Fine Scale Meteorology & Air Quality Models - School of Architecture ...

ASI2_Lecture 3_7/12/2011
Summary: Fine grid modeling issues
• A Conundrum: Real features of 1-10 km scales require grid resolution
about an order of magnitude finer in size; however, much stochastic
noise is present in current mesoscale simulations at fine scales (Terra
Incognita regime).
• Quasi Stationary, persistent 2-D and 3-D circulation in weakly forced flows
• Simulations are physics option dependent
• K H using SMAG (allows rolls), K H SMAG + Constant (supresses rolls)
• Horizontal vortices and/or structures:
– Become more pronounced with decreasing ∆x = 1km, 333m & 111m
– Dissipate for decreasing ∆t= 6s,1s, 0.66s, 0.33s (features are non convergent)
– Reaches max strength in afternoon, dissipate at night, are computationally stable
• Numerical filters may helpful in reducing stochastic model noise, but
there is no fundamental guidance that currently apply.
• Other aspects/sensitivities, approaches still need to be explore:
Even vs odd order filters.
WRF LES
ASI 2: URBAN CLIMATE AND AIR POLLUTION School of Architecture, The Chinese University of Hong Kong, Hong Kong, 7-8 Dec 2011
Part 3: Fine grid air quality modeling (examples CMAQ)
1 km UCP 4km Native
Ozone NOx
• 1 km results are
significantly more
textured than 4
km
• The titrating effect
of NOx on ozone
(especially along
highways and
major point
sources) are
captured at 1km
but not at 4 km
• Photochemistry at
4 km results in
part from overdilution
of withingrid
sources
ASI 2: URBAN CLIMATE AND AIR POLLUTION School of Architecture, The Chinese University of Hong Kong, Hong Kong, 7-8 Dec 2011
26