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.

Comparison of microwave radiometer observations and snow grain size in Sodankylä

[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.

Description Sample frequency

Method

Traditional grain size

Dmax Physical diameter of a typical snow grain

One sample from every layer

Estimated visually from macro-photographs by accuracy of 0.25 mm. Value for the whole snowpack is calculated by weighted mean with layer height.

Optical grain size

D0 Diameter of ice spheres which have the same optical properties as original grains

3 cm Derived from IceCube-instrument specific surface area (SSA) measurements [2]. Value for the whole snowpack is calculated by weighted mean with sample height (and height from ground for the lowest measurement).

Correlation length

Pex Length scale of porous media

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

Retrieved using inversion of HUT snow emission model from radiometer observations at 18.7 – 36.5 GHz

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

Snow Micro Pen [3]

IceCube [2]

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 mm, between Deff and D0 is 0.55 mm and between Deff and pex is 0.85 mm.

• RMS error between scaled D0 and Deff is only 0.29 mm.

• Deff depends on used frequency, values 18.7 GHz minus 36.5 GHz are mostly used.

Grain size photography

Radiometers

Snowpit measurements

Measurement area of radiometers

Grain size definitions

ConclusionEffective 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.

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

Figure 1. Height-weighted averages over whole snowpack of Dmax, D0 and Pex are 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).

Snowpit measurements include grain size macro-photography, IceCube and Snow Micro Pen