@article{wes-4-193-2019,
title = {Low-level jets over the North Sea based on ERA5 and observations: together they do better},
author = {Peter C. Kalverla and J. B. Duncan Jr. and Gert-Jan Steeneveld and Albert A. M. Holtslag},
doi = {10.5194/wes-4-193-2019},
year = {2019},
date = {2019-04-03},
journal = {Wind Energy Science},
volume = {4},
number = {2},
pages = {193--209},
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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{doi:10.1002/we.2267,
title = {Evaluation of three mainstream numerical weather prediction models with observations from meteorological mast IJmuiden at the North Sea},
author = {Peter C. Kalverla and Gert-Jan Steeneveld and Reinder J. Ronda and Albert A. M. Holtslag},
doi = {10.1002/we.2267},
year = {2018},
date = {2018-00-00},
journal = {Wind Energy},
volume = {0},
number = {0},
abstract = {Abstract Numerical weather prediction models play an important role in the field of wind energy, for example, in power forecasting, resource assessment, wind farm (wake) simulations, and load assessment. Continuous evaluation of their performance is crucial for successful operations and further understanding of meteorology for wind energy purposes. However, extensive offshore observations are rarely available. In this paper, we use unique met mast and Lidar observations up to 315 m from met mast “IJmuiden,” located in the North Sea 85 km off the Dutch coast, to evaluate the representation of wind and other relevant variables in three mainstream meteorological models: ECMWF-IFS, HARMONIE-AROME, and WRF-ARW, for a wide range of weather conditions. Overall performance for hub-height wind speed is found to be comparable between the models, with a systematic wind speed bias <0.5 m/s and random wind speed errors (centered RMSE) <2 m/s. However, the model performance differs considerably between cases, with better performance for strong wind regimes and well-mixed wind and potential temperature profiles. Conditions characterized by moderate wind speeds combined with stable stratification, which typically produce substantial wind shear and power fluctuations, lead to the largest misrepresentations in all models.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Kalverla2017,
title = {An observational climatology of anomalous wind events at offshore meteomast IJmuiden (North Sea)},
author = {Peter C. Kalverla and Gert-Jan Steeneveld and Reinder J. Ronda and Albert A. M. Holtslag},
doi = {10.1016/j.jweia.2017.03.008},
year = {2017},
date = {2017-06-00},
journal = {Journal of Wind Engineering and Industrial Aerodynamics},
volume = {165},
pages = {86–99},
abstract = {Uncertainty reduction in offshore wind systems heavily relies on meteorological advances. A detailed characterization of the wind climate at a given site is indispensable for site assessment, and its accurate representation in load assessment models can reduce costs of turbine design and the risk of failure. While regular wind conditions are reasonably described by established methods, some atypical wind conditions are poorly understood and represented, although they contribute substantially to load on turbines. In this study, 4 years of high-quality observations gathered up to 300 m are analyzed to characterize the wind climate at the IJmuiden tower, focusing on these ill-defined conditions. Following a systematic approach, six ‘anomalous wind events’ are identified and described: low-level jets, extreme wind speeds, shear, veer, turbulence and wind ramps. In addition, we identify typical weather conditions that favour their formation. Stable stratification in spring and summer leads to low-level jets (up to 12% of the time) for moderate wind conditions, and to extreme wind shear for stronger wind regimes. Typical wind ramps lead to a change in wind speed of 2 m/s in one hour. The applicability of turbulence intensity as a measure of turbulence and gusts is found to be questionable.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}