Introduction

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, 99600 Sodankylä, Finland 2WSL 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 ofnatural seasonal snowpack • Weekly manual snowpit measurements (grain size, grain type, SSA , correlation length and reference measurements) and continuous microwave radiometermeasurementsare 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. Snowpit measurements include grain size macro-photography, IceCube and Snow Micro Pen Measurement area of radiometers Radiometers Grain size photography • 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 Snowpit measurements IceCube [2] Snow Micro Pen [3] • 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 D0are 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 mm, between Deff and D0 is 0.55 mm and between Deff and pexis 0.85 mm. • RMS error between scaled D0 and Deffis only 0.29 mm. • Deff depends on used frequency, values 18.7 GHz minus 36.5 GHz are mostly used. Grain size definitions 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) with optical grain size multiplied by scaling factor. Figure 1. Height-weightedaveragesoverwholesnowpack of Dmax, D0and Pexare compared to Deff. Winter 2014 has Deff. inverted from 18.7 GHz observations and also from 18.7 - 36.5 GHz observations in January (lower dots). [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, and M. Schneebeli, 2014.: “Density, specific surface area and correlation length of snow measured by high resolution penetrometry”, Journal of Geophysical Research Earth surface, submitted.