Guo-Yue Niu and Zong-Liang Yang

The Department of Geological Sciences

The University of Texas at Austin

Evaluation of snow simulations Evaluation of snow simulations from CAM2/CLM2.0from CAM2/CLM2.0

Research was funded under NASA grant NAG5-12577

OutlineOutline

Brief Introduction of Snow-related Schemes Snowfall Snowpack physics Snow albedo Snow cover fraction

Evaluation of Wintertime Surface Albedo and Snow Depth Wintertime surface albedo and snow depth Main factors affecting wintertime surface albedo Sensitivity studies

A new Snow Cover Fraction Scheme Evaluation of Snowfall Summary and Future work

Introduction

1. Over the past century, the largest global warming has occurred in the high latitude areas (Houghton et al, 2001)

2. The CAM2/CLM2 produced a significant warm bias in snow-covered regions during wintertime

3. The goal of this study is to investigate how this warm bias is related to the representation of snow processes in CAM2/CLM2

Snow-related Parameterization Schemes inCAM2/CLM2 in Comparison with CCM3/LSM1.0

CCM3/LSM CAM2/CLM2

Snowfall

Snowpack Physics

Snow Albedo

SCF

Ts<Tfrz+2.0 T(b3)<Tfrz–2.0

1-L No Liquid water No densification

5-L Liquid water Densification

Marshall (1989) BATS

LSM1.0 BATS

Consequences in SWE

–

+ +

+

– – –

Snow Cover Fraction Schemes

Two 12-year Experiments: 1. Control run with default

SCF scheme

2. Modified run with the Yang et al. (1997) SCF scheme

Yang et al (1997) scheme has been tested by Roesch et al (2000) at T106.

Applicable to flat, non-forested areas.

Wintertime Surface Albedo (FEB)

MODIS

CLM2/Yang

CLM2/ Default

CLM2/default produced higher surface albedo in boreal forest regions, lower surface albedo in mid-latitude regions. However, CLM2/Yang improved the surface albedo simulations in mid-latitude regions.

Wintertime Surface Albedo – regional averages

Mid-latitude grass/cropland Regions

Boreal Forest Regions

OBS

Default

Yang

Snow Depth – regional averages

Mid-latitude grass/cropland Regions

Boreal Forest Regions

OBS

Default

Yang

Factors Affecting Wintertime Land Surface Albedo

Vegetation shading factors Tree cover fraction Leaf/stem area index Vegetation height (canopy fraction buried by snow)

Snow on canopy Tree cover fraction Interception capacity Meteorological conditions (wind, temperature)

Ground snow covered fraction (SCF) Ground surface roughness length Snow depth

Snow properties Grain radius Impurity

The model prescribed tree (needleleaf & broadleaf) cover fraction from the 1-km IGBP DISCover dataset and the 1-km University of Maryland tree cover dataset (Bonan et al, 2002)

The tree cover fraction is between 40-60% meaning a significant fraction of open areas for potentially high surface albedo.

Tree Cover Fraction in CAM2/CLM2

The modeled effective leaf area index (after buried by snow) is higher than the AVHRR values, especially in boreal forest regions.

LAI & SAI in CAM2/CLM2 (February)

AVHRR

CAM2/CLM2

CCM3/LSM

Over 80 percent of canopy is buried by snow in the high latitude tundra areas.

Fraction of Canopy Buried by Snow in CAM2/CLM2

Four 1-month Sensitivity Experiments

Mid-latitude grass/cropland Regions

Boreal Forest Regions

default: CAM2/CLM2 default runrun1: No snow effects on canopy albedorun2: Yang et al. (1997) SCF schemerun3: Doubling LAI & SAI

Spectral-integrated albedo as function of (LAI+SAI)

Surface Air Temperature (February)

The significant warm bias both in Eurasia and North America from the default CAM2/CLM2 is largely reduced by using the Yang et al (1997) SCF scheme

CLM2/Yang – OBS

CLM2/Default – OBS

Regional Averaged Snow Depth

Yang et al (1997) SCF scheme produced too much snow and a cold bias in melting season

Mid-latitude grass/cropland Regions

Boreal Forest Regions

Eurasia North America

Observed Depletion Curve in Melting Season in a Small WatershedF

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over

ed A

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Basin Avg. SWE / Maximum Basin Avg. SWE

Upper Sheep Creek Watershed, Idaho (26 ha)

From Luce & Tarboton (1999)

A New Snow Cover Fraction

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tanh(

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max

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h

h

z

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SCFsno

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sno

daymmq

daymmq

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melt

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Need further validation with high resolution snow cover and snow depth data

Wintertime Surface Air Temperature

Default-OBS (DJF)

NEW-OBS (DJF)

Melting Season Surface Air Temperature

Default-OBS (MAM)

NEW-OBS (MAM)

Regional Averaged Snow Depth

Mid-latitude Grass/cropland Regions

Boreal Forest Regions

Eurasia North America

Improved simulations of snow depth in mid-latitude areas

Snowfall Estimation using Monthly T and Prec Observations

)15.273(35.1*6.11

1

*Pr

TR

RecSnowfallLegates & Willmott (1990)

Evaluation of Snowfall (MAM)

Global Air Temperature and Precipitation (V. 2.01) (Willmott and Matsuura, 1995)

CAM2-CCM3

CCM3-OBS

CAM2-OBS

Regional Averages of Snowfall

Europe

North AmericaEurasia

Northwest NA

CAM2/CLM2 improved the snowfall simulations in fall and spring seasons compared to CCM3/LSM, but underestimated snowfall in winter season.

Ongoing Work

Using high resolution datasets of snow cover, land surface albedo and snow depth

To analyze the relationship between SCF and snow depth

To analyze the relationship between SCF and seasons

To analyze the relationship between SCF and sub-grid topography

To parameterize SCF as function of snow depth, canopy height (land cover), season, and sub-grid topography

Funded by a 3-year NASA grant

Summary

• The snow simulations have been assessed using observed climatology of snow depth, temperature and MODIS-derived surface albedo.

• The CAM2/CLM2 produced shallower than observed snow depth and lower surface albedo, which coincide with a warm bias in mid-latitude regions in wintertime.

• The shallower snow depth and lower surface albedo may be largely due to an inappropriate SCF parameterization scheme.

• A new SCF scheme, which differentiates snow cover patterns during accumulation and melt periods, improves the simulations of snow depth, surface albedo, and surface air temperature.

• CAM2/CLM2 improved snowfall simulations in fall and spring seasons compared to CCM3/LSM, but underestimated snowfall in winter season.