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

1 Introduction
The study of our Earth and its atmosphere has led to an understanding that the atmosphere
is far from homogeneous, but instead complex and dynamical. The atmospheric boundary
layer (ABL) may be defined as the lowest part of the troposphere, where the atmospheric
flow is directly influenced by the surface, and responds to changes in the surface forcings
within an hour or less Stull (1988). It is an important factor in understanding the lowest
part of the atmosphere, and in many different branches of atmospheric research it is used
for modelling and experimental considerations. Similarly the height of the ABL, called the
boundary-layer height (BLH) separates the turbulent ABL from the free atmosphere (FA)
above, and is an essential parameter in defining the ABL. As transport of many quantities
such as pollutants, energy, water vapor and aerosols is governed by the dynamics of wind,
the BLH plays an important role in both theoretical and numerical meteorological studies, e.g.
- When working with wind energy and the placement of wind turbines, wind profile models
are applied, where the BLH is an important parameter of the vertical wind speed profile
(Peña et al., 2010).
- Pollution dispersion models. The BLH is a necessary parameter in models simulating
turbulent diffusion and transport of pollutants in the atmosphere (Seibert et al., 1998).
- Improvement of mesoscale numerical weather prediction models to predict the BLH at a
regional scale (Gryning and Batchvarova, 2002).
Despite the importance of the BLH, it does not have an exact definition, or straightforward
method of measurement. It is a parameter hard to define, and cannot always be inferred from
a simple equation, especially not in convective conditions (Batchvarova and Gryning, 1991).
Synoptic weather, vertical wind shear and stability conditions at the surface all influence the
value of the BLH, which may vary significantly in time and space.
Also, as the BLH is often found in the range of hundreds of meters to kilometers, conventional
instruments such as radio sondes and masts often lack either the temporal or spatial
resolution to provide sufficient data for determining the BLH satisfactorily.
Remote sensors such as lidar are therefore a valuable tool in measuring the BLH. Even so,
a remote sensor cannot measure all meteorological conditions. Therefore, when studying the
atmospheric BLH, it may be advantageous to include different methods and approaches for
a complete picture of the key atmospheric processes.
The method for BLH detection in this thesis is primarily by use of a ceilometer, which
will be presented in detail in section 3. A major advantage of using remote sensors such as
a ceilometer, is that it measures continuously and over a large vertical range. In contrast to
radio sondes which are limited by the rate of release and masts being limited by their height.
In this study a new BLH detection method is described and a new efficient way of handling
complications arising in the presence of clouds is presented and tested. For the first time a
detailed analysis of frequency distributions of BLH estimates is performed on data from an
entire year of measurements at the measuring site Høvsøre. Further, the BLH and its relation
to specific conditions of wind, temperature and season is investigated.
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