Development and investigation of a
new numerical technique for
simulating flow over steep terrain
Beth Good
Supervisors :
Alan Gadian,
Sarah-Jane Lock,
Andrew Ross
• 2007-2010 - BSc Mathematics from Warwick
• 2010-2011 - MSc Atmosphere Oceans and
climate Reading
• 2011 - Started PhD at Leeds looking at
simulating flow over steep terrain.
Introduction
• The shape of the underlying surface has an
effect on the local and in some cases synoptic
weather.
• Mountain ranges channel and direct winds.
• Produce rain shadows down stream
• Large down slope winds
• Foehn effect – fires
• Set up waves which can propogate for miles
downstream. e.g. Rocky’s
Terrain Following Approach
• Models commonly use terrain
following coordinates to
represent the surface.
• Where there are steep gradients errors occur in the computation of
the horizontal component of the pressure gradient force.
• These errors impact on the winds and can lead to
– Generation of spurious winds in the vicinity of mountains (Jebens et
al, 2011)
– Incorrect cloud structure and precipitation patterns. E.g. The LM-tf
model systematically fails to forecast low stratus north of the Alps in
low wind situations. (Steppeler et al, 2006)
• The higher the resolution of the model the more the detail of the
orography influences the simulated atmospheric processes
Cut-cell Approach
• The vertical levels are
horizontal.
• The terrain intersects the grid.
• More accurate representation
of the horizontal pressure
gradient.
• In theory avoids errors found
in terrain following models.
Representation of terrain in a cut-cell
model
Resting Atmosphere Test
Terrain Following Models
Figure 5: Maximum vertical velocities from four
terrain following models. (Klemp,MWR, 2011)
Cut-cell Model
Figure 6: Maximum vertical velocity with time from the
Cut-cell model
•Best case - maximum velocity 10-2 m/s
•Maximum velocity – 10-12 m/s
•Less sophisticated models - 1-6m/s
•Due to machine accuracy
Thanks for Listening