Radar Backscattering of Lake Ice During Freezing and Thawing Stages Estimated by Ground-Based Scatterometer Experiment and Inversion From Genetic Algorithm

Lake ice under phase transition shows large variation on radar backscattering due to the changes of dielectric constant and roughness of ice surface and thus the transmissivity of microwave into ice body. To study the effects of freezing/thawing of ice on radar backscattering in a short time, we spread water over lake ice and continuously measured radar backscattering by using a ground-based microwave scatterometer system operated in C-band HH polarization. By establishing scattering models and applying inversion from genetic algorithm, radar returns were separated into ice-surface, volume, and ice-bottom scatterings, and the changes in dielectric constant and roughness parameters of ice surface were estimated as well. Immediately after spreading water on ice surface, ice-surface scattering was strongest due to high dielectric constant of surface water while volume and ice-bottom scatterings were very weak due to low microwave transmissivity into ice body. As surface water was being frozen, ice-surface scattering became weak with decreasing dielectric constant while volume and ice-bottom scattering increased due to higher transmissivity into ice body. In a transition stage, when surface water was almost frozen, all three scatterings increased simultaneously. Crystallization of ice produced rougher surface overcoming the decrease in dielectric constant, resulting in the increase of ice-surface scattering, while volume and ice-bottom scattering was continuously increased due to increasing transmissivity. At the end of the experiment, air temperature rose above freezing point, and ice surface thawed again so that ice-surface scattering increased while volume and ice-bottom scattering were decreased.