A Study of the Surface Layer Atmospheric Stability

at Two UK Offshore Sites

Mr. Peter ArgyleCRESTLoughborough UniversityProf. Simon Watson

CRESTLoughborough University

Outline

• Site description and comparison

• Brief comment about humidity and temperature within the Monin-Obukhov similarity theory

• Discussion on stability distribution with respect to• Stability changes with wind speed • Stability changes with wind direction• Stability changes with time

Site Information

Shell Flats

Scroby Sands

Wind Speed & Direction

Temperature

Humidity

Atmospheric Pressure

Shell Flats

80m82m

70m

50m

30m

20m

12m

Scroby Sands

51m47m

33m

19m14m

•1.5 year’s data at Shell Flats

•7 year’s data at Scroby Sands

•10 minute averaged data

•Filter data to satisfy Monin– Obukhov assumptions

Considering Temperature

Virtual Temperature

(°C)

Potential Temperature (°C)

Data from Shell Flats site at 12m

Potential Temperature• Barthelmie 1999

Virtual Temperature• Lange et al 2004

Virtual Potential Temperature• Edson and Fairall 1998• Peña et al 2008• Sathe et al 2011

Initial Results

Obukhov Length Stability Class Notation

-200 < L < 0 Very Unstable VU

-1000 < L < -200 Unstable U

1000 < L < -1000 Neutral N

200 < L < 1000 Stable S

0 < L < 200 Very Stable VS

Scroby Sands

Shell Flats

Number of events used for analysis

22585 7569

Percentage of collected dataset

38% 14%

Fre

quen

cy o

f O

ccur

renc

e (%

)

Scroby Sands Shell Flats

Wind Speed Results

Shell FlatsScroby SandsShell Flats

Wind Direction Results

Wind Direction

Dis

tanc

e to

Sho

re (

km)

Fre

quen

cy o

f O

ccur

renc

e (%

)Wind Direction

Dis

tanc

e to

Sho

re (

km)

Fre

quen

cy o

f O

ccur

renc

e (%

)

Scroby Sands Shell Flats

Monthly ResultsF

requ

ency

of O

ccur

renc

e (%

)

MonthF

requ

ency

of O

ccur

renc

e (%

)Month

Scroby Sands Shell Flats

Hourly Results

HourF

requ

ency

of O

ccur

renc

e (%

)Hour

Scroby Sands Shell Flats

Fre

quen

cy o

f Occ

urre

nce

(%)

Conclusions

• Greater consistency is needed between site measurements• Similar instruments means similar variables can be compared• Similar heights makes comparison simpler

• The marine atmosphere is rarely in neutral conditions

• Monin – Obukhov similarity theory discards too much data

References•Barthelmie: “The Effects of Atmospheric Stability on Coastal Wind Climates” Meteorol. Appl., 1999, 6, 39-47•Lange, Larsen, Højstrup and Barthelmie: “The Influence of Thermal Effects on the Wind Speed Profile of the Coastal Marine Boundary Layer” Boundary-Layer Meteorol., 2004, 112, 587-617•Peña, Gryning and Hasager: “Measurements and Modelling of the Wind Speed Profile in the Marine Atmospheric Boundary Layer” Boundary-Layer Meteorol., 2008, 129, 479-495•Sathe, Gryning and Peña: “Comparison of the Atmospheric Stability and Wind Profiles at Two Wind Farm Sites Over a Long Marine Fetch in the North Sea” Wind Energy, 2011, 14, 767-780•Edson and Fairall: “Similarity Relationships in the Marine Atmospheric Surface Layer for Terms in the TKE and Scalar Variance Budgets” J. Atmos. Sci., 1998, 55, 2311-2328•Zilitinkevich, Johansson, Mironov and Baklanov: “A Similarity-theory Model for Wind Profile and Resistance Law in Stably Stratified Planetary Boundary Layers” J. Wind Eng. Ind. Aerodyn., 1998, 74-76, 209-218

Questions

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