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					 MODELING OF COLD SEASON
       PROCESSES

• Snow Ablation and Accumulation

• Frozen Ground Processes
    Snow Ablation and Accumulation
•   Snow physics
•   Commonly Used Parameterizations
•   How well do we model snow cover
•   NOAh and SNOW-17 Models
•   Input Data Uncertainties
•   Model Structure/Parameter Uncertainties
•   Effect of Weather Conditions
•   Summary
•   Where to go in Snow Modeling?
                 Snow Physics
          Sources of Energy Exchange

• Net Radiation, Qn
  – Solar Incoming – Reflected
  – Incoming from Atmosphere – Outgoing from Snow
• Turbulent Exchange
  – Sensible Heat: Based on the Temperature Gradient, Qh
  – Latent Heat: Based on the Moisture Gradient, Qe
• Mass Transfer: Heat from Precipitation, Qm
• Heat Exchange with the Underlying Soil, Qg
            Qn + Qh + Qe + Qm + Qg = ΔQ
                 Snow Physics
              Problem Complexities
• Net Solar Radiation is not easy observable and it
  varies significantly with weather and snow conditions
  (sky, snow reflectance (albedo), forest, topography)
                 Snow Physics:
          Problem Complexities (cont.)

• Turbulent Transfer is a very non-linear Function of
  the Wind Speed. A Turbulent Transfer Coefficient of
  this Function is defined under a Number of
  Assumptions.

• Large Uncertainties in Internal Snow Cover
  Processes: Density Changes in Time and Depth,
  Liquid Water Refreezing, Liquid Water Retention and
  Transmission, Rain Effect.

• Spatial Variability and Redistribution due to Wind
Commonly Used Parameterizations:
        Energy Based vs. Temperature Index

• Energy Based
  –   Snowpack Layers
  –   Albedo Definition
  –   Snowpack Property Dynamics
  –   Wind Function Approximation
• Temperature Index
  –   Fully Index Based or Combination
  –   Snow Melt Rate Dynamics
  –   Wind Function Approximation
  –   Snowpack Property Dynamics
How well do We Model Snow Cover
How well do We Model Snow Cover
      NOAh Energy Based Model
• One Layer snowpack
• Variable Snow Properties
• Multi-Layer Soil Profile


                      Snow
                                      Surface
       Soil surface                   runoff
                      Frost      Infiltration



                      Unfrozen
NOAh Energy Based Model
     Snow ablation and accumulation

  net             latent          sensible
radiation          heat             heat




           Snow density                        Varies depending on SWE
       Thermal Conductivity                  and snow temperature
                                             [Anderson-Yosida’s snow
                                             compaction model].
                                               Thermal conductivity varies
                                             with snow density

            Spatial variability              Non-linear function of SWE



            100% coverage                    Varies depending on vegetation
               threshold



              Albedo                         Weighted snow and bare soil
                                             albedos




                ground
                 heat
    SNOW-17: Temperature Index Model


• Conceptual model
  • Simplified Heat Balance During Rainfall Events
  • Degree-Day Melt Factor During Non-Rain Events
• Input Variables:         Air Temperature & Precipitation
• Output:                  Melt, WE, Depth, Areal Extend
• Snow Cover Processes
   •   Snow accumulation
   •   Surface Energy Exchange
   •   Snow Compaction
   •   Liquid Water Transmission
• Watershed /Areal Application
  • Areal Extent of Snow Cover
   SNOW-17: Surface Energy Exchange

Ta > 0oC – Snowmelt
• If Rainfall > 0.25 mm/hr
      - Simplified Heat Balance Equation:
            - no Solar Radiation,
            - Atmosphere & Snow is a Black Body,
            - Relative Humidity = 90%,
            - Rainwater at an Air Temperature
• Else
      - Degree-day Melt Factor (Seasonal Variable)

Ta < 0oC – No Snowmelt
      - Estimate Change in Snow Heat Deficit
     SNOW-17: Model Parameters
                    Major             Minor
• Accumulation      SCF (1-1.2)       PXTEMP (0.6-0.2)

• Surface Melt      MFMAX (1.7-2.0)   MBASE (0.0)
                    MFMIN (0.2-0.6)
                    UADJ (0.002*U)
• Heat Storage &                      NMF (0.15)
  Water Retention                     TIPM (0.05)
                                      PLWHC (0.02-0.05)

• Areal Coverage    SI ( > Wmax )
                    Depletion Curve


• Ground Melt                         DAYGM
SWE (Plot 3) and Depth (Plot 4) from NOAh (red lines) &
 SNOW-17 (white lines), Swiss Alps Site, 1992-1993
          Input Data Uncertainties
    Required Data for Energy Budget Models
•   Incoming Solar Radiation
•   Reflected Solar Radiation or Albedo
•   Incoming Long-Wave Radiation
•   Snow Surface Temperature
•   Wind Speed
•   Air Temperature
•   Dew-Point
•   Precipitation and Phase
•   Wet Bulb Temperature
•   Snow Cover Profile (Density, Temperature, etc.)
•   Soil Temperature
                                   Input Data Uncertainties


1000                                                                        700
                                    (a)                                                                      (b)
                                                                            600
    800

                                                                            500

    600
SWE, mm




                                                                           SWE, mm
                                                                            400


                                                                            300
    400

                                                                            200

    200
                                                                            100


          0                                                                          0
              60   90   120   150  180    210   240    270     300   330                 0   30   60   90    120    150     180   210   240
                                 Days from A ugus t 1992                                                Days from Oc tober 1981


                          NOA H-LS M   S NOW -17   Meas ured                                           NOA H-LS M   S NOW -17




              Swiss Alps Site, 1992-1993                                   Goose Bay Site, Canada, 1981-1982
                                                 Input Data Uncertainties
                                                               Effect of Net Radiation


     350                                                                          1000



     300
                                                                                                  800
Snow water equivalent, mm




                                                                                 Snow water equivalent, mm
     250


                                                                                                  600
     200



     150
                                                                                                  400


     100

                                                                                                  200
                     50



                            0                                                                                0
                                0   30   60         90      120      150   180                                   70   100   130   160    190    220   250    280   310   340
                                              Days from Novem ber 1996                                                                  Days from A ugus t 1992




   Sleepers River Site, USA, 1996-1997                                                                                Swiss Alps Site, 1992-1993

Observed fluxes (solid lines), Incoming solar radiation with a 20 Wm-2 error
added (dotted lines), and the constant albedo of 0.7 (dashed lines).
          Input Data Uncertainties
Effect of Wind Speed: V=1m/s (white), V=15m/s (colored)
                                        Goose Bay Site, Canada
                                      Input Data Uncertainties
 Effect of Precipitation Phase: with use (red), w/o use (blue)
    150                                                                                                                                      150
                                                                                                  300                                                                                                                                                 300
                        Danville, Vermont, USA
    125                                                                                           200                                        125                                                                                                      200




                                                                                                                                                                                                                                                            Snow water equiwalent (WE), mm
                                                                                                        Snow water equiwalent (WE), mm
                                                                                                  100                                                                                                                                                 100
    100                                                                                                                                      100




                                                                                                                                         Snow depth, cm
Snow depth, cm




                                                                                                  0                                                                                                                                                   0

             75                                                                                                                                       75
                                                                                                  -100                                                                                                                                                -100


                                                                                                  -200                                                                                                                                                -200
             50                                                                                                                                       50

                                                                                                  -300                                                                                                                                                -300

             25                                                                                                                                       25
                                                                                                  -400                                                                                                                                                -400


                 0                                                                                -500                                                    0                                                                                           -500
                  225   240   255    270   285   300     315   330   345   360   375    24   39                                                            194   209   224   239   254     269   284 299 314      329   344   360    375    24   39
                                                       Time, day s                                                                                                                                  Time, day s



                                    Hobs               Hsim, w/o P-type    Hsim, w P-type                                                                                     Hobs                  Hsim, w/o P-type      Hsim, w P-type


    175                             Wobs               Ws im, w/o P-type   Ws im,w- Ptype         400                                        150                              Wobs                  Ws im, w/o P-type     Ws im,w- Ptype

                                                                                                                                                                                                                                                      200
                                                                                                  300
    150
                                                                                                                                             125
                                                                                                                                                                                                                                                      100
                                                                                                  200
                                                                                                        Snow water equiwalent (WE), mm




                                                                                                                                                                                                                                                            Snow water equiwalent (WE), mm
    125
                                                                                                  100                                        100                                                                                                      0
Snow depth, cm




    100                                                                                           0                                      Snow depth, cm                                                                                               -100
                                                                                                                                                      75

             75                                                                                   -100
                                                                                                                                                                                                                                                      -200
                                                                                                  -200                                                50
             50
                                                                                                                                                                                                                                                      -300
                                                                                                  -300
                                                                                                                                                      25
             25                                                                                                                                                                                                                                       -400
                                                                                                  -400


                 0                                                                                -500                                                    0                                                                                           -500
                  255   270   285    300   315   330     346   361   376    25    40    55   70                                                            255    270        285     300     315      330     346       361    376         25    40
                                                       Time, day s                                                                                                                                  Time, day s



                                    Hobs               Hsim, w/o P-type    Hsim, w P-type                                                                                     Hobs                  Hsim, w/o P-type      Hsim, w P-type


                                    Wobs               Ws im, w/o P-type   Ws im,w- Ptype                                                                                     Wobs                  Ws im, w/o P-type     Ws im,w- Ptype
Model Structure/Parameter Uncertainties
           Effect of Weather Conditions




SWE and Depth from NOAh (red) & SNOW-17 (white), Col de Porte Site, France
Effect of Weather Conditions
                Simulated snowmelt variables from
                NOAH-LSM (dashed lines)
                & SNOW-17 (solid lines)
                Col de Porte, France
                February 10-22, 1998.

                Weather:
                           Very low wind;
                           High diurnal amplitude
                           of air temperature

                Effect:
                           (a) Much faster snowmelt
                              from SNOW-17;

                           (b) Significant effect of
                               net radiation and liquid
                              water refreezing
Effect of Weather Conditions
                   Weissfluhjoch Site
                   Switzerland, July 11-16, 1993

                   Weather:
                          Negative Tair
                          High wind speed

                   Effect:
                             Significant snowmelt
                             from NOAH,

                             Accumulation from
                             SNOW-17
                   Summary
• Model Complexity does not Guaranty Accuracy
• Temperature Index Models Provide Practically
  Reasonable Results, however They are
  Sensitive to Weather Conditions, Specifically
  Wind and Solar Radiation Conditions
• Energy Based Models are Sensitive to Input
  Data Errors, Specifically Wind, Solar Radiation
  and Albedo Treatment
• Some Calibration/Tunning is Needed to get
  Better Results from Both Simple and Complex
  Models
  Where to go in Snow Modeling?
• Energy and Temperature Based Models may
  Coexist for a Long Time, Specifically in River
  Runoff Prediction
• Improvements to Temperature Based Models
  can be Achieved by
  – Incorporating Wind and Humidity Data
  – Regionalization of the Most Critical Parameters to run
    in a Distributed Mode
• Energy Based Model Improvements
  – Define the Most Reliable Data Sources Using
    Sensitivity Tests
  – Better Parameterizations of Albedo and Wind
    Function
  – Improvement in Prediction of Weather Variables
Appendix 1
Appendix 2
Appendix 3
Appendix 4

				
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