Ce diaporama a bien été signalé.
Nous utilisons votre profil LinkedIn et vos données d’activité pour vous proposer des publicités personnalisées et pertinentes. Vous pouvez changer vos préférences de publicités à tout moment.

Climates of Non-uniform Terrain

From Boundary Layer Climates, T.R. OKE

  • Soyez le premier à commenter

  • Soyez le premier à aimer ceci

Climates of Non-uniform Terrain

  1. 1. Pabitra Gurung CHAPTER 5 CLIMATES OF NON- UNIFORM TERRAIN
  2. 2. CONTENTS 1. Effects of spatial inhomogeneity (a)Advective effects (b)Thermal circulation systems 2. Effects of topography (a)Radiation loading effects (b)Topographically-generated winds (c)Topographically-modified winds
  3. 3. 1. EFFECTS OF SPATIAL INHOMOGENEITY Moisture advection from a dry to a wet surface. (a) Evaporation rates and the vapour balance of a surface air layer, (b) Surface evaporation rate (𝐸0), and mean water vapour concentration of the air layer, (c) Vertical profile of water vapour in relation to the developing boundary layer The development of an internal boundary layer as air flows from a smooth, hot, dry, bare soil surface to a rougher, cooler and more moist vegetation surface A = the rate of horizontal moisture transport Advective effects  Clothesline effect: The flow of air through a vegetative canopy  Leading-edge or fetch effect: Air passes from one surface- type to a new and climatically different surface  Oasis effect: Due to evaporation cooling, an isolated moisture source always cooler than its surroundings (Desert oasis)
  4. 4. 1. EFFECTS OF SPATIAL INHOMOGENEITY (a) Adjustment of surface sensible heat flux (𝑄 𝐻0 ) and mean air temperature ( 𝑇) as air passes from a hot to a cooler surface. (b) Change in surface shearing stress (𝜏0) and mean wind speed ( 𝑢) as air flows from a smooth to a rougher surface. Associated modification of the vertical profiles of (c) air temperature, and (d) wind speed at different distances downward of the leadi ng edge Advective effects  Clothesline effect: The flow of air through a vegetative canopy  Leading-edge or fetch effect: Air passes from one surface- type to a new and climatically different surface  Oasis effect: Due to evaporation cooling, an isolated moisture source always cooler than its surroundings (Desert oasis) The development of an internal boundary layer as air flows from a smooth, hot, dry, bare soil surface to a rougher, cooler and more moist vegetation surface
  5. 5. (a) Horizontal profile of surface radiation temperature, and (b) hot and cold ‘plumes” over a diverse prairie landscape. Based on aircraft observations on the afternoon of 6 August 1968 near Brooks, Alberta 1. EFFECTS OF SPATIAL INHOMOGENEITY Average daily energy balance of an alfalfa crop in June 1964 near Phoenix, Arizona (33°N), The crop was irrigated by flooding in late May and this followed by drought throughout June Advective effects  Clothesline effect: The flow of air through a vegetative canopy  Leading-edge or fetch effect: Air passes from one surface- type to a new and climatically different surface  Oasis effect: Due to evaporation cooling, an isolated moisture source always cooler than its surroundings (Desert oasis)
  6. 6. 1. EFFECTS OF SPATIAL INHOMOGENEITY Thermal circulation systems  Land and sea (Lake) breezes o Water allows transmission of SW radiation to considerable depths o Water is able to transfer heat by convection and mixing o Water converts much of its energy surplus into latent rather than sensible heat o Water has large thermal inertia due to its higher heat capacity  Other thermal breezes o A city can generate ‘country breezes’ Land and sea (lake) breeze circulations across a shoreline (a) by day and (b) at night, during anticyclonic weather
  7. 7. 2. EFFECTS OF TOPOGRAPHY Radiation loading effects The diurnal variation of direct-beam solar radiation upon surfaces with different angles of slope and aspect at latitude 40°N for (a) the equinoxes (21 Mar, 21 Sep), (b) summer solstice (22 Jun), and (c) winter solstice (22 Dec) (a) Diagrammatic representation of the angle 𝜃 between the surface and the incident direct-beam short-wave radiation, 𝑆, (b) The form of the cosine law of illumination 𝑺 = 𝑺𝒊 𝒄𝒐𝒔 𝜽 Total daily direct-beam solar radiation ( 𝑆) incident upon slopes of differing angle and aspect at latitude 45°N at the times of the equinoxes
  8. 8. 2. EFFECTS OF TOPOGRAPHY Topographically-generated winds  By day: The slopes and floor of the valley will be heated. [Anabatic winds: Unstable upslope flow]  By night: The lower air layers cool and slide down-slope under the influence of gravity. [Katabatic winds] Mountain and valley wind system viewed with the reader looking up-valley. (a) By day slope winds are anabatic, and the valley wind fills the valley and move upstream with the anti-valley wind coming downstream. (b) At night the slope winds are katabatic and reinforce the mountain wind which flows downstream, with the anti-mountain wind flowing in the opposite direction above. Time sequence of valley inversion destruction including potential temperature profile at valley centre (left) and cross-section of inversion layer and motions (right), at each time. (a) Nocturnal valley inversion, (b) start of surface warming after sunrise, (c) shrinking stable core and start of slope breezes, (d) end of inversion 3-5 h after sunrise.
  9. 9. 2. EFFECTS OF TOPOGRAPHY Topographically-generated winds The vertical distribution of along-valley winds in a 1 km deep valley on Mt. Rainer, Washington. Horizontal scale is graduated in units of wind speed and separated into two wind directions (up and down valley) Variation of air temperature with distance along a traverse route over hilly terrain in the early morning following a good radiation night. Note the correspondence of elevation and temperature. The vertical distance scale is exaggerated to aid comparison.
  10. 10. 2. EFFECTS OF TOPOGRAPHY Topographically-modified winds  Flow over moderate topography  Flow over steep topography  Flow over roughness changes Typical patterns of airflow over moderate topography. The point maximum (●) and minimum (○) is also indicated [Slope ≤17°] Typical patterns of airflow over steep topography [Slope >17°] (a) Wind from rough to smooth, (b) Wind from smooth to rough, (c) Wind parallel to the boundary with the rougher area to the right of the wind, (d) Wind parallel to the boundary with the smoother area to the right of the wind, and (e) Airflows across an isolated area of greater roughness
  11. 11. 2. EFFECTS OF TOPOGRAPHY Topographically-modified winds  Flow over moderate topography  Flow over steep topography  Flow over roughness changes Problems of pollution dispersal on the windward slope of a steep sided valley. In (a) plume contents are trapped in the lee eddy, and in (b) are forced to ground level by ‘downwash’
  12. 12. ADDITIONAL SLIDES FROM PREVIOUS CHAPTERS
  13. 13. o Diurnal variation of the important radiation budget components o Diffusion of radiation (Cloud, water vapour haze, smoggy areas, distance between sun and atmosphere) o Net radiation budget Net SW radiation, 𝐾∗ = 𝐾 ↓ −𝐾 ↑= 1 − 𝛼 𝐾 ↓ Net LW radiation, 𝐿∗ = 𝐿 ↓ −𝐿 ↑= 𝜀 𝐿 ↓ −𝜎𝑇0 4 o Total net radiation budget on the earth surface (𝑄∗) At day time, 𝑄∗ = 𝐾∗ + 𝐿∗ At night time, 𝑄∗ = 𝐿∗ Radiation budget components for 30July1971, at Matador, Saskatchewan over a 0.2 m stand of native grass in cloudless [𝐾 ↑= 𝛼𝐾 ↓] [𝐿 ↑= 𝜀𝜎𝑇0 4 + 1 − 𝜀 𝐿 ↓] DIURNAL ENERGY BALANCES AND RADIATION BUDGET AT AN ‘IDEAL’ SITE
  14. 14. ANNUAL ENERGY BALANCES AND RADIATION BUDGET OF E-A SYSTEM  Energy exchanges between the earth, the atmosphere and space o For single wavelength 𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑣𝑖𝑡𝑦(ψλ) + 𝑅𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦(𝛼λ) + 𝐴𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑣𝑖𝑡𝑦(ζλ) = 1  Most of natural surfaces: 𝜀 ≈ 1  Earth mean annual temp. ≈ 288 K  Energy emitted by earth surface = 𝜀𝜎𝑇0 4 ≤ 390 𝑊𝑚−2  Solar input (100%) = 𝑆𝑊 𝑅𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 28% + 𝐿𝑊 𝐸𝑚𝑖𝑠𝑠𝑖𝑜𝑛(72%)  Equilibrium: the E-A system and the E-A sub- system  Annual net sub-surface storage is zero  Net 𝑄 𝐺 in annual balance is also zero 𝐾 𝐸𝑥 = 𝐾 ↑(𝐴𝑐)+ 𝐾 ↑(𝐴𝑎)+ 𝐾 ∗ (𝐴𝑐)+𝐾 ∗ (𝐴𝑎) +𝐾 ↑(𝐸) +𝐾 ∗ (𝐴𝐸) 100% = 19% + 6% + 5% + 20% + 3% + 47% 𝐾 𝐸𝑥 = Spatial mean energy input ≈ 338 W m-2 (Values are in %)
  15. 15. RADIATION BUDGET: ORCHARDS AND FORESTS  The principal radiative exchanges occur at the canopy layer (upper and lower boundary)  Approximate attenuation of SW with height is given Beer’s law [𝐾 ↓ 𝑧= 𝐾 ↓0 𝑒−𝑎𝑧 ]  Amount of SW transmission depend on the height, density and species of the stand, the angle of solar incidence (generally 5% - 20% of flux 1 reaches floor of a stand) Schematic model of radiation exchanges above and within a forest. (𝐊 ↓) (𝐊 ↑) (𝐋 ↑)(𝐋 ↓) (𝐊 ↓) (𝐊 ↑) (𝐋 ↓) (𝐋 ↑) SW radiation budget of (a) an orange orchard, and (b) a single-layer mosaic of fresh orange leaves. All values expressed as percentages of the incident radiation
  16. 16.  Dry adiabatic lapse rate (Γ): Constant (9.8 ℃ 𝑘𝑚−1) for dry/unsaturated air  Environmental lapse rate (ELR): Based on actual observed temperature structure above a given location  (a) Unstable (ELR > Γ), (b) Stable (ELR < Γ), & (c) Neutral (ELR = Γ) Lapse rates and stability Warmer Colder Warmer Colder  With fine weather: Unstable by day and Stable by night  Over high latitude snow surfaces in winter: Stable boundary layer for longer period  Over tropical ocean surfaces: Unstable boundary layer for longer period Height vs temperature (a) unstable atmosphere on sunny days and (b) stable atmosphere at night SURFACE LAYER CLIMATES & EXCHANGES
  17. 17. THANK YOU

×