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, 99600 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 150 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 19-37 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. 1378-1390, 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. 167-182, 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: EGU2012-14125., 2012 [4] SMP