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

1400
β profile 01/05/2010 15:20
1400
δβ smooth
/δz profile 01/05/2010 15:20
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Height [m]
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Height [m]
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Backscatter [1/(10 8 *srad*m)]
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Backscatter gradient [1/(10 8 *srad*m 2 )]
Figure 10: On the left: The vertical backscatter profile. On the right: The vertical gradient
of the smoothed backscatter profile. Dashed line shows the BLH estimate. Data is from the
ceilometer on May 1 2010.
Fluctuations due to noise in the backscatter profile may give large noise induced gradients.
These may sometimes be misinterpreted in automated BLH detection programs as the BLH.
These misinterpretations may be reduced by smoothing of the backscatter profile and then
calculating the vertical gradient of the smoothed profile. In this study the backscatter profile
is smoothed with a moving average filter spanning 7 measuring heights before calculating the
gradient. The vertical gradient method is computationally simple and therefore comparatively
fast in automatic BLH detection programs. In Figure 10 is seen the vertical backscatter
profile along with the calculated vertical gradient of the smoothed backscatter profile. The
BLH estimate is made where the minimum of the backscatter gradient is found.
4.2 Critical threshold value of the aerosol profile
In the ABL the aerosol concentration and the backscatter values are relatively high. Therefore
one method of estimating the BLH is by applying a critical threshold value to the backscatter
profile. This value will lie in the transition from high backscatter values to low, and would best
represent the BLH as approximately half the mean mixed layer backscatter. As the mean mixed
layer backscatter varies considerably (∼50–300/(10 8 m srad)), a constant critical value will
overestimate the BLH when aerosol concentrations are relatively high, and underestimate, or
even not estimate the BLH at all, when aerosol concentrations are low.
An automated BLH detection algorithm is therefore developed for this study with a variable
critical threshold value. The approach of the method is to define a critical threshold value,
β crit and a ’maximum’ mean backscatter value β m . Here set to β crit = 100/(10 8 m srad)
and β m =150/(10 8 m srad). The first height with backscatter values below β crit is then
taken as a preliminary BLH estimate. Next, the mean backscatter from ground level to that
height is calculated. If this value is above β m the BLH estimate is used, else it is discarded
and the process is repeated with lower β crit and β m values (here 60/(10 8 m srad) and
DTU Wind Energy Master Thesis M-0039 23