Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 127

Fig. 4.33 Water vapour flux profiles from DIAL measurements (Wulfmeyer 1999 )
4.5 Regional and Local Flow Systems
Up to now we have statically analyzed the vertical structure of the ABL and verti-
cal profiles of different variables with the ABL. But there are also dynamical flow
processes taking place in the ABL, caused by temporal and spatial variations of
the surface conditions. Some of these regional and local flows can considerably
modify again the vertical structure of the ABL. The first two subsections in this
section deal with non-stationary phenomena. Low-level jets occur due to diurnal
variations in the static stability of the ABL in calm weather conditions but still
existing large-scale horizontal pressure gradients, while gust fronts and cold air out-
breaks are boundary-layer phenomena related to severe convective events such as
thunderstorms.
The second three subsections present flow features due to horizontal differences
in surface properties. While land-sea differences are mainly provoked by the dif-
ference in thermal inertia of these two surface types, mountainous flow features are
caused by mechanical and thermal influences.
4.5.1 Low-Level Jets
Low-level jets are a dynamical feature occurring near the top of nocturnal stable
boundary layers in synoptically undisturbed weather situations. Low-level jets have
already been mentioned in Section 4.3.1.2 , where optical wind sounding over flat
terrain was presented. More detailed measurements can often be made by acoustic