Results from the Radiation, Snow Characteristics and Albedo at Summit (RASCALS) campaign Aku Riihelä, Panu Lahtinen Finnish Meteorological Institute Teemu Hakala Finnish Geodetic Institute IGARSS 2011 – Vancouver, Canada

I.Motivation and campaign introduction A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver2

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. A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver3 CM SAF Arctic SAL product,

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. EARSeL Snow & Ice Workshop, Bern,

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 EARSeL Snow & Ice Workshop, Bern,

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 A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver6

II.Applications for data A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver7

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. A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver8 ECMWF and NCEP NG snow albedo schemes also calculated for illustrative purposes.

Evaluation of snow albedo parameterizations A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver9 Snow surface roughness effect in albedo; surface hoar manifesting at SZA > 68 deg.

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. A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver13

Direct validation of satellite reflectance/albedo retrievals EARSeL Snow & Ice Workshop, Bern, 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 EARSeL Snow & Ice Workshop, Bern, 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 EARSeL Snow & Ice Workshop, Bern, Sun Sat

Results against AVHRR / CM SAF SAL A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver17 Polarized FF measurement, half power

III. Future directions & conclusions A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver18

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 nm waveband Also allows the evaluation of narrow-to-broadband conversion accuracy A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver19

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. A. Riihelä / RASCALS campaign / IGARSS 2011, Vancouver20