Ninth International Conference on Permafrost ... - IARC Research

The Sensitivity of a Model Projection of Near-Surface Permafrost Degradation toSoil Column Depth and Representation of Soil Organic MatterDavid M. LawrenceNational Center for Atmospheric Research, Boulder, CO, USAAndrew G. SlaterCooperative Institute for Research in the Environmental Sciences, Boulder, CO, USAVladimir RomanovskyUniversity of Alaska Fairbanks, AK, USADmitry NicolskyUniversity of Alaska Fairbanks, AK, USAIntroductionCoupled global climate models (GCMs) are advancing tothe point that many of the biogeophysical, biogeochemical,and hydrological interactions and feedbacks that are directlyor indirectly related to permafrost degradation are or will soonbe captured. Here, we describe and analyze improvements inthe depiction of permafrost in the Community Land Model(CLM), CLM is the global land-surface scheme that isincluded in the Community Climate System Model (CCSM).These improvements to CLM represent another step towardsa more complete depiction of the integrated Arctic processesin a global modeling system.In Lawrence and Slater (2005), we presented data fromCCSM3 simulations indicating that the extent of near-surfacepermafrost may contract substantially during the 21 st centuryas arctic temperatures soar. Here, we examine the sensitivityof these near-surface permafrost degradation projections tothe incorporation of a deeper soil column and the explicittreatment of the thermal and hydrologic properties of soilorganic matter. The results presented here are excerptedfrom our recently published study (Lawrence et al. 2008).ModelCLM (for a detailed technical description see Oleson etal. 2004) can be run in both offline mode or as a componentof CCSM. The land surface is represented by fractionalcoverage of lakes, wetland, bare soil, glacier, and up to fourplant functional types (PFT) for each grid box. Processessimulated by CLM include heat transfer in soil and snow,hydrology of canopy, soil, and snow, and stomatal physiologyand photosynthesis. Fluxes of energy and moisture aremodeled independently for each surface type and aggregatedbefore being passed to the atmosphere model. CLM3includes a five-layer snow model which simulates processessuch as accumulation, melt, compaction, snow aging, andwater transfer across layers. Simulations with the standardversion of CLM are referred to as CONTROL.Organic soilNicolsky et al. (2007) show that accounting for the physicalproperties of soil organic matter significantly improves soiltemperature simulations. In Lawrence and Slater (2007), wedescribe how organic soil and its impact on soil thermal andhydraulic properties can be implemented into CLM. Briefly,a geographically distributed and profiled soil carbon densitydataset for CLM is derived by taking the gridded GlobalSoil Data Task soil carbon content dataset and distributingthe carbon content for each grid box vertically through theCLM This dataset is then used to calculate the organic soilor mixed organic and mineral soil thermal and hydrologicproperties for each soil layer. Simulations using this organicmatter dataset along with the revised parameterizations arereferred to as SOILCARB.Deep soilAlexeev et al. (2007) demonstrate that the depth of thebottom boundary condition strongly influences seasonal andlonger time-scale soil temperature dynamics. Soil depths ofgreater than 30 m are preferred to reasonably simulate theannual cycle and decadal trends of subsurface temperatures.We test CLM with soil depths ranging from 25 m to 125m by adding from 4 to 7 exponentially thicker layers tothe original 10 level soil model. Experiments with a deepsoil configuration (and organic matter) are referred to asSOILCARB_DS50 and SOILCARB_DS125.ResultsFigure 1 shows annual cycle-depth temperature plotsfor CONTROL, SOILCARB, and SOILCARB_DS50compared to observed annual cycle-depth temperatures.The broad qualitative improvements in the simulation areimmediately apparent. The active layer thickness (ALT),defined as the depth to which the soil thaws each summer,is much shallower in SOILCARB and SOILCARB_DS50,and its level is in much closer agreement with observations.Soil temperatures below the active layer are also improved,especially in SOILCARB_DS50, where the removal of thezero flux boundary at 3.5 m results in smaller and morerealistic seasonal temperature variations at depths below 2m.We then force the improved CLM with 6-hourly data froma fully coupled CCSM3 20 th and 21 st century simulation.The resulting time series of near-surface permafrost extentare shown in Figure 2 for the CONTROL, SOILCARB,SOILCARB_DS50, and SOILCARB_DS125 experiments.169