Evaluation of methods for estimating atmospheric stability at two coastal sites

Bardal, Lars Morten; Onstad, Anja Eide; Sætran, Lars Roar; Lund, John Amund

doi:10.1177/0309524x18780378

“…The gradient method is due to the postulated thermal layering of the MABL also found by Argyle and Watson (2014) to be dependant on the measuring heights used. There is therefore a degree of uncertainty related to the stability analysis, at several sites the stability distribution was therefore compared to previous studies and showed reasonable agreement (Høvsøre: Peña et al 2016, Skipheia: Bardal et al (2018), FINO1 and FINO3: Argyle and Watson (2014)). The use of different measurement heights was in addition thoroughly tested at FINO1, FINO2 and Skipheia, and although the stability distributions showed variation, the same tendencies prevailed and the same conclusions were drawn regardless of the measurement heights used.…”

Section: Atmospheric Stability Calculationmentioning

confidence: 75%

Review for WES-2019-40

Anonymous

2019

Preprint

0

View full text Add to dashboard Cite

Understanding the vertical wind profile is paramount for design & operation of wind turbines. It is needed not only for extrapolation of the wind velocity to hub height but also for structural load calculations, to name the most obvious issues. As wind turbines grow in size and development transitions offshore, issues such as shallow surface layers, low-level jets and internal boundary layers are raising questions to the applicability of the commonly used Monin-Obukhov similarity theory to accurately describe the vertical wind development to modern wind turbine hub heights. In this study the 10-minute averaged vertical wind profile up to a minimum elevation of 100m is analyzed through measurements collected from seven sites which represent a span of conditions. Three sites are located offshore in the North/Baltic Sea with varying fetch, two onshore by the Norwegian coast, one further onshore by the Danish coast, and one is an inland forested site in Sweden. Through analysis of data series ranging from 8 months to several years depending on the site, the wind profile has been quantitatively categorized according to the number of exhibited local maxima which are not possible within Monin-Obukhov similarity theory. The results reveal that the occurrence of local maxima scales inversely to the roughness length, causing 65 75% abnormal profiles offshore which decreases as the location transitions from offshore to coastal to further inland, and is lowest at the forested site. The results indicate that issues in predicting the vertical wind profile are most prevalent offshore, where very stable inflections cause severe deviations which may be related to an offshore internal boundary layer. These findings suggest that there is evident need of an improved vertical wind profile description in order to improve the accuracy of power predictions and load calculations, especially at offshore and coastal sites.

show abstract

“…The gradient method is due to the postulated thermal layering of the MABL also found by Argyle and Watson (2014) to be dependant on the measuring heights used. There is therefore a degree of uncertainty related to the stability analysis, at several sites the stability distribution was therefore compared to previous studies and showed reasonable agreement (Høvsøre: Peña et al (2016), Skipheia: Bardal et al (2018), FINO1 and FINO3: Argyle and Watson (2014)). The use of different measurement heights was in addition thoroughly tested at FINO1, FINO2 and Skipheia, and although the stability distributions showed variation, the same tendencies prevailed and the same conclusions were drawn regardless of the measurement heights used.…”

Section: Atmospheric Stability Calculationmentioning

confidence: 87%

“…A part of this study includes the investigation of the correlation between atmospheric stability and abnormal vertical wind profiles. The stability analysis was conducted using the Richardson number (Arya, 1988) to calculate the Obukhov length and subsequently dividing the occurrences into the 5 stability classes (very stable, stable, neutral, unstable, very unstable) using Obukhov length bins given by Bardal et al (2018). Certain sites in the analysis had ultrasonic anemometers which would have enabled a sonic method of stability calculation.…”

Section: Atmospheric Stability Calculationmentioning

confidence: 99%

Reply to Anonymous Referee 1

Møller¹

2019

Preprint

0

View full text Add to dashboard Cite

Understanding the vertical wind profile is paramount for design & operation of wind turbines. It is needed not only for extrapolation of the wind velocity to hub height but also for structural load calculations, to name the most obvious issues. As wind turbines grow in size and development transitions offshore, issues such as shallow surface layers, low-level jets and internal boundary layers are raising questions to the applicability of the commonly used Monin-Obukhov similarity theory to accurately describe the vertical wind development to modern wind turbine hub heights. In this study the 10-minute averaged vertical wind profile up to a minimum elevation of 100m is analyzed through measurements collected from seven sites which represent a span of conditions. Three sites are located offshore in the North/Baltic Sea with varying fetch, two onshore by the Norwegian coast, one further onshore by the Danish coast, and one is an inland forested site in Sweden. Through analysis of data series ranging from 8 months to several years depending on the site, the wind profile has been quantitatively categorized according to the number of exhibited local maxima which are not possible within Monin-Obukhov similarity theory. The results reveal that the occurrence of local maxima scales inversely to the roughness length, causing 65 75% abnormal profiles offshore which decreases as the location transitions from offshore to coastal to further inland, and is lowest at the forested site. The results indicate that issues in predicting the vertical wind profile are most prevalent offshore, where very stable inflections cause severe deviations which may be related to an offshore internal boundary layer. These findings suggest that there is evident need of an improved vertical wind profile description in order to improve the accuracy of power predictions and load calculations, especially at offshore and coastal sites.

show abstract

“…A part of this study includes the investigation of the correlation between atmospheric stability and abnormal vertical wind profiles. The stability analysis was conducted using the Richardson number (Arya, 1988) to calculate the Obukhov length and subsequently dividing the occurrences into the 5 stability classes (very stable, stable, neutral, unstable, very unstable) using Obukhov length bins given by Bardal et al (2018). Certain sites in the analysis had ultrasonic anemometers which would have enabled a sonic method of stability calculation.…”

Section: Atmospheric Stability Calculation 30mentioning

confidence: 99%

“…The gradient method is due to the postulated thermal layering of the MABL also found by Argyle and Watson (2014) to be dependant on the measuring heights used. There is therefore a degree of uncertainty related to the 15 stability analysis, at several sites the stability distribution was therefore compared to previous studies and showed reasonable agreement (Høvsøre: Peña et al (2016), Skipheia: Bardal et al (2018), FINO1 and FINO3: Argyle and Watson (2014)). The use of different measurement heights was in addition thoroughly tested at FINO1, FINO2 and Skipheia, and although the stability distributions showed variation, the same tendencies prevailed and the same conclusions were drawn regardless of the measurement heights used.…”

mentioning

confidence: 90%

Comparing Abnormalities in Onshore and Offshore Vertical Wind Profiles

Møller¹,

Domagalski²,

Sætran³

2019

Preprint

Self Cite

View full text Add to dashboard Cite

Understanding the vertical wind profile is paramount for design & operation of wind turbines. It is needed not only for extrapolation of the wind velocity to hub height but also for structural load calculations, to name the most obvious issues.As wind turbines grow in size and development transitions offshore, issues such as shallow surface layers, low-level jets and internal boundary layers are raising questions to the applicability of the commonly used Monin-Obukhov similarity theory to accurately describe the vertical wind development to modern wind turbine hub heights. In this study the 10-minute averaged 5 vertical wind profile up to a minimum elevation of 100m is analyzed through measurements collected from seven sites which represent a span of conditions. Three sites are located offshore in the North/Baltic Sea with varying fetch, two onshore by the Norwegian coast, one further onshore by the Danish coast, and one is an inland forested site in Sweden. Through analysis of data series ranging from 8 months to several years depending on the site, the wind profile has been quantitatively categorized according to the number of exhibited local maxima which are not possible within Monin-Obukhov similarity theory. The 10 results reveal that the occurrence of local maxima scales inversely to the roughness length, causing 65−75% abnormal profiles offshore which decreases as the location transitions from offshore to coastal to further inland, and is lowest at the forested site.The results indicate that issues in predicting the vertical wind profile are most prevalent offshore, where very stable inflections cause severe deviations which may be related to an offshore internal boundary layer. These findings suggest that there is evident need of an improved vertical wind profile description in order to improve the accuracy of power predictions and load 15 calculations, especially at offshore and coastal sites.

show abstract

Evaluation of methods for estimating atmospheric stability at two coastal sites

Cited by 20 publications

References 36 publications

Review for WES-2019-40

Review for WES-2019-40

Reply to Anonymous Referee 1

Comparing Abnormalities in Onshore and Offshore Vertical Wind Profiles

Contact Info

Product

Resources

About