Flux Footprints in the Convective Boundary Layer: Large-Eddy Simulation and Lagrangian Stochastic Modelling |
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Authors: | Xuhui Cai Jiayi Chen Raymond L Desjardins |
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Institution: | 1.Department of Environmental Sciences,Peking University,Beijing,China;2.Agriculture and Agri Food Canada, Research Branch,Ottawa,Canada |
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Abstract: | We investigated the flux footprints of receptors at different heights in the convective boundary layer (CBL). The footprints
were derived using a forward Lagrangian stochastic (LS) method coupled with the turbulent fields from a large-eddy simulation
model. Crosswind-integrated flux footprints shown as a function of upstream distances and sensor heights in the CBL were derived
and compared using two LS particle simulation methods: an instantaneous area release and a crosswind linear continuous release.
We found that for almost all sensor heights in the CBL, a major positive flux footprint zone was located close to the sensor
upstream, while a weak negative footprint zone was located further upstream, with the transition band in non-dimensional upwind
distances −X between approximately 1.5 and 2.0. Two-dimensional (2D) flux footprints for a point sensor were also simulated. For a sensor
height of 0.158 z
i, where z
i is the CBL depth, we found that a major positive flux footprint zone followed a weak negative zone in the upstream direction.
Two even weaker positive zones were also present on either side of the footprint axis, where the latter was rotated slightly
from the geostrophic wind direction. Using CBL scaling, the 2D footprint result was normalized to show the source areas and
was applied to real parameters obtained using aircraft-based measurements. With a mean wind speed in the CBL of U = 5.1 m s−1, convective velocity of w
* = 1.37 m s−1, CBL depth of z
i = 1,000 m, and flight track height of 159 m above the surface, the total flux footprint contribution zone was estimated to
range from about 0.1 to 4.5 km upstream, in the case where the wind was perpendicular to the flight track. When the wind was
parallel to the flight track, the total footprint contribution zone covered approximately 0.5 km on one side and 0.8 km on
the other side of the flight track. |
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