06PrecipitationEvaporation

06PrecipitationEvaporation - ESM 203 Precipitation and...

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ESM 203: Precipitation and ESM 203: Precipitation and evaporation evaporation Jeff Dozier and Thomas Dunne Fall 2007
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Driver of precipitation 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) 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 2
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Precipitation results from cooling of air to its Precipitation results from cooling of air to its dewpoint temperature in the presence of dewpoint temperature in the presence of condensation nuclei condensation nuclei Rising air encounters lower 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 1° C/100m, the dry adiabatic lapse rate adiabatic means without the introduction of heat from external sources 3 0 1 2 3 4 5 -10 0 10 20 T, °C ht, km dry adiabatic ambient
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4 Variation of atmospheric temperature with elevation reflects Variation of atmospheric temperature with elevation reflects absorption of radiation emitted from surface and absorbed by absorption of radiation emitted from surface and absorbed by atmospheric gases 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 – 0.65° C/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
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5 Lapse rates in rising air Lapse rates in rising air Dry adiabatic lapse rate 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 = − = = = = gravity specific heat of air at constant pressure = − 9 . 8 ms 2 1005 J kg 1 deg 1 ≈ − 0 . 01 degm 1 9 same as 10 degkm 1 9
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Atmospheric stability 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 pressure-gradient force pushing air over a mountain range) Most stable is a temperature inversion 6 0 1 2 3 4 5 -10 0 10 20 T, °C ht, km dry adiabatic ambient
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