Boundary-layer height detection with a ceilometer at a coastal ... - Orbit
Boundary-layer height detection with a ceilometer at a coastal ... - Orbit
Boundary-layer height detection with a ceilometer at a coastal ... - Orbit
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5.9 Intra-annual vari<strong>at</strong>ion<br />
The BLH estim<strong>at</strong>es made <strong>with</strong> the four different BLH <strong>detection</strong> methods are used to derive<br />
a monthly vari<strong>at</strong>ion of the mean BLH. Figure 55 illustr<strong>at</strong>es the mean BLH estim<strong>at</strong>ed in westerly<br />
and easterly wind conditions. The four different methods show very similar curves. This<br />
analysis shows generally the highest BLH estim<strong>at</strong>es <strong>with</strong> the critical threshold method and<br />
the lowest <strong>with</strong> the vertical gradient method. The exponent idealized profile tends to show<br />
higher BLH estim<strong>at</strong>es than the idealized profile method.<br />
All the BLH <strong>detection</strong> methods show the highest mean BLH in November when winds<br />
are predominantly from the west. The mean BLH in westerly winds increases from June to<br />
November. The high BLHs in the autumn may be explained by the temper<strong>at</strong>ure difference<br />
of the air and sea surface. The rel<strong>at</strong>ively warm sea surface in autumn may lead to unstable<br />
conditions <strong>with</strong> deep boundary-<strong>layer</strong> observ<strong>at</strong>ions.<br />
The curves showing the mean BLHs in easterly wind conditions are more fluctu<strong>at</strong>ing compared<br />
<strong>with</strong> those for westerly winds. The BLH <strong>detection</strong> methods all show the highest mean<br />
BLH in April 2010. From October 2010 to January 2011 the mean BLH estim<strong>at</strong>es are generally<br />
low and decrease slightly throughout the period.<br />
BLH [m]<br />
Critical threshold<br />
800<br />
750<br />
700<br />
650<br />
600<br />
550<br />
500<br />
450<br />
400<br />
350<br />
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar<br />
Month<br />
BLH [m]<br />
Idealized profile<br />
800<br />
750<br />
700<br />
650<br />
600<br />
550<br />
500<br />
450<br />
400<br />
350<br />
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar<br />
Month<br />
BLH [m]<br />
Exponent id. profile<br />
800<br />
750<br />
700<br />
650<br />
600<br />
550<br />
500<br />
450<br />
400<br />
350<br />
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar<br />
Month<br />
BLH [m]<br />
Vertical gradient<br />
800<br />
750<br />
700<br />
650<br />
600<br />
550<br />
500<br />
450<br />
400<br />
350<br />
Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar<br />
Month<br />
Figure 55: Intra-annual vari<strong>at</strong>ion of the BLH estim<strong>at</strong>es from April 2010 – March 2011. The<br />
green line denotes the easterly wind BLH estim<strong>at</strong>es and the purple line denotes the westerly<br />
wind BLH estim<strong>at</strong>es.<br />
66 DTU Wind Energy Master Thesis M-0039