Measuring Meteorology in Urban Areas – Some Progress and Many Problems - Meteorological and Air Quality Models for Urban Areas - page 116

variability in the spatial wind field and without a systematic horizontal pattern.
It would be very interesting in a similar way to study the vertical wind profile
and horizontal variability of the wind over the roofs of buildings in an urban
area.
12.2 The Height of the Boundary Layer
The best way to estimate the height of the boundary layer is from profiles of
turbulence, momentum and heat flux, but this is generally not possible because
meteorological masts do not reach far enough into the boundary layer except under
special conditions. The height of the boundary layer is a parameter that despite its
obvious importance (Gryning et al., 1987) is often unfairly treated or even neglected
in experimental campaigns. This also holds for campaigns in urban areas. Neverthe-
less it is an important meteorological parameter in the urban context because the
behaviour and characteristics of the urban boundary layer are quite different from
the usual perception over rural terrain, the main difference being the rarity of stable
conditions in a wide range of urban settlements.
The use of a remote sensing for routine measurements of the boundary layer
height is an interesting challenge. The ceilometer is a new not yet fully explored
instrument for boundary layer depth measurements. It is an inexpensive and sturdy
instrument originally developed for routine cloud height observations, consisting of
a vertically pointing laser and a receiver in the same location. It determines pro-
files by measuring the time required for a pulse of light to be scattered back from
the particles in the air. Since the instrument will note any returns, it is possible
to determine particle profiles by looking at the whole pattern of returned energy.
This has been developed in research and could be applied for operational purposes.
Assuming that particles within the boundary layer originate from the ground and
that the particle concentration above the boundary layer is comparatively small, the
height of the boundary layer can be determined from particle profiles measures by a
ceilometer.
In the Galathea expedition (http://www.galathea.nu/) a ceilometer was used suc-
cessfully to determine the depth of the marine boundary layer (Fig. 12.6). The mea-
surements were performed in the up welling zone west of Namibia. It can be seen
that the marine boundary layer is shallow, having a depth of about 200 m. Part of
the time atmospheric waves form on the top of the boundary layer. Obviously the
sea-spray is confined to the boundary layer and does not penetrate up into the free
atmosphere, constituting near ideal conditions for measurements of the depth of the
boundary layer by use of a ceilometer. The traditional way of measuring the bound-
ary layer height (Fig. 12.6c,d,e) is to look for jumps in the profiles from radiosound-
ings; especially in the potential temperature, but also humidity and wind speed and
direction, and the variability of the wind direction. In the radiosoundings performed
outside Namibia the top of the boundary layer is made known by 15 K increase of