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

Threshold
Westerly winds
Ideal profile
120
100
NW
SW
120
100
NW
SW
counts
80
60
counts
80
60
40
40
20
20
0
0 500 1000 1500 2000
BLH
0
0 500 1000 1500 2000
BLH
Exponent−ideal profile
Vertical gradient
120
100
NW
SW
120
100
NW
SW
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 46: Frequency distribution of all BLH estimates in April 2010. The estimates are divided
into winds from SW (green) and NW (blue).
are observed with peaks at approximately the same heights.
May 2010
Frequency distributions for the westerly winds in May 2010 are here analysed. Easterly winds
are rarely observed during this month, only in 231 out of 3310 10 min observations. Dividing
the westerly wind into SW and NW shows that winds from SW only occur in 75 cases, while
winds from NW occur in 2058 cases. North west is the dominating wind direction this month.
Method Day mean BLH [m] Night mean BLH [m]
Critical threshold 506 505
Idealized profile 485 481
Exp. ideal. profile 495 487
Vertical gradient 495 490
Table 7: Mean BLH in the daytime and nighttime May 2010 north westerly winds.
Figure 47 shows the BLH detections made when the wind direction is from west and further
divided into day and night. The distributions are bi-modal but this is more clearly seen in
the daytime estimates. The mean BLH estimates are approximately the same for day and
nighttime, as the day and night sea surface temperature difference is small, due to the large
heat capacity of the ocean mixed layer. All the BLH detection methods detect a distinct peak
at 750–800 m. At 200–250 m a peak is detected for the exponent- and idealized profiles. The
critical threshold method shows a peak at 250–300 m and the vertical gradient shows a peak
at 350–400 m.
DTU Wind Energy Master Thesis M-0039 57