NORA10EI: A revised regional atmosphere‐wave hindcast for the North Sea, the Norwegian Sea and the Barents Sea

Hilde Haakenstad,Øyvind Breivik,Ole J Aarnes,Magnar Reistad

doi:10.1002/joc.6458

Abstract

AbstractNORA10EI, a new atmosphere and wave hindcast for the Norwegian Sea, the North Sea and the Barents Sea is presented. The hindcast uses ERA‐Interim as initial and boundary conditions and covers the period 1979–2017. The earlier NORA10 hindcast used ERA‐40 as initial and boundary conditions before September 2002 and operational analyses from the European Centre for Medium‐Range Weather Forecasts (ECMWF) in the continuation. This change in initial and boundary conditions may lead to non‐stationarities in bias and random errors, and it is a question of some concern whether this also leads to spurious trends. We investigate this by comparing the two hindcasts. We find only minor differences in the statistics of means and upper percentiles, but somewhat larger differences in the extremes (100‐year return values) of significant wave height and 10‐m winds. Generally, NORA10EI outperforms NORA10 in the ERA‐40 period (before September 2002) since ERA‐Interim outperforms ERA‐40. Conversely, NORA10 outperforms NORA10EI after 2006, since the operational ECMWF analyses here outperform ERA‐Interim. Years 2002–2006 is a transition period with minor differences between the NORA10 and NORA10EI where the resolution of ERA‐Interim is lower than that of the ECMWF analyses, but its physics are from a more recent model (2006). An important finding is that the regional hindcasts appear quite insensitive to changes in the host reanalysis with no statistically significant differences in mean and upper percentile trends of wind speed and wave height. A comparison of four polar low cases confirms that using ERA‐Interim as host reanalysis yields a slightly better representation of evolution and intensity of polar lows than NORA10 in the ERA‐40 period and the opposite after 2006.

Highlights

Regional downscaling of global reanalyses of the atmosphere and the wave field, known as hindcasts, are a cheap and useful supplement to regional reanalyses as they are affordable on much higher lateral resolutionInt J Climatol. 2020;1–27.wileyonlinelibrary.com/journal/jocHAAKENSTAD ET AL.without the need for an expensive data assimilation system
This hindcast study has deliberately been set up on the same model domain and with a configuration which closely matches that of NORA10. This is because we aim to investigate the sensitivity of a regional hindcast to host analysis forcing, and to what extent NORA10 is affected by the change in forcing data in 2002 where the transition from ERA-40 to European Centre for Medium-Range Weather Forecasts (ECMWF) analyses takes place
This study, and the generation of the NORA10EI hindcast, was in part motivated by a need to test the impact of an abrupt change in the forcing fields in the NORA10 hindcast as it is extensively used for extreme value analysis and climatological studies of wind and wave height in the North Sea, the Norwegian Sea and the Barents Sea (Aarnes et al, 2012; Furevik and Haakenstad, 2012; Bruserud and Haver, 2016), but more generally to look for the impact of host analysis on regional hindcasts

Summary

Introduction

Regional downscaling of global reanalyses of the atmosphere and the wave field, known as hindcasts (a model run without data assimilation but constrained by a reanalysis on the boundaries and as initial conditions), are a cheap and useful supplement to regional reanalyses as they are affordable on much higher lateral resolutionInt J Climatol. 2020;1–27.wileyonlinelibrary.com/journal/jocHAAKENSTAD ET AL.without the need for an expensive data assimilation system. Regional downscaling of global reanalyses of the atmosphere and the wave field, known as hindcasts (a model run without data assimilation but constrained by a reanalysis on the boundaries and as initial conditions), are a cheap and useful supplement to regional reanalyses as they are affordable on much higher lateral resolution. Wave hindcasts are useful since the regional wave climate requires relatively high resolution to resolve topographical features that modify the wind field and obstruct the wave field. A number of regional (e.g., Bromirski et al, 2013; Izaguirre et al, 2013; Appendini et al, 2014; Semedo et al, 2015), basin-scale (Wang et al, 2012) and global (Semedo et al, 2011; Aarnes et al, 2015; Meucci et al, 2019) studies on wave climate variability and trends from hindcasts and reanalyses have recently been presented. Common to all of them is that they take their boundary conditions from global reanalyses

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