David Lawrence

Project Scientist II
TIIMES - CGD
WCAS

Contact Information:
PO Box 3000, Boulder, CO 80307-3000
Office: ML 214
Telephone: 303-497-1384
Email: dlawren@ucar.edu
Home Page

Project Summary:

Click on picture to view the entire figure.

Figure 1. Mean annual cycle of components of the hydrologic cycle for CONTROLoff, VEGoff, and VEGHYDoff experiments. QSURF is surface runoff, QDRAIN is drainage. GPP is gross primary production.

Over the past year, we have had two papers published that are relevant to TIIMES research.  The first paper studies how the partitioning of evapotranspiration affects land-atmosphere interactions in a GCM.  The second describes a new parameterization for the Community Land Model that explicitly accounts for the thermal and hydrologic properties of organic soil.  More details, figures, and references are included below.

Although the global partitioning of evapotranspiration (ET) into transpiration, soil evaporation, and canopy evaporation is not well-known, most current land-surface schemes and the few available observations indicate that transpiration is the dominant component on the global scale, followed by soil evaporation and canopy evaporation.  The Community Land Model (CLM3), however, does not reflect this global view of ET partitioning with soil evaporation and canopy evaporation far outweighing transpiration.  One consequence of this unrealistic ET partitioning in CLM3 is that photosynthesis, which is linked to transpiration through stomatal conductance, is significantly underestimated on a global basis.  A number of modifications to CLM3 vegetation and soil hydrology parameterizations are described that improve ET partitioning and reduce an apparent dry soil bias in CLM3.   The modifications reduce canopy interception and evaporation, reduce soil moisture stress on transpiration, increase transpiration through a more realistic canopy integration scheme, reduce within canopy soil evaporation, eliminate lateral drainage of soil water, increase infiltration of water into the soil, and increase the vertical redistribution of soil water.  The partitioning of ET is improved, with notable increases seen in transpiration (13% to 41% of global ET) and photosynthesis (65 to 148 Pg C yr-1).  Soils are wetter and exhibit a far more distinct soil moisture annual cycle and greater interseasonal soil water storage which permits plants to sustain transpiration through the dry season.

The broader influences of improved ET partitioning on land-atmosphere interaction are diverse.  Stronger transpiration and reduced canopy evaporation yield an extended ET response to rain events and a shift in the precipitation distribution towards more frequent small to medium size rain events.   Soil moisture memory timescales decrease particularly at deeper soil levels.  Sub-surface soil moisture exerts a slightly greater influence on precipitation.  These results indicate that partitioning of ET is an important task for land surface schemes, a task that will gain in relevance as GCMs evolve to incorporate ever more complex treatments of the earth’s carbon and hydrologic cycles.

Lawrence, D.M., P.E. Thornton, K.W. Oleson, and G.B. Bonan, 2007: The partitioning of evapotranspiration into transpiration, soil evaporation, and canopy evaporation in a GCM: Impacts on land-atmosphere interaction.. J. Hydromet., 8, 862-880.

Click on picture to view the entire figure.

Figure 1. (a) Global Soil Data Task (2000) soil carbon content regridded onto CLM 2.8° x 2.8° grid. (b) Cumulative carbon storage with depth for the two major classes of soils identified in Zinke et al. (1986) used to determine vertical soil carbon distribution in new CLM soil carbon dataset. (c) Sample soil carbon profiles for Siberian peatlands (60°-70°N, 70°-80°E), Alaska (60°-70°N, 140°160°W), and Tropical Africa (Eq-10°N, 25°-35°E).

Organic matter significantly alters a soil's thermal and hydraulic properties but is not typically included in land-surface schemes used in global climate models. This omission has consequences for ground thermal and moisture regimes, particularly in the high-latitudes where soil carbon content is generally high. Global soil carbon data is used to build a geographically distributed, profiled soil carbon density dataset for the Community Land Model (CLM). CLM parameterizations for soil thermal and hydraulic properties are modified to accommodate both mineral and organic soil matter. Offline simulations including organic soil are characterized by cooler annual mean soil temperatures (up to ~2.5°C cooler for regions of high soil carbon content). Cooling is strong in summer due to modulation of early and mid-summer soil heat flux. Winter temperatures are slightly warmer as organic soils do not cool as efficiently during fall and winter. High porosity and hydraulic conductivity of organic soil leads to a wetter soil column but with comparatively low surface layer saturation levels and correspondingly low soil evaporation. When CLM is coupled to the Community Atmosphere Model, the reduced latent heat flux drives deeper boundary layers, associated reductions in low cloud fraction, and warmer summer air temperatures in the Arctic. Lastly, the insulative properties of organic soil reduce interannual soil temperature variability, but only marginally. This result suggests that, although the mean soil temperature cooling will delay the simulated date at which frozen soil begins to thaw under transient surface warming, organic matter may provide only weak insulation from surface warming.

Lawrence, D.M. and A.G. Slater, 2007: Incorporating organic soil into a global climate model. Clim. Dyn., doi:10.1007/s00382-007-0278-1.

Publications:

Lawrence, D. M., P. E. Thornton, K. W. Oleson, G. B. Bonan, 2007: The partitioning of evapotranspiration into transpiration, soil evaporation, and canopy evaporation in a GCM: Impacts on land-atmosphere interaction. J. Hydrometeorol., 8, 862-880, doi: 10.1175/JHM596.1.

Lawrence, D. M., A. G. Slater, 2007: Incorporating organic soil into a global climate model. Clim. Dyn., doi: doi:/10.1007/s00382-007-0278-1.

Nicolsky, D. J., V. E. Romanovsky, V. A. Alexeev, D. M. Lawrence, 2007: Improved modeling of permafrost dynamics in a GCM land-surface scheme. Geophys. Res. Lett., 34, L08501, doi: doi:/10.1029/2007GL029525.

Alexeev, V. A., D. J. Nicolsky, V. E. Romanovsky, D. M. Lawrence, 2007: An evaluation of deep soil configurations in the CLM3 for improved representation of permafrost. Geophys. Res. Lett., 34, L09502, doi: doi:/10.0129/2007GL029536.

Osborne, T. M., D. M. Lawrence, A. J. Challinor, J. M. Slingo, T. R. Wheeler, 2007: Development and assessment of a coupled crop-climate model. Global Change Biology, 13, 169-183.