Comparison of the O3MSAF Total Ozone and Ozone Profile products

Transcription

1 Comparson of the O3MSAF Total Ozone and Ozone Profle products Zsófa Kocss, Zoltán Tóth Hungaran Meteorologcal Servce, Ktabel Pál utca 1, H-1024 Budapest, Hungary Abstract The depleton of the atmosphere s protectve ozone layer s of partcular envronmental concern. The resultng hgh levels of ultravolet radaton have harmful effects on agrculture, forests, water ecosystems, and human bengs. The measurement of ozone from space s extremely mportant as only satellte measurements can gve a global pcture of the ozone layer. The EUMETSAT s Satellte Applcaton Faclty on Ozone and Atmospherc Chemstry Montorng (O3MSAF) provdes among others the Total Column Ozone and the Ozone Profle from the EUMETSAT s polar satellte, MetOp. These products are declared operatonal, avalable both onlne and offlne. From the Ozone Profle product total column ozone can be determned by addng up the ozone values of each level. As a result, there wll be two dfferent total ozone amounts, retreved by two dfferent methods. Thus, they or mght gve dfferent estmatons of the ozone content. Both the Total Column Ozone and the Ozone Profle products have already been valdated by ground-truth/n stu measurements n other countres, however, the total ozone amount calculated from the Ozone Profle has not yet been compared wth the Total Column Ozone product tself. We have studed how the two dfferent types of total ozone estmaton are related to each other and also have verfed them wth hgh-accuracy ground-based ozone measurements of Budapest. The paper presents our results on the comparson of the total ozone contents and also ther relatonshp to ground-truth measurements n Hungary. INTRODUCTION The EUMETSAT s Satellte Applcaton Faclty on Ozone and Atmospherc Chemstry Montorng (O3MSAF) provdes - among others - the Total Ozone Column and the Ozone Profle from the EUMETSAT s polar satellte, MetOp. These products are declared operatonal, avalable both onlne and offlne. From the Ozone Profle product total column ozone can be determned by addng up the ozone values of each level (we wll call t as ntegrated ozone profle product). As a result, there wll be two dfferent total ozone amounts, produced by two dfferent methods. Thus, they mght gve dfferent estmatons of the ozone content. The purpose of our work was to compare these ozone retrevals to each other and also to see how they are related to the hgh resoluton ground-based ozone measurements whch take place n Budapest. THE OZONE PROFILE AND OZONE COLUMN PRODUTS Both the Ozone Profle and the Total Ozone Column products are derved from the GOME-2 measurements by O3MSAF.

2 Total column amounts of ozone are made by a classcal DOAS-AMF fttng algorthm at DLR. Ths algorthm has two major steps: a DOAS least-squares fttng for the trace gas slant column, followed by the computaton of a sutable Ar Mass Factor to make the converson to the vertcal column densty (Valks et al., 2009). The Total Ozone Column has 24 forward-scan pxels wth a nomnal resoluton of 40 km x 80 km, and 8 back-scan pxels wth a nomnal resoluton of 40 km x 240 km (Loyola et al., 2009). Only the forward-scan pxel values were used. Ozone Profle product s generated by the Ozone ProflE Retreval Algorthm (Opera) at KNMI. Ths s capable of calculatng an ozone profle from backscattered lght n the UV-VIS spectral range measured by the satellte nstrument. The method uses for ts retreval the laws of radatve transfer to calculate the radance values from atmospherc parameters (Tunder, 2009). The product has 40 km x 640 km resoluton. The Total Ozone Column and the Ozone profle products don t have the same resolutons, we had to ft them to each other. The Ozone Profle pxels are 8 tmes larger than the Column product s so we reduced the resoluton of the total column product to get the same pxels as there are n the profle product (Fgure 1). Thus we got the same resoluton for both the Profle and the Column products. Fgure 1: Example for the ntegrated ozone profle (left) and for the total ozone column (rght) products. The dfference between the resolutons can be seen n the fgure. STATISTICS USED FOR THE COMPARISONS We calculated the Error (ME), the Absolute Error (MAE) and the Root Square Error (RMS) to compare the Integrated Ozone Profle to the Ozone Column after convertng the Ozone Column product to the same resoluton as the Ozone profle product. (From now on we wll call t enlarged ozone column.) In the formulas P stands for the Integrated Ozone Profle and M ndcates the enlarged Ozone Column (both n Dobson Unt): 1 ME N 1 MAE N P M P M RMS 1 P 2 M. N

3 January February March Aprl Ma June July August September October November December COMPARISON OF THE INTEGARTED OZONE PROFILE AND THE ENLARGED OZONE COLUMN PRODUCTS The comparson was performed for the year 2009 for the whole globe. The results are seen n Table 1. We calculated all the statstcs for the whole perod and for each month as well. We found that the ntegrated ozone profle gves smaller values than the enlarged ozone column. The ME s about -1.6 Dobson Unt, wth a 9.8 Dobson Unt standard devaton. From the monthly statstcs one can see that n November and December the ntegrated ozone profle has larger values than the enlarged ozone column (see Table 2 and Fgure 2), whle n the rest of the year t s the reverse. The absolute mean error s almost same for every month. The RMS s or slghtly larger from October to February; the hghest value can be seen n February. The mean values and the numbers of the data pars can be seen n Table 2. ME MAE RMS ntegrated profle enlarged Column Number of data Table 1: Statstcs for Fgure 2: Monthly statstcs for ntegrated profle enlarged Column Number of data Table 2: values and data par numbers for the monthly statstcs. Statstcs were calculated also for dfferent areas: polar, md-lattude and tropc areas as well as Northern and Southern hemsphere.

4 In the tropcs the ntegrated ozone profle gves hgher values than the enlarged ozone column whle at the md lattudes and n the poles t s the opposte (Fgure 3). The smallest ME s n the md lattude areas, at the tropcs t gets larger and the n the poles the ME gves the hghest value. The RMS and the MAE s ncreasng from the tropc areas to the poles. There s no bg dfference between the Northern and the Southern hemspheres, except between the North Pole and the South Pole. The ME gets larger at the North Pole whle the devaton s larger at the South Pole. Table 3 shows the mean values and the number of data pars used to calculate the statstcs. Fgure 3: Statstcs for the Tropcs, Md lattudes, Poles for the Northern and Southern hemsphere. Northern hemsphere Md Tropcs Pole lattudes Southern hemsphere Md Tropcs Pole lattudes ntegrated profle enlarged Column Number of data Table 3: values and data par numbers for the dfferent areas. COMPARISON OF THE SATELLITE-RETRIEVED AND GROUND-BASED-MEASURED TOTAL OZONE AMOUNTS We compared the ntegrated ozone profle and ozone column values (both the enlarged pxel and normal pxel values) to the ozone measurements n Budapest. The total ozone observaton s carred out by a Brewer MKIII double monochromatc spectrophotometer (No. 152) n the Hungaran Meteorologcal Servce. It s calbrated to the travellng world reference Brewer nstrument (operated by the IOS, Canada) n every two years n accordance wth the manufacturer s recommendaton. Several tests run on daly and monthly bass to keep the nstrument on ts hghest possble calbraton level. The nstrument makes the measurements and tests at fxed solar zenth angles, so ozone values are avalable ca n every 15 mnutes. For ths comparson we selected those satellte pxels where Budapest s located. We also selected those ground-based measurements whch were closest n tme to the Metop overpass for each day. If the tme dfference between the satellte and ground-based measurement was more than 5 mnutes, an nterpolated value between the prevous and subsequent ground-based measurements was used. After these preparatons, we calculated the statstcs descrbed before.

5 We had only 282 data pars to calculate the statstcs; we used only those days, when smultaneously Ozone Profle, Total Ozone Column products and spectrophotometer measurements were avalable (Table 4). The results show that all satellte retrevals underestmate the ground-based measurements. The Total Ozone Column product s nearer to the spectrophotometer measurements and the ntegrated profle s less accurate. Ths s also true for the absolute mean error and the root mean square error (Fgure 4). The orgnal resoluton Total Column Ozone product fts the best the spectrophotometer measurements. The dfference between the satellte retrevals are less than between the retrevals and the ground-based measurements. Fgure 4: Comparson to the ground-based measurement n Budapest. Integrated Enlarged Profl Column Column Measurement Number of data Table 4: values and data par numbers for the comparson to the ground-based measurements. The underestmaton of the measurements can be seen on the scatter plot dagrams (Fgure 5). When the ozone amount measured by the Brewer spectrophotometer s ncreasng, the underestmaton of the satellte measurements ncreases too. We also vsualzed the correlaton effcent n the fgures. Ths effcent s over 0.9 n each case. Comparng the spectrophotometer measurements to the ntegrated ozone profle values, we got the smallest correlaton coeffcent, whle comparng them to the orgnal resoluton ozone column values the correlaton effcent s the hghest. The orgnal resoluton Total Ozone Column product fts the best to the ground based measurements. Fgure 5: Scatter plot dagrams for the ntegrated ozone profle (left), for the enlarged ozone column (mddle) and for the ozone column (rght) products.

6 CONCLUSION We vertcally ntegrated the ozone profle product so we got an estmaton for the total ozone content for the atmosphere. We compared ths ntegrated ozone profle to the total ozone column product for the year We found that the ntegrated ozone profle gave lowest values than the enlarged ozone column. There were only two month, n November and December where t was the opposte way. We calculated the statstcs also for dfferent areas. The ntegrated ozone profle gave larger values than the enlarged ozone column for the tropcs, however, for the md lattudes and poles we found that t was reversed. The largest dfference occurred on the poles. Bg dfferences ddn t appear between the Northern and the Southern hemsphere, expect for the poles where the ME was larger at the North Pole whle the RMS was larger at the South Pole. We also nvestgated how these amounts relate to the Brewer measurements whch take place n Budapest. The satellte retrevals make a good estmaton of the total ozone amount; the underestmaton of the ground-based measurements s only 2-4%. The smallest underestmaton s found for the orgnal ozone column, the largest one for the ntegrated ozone profle. We found that wth ncreasng ground measured ozone amount values the underestmaton of the satellte retrevals also ncrease. The dfference between the satellte retrevals s less than between the retrevals and the ground-based measurements. ACKNOWLEDGEMENT: We thank to the O3MSAF project of the EUMETSAT for provdng data and products used n ths paper. REFERENCES: Loyola, R. D., Zmmer, W., Kemle, S. and Valks, P., Ruppert, T., 2009, Product User Manual for GOME Total Columns of Ozone, Mnor Trace Gases, and Cloud Propertes. Avalable on-lne on the O3MSAF home page: Tunder, O., 2009, Product User Manual for the Near Real Tme and Offlne Ozone Profle. Avalable on-lne on the O3MSAF home page: Valks, P., Loyola, D., Hao, N., Rx, M. and Sljkhus, S., 2009, Algorthm Theoretcal Bass Document for GOME-2 Total Column Products of Ozone, NO2, SO2, BrO, H2O, tropospherc NO2 and Cloud Propertes. Avalable on-lne on the O3MSAF home page: