Low-level jets over the North Sea based on ERA5 and observations: together they do better

Abstract

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.

Quality of wind characteristics in recent wind atlases over the North Sea

“…A remarkable discontinuity in the diurnal cycle of wind speed was revealed in the ERA‐5 data by Kalverla et al . (). At 1000 UTC, the wind speed bias suddenly strengthens.…”

“…More details can be found in (e.g. Kalverla et al ., , ). The large temporal extent, distance to shore and vertical measurement range makes this dataset optimally suited to characterise the wind climate.…”

“…In this study, we present a first evaluation of wind and anomalous wind events in the aforementioned three wind atlases against observations from a prospective wind farm site in the North Sea, 85 km off the Dutch coast – met mast IJmuiden (MMIJ). MMIJ is located far enough from the coast and spans a long enough period of time to show reasonable agreement with the ERA‐5 data (Kalverla et al ., ). The relatively long time span of the MMIJ dataset allows for reliable statistics (better than other platforms).…”

“…Moreover, Kalverla et al . () extensively compared the ERA‐5 wind speed data against observations at multiple sites over the North Sea and found that the overall representation of wind speed was quite good, with a maximum root mean square error of 1.5 m·s −1 . The superior performance of ERA‐5 as compared to short forecasts in Kalverla et al .…”

“…In their validation of ERA‐5, Kalverla et al . () focused on one of these events: the LLJ. They showed that the representation of LLJs in ERA‐5 was mediocre: one‐to‐one correspondence was poor, the LLJs seemed to be vertically displaced (too high) and their magnitude underestimated, but the climatological frequency representation of LLJ characteristics was reasonable.…”