06PrecipitationEvaporation - 10/16/2007 Driver of...

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Unformatted text preview: 10/16/2007 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) 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 2 4 Droplets coalesce and, when large enough, fall to ground. So ... how do we cool the air? 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 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 Lapse rates in rising air Dry adiabatic lapse rate ambient 4 3 2 1 0 -10 0 gravity specific heat of air at constant pressure 9.8 ms 1 2 ht, km 1005 J kg deg 1 0.01 deg m 1 same as 10 deg km 1 But if water vapor condenses from the air during the cooling, latent heat is released, it warms the ascending air 10 T, C 3 20 Wet (saturated) adiabatic lapse rate = Dry ALR + heat added by condensing water, 0.4 to 0.9 C /100 m 5 ESM 203: Precipitation and evaporation 1 10/16/2007 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 Conditional instability Air forced to rise cools g y along the dry ALR and is 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 20 6 8 5 Dry Wet adiabatic 4 3 2 1 Ambient 4 3 2 1 0 -10 0 ambient Elevation, km ht, km Most stable is a temperature inversion 0 -10 0 10 20 TC 10 T, C 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 Mechanisms that cool air to generate precipitation 1: Convective/convergent 5 Dry adiabatic 4 3 2 1 Ambient Air rises because it is unstable Often caused by surface heating Elevation, km 0 -10 0 10 20 TC 7 9 ESM 203: Precipitation and evaporation 2 10/16/2007 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 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 Aguado & Blunt 10 12 Orographic precipitation in the Sierra Nevada Mechanisms that cool air to generate precipitation 3: Cyclonic/frontal lifting Common at fronts within mid-latitude cyclones (depressions) http://geography.sierra.cc.ca.us/ 11 13 ESM 203: Precipitation and evaporation 3 10/16/2007 Current weather, eastern Pacific http://squall.sfsu.edu/ Fluxes of water between surface and atmosphere volume 1012m3/ /yr Land (29.1% of area) Precipitation Evaporation & transpiration Ocean (70.9% of area) Precipitation Evaporation E ti Total Precipitation Evaporation & transpiration 14 depth mm/yr / 728 -418 310 1134 -1261 1261 -127 1016 -1016 0 16 108 -62 46 410 -456 456 -46 518 -518 0 Picture is P hyperlinked to latest image Water balance for a landscape The energy balance equation (flux per unit area, W m2) applied to a surface S(1 - ) + F - s Ts4 = H + L + G net radiation Rnet Precipitation (P) Evapotranspiration (E) slope water table Runoff (R = Quickflow + Delayed Flow) S = solar radiation = albedo: water 0.06; conifer forest 0.09; Amazon broadleaf forest 0.12; grassland 0.20.4; snow 0.60.8 F = downward infrared radiation, , depends on temperature, water vapor, and clouds Ts = surface temperature H = sensible heat transfer (+ is surface to atmosphere) L = latent heat transfer in water evaporating or condensing (+ is evapotranspiration, is condensation) G = heat conducted into soil s = surface emissivity = Stefan-Boltzmann constant = 5.67108 W m2 deg4 (Quickflow leaves the landscape within a few hours of rainfall. Delayed flow represents slow drainage from soil water and ground water.) 15 17 ESM 203: Precipitation and evaporation 4 10/16/2007 Hard to measure evaporation directly, so we estimate the energy L and then convert to get E L = w v E Wm-2 (Jm-2s-1)= or E = kg J m m3 kg s L Latent heat exchange per unit area (p converts a volume of water (per unit area) to vapor The energy required for this conversion is the volume of water per unit area (E) multiplied by the latent heat of vaporization energy required to convert a kg of water to vapor End points (for day or month, G=0) G=0) Waterless Earth: L = 0 so Rnet = H Mid-ocean or mid-Congo: H = 0 so Rnet = L and E = Rnet w v w v L = latent heat flux E = evapotranspiration rate (m s1) w = density of water (1000 kg m3) v = latent heat of vaporization (2.5106 J kg1) and by the density of water which converts the mass per unit area to a volume per unit area The "potential evapotranspiration" is the p p p energy-limited rate for a wet soil Actual evapotranspiration is limited by energy and water availability 18 20 Saturation vapor pressure and evaporation 45 40 sa aturation vapor pressure, mb water t ice Basic principle Net radiation (Rnet) drives the sum of sensible (H) and latent (L) heat exchange with the atmosphere and heat flow into or out soil (G) G is normally small 35 30 25 20 15 10 5 0 -40 -35 Temperature difference KG is thermal conductivity, z is depth Vapor pressure difference -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 Temperature, vapor pressure, and soil moisture determine how Rnet is partitioned between H and L, depending on the magnitude of the temperature gradient vs. the vapor g p g p pressure gradient the rate at which the atmosphere in the boundary layer mixes temperature, C 19 21 ESM 203: Precipitation and evaporation 5 10/16/2007 Canonical form of equations for sensible and latent heat flux some constants, temperature difference typically including some description yp y g p H = (or gradient) air density and of the mixing between surface and air specific heat of air 1 some constants, humidity difference typically including some description (or gradient) L= of the mixing air density and between surface and air enthalpy of vaporization 1 (variables) a air density Rnet net radiation H sensible heat exchange L latent heat exchange G soil heat flow S solar radiation c p specific heat of air Ts surface temperature Ta air temperature ra aerodynamic resistance rc surface resistance albedo F longwave radiation soil moisture availability function psychrometric constant = p y cpP m v w ma s surface emissivity Stefan-Boltzmann constant e * (Ts ) equilibrium vapor pressure at Ts ea vapor pressure KG thermal conductance of soil Tz temperature at depth z 22 24 Partitioning between sensible and latent heat exchange In equation form using equations Rnet = H + L + G expands to 4 s Problem 1, es (vapor pressure at leaf) Sellers et al. suggest S(1 - ) + F - s T = in the form of Sellers et al. acp Ts - Ta e * (Ts ) - ea Ts - Tz + + KG ra + rc z ra ( es - ea ) = e* (Ts ) - ea is a "moisture availability function" = 1 when soil moisture W = Wmax e * (Ts ) = saturation vapor pressure at Ts e * (Ts ) - ea ac p So L = ( (their equation 4) q ) ra Controls latent heat exchange 23 So we're concerned with how Controls sensible heat exchange Ts - Ta e * (Ts ) - ea compares to ra + rc ra The "potential evapotranspiration" is when =1 Common suggestion is = W - Wwp [wp : wilting point] Wmax - Wwp 25 ESM 203: Precipitation and evaporation 6 10/16/2007 Problem 2, resistance Sellers et al. (eq. 6) suggest both an aerodynamic resistance ra and a surface resistance rc Evaporation over deep water e * (Ts ) - ea ac p L= ra + rc In summer, evaporation over deep water is usually less than potential evapotranspiration over soil In i t I winter, evaporation over d ti deep water is usually greater than t i ll t th potential evapotranspiration over soil In fact, over large water bodies (Lake Superior, Pacific Ocean) evaporation in usually greater in winter than in summer It takes more heat to keep a swimming pool warm in the winter because of evaporation, i.e. because e*(Ts)ea is large, not because TsTa i l T is large 26 28 Evaporation from water bodies Over the land, we can usually neglect G in the equation Rnet=H+L+G This is not the case over water, especially deep water , p y p Because G occurs by convection rather than conduction, it can be a significant term in the energy balance equation, either into or out from the water If out, G is a source of energy for evaporation Water and energy balance of a vegetated, soil-covered land surface soilExchange of sensible (H) and latent heat l t t h t (L) Quickflow Soil Recharge Ground water Rnet E P For some t (e.g day, or month) on a unit area of land, the l d th mass b l balance equation for water is P - E = Quickflow - Delayedflow = U t Finally, advection (horizontal transport of heat in ocean ) g currents) is significant U is the water content of the " d th "underground store" d t " (soil or ground water) Units are [L3/(L2T)] or depth/time (e.g., m/mo) Delayed flow 27 ESM 203: Precipitation and evaporation 7 ...
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This note was uploaded on 08/06/2008 for the course ESM 203 taught by Professor Dozier,dunne during the Fall '07 term at UCSB.

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