Principles of soil water and heat transfer in JULES

Transcription

1 Principles of soil water and heat transfer in JULES Anne Verhoef 1, Pier Luigi Vidale 2, Raquel Garcia- Gonzalez 1,2, and Marie-Estelle Demory 2 1. Soil Research Centre, Reading (UK); 2. NCAS-Climate, Reading (UK) 1

2 Overview of presentation Summary of theory Calculation of water and heat flow Parameterisation of water retention curve Parameterisation of thermal soil properties Importance of texture maps Examples of some research in this area 2

3 JULES Schematic Schematic of JULES model (Blyth et al.; 2006)

4 JULES Soil hydrology and thermodynamics Within-soil transfer R n LE G H Ψ 2 Ψ 3 T 2 T 3 Ψ 4 T 4 Layers (1-4): 0.10, 0.25, 0.65, 2.0 m W = K ψ z +1 G = λ T z Plus advective heat transfer 4

5 How does JULES simulate soil hydrology and heat transfer? M = ρ w zθ s S u + S { f } W = K ψ z +1 Darcy s law surface E n n layer W n-1 Capillary capacity W n dm n dt = W n 1 W n E n (as in JULES documentation) (standard Richard s equation) Some models consider coupled heatand water movement in the soil Diffusive flux, Fourier s law Advective flux Apparent volumetric heat capacity 5

6 Recent addition of vapourtransport.. Due to soil water potential (isothermal) and thermal gradients Isothermal vapour conductivity Thermal vapour diffusivity C A T n t = z λ n T n z + ρ w LD ψ,v,n ψ n z c W T n w z These gradients will induce soil moisture transport and affect soil moisture distribution, which in turn will affect heat flow Milly, et. 1982,

7 Water retention curve How tightly is water held in the soil? Soil matric potential, Ψ (kpa) WP CRIT Soil moisture content, θ (%)

8 How do we get Ψ and K?? ( ) ψ = ψ θ / θ b ( ) 2b+ 3 K = θ / θ s Clapp & Hornberger (1978): s K s s log Ψ (m) WP CRIT 0 Loamy sand Loam Clay 10 log K (mm day -1 ) Loamy sand Loam Clay θ 0.0 w θ 0.1 c θ s 0.5 θ (m3 m -3 ) θ (m3 m -3 ) Soil texture f sand (%) f silt (%) f clay (%) Loamy sand Loam Clay

9 How do we get Ψ and K?? Van Genuchten (1981): ( ψ = Θ 1/m 1) 1/n α K = K s 1 αψ { } m ( ) n 1 1+ ( αψ) n 1+ αψ ( ) n m/2 2 Θ = θ θ r θ s θ r m =1 1 n When using van Genuchten soil hydraulics, the UM/JULES only stores the free soil water θ-θ r and uses the approximation Θ = θ θ r θ s θ r 9

10 Soil Hydraulic parameters Clapp and Hornberger parameters Cosby et al. (1984) parameters Soil texture b θ s ψ s QC K s (-) (m 3 m -3 ) (m) (-) (cm/min) Sand Loamy sand Sandy loam Silt loam Loam Sandy clay loam Silty clay loam Clay loam Sandy clay Silty clay Clay

11 Pedotransfer functions Cosby pedotransfer functions (PTFs) based on MLRA (multiple linear regression analysis) dependent on percentages of sand, silt and clay b = f clay f sand θ s = ( f f )/ sand clay ψ s = 0.01e f sand f silt ψ s = f sand f silt K s = a e f clay f sand K s = a f clay f sand

12 Pedotransfer functions Van Genuchten pedotransfer functions (PTFs) Wosten et al. (1999) Geoderma 90,

13 Global soils distribution, effect of soil map IGBP-DIS map has a higher resolution than Wilson-Henderson- Sellers map and retains more heterogeneity in soil types. ( ) ψ = ψ θ / θ b ( ) 2b+ 3 K = θ / θ s s K s s

14 Importance of using the right PTFs θ c θ w Ψ s K s

15 JULES soil physical parameters Original table courtesy of Jon Finch 15

16 Role of key moisture contents g soil = θ1 θ c 2 Affects soil evaporation g soil (m s -1 ) sandy loam (PTF: ln) loam (PTF: ln) clay (PTF: ln) sandy loam (PTF: 10 log) loam (PTF: 10 log) clay (PTF: 10 log) 0.00 β (-) θ w θ w θ c θ c θ (m 3 m -3 ) sandy loam (PTF: ln) loam (PTF: ln) clay (PTF: ln) sandy loam (PTF: 10 log) loam (PTF: 10 log) clay (PTF: 10 log) β = Θ Θ w Θ c Θ w 0 θ (m 3 m -3 ) Affects photosynthesis, g c and transpiration A = ' A β ( θ )

17 Thermal conductivity parameterisation Original JULES model x θ λ = + θ s ( λ ) sat λ dry λdry Lu et al. model λ = ( λ λ K + λ sat dry ) e dry K = exp α e 1 θ θ s α 1. 33

18 Theoretical background Thermal soil property values of soil components at 10 C C h λ MJ m -3 K -1 W m -1 K -1 quartz clay minerals organic matter water ice (0 C) air (saturated with water vapour)

19 Thermal conductivity JULES versus Lu et al.

20 Theoretical background Standard soils: Cosby et al. (1986), thermal conductivity λ = ( λ K + λ K e = exp α 1 θ sat λdry ) e dry θ sat α 1.33 Original Jules λ 20

21 Theoretical background Standard soils: Cosby et al. (1986), heat capacity C h = ( 1 θ sat )C h,s +θc h,w Associated with phase changes C A = C s + ρ w c w θ u + ρ i c i θ f + ρ w {( c w c i )T + L f } θ u T 21

22 Timeseries comparison

23 Timeseries comparison

24 Timeseries comparison

25 HadGAM1 AGCM runs, soil temperature Average JJA Soil temperature difference (deg C) Layer 1 ( m) Layer 4 ( m) Average DJF Soil temperature difference (deg C) Layer 1 ( m) Layer 4 ( m)

26 HadGAM1 AGCM runs, soil moisture content Average JJA Soil moisture difference (mm) Layer 1 ( m) Layer 4 ( m) Average DJF Soil moisture difference (mm) Layer 1 ( m) Layer 4 ( m)

27 Reduction of UM 2m Temp cold bias UKMO R&D Technical report 528, New soil physical properties implemented in the Unified Model at PS18 by Dharssi et al.

28 Reduction of UM winter 2m Temp RMSE

29 Final remarks Other processes that require further testing/development Infiltration Parameterisation of groundwater table Within/below canopy aerodynamic transfer Soil gas transfer