ENVISAT Data Validation with Ground-based Differential Absorption Raman Lidar (DIAL) at Toronto (73.8N, 79.5W) under A.O. ID 153

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1 ENVISAT Data Validation with Ground-based Differential Absorption Raman Lidar (DIAL) at Toronto (73.8N, 79.5W) under A.O. ID 153 Shiv R. Pal 1, David I. Wardle 2, Hans Fast 2, Richard Berman 3, Richard L. Mittermeier 2 and Jonathan Davies 2 1 Science and Art Innovations Inc (SAAI Inc), 2 Meteorological Service of Canada (MSC) 3 Spectral Applied Research 1.0 Introduction This report provides an analytical overview of data validation for the atmospheric sensors on board ESA s ENVISAT in its Commissioning Phase since its launch on March 1, 2002, carried out by MSC and SAAI Inc with ground-based Differential Absorption Raman Lidar (DIAL) at Toronto. This participation, under A.O. ID 153, was under ESA s Atmospheric Chemistry Validation Team (ACVT). The first two authors are the PI and Co-PI on the project. Fast was measurement program coordinator. Mittermeier and Davies took Lidar measurements and Richard Berman took part in measurements and developing the Lidar data analysis processor. The Toronto Lidar is a night-time only four channel (308 nm, 353nm, 332nm, 385nm) Raman DIAL with 1m receiver telescope (Pal et al. 1996). The same system has been extensively used in the past under the NDSC measurement program. Under the ACVT our participation was required to measure, report to ESA and utilize for validation the three geophysical parameters as listed is the Table 1 below. Table 1: Geophysical parameters measured by Toronto Lidar. AOID Geophysical Instrument Location Lat Long parameter 153 O3 profile Raman DIAL Toronto Ozone 153 Aerosol profile Raman DIAL Toronto Ozone 153 Temp profile Raman DIAL Ozone Toronto Due to system malfunction and unfavourable weather conditions by the end of November ) only 7 Lidar profiles were measured of which two have been analysed in detail with respect to with respect to three coincident occultations measured by the GOMOS instrument on ENVISAT. For the remaining, no GOMOS occultations were available and no MIPAS data were available at this time. The measurement overview is presented in Table 2. Proc. of Envisat Validation Workshop, Frascati, Italy, 9 13 December 2002 (ESA SP-531, August 2003) 1

2 Table 2: Measurement Overview, Toronto Lidar Site (43.8, -79.5). Lidar Measurements Duration Orbit GOMOS Occultations Star ID Availability MIPAS Data h (9700K) Av NAv (9700K) Av h 2127 NAv NAv h (4500K) NAv NAv 172 NAv h (4500K) Av NAv h NAv NAv NAv ( ) NAv In Table 2 are shown the occultation star ID and the star temperature in brackets. The Av and NAv indicate availability and non-availability of Envisat data. A nightly averaged Lidar profile is available for the indicated measurement duration. This time duration encompasses whenever available a GOMOS occultation time or the MIPAS overpass time in the vicinity of Toronto. For this report only two GOMOS occultations were available in the vicinity of Toronto as shown in Figure 1(a, b) that form the basis for this correlative analysis. 2

3 Measurement Duration (UTC) Measurement Sequence GOMOS Coincidence UTC 84 (a) 1 Latitude (deg) Orb2092 T (43.8, -79.5) Orb km 1000km Orb Longitude (deg) (b) Figure 1(a): (top panel) shows the Lidar measurement duration and star occultation position. (b) (bottom panel) shows the ground circles for 500 km and 1000 km for Toronto (T) and the position of occultation by red traces. In Fig1(a) the vertical bar presents the duration of Lidar measurements where for example over the duration of a 6 hour over 13 million Lidar returns are averaged to produce one nightly average Lidar profile. The GOMOS star occultation for this period is marked by positioning the star at the Lidar time bar. The size of the star is a relative measure of its temperature. The corresponding occultation tangent Lat-Long traces are given in Fig. 1(b) with corresponding Orbit number with respect to the position of Toronto (T) and the ground circles at 500 km and 1000 km. 3

4 2.1 Lidar Data Validating data have to be as good as or better than data to be validated With this principle in mind it was thought essential to show how good the Lidar data, used for this validation, was. As mentioned above a Lidar profile is derived from several million Lidar shots; depending on the measurement duration. In our Lidar system the hardware averaging produces 10 minute averages of raw data. The subsequent data processing can produce profiles of O3, air density and temperature for each 10 min average as well as the overall average profiles of these geophysical parameters. The 10 min air density profiles for July 25, 2002 corresponding to the Orbits 2091 and 2092 are superimposed in Fig Nightly Average of Rayleigh Density Altitude [km] Error Cirrus Cloud 20 Rayleigh Height Density [molecules/cm 3 ] Radio Sonde Figure 2: Ten minute average profiles of Air Density derived from Lidar Rayleigh channels. The accuracy of measurement is shown in red in terms of one standard deviation. In Fig. 2 the Rayleigh height at about 17 km altitude represents the region where top-side Rayleigh and the bottom side Raman profiles (not included in this figure) are merged to derive a composite profile. To derive air density although a reasonable temperature at 70 km is assumed (220 K) of which the analysis becomes independent after several altitude range bins ( range bin is 300 m). Additionally the radiosonde data are used to derive normalization factor for Raman channel analysis. The upper altitude label just below 50 km is the limit of radiosonde data. 4

5 In spite of the presence of a thin cirrus at about 12 km altitude the Raman signals (unaffected by cirrus) help provide highly accurate O3 profiles. The air density profiles in Fig. 2 show that the data utilized for the correlative analysis is highly reliable. The corresponding 10 min averages of O3 profiles from Rayleigh channels only are shown in Fig. 3. The corresponding error in O3 is shown by the single curve. Since low values of error does not permit proper exhibition of it on the same scale, it is shown 10 times in this figure Lidar Ozone Profiles for July 25, Minute Average of Rayleigh Ozone 45 Error x Altitude [km] Ozone [molecules/cm 3 ] Figure 3: Ten minute average profiles of O3 for July 25, 2002 from Rayleigh channels only. Although there is a fair bit of variation between the individual O3 profiles most of them fall close to an average. The error curve plotted 10x indicates that for Rayleigh part of the O3 profile as derived from the Lidar is highly reliable between the preset lower Rayleigh limit and an arbitrary upper limit near 45 km altitude. Although the Lidar data above 45 km gets to be noisy and unreliable the contribution to ozone by the part of the profile above 45 km is not substantial. A combined O3 profile using Rayleigh and Raman Lidar signals is utilized to study comparisons with GOMOS O3 profiles as in the following sections. 2.2 GOMOS and Lidar Comparison The comparisons between the ozone, air density and temperature profiles from GOMOS and Toronto Lidar are made for two night time measurements on July 25, 2002 with two GOMOS Orbits (2091, 2092) and on October 25, 2002 with GOMOS one orbit (3407) as depicted in Fig 1(a,b). These are studied in the following on a case by case basis. x

6 2.2.1 Ozone profiles Orbits 2091 and 2092 The average Lidar ozone profile for a measurement period of about 5.21 hours is shown in Fig. 4 with ± ó (standard deviation). As is obvious the Lidar profile is highly accurate between the entire stratosphere up to about 45 km altitude. The ozone peak is clearly defined near 25 km altitude in the average as well as in the 10 min averages. The corresponding GOMOS profile for the orbit 2091 is shown in Fig Toronto Lidar O3+-1 sigma A ltitude (m) E+0 1.0E E E E E E+12 Ozone concentration Figure 4: Lidar ozone profile for with ± ó bound. 6

7 Orbit Altitude (m) E+0 1.0E E E E E E+12 Ozone Concentration (cm-3) Figure 5: GOMOS ozone profile for the orbit 2091, Star ID 18. From the Lidar data it is quite obvious that a smooth O3 profile was expected from the GOMOS data as well. The profile above the ozone peak near 25 km altitude, with the exception of pronounced steps overriding the profile, seems quite reasonable. However the data below the peak seem too noisy and unreliable. Although the error bounds in this range do not seem too high around the mean profile the big oscillations of ozone values however are not expected. This could be an artefact of the GOMOS data processor otherwise high error bars were expected in this region. A closer comparison for Lidar and the two corresponding GOMOS profiles is provided in Fig. 6(a,b) where data have been presented on a logarithmic scale. The steps in GOMOS profile are more pronounced in the Orbit 2091 than in the Orbit It can be seen that the Lidar data also exhibit this oscillatory behaviour to a small degree. It is possible that in case of Lidar the oscillations are smoothed out in a 6- hour nightly average. One more departure between Lidar and GOMOS is slight drooping of Lidar profile above 40 km altitude to 45 km; above which the Lidar data becomes unreliable. 7

8 Orbit 2091 Lidar (250702) Altitude (m) E E E+13 O3 Concentration (cm-3) Figure 6(a,b): (a) Lidar (red) and GOMOS (blue) Orbit 2091, (b) Lidar (red) and GOMOS (purple) Orbit To study the departure between Lidar and GOMOS, a difference parameter in terms of percentage difference between GOMOS (G) and Lidar (L), ie. 100(G L)/L was examined. To do this all data points are required to have similar range bins. Since the Lidar and GOMOS altitude resolutions were different and that the GOMOS range bin being much larger (~ 2 km ) than Lidar s (300 m), GOMOS data were interpolated at Lidar altitude bins using a spline-fit. The difference parameter thus estimated for the two orbits is shown in the Fig. 7 (a,b). In the Orbit 2091 (Fig. 7(a)), below the peak of the ozone layer (25 km) the difference parameter indicates that the GOMOS profile cannot be considered reliable. The difference parameter between the peak to about 40 km altitude shows that on an average the GOMOS and Lidar provide amazingly well correlated data with a small negative bias of a few percent. The positive bias above 40 km altitude however is due to perhaps an over subtraction of background in Lidar analysis which further needs to be investigated. The oscillations on the difference parameter are due to the steps as pointed out in the previous figures. Fig. 7(b) for the Orbit 2092 shows more or less the same behaviour as the Orbit 2091 with intensity of oscillation in the difference parameter being relatively less. On an average the negative bias between 25 km and 40 km is of the order of - 5%. Although the data utilized here is statistically not significant to draw a general conclusion, it can be concluded that GOMOS consistently provides good data for the top-side of the ozone profile while poor data for the bottom-side of the layer. 8

9 45000 Orbit 2091, (a) Altitude (m) Figure 7 (a,b): Difference parameter for orbit 2091 (a) and 2092 (b). 9

10 This analysis was extended to determining correlation coefficient R and the intercept A for a linear best-fit between the Lidar and GOMOS data. Since the difference parameter in the bottom part of the profile was in discordance the lower altitude data were removed in from the set in arbitrary steps to see improvement in R and reduction in A. A set of figures thus were generated as shown in Fig. 8. 6E+12 5E+12 Orbit 2091 Lidar(250702) (a) All Data (12112m m) R = A = 2.382E11 0E+0 Figure 8: Best fit for 2091 and Lidar data. 10

11 In Fig. 8(a) all data from the lowest data point in GOMOS profile to about 60 km altitude where Lidar data ends were used. The correlation coefficient R for this set is R= with relatively high intercept at Lidar axis of 2.382E+11 cm -3. The next four sets (b to e) show considerable improvement in the correlation when the top-side of the Lidar/GOMOS profiles was dropped to 45 km and the bottom side were dropped successively moving closer to the peak of the ozone layer while keeping the upper altitude to 45 km. The value of R in the frame (e) reaches a with A= E+09 cm -3 crossing over the zero intercept. The range of correlation is marked on each figure (a to e) along with R and A. 6E+12 5E+12 Orbit 2092 Lidar(250702) (15120 m to m) R = A = E11 6E+12 5E+12 Orbit 2092 Lidar(250702) m to m R = A = E11 0E+0 0E+0 0.0E+0 1.0E E E E E E+12 GOMOS O3 (cm-3) 6E+12 5E+12 Orbit 2092 Lidar(250702) m to m R = A = E10 6E+12 5E+12 Orbit 2092 Lidar( ) m to m R = A = E10 0E+0 0E+0 0.0E+0 1.0E E E E E E+12 GOMOS O3 (cm-3) Figure 9: Best fit for Orbit 2092 and Lidar data ( ). 11

12 O r b i t O r b i t Similar to the data of orbit 2091 the R and A for the orbit 2092 are determined and shown on the plotts in the Fig. 9. The results show a situation similar to the one in Fig. 8. The R and A trends for the two successive orbits are given in the Fig. 10 which shows that as the R increases fast towards the Top-side of the ozone layer and the intercept A goes closer to zero for the best fit. 1.1 Orbit 2091 Orbits 2092 Lidar(250702) 1.0 R 0.9 R Step Figure 10: Correlation coefficient R (left) and intercept A (right) between Lidar data and GOMOS Orbit 2091 and 2092 for the steps in Figs. 8 and Orbit 3407 The ozone profile for the Orbit 3407 on the night of and the corresponding lidar ozone profiles are presented in the Fig. 11 (left). There seems to be a slight downward shift in the GOMOS O3 profile relative to the Lidar profile. The difference parameter for this set is presented in the same figure(right) indicating oscillations similar to the those observed in the previous measurements. The relative bias also seems to concentrate near about 5% for the main part of the ozone profile. 12

13 Orbit 3407 Lidar(251002) Altitude (m) E+0 1.0E E E E E E+12 Ozone Concentration (cm-3) Figure 11: (Left) Lidar and GOMOS O3 profiles for the orbit 3407 on Oct. 25, 2002 and (Right) Difference Parameter. The correlation analysis for this case is shown in the Fig. 12 where the first panel included all data and the last includes data between about 15 km and 45 km. The correlation coefficient R improves from to This once again indicates that for the three GOMOS sets and two corresponding Lidar sets the Top-side of the ozone profile matches very well while the bottom-side of it does not. This indicates that the GOMOS, at least for these sets, did not provide reliable ozone values. This should also affect the total ozone measurements determined for the ozone layer from GOMOS data. 13

14 5E+12 Orbit 3407 Lidar( ) 5E+12 All Data (12421m m) R = A = E11 Lidar( ) (15119 m m) Orbit 3407 R = A = E11 5E+12 0E+0 Orbit 3407 Lidar( ) (19916 m m) R = A = E11 0E+0 5E+12 Orbit E+0 1.0E E+12 (25012 m m) 3.0E E E+12 Lidar( ) GOMOS O3 (cm-3) R = A = E10 0E+0 0E+0 0.0E+0 1.0E E E E E+12 GOMOS O3 (cm-3) Figure 12: Best fit for Orbit 3407 ( ) and Lidar data between given altitude ranges listed on the panels. 2.3 Air Density comparison The atmospheric air density determined by Lidar and GOMOS is presented in the Figure 13 for the Orbits 2091 and 2091 (left) and 3407(right). Although the lidar provides the density measurements right up to about 5 km altitude the GOMOS data stops near the tropopause height. The density measurements with two systems provide a very good coincidence with the exception of overriding fluctuations in GOMOS data. 14

15 70000 Altitude (m) Orbit 2091 Orbit 2092 Lidar (250702) 1.0E E E E E E+20 Air Density (cm-3) Altitude (m) Orbit 3407 Lidar (250702) 1.0E E E E E E+20 Air Density (cm-3) Figure 13: Air Density comparison between Lidar and GOMOS data for the Orbits 2091/2092 (left) and Orbit 3407(right) 2.4 Temperature Comparison The Toronto Lidar being a Raman DIAL provides temperature profiles up to about 5 km altitude. The Raman measurements require a normalizing air density somewhere near 17 km altitude for our analysis. This is inputted from the NCEP data, also acquired for the day of the measurement. The NCEP data are based on a model interpolation that utilizes data from daily radiosonde flights over a wide radiosonde network. A comparison between the lidar derived temperature (T) and air density and the one provided by NCEP are generally very good. The Lidar T profile for the and GOMOS for the Orbits 2091 and 2092 are show up to an altitude of 60 km in the left panel of Figure 14. The data for and the corresponding Orbit 3407 are shown in the right panel. The Lidar T above this altitude gets noisy and less reliable. A careful examination of data revealed the altitude of T minimum in the region of tropopause. These altitudes are marked as the Topopause Heights. For Lidar this height is coincident with that in the NCEP data also. 15

16 Altitude (m) GOMOS-Toronto Lidar Orb 2091 Orb 2092 Lidar (250702) Tropopause Height (GOMOS: 17250m, 216.9K) Tropopause Height (GOMOS: 17357m, 221.1K) Lidar Tropopause Height ( 14070m, 213.3K) Temperature (K) Figure 14: Temperature profiles derived from Toronto lidar measurements and GOMOS for ( Orbits 2091, 2092 )(left) and for ( Orbit 3407) (right). The horizontal lines represent expected tropopause heights. It is obvious that there is a considerable difference in the T profiles measured with Lidar and GOMOS for Orbits 2091/2092. The T in the thermosphere is over-estimated by GOMOS model, under-estimated in stratosphere and over-estimated near tropopause. Although there is a clear reversal in T at km altitude in GOMOS data as measured in both successive orbits, this height is unrealistically too high for defining a tropopause. The Lidar determined tropopause however is very realistic and this is also supported by the NCEP data. The right hand panel in the Fig. 14 represents the T profiles for Lidar ( ) and Gomos (Orbit 3407). These observations also show a behaviour similar to the one in Orbits 2091/2092, with the exception that the GOMOS data in Orbits 2091/2092 crossed over the tropopause height to determine its tropopause. On the basis of T minimum the tropopause height associated with the GOMOS data is unrealistically too high as shown. One important observation is the large difference in T at the stratopause height. Compared to GOMOS considerably lower T measured by Toronto Lidar has been a subject of discussion (private communication with Ulrich Blum) and re-examination of our Lidar analysis. We have found that the T profiles presented in the Fig. 14 are very realistic. 16

17 2.5 Conclusions The data utilized in this analysis is statistically mot significant. A detailed case study for three GOMOS orbits and the corresponding two sets of lidar profiles for O3, Air Density and Temperature gives considerable preliminary insight into the behaviour of GOMOS data and the GOMOS data processing. The GOMOS data provide good correlation in the topside of the O3 profile while it was very noisy and unreliable below the O3 peak. This means that to utilize GOMOS data one has to be careful because GOMOS data in its present form does not seem to account for total ozone in the ozone layer. This point was raised at the Validation Workshop and steps will be taken to improve GOMOS measurements and processing. The large oscillations in the lower part of the profile in GOMOS were expected to be associated with large error bars in this region contrary to the very small error bars presently in GOMOS data. This author also pointed out this discrepancy. The GOMOS team has made a note of this and improvements are forthcoming. The oscillations in the difference parameter observed in all three profiles that are included in this analysis indicate that they relate to step function introduced in the O3 profile by the GOMOS processor. These oscillations seem enhanced when correlated with data having finer range resolution such as from a Lidar. This requires further examination by the GOMOS team. It was learnt at the Validation Workshop that the large differences in temperature profiles between GOMOS data and Lidar were the result of the atmospheric model adopted in the GOMOS processor that requires a fresh look. From GOMOS the high resolution T data are also available that covers a reasonable high altitude range which should be utilized for validation. 2.6 Acknowledgement: The Lidar measurement program at Toronto is run by the Meteorological Service of Canada. The financial support for the ACVT participation has been provided by the Canadian Space Agency. 2.7 References Pal, Shiv R., Allan I. Carswell, John Bird, David Donovan, Thomas Duck and James Whiteway, Lidar Measurements of the Stratosphere at the Eureka and Toronto NDSC Stations, SPIE Proc., V2833, 28-39,