Aerosol and Cloud measurements by means of the M-55 Geophysica in the frame of the ENVISAT Validation campaigns (July 2002 and October 2002) M. Cacciani )1( , F. Cairo )2( , V. Mitev )3( , R. Matthey )3( , L. Stefanutti )4( , S. Borrmann )5( , G. Fiocco )1( ,

P. Mazzinghi )6( , G. Pace )1( , C. Buontempo )2( , H. Voessing )5( , J. Curtius )5( , R. Weigel )5( , F. Terenzi )1( ,

T. Di Iorio )1( , G. Martucci )3( , G. Redaelli )7( , A. Kentarchos )8(

)1( University of Rome, Rome, Italy (cacciani@g24ux.phys.uniroma1.it) )2( CNR-ISAC, Rome, Italy (f.cairo@isac.cnr.it) )3( Observatory of Neuchatel, Swizerland (valentin.mitev@ne.ch) )4( Geophysica-GEIE, Florence, Italy (Lidar@iroe.fi.cnr.it) )5( University of Mainz, Mainz, Germany (borrmann@mail.uni-mainz.de) )6( Instituto Nationale di Ottica Applicata (INOA), Florence, Italy (mazzinghi@ino.it) )7( University of L’Aquila, L’Aquila, Italy (gianluca.redaelli@aquila.infn.it) )8( Environmental Research and Services, Florence, Italy (A.Kentarchos@iroe.fi.cnr.it)

Abstract: The present paper outlines the aerosol and cloud flights performed by the high-altitude aircraft M55-Geophysica during July 2002 (Test and Validation campaign) and October 2002 (Mid-latitude validation campaign). The rational behind the flight planning is discussed, together with the flight paths and indicative results. Data analysis from the first campaign (July) as well as preliminary results from the second campaign (October) show good internal consistency between the various cloud and aerosol instruments and demonstrate the detection of aerosol layers (associated with Saharan dust outbreaks in the southern Mediterranean) and pronounced cloud formation (middle/high clouds). In that respect, level-2 aerosol/cloud products from Envisat (e.g. cloud top, optical depth etc.) can be validated when available. AEROSOL/CLOUD FLIGHTS Two validation campaigns by means of the high altitude aircraft M-55 Geophysica have been performed during 2002 from Forlì (Italy); the first one during the month of July and the second during the month of October. The campaigns have been performed in the frame of a project sponsored by ESA, ASI and the European Commission. The campaign of July served also as a test campaign as new and upgraded instruments were flying again after a long period of quiescence. During all the flights (4 in July and 7 in October) aerosol and cloud instruments as well as chemical instruments were allocated on the aircraft. For the so called ‘aerosol/cloud’ flights, the large remote sensing instruments, i.e. MIPAS and SAFIRE were dismounted. Due to time period of the first campaign (summer) it was decided that it would be particularly interesting to test if an instrument like SCIAMACHY could monitor tropospheric aerosol loading as in the case of Saharan dust outburst over the Mediterranean. It was therefore decided to look at meteorological situations favourable of such events, and as such information from relevant forecasted fields were utilised for the flight planning (see overview paper for more details). It was also chosen to fly in these cases at night, since the aerosol instruments, on board the M-55 Geophysica, (i.e. the three lidars and MAS) operate at best under low background radiation. Due to the relatively long lifetime of a phenomenon like Sahara dust outbreak (usually 2-3 days) it was considered that measurements carried out at night, could be compared with daytime SCIAMACHY measurements taken from the subsequent or the previous overpass over the same geographical region. Two flights were therefore programmed to monitor Sahara dust outbreak and coordinated activity was agreed with the personnel at the ENEA scientific station in Lampedusa (35.5°N, 12.6°E) in order to operate the Lidar belonging to the University of Rome “La Sapienza”.

__________________________________________________________________________________________________________Proc. of Envisat Validation Workshop, Frascati, Italy, 9 – 13 December 2002 (ESA SP-531, August 2003)

On July 15, the first ‘aerosol’ flight (test flight) was scheduled. MIPAS was kept on board to give an additional possibility for a test in order to solve the problems encountered during the previous (‘chemistry’) flight. The flight was mostly a night flight (to permit better detection of aerosol properties from the remote sensing instruments). The plane took off at 17:45 UTC and landed at 21:25 UTC. Fig. 1 shows the followed path. The daytime overpass of ENVISAT was at 8:40 UTC, and the night-time overpass at 20.40 UTC. A dive was planned north-east of Sicily, the only area where the aviation authorities allowed us to perform dives. The forecasted Sahara dust outbreak is shown in Fig. 2. The dive was planned to bring the aircraft from an altitude of 17.5 km to 6 km. The flight proceeded as planned, however the dive had to be truncated at an altitude of 9 km due to high consumption of fuel (due to the high payload: 2107 kg) and the very strong winds encountered in that area.

Fig. 1. Flight track for the M55 Geophysica flight of 15 July 2002 (17:45 - 21: 25 UTC) (‘F’: Forli airport). Blue lines indicate cruise altitude of 17.5 km, while red lines indicate the performed ‘dive’ (down to 9 km and back to cruise altitude of 17.5 km) from initial point ‘V’ (Vento) towards a reference point to the west of Sicily and ‘climb’ back to original cruise altitude at Vento. The green lines indicate the swath of MERIS.

Fig. 2. Dust load forecast relevant to the flight of 15th July 2002, from the University of Athens’ SKIRON model ABLE data indicate (Fig. 3) that a dust aerosol layer was detected during the southern part of the flight near the area where a dive was performed. Cloud layers below 10 km of altitude were observed along the entire flight. Remote sensing measurements place the top of the dust layer at about 4 km, so that in-situ measurements of this Sahara dust outburst with the Geophysica would not have been possible even had the dive reached its lowest permitted altitude. In order to retrieve the vertical limits of a layers a single algorithm is not suitable for any situation. The specific algorithm used in this case, works successfully with layers producing an appreciable and sudden increase of the signal above the molecular and noise background, as in the case of tropospheric clouds [1,2]. Nevertheless it is not capable automatically to detect the beginning of aerosol layers, as in the case of Sahara dust, where the transition from the pure

molecular signal to the aerosol-plus-molecular signal is smoother. A more suitable algorithm for this last case will be implemented in the next future. The optical thickness at the visible wavelength has also been calculated for those layers that do not extinguish completely the laser pulse and do not extend down to the ground. For these types of cloud, not including layers with visible optical depths larger than 3 and the Saharan dust, the optical depth have been retrieved from the backscattering vertical profile using the transmission jump between the cloud top and cloud bottom. Assuming a constant value across each layer of the extinction-to-backscatter ratio, it is possible to retrieve profiles of the extinction coefficient from the backscattering vertical profiles [1,2]. Fig. 3. First aerosol flight (July15); top:total aerosol backscatter coefficient vs. time and altitude (the color bar on the left side represents the typical error in the value of the backscatter ratio; the flight altitude is indicated by a black line); middle: cloud top altitude (±30 m); bottom: optical depth at 532 nm (error bars are shown; red color is used when a second layer is present in the same profile). Trajectory studies (see Fig. 4) provide additional information on the origin of the air masses measured by both the lidar at Lampedusa and the one on board the M-55 Geophysica. The air masses observed by ABLE at the lower point of the dive passed above Lampedusa few hours in advance and were observed by the ground-based lidar. The air masses between 1 and 4 km of altitude travelled above North Africa before reaching Lampedusa and transported the dust collected in the north part of the desert. The backscattering vertical profiles observed by the two instruments are compared in figure 5. The values measured in Lampedusa at 9:30 (black dots) agree very well with the ABLE profiles above 2 km of altitude, while below 2 km there is a better agreement between ABLE and the midday Lampedusa profile (green dots). This is consistent with the passage of the air masses above Lampedusa: at higher altitudes the air masses travelled faster and passed above Lampedusa earlier in the morning.

Fig. 4. Backward trajectories starting from the lowest point of the dive and passing over Lampedusa at different times. The July 15 flight path is shown with a color scale proportional to the altitude

Fig. 5. Backscatter ratio vertical profiles observed by ABLE at the lowest point of the dive (lines) and by the Lampedusa lidar in coincidence with the air mass passage (dots) The second “aerosol-flight” was scheduled for July 18, due to the coincidence of forecasted dust outbreak, cloud activity over NE Italy (Fig. 6) and suitable satellite overpass. The M55-Geophysica was directed first towards Lampedusa, then back towards northern Sicily to a reference point (called VENTO) and from there to the area authorised for the dive (Fig. 7). In order to perform this complex flight MIPAS was disembarked. The total weight was 1780 Kg. The plane took off at 16:40 UTC and landed at 21:38 UTC.

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Fig. 6. Forecasted cloud occurrence (upper panel) (from the University of L’Aquila) and Sahara dust load height cross section (2nd panel) and geographical extent (last panel) (from the Euro-Mediterranean centre on Insular Coastal Dynamics) for the 18th of July 2002.

Fig. 7. Flight track for the M55 Geophysica flight of 18 July 2002 (16:40 - 21: 38 UTC) (‘F’: Forli airport). Blue lines indicate cruise altitude of 17.5 km, green lines a cruise altitude of 14 km, while red lines indicate the performed ‘dive’ (down to 6 km and back to cruise altitude of 17.5 km) from initial point ‘V’ (Vento) towards a reference point to the west of Sicily and climb back to original cruise altitude at Vento before returning back to Forli The flight was successful. Lidar data from ABLE indicate (Fig. 8) detection of a dust cloud during the southern leg of the flight, and possible detection of clouds during the ‘return’ part of the flight towards Forli. Figure 6. Second aerosol flight (July 18). Same of figure 2

Fig. 8. Second aerosol flight (July 18). Same of figure 2.

Fig. 9. Second aerosol flight (July 18): comparisons between observations of the total backscatter ratio obtained by in situ (MAS) and remote sensing (MAL-down) instruments. Furthermore, the intercomparison between ABLE, MAS and MAL is very promising, especially if one looks directly at individual lidar signatures. The precision for identification of cloud layers is of the order of 30 meter, i.e. the resolution of the lidar detection unit. Small discrepancies may occur occasionally, due to different integration time in MAL and ABLE as for instance in the backscattering signature shown in Fig. 9. (right plot). Still the agreement on cloud height and width is perfect. Multi-layer clouds and multiple clouds may be detected if the clouds are not optically too thick. Once the processed ENVISAT data become available, (particularly those of SCIAMACHY) the intercomparison/validation will be performed. While during July the goal has been to monitor Sahara dust outburst, in October all aerosol flights were dedicated to cloud measurements, including flights inside the clouds. Here we show the example of October 11, 2002, with a flight over the northern Mediterranean, Gulf of Genoa and a dive close to Monaco (Fig. 10). The primary objective of this particular flight was the detection and characterisation of high clouds forecasted in the area north of Corsica/southern France (Fig. 11) in association with the footprint of SCIAMACHY, MERIS and AATSR. The cruise altitude was 17.5 km, while a dive down to 9 km was performed, in coincidence with the ENVISAT overpass, to acquire vertical profiles of chemical species. Take off time was 8:30 UTC while the duration of the flight was 3 hours and 5 minutes. Quick look data and the pilot’s report show that clouds were indeed detected during most of the flight and also during the dive.

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Fig. 10. Followed path of M55-Geophysica (circular flight from Forli) for the flight of the 11th of October 2002 (orbit: 3211) (cloud/aerosol payload). Blue lines indicate cruise altitude, while red lines indicate the area of the ‘dive’ (down to 9 km.

Fig. 11. Predicted total cloud cover for the 11th of October 2002, (high clouds) using NCEP data. Forecasts were provided by the University of L’Aquila. The following figure shows the ABLE, MAL and MAS data. The full correspondence can be seen. The continuous measurement of clouds during the daytime is demonstrated. The dive shows the direct penetration into clouds. The height profile of the aircraft is practically given by the upper contour of the figure, as ABLE starts to measure from less than one kilometre below the aircraft.

Fig. 12 First aerosol flight (October 11). Plot of the aerosol backscatter ratio at 1064 nm vs. time and altitude. (preliminary results) CONCLUSIONS The first two campaigns utilising the M55-Geophysica have been successfully performed from Forli, northern Italy. The analysis of aerosol and cloud measurements during the Test and Validation campaign (but also preliminary results from the Mid-latitude Validation campaign) showed good internal constituency, between the various instruments, and the detection of aerosol layers associated with Sahara dust outbreak events as well as detection of middle/high clouds. Validation of Envisat level-2 cloud/aerosol parameters (such as cloud flags, cloud top pressure, optical depth etc.) can be immediately performed when the corresponding Envisat data will be available. REFERENCES [1] Klett J.D., Stable analytical inversion solution for processing lidar returns, Appl. Optics., 20, 211-220, 1991. [2] Platt C.M. et al., The Experiment Clouds Lidar Pilot Study (ECLIPS) for cloud radiation research. Bulletin of the American Meteorological Society, vol. 75, 1635-1654, 1994.