3 research outputs found
Validating CFD predictions of flow over an escarpment using ground-based and airborne measurement devices
Micrometeorological observations from a tower, an eddy-covariance (EC) station and an unmanned aircraft system (UAS) at the WINSENT test-site are used to validate a computational fluid dynamics (CFD) model, driven by a mesoscale model. The observation site is characterised by a forested escarpment in a complex terrain. A two-day measurement campaign with a flow almost perpendicular to the escarpment is analysed. The first day is dominated by high wind speeds, while, on the second one, calm wind conditions are present. Despite some minor differences, the flow structure, analysed in terms of horizontal wind speeds, wind direction and inclination angles shows similarities for both days. A real-time strategy is used for the CFD validation with the UAS measurement, where the model follows spatially and temporally the aircraft. This strategy has proved to be successful. Stability indices such as the potential temperature and the bulk Richardson number are calculated to diagnose atmospheric boundary layer (ABL) characteristics up to the highest flight level. The calculated bulk Richardson values indicate a dynamically unstable region behind the escarpment and near the ground for both days. At higher altitudes, the ABL is returning to a near neutral state. The same characteristics are found in the model but only for the first day. The second day, where shear instabilities are more dominant, is not well simulated. UAS proves its great value for sensing the flow over complex terrains at high altitudes and we demonstrate the usefulness of UAS for validating and improving models
Quantifying horizontal and vertical tracer mass fluxes in an idealized valley during daytime
The transport and mixing of pollution during the daytime evolution of a
valley boundary layer is studied in an idealized way. The goal is to quantify
horizontal and vertical tracer mass fluxes between four different valley
volumes: the convective boundary layer, the slope wind layer, the stable
core,
and the atmosphere above the valley. For this purpose, large eddy simulations
(LES)
are conducted with the Weather Research and Forecasting (WRF) model for a
quasi-two-dimensional valley. The valley geometry consists of two slopes with
constant slope angle and is homogeneous in the along-valley direction. The
surface sensible heat flux is horizontally homogeneous and prescribed by a
sine function. The initial sounding is characterized by an atmosphere at rest
and a constant Brunt–Väisälä frequency. Various experiments are conducted
for different combinations of surface heating amplitudes and initial
stability conditions. A passive tracer is released with an arbitrary but
constant rate at the valley floor and resulting tracer mass fluxes are
evaluated between the aforementioned volumes.As a result of the surface heating, a convective boundary layer is
established in the lower part of the valley with a stable layer on top –
the so-called stable core. The height of the slope wind layer, as well as the wind
speed within, decreases with height due to the vertically increasing stability.
Hence, the mass flux within the slope wind layer decreases with height as
well. Due to mass continuity, this along-slope mass flux convergence leads to
a partial redirection of the flow from the slope wind layer towards the
valley centre and the formation of a horizontal intrusion above the
convective boundary layer. This intrusion is associated with a transport of
tracer mass from the slope wind layer towards the valley centre. A strong
static stability and/or weak forcing lead to large tracer mass fluxes
associated with this phenomenon. The total export of tracer mass out of the
valley atmosphere increases with decreasing stability and increasing forcing.
The effects of initial stability and forcing can be combined to a single
parameter, the breakup parameter B. An analytical function is presented
that describes the exponential decrease of the percentage of exported tracer
mass with increasing B. This study is limited by the idealization of the
terrain shape, stratification, and forcing, but quantifies transport
processes for a large range of forcing amplitudes and atmospheric stability