Reply on RC1

Graig Sutherland

doi:10.5194/tc-2021-289-ac1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Understanding the transport of objects and material in the marginal ice zone (MIZ) is critical for human operations in polar regions. This can be the transport of pollutants, such as spilled oil, or the transport of objects, such as drifting ships and search and rescue operations. For emergency response, the use of environmental prediction systems are required which predict ice and ocean parameters and are run operationally by many centres in the world. As these prediction systems predict both ice and ocean velocities, as well as ice concentration, it must be chosen how to combine these data to best predict the mean transport velocities. In this paper we present a case study of four drifting buoys in the MIZ deployed at four distinct ice concentrations. We compare short-term trajectories, i.e. up to 48âh lead times, with standard transport models using ice and ocean velocities from two operational prediction systems. A new transport model for the MIZ is developed with two key features aimed to help mitigate uncertainties in iceâocean prediction systems: first, including both ice and ocean velocities and linearly weighting them by ice concentration, and second, allowing for a non-zero leeway to be added to the ice velocity component. This new transport model is found to reduce the error by a factor ofÂ 2 toÂ 3 for drifters furthest in the MIZ using ice-based transport models in trajectory location after 48âh.

Highlights

Estimating the drift and transport of material in the marginal ice zone (MIZ) is a crucial aspect of polar operations
Presented here is a general leeway model, which assumes a linear weighting of the ice and ocean velocities based on ice concentration and allows for a non-zero leeway in both the ice and ocean components and this leeway can vary in magnitude and direction
Optimal leeway coefficients are calculated by minimizing the error between observed drifter velocities in the MIZ and the model velocities calculated using ice-ocean velocities provided by two different coupled ice-ocean prediction systems: Canadian Arctic Prediction System (CAPS) and TOPAZ

Summary

Introduction

Estimating the drift and transport of material in the marginal ice zone (MIZ) is a crucial aspect of polar operations. While free drift models exist, and they perform quite well when compared with observations, they do require data in order to tune the drag coefficients (Schweiger and Zhang, 2015) which can be difficult to obtain in the arctic It is common for operational prediction of drifting objects to include a leeway term, which is a fraction of the wind that is added to the predicted water velocity. 45 it’s long been established that the competing wind and wave effects combine for a net leeway of about 3% of the velocity (Wu, 1983) For sea ice it is not clear whether a leeway term should be included in the drift, operational models such as CAPS and TOPAZ do not explicitly include processes associated with surface waves or form drag due to the ice roughness.

Oil transport equation in the MIZ

General transport equation in the MIZ

Drifters

Ice-ocean prediction systems

Calculating leeway coefficients

Ice and water leeway coefficients

Short-term trajectory predictions

Conclusions