Results from the Radiation, Snow Characteristics and Albedo at Summit (RASCALS) campaign

Aku Riihelä, Panu LahtinenFinnish Meteorological Institute

Teemu HakalaFinnish Geodetic Institute

IGARSS 2011 – Vancouver, Canada

I. Motivation and campaign introduction

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Surface albedo of snow

• A significant driver of the surface radiation budget of the polar regions

• Recent studies indicate that snow/ice albedo treatment in climate models will explain much of the inter-model scatter.

• Accurate and robust snow albedo descriptions are needed for detailed studies of polar and even global climate.

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CM SAF Arctic SAL product, 20.6.-27.6.2011

How to get snow albedo?• Spot coverage from AWS stations is not enough;

regional and global coverage are required.

One may opt to snow parameterizations or direct airborne/spaceborne observations.

Airborne coverage sparse in both space and time -> satellite remote sensing is the most viable means to have global, cost-effective coverage.

• Regardless of this choice, both parameterization and satellite retrieval are estimates of snow albedo.

Careful in-situ observations are needed to validate and develop both.

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The RASCALS campaign• RAdiation, Snow

Characteristics and Albedo at Summit• Perennial snow during

polar summer 2010

• Summit station on top of Greenland Ice Sheet (3200 m)

• Pristine snow, no vegetation, no topography

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The observations• Surface broadband albedo

• Hemispherical-directional snow reflectance, spectrally resolved

• Spectrally resolved irradiance

• Snow physical characteristics

• SSA estimate from macrophotographs

• Density, temperature profiles

• Surface roughness

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CM 14 observations of snow albedo at Summit, Greenland

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Spectrally resolved irradiance at 11:22 UTC on July 5, 2010 at Summit station

II. Applications for data

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Evaluation of snow albedo parameterizations• The recent Gardner-Sharp snow albedo

parameterization was constructed from our observations of snow grains (SSA) and the atmospheric state on site.

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ECMWF and NCEP NG snow albedo schemes also calculated for illustrative purposes.

Evaluation of snow albedo parameterizations

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Snow surface roughness effect in albedo; surface hoar manifesting at SZA > 68 deg.

Evaluation of snow albedo parameterizations

Evaluation of snow albedo parameterizations

Evaluation of snow albedo parameterizations

Evaluation of snow albedo parameterizations

• The GS parameterization performed well across all 4 days (RMSE ~ 0.01 to 0.02).

• The BC concentrations used to tune the fit were in line with previous knowledge (2 ppb) – but the atmospheric pressure correction term is needed for high glacier snow.

• The GCM parameterizations also predicted the general level of albedo, although less accurately.

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Direct validation of satellite reflectance/albedo retrievals

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Spatial and temporal matching of the observations required! Temporal decorrelation possible – but less of an issue for Arctic glacier snow.

The HDRF/BRF library from Summit

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The clear-sky BRF dataset covers all Sun zenith angles that are applicable for radiation studies or satellite reflectance retrievals at Summit for this period!

Application; direct validation of satellite reflectance/albedo retrievals

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Sun

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Results against AVHRR / CM SAF SAL

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Polarized FF measurement, half power

III. Future directions & conclusions

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Next steps• MODIS instantaneous surface reflectance data for the

evaluation would be highly valuable (any MODIS staff present?)

• Proceed from broadband reflectance to channel-specific surface reflectance study

• FIGIFIGO data covers 300-2500 nm waveband

• Also allows the evaluation of narrow-to-broadband conversion accuracy

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To conclude…• The RASCALS campaign was successfull.

• A sizable database of snow albedo and related quantities was gathered from the Greenland Ice Sheet.

• The data is now being applied to improve our understanding of both snow albedo parameterizations and remotely sensed snow albedo estimates.

• The snow physics-based Gardner-Sharp parameterization works well with arctic perennial snow (and subarctic snow too).

• Comparison of directional reflectance measurements on site versus the CM SAF SAL product shows improvement in the SAL satellite product as the algorithm develops; some underestimation still evident.

• Next steps will be to bring new satellite data into the study, and also prepare for the release of data to the scientific community once first publications are out.

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