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An Overview of the Noah-Distributed Land Surface Model

An Overview of the Noah-Distributed Land Surface Model

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An Overview of the Noah-Distributed Land Surface Model

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  1. An Overview of the Noah-Distributed Land Surface Model David J. Gochis, Wei Yu, Fei Chen, Kevin Manning WRF Land Surface Modeling Workshop Sep. 13, 2005

  2. Brief Rationale for Noah-Distributed • Standard land surface parameterizations characterize exchanges of radiation, heat, mass and momentum between the land and atmosphere • Historically, treatment of terrestrial hydrology has been simplified 1-d formulations • With high resolution implementations/applications there is now a need to explicitly account for enhanced hydrological processes: Violating early assumptions… 1. Surface runoff can not assumed to be “captured” by a stream channel 2. Lateral transfers from one cell may form significant input to adjacent cell

  3. Brief Rationale for Noah-Distributed • Standard land surface parameterizations characterize exchanges of radiation, heat, mass and momentum between the land and atmosphere • Historically, treatment of terrestrial hydrology has been simplified 1-d formulations • With high resolution implementations/applications there is now a need to explicitly account for enhanced hydrological processes: • Higher resolution capabilities  land surface heterogeneity • Earth systems-biogeochemcial cycling • Mitigation of “high-impact” weather events (e.g. floods)

  4. Outline • Brief overview of the Noah LSM • Noah-distributed core features • Implementation of Noah-distributed into the NCAR/HRLDAS framework • Ongoing and planned upgrades to Noah-distributed

  5. Community Noah Land Surface Model – Recent Enhancements • Recent Enhancement of the Community Noah LSM (released in WRF V2.0, May 2004) ‘Noah-Unified’ • “Nearly”-identical implementations of Noah LSM development effort: NCAR, NCEP, U.S. Air Force Weather Agency, NASA, university community • Fully modularized, F90 code conventions • Seasonal surface emissivity • surface emissivity is introduced as function of landuse • Added surface emissivity in surface energy balance equation for both snow and non-snow surfaces • Urban model improvements (the simple approach) such as • Large roughness length • Low surface albedo • Large thermal capacity and thermal conductivity

  6. IF (Surface Head > Retention Depth)  Route Water as Overland Flow 2-Dimensional Diffusive Wave Overland Flow Routing Ogden, 1997 Overland Flow Processes in Noah-Router (NCAR Tech Note: Gochis and Chen, 2003) • New Parameters: retention depth, surface roughness • Ponded water in excess of retention depth subject to overland flow • Overland flow: fully-unsteady, explicit, finite-difference, 2-dimensional diffusive wave (generally applicable to length scales < 1km)

  7. Surface Runoff  Surface Head Direct Evaporation Re-infiltration Ponded Water Evaporation and Re-infiltration Dynamic modeling of land-surface hydrology with ‘Noah-Router’: Ponded Water Processes (NCAR Tech Note: Gochis and Chen) • New Parameters: None • Currently no formulation for partial area coverage • Ponded water consists of: residual of ‘infiltration excess’ from previous time step and routed surface water • Direct evaporation of ponded water reduces potential evaporation (no adj. for temp/albedo) • Ponded water not evaporated is subject to infiltration Issue: May need to revise infiltration formulation when using routed runoff to calibrate: Surface runoff can not assumed to be “captured” by a stream channel

  8. Subsurface Flow Routing Noah-Router (NCAR Tech Note: Gochis and Chen) • New Parameters: Lateral Ksat, n – exponential decay coefficient • Critical initialization value: water table depth • 8-layer soil model (2m – depth, sealed bottom boundary) • Quasi steady-state saturated flow model, 2-d (x-,y-configuration) • Exfiltration from fully-saturated soil columns Surface Exfiltration from Saturated Soil Columns Lateral Flow from Saturated Soil Layers Saturated Subsurface Routing Wigmosta et. al, 1994

  9. Noah-Distributed Core Features • Present issues in treatment of subsurface routing: • Frozen soil adjustment to soil water • Remove soil ice from total soil moisture and route only liquid component • Update of conductivity as a function of soil temp/fraction of frozen soil • Inclusion of variable depth soils

  10. Noah land surface model grid Routing Subgrids AGGFACTR = 4 Subgrid Routing • Noah LSM is run at a variety of grid spacings • Subsurface and overland flow routing need to be performed on a terrain grid (< 1 km) • Required fields are aggregated/disaggregated using a simple averaging scheme • Soil water, infiltration excess, routing parameters • Can offer significant computational savings compared to full resolution implementations of Noah LSM • Sacrifice detail in current formulation

  11. Noah-Distributed Core Features • Subgrid disaggregation: proposed new method carrying over weighting factors between LSM model executions • Eliminates the “loss” of distributed information between routing time-steps Noah land surface model grid Routing Subgrids

  12. Noah-Distributed Software Features • F90, up to date with recent version in HRLDAS • Routing routines (1-d and 2-d) are contained within a single module (all agg./disagg. Routines will be included into routing module) • Routing and sub-grid options are switch-activated though a namelist file • Options to output sub-grid state and flux fields to WRF consistent netcdf files • Basic Flow: LSM > Disagg. > Subsfc > Overland > Agg. > LSM > …

  13. Outline • Brief overview of the Noah LSM • Noah-distributed core features • Implementation of Noah-distributed into the NCAR/HRLDAS framework • Ongoing and planned upgrades to Noah-distributed

  14. NCAR-HRLDAS (High Res. Land Data Assim. System) • Rationale and basics of HRLDAS: • Create globally-deployable variable resolution equilibrated land surface conditions for NWP initializations • Current static & forcing data:

  15. HRLDAS • Recent tests over International H2O Project (IHOP) domain • 18 month execution 1 Jan, 2001 – 30 Jun, 2002

  16. Top Layer Soil Moisture (fraction):

  17. Total Column Soil Moisture (mm):

  18. Total Surface Evapotranspiration (mm):

  19. Ponded Water Evaporation (mm):

  20. Outline • Brief overview of the Noah LSM • Noah-distributed core features • Implementation of Noah-distributed into the NCAR/HRLDAS framework • Ongoing and planned upgrades to Noah-distributed

  21. Future Upgrades to Noah Distributed • Improve runtime performance: • 2-d vs 1-d formulations • DEM-based steepest descent method is much faster • Strictly DEM based routing (kinematic) problematic in flat areas where change in sfc water influences flow direction (e.g. backwater) • Working on a compromise algorithm • default to DEM based routing • check for backwater • Perform search • 1-d: 129 model days/wall clock d vs. 2-d: 97 model days/wall clock day (~1/3 faster)

  22. Future Upgrades to Noah Distributed • Complete parallelization • Couple to stream channel model (via DESWAT project) • Develop better method to nudge/assimilate groundwater and river/stream stage into modeling system • Develop enhanced method to characterize stream-aquifer exchange