4.4.3 Testing CTM Simulation of the Current (Y2000) Atmosphere
The OxComp workshop defined a series of atmospheres and emission scenarios.
These included Y2000, a new reference atmosphere meant to represent year 2000
that provides a baseline from which all changes in greenhouse gases were calculated.
For Y2000, abundances of long-lived gases were prescribed by 1998 measurements
(Table 4.1a), and emissions of short-lived pollutants,
NOx, CO and VOC, were based primarily on projections to the year
2000 of GEIA/EDGAR emissions for 1990 (Olivier et al., 1998, 1999), see Section
4.3.1. Stratospheric O3 was calculated in some models and prescribed
by current observation in others. The predicted atmospheric quantities in all
these simulations are therefore short-lived tropospheric gases: O3,
CO, NOx, VOC, OH and other radicals. Following the GIM model study
(Kanakidou et al., 1999), we use atmospheric measurement of O3 and
CO to test the model simulations of the current atmosphere. The Y2000 atmosphere
was chosen because of the need for an IPCC baseline, and it does not try to
match conditions over the l980s and 1990s from which the measurements come.
Although the observed trends in tropospheric O3 and CO are not particularly
large over this period and thus justify the present approach, a more thorough
comparison of model results and measurements would need to use the regional
distribution of the pollutant emissions for the observation period.
The seasonal cycle of O3 in the free troposphere (700, 500, and
300 hPa) has been observed over the past decade from more than thirty ozone
sonde stations (Logan, 1999). These measurements are compared with the OxComp
Y2000 simulations for Resolute (75°N), Hohenpeissenberg (48°N), Boulder
(40°N), Tateno (36°N), and Hilo (20°N) in Figure
4.10. Surface measurements from Cape Grim (40°S), representative of
the marine boundary layer in southern mid-latitudes, are also compared with
the models in Figure 4.10. With the exception of a few
outliers, the model simulations are within ±30% of observed tropospheric
O3 abundance, and they generally show a maximum in spring to early
summer as observed, although they often miss the month of maximum O3.
At 300 hPa the large springtime variation at many stations is due to the influence
of stratospheric air that is approximately simulated at Resolute, but, usually
overestimated at the other stations. The CTM simulations in the tropics (Hilo)
at 700 to 500 hPa show much greater spread and hence generally worse agreement
with observations. The mean concentration of surface O3 observed
at Cape Grim is well matched by most models, but the seasonality is underestimated.
Observed CO abundances are compared with the Y2000 model simulations in Figure
4.11 for surface sites at various altitudes and latitudes: Cape Grim (CGA,
94 m), Tae Ahn (KOR, 20 m), Mauna Loa (MLH, 3397 m), Alert (ALT, 210 m), and
Niwot Ridge (NWR, 3475 m). The Alert abundances are well matched by most but
not all models. Niwot Ridge and Mauna Loa are reasonably well modelled except
for the February to March maximum. At Tae Ahn, the models miss the deep minimum
in late summer, but do predict the much larger abundances downwind of Asian
sources. At Cape Grim the seasonal cycle is matched, but the CO abundance is
uniformly overestimated (30 to 50%) by all the models, probably indicating an
error in Southern Hemisphere emissions of CO.
Overall, this comparison with CO and O3 observations shows good
simulations by the OxComp models of the global scale chemical features of the
current troposphere as evidenced by CO and O3; however, the critical
NOx chemistry emphasises variability on much smaller scales, such
as biomass burning plumes and lightning storms, that are not well represented
by the global models. With this large variability and small scales, the database
of NOx measurements needed to provide a test for the global models,
equivalent to CO and O3, would need to be much larger.
The current NOx database (e.g., Emmons et al., 1997; Thakur et al.,
1999) does not provide critical tests of CTM treatment of these sub-grid scales.
Figure 4.10: Observed monthly mean O3 abundance
(ppb) from sondes at 700 hPa (left column), 500 hPa (centre) and 300
hPa (right) from a sample of stations (thick black line) compared
with Y2000 model simulations from OxComp (thin coloured lines, see
model key in legend and Table 4.10).
The sonde stations include RESolute (75°N, 95°W), HOHenpeissenberg
(48°N, 11°E), BOUlder (40°N, 105°W), TATeno (36°N,
140°E), and HILo (20°N, 155°W). Surface monthly O3
observations (thick black line) at Cape Grim Observatory (CGA, 40°S,
144°E, 94 m above mean sea level) are also compared with the models. |
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Figure 4.11: Observed seasonal surface CO abundance (ppb,
thick black lines) at Cape Grim (CGA: 40°S, 144°E, 94 m above
mean sea level), Tae Ahn (KOR: 36°N, 126°E, 20 m), Mauna Loa
(MLH: 19°N, 155°W, 3397 m), Alert (ALT: 82°N, 62°W,
210 m), and Niwot Ridge (NWR: 40°N, 105°W, 3475 m) are compared
with the OxComp model simulations from Y2000, see Figure
4.10. |
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