A novel strategy to analyse the form drag on pressure ridges and the air–ice drag coefficient in the north-western Weddell Sea

Abstract

• A novel strategy for the form drag considering ridge morphologies and ice surface roughness is proposed. • A novel strategy for drag coefficient considering ridge morphologies and ice surface roughness is proposed. • The related conclusions of the form drag and drag coefficient is proved. • The variations of form drag and drag coefficient with ridging intensity and ice surface roughness is analysed. As the most important quantities needed in numerical modelling of sea ice dynamics, the form drag on pressure ridges and the air–ice drag coefficient are generally analysed based on wind velocities and sea-ice conditions. However, it is difficult to give exact values because these quantities have not been measured. Based on a large data set measured by a helicopter-borne laser altimeter during the Winter Weddell Outflow Study 2006, the influences of pressure-ridge morphologies and sea-ice surface roughness on the form drag of pressure ridges and the air–ice drag coefficient were considered to seek a novel strategy for analysing these two essential quantities. The results show that the contribution of form drag on pressure ridges to total air–ice drag increases at greater ridging intensity (the ratio of mean ridge height to spacing), but decreases with increasing roughness length. The air–ice drag coefficient shows a similar trend to the contribution of form drag with increasing ridging intensity. Moreover, a ridging intensity threshold was determined for the air–ice drag coefficient. These results imply that large values of the air–ice drag coefficient could be caused by form drag on pressure ridges as well as by skin drag over rough ice surfaces. Finally, the present results are consistent with reported measurements in the north-western Weddell Sea.