Scientific Theme: Advanced Modeling and Observing Systems

Land Surface Hydrology and Global Climate
Tom Chase
Complementary Research: Faculty Fellows Research
In recent climate sensitivity experiments with the Community Climate System Model
(CCSM 3.0) we found that the Community Land Model (CLM 3.0) simulates mean global
evapo-transpiration with low contributions from transpiration (15%), and high
contributions from soil and canopy evaporation (47% and 38%). This evapo-transpiration
partitioning is inconsistent with the average of other land-surface models used in GCMs.
To understand the high soil and canopy evaporation, and the low transpiration observed in
the CLM 3.0, we compared select individual components of the land-surface
parameterizations of the Simple Biosphere Model (SiB 2.0) against the equivalent
parameterizations used in CLM 3.0. We also compared alternative parameterizations from literature that simulated
the same processes that control transpiration, canopy and soil evaporation, and soil hydrology in both models. The
behavior of the CLM 3.0, SiB 2.0 and alternative parameterizations were assessed with each other through idealized
off-line simulations performed over a range of soil moisture, radiation and atmospheric conditions. We used the
findings of these investigations to develop new parameterizations for CLM 3.0 that would reproduce the functional
dynamics of land-surface processes found in SiB 2.0 and other alternative land surface parameterizations. We
performed global climate sensitivity experiments with the new land-surface parameterizations to assess how the new
SiB 2.0-consistent CLM land surface parameterizations impact the surface energy balance, hydrology and
atmospheric fluxes in CLM 3.0, and the larger scale climate modeled in CCSM 3.0. The new parameterizations
enable CLM to simulate evapo-transpiration partitioning consistently with the multi-model average of other land
surface models used in GCMs as evaluated by Dirmeyer et al. [2005]. The changes in surface fluxes also resulted in
a number of improvements in the simulation of precipitation and near surface air temperature in CCSM 3.0,
although substantial biases remain. The improvements in evapo-transpiration partition however, do provide a
substantially more robust framework for performing land cover change experiments in CLM and CCSM than the
existing CLM 3.0 parameterizations. Temperature maps of climate anomalies associated with the changed land
surface hydrology are shown below. A major result is that correcting the land surface hydrology resulted in an
annually, globally averaged change in near-surface temperature of 0.55 degrees, or approximately the observed
change in surface temperature over the last century. This is an indication that climate change cannot be adequately
simulated without a reasonable hydrologic cycle ― something which remains elusive.
Changes in near surface temperature due to changes in land-surface hydrology.
Left panel: Dec., Jan., Feb. average. Right panel: Jun., Jul., Aug. average.
Note that errors in land surface hydrology were responsible for
larger climate changes than observed in recent years.
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