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

4 Boundary-layer height detection
Traditionally, BLH is derived from data measured with radiosondes. The radiosonde measurements
provide measurements of temperature, dewpoint and pressure through the lower
troposphere. From these measurements vertical profiles of potential temperature and water
vapour mixing ratio may be attained. The BLH is associated with sharp changes in the vertical
profiles and is often identified subjectively from the profiles. Objective BLH detection from
radiosonde data exist as well and are described by Seibert et al. (1998). The BLH estimates
derived from radiosonde data are limited to the launch time of the radiosondes, which is
typically 2–4 times a day.
In this way remote sensing systems have an advantage in BLH estimations, as they provide
continuous measurements. Besides lidar measurements, remote sensing is often performed
with wind profilers or sodars. The sodar basically may be used to derive the BLH from the
backscatter of sound waves. The intensity of the backscatter mainly depends on small-scale
temperature inhomogeneities, used to derive the BLH.
The wind profiler is similar to the acoustic sodar, with the main difference being its use
of electromagnetic waves instead of sound waves and that it apart from temperature also
measures moisture inhomogeneities (Seibert et al., 1998).
As mentioned, the lidar measurements rely on the aerosol concentration in the atmosphere.
In the ABL the aerosol concentration is high compared to the free atmosphere above and this
contrast is the basis for BLH detection from lidar measurements (Cohn and Angevine, 2000).
It is often seen that the aerosol content is closely related to the temperature profile, and the
different methods therefore provide similar BLH estimates. Even so, there may be notable differences
in the thermal structure and the aerosol content as mentioned by Emeis et al. (2004).
This is seen on clear days, near sunset, when a SBL starts to form near the surface, changing
the temperature profile, while the aerosol content in the residual layer is not immediately
affected by this formation. The SBL is therefore generally detected later during the nighttime
with aerosol measurements. Although most BLH detection methods focus on mixed layer detection,
it is also possible to detect the SBL with lidars and ceilometers (Martucci et al., 2007).
When looking at an aerosol backscatter profile under ideal cloud free conditions during
daytime, the entrainment zone and the mixed layer may clearly be seen. In these situations
the BLH may be identified by visual inspection. However, when the amount of backscatter
data is large, this method will be time consuming. In that case an automated BLH detection
program will be preferable.
There are numerous ways to detect the BLH from aerosol backscatter data, e.g. wavelet
analyses (Davis et al., 1997), variance method (Martucci et al., 2010), the vertical gradient
(Schäfer et al., 2004; Emeis et al., 2008), critical threshold (Batchvarova et al., 1999) and
fitting of an ideal profile (Steyn et al., 1999). Three of the methods are presented here.
4.1 The vertical gradient of the aerosol profile
The vertical gradient of the backscatter profile is calculated with a central-difference formula
∂β
∂z (z i) ≈ β(z i + ∆z) − β(z i − ∆z)
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2∆z
where β is the backscatter, z i a reference height and ∆z = 20m (from the ceilometer settings).
The minimum value of the gradient often indicates the BLH.
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