force large enough to drive air up and over a mountain range High pressure PGF Low pressure Elevation, km 0 -10 0 10 20 TC 29 Aguado & Blunt 31 Mechanisms that cool air to generate precipitation 1: Convective/convergent Air rises because it is unstable Often caused by surface heating Orographic precipitation in the Sierra Nevada http://geography.sierra.cc.ca.us/ 30 32 ESM 203: Atmospheric circulation and precipitation 8 10/09/2007 Mechanisms that cool air to generate precipitation 3: Cyclonic/frontal lifting Colder, denser air flows under warmer air, lifting it and forcing it to cool at the dry and then wet ALR Current weather, eastern Pacific http://squall.sfsu.edu/ Picture is P hyperlinked to latest image 33 35 Mechanisms that cool air to generate precipitation 3: Cyclonic/frontal lifting Common at fronts within mid-latitude cyclones (depressions) Some useful links Geog 110 Introductory Meteorology (Michaelsen) Links to current satellite images California Data Exchange Center http://cdec.water.ca.gov/ National Climate Data Center http://lwf.ncdc.noaa.gov/oa /ncdc.html Links to other national centers through the World Meteorological Organization http://www.wmo.ch/weben/member.html 36 California weather and water data River conditions, weather forecasts, precipitation World Weather 2010 project has a nice outline-style navigation bar 34 Also see links on the ESM 203 web page to some animations ESM 203: Atmospheric circulation and precipitation 9 ...
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