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Diagnosis of Performance of the Noah LSM Snow Model

Diagnosis of Performance of the Noah LSM Snow Model. *Ben Livneh, *D.P. Lettenmaier, and K. E. Mitchell *Dept. of Civil Engineering, University of Washington. Outline. Large-scale hydrologic model Problem Definition and Key Concepts Snow albedo Turbulent Exchange Energy Balance Issues

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Diagnosis of Performance of the Noah LSM Snow Model

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  1. Diagnosis of Performance of the Noah LSM Snow Model *Ben Livneh, *D.P. Lettenmaier, and K. E. Mitchell *Dept. of Civil Engineering, University of Washington

  2. Outline • Large-scale hydrologic model • Problem Definition and Key Concepts • Snow albedo • Turbulent Exchange • Energy Balance Issues • Study Area [Noah – VIC models] • Results & Conclusions

  3. Background – NOAH LSM • NOAH LSM: (NCEP, OSU, Air Force, Hydrologic research lab) • Land scheme for suite of weather and atmospheric models (NOAA). • Partitioning of radiation (H, LE) • Characterizes soil moisture, snow cover, etc… • Focus: snow cover very important to both land and atmosphere.

  4. Problem: Excessive Snow Ablation • SEB = Rnet + H + LE + G + F • Numerous authors identified an excessive ablation (*melt + sublimation) problem • Chose to focus on: • Snow Albedo • Turbulent Exchange • Energy Balance Closure Noah SWE (mm) Observed SWE (mm) Mitchell et al, 2004

  5. Snow Albedo • % of reflected incident radiation • Determined mostly by character of snow surface • Effectively changes over time • *Noah presently uses fixed value (satellite) • *Problem: doesn’t capture the evolving amount of melt energy available Corps of Engineers, 1956; CA DMSP satellite image, 1979

  6. As used in VIC: Snow Albedo Decay Scheme • Maximum snow albedo: αmax = 0.85 • Daily value: • Accumulation season: • Melt Season: * t-days since last snowfall

  7. Turbulent Exchange • Highly efficient means for the transfer of temperature, vapor, and momentum • Important features: -surface roughness: hvegetation -air stability *Indicators of instability are: 1. Richardson#: 2. Monin-Obhukov stability length ratio : *Noah uses combined Monin-Obhukov stability correction method for Ch

  8. Noah: Surface Exchange and Stability Parameterization • Complex, iterative parameterization for combined surface exchange and stability! • Agglomeration of numerous components of boundary layer theory, runaway feedbacks requiring superimposed bounding. Surface Exchange Coeff. Temperature Roughness Length (Zilitenkovich (1995) Monin-Obukkhov Similarity Theory Friction Velocity Paulson (1970) functions, Monin-Obukkhov similarity, to derive height for surface exchange and stability

  9. As used in VIC: Surface Exchange • Straightforward, non-iterative scheme, considers CH and stability separately. • Surface Exchange based on Aerodynamic Resistance: Ra=1/CH (DHSVM: Soer, 1980) • Yields more realistic snow ablation: - timing and amount; - no run-away feedback.

  10. As used in VIC: Stability Correction • Non-neutral conditions, account for either enhanced or restricted heat transfer • Applied to aerodynamic resistance, Ra • Atmospheric instability (turbulence) Critical Richardson #: Stable (Ri > 0): Unstable (Ri < 0):

  11. Study areas • SNOTEL stations (AK, CA, CO, WA) *Daily: Tmin, Tmax, Pacc, SWE • Verify modifications in different snow types (alpine, maritime, low and high latitude, etc…) • SWE used as comparison tool • Panarctic (gridded data)

  12. Pan-arctic • Investigation of snow cover extents (Apr 1, 1999) • Noah slightly under-predicted snow cover, based on vegetation specific threshold (stability adj.) Noah Satellite Noah

  13. Conclusions • Major improvement seen with albedo decay scheme especially lower latitude sites… • Energy balance issues create further ablation problems, requiring additional analysis… • Pan-arctic simulations provide a reasonable estimation of snow cover extents, especially with an alternate stability correction.

  14. Thank you

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