MIPAS WATER VAPOUR MIXING RATIO AND TEMPERATURE VALIDATION BY RAMAN-MIE-RAYLEIGH LIDAR

Transcription

1 MIPAS WATER VAPOUR MIXING RATIO AND TEMPERATURE VALIDATION BY RAMAN-MIE-RAYLEIGH LIDAR T.Colavitto (1) (2), F.Congeduti (1), C.M. Medaglia (1), F. Fierli (1), P. D Aulerio (1) (1) Istituto di Scienze dell Atmosfera e del Clima CNR, Via del Fosso del Cavaliere, 133 Rome, Italy (2) Università di Roma La Sapienza, Via Eudossiana 18, 184 Rome, Italy ABSTRACT Water vapor mixing ratio and temperature profiles carried out by a Raman-Mie-Rayleigh lidar have been used for the validation of the MIPAS instrument on board of ENVISAT. The measurements have been performed at the Institute of Atmospheric Sciences and Climate of the Italian National Research Council (ISAC-CNR) in Rome-Tor Vergata. Measurement sessions were carried out during the years 2 and 3, and some at the beginning of 4. The lidar records in coincidence with ENVISAT overpasses were, but only 4 comparisons with MIPAS 4.61 version have been possible up to now, because the reprocessing of the MIPAS data for 3 is still in progress. The lidar profiles are complemented by radiosonde observations. In the altitude range where the lidar profile overlaps MIPAS, no marked differences were found in the water vapor mixing ratio. From the tropopause up, MIPAS data appear being generally lower than lidar and radiosonde values. The comparison of the temperature shows some differences mostly close to the stratopause, where the MIPAS data appear systematically lower than the lidar ones. 1. LIDAR SYSTEM OVERVIEW A powerful Rayleigh-Mie-Raman (RMR) lidar is operating in Rome-Tor Vergata (41.83 N, E and 115 a.s.l.) to measure water vapor mixing ratio and temperature vertical profiles [1,2,3]. The features of the system are listed in Table 1. The light source is a Nd:YAG pulsed laser with second and third harmonic generators. The laser transmits two beams (532nm and 355nm) vertically in the atmosphere through two digitally controlled 45 mirrors. To overcome the problem of the very high dynamical range of the signal through the entire sounding depth, the backscattered radiation is received by three collectors of different area. This allows dividing the altitude range into smaller and partially overlapping intervals. The construction of a unique profile from the boundary layer up to the tropopause (for the Raman signal) and the mesopause (for the Rayleigh signal) is obtained numerically offline. The first collector is an array of 9 telescopes of cm diameter in Newtonian configuration, with 1 cm focal length (f-number = 3). It is designed to sound the upper part of the interesting atmospheric layer (i.e. upper stratosphere and mesosphere, for the elastic backscattering signal, and the middle and upper troposphere, for the Raman). During the validation campaign only one telescope of the array was operative, because the set-up of the others is still in progress. The second and third collectors are single Newtonian telescopes with cm and 15 cm diameter, respectively, and f-number = 3. The useful range of the latter starts in the lower part of the atmosphere so that the sounding is extended down to the first levels of the boundary layer. As the MIPAS profiles refer to the upper troposphere stratosphere, only the signal from the larger collector is used in the actual validation activity. Each mirror focuses onto a dichroic beam splitting system separating the visible component (532nm) from all radiation below 4nm (i.e. 355nm and Raman returns). The positions of these focal plane systems are computer-controlled by X-Y-Z micrometric translators. The resulting two beams are focused onto optical fibers that transfer the light toward the photodetection system. In the optical channels dedicated to the upper altitudes, a signal modulation system (a chopper, i.e. a rotating wheel synchronous with the light pulse emission) is utilized at the output of each fiber to prevent blinding of the photomultiplier (PMT) by the too intense signal scattered by the lower layers. The output of the PMTs is acquired both by photoncounting mode and by analogue-to-digital conversion. Finally, the data are transferred to a PC and stored. The characteristics of the transmitter and the receiver are listed in Table 1. The entire system is installed in two containers for possible deployment to remote locations. In the frame of the validation activity the lidar observations are complemented with radiosonde profiles collected by the National Meteorological Service at Pratica di Mare, about 25 km South-West of Rome. The radiosonde is a RS9 Vaisala PTU model. Proceedings of the Second Workshop on the Atmospheric Chemistry Validation of ENVISAT (ACVE-2) ESA-ESRIN, Frascati, Italy, 3-7 May 4 (ESA SP-562, August 4) EPOMITC

2 Table 1. Transmitter and receiver characteristics of the RMR lidar system. Transmitter Laser Nd:Yag Wavelenghts 532 and 355 nm Pulse energy 3 and 4 mj Repetion rate Hz Pulse duration (532 and 355 nm) 5-7 ns Beam divergence ~.15 mrad Beam diameter ~ mm Receiver Characteristics Channel 1 Channel 2 Channel 3 Number of telescopes Diameter (cm) 15 Focal length (cm) Field of view (mrad) Bandwith (nm) 1 nm (532) 1 nm (532) 1 nm (532) 2 nm (355) 2 nm (355) 2 nm (355) Sounding Range (km) MIPAS VALIDATION CAMPAIGN The ISAC-CNR measurements within the campaign for the validation of ENVISAT products started on August 2 and continued until March 4. During this period, 43 lidar measurements in night-time conditions were carried out: 13 during 2, 26 during 3 and 4 at the beginning of 4. In the same period, the MIPAS processor for retrieving the atmospheric quantities has gone through different changes (from version 4.57 to 4.61). At the beginning of the 4, shortly before the ACVE 2 Workshop, the data were reprocessed with the latest processor version, the Up to now, the reprocessing has been limited to the data of 2 and of February/March 4. ENVISAT overpasses within a -km range from the ground station have been considered. Among these, MIPAS elevation scans with the greatest number of data points available have been selected for the comparison of the water vapor mixing ratio, to extend as far as possible the overlap between the satellite and ground station profiles. This choice is mandatory because of the generally very limited overlapping range. Among the MIPAS files selected with those criteria, only 4 comparisons with MIPAS/4.61 were possible. No further inter-comparisons were possible, essentially due to technical upgrade of the lidar system during the 2, lack of the reprocessed MIPAS data for the year 3 and rainy weather in the first months of the 4. After March 26, 4 MIPAS measurements were suspended by ESA as a consequence of an increasing number of anomalies in the functioning of the interferometer slides. Table 2. Lidar measurements carried out at ISAC - CNR in coincidence with ENVISAT overpasses. Lidar measurements MIPAS overpasses Date Time (UTC) Time (UTC) 2/8/2 4:39: 6:5:15 1/8/2 21:45:49 /11/2 21::-23:55:49 ::46 9/7/3 :5:-22:33: 21:14:14 11/7/3 21:56:-23:1: 21::7 15/7/3 :24:-22:43: 21:25:35 22/7/3 :27:-23:11: 21:5:28 28/7/3 :27:-22:37: 21:16:58 31/7/3 19:38:-22:8: 21:22:43 4/8/3 :2:-1:3:7 :56:51 4/9/3 21:59:-23:3: 21:16:3 8/9/3 18:51:-22:6: :57:6 29/9/3 18:9:-21:19:48 21::21 //3 17:45:-:4:45 :51:17 16//3 18:35:-21:36: 21:2:46 /11/3 18:5:-21:12:48 :59:34 1/12/3 17:57:-22:27:53 21:39:51 15/1/4 18:47:-22:7:11 21:36:4 22/1/4 18:3:-22:21: 21:15:55 12/3/4 19:15:-22:28:32 21:34:6 15/3/4 18:42:-21:55:8 21:39: The lidar measurements carried out in coincidence with ENVISAT overpasses are listed in Table 2. For each day, start and end time of the lidar measurements are listed in the Lidar measurements column, while the time corresponding to the beginning of the first elevation scan for the MIPAS file considered (i.e. the hour indicated in the file name) is listed in the MIPAS overpasses column. In all selected cases, the time difference between MIPAS scan and median time of the lidar profile is less than 2h, except for August 2, 2 when the time difference was about 6h. 3. MIPAS LIDAR INTERCOMPARISONS In this section some examples of comparison between MIPAS and lidar profiles both for water vapor mixing ratio and for temperature are presented. The altitude associated to the MIPAS data points is the ESA corrected altitude (better hydrostatic altitude [4]). All observations are complemented with radiosonde profiles. In the case of the temperature validation, MSIS 9 model is used to support the comparison between MIPAS and lidar. 3.1 Water Vapor Mixing Ratio Comparisons among water vapor mixing ratio profiles of MIPAS, lidar and radiosonde are presented. In two cases (August 2, 2 and March 15, 4), the MIPAS data were generated with the last processor

3 version 4.61; in the other three examples relating to the year 3 (July 9, November and December 1) the data were generated by the versions 4.57 (NRT), 4.59 (OFL), 4.59 (OFL), respectively. The lidar data are represented with a vertical resolution of 75 m up to 6 km and 525 m above that altitude Lidar (4:39: - 6:5:15 UT) Radiosonde (: UT) Mipas (1 August 23:25:24-23:26:35 UT) Distance_GS 537 km Rome, 2 August 2 the MIPAS and radiosonde profiles overlap, there is a good agreement between them. On March 15, 4 (Fig. 2) the lidar measurements are integrated over min. The time difference between MIPAS and lidar is about min and the spatial distance 767 km. As shown in Fig. 2 the comparison is possible only at the lowest MIPAS data point at about 9 km height, where the agreement with the radiosonde is very good too. Above that altitude, the comparison is possible only between MIPAS and the radiosonde profiles reported for 18: UTC and for 24: UTC. In both cases, the difference between the two instruments is evident. In particular, around 12 km, MIPAS water vapor mixing ratio values are almost times smaller than the ones measured by radiosonde. This effect should be attributed to the loss of accuracy of the MIPAS measurements in atmospheric layers with high water vapor gradient. 1E-4 1E-3,1,1 1 Rome, 9 July 3 Rome,1 December 3 Fig.1 Comparison among MIPAS, lidar and radiosonde water vapor profiles on August 2, 2. Rome,15 March 4 15 Mipas (21:14:14-21:19:28 UT) Lidar (21:25-21:55 UT) Mipas (21:58:46-21:59:57 UT) Lidar(17:57: - 22:27:53 UT) 5 Lidar(:42: - 21:55:8 UT) Radiosonde (18: UT) Mipas (21:57:25-21:58:36 UT) Distance_GS 767 km 1E-3,1,1 1 1E-3,1,1 1 1E-4 1E-3,1,1 1 Fig.3 Two examples of MIPAS water vapor mixing ratio compared to lidar and radiosonde profiles. Fig.2 - Comparison among MIPAS, lidar and radiosonde water vapor profiles on March 15, 4. In Fig. 1, the comparison relative to the data of August 2, 2 is presented. The lidar data are integrated over an 8min acquisition period. The mean distance of the MIPAS measurement from the lidar station is 537 km and the time difference between the MIPAS scan and the median time of the lidar profile is 5h 57min. The lidar appears in good accordance with the radiosonde profile. In this case, as in others analyzed, a direct comparison with MIPAS is not possible because the MIPAS vertical scan extends down to about 9 km while the lidar profile does not reach that altitude. On the other hand, in the altitude range 9 12 km, where In Fig.3 other two comparisons are shown using the entire MIPAS profile. In these cases the overlapping zone of MIPAS and lidar is very small or almost absent. On July 9, 3 the lidar data are integrated over min and the temporal difference between the MIPAS scan and the median time of lidar is min. The distance from the ground station is about 291 km. There is a good agreement between the radiosonde and the lidar, while a comparison with MIPAS is possible only around 11 km where the match appears good. The overlapping zone is confined around the lowest MIPAS data point. On December 1, 3 the lidar data are integrated for 4h min, the temporal difference with MIPAS scan is 1h 45min and the distance is about 838 km. Although the MIPAS profile extends down to 9 km, the comparison with the lidar is possible only

4 around one MIPAS experimental point, approximately in the altitude range 9 km. Nevertheless, an evident discrepancy between MIPAS and the ground based instruments comes out; indeed the MIPAS lowest point displays a water vapor content whose value is almost.1 times the value measured by the other two instruments. This could be ascribed to the poor measurement coincidence in space and time and to high local variability conditions Rome, November 3 records. An improvement was obtained after the chopper system was set up. Only two measurement sessions on March 12 and 15, 4 were possible before the regular MIPAS operations were suspended by ESA on March 26. During the measurements of March 12, the presence of cirrus clouds around.5 km considerably reduced the measurement sensitivity above that altitude; as the thickness of the cirrus increased rapidly, consistent results in the upper layers were obtained only during the first hour of measurement. On March 15, despite a conspicuous turbidity up to 9 km altitude produced by desert dust above Tor Vergata, a three hour measurement including the MIPAS overpass was possible. Consistent results can be obtained by integrating over a single hour as well. Rome, 12 March 4 Rome, 12 March 4 Rome, 12 March Mipas (21:17:7-21:18:18 UT) Lidar (:17: - 21:12:48 UT) 5,1,1 1 Fig.4 Overlapping zone among MIPAS, lidar and radiosonde profiles on November, 3. Mipas (21::18-21:51: UT) Lidar (22:15-23:15 UT) MSIS-9 model Mipas (21:51:39-21:52:51 UT) Lidar (22:15-23:15 UT) MSIS-9 model Mipas (21:53: - 21:54:12 UT) Lidar (22:15-23:15 UT) MSIS-9 model In Fig.4 the overlapping zone between MIPAS and lidar profiles for the measurements of November, 3 is shown. The lidar data are integrated for 55 minutes, the time difference with MIPAS scan is 33 min and the distance is about 259 km. In this case the comparison is possible approximately in the range 6 9 km. The presence of cirrus at higher altitude interrupts the lidar profile. However, the lidar measurements seem to be in fair accordance with the two lowest MIPAS points. Above these, the radiosonde and MIPAS profiles are dissimilar in correspondence with the atmospheric layers with high water vapor gradient, as in Fig Rome, 15 March Rome, 15 March 4 Height(km) Rome, 15 March Temperature Mipas (21:56:4-21:57:15 UT) Lidar (19:42 - :42 UT) MSIS-9 model Mipas (21:57:25-21:58:36 UT) Lidar (19:42 - :42 UT) MSIS-9 model Mipas (21:58:46-21:59:57 UT) Lidar (19:42 - :42 UT) MSIS-9 model The lidar temperature profiles were obtained by using the Rayleigh lidar inversion technique, in the aerosol free part of the atmosphere. This means that the useful temperature profile begins above the stratospheric aerosol layer, namely from 25- km up. The inversion procedure was successfully implemented and tested using simulated lidar data; however, up to the first months of the current year, the results obtained from the actual lidar signal were unsatisfactory, due to a noise induced by high signal in the elastic backscatter Fig.5 - Comparisons of lidar with MIPAS temperature profiles taken in a km range and within a ±2 hours time window. The temperature needed by the Rayleigh inversion procedure at the top boundary was assumed to be equal

5 to the MIPAS temperature rather than to the climatological value as usual. An actual discrepancy in the upper bound assumption, indeed, would result in a bias propagating to the underlying profile. The lidar profiles, originally sampled on a 75 m grid, were integrated over 3 km intervals centered on MIPAS data points. Comparisons have been made using MIPAS profiles within km from the lidar station and in a ±2 hour time-window. For both the days considered, this interval includes scans 14, 15 and 16 of MIPAS interferometer, with overpasses centered at 36.54, and latitude North respectively, and at longitudes included within the interval East. The comparisons of the lidar profiles with the MIPAS are reported in Fig.5. In each of the six plots, the midnight radiosoundings of Pratica di Mare and the profile from the climatological model MSIS 9 are reported as well. As it appears in both days, the MIPAS 16 th scan presents temperatures markedly colder than lidar along the entire overlapping range. This can be attributed to the difference in latitude between the measurements. Because of this evident difference, this scan was rejected in the further analysis. The apparent similarity of MIPAS 14 th and 15 th scans with the corresponding lidar profiles allows cross comparison between them. The results are summarized in Fig.6 where at each MIPAS altitude the mean value (triangle), the root mean square deviation (error bar), the minimum and maximum deviation (x-symbol) for the percent temperature difference (MIPAS minus lidar) are reported. Each data point arises from an 8 element sample: two from March 12 (two MIPAS scans and one lidar profile), and 6 from March 15 (cross comparisons between two MIPAS scans and three lidar profiles). Larger deviations in the upper part of the profiles must be ascribed to peculiar behavior of some MIPAS profiles exhibiting multiple slope changes around the stratopause altitude. The overall average difference is -1.6 K, with MIPAS being colder than the lidar, and the total root mean square deviation is 3.9 K. The results of this analysis can be affected by possible variability inside the space and time windows of the different measurements, thus the retrieved values could result to some degree overestimated. 4. CONCLUSIONS The main objective of this study was to validate MIPAS water vapor mixing ratio and temperature profiles with measurements made with the Raman- Mie-Rayleigh lidar located in Rome, Tor Vergata. These comparisons have been complemented by radiosonde data and, for the temperature validation, by MSIS-9 model as well. Five examples of comparisons between water vapor mixing ratio profiles have been presented. Even after the last reprocessing operation, the MIPAS profiles rarely extend below 9 km, while the lidar water vapor profile usually does not extend above -11 km altitude. Thus, a direct comparison satellite ground station was possible only around the lowest (one or two) MIPAS data points, where the agreement is generally good. Isolated cases of high discrepancy (MIPAS values sometimes almost times smaller than lidar) should be better analyzed to decide whether they must be ascribed to measurement bias or to changes in the real water vapor concentration. At higher altitude the comparison was generally possible only with radiosonde. Here MIPAS was frequently found underestimating the values of both radiosonde and lidar (in the very few cases of lidar measurements extending enough upward). For the temperature validation, it was possible to utilize only two measurement sessions. The comparison was possible in the km altitude range. Compared to the lidar temperatures the MIPAS values appear colder and the overall average deviation is -1.6 K with a 3.9K root mean square deviation and extreme differences of +7.5, -12 K. mean value minimum and maximum Temperature Deviations (%) Fig.6 - Mean value, root mean square deviation, minimum and maximum deviation for the percent temperature difference of MIPAS minus lidar values. 5. REFERENCES [1] F. Congeduti, et al., The multiple-mirror lidar 9- eyes, J. Opt. A: Pure Appl. Opt., vol. 1, pp , [2] F. Congeduti, C.M. Medaglia, P. D'Aulerio, F. Fierli, S. Casadio, P. Baldetti, F.Belardinelli, A powerful trasportable Rayleigh-Mie-Raman Lidar

6 System, in Proceedings of 21st International Laser Radar Conference, L.R. Bissonette, G.Roy, and G. Vallèe eds., Québec City, 2. [3] D'Aulerio P., F. Fierli, F. Congeduti, C.M. Medaglia, M.Baldi, S. Casadio, Lidar water vapour measurements during the MAP campaign, in Proceedings of 21st International Laser Radar Conference, L.R. Bissonette, G.Roy, and G. Vallèe Editors, Québec City, 2. [4] K.H. Fricke, U. Blum, G. Baumgarten, F. Congeduti, G. Hansen, L. Mona, V. Cuomo, and H. Schets, MIPAS Temperature Validation by Radiosonde and Lidar, in Proceeding of the Second Workshop on the Atmospheric Chemistry Validation of ENVISAT (ACVE-2) held at Frascati, Italy, May 4, vol. SP- 562 (this volume), European Space Agency, 4.