Ph.D. Seminar, Risoe 4.-5.10.2006
The influence of atmospheric circulation
patterns on surface winds above North Sea
Kay Sušelj, Abha Sood, Detlev Heinemann
Ph.D seminar Risoe 4.-5.10.2006
Goals of the study
Investigation of surface wind (at 10 m height) variability
above North Sea: Comparison of the reanalysis data (ECMWF ERA-40 and NCEP/NCAR)
Statistics of wind speed and wind time series
Understanding the relationship between wind and circulation
patterns: Wind Speed and North Atlantic Oscillation (NAO)
Empirically derived circulation patterns from Sea Level Pressure (SLP)
Ph.D seminar Risoe 4.-5.10.2006
Reanalysis data used
ECMWF ERA-40
Time span: Sep. 1957- Dec. 2001
Spatial resolution: 1°×1°
NCEP/NCAR Reanalysis 2
Time span: Jan. 1948 - present
Spatial resolution: 1.9°×1.85° (wind) and 2.5°×2.5° (pressure)
Spatial resolution of both datasets: 6 hoursArea under investigation
Ph.D seminar Risoe 4.-5.10.2006
Comparison of ERA-40 and NCEP/NCAR
data (1958-2001)ERA 40 NCEP/NCARMean wind speed
Time series of spatially averagedWS (3 yrs running average filter)
Whole year Winter (DJFM) Summer (JJAS)
Ph.D seminar Risoe 4.-5.10.2006
ERA 40
Comparison (cont.)NCEP/NCAR
Probability density of mean monthly wind speed
Probability density of mean monthly wind angle
Ph.D seminar Risoe 4.-5.10.2006
EOF analysis (dominant modes) WSA - wind speed anomaly from the annual cycle
Empirical Orthogonal Funcitions (EOF) - isolate main field variability into
spatial EOF patterns and time series (principal components, PCs) EOF 1 (41.5%) EOF 2 (13.5%)
EOF 3 (12%) EOF 4 (6%)
Ph.D seminar Risoe 4.-5.10.2006
Large scale circulation patterns derived from
SLPA and WSA
Separate cases (days) for low (<-1), medium (btw. -1 and 1) and high (>1) PCs
Plot SLPA for all three cases
Derive circulation patterns from plots
Use indices (time series of circulation patterns) to predict WSA
Compare predicted and 'real' WSA
How good is North Atlantic Oscilation Index (NAOI) in predicting NS
WSASummer NAOI and PC 1 of WSAWinter NAOI and PC 1 of WSA
Correlation btw. winter NAOI and
WSA
Can we find better predictors for WSA?
Ph.D seminar Risoe 4.-5.10.2006
PCs of WSA and its relation to
circulation patterns ... 1st PC of WSA
SLPA for low PC of WSA SLPA for medium PC of WSA SLPA for high PC of WSA
First PC, measured & estimated
from circulation pattern
Correlation btw. WSA and WSA
estimated from circ. pat.
Correlation btw. winter WSA and
WSA estimated from circ. pat.
Ph.D seminar Risoe 4.-5.10.2006
... and 2nd & 3rd PCs of WSA
Low values High values
2nd P
C3rd
PC
Ph.D seminar Risoe 4.-5.10.2006
Correlation btw. WSA and WSA predicted
from three circulation patterns
Whole year
Summer
Winter
Ph.D seminar Risoe 4.-5.10.2006
The WS above NW Europe has a pronounced annual cycle, with the highest
values in the cold part of the year
Greater part of the WSA varies coherently over studied domain
The winter WSA shows the pronounce increase over the last half of the
century, slight decrease over the last decade
The WSA over the NW Europe is parly determined by circulation indices as
SLPA difference between:
West of Iberia and west of Iceland
Scandinavia and Iceland
Above Great Britain
Conclusions and plans for further work
Improvement of the wind surface data base (downscaling reanalysis data)
Special effort toward reliable boundary layer parameterizations
Detailed estimation of the past and future climate and its relation to the
wind resources (variability, trends)
Ph.D seminar Risoe 4.-5.10.2006
Acknowledment
The work is founded by the EU Commission within
the ModObs project (MRTN-CT-2005-019369).
ECMWF and NCEP/NCAR gratefully provided the
reanalysis data.