1 / 46

Parameterization of surface fluxes

Parameterization of surface fluxes. Bart van den Hurk (KNMI/IMAU). General form of land surface schemes. Q*. H.  E. P SN. E SN. Accumulation. G. M. Energy balance equation K  (1 – a ) + L  – L  +  E + H = G Water balance equation  W / t = P – E – R s – D

shanon
Download Presentation

Parameterization of surface fluxes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Parameterization of surface fluxes Bart van den Hurk (KNMI/IMAU) HTESSEL parameterization

  2. General form of land surface schemes Q* H E PSN ESN Accumulation G M • Energy balance equation K(1 – a) + L – L + E + H = G • Water balance equation W/t = P – E – Rs – D S/t = Psn – Esn – M P E Rs Infiltration D HTESSEL parameterization

  3. Soil hydrology • Top: F [kg/m2s] = T – Esoil – Rs + M • Bottom (free drainage) F = Rd = wK • with • T = throughfall (Pl – Eint – Wl/t) • Esoil = bare ground evaporation • Eint = evaporation from interception reservoir • Rs = surface runoff • Rd = deep runoff (drainage) • M = snow melt • Pl = liquid precipitation • Wl = interception reservoir depth • S = root extraction Pl Eint T Wl Esoil M Rs S Rd HTESSEL parameterization

  4. Soil heat flux • Multi-layer scheme • Solution of diffusion equation • with • C [J/m3K] = volumetric heat capacity • T [W/mK] = thermal diffusivity • with boundary conditions • G [W/m2] at top • zero flux at bottom HTESSEL parameterization

  5. Main sections • Surface tiling • Surface energy balance & vegetation • Soil heat transfer • Soil hydrology • Snow hydrology & albedo • Surface characteristics (“climate fields”) HTESSEL parameterization

  6. Tile structure of HTESSEL • 6 fractions (“tiles”) • Aerodynamic coupling • Vegetatie • Verdampingsweerstand • Wortelzone • Neerslaginterceptie • Kale grond • Sneeuw HTESSEL parameterization

  7. Tile structure of HTESSEL • 6 fractions (“tiles”) • Aerodynamic coupling • Wind speed • Roughness • Atmospheric stability • Vegetatie • Verdampingsweerstand • Wortelzone • Neerslaginterceptie • Kale grond • Sneeuw HTESSEL parameterization

  8. Tile structure of HTESSEL • 6 fractions (“tiles”) • Aerodynamic coupling • Wind speed • Roughness • Atmospheric stability • Vegetation • Canopy resistance • Root zone • Interception • Kale grond • Sneeuw HTESSEL parameterization

  9. Tile structure of HTESSEL • 6 fractions (“tiles”) • Aerodynamic coupling • Wind speed • Roughness • Atmospheric stability • Vegetation • Canopy resistance • Root zone • Interception • Bare ground • Sneeuw HTESSEL parameterization

  10. Tile structure of HTESSEL • 6 fractions (“tiles”) • Aerodynamic coupling • Wind speed • Roughness • Atmospheric stability • Vegetation • Canopy resistance • Root zone • Interception • Bare ground • Snow HTESSEL parameterization

  11. Tile fractions (calculated every time step) • 3 ‘static’ tiles • high vegetation • low vegetation • bare ground • 3 ‘dynamic’ tiles • interception reservoir • snow low/bare • snow forest HTESSEL parameterization

  12. Parameterization of surface energy balance and evaporation HTESSEL parameterization

  13. Aerodynamic exchange • Turbulent fluxes are parameterized as (for each tile): • Solution of CH requires iteration: • CH = f(L) • L = f(H) • H = f(CH) L = Monin-Obukhov length HTESSEL parameterization

  14. Treatment of tiled evaporation • Potential evaporation (P): a = s = CHU = 1/raH • Transpiration (T) a = s = 1/(raH + rc) • Combined snow tile (S) T P T S T P HTESSEL parameterization

  15. More on the canopy resistance • Active regulation of evaporation via stomatal aperture • Empirical (Jarvis-Stewart) approach: rc = (rc,min/LAI) f(K) f(D) f(W) HTESSEL parameterization

  16. Jarvis-Stewart functions • Shortwave radiation: • Atmospheric humidity deficit (D): f3 = exp(-cD) (c  0 for forest only) HTESSEL parameterization

  17. Jarvis-Stewart functions • Soil moisture ( = weighted mean liquid water over root profile): • Standard approach: linear profile 1 HTESSEL parameterization

  18. Specification of vegetation types HTESSEL parameterization

  19. Soil heat flux HTESSEL parameterization

  20. Numerical solution • Solution of energy balance equation • With (all fluxes positive downward) • Express all components in terms of Tsk (with Tp = Tskt -1) netradiation sensible heat flux latent heat flux soil heat flux HTESSEL parameterization

  21. Numerical solution • Substitute linear expressions of Tsk into energy balance equation • Sort all terms with Tsk on lhs of equation • Find Tsk = f(Tp , Tsoil , CH ,forcing, coefficients) HTESSEL parameterization

  22. Soil heat transfer HTESSEL parameterization

  23. Heat transport in soil • Multi-layer scheme • Solution of diffusion equation • with • C [J/m3K] = volumetric heat capacity • T [W/mK] = thermal diffusivity • with boundary conditions tiled soil heat flux direct absorption snow base heat flux HTESSEL parameterization

  24. Heat capacity and thermal diffusivity • Heat capacity • sCs  2 MJ/m3K, wCw  4.2 MJ/m3K • Thermal diffusivity depends on soil moisture • dry: ~0.2 W/mK; wet: ~1.5 W/mK HTESSEL parameterization

  25. Freezing of soil water • In case of melt/freezing, and extra heat capacity term is added: • The ice fraction is a diagnostic variable: fixed value, to decouple water and temperature eqs HTESSEL parameterization

  26. Parameterization of soil hydrology HTESSEL parameterization

  27. Soil water flow • Water flows when work is acting on it • gravity: W = mgz • acceleration: W = 0.5 mv2 • pressure gradient: W = m  dp/ = mp/ • Fluid potential (mechanical energy / unit mass) • = gz + 0.5 v2 + p/ p = gz •  g(z+z) = gh • h = /g = hydraulic head = energy / unit weight = • elevation head (z) + • velocity head (0.5 v2/g) + • pressure head ( = z = p/g) HTESSEL parameterization

  28. Relation between pressure head and volumetric soil moisture content strong adhesy/ capillary forces dewatering from large to small pores retention curve HTESSEL parameterization

  29. Darcy and Richards equation qz = flux HTESSEL parameterization

  30. Darcy and Richards equation  = vol. soil moisture content (m3/m3) K = hydraulic conductivity (m/s) D = hydraulic diffusivity (m2/s) HTESSEL parameterization

  31. Implementation in discrete form • In (discrete) flux form: • With F specified as: root extraction diffusion term gravity term HTESSEL parameterization

  32. Parameterization of K and D • 2 ‘schools’ • Clapp & Hornberger ea • single parameter (b) • Van Genuchten ea • more parameters describing curvature better • Defined ‘critical’ soil moisture content • wilting point ( @  = -150m or -15 bar) • field capacity ( @  = -3m or -0.33 bar) HTESSEL parameterization

  33. Boundary conditions • Top: F [kg/m2s] = T – Esoil – Rs + M • Bottom (free drainage) F = Rd = wK • with • T = throughfall (Pl – Eint – Wl/t) • Esoil = bare ground evaporation • Eint = evaporation from interception reservoir • Rs = surface runoff • Rd = deep runoff (drainage) • M = snow melt • Pl = liquid precipitation • Wl = interception reservoir depth • S = root extraction Pl Eint T Wl Esoil M Rs S Rd HTESSEL parameterization

  34. Parameterization of interception • Simple budget equation • with • El = evaporation • D = dew collection • I = interception from precipitation • Points for attention: • maximum storage reservoir ~ 0.2 mm per m2 leaf/ground area • rapid process (water conservation in discrete time step needs care) • interception efficiency depends on type of precipitation (large scale precip: very efficient. convective precip: more falls off) HTESSEL parameterization

  35. Parameterization of runoff • Simple approach • Infiltration excess runoff Rs = max(0, T – Imax), Imax = K() • Difficult to generate surface runoff with large grid boxes • Explicit treatment of surface runoff • ‘Arno’ scheme Infiltration curve (dep on W and orograpy) Surface runoff HTESSEL parameterization

  36. Parameterization of snow HTESSEL parameterization

  37. Snow parameterization • Effects of snow • energy reflector • water reservoir acting as buffer • thermal insolator • Parameterization of albedo • open vegetation/bare ground • fresh snow: albedo reset to amax (0.85) • non-melting conditions: linear decrease (0.008 day-1) • melting conditions: exponential decay • (amin = 0.5, f = 0.24) • For tall vegetation: snow is under canopy • gridbox mean albedo = fixed at 0.2 HTESSEL parameterization

  38. Parameterization of snow water • Simple approach • single reservoir • with • F = snow fall • E, M = evap, melt • csn = grid box fraction with snow • Snow depth • with • sn evolving snow density (between 100 and 350 kg/m3) HTESSEL parameterization

  39. Snow energy budget • with • (C)sn = heat capacity of snow • (C)i = heat capacity of ice • GsnB = basal heat flux (T/rs) • Qsn = phase change due to melting (dependent on Tsn) HTESSEL parameterization

  40. Snow melt • Is energy used to warm the snow or to melt it? In some stage (Tsn 0C) it’s both! • Split time step into warming part and melting part • first bring Tsn to 0C, and compute how much energy is needed • if more energy available: melting occurs • if more energy is available than there is snow to melt: rest of energy goes into soil. HTESSEL parameterization

  41. Surface characteristics(surface ‘climate fields’) HTESSEL parameterization

  42. Surface climate fields • Vegetation types • Vegetation cover • Surface geopotential • Land/sea mask • oro (for runoff and for z0m(orographic part) • vegetation roughness z0m • thermal roughness z0h • monthy background (snowfree) albedo • Soil type (for hydraulic properties) HTESSEL parameterization

  43. Vegetation distribution HTESSEL parameterization

  44. Climatological albedo (static vegetation) Jan Jul HTESSEL parameterization

  45. Prognostic quantities • 4 soil temperatures • 4 soil moisture contents • interception reservoir depth • snow depth • snow albedo • snow density • snow temperature • (skin temperature) (adjusts rapidly) HTESSEL parameterization

  46. More information • Bart van den Hurk • hurkvd@knmi.nl HTESSEL parameterization

More Related