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Improving the Snow Processes in the NCEP Noah Land Model

Xubin Zeng, Zhuo Wang, and Mark Decker Department of Atmospheric Sciences University of Arizona xubin@atmo.arizona.edu. Improving the Snow Processes in the NCEP Noah Land Model. JCSDA Annual Meeting, May 2009. Progress.

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Improving the Snow Processes in the NCEP Noah Land Model

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  1. Xubin Zeng, Zhuo Wang, and Mark Decker Department of Atmospheric Sciences University of Arizona xubin@atmo.arizona.edu Improving the Snow Processes in the NCEP Noah Land Model JCSDA Annual Meeting, May 2009

  2. Progress (1) Land skin temperature over semiarid regions (Mike Ek’s talk): ----A successful university-JCSDA-EMC partnership in reducing forecasting bias and increasing the use of satellite data (2) Intercomparison of snow albedo formulations from NCEP, NCAR, and ECMWF (Wang and Zeng 2009)‏ (3) Revised form of the soil moisture Richards equation (Zeng & Decker 2009; Decker & Zeng 2009)‏ (4) Evaluation of the CRTM emissivity model over snow covered forest and short vegetation sites (interaction with Fuzhong Weng’s Team at JCSDA and Mike Ek’s Team at EMC)‏ (5) Improving the snow treatment in Noah (This talk): ----identified a significant bias‏ ----did a detailed diagnostics and understood the processes ----came up with the preliminary solution and testing

  3. Major snow deficiencies of Noah over forest areas • Snowmelt too early • Abrupt drop in snow depth • Small fraction of snow difficult to melt in spring • Downward SH too large for some days in mid-winter • Later winter-early spring LH (primarily sublimation) too large Top panel (snow depth): Noah Middle panel: Rnet LH SH Bottom panel: LH SH

  4. ECMWF example a constant albedo (-0.2) for snow under trees; significantly reduced T2m cold bias; significantly reduced cold bias at 850 mb Viterbo and Betts (1999)‏ Control New

  5. Overall reasons ----single combined temperature of ground, vegetation, & snow ----vegetation shading effect on underlying snow sublimation & melt not considered (while effect on albedo considered)‏ Potential specific reasons: ----snow albedo ----snow aging ----snow albedo dependence on solar zenith angle ----snow fraction ----roughness over snow-covered surface ----turbulent exchange coefficient ----water-holding capacity of snow ----turbulence scheme fails to converge ----vegetation shading effect on underlying snow sublimation & melt ----roughness over snow-covered surface with correct asympototic behaviors for short and tall vegetation ----snow density computation near melting point ----snow sublimation at small snow fraction

  6. Albedo adjustment Control: max albedo (from MODIS) of 0.34 Test: 0.52, 0.70 0.34 (control)‏ 0.52 0.70 • not much effect on early snowmelt; • negative effect on atmospheric processes (e.g., ECMWF); • not included in our revisions

  7. Turbulence scheme fails to converge Control: 5 iterations ( )‏ New: 30 iterations ( )‏

  8. Our revisions Control + turbulence convergence + Zo convergence Zo = f(Zo,sn, Zo,v, non-buried GVF)‏ + shading effect on snow melt and sublimation (Primary reason)‏ - - - + snow density near 0oC using T1 and Tsoil + sublimation at small fsn using max(0.3,fsn)‏

  9. New version with all revisions (1) Con New

  10. New version with all revisions (2) Con New

  11. How about validation over other forest sites? • Assuming no additional revision, if the new version is directly applied to short vegetation with snow, what do you expect? • better, • worse, • no change, or • don’t know

  12. Over grass (Valdai, Russia)‏ Con New

  13. Can we solve the snow problem in Noah by using more complicated structures (e.g., separate canopy layer, multiple snow layers, …)? • Theoretically, it is possible because the • relevant physics is explicitly considered • In practice, following Gershwin’s tune: • “it ain’t necessarily so”

  14. NCAR CLM as an example • Snow burial fraction (Wang & Zeng 2009)‏ • Ground evaporation (Sakaguchi&Zeng 09)‏ • Zom convergence (Zeng & Wang 2007)‏ • Revised form of the soil moisture Richards equation (Zeng & Decker 2009; Decker & Zeng 2009)‏ • Shrub submodel in the dynamic global vegetation model (Zeng et al. 2008)‏ Noah-Con Noah-New CLM3.5 CLM3.0 snow burial fraction

  15. Summary and future work made preliminary but significant progress in solving the snow problem in Noah: largely removed the early snowmelt; improved the sensible and latent heat fluxes that are crucial for land-atmosphere coupling; and maintained existing code structure and introduced no prognostic variables so that operational implementation is relatively easy will work with EMC Land Team and CRTM emissivity model Team and other partners to further evaluate and refine the revisions; to assess the (expected significant) effect on weather forecasting; to address its effect on satellite data assimilation

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