Claudia Cenedese

During the last ice age, icebergs transported Ice‐Rafted Debris (IRD) more than 3,000 km across the North Atlantic. However, their effect on the deterioration of icebergs and thus where the deposits form is poorly understood. Laboratory experiments and a predictive model (with no fitting parameters) investigating the effects of sediment on iceberg deterioration found that IRD increase the melt rate in three ways. First, the melt rate of the sediment layer is larger because there is less ice to melt, where is the sediment volume fraction. Second, the melt rate of the sediment layer and ice layer below is almost three times larger due to a sediment plume which enhances the turbulent heat flux toward the ice. Third, a sediment layer can cause an iceberg to break into two pieces, increasing the total surface area and causing the overall melt rate to increase as much as 50%–60%.

The 2019 Geophysical Fluid Dynamics Summer Study Program theme was Stratified Turbulence and Ocean Mixing Processes. Two principal lecturers, Stephanie Waterman (University of Victoria) and Colm-cille Caulfield (University of Cambridge), were our expert guides for the first two weeks. Colm covered the theoretical aspects of stratified turbulence, from simple scaling laws to sophisticated, modern numerical results. Stephanie kept the enthusiastic audience grounded with her lectures on ocean mixing processes, observations, and their many technical challenges.

Shallow cumuli are cloud towers that extend a few kilometres above the atmospheric boundary layer without significant precipitation. We present a novel laboratory experiment, boiling stratified flow, as an analogy to study turbulent mixing processes in the boundary layer by shallow cumulus convection. In the experimental beaker, a syrup layer (representing the atmospheric boundary layer) is placed below a freshwater layer (representing the free troposphere) and heated from below. The temperature is analogous to the water vapour mixing ratio in the atmosphere, while the freshwater concentration is analogous to the potential temperature. When the syrup layer starts boiling, bubbles and their accompanying vortex rings stir the two-layer interface and bring colder fresh water into the syrup layer. Two distinct regimes are identified: transient and steady boiling. If the syrup layer is initially sufficiently thin and diluted, then the vortex rings entrain more cold water than needed to quench superheating in the syrup layer, ending the boiling. If the syrup layer is initially deep and concentrated, then the boiling is steady since the entrainment is weak, causing the entrained colder water to continuously prevent superheating. A theory is derived to predict the entrainment rate and the transition between the two regimes, validated by experimental data. Finally, analogies and differences with the atmospheric processes are discussed.

The entrainment hypothesis states that the mean inflow velocity across the boundary of a turbulent flow is proportional to a characteristic velocity of the flow. Proposed by G. I. Taylor approximately 80 years ago, it is still a common model of turbulence closure widely used in environmental engineering and geophysical fluid mechanics. Although it is a very simple concept and mathematical model, it has proven to be able to predict the entrainment in a variety of geophysical flows, e.g. convective clouds and plumes from erupting volcanoes in the atmosphere; dense water overflows and turbidity currents in the ocean; magma injection in a magma chamber in the interior of the Earth, to name just a few. In a seminal paper, Turner (J. Fluid Mech., vol. 173, 1986, pp. 431–471) presents a variety of laboratory and geophysical flows to illustrate the success of the entrainment hypothesis and discusses why such a simple hypothesis works so well even when the original assumptions are no longer valid.

Icebergs are a primary mechanism for the long-distance transport of terrestrial sediment in the marine environment. During the last ice age, icebergs transported sediment more than 2000 km across the North Atlantic. In addition, sediment plays a key role in marine nutrient fertilization, phytoplankton growth, and CO2 drawdown. However, the effect sediment has on the deterioration of icebergs, as well as rates of sediment loss to the marine environment, are poorly understood. Here we present a completely new mechanism for iceberg deterioration which has previously gone undiscovered. We conducted the first laboratory investigation into the effects of sediment on iceberg deterioration and found that it increases the melt rate in three ways. First, as expected, there is less ice to melt. Second, a sediment plume may form which enhances the heat flux towards the ice. Third, sediment layers induce an iceberg to break into multiple pieces, dramatically increasing the overall rate of ice melt. The latter mechanism has never been reported previously. Our results provide a parametrization that includes the effects of sediment on iceberg deterioration. Our findings have significant implications for more accurately modeling iceberg break-up and for the potential of icebergs melt water to trigger abrupt climate change.

Editorial: Multi-scale fluid physics in oceanic flows: new insights from laboratory experiments and numerical simulations

This paper summarizes the results of the first investigation into the effect of particle‐laden plumes on glacier melting using laboratory experiments. We find that the melt rate, when the ice is exposed to a particle‐laden plume, can be larger than when exposed to an equivalent plume without particles. The increased melt rate is linked to an increase in the plume velocity in response to the presence of suspended particles. Including this increased velocity in a plume model improves melt rate predictions from the “three‐equation model” by approximately 45% for the range of particle concentrations used in this study.

We present laboratory experiments that investigate the structure and flow characteristics of gravity currents traveling through an array of roughness elements. The roughness elements are of comparable height to the gravity current such that the current flows through the roughness array rather than over it. The frontal velocity and density structure are measured as the current transitions from flowing along a smooth bed to flowing through the roughness array and then back to a smooth bed. We find that, upon entering the roughness array, the gravity current decelerates and the density structure changes from the head-and-tail structure typical of smooth bed gravity currents to a wedge shape. A model is presented that explains the deceleration and change in shape based on a dynamic balance between a pressure gradient within the current tail and a drag force associated with individual roughness elements. This model accurately predicts the deceleration of the gravity current, supporting the proposed dynamic balance.