surf2

Layout : Introduction General remarks Model development and validation The surface energy budget The surface water budget Soil heat transfer Soil water transfer Snow Initial conditions Conclusions and a look ahead Layout


Thermal budget of a ground layer at the surface : Thermal budget of a ground layer at the surface


Energy budget: Summer examples : Energy budget: Summer examples Arya, 1988


The surface radiation : The surface radiation In some cases (snow, sea ice, dense canopies) the impinging solar radiations penetrates the “ground” layer and is absorbed at a variable depth. In those cases, an extinction coefficient is needed. Arya, 1988


The other terms : The other terms


Recap: The surface energy equation : Recap: The surface energy equation Equation for For: a thin soil layer at the top G (Ts,Tsk) is known, or parameterized or G << Rn we have a non-linear equation defining the skin temperature


Tiles : Tiles


TESSEL geographic characteristics : TESSEL geographic characteristics


High vegetation fraction at T511 (now at T799) : High vegetation fraction at T511 (now at T799) Aggregated from GLCC 1km


Low vegetation fraction at T511 (now at T799) : Low vegetation fraction at T511 (now at T799) Aggregated from GLCC 1km


High vegetation type at T511 (now at T799) : High vegetation type at T511 (now at T799) Aggregated from GLCC 1km


Low vegetation type at T511 (now at T799) : Low vegetation type at T511 (now at T799) Aggregated from GLCC 1km


Layout : Introduction General remarks Model development and validation The surface energy budget The surface water budget Soil heat transfer Soil water transfer Snow Initial conditions Conclusions and a look ahead Layout


Evaporation: Idealized surfaces : Evaporation: Idealized surfaces


Potential evaporation, bucket model : Potential evaporation, bucket model Potential evaporation The evaporation of a large uniform surface, sufficiently moist or wet (the air in contact to it is fully saturated)


A general, algebraic formulation : A general, algebraic formulation Two limit behaviours Bare soil: Evaporation dependent on soil water (and trapped water vapor) in a top shallow layer of soil (~ 20 mm). Vegetated surfaces: Evaporation controlled by a canopy resistance, dependent on shortwave radiation, water on the root zone (~ 1-5 m deep) and other physical/physiological effects.


Transpiration: The big leaf approximation : Transpiration: The big leaf approximation Sensible heat (H), the resistance formulation Evaporation (E), the resistance formulation (the big leaf approximation, Deardorff 1978, Monteith 1965)


Some plant science : Some plant science rs, the stomatal resistance of a single leaf. Physiological control of water loss by the vegetation. Stomata (valve-like openings) regulate the outflow of water vapour (assumed to be saturated in the stomata cells) and the intake of CO2 from photosynthesis. The energy required for the opening is provided by radiation (Photosynthetically Active Radiation, PAR). In many environments the system appears to be operate in such a way to maximize the CO2 intake for a minimum water vapour loss. When soil moisture is scarce the stomatal apertures close to prevent wilt and dessication of the plant.


Jarvis approach(1) : Jarvis approach(1) Dickinson et al 1991


Jarvis approach(2) : Jarvis approach(2) Shuttleworth 1993


Bare ground evaporation : Bare ground evaporation Soil (bare ground) evaporation is due to: Molecular diffusion from the water in the pores of the soil matrix up to the interface soil atmosphere (z0q) Laminar and turbulent diffusion in the air between z0q and screen level height All methods are sensitive to the water in the first few cm of the soil (where the water vapour gradient is large). In very dry conditions, water vapour inside the soil becomes dominant


TESSEL transpiration : TESSEL transpiration


TESSEL root profile : TESSEL root profile


TESSEL evaporation: shaded snow : TESSEL evaporation: shaded snow Evaporation is the sum of the contribution of the snow underneath (with an additional resistance, ra,s ,to simulate the lower within canopy wind speed) and the exposed canopy. The former is dominant in early spring (frozen soils) and the latter is dominant in late spring.


TESSEL bare ground evaporation : TESSEL bare ground evaporation


Tiles : Tiles


TESSEL geographic characteristics : TESSEL geographic characteristics


Interception (1) : Interception (1) Interception layer represents the water collected by interception of precipitation and dew deposition on the canopy leaves (and stems) Interception (I) is the amount of precipitation (P) collected by the interception layer and available for “direct” (potential) evaporation. I/P ranges over 0.15-0.30 in the tropics and 0.25-0.50 in mid-latitudes. Leaf Area Index (LAI) is (projected area of leaf surface)/(surface area) 0.1 < LAI < 6 Two issues Size of the reservoir Cl, fraction of a gridbox covered by the interception layer T=P-I; Throughfall (T) is precipitation minus interception


Interception: Canopy water budget : Interception: Canopy water budget


TESSEL: interception : TESSEL: interception Interception layer for rainfall and dew deposition


Example: Deep tropics interception : Example: Deep tropics interception Viterbo and Beljaars 1995 Interception Evaporation Interception ARME, 1983-1985, Amazon forest Accumulated water fluxes


Case study: Aerodynamic resistance (1) : Case study: Aerodynamic resistance (1) Cabauw (Netherlands), is a grass covered area, where multi-year detailed boundary layer measurements have been taken Observations were used to force a stand-alone version of the surface model for 1987 The first model configuration tried had z0h=z0m


Case study: Aerodynamic resistance (2) : Case study: Aerodynamic resistance (2) Beljaars and Viterbo 1994


Case study: Aerodynamic resistance (3) : Case study: Aerodynamic resistance (3)


Case study: Aerodynamic resistance (4) : Case study: Aerodynamic resistance (4)


Case study: Aerodynamic resistance (5) : Case study: Aerodynamic resistance (5)


Runoff and infiltration : Runoff and infiltration Infiltration is that part of the precipitation flux that contributes to wet the soil Runoff occurs in Parts of the watershed where hydraulic conductivities are lowest (Horton mechanism, in upslope areas) Parts of the watershed where the water table is shallowest (Dunne mechanism, in near channel wetlands) Runoff depends on orography, nature and moisture state of the soil, precipitation intensity, and sub-grid scale effects Runoff in NWP/climate models should be called runoff generation (upon application of a routing algorithm becomes “runoff”)


TESSEL: runoff generation : TESSEL: runoff generation Surface runoff is based on a maximum infiltration limit concept, but with no sub-grid scale variability of either the precipitation flux or the top soil water content. Physically, it is the Hortonian concept, but applied at the wrong spatial scales (the model grid-box) Deep runoff is free drainage at the bottom All computations are performed with soil liquid water only. The frozen fraction of the surface is impervious to vertical water fluxes