1. Comparison of microwave radiometer observations and snow grain size in Sodankylä
Leena Leppänen1, Anna Kontu1, Juha Lemmetyinen1, Martin Proksch2
1 Finnish Meteorological Institute, Arctic Research, Tähteläntie 62, Sodankylä, Finland
2 WSL Swiss Federal Institute for Snow and Avalanche Research SLF, CH-7260 Davos, Switzerland
Introduction
Knowledge of the snow microstructure (correct a priori parameterization of grain size) is relevant for successful retrieval of snow parameters (e.g. SWE) from microwave observations.
Sodankylä has extent measurement site for monitoring development of natural seasonal snowpack
Weekly manual snowpit measurements (grain size, grain type, SSA , correlation length and reference measurements) and continuous microwave radiometer measurements are made.
Microwave observations are modeled mainly with HUT snow emission model [1].
Different measures of snow structure is compared to inverted values from passive microwave observations to analyze suitability of different grain size definitions in simulation of microwave emission.
Measurement area of radiometers
Radiometers
Grain size photography
Snowpit measurements
Passive microwave radiometers
Measures brightness temperature of snow and ground.
Based on 5-m high tower
Measurements only from dry snow
Frequency channels 10.65, 18.7, 21.0, 36.5, 89 and GHz with horizontal and vertical polarization.
Measurements since 2009
IceCube [2]
Snow Micro Pen [4]
Grain size definitions
Results
Magnitude of traditional grain size (Dmax) is larger than magnitude of optical grain size (D0) and correlation length (Pex) (Fig. 1).
Trends of the Dmax and D0 are quite similar. Difference (scaling factor) between Dmax and D0 is approximately 2.5.
RMS error between effective grain size (Deff) and Dmax is 0.45, between Deff and D0 is 0.55 and between Deff and pex is 0.85.
RMS error between scaled D0 and Deff is only 0.29.
More observations of pex are needed for final results.
Deff depends on used frequency, values 19 GHz minus 37 GHz are mostly used.
Description
Sample frequency
Time and place
Method
Traditional grain size
Dmax
Physical diameter of a typical snow grain
One sample from every layer
Measured weekly in measurement field.
Estimated visually from macro-photographs by accuracy of 0.25 mm
Optical grain size D0
Diameter of ice spheres which have the same optical properties as original grains
3 cm
Measured weekly in measurement field
Derived from IceCube-instrument specific surface area (SSA) measurements [2]
Correlation length
Pex
Describes the distribution of
scattered radiation, and it is related to grain size
0.125 cm
Derived from Snow Micro Pen (SMP) measurements [3]
Effective grain size
Deff
Parameter which describes microstructure of snowpack in radiative transfer models
One value for the whole snowpack
Continuous measurements in measurement field
Inverted with HUT snow emission model from radiometer observations
Conclusion
Effective grain size inverted with HUT snow model from radiometer observations in Sodankylä has magnitude closest to traditional grain size, but best similarity (smallest RMSE) to optical grain size multiplied by scaling factor.
Figure 1. Height-weighted averages over whole snowpack of Dmax, D0 and Pex are compared to Deff. Winter 2014 has Deff. inverted from 19 GHz observations and also from GHz observations in January.
[1] J. Pulliainen, J. Grandell, and M. Hallikainen, “HUT snow emission model and its applicability to snow water equivalent retrieval,” IEEE Trans. Geosci. Remote Sens, vol.37(3), pp , 1999.
[2] J.-C. Gallet, F. Domine, C. Zender, and G. Picard, “Measurement of the specific surface area of snow using infrared reflectance in an integrating sphere at 1310 and 1550 nm,” The Cryosphere, vol.3, pp , 2009.
[3] M. Proksch, H. Löwe, M. Schneebeli, “Statistical model for the correlation length of snow derived from snow-micro-pen measurements”. [Abstract] Geophys. Res. Abstr. 14: EGU , 2012
[4] SMP