By Mel Sandells
One of the limitations for measurement of global snow mass is that the satellite observations at microwave frequencies are affected not only by how much snow is on the ground, but also the size of the snow crystals. In August 2014, the snow microwave remote sensing community descended upon Reading University. Over 40 participants from 11 countries came to the MicroSnow workshop to discuss the role of snow microstructure in microwave radiative transfer. We were very lucky with the perfect Reading weather for the meeting, although I think my colleagues now have an unrealistic view of the UK climate.
The workshop opened with a summary of new ways of measuring snow microstructure. Over the last few years, we have come a long way from observer-dependent measurements of maximum extent of snow grains placed on a grid. New objective methods include x-ray tomography, spectral or single wavelength reflectance, gas adsorption (OK, that’s an old method, but there is a new field instrument!) and penetrometer observations. However, snow grain size measured with these instruments (assumptions of spheres) is not the right number to put into microwave emission models. They need to be scaled by a factor to account for the fact that snow crystals are odd shapes, sometimes bonded together, and have a distribution of sizes even within seemly homogenous layers. The magic number is 2.8, I’ve been told, although other empirical corrections are also available.
The aim of the workshop was to understand how to use the observations from the new grain size measurement techniques and how we can move away from grain scale factors. The answer, it would seem, lies in the autocorrelation function of snow – the probability of still being in the same medium (either ice or air) as a function of distance from a starting point. The evening model clinic on the first day enabled researchers to install different microwave models on their own machines, whilst the model developers were around to assist and answer questions. There were many exciting presentations at the meeting covering representation of the soil, ice lenses, impact of layering, fundamental differences between microwave models, radiance assimilation and satellite retrieval applications. There was even a presentation on novel use of the SMOS L-band satellite to sense density variations within ice sheets, which opens the door to obtain climate information by this method. It seems that we have yet to collect the perfect dataset for snow microwave model evaluation, and the community will work towards addressing that – to be discussed at MicroSnow2 in Annapolis, USA, 2015.
The meeting was held in the fabulous Meadow Suite, and we were able to provide excellent lunches and evening meals for all, as well as award travel bursaries for 10 early career scientists. It is thanks to funding from the ESA-DA projects, National Centre for Earth Observation, Micro-DICE (European Science Foundation), International Association of Cryospheric Sciences and the Software Sustainability Institute that made this meeting so inclusive and so productive. We have come a long way in defining the science needed for the next snow monitoring satellite, and we’re all fired up to focus on a new mission.