Lidar and radiosonde measurement campaign for the validation of ENVISAT atmospheric products

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1 Lidar and radiosonde measurement campaign for the validation of ENVISAT atmospheric products V. Cuomo, G. Pappalardo, A. Amodeo, C. Cornacchia, L. Mona, M. Pandolfi IMAA-CNR Istituto di Metodologie per l Analisi Ambientale Consiglio Nazionale delle Ricerche, C.da S. Loja I-55 Tito Scalo (PZ), Italy pappalardo@imaa.cnr.it ABSTRACT An intensive aerosol and water vapour lidar measurement campaign, started on July, is in progress at IMAA in Tito Scalo (PZ) (Southern Italy, 3 N, 15 E, m above sea level) in the frame of the validation program of ENVISAT. A Raman lidar system is used to perform both aerosol and water vapour measurements; aerosol backscatter and extinction coefficients are retrieved from simultaneous elastic signals at 355 nm and inelastic N Raman backscatter lidar signals at 3. nm, whereas, water vapour mixing ratio measurements are retrieved from simultaneous H O and N Raman signals. All the observations are complemented with radiosonde launches. First results of the intercomparison between water vapour lidar profiles and MIPAS profiles are presented. Radiosonde measurements of pressure and temperature have been compared with MIPAS and GOMOS profiles. Keywords: ENVISAT, MIPAS, GOMOS, SCIAMACHY, water vapour, aerosol, lidar 1. INTRODUCTION Lidar techniques are widely used to measure atmospheric aerosols and water vapour concentration and therefore they can be efficiently employed to validate ENVISAT products [1]. An intensive aerosol and water vapour lidar measurement campaign, started on July, is in progress at IMAA (Istituto di Metodologie per l Analisi Ambientale, Tito Scalo (PZ), Southern Italy, 3 N, 15 E, m above sea level) in the frame of the calibration and validation program (CAL/VAL) of ENVISAT. Systematic measurements will be performed for a period of months. Two measurements per week are scheduled for the first six months of the validation campaign, while one measurement per week is scheduled for the last six months. A Raman lidar system is used to perform both aerosol and water vapour measurements; a detailed description of the system is reported in the next section. Aerosol backscatter measurements are performed at 355 nm [], aerosol extinction coefficient are retrieved from simultaneous N Raman backscatter signals at 3. nm [3,], water vapour mixing ratio is retrieved from simultaneous H O and N Raman signals [5]. All the observations are complemented with radiosonde launches (Vaisala type). In this paper, first results of the validation measurement campaign are presented.. EXPERIMENTAL SET-UP The Raman Lidar system, operational at IMAA, is based on a Nd:YAG laser equipped with third harmonic generator (355 nm) with a repetition rate up to 1 Hz. The third harmonic is transmitted into the atmosphere in a coaxial mode. The receiver consists of a vertically pointing telescope in Cassegrain configuration with a.5 m diameter primary mirror and a combined focal length of 5 m. The collected radiation is split into three channels by means of dichroic mirrors. Interferential filters are used to select the elastic backscattered radiation at 355 nm, the N Raman shifted signal at 3. nm and the water vapour Raman shifted signal at 7 nm. Each wavelength is then split into different channels for both low and high range signals. Photomultiplier tubes are used as detectors. Low range signals are sampled in analog mode by means of a digital oscilloscope (5 MHz). High range signals are acquired in photon counting mode by using a Multi Channel Scaler (MCS) with a dwell time of 1 ns. The overall characteristics of the Raman lidar are reported in Table 1. Balloon-borne sensors of humidity (Vaisala type) are used to calibrate lidar measurements of water vapour. These are capacitive sensors with a precision of 3 % RH in the measurement range of 1 % RH and in the temperature range of -5 5 C. The sonde is also equipped with sensors for temperature and pressure. Pressure sensor precision is.1 Proc. of Envisat Validation Workshop, Frascati, Italy, 9 13 December (ESA SP-531, August 3)

2 mb with an accuracy of.5 mb in the measurement range of 3 1 mb. Temperature sensor precision is.1 C with an accuracy of. C in the measurement range of -9 C. Table 1. Raman lidar system LASER SOURCE: Nd:YAG laser (Coherent) Wavelength and pulse energy 355 nm E MAX = 17 mj Maximum pulse repetition rate 1 Hz Beam divergence <.5 mrad Pulse duration 3 ns RECEIVER: Cassegrain telescope Primary mirror diameter.5 m Combined focal length 5 m Field of view. mrad SPECTRAL SELECTION: Interference filter Wavelengths 355 nm, 37 nm, 7 nm Bandwidth < 1. nm (night time operation) <.5 nm (daytime operation) Out of band rejection 1 -, 1-1 Transmission efficiency 33%, %, % DETECTORS : Photomultipliers THORN EMI 933 QB THORN EMI 9 QB 355 nm 37 nm, 7 nm ACQUISITION Photon counting Analog EG&G Ortec MCS TEKTRONIX digital oscilloscope Minimum dwell time 1 ns Number of channels 3 Bandwidth 15 MHz Bandwidth 5 MHz PHILLIPS SCIENTIFIC fast discriminator Bandwidth 3 MHz. VALIDATION LIDAR MEASUREMENT CAMPAIGN AT IMAA At IMAA the measurement campaign for the validation of ENVISAT products started on July and it will be continued for one year. This measurement campaign foresees radio soundings and lidar measurements per week, in coincidence with ENVISAT overpasses, during the first six months period (Commissioning phase) after the onboard instruments have been switched on; in the second six months, 1 radio sounding and 1 lidar measurement per week have been scheduled. Lidar aerosol observations are performed in terms of aerosol backscatter and extinction in the UV region by using the third harmonic of the Nd:YAG laser. Lidar data are integrated over 1 minute, corresponding to 3 laser shots with a laser repetition rate of 5 Hz, and with a spatial resolution of 15 m. So, the system allows to measure the temporal evolution of atmospheric aerosol extinction and backscatter with high time and spatial resolutions. A typical vertical profile of the aerosol backscatter at 355 nm measured at IMAA is shown in Fig. 1(a). It refers to measurements performed on September and it has been obtained by averaging lidar signals over 1 minutes corresponding to 3 laser shots. The profile extends from the height (7 m above lidar station) where full overlap between the laser beam and the telescope field of view is reached up to an height of 35 m above sea level (a.s.l.), with a vertical resolution of m. In Fig. 1(b) the corresponding aerosol extinction profile is reported extending from 175 m up to m, with a vertical resolution of 35 m up to 3 m, 9 m up to m and 5 m in the cirrus cloud from m up to m. Error bars are reported in both figures. Typical statistical errors for the aerosol backscatter coefficient are not exceeding % up to m and reach 3 % up to 35 m while aerosol extinction errors are within 3% up to m, and around % up to 15 m for 1 minute integration time and for the reported vertical resolution. Aerosol optical thickness can be retrieved directly from the aerosol extinction profile up to the free troposphere; in the stratosphere this can be retrieved from the aerosol backscatter profile by making an assumption on the value for the lidar ratio (extinction-to-backscatter ratio). Lidar measurements of aerosol optical thickness can be compared with optical thickness profiles provided by GOMOS.

3 35 (a) 35 (b) x1-1.x x1-5 Aerosol Backscatter Coefficient (m -1 sr -1 ). 5.x1-5 1.x1-1.5x1 -.x1 - Aerosol Extinction Coefficient (m -1 ) Fig.1. Aerosol backscatter (a) and extinction (b) profiles at 355 nm measured at IMAA-CNR on September (:3 : UTC). Water vapour Raman lidar measurements are expressed in terms of mixing ratio, that is the ratio between the mass of water vapour and the mass of dry air in a given volume. Lidar data are integrated over 1 minute, corresponding to 3 laser shots, and with a spatial resolution of 15 m. Raman lidar measurements are affected by a statistical uncertainty related to the statistics of the signal, and by a systematic error due to the uncertainty on the knowledge of Raman crosssections (typically ~1%), and to the uncertainty on the determination of the transmissivity profile at H O and N Raman shifted wavelengths. This systematic effect has been removed by applying a calibration procedure based on simultaneous radiosonde measurements. An intensive calibration measurements campaign of water vapour mixing ratio has been performed in the period May - June. Fig. shows a typical calibrated water vapour vertical profile for the night of October, obtained through the application of the Raman technique (blue solid line), compared with almost simultaneous radiosonde data (red dashed line). In this case, Raman lidar measurements are reported with a vertical resolution of m up to 5.7 km, 9 m up to.5 km, m up to 9. km and 3 m up to 13 km and with a time integration of 1 minutes corresponding to 3 laser shots. Both lidar and radiosonde measurements show a good agreement up to an height of about 13 km. Typical statistical errors for night-time lidar water vapour measurements are within 1% up to 1 km, with an integration time of 1 minutes and for the typical vertical resolutions ( m, 9 m and m), and 15% up to 13 km of height, with an integration time of minutes and for a typical vertical resolution of about 3 m. Due to the weakness of the Raman scattering, typical statistics of the lidar signal in the tropopause is low and for this reason longer integration time is needed in order to retrieve the water vapour mixing ratio at that height; anyway in the tropopause there is not a large variability for the water vapour mixing ratio as in case of the low troposphere. Figure 3 reports all the water vapour mixing ratio profiles obtained starting from the radiosonde data for the whole measurement campaign; in the figure it is evident a large variability of the water vapour mixing ratio only in the low troposphere below 1 km of height. Lidar water vapour mixing ratio profiles can be compared with MIPAS profiles while the water vapour column content can be compared with SCIAMACHY data.

4 13 11 October Lidar (:9 - :5 UTC) Radiosonde (:5 UTC) Fig.. Water vapour mixing ratio profile measured at IMAA-CNR on October by using Raman lidar technique (:9 :5 UTC) compared with a simultaneous radiosonde measurement (:5 UTC launch time). Height a.s.l. (m) July :5 UTC 19 August 19:35 UTC 1 August : UTC August 19:35 UTC 9 August 1: UTC September : UTC September :31 UTC 5 September 19: UTC 1 September : UTC 1 Septemberber :3 UTC October :5 UTC 1 October :15 UTC 1 October :17 UTC October :5 UTC 3 October : UTC October :5 UTC 9 October :1 UTC 7 November :7 UTC 11 November 19:5 UTC 13 November : UTC 5 1 Fig. 3. Water vapour mixing ratio profiles obtained by radiosonde data at IMAA-CNR in the period July November.

5 3. VALIDATION RESULTS 3.1 MIPAS The timetable of the lidar and radiosonde measurements in coincidence of MIPAS overpasses is listed in table. Comparisons have been performed between MIPAS and lidar measurements of water vapour mixing ratio, while both temperature and pressure MIPAS profiles have been compared with radiosonde data []. For these comparisons we have considered MIPAS data within a radius of 15 km from the lidar station. Table. Timetable of the lidar and radiosonde measurements performed at IMAA-CNR in coincidence of MIPAS overpasses DATE LIDAR RADIOSONDE MIPAS 1 September 1:- 3:1 :3 :1 1 October 19:53-: :15 19:57 October :5-3: :5 :57 9 October :1 : 3 October 9:33-: 9:51 - : 1:3 31 October 17:9-: - 1:3 7 November :-3: :7 : 11 November 19:3-: 19:5 :1 1 November 1:51-:1 - :3 17 November 15:31-1: - :7 November :-: 1:17 1: MIPAS water vapour profiles: MIPAS-lidar-radiosonde intercomparison Figures and 5 report some preliminary comparisons between lidar, radiosonde and MIPAS water vapour profiles. Water vapour profiles are expressed in terms of water vapour mixing ratio as directly measured by the Raman lidar. Water vapour mixing ratio for radiosonde have been retrieved starting by the measured relative humidity, pressure and temperature profiles; for MIPAS data, we calculated water vapour mixing ratio starting from the provided water vapour concentration, temperature and pressure profiles. This is only a very preliminary comparison, but we can say that there is a good agreement in terms of water vapour mixing ratio between MIPAS and radiosonde data in the height range 13 3 km of height above sea level. In the comparison lidar-mipas, unfortunately at moment we have the possibility to compare just one point at an height of about m a.s.l.; this point corresponds to about the maximum height range covered by the Raman lidar and it is the lowest data point for MIPAS provided with a not really high accuracy MIPAS pressure and temperature profiles: MIPAS-radiosonde intercomparison Figures and 7 show some comparisons between MIPAS and radiosonde pressure and temperature profiles, respectively. We have only eight profiles available for the comparison with the radiosonde and then we can obviously speak about very preliminary results. In this sense, we can say that the comparisons show good agreement between MIPAS and radiosonde pressure profiles above 5 km of height. For the temperature we can say that the comparisons between MIPAS and radiosonde data show a quite good agreement from the tropopause up to 35 km of height with a mean deviation of 1.3%. 3. GOMOS GOMOS pressure and temperature profiles: GOMOS-radiosonde intercomparison We have only three GOMOS temperature and pressure profiles available for the comparison with the radiosonde [7]; an example is reported in figure. We have considered GOMOS data within a radius of 1 km from the IMAA station. The comparisons show a good agreement for the pressure profiles along all the height range. Regarding the temperature profiles, we find a good agreement from up to 35 km of height range.

6 3 3 1 September MIPAS :1 UTC Lidar :7-:3 UTC Lidar :-1:1 UTC Radiosonde launched at : October MIPAS 19:57 UTC Lidar :1-:19 UTC Lidar 19:53-: UTC Radiosonde launched at :15 UTC October MIPAS :57 UTC Lidar :5-1: UTC Lidar :5-3:7 UTC Radiosonde launched at :5 UTC October MIPAS 1:3 UTC Lidar :57-1: UTC Lidar :33-1:3 UTC Fig.. Comparisons of MIPAS water vapour mixing ratio profiles with Raman lidar and radiosonde measurements performed at IMAA-CNR.

7 3 3 7 November MIPAS : UTC Lidar :13-1: UTC Lidar :-:31 UTC Radiosonde launched at :7 UTC November MIPAS :1 UTC Lidar 19:-:3 UTC Lidar 19:-19:57 UTC Radiosonde launched at 19:5 UTC Altitude a.s.l. (km) November MIPAS :7 UTC Lidar 19:5-:53 UTC Lidar :-:9 UTC 3 3 November MIPAS 1: UTC Lidar :-:1 UTC Lidar :-1:1 UTC Radiosonde launched at 1:17 UTC Fig. 5. Comparisons of MIPAS water vapour mixing ratio profiles with Raman lidar and radiosonde measurements performed at IMAA-CNR.

8 1 September October 7 Lidar Station.N 15.73E - m a.s.l. MIPAS N E MIPAS (:1 UTC) RADIOSONDE (:3 UTC) 7 Lidar Station.N 15.73E - m a.s.l. MIPAS N E MIPAS (:57 UTC) RADIOSONDE (:5 UTC) Pressure (hpa) Pressure (hpa) 7 7 November Lidar Station.N 15.73E - m a.s.l. MIPAS N.39-.E 7 November Lidar Station.N 15.73E - m a.s.l. MIPAS N.39-.E MIPAS (:3 UTC) RADIOSONDE (:7 UTC) MIPAS (:17 UTC) RADIOSONDE (1:17 UTC) Pressure (hpa) Pressure (hpa) Fig.. Comparisons of MIPAS pressure profiles with radiosonde measurements performed at IMAA-CNR.

9 7 1 September Lidar Station.N 15.73E - m a.s.l. MIPAS N E MIPAS (:1 UTC) RADIOSONDE (:3 UTC) 7 October Lidar Station.N 15.73E - m a.s.l. MIPAS N E MIPAS (:57 UTC) RADIOSONDE (:5 UTC) 5 5 Height a.s.l. (m) Temperature (K) 3 Temperature (K) 7 7 November Lidar Station.N 15.73E - m a.s.l. MIPAS N.39-.E MIPAS (:3 UTC) RADIOSONDE (:7 UTC) 7 November Lidar Station.N 15.73E - m a.s.l. MIPAS N.39-.E MIPAS (: UTC) RADIOSONDE (1:17 UTC) Temperature (K) 3 Temperature (K) Fig. 7. Comparisons of MIPAS temperature profiles with radiosonde measurements performed at IMAA-CNR.

10 7 19 September Lidar Station.N 15.73E - m a.s.l. GOMOS.9N-1.E GOMOS (1:5 UTC) RADIOSONDE (1:17 UTC) 7 19 September Lidar Station.N 15.73E - m a.s.l. GOMOS.9N-1.E GOMOS (1:5 UTC) RADIOSONDE (1:17 UTC) Pressure (hpa) 5 3 Temperature (K) Fig.. Comparisons of GOMOS pressure and temperature profiles with radiosonde measurements performed at IMAA. A preliminary analysis of the comparison between MIPAS, radiosonde and lidar for water vapour mixing ratio measurements has been performed. Water vapour lidar data can be used only for altitudes below 1 km and for this height region MIPAS data seem to indicate an understimation of the water vapour mixing ratio, while at higher height the agreement with radiosonde data seems to be good. The differences between MIPAS and radiosonde data are within % for both temperature and pressure profiles. First preliminary comparisons between GOMOS and radiosonde measurements show a good agreement for both pressure and temperature profiles. However, at this stage of the validation campaign with a small available dataset, it is difficult to draw some final conclusions; the measurement campaign at IMAA is still in progress and we ll have more data available for comparisons in the next future.. REFERENCES 1. V. Cuomo, P. Di Girolamo, G. Pappalardo, N. Spinelli, M. Armenante, V. Berardi, M.P. McCormick, LITE Correlative Measurements by Lidar in Potenza, Southern Italy, J. Geophys. Res., 13, D1, pp , F.G. Fernald, Analysis of atmospheric lidar observations: some comments, Appl. Opt., 3, pp. 5-53, A. Ansmann, U. Wandinger, M. Riebesell, C. Weitkamp and W. Michaelis, Independent measurement of extinction and backscatter profiles in cirrus clouds by using a combined Raman elastic-backscatter lidar, Appl. Opt., 31, pp , G. Pappalardo, M. Pandolfi, P.F. Ambrico, A. Amodeo, P. Di Girolamo, A. Boselli, Lidar measurements of aerosol extinction and backscatter in the PBL, Advances in Laser Remote Sensing, Alain Dabas, Claude Loth and Jacques Pelon Editors, École Polytechnique, Palaiseau Cedex, pp. 7-1, D.N. Whiteman, S.H. Melfi, and R.A. Ferrare, Raman lidar system for the measurement of water vapor and aerosols in the Earth s atmosphere, Appl. Opt., 31, pp. 3-3, H. Schets, D. De Muer, H.K. Fricke, U. Blum, V. Cuomo, and G. Pappalardo, Validation of MIPAS temperature, Density and Water Vapour Profiles, Proceedings of the Envisat Validation Workshop, ESA-SP531, S. Marchand, P. Keckhut, K.H. Fricke, U. Blum, H. Schets, S. Pal, A. Hauchecorne, GOMOS validation air-density and temperature profiles, Proceedings of the Envisat Validation Workshop, ESA-SP531, 3.