Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 129

Fig. 4.35 Time-height cross sections of sodar wind measurements featuring a nocturnal low-level
jet over Essen, Germany, on May 28/29, 1997. Left : wind speed (purple: more than 10 m s − 1 wind
speed, blue: less than 1 m s − 1 wind speed). Right : Variance of the vertical velocity component as
measure for the turbulence ( blue : low values, green : intermediate values, red :highvalues)
of a low-level jet. Low-level jets have also coined the nocturnal wind profiles dis-
played in Figs. 4.15 and 4.17 . Low-level jets had occurred so frequently in the spring
and summer months depicted in these figures that they were detectable even from
monthly mean wind profile data. Furthermore, this example is a proof that low-level
jets are a regional phenomenon that develops on a horizontal scale of several hun-
dreds of kilometres. Figures 4.15 and 4.17 show wind profiles obtained over a town
which is situated quite isolated in a larger relatively flat rural area. The nocturnal
urban surface layer conditions themselves (enhanced turbulence due to the high sur-
face roughness and near-neutral to even slightly unstable thermal stratification due
to the large heat capacity of the urban surface) would not have been suited for the
development of low-level jets. The low-level jets rather developed over the larger
rural environment of this city. The city itself is too small to eradicate the low-level
jets, thus the jets are discernable from these urban wind profiles.
4.5.2 Gust Fronts and Cold Air Outflows
Gust fronts are a typical boundary layer phenomenon of larger thunderstorms
(Emeis 1983 ). These fronts mark the leading edge of the spreading cold air under-
neath the thunderstorms. The boundary layer cold air pools of thunderstorms are
fed by cold downdraughts and by evaporational cooling. Another source for cold air
outflows is the formation of cold surface air due to nocturnal radiative cooling in
mountainous terrain. This second type of cold air outflow is addressed in Sections
4.5.4 and 4.5.5 below.
Figure 4.36 shows Doppler wind LIDAR observations of an evolving thunder-
storm gust front (Intrieri et al. 1990 ) from range-height indicator (RHI) scans. In
the figure the cold air flows from the left-hand side towards the LIDAR and the
line demarks the boundary of the cold air. The depicted wind arrows in this ver-
tical cross section indicate the horizontal and vertical wind components within the
plane of this cross section. The vertical flow component was derived from the quasi-
horizontal wind data using the continuity equation and assuming vanishing vertical