Abstract. Ten years of ERA5 reanalysis data are combined with met-mast and lidar
observations from 10 offshore platforms to investigate low-level jet
characteristics over the Dutch North Sea. The objective of this study is to
combine the best of two worlds: (1) ERA5 data with a large spatiotemporal
extent but inherent accuracy limitations due to a relatively coarse grid and
an incomplete representation of physical processes and (2) observations that
provide more reliable estimates of the measured quantity but are limited in
both space and time. We demonstrate the effect of time and range limitations
on the reconstructed wind climate, with special attention paid to the impact
on low-level jets. For both measurement and model data, the representation of wind speed is
biased. The limited temporal extent of observations leads to a wind speed
bias on the order of ±1 m s−1 as compared to the long-term mean. In
part due to data-assimilation strategies that cause abrupt discontinuities in
the diurnal cycle, ERA5 also exhibits a wind speed bias of approximately 0.5 m s−1. The representation of low-level jets in ERA5 is poor in terms of a
one-to-one correspondence, and the jets appear vertically displaced (“smeared
out”). However, climatological characteristics such as the shape of the
seasonal cycle and the affinity with certain circulation patterns are
represented quite well, albeit with different magnitudes. We therefore
experiment with various methods to adjust the modelled low-level jet rate to the
observations or, vice versa, to correct for the erratic nature of the short
observation periods using long-term ERA5 information. While quantitative
uncertainty is still quite large, the presented results provide valuable
insight into North Sea low-level jet characteristics. These jets occur
predominantly for circulation types with an easterly component, with a clear
peak in spring, and are concentrated along the coasts at heights between 50 and 200 m.
Further, it is demonstrated that these characteristics can be used as
predictors to infer the observed low-level jet rate from ERA5 data with
reasonable accuracy.
Climatic factors contributing to long-term variations of fine dust concentration in the United States
