Global Change Abstracts The Swiss Contribution - SCNAT

Global Change Abstracts – The Swiss Contribution | Atmosphere 65
08.1-75
The effect of mountainous topography on
moisture exchange between the “surface” and
the free atmosphere
Weigel A P, Chow F K, Rotach M W
Switzerland, USA
Meteorology & Atmospheric Sciences
Typical numerical weather and climate prediction
models apply parameterizations to describe
the subgrid-scale exchange of moisture, heat and
momentum between the surface and the free
atmosphere. To a large degree, the underlying
assumptions are based on empirical knowledge
obtained from measurements in the atmospheric
boundary layer over flat and homogeneous topography.
It is, however, still unclear what happens if
the topography is complex and steep. Not only is
the applicability of classical turbulence schemes
questionable in principle over such terrain, but
mountains additionally induce vertical fluxes on
the meso-gamma scale. Examples are thermally
or mechanically driven valley winds, which are
neither resolved nor parameterized by climate
models but nevertheless contribute to vertical exchange.
Attempts to quantify these processes and
to evaluate their impact on climate simulations
have so far been scarce. Here, results from a case
study in the Riviera Valley in southern Switzerland
are presented. In previous work, measurements
from the MAP-Riviera field campaign have been
used to evaluate and configure a high-resolution
large-eddy simulation code (ARPS). This model is
here applied with a horizontal grid spacing of 350
m to detect and quantify the relevant exchange
processes between the valley atmosphere (i.e. the
ground “surface” in a coarse model) and the free
atmosphere aloft. As an example, vertical export
of moisture is evaluated for three fair-weather
summer days. The simulations show that moisture
exchange with the free atmosphere is indeed
no longer governed by turbulent motions alone.
Other mechanisms become important, such as
mass export due to topographic narrowing or the
interaction of thermally driven cross-valley circulations.
Under certain atmospheric conditions,
these topographical-related mechanisms exceed
the “classical” turbulent contributions a coarse
model would see by several times. The study
shows that conventional subgrid-scale parameterizations
can indeed be far off from reality if
applied over complex topography, and that largeeddy
simulations could provide a helpful tool for
their improvement.
Boundary Layer Meteorology, 2007, V125, N2,
NOV, pp 227-244.
08.1-76
Source apportionment of PM2.5 and selected
hazardous air pollutants in Seattle
Wu C F, Larson T V, Wu S Y, Williamson J, Westberg
H H, Liu L J S
Taiwan, USA, Switzerland
Urban Studies , Meteorology & Atmospheric Sciences
, Modelling
The potential benefits of combining the speciated
PM2.5 and VOCs data in source apportionment
analysis for identification of additional sources
remain unclear. We analyzed the speciated PM2.5
and VOCs data collected at the Beacon Hill in Seattle,
WA between 2000 and 2004 with the Multilinear
Engine (ME-2) to quantify source contributions
to the mixture of hazardous air pollutants
(HAPs). We used the ‘missing mass’, defined as
the concentration of the measured total particle
mass minus the sum of all analyzed species, as
an additional variable and implemented an auxiliary
equation to constrain the sum of all species
mass fractions to be 100%. Regardless of whether
the above constraint was implemented and/
or the additional VOCs data were included with
the PM2.5 data, the models identified that wood
burning (24%-31%), secondary sulfate (20%-24%)
and secondary nitrate (15%-20%) were the main
contributors to PM2.5. Using only PM2.5 data, the
model distinguished two diesel features with the
100% constraint, but identified only one diesel
feature without the constraint. When both PM2.5
and VOCs data were used, one additional feature
was identified as the major contributor (26%) to
total VOC mass. Adding VOCs data to the speciated
PM2.5 data in source apportionment modeling
resulted in more accurate source contribution
estimates for combustion related sources as evidenced
by the less ‘missing mass’ percentage in
PM2.5. Using the source contribution estimates,
we evaluated the validity of using black carbon
(BC) as a surrogate for diesel exhaust. We found
that BC measured with an aethalometer at 370
nm and 880 nm had reasonable correlations with
the estimated concentrations of diesel particulate
matters (r > 0.7), as well as with the estimated concentrations
of wood burning particles during the
heating seasons (r=0.56-0.66). This indicates that
the BC is not a unique tracer for either source. The
difference in BC between 370 and 880 nm, however,
correlated well exclusively with the estimated
wood smoke source (r=0.59) and may be used to
separate wood smoke from diesel exhaust. Thus,
when multiple BC related sources exist in the
same monitoring environment, additional data
processing or modeling of the BC measurements