Atmospheric sensitivity to marginal‐ice‐zone drag: Local and global responses

Ian A Renfrew,Andrew D Elvidge,John M Edwards

doi:10.1002/qj.3486

Ian A Renfrew, Andrew D Elvidge + Show 1 more

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https://doi.org/10.1002/qj.3486

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Abstract

The impact of a physically based parametrization of atmospheric drag over the marginal ice zone (MIZ) is evaluated through a series of regional and global atmospheric model simulations. The sea‐ice drag parametrization has recently been validated and tuned based on a large set of observations of surface momentum flux from the Barents Sea and Fram Strait. The regional simulations are from March 2013 and make use of a collection of cold‐air outbreak observations in the vicinity of the MIZ for validation. The global model analysis uses multiple 48 h forecasts taken from a standard test suite of simulations. Our focus is on the response of the modelled atmosphere to changes in the drag coefficient over the MIZ. We find that the parametrization of drag has a significant impact on the simulated atmospheric boundary layer; for example, changing the surface momentum flux by typically 0.1–0.2 N m−2 (comparable to the mean) and low‐level temperatures by 2–3 K in the vicinity of the MIZ. Comparisons against aircraft observations over and downwind of the MIZ show that simulations with the new sea‐ice drag scheme generally have the lowest bias and lowest root‐mean‐square errors. The wind speed and temperature biases are reduced by up to 0.5 m s−1 and 2 K respectively, compared to simulations with two settings of the previous drag scheme. In the global simulations the atmospheric response is widespread – impacting most of the Arctic and Antarctic sea‐ice areas – with the largest changes in the vicinity of the MIZ and affecting the entire atmospheric boundary layer.

Highlights

The marginal ice zone (MIZ) is the band of partially ice-covered water that separates the ice-free ocean and the main Arctic or Antarctic sea-ice pack
The impact of MIZ surface drag on the atmosphere has been evaluated through three sets of numerical weather prediction model simulations
The "L12E16" simulations make use of a physically based drag scheme (Lüpkes et al, 2012) that has recently been tuned with a relatively large set of observations (Elvidge et al, 2016b); while the other two simulations make use of the drag scheme used in the former operational forecast configuration of the Met Office Unified Model (Rough), and that commonly used in climate model configurations of the MetUM (Smooth)

Summary

INTRODUCTION

The marginal ice zone (MIZ) is the band of partially ice-covered water that separates the ice-free ocean and the main Arctic or Antarctic sea-ice pack. Atmospheric boundary-layer changes across the MIZ are a result of changes in the surface exchange of momentum, heat and moisture These are challenging quantities to measure directly, but observations are vital to understand BL processes and represent these fluxes in numerical models. One Lüpkes et al (2012) scheme is becoming widely adopted (Table 1; Figure 2); for example, it is available as an option in the CICE sea-ice model (Tsamados et al, 2014; CICE Consortium, 2019) and has recently been implemented by us in the Met Office Unified Model (MetUM) This scheme is summarized by E2016, who show that the functional form of this parametrization is a good fit to their large set of aircraft-based observations.

The model

Observational data

Experimental design

Findings

CONCLUSIONS AND DISCUSSION