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

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

Problem: Excessive Snow Ablation SEB = R net + H + LE + G + F SEB = R net + H + LE + G + F Numerous authors identified an excessive ablation (*melt + sublimation) problem Numerous authors identified an excessive ablation (*melt + sublimation) problem Chose to focus on: 1.Snow Albedo 2.Turbulent Exchange 3. Energy Balance Closure Observed SWE (mm) Noah SWE (mm) Mitchell et al, 2004

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

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

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

Noah: Surface Exchange and Stability Parameterization Complex, iterative parameterization for combined surface exchange and stability! Complex, iterative parameterization for combined surface exchange and stability! Agglomeration of numerous components of boundary layer theory, runaway feedbacks requiring superimposed bounding. 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

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

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

Study areas SNOTEL stations (AK, CA, CO, WA) SNOTEL stations (AK, CA, CO, WA) *Daily: T min, T max, P acc, SWE Verify modifications in different snow types (alpine, maritime, low and high latitude, etc…) Verify modifications in different snow types (alpine, maritime, low and high latitude, etc…) SWE used as comparison tool SWE used as comparison tool Panarctic (gridded data) Panarctic (gridded data)

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

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

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