Total ozone column

To know the capability of two linear schemes to simulate stratospheric ozone in time and space, the monthly zonal mean total column of ozone in Dobson Units (DU) from the two model sim-

3.4. RESULTS

Figure 3.2: Ozonesondes locations used in this model evaluation

ulations are compared with the SCIAMACHY observations. CAR (first column), COP (second column) and, SCIAMACHY satellite retrievals (third column) for January to June are displayed in Figure 3.3, and July to December 2004 in Figure 3.4. To make a proper comparison, data from the model simulation is compared only when satellite data is available at each time and location of each overpass.

In general, both COP and CAR simulations show a realistic seasonal cycle, where: (1) in the NH extratropics, ozone values are highest during spring and lower during Sept-Oct, due to the poleward and downward transport of the ozone by the large-scale Brewer-Dobson circulation (Brewer, 1949; Dobson, 1956; Weber et al., 2011), (2) in the tropics, where there is slow large- scale ascent, the ozone columns are lower mostly during Nov-Jan when the upwelling branch of the Brewer-Dobson circulation is strongest, (3) in the south pole, low values of the Antarctic ozone hole are seen during the Antarctic spring, (4) over the circum-Antarctic belt the highest ozone concentrations are seen during Sep-Nov when ozone rich air is brought down by a large- scale descendent.

Figure 3.3 shows that during the first part of the year (January-April), both simulations result in low column values over northern high latitudes with respect to the satellite data. This underes- timation is particularly large for the COP simulation (50 DU less). During late spring and the beginning of summer, a reduction in ozone concentrations is observed in northern high latitudes. COP is able to capture this ozone descendent with similar values, and CAR has higher values in comparison with the satellite data. Moreover, COP modelled ozone values are in agreement with the satellite data over the north hemisphere during autumn and summer. Larger biases be- tween April and November are noticeable through CAR simulation (∼ 25-100 DU). Over the Equator latitudes (-5◦ to 5◦) both simulations have positive bias throughout the year, except in

3.4. RESULTS

November-January, and is more significant in February-March and July-August of the order of 25-50 DU. Total column differences (DU) with respect to TOMS satellite data for SLIMCAT runs using the COPCAT and the Cariolle v2a are presented in Monge-Sanz et al. (2011). Results from this comparison show a clear underestimation: from -30 to -20 DU for COPCAT and less than 10 DU in the case of Cariolle v2a over the Equator latitudes. These results are different from what we see in our model simulation suggesting a possible limitation in the parameterization of the radiation scheme (underestimation of the aerosol attenuation) in our model over this area. Having said that, the positive O3bias over the tropics requires further investigation. Lower con-

centrations over the tropics are observed during December-February when the Brewer-Dobson circulation is strongest. Over the north tropical Pacific, both simulations underestimate ozone concentrations during the year, more significantly in late autumn and winter seasons. From De- cember to May, CAR and COP simulations have a tendency to overestimate concentrations over southern mid and high latitudes. Higher values are particularly large by 50 DU in these latitudes for the COP simulation during December-February. Moreover, Figure 3.4 shows that from June- August CAR simulation has a tendency to overestimate O3at southern high latitudes by 25 DU.

The Antartic ozone hole is most commonly defined as the region at high southern latitudes en- closed by the 220 DU of ozone (Newman et al., 2004). Figure 3.4 shows that over the Antarctic, the ozone hole during September-October is well-simulated by COP capturing its extension and magnitude. On the other hand, CAR captures the ozone decrease during the Antarctic spring but clearly overestimates ozone concentrations by 25 DU compared to satellite data. This result may be due to the different heterogeneous chemistry treatment adopted in each scheme. Note that, both simulations have a shorter duration of the ozone hole in comparison to the satellite data that has lower concentrations during November. In addition, higher values in the belt above the Antarctic are well-simulated by both CAR and COP models with significant overestimation by CAR during Sept-Nov, and also by COP only during November.

Total column ozone field is also computed in previous studies such as Inness et al. (2013) using the MACC reanalysis. The MACC reanalysis system assimilates ozone satellite retrievals from several satellite sensors. Over the NH, the MACC reanalysis is able to simulate higher values (> 400 DU) during winter and spring; lower values are seen in CAR (only for winter) and COP in comparison with the MACC reanalysis. Similar values between the NMMB/BSC-CTM simulations and MACC reanalysis are seen from -45◦to 45◦latitudes throughout the year, with the exception of the positive bias already observed in the Equator for February-March and July- August. Over the SH, the MACC reanalysis has lower values in comparison to COP and CAR from December-May, but it is in agreement with COP successfully simulating the Antarctic ozone hole during the months of September-October.

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Figure 3.3: Total O3(DU) between the two simulations, CAR (first column) and COP (second column), and the

SCIAMACHY satellite data (third column) for the months of January to June (from the first row to the last, respec- tively) .

3.4. RESULTS

Figure 3.4: Total O3(DU) between the two simulations, CAR (first column) and COP (second column), and the

SCIAMACHY satellite data (third column) for the months of July to December (from the first row to the last, respec- tively) .

3.4. RESULTS