Land Surface Models for Climate Models: Description and Application

Transcription

1 Land Surface Models for Climate Models: Description and Application Gordon Bonan National Center for Atmospheric Research Boulder, Colorado Advanced Study Program Summer Colloquium National Center for Atmospheric Research Boulder, CO June 26

2 Role of land surface models in GCMs Provides the boundary conditions at the land-atmosphere interface e.g. albedo, surface temperature, surface fluxes Partitions available energy at the surface into sensible and latent heat flux components Partitions rainfall into runoff and evaporation Evaporation provides surface-atmosphere moisture flux River runoff provides freshwater input to the oceans Provides the carbon fluxes at the surface (photosynthesis, respiration) Updates state variables which affect surface fluxes e.g. snow cover, soil moisture, soil temperature, vegetation cover, leaf area index LSM cost is actually not that high ( ~1% of full coupled model)

3 Role of land surface models in GCMs The land-surface model solves (at each timestep) Surface energy balance (and other energy balances, e.g. in canopy, snow, soil) S + L = S + L + λe + H + G S, S are down(up)welling solar radiation L, L are down(up)welling longwave radiation λ is latent heat of vaporization, E is evaporation H is sensible heat flux G is ground heat flux Surface water balance (and other water balances such as snow and soil water) P = E S + E T + E C + R Surf + R Sub-Surf + SM / t P is rainfall E S is soil evaporation, E T is transpiration, E C is canopy evaporation R Surf is surface runoff, R Sub-Surf is sub-surface runoff SM / t is the change in soil moisture over a timestep Carbon balance (and plant and soil carbon pools) NPP = GPP Ra = ( C f + C s + C r ) / t NEP = NPP Rh NBP = NEP - Combustion NPP is net primary production, GPP is gross primary production Ra is autotrophic (plant) respiration, Rh is heterotrophic (soil) respiration C f, C s, C r are foliage, stem, and root carbon pools NEP is net ecosystem production, NBP is net biome production Combustion is carbon loss during fire

4 Surface energy balance and surface temperature Surface energy balance (S -S ) + εl = L [T s ] + H[T s ] + λe[t s ] + G[T s ] L = εσ T + L H = ρc 4 ( s ) + (1 ε ) p ρc λe = γ G= k ( Ta Ts) r p ah ( Ts Tsoil ) Δz ( ea e Ts ) r aw * [ ] Atmospheric forcing S - incoming solar radiation L - incoming longwave radiation T a air temperature e a vapor pressure Surface properties S - reflected solar radiation (albedo) ε - emissivity rah aerodynamic resistance (roughness length) raw aerodynamic resistance (roughness length) T soil soil temperature k thermal conductivity Δz soil depth With atmospheric forcing and surface properties specified, solve for temperature T s that balances the energy budget

5 Turbulent fluxes Logarithmic wind profile in atmosphere near surface Height above ground (m) Day 14 2 Day 12 Day Wind speed ( m s -1 ) ( z/ u ) u / z = 1/ k u2 u1 = ( u / k) ln ( z2/ z1) u( z) = ( u / k) ln ( z/ z ) * * ( ) ( ) u( z) = u / k ln z d / z * M * M Similar logarithmic profiles for temperature and vapor pressure Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

6 Turbulent fluxes τ = ρ( u u )/ r = ρu / r H = ρc z s am am p ( θa Ts) r ah r am r r ah aw 1 = 2 ln z d ψm ( ζ) ku z M 2 ln z d m( ) ln z d ψ ζ ψh( ζ) zm zh 1 = ku 2 ln z d m( ) ln z d ψ ζ ψw( ζ) zm zw 1 = ku 2 Height r am r am r ah = r am + r * b z M z H r * b u s = Wind speed u z θ z Temperature θ s Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

7 Hydrometeorology Community Land Model Biogeophysics Hydrology Direct Solar Radiation Diffuse Solar Radiation Longwave Radiation Latent Heat Flux Sensible Heat Flux Photosynthesis Momentum Flux Wind Speed u a Precipitation Interception Canopy Water Evaporation Transpiration Reflected Solar Radiation Absorbed Solar Radiation Emitted Longwave Radiation Soil Heat Flux Melt Sublimation Snow Throughfall Stemflow Evaporation Infiltration Surface Runoff Soil Water Heat Transfer Redistribution Community Land Model Land model for Community Climate System Model Developed by the CCSM Land Model Working Group in partnership with university and government laboratory collaborators Bonan et al. (22) J Climate 15: Oleson et al. (24) NCAR/TN-461+STR Dickinson et al. (26) J Climate 19: Drainage Energy fluxes: radiative transfer; turbulent fluxes (sensible, latent heat); heat storage in soil; snow melt Hydrologic cycle: interception of water by leaves; infiltration and runoff; snow accumulation and melt; multi-layer soil water; partitioning of latent heat into evaporation of intercepted water, soil evaporation, and transpiration

8 Community Land Model Dynamic vegetation μg CO 2 g -1 s -1 μg CO 2 g -1 s -1 μg CO 2 g -1 s Photosynthesis -1-2 Foliage Water Potential (MPa) PPFD (μmol m -2 s -1 ) 153 Vapor Pressure Deficit (Pa) Ecosystem carbon balance Growth Respiration 15 3 Temperature ( C) 5 1 Ambient CO 2 (ppm) 1 2 Foliage Nitrogen (%) Nutrient Uptake μg CO 2 g -1 s -1.5 Foliage μg CO 2 g -1 s Temperature ( C) Relative Rate 8 Litterfall Temperature ( C) Sapwood Temperature ( C) Relative Rate μg CO 2 g -1 s -1 Autotrophic Respiration 1 1 Soil Water (% saturation).3 Root Temperature ( C) Heterotrophic Respiration Leaf Area Index Leaf phenology Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Needleleaf evergreen tree Broadleaf deciduous tree Vegetation dynamics Bonan et al. (23) Global Change Biology 9: Levis et al. (24) NCAR/TN-459+IA Bonan & Levis (26) J Climate 19:

9 First-generation models Sensible heat Latent heat Precipitation Evaporation E = β E p β = 1 for w w β = w/w for w<w H = ρc p ( T T ) a r a ( ) ρc [ ] p ea e* Ts λe = γ r / β a s T a r a T s e a r a /β e * (T s ) Water depth, w Critical depth, w Runoff Simple energy balance model: (1-r)S + εl = L [T s ] + H[T s ] + λe[t s ] Prescribed surface albedo Bulk parameterizations of sensible and latent heat flux No influence of vegetation on surface fluxes Prescribed soil wetness factor β or calculated wetness from bucket model No soil heat storage Manabe (1969) Monthly Weather Review 97: Williamson et al. (1987) NCAR/TN-285+STR

10 Green world vs desert world Two climate model experiments Wet evapotranspiration not limited by soil water; vegetated planet Dry no evapotranspiration; desert planet July surface temperature ( C) July precipitation (mm/day) Wet soil Wet soil Dry soil Dry soil Dry soil warmer than wet soil Dry soil has less precipitation Shukla & Mintz (1982) Science 215:

11 Vegetation and hydrologic cycle Second-generation models Biosphere-Atmosphere Transfer Scheme (BATS) Simple Biosphere Model (SiB) Dickinson et al. (1986) NCAR/TN-275+STR Sellers et al. (1986) J Atmos Sci 43:55-531

12 Radiative transfer Fractional Canopy Absorption Visible Near-infrared Leaf Area Index (m 2 m -2 ) Canopy Albedo Visible Snow-Covered Ground Random leaf orientation Zenith angle = 45 Near-infrared Leaf Area Index (m 2 m -2 ) Sunlit Leaf Area Index random horizontal vertical Zenith Angle = Leaf Area Index Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

13 Plant canopy Sensible Heat T a T a T a T a r ah r ah r ah r cu r ah r s T ac r c T v T acu r acu r cl T vu T s T acl T vl T s r ac T g r acl T g Bulk Surface No Vegetation Bulk Surface With Vegetation One Vegetation Layer Two Vegetation Layers Latent Heat e a e a e a e a r aw r aw r s r aw r aw e acu r cu e vu r c e v r acu r cl e vl e s e ac e s e g e g r ac e acl r s r acl Bare Surface No Vegetation Bulk Surface With Vegetation One Vegetation Layer Two Vegetation Layers Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

14 Leaf stomatal resistance Stomatal Gas Exchange Leaf Cuticle Guard Cell High CO 2 CO 2 Dry Air H 2 O Guard Cell Photosynthetically Active Radiation Chloroplast light CO H 2 O CH 2 O + O 2 + H 2 O Low CO 2 Moist Air Stomata Open: High Light Levels Moist Leaf Warm Temperature Moist Air Moderate CO 2 High Leaf Nitrogen Stomata Close (Smaller Pore Opening): Low Light Levels Dry Leaf Cold Temperature Dry Air High CO 2 Low Leaf Nitrogen Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

15 Leaf boundary layer High CO 2 Dry Air Sensible Heat CO 2 H 2 O r b Boundary Layer Thickness: 1 to 1 mm r b Height Leaf Cuticle Guard Cell Guard Cell High Low High Wind Speed Temperature Photosynthetically Active Radiation Chloroplast r s Low CO 2 Moist Air light CO H 2 O CH 2 O + O 2 + H 2 O Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

16 Plant canopy Stomata T s Leaf Boundary Layer r b /2 T a Total Resistance Sensible Heat r b /2 Canopy r b /(2L) Atmosphere Surface Layer T s T ac T a r a Total Resistance Sensible Heat r b /(2L)+r a e i =e*(t s ) r s r b Latent Heat e s e a (r s +r b ) e i =e*(t s ) r s /L e s r b /L e ac r a e a Latent Heat (r s +r b )/L+r a c i 1.65r s c s 1.37r b c a Photosynthesis (1.65r s +1.37r b ) 1.65r s /L 1.37r b /L c i c s c a Photosynthesis (1.65r s +1.37r b )/L Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

17 Soil temperature Vertical Heat Transfer F in F k = F H I ΔT k Δz K = F H T z I K Δz Δy Δx change in storage = flux in - flux out ΔT Δt F ρ H G I c K J = ( ) F HG ρ c Δx Δy Δz Fin Fout Δx Δy F ΔT ρ H G I K J F = H G ΔF I c K J Δt Δz T I K J = F T I F k HG K J = k H G t z z 2 T z 2 I K J 5 mm 15 mm A 41 C B 37 C F F W = 15. m o C F HG = 6 W/m 2 41 o C 37 o C 1. m I KJ F out Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

18 Hydrologic cycle Biogeophysics Hydrology Direct Solar Radiation Diffuse Solar Radiation Longwave Radiation Latent Heat Flux Sensible Heat Flux Photosynthesis Momentum Flux Wind Speed u a Precipitation Interception Canopy Water Evaporation Transpiration Reflected Solar Radiation Absorbed Solar Radiation Emitted Longwave Radiation Soil Heat Flux Melt Sublimation Snow Throughfall Stemflow Evaporation Infiltration Surface Runoff Soil Water Heat Transfer Redistribution Drainage Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

19 Soil water Richards equation Vertical Water Flow F in F = k F = k L NM L NM O F H Δ( ψ + z) k z QP = Δψ z + 1 Δ Δ ( ψ + z) O F k k z QP = ψ I F H z + K = θ H z ψ 1 θ + 1 I K I K Δy Δx Δz change in storage = flux in - flux out F ΔθI HG Δt K J = Δx Δy Δz ( Fin Fout ) Δx Δy Δθ / Δt = ΔF / Δz L F I HG K J O + 1 NM QP θ = θ ψ t z k z θ F out 5 mm 55 mm A B ψ=-478 mm z=5 mm ψ=-843 mm z= mm F F mm = 2 hr L NM = 346. mm / hr ( 478 mm + 5 mm ) ( 843 mm+ mm) 5 mm O QP Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

20 Land degradation Climate feedback Overgrazing Drought Dead vegetation in drought-stricken area, Sol-Dior area, Senegal (FAO, Ch. Errath) Reduced Vegetation Cover Less Rainfall Increased Albedo Decreased Clouds And Convection Decreased Net Radiation Goat seeks food in the sparsely vegetated Sahel of Africa (US AID) Subsidence Surface Cooling Charney (1975) QJRMS 11: Charney et al. (1975) Science 187:

21 Climate model experiments Land degradation Degradation scenario - the vegetation type within the shaded area was changed to type 9 to represent degradation: less vegetation, lower LAI, smaller surface roughness length, higher albedo, sandy soil Broadleaf evergreen tree Broadleaf shrub/ground cover Broadleaf tree/ground cover Broadleaf shrub/bare soil Clark et al. (21) J Climate 14:

22 Climate impacts July-August-September precipitation differences (mm/day) due to degradation. Differences that are significant at the 95% confidence level are shaded and the degraded area is enclosed by a solid line. Land degradation July-August-September mean differences due to degradation. Values are means over the degraded area. D C is the difference between degraded and control values. Clark et al. (21) J Climate 14:

23 Tropical deforestation Settlement and deforestation surrounding Rio Branco, Brazil (1 S, 68 W) in the Brazilian state of Acre, near the border with Bolivia. The large image covers an area of 333 km x 333 km. (NASA/GSFC/LaRC/JPL) (National Geographic Society)

24 Warmer, drier tropical climate Tropical deforestation Annual response to Amazonian deforestation in various climate model studies. Δalbedo and Δz indicate the change in surface albedo and roughness due to deforestation (+, increase; -, decrease). ΔT, ΔP, and ΔET are the simulated changes in temperature, precipitation, and evapotranspiration. Shading denotes warmer, drier climate. Surface Change Climate Change Study Δalbedo Δz ΔT ( C) ΔP (mm) Dickinson and Henderson-Sellers (1988) Lean and Warrilow (1989) Nobre et al. (1991) Dickinson and Kennedy (1992) Mylne and Rowntree (1992) + unchanged Henderson-Sellers et al. (1993) Lean and Rowntree (1993) Pitman et al. (1993) Polcher and Laval (1994a) + unchanged Polcher and Laval (1994b) Sud et al. (1996) McGuffie et al. (1995) Lean and Rowntree (1997) Hahmann and Dickinson (1997) Costa and Foley (2) ΔET (mm) Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

25 Third-generation models Net Photosynthesis (μmol m -2 s -1 ) Stomatal Gas Exchange Photosynthetic Photon Flux Density (μmol m -2 s -1 ) Leaf Cuticle Photosynthetically Active Radiation Chloroplast Guard Cell CO 2 light CO H 2 O CH 2 O + O 2 + H 2 O Low CO 2 H 2 O Moist Air Guard Cell Stomata Open: High Light Levels Moist Leaf Warm Temperature Moist Air Moderate CO 2 High Leaf Nitrogen Stomatal Conductance (mmol CO 2 m -2 s -1 ) Photosynthetic Photon Flux Density (μmol m -2 s -1 ) Net Photosynthesis (μmol CO 2 m -2 s -1 ) Photosynthetic Photon Flux Density.9 Photosynthesis.8 Transpiration Stomatal Conductance (mmol CO 2 m -2 s -1 ) Transpiration (mmol H 2 O m -2 s -1 ) Bonan (1995) JGR 1: Denning et al. (1995) Nature 376: Denning et al. (1996) Tellus 48B: ,

26 Leaf stomatal resistance A ( h /1) P = gs = m + b c 1 n s r s A = min( w, w ) R w n c j d c V ( c Γ ) = c + K (1 + O / K ) max i * i c i o s w c is the rubisco-limited rate of photosynthesis, w j is light-limited rate allowed by RuBP regeneration rubisco-limited rate is Photosynthesis (μmol CO 2 m -2 s -1 ) Light Response Curve W j W c W j Photosynthetic Photon Flux Density (μmol photons m -2 s -1 ) RuBP regeneration-limited rate is w j Jc ( i Γ ) = 4( c + 2 Γ ) i * * Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

27 Canopy resistance Percent Of Full Solar Radiation 7 Oak Forest Height (meters) Height (meters) LAI Radiation Leaf Area Index Cumulative Leaf Area Index Bonan (28) Ecological Climatology. 2nd edition (Cambridge Univ. Press)

28 CO 2 fertilization and stomatal conductance Leaf photosynthesis and conductance response to atmospheric CO 2 concentration, light-saturated (a) Dependence of leaf-scale photosynthesis for C 3 and C 4 vegetation on external CO 2 concentration (b) The C 3 photosynthesis curves for unadjusted (C and P) and down-regulated (PV) physiology (c) Dependence of stomatal conductance on CO 2 concentration for the unadjusted and downregulated cases. Photosynthesis increases and stomatal conductance decreases with higher atmospheric CO 2 Bounoua et al. (1999) J Climate 12:39-324

29 CO 2 fertilization and stomatal conductance CO 2 fertilization (RP, RPV) reduces canopy conductance and increases temperature compared with radiative CO 2 (R) Amazonian evergreen forest, diurnal cycle January Canadian evergreen forest, diurnal cycle July Bounoua et al. (1999) J Climate 12:39-324

30 CO 2 fertilization and stomatal conductance Global climate: Reduced conductance Reduced evaporation Reduced precipitation Warmer temperature Bounoua et al. (1999) J Climate 12:39-324

31 Dynamic vegetation Fourth-generation of models μg CO 2 g -1 s -1 μg CO 2 g -1 s -1 μg CO 2 g -1 s Photosynthesis -1-2 Foliage Water Potential (MPa) PPFD (μmol m -2 s -1 ) 153 Vapor Pressure Deficit (Pa) Ecosystem carbon balance Growth Respiration 15 3 Temperature ( C) 5 1 Ambient CO 2 (ppm) 1 2 Foliage Nitrogen (%) Nutrient Uptake μg CO 2 g -1 s -1.5 Foliage μg CO 2 g -1 s Temperature ( C) Relative Rate 8 Litterfall Temperature ( C) Sapwood Temperature ( C) Relative Rate μg CO 2 g -1 s -1 Autotrophic Respiration 1 1 Soil Water (% saturation).3 Root Temperature ( C) Heterotrophic Respiration Leaf Area Index Leaf phenology Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Needleleaf evergreen tree Broadleaf deciduous tree Vegetation dynamics Foley et al. (1996) GBC 1: Levis et al. (1999) JGR 14D: Levis et al. (2) J Climate 13: Cox et al. (2) Nature 48:

32 BIOGEOPHYSICS radiative transfer ATMOSPHERE T, u,v, q, P S, L, (CO 2 ) canopy physics energy balance temperature λ E, H, τ x,τ y, S, L, (CO 2 ) aerodynamics water balance Greening of a land surface model plant functional type (presence, extent) height plant carbon litter and soil carbon VEGETATION DYNAMICS allocation turnover leaves leaf litter stems sapwood to roots heartwood seeds root litter ecophysiology mortality growth efficiency bioclimatology frost tolerance heat stress canopy physiology photosynthesis (GPP) stomatal conductance soil/snow/ice physics energy temperature water balance balance BIOGEOCHEMISTRY autotrophic respiration (R A ) maintenance foliage,stem,root growth heterotrophic respiration (R H ) litter carbon soil carbon 2-minutes GPP soil water leaf temperature soil temperature net primary production GPP-R A Net CO 2 GPP-R A -R H daily leaf area index PHENOLOGY summer green rain green DAILY STATISTICS phenology 1-day mean temperature 1-day mean photosynthesis growing degree-day accumulation fire probability Daily maximum leaf area index litter soil soil organic matter fire occurrence moisture fuel load mortality fire resistance combustion plant, litter ANNUAL STATISTICS fire season length net primary production GPP and potential GPP establishment potential rate canopy gap bioclimatology frost tolerance heat stress winter chilling growing season warmth low precipitation bioclimatology minimum monthly temperature (2-year mean) growing degree-days above 5 C (2-year mean) precipitation growing degree-days above heat stress Yearly competition aboveground space Bonan et al. (23) Global Change Biology 9:

33 Model validation tower fluxes Boreal Ecosystem Atmosphere Study (BOREAS) Model Tower Observations Bonan et al. (1997) J Geophys Res 12D:

34 Simulated Leaf Area Index Three types of phenology Evergreen Raingreen Summergreen Bonan et al. (23) Global Change Biology 9:

35 Model validation global net primary production Annual net primary production (g C m -2 yr -1 ) Vegetation Type Simulated Observed Tropical broadleaf evergreen forest ±9 Tropical broadleaf deciduous forest ±475 Temperate broadleaf deciduous forest 579 6±325 Boreal deciduous forest ±2 Boreal needleleaf evergreen forest ±2 Temperate/boreal mixed forest ±275 Grassland ±475 Tundra ±2 Bonan et al. (23) Global Change Biology 9:

36 Vegetation dynamics Boreal forest succession Global biogeography Bonan et al. (23) Global Change Biology 9:

37 Greening of North Africa Climate 6 years BP Increased Northern Hemisphere summer solar radiation Strengthened African monsoon Wetter North African climate allowed vegetation to expand Two climate model experiments Desert North Africa Green North Africa Kutzbach et al. (1996) Nature 384: Climate model experiments show: Strengthened monsoon due to radiative forcing Vegetation forcing similar in magnitude to radiative forcing

38 Greening of North Africa Present Day Biogeography (percent of grid cell) 6kaBP DynVeg Soil Texture kabp Precipitation Change From Present Day Dominant forcing Increase in evaporation Decrease in soil albedo Albedo Vegetation and soil Orbital geometry Levis et al. (24) Climate Dynamics 23:791-82

39 Effect of boreal forests on climate Vegetation masking of snow albedo Maximum satellite-derived surface albedo during winter Robinson & Kukla (1985) J Climate Appl Meteor 24: Tree-covered land has a lower albedo during winter than snow-covered land Colorado Rocky Mountains

40 Effect of boreal forests on climate Climate model simulations show boreal forest warms climate Climate Model Simulations: Forested - Deforested Temperature Difference ( C) January April July October Latitude (degrees N) Forest warms climate by decreasing surface albedo Warming is greatest in spring but is year-round Warming extends south of boreal forest (about 45 N) Bonan et al. (1992) Nature 359:

41 Effect of boreal forests on climate Boreal forest expansion with 2 CO 2 warms climate Mean annual temperature (2 CO 2 ) Dominant forcing Decrease in albedo [Carbon storage could mitigate warming] Additional temperature change with vegetation Bonan & Levis, unpublished

42 Land cover change as a climate forcing

43 Land cover change as a climate forcing Future IPCC SRES Land Cover Scenarios for NCAR LSM/PCM Forcing arises from changes in Community composition Leaf area Height [surface roughness] Surface albedo Turbulent fluxes Hydrologic cycle Also alters carbon pools and fluxes, but most studies of land cover change have considered only biogeophysical processes Feddema et al. (25) Science 31:

44 Land use climate forcing SRES B1 SRES A Dominant forcing Brazil albedo, ET U.S. albedo Asia - albedo PCM/NCAR LSM transient climate simulations with changing land cover. Figures show the effect of land cover on temperature (SRES land cover + SRES atmospheric forcing) - SRES atmospheric forcing Feddema et al. (25) Science 31:

45 Carbon cycle feedback Three climate model simulations to isolate the climate/carbon-cycle feedbacks Prescribed CO 2 and fixed vegetation (a 'standard' GCM climate change simulation) Interactive CO 2 and dynamic vegetation but no effect of CO 2 on climate (no climate/carbon cycle feedback) Fully coupled climate/carbon-cycle simulation (climate/carbon cycle feedback) Effect of climate/carbon-cycle feedbacks on CO 2 increase and global warming Prescribed CO 2 and fixed vegetation Interactive CO 2 and vegetation, no climate change Fully coupled Carbon budgets for the fully coupled simulation Cox et al. (2) Nature 48: