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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. University of Bremen, Bremen, Germany University of Maryland, Baltimore County, USA NASA – Goddard Space Flight Center, Maryland, USA
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The retrieval of snow properties from space: theory and applications A. A. Kokhanovsky1 , M. Tedesco2,3,G. Heygster1 , M. Schreier1, E. P. Zege4 • University of Bremen, Bremen, Germany • University of Maryland, Baltimore County, USA • NASA – Goddard Space Flight Center, Maryland, USA • Institute of Physics, Minsk, Belarus alexk@iup.physik.uni-bremen.de
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 Satellite • clear sky: • gases • aerosols Sun semi-infinite horizontally homogeneous plane-parallel medium composed of fractal ice grains suspended in air
2. Snow optical model phase function g=0.75 in the visible Macke et al., 1996; Mishchenko et al., 1997
3. Snow radiative transfer model Kokhanovsky and Zege, 2004; Appl. Optics Snow spectral reflectivity Escape function R0= Reflectivity of a semi-infinite snow layer at zero absorption Kokhanovsky, 2006; Optics Letters (methane adsorption; Domine et al., 2006) • a=1.247, b = 1.186 and c= 5.157 • the function p is the snow grain phase function
3. Snow radiative transfer model: albedo determination Reflectivity: Albedo:
3. Snow radiative transfer model fresh snow: no pollution • grain size d • spectral snow albedo r
4. Validation: Hokkaido Solar zenith angle=54deg
4. Validation: Antarctica Experiment Hudson et al., 2006 JGR
4. Validation: North Pole DAMOCLES IP 2005-2009: Developing Arctic Modeling and Observing Capabilities for Long-term Environmental Studies
4. Satellite retrievals: grain size from MODIS data Band 4 Band 5 2002 Band 6 1999 MODIS Band 5 offers sensitivity to grain size Sensitivity of albedo to grain size
First application to MODIS data: The CLPX dataset Multi-scale, multi-sensor approach to build comprehensive data set needed to meet NASA Earth Science Enterprise science objectives.
Elevation and forest cover of the test area Global Land Cover mountain Colorado Meters Forest cover fraction Green very sparse Blue/Black very dense White no forest Elevation [m] ground measurements
Grain size retrieval: Feb.19, 2003 TERRA (AM) AQUA (PM) forest snow cloud micrometers 1:30pm 10:30am morning: snowfall MODIS BRDF product
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) elevation:2.5km Aqua Feb_21 Terra Feb_22 CLPX-1 campaign, North Park, Colorado, USA, 2003
5. Satellite retrievals: AATSR Reflectances
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).
