MET 3102-U01 PHYSICAL CLIMATOLOGY (ID 17901) Lecture 14

Transcription

1 MET 3102-U01 PHYSICAL CLIMATOLOGY (ID 17901) Lecture 14

2 The hydrologic cycle evaporation vapor transport precipitation precipitation evaporation runoff Evaporation, precipitation, etc. in cm Vapor transported to land represents an 11 cm loss to the sea, but a 27 cm gain to the smaller area of the land; and similarly for runoff which represents a 27 cm loss to land, but only an 11 cm gain to the larger area of the oceans.

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4 g = P+ D-E-Df w Water Balance The surface water balance includes, storage, precipitation, evaporation, dewfall and runoff Generally storage & dewfall are small, so D f = P-W Horizontal export of water (vapor) by atmospheric motions A similar relation for the atmosphere is written g = -( P+ D-E) -Df wa Adding the atmosphere to the surface shows that column storage is the sum of the net transports in the atmosphere and at or below the surface g + g =-Df - Df Generally small w wa a a Water carried to continents must equal runoff

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6 P-E (P-E)/P Runoff ratio Measures wetness Large arid Regions, Ice, snow Exporters Of H 2 O High runoff ratio, wet continent Typically Evaporation is 60% of P, runoff 40% D f / P = Runoff ratio

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8 Water transported by the atmosphere into and out of the Atlantic. Basins draining into the Atlantic are solid grey, deserts are white, and other drainage basins are stipled. Arrows give direction of water transport by the atmosphere, and values are in Sverdrups. Bold numbers give the net transport for the Atlantic. Overall, the Atlantic loses 0.32Sv, an amount approximately equal to the flow in the Amazon River. From Broecker (1997).

9 Zonal Mean E, P, R small P over polar regions due to low temperatures and low water-carrying capacity of the atmosphere Zonal mean E, P, and R P peaks at equator due to heavy convection in ITCZ P has 2 nd maxima over midlatitudes due to vertical motion associated with baroclinic disturbances from: Hartmann 1994 R P has minimum over subtropical arid regions: suppressed P by mean downward motion of quasi permanent H s latitude Transport of water vapor from the subtropics (oceans) to low and high latitudes

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11 Zonal Mean Precipitation

12 Inter-tropical convergence Zone (ITCZ)

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14 subtropical arid regions Precipitation Rate: CMAP midlatitude stormtracks annual midlatitude stormtracks Indonesian warm pool 3650 mm/year ITCZ, north of equator 365 mm/year South Pacific convergence zone (SPCZ) JFM JAS subtropical arid regions data: CMAP, (Xie & Arkin)

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16 Seasonal Cycle of Precipitation What determines seasonal cycle of rainfall? JFM-JAS (CMAP) Polar regions (~65-90º) little rain fall since cold, summer maximum when stormtracks north Midlatitudes (~45-65º) west coasts: oceanic regime, winter maximum due to stormtracks interior of continents: continental regime, summer maximum due to convective activity Subtropics (~25-45º) generally dry, poleward: winter maximum due to extratropics; equatorward: summer maximum due to northward shift of ITCZ Tropics (~0-25º) maximum where ITCZ is, convective, usually semi-annual cycle

17 Geographic Distribution of Annual P-E (mm) Evaporation excess nearly ubiquitous over sub-tropical oceans, with a sharp contrast at coastal regions. Equatorial ocean evaporation minimum consistent with other findings (e.g. Seager et al., 2003). Tropical land areas show richest excess in precipitation. Major desert regions, tundra, and mountainous regions all indicate deficit to marginally-balanced conditions. Mid-latitude and boreal coastal/maritime environments exhibit adequate precipitation supply over evaporation. Liang Yang

18 Precipitation Liang Yang

19 Precipitation Liang Yang

20 Storage lakes Surface Water snow pack, released during spring and summer underground (ground water) Field capacity amount of water that can be held by soil against gravity else: flow into ground water depends on soil type Infiltration-transfer of water from surface to soil Evaporation 1. from surfaces, often includes sublimation (conversion of ice to vapor) 2. from vegetation (transpiration), roots can go deep Evapotranspiration from surface and vegetation Potential evaporation maximum possible evapotranspiration if surface was wet

21 Some Concepts Storage is important on a seasonal basis Soil moisture accumulates during the rainy/cool season Evaporates from the surface or from plants even when no rain falls Transpiration: Evaporation from leaves of growing plants. Evapotranspiration Combined evaporation from water, land and transpiration from growing plants Soil/field capacity: The maximum amount of water that the soil can hold. When it is exceeded, the additional water becomes recharge of the ground water aquifer or runoff.

22 plant canopy: collection of vegetable matter covering the surface roots reach deep into soil, which effectively increases evapotranspiration Effects of Vegetation interception and evaporation of rain by leaves can greatly decrease runoff, in particular if rain is not too intense Infiltration-transfer of water into soil If P>>E+infiltration =>ponding and runoff

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24 Potential Evapotranspiration Book calls it Potential Evaporation The rate of evapotranspiration that would occur if the surface were wet Depends upon Energy supply (ie heating) Ability of the air to take up moisture from the surface (ie humidity) Other factors (i.e. windspeed) Exceeds actual evapotranspiration when the surface dries out

25 Bucket Model for Soil Moisture Represent soil moisture as a depth of water, up to the field capacity, and account for changes through rainfall, evaporation, snowmelt & runoff w w t = ρ w h w t = P r E + M s Δf, (h w h c ) Use a similar model for snow depth expressed as equivalent amount of liquid water w s t = ρ w h s t = P s E sub M s

26 Winter max P Summer dry season PE > E soil is dry! Tropics! (17N) T,Q fairly constant here LA is always dry Spring summer max P Warm, wet, sunny great agriculture If precipitation does not appear it is equal to evaporation. If potential evaporation does not appear it is equal to evaporation

27 Desert, little P, PE >> E soil is very dry! WV transported here, evapotranspiration is energy limited so soils are moist(frozen) Winter P frontal, Summer P Convective Southeast modest winter peak Wet season, solar heating and thunderstorms Subtropics, PE always high!

28 Summary Water cycle: Evaporationà Precipitation à Runoff. Land is a net source of liquid H 2 O; sea is a net source of vapor Most (97%) of water is in the sea, 2% in ice, 0.6 % fresh water, 0.001% vapor Water balance: storage = precip. -evap. -net transport out Latitude dependence Heaviest precip. in ITCZ and polar front Evaporation over subtropical and tropical seas Atlantic and Indian oceans have excess evap over precip. Soil moisture essential to agriculture Soil Capacity: Maximum amount of water that the soil can hold, typically ~15 cm Evapotranspiration: Water from surface and growing plants becomes vapor in the atmosphere Potential Evapotranspiration: Amount that would occur if the soil were wet. Bucket model: Precip + Snowmelt -Evap & Runoff fill a bucket that s soil capacity deep Similar model for show depth Annual cycles in various climates

29 For Next Time Exam, 08 March 2018 Start Reading Chapter 6 Do problems 1 and 2 on page 134 & 135, due Thursday, 08 March 2009