Boundary-layer height detection with a ceilometer at a coastal ... - Orbit

140
120
Threshold
night
day
Easterly winds
140
120
Ideal profile
night
day
100
100
counts
80
60
counts
80
60
40
40
20
20
0
0 500 1000 1500 2000
BLH
0
0 500 1000 1500 2000
BLH
140
120
Exponent−ideal profile
night
day
140
120
Vertical gradient
night
day
100
100
counts
80
60
counts
80
60
40
40
20
20
0
0 500 1000 1500 2000
BLH
0
0 500 1000 1500 2000
BLH
Figure 49: Frequency distribution of all BLH estimates during October 2010, easterly wind
direction, divided in day and night.
wind directions are shown in table 10. The mean BLH is much higher when winds are north
westerly according to all methods. The explanation of the higher BLHs when winds are from
NW is given above for April 2010, but here the difference is more noticeable and the mean
BLHs significantly higher, as the sea surface temperature is higher in October than in April.
The detections of the critical threshold method show the highest mean BLH both with south
westerly and north westerly winds compared with the other methods. The vertical gradient
method results in the lowest mean BLH for both wind direction criteria.
Method SW mean BLH [m] NW mean BLH [m]
Critical threshold 633 884
Idealized profile 577 859
Exp. ideal. profile 602 870
Vertical gradient 558 826
Table 10: Mean BLH October 2010 north westerly- and south westerly winds.
February 2011
In Figure 51 the frequency distributions of the BLH estimates are shown for easterly wind
directions, divided into day and night for February 2011. The BLH estimates are found between
0 and 1800 m. The four different BLH detection methods show similar distributions and
generally higher BLHs during night than during daytime. The mean BLHs are seen in Table
11 and it is seen that the nighttime BLHs are not much higher, only ∼60 m higher than the
daytime BLHs. The lowest mean BLH estimates are made with the idealized profile, both day
60 DTU Wind Energy Master Thesis M-0039