The retrieval of snow properties from space: theory and applications A. A. Kokhanovsky 1, M. Tedesco 2,3, G. Heygster 1, M. Schreier 1, E. P. Zege 4 1)University of Bremen, Bremen, Germany 2)University of Maryland, Baltimore County, USA 3)NASA – Goddard Space Flight Center, Maryland, USA 4)Institute of Physics, Minsk, Belarus

Introduction A new snow retrieval algorithm that makes use of visible and near-infrared measurements in which snow is modeled as a semi-infinite weakly absorbing medium is developed The shape of grains is accounted for by means of a fractal snow grain model The technique is applied to study the changes of snow properties before and just after snow fall in Colorado as seen by two MODIS sensors on TERRA and AQUA satellites The snow grain size and snow albedo have been retrieved from AATSR onboard ENVISAT data over Greenland Preliminary comparisons with ground measurements have been performed

1. Snow physical model

semi-infinite horizontally homogeneous plane-parallel medium composed of fractal ice grains suspended in air Sun Satellite clear sky: gases aerosols

2. Snow optical model g=0.75 in the visible

3. Snow radiative transfer model - a=1.247, b = and c= the function p is the snow grain phase function Snow spectral reflectivity R 0 = Reflectivity of a semi-infinite snow layer at zero absorption Escape function Kokhanovsky and Zege, 2004; Appl. Optics Kokhanovsky, 2006; Optics Letters (methane adsorption; Domine et al., 2006)

3. Snow radiative transfer model: albedo determination Reflectivity: Albedo:

3. Snow radiative transfer model grain size d spectral snow albedo r

4. Validation: Hokkaido Solar zenith angle=54deg

4. Validation: Hokkaido

4. Validation: Antarctica Experiment Hudson et al., 2006 JGR

4. Validation: North Pole DAMOCLES IP : Developing Arctic Modeling and Observing Capabilities for Long-term Environmental Studies

Sensitivity of albedo to grain size 4. Satellite retrievals: grain size from MODIS data Band 5 Band 4 Band 6 MODIS Band 5 offers sensitivity to grain size

Multi-scale, multi-sensor approach to build comprehensive data set needed to meet NASA Earth Science Enterprise science objectives. First application to MODIS data: The CLPX dataset

Elevation and forest cover of the test area Meters Forest cover fraction Green very sparse Blue/Black very dense White no forest Elevation [m] ground measurements Global Land Cover

Grain size retrieval: Feb.19, 2003 TERRA (AM)AQUA (PM) micrometers 10:30am 1:30pm cloud snow forest morning: snowfall

Forest effect Grain size values retrieved from MODIS-TERRA vs. those retrieved from MODIS-AQUA on February 19, 2003

Preliminary validation(d=(a+b)/2) CLPX-1 campaign, North Park, Colorado, USA, 2003 Terra Aqua Feb_21 Feb_22 elevation:2.5km

5. Satellite retrievals: AATSR Reflectances

5. Satellite retrievals: AATSR

5. Satellite retreivals

6. Account for snow pollution in the visible

Observations and future work A new approach is to be developed (A. Lyasputin, UMBC/NASA; von Hoyningen-Huene, University of Bremen) for simultaneous retrieval of AOT and surface BRDF. This will improve MODIS snow BRDF product. The cloud mask must be improved. A comprehensive validation and calibration campaign is needed. This will be performed using measurements in Greenland (M. Tedesco, PI of the Proposal submitted to NASA NNH06ZDA001N-IPY).