ESM 203: Precipitation and evaporation

1. ESM 203: Precipitation and evaporation Jeff Dozier and Thomas DunneFall 2007

2. 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

3. Precipitation results from cooling of air to its dewpoint temperature in the presence of 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

4. 4 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 � 0.65�C/100m, but varies tremendously, can even be positive (inversion). Variations driven by recent history of mixing, conduction, and radiation

5. 5 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

6. 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

7. 7 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

8. 8 Conditional instability Air forced to rise cools 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 becomes warmer than its surroundings and thereafter rises unstably

9. 9 Mechanisms that cool air to generate precipitation 1: Convective/convergent Air rises because it is unstable Often caused by surface heating

10. 10 Mechanisms that cool air to generate precipitation 2: Orographic lifting Pressure-gradient force large enough to drive air up and over a mountain range

11. 11 Orographic precipitation in the Sierra Nevada

12. 12 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

13. 13 Mechanisms that cool air to generate precipitation 3: Cyclonic/frontal lifting Common at fronts within mid-latitude cyclones (depressions)

14. 14 Current weather, eastern Pacific http://squall.sfsu.edu/

15. 15 Water balance for a landscape

16. 16 Fluxes of water between surface and atmosphere

17. 17 The energy balance equation (flux per unit area, W m�2) applied to a surface S = solar radiation a = albedo: water 0.06; conifer forest 0.09; Amazon broadleaf forest 0.12; grassland 0.2�0.4; snow 0.6�0.8 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 es = surface emissivity s = Stefan-Boltzmann constant = 5.67�10�8 W m�2 deg�4

18. 18 Hard to measure evaporation directly, so we estimate the energy L and then convert to get E L = latent heat flux E = evapotranspiration rate (m s�1) ?w = density of water (1000 kg m�3) ?v = latent heat of vaporization (2.5?106 J kg�1) Latent heat exchange per unit area 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 and by the density of water which converts the mass per unit area to a volume per unit area

19. 19 Saturation vapor pressure and evaporation

20. 20 End points (for day or month, G=0)

21. 21 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 KG is thermal conductivity, z is depth 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 pressure gradient the rate at which the atmosphere in the boundary layer mixes

22. 22 Canonical form of equations for sensible and latent heat flux

23. 23 Partitioning between sensible and latent heat exchange In equation form So we�re concerned with how

24. 24 (variables)

25. 25 Problem 1, es (vapor pressure at leaf) Sellers et al. suggest The �potential evapotranspiration� is when �=1 Common suggestion is

26. 26 Problem 2, resistance Sellers et al. (eq. 6) suggest both an aerodynamic resistance ra and a surface resistance rc

27. 27 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 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 Finally, advection (horizontal transport of heat in ocean currents) is significant

28. 28 Evaporation over deep water In summer, evaporation over deep water is usually less than potential evapotranspiration over soil In winter, evaporation over deep water is usually greater than 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 Ts�Ta is large

29. Water and energy balance of a vegetated, soil-covered land surface For some ?t (e.g day, or month) on a unit area of land, the mass balance equation for water is U is the water content of the �underground store� (soil or ground water) Units are [L3/(L2�T)] or depth/time (e.g., m/mo)