Dr Agnieszka Herman
Institute of Oceanography
University of Gdañsk
Al. Pi³sudskiego 46, 81-378 Gdynia
Phone: +48 (0)58 5236887
Fax: +48 (0)58 5236678
Herman, A., 2018. Wave-induced surge motion and collisions of sea ice floes: finite-floe-fize effects. J. Geophys. Res., 123, 7472-7494, doi: 10.1029/2018JC014500 (paper).
Abstract: Among many mechanisms potentially contributing to wave energy attenuation in sea ice are wave-induced ice floe collisions. At present, little is known about collision patterns and their phase-averaged effects under different combinations of sea ice properties (ice thickness, floe size, etc.) and wave forcing (wavelength and steepness). The existing parameterizations of collision-related effects are therefore based on several simplifying, unverified assumptions. In this work, wave-induced motion and collisions of ice floes are analyzed numerically with a model based on momentum equations for an arbitrary number of floes, with source terms computed by integrating local forcing (wave-induced dynamic pressure, surface drag, etc.) over the surface area/volume of each floe. It is shown that this simple model, with prescribed wave forcing (i.e., no wave-ice interactions), is capable of reproducing observed surge amplitudes up to floe sizes comparable with wavelength. A full Hertzian contact model is used instead of a simple hard-disk algorithm, which makes the model suitable for simulating both rapid collisions and prolonged contact between floes. The model equations are used to formulate heuristic collision criteria based on relative floe size, ice concentration, and wave steepness. The model is then run for different combinations of those three parameters, together with different restitution and drag coefficients, in order to analyze possible motion/collision patterns within the multidimensional parameter space, and phase-averaged effects of collisions: kinetic and contact stress, granular temperature, and work done by forces acting on the ice.
Wenta, M., Herman, A., 2018. The influence of the spatial distribution of leads and ice floes on the
atmospheric boundary layer over fragmented sea ice. Ann. Glaciol., 59, 213-230, doi: 10.1017/aog.2018.15 (paper)
Abstract: The response of the atmospheric boundary layer (ABL) to subgrid-scale variations of sea ice properties and fracturing is poorly understood and not taken into account in mesoscale Numerical Weather Prediction (NWP) model parametrizations. In this paper we analyze three-dimensional air circulation within the ABL over fragmented sea ice. A series of idealized high-resolution simulations with the Weather Research and Forecasting (WRF) model is performed for several spatial distributions of ice floes and leads for two values of sea ice concentration (0.5 and 0.9) and several ambient wind speed profiles. The results show that the convective circulation within the ABL is sensitive to the subgrid-scale spatial distribution of sea ice. Considerable variability of several domain-averaged quantities – cloud liquid water content, surface turbulent heat flux (THF) – is found for different arrangements of floes. Moreover, the organized structure of air circulation leads to spatial covariance of variables characterizing the ABL. Based on the example of THF, it is demonstrated that this covariance may lead to substantial errors when THF values are estimated from area-averaged quantities, as it is done in mesoscale NWP models. This suggests the need for developing suitable parametrizations of ABL effects related to subgridscale sea ice features for these models.
Herman, A., 2017. Wave-induced stress and breaking of sea ice in a coupled hydrodynamic–discrete-element wave–ice model. The Cryosphere, 11, 2711-2725, doi: 10.5194/tc-11-2711-2017 (paper).
Abstract: In this paper, a coupled sea ice–wave model is developed and used to analyze wave-induced stress and breaking in sea ice for a range of wave and ice conditions. The sea ice module is a discrete-element bonded-particle model, in which ice is represented as cuboid “grains” floating on the water surface that can be connected to their neighbors by elastic joints. The joints may break if instantaneous stresses acting on them exceed their strength. The wave module is based on an open-source version of the Non-Hydrostatic WAVE model (NHWAVE). The two modules are coupled with proper boundary conditions for pressure and velocity, exchanged at every wave model time step. In the present version, the model operates in two dimensions (one vertical and one horizontal) and is suitable for simulating compact ice in which heave and pitch motion dominates over surge. In a series of simulations with varying sea ice properties and incoming wavelength it is shown that wave-induced stress reaches maximum values at a certain distance from the ice edge. The value of maximum stress depends on both ice properties and characteristics of incoming waves, but, crucially for ice breaking, the location at which the maximum occurs does not change with the incoming wavelength. Consequently, both regular and random (Jonswap spectrum) waves break the ice into floes with almost identical sizes. The width of the zone of broken ice depends on ice strength and wave attenuation rates in the ice.
Wenta, M., Herman, A., 2017. Submesoscale atmospheric boundary layer processes over fragmented sea ice, 97th AMS Annual Meeting, 14th Conference on Polar Meteorology and Oceanography, Seattle, USA, 22-26 I 2017 (poster).