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Observations of convective clouds generated by solar heating of dark smoke plumes L. Klüser, D. Rosenfeld, A. Macke, and T. Holzer-Popp German Aerospace Center (DLR), German Remote Sensing Data Center (DFD), Weßling, Germany Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem, Israel Leibniz-Institute of Marine Sciences, IFM-GEOMAR, Kiel, Germany Received: 22 November 2007 – Accepted: 6 December 2007 – Published: 10 January 2008 Correspondence to: L. Klüser (lars.klueser@dlr.de)

absorption and reduced reflection of solar radiation. By these means they can alter the temperature profile of the atmosphere and trigger convective clouds. A heavy smoke plume emerging from burning Lebanese oil tanks and spreading over adjacent deserts on 17 July 2006 has been observed as an example of such an effect. This study suggests a physical explanation of the observed convection along the edge of the smoke 10 plume, namely the strong thermal contrast resulting from solar heating of the smoke layer.

Introduction
Many ways have been suggested, how atmospheric aerosols can interact with clouds and alter their development and properties (e.g. Twomey, 1974;Kaufman and Fraser, 15 1997; Ramanathan et al., 2002;Rudich et al., 2003;Rosenfeld, 2006). The Spinning Enhanced Visible and Infrared Imager (SEVIRI) on the geostationary Meteosat Second Generation (MSG) satellite measures radiation in 11 wavelength bands in the visible and infrared part of the spectrum and with one broadband solar channel with an improved spatial resolution (High Resolution Visible, HRV). This combines the ad- 20 vantages of a fine spatial resolution and of the geostationary orbit, i.e. being able to temporarily resolve dynamical cloud processes. Observing consecutive SEVIRI images enables to study the dynamics of clouds during their lifecycles. On 17 July 2006 SEVIRI image loops show a heavy smoke plume emerging from burning Lebanese oil tanks and spreading over the adjacent deserts. Convective clouds can be observed 25 forming at the edge of this smoke plume.
550 Figure 1 shows the middle-east region as a Red-Green-Blue composite image of the MSG solar channels with central wavelengths at 1.6µm, 0.8µm and 0.6µm, respectively, from 17 July 2006, 12:00 UTC. The smoke plume and convective clouds are marked with arrows in this image.
Section 2 introduces the presented phenomenon in images of the HRV channel and 5 of infrared brightness temperatures and by comparison of the reflectance of the smoke plume and the convective clouds with smoke-and cloud-free images obtained from some days prior to the smoke event. In addition to the consecutive SEVIRI images data from the MODerate resolution Imaging Spectroradiometer (MODIS) on the polarorbiting Aqua satellite (Barnes et al., 1998) are used for further analysis of the clouds.

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Sect. 3 suggests a physical explanation of the observed cloud formation whereas the conclusions are discussed in Sect. 4.

Smoke and cloud observations from SEVIRI and MODIS
The data mainly used in this study are radiances from the SEVIRI imaging radiometer on the MSG-1 satellite.

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The SEVIRI instrument is a 12-channel imager consisting of 11 channels with "normal" spatial resolution (3 km×3 km at nadir) and one "high resolution" (1 km×1 km at nadir) channel (HRV). The normal resolution channels measure radiation in various wavelength-bands, of which three are in the solar part of the spectrum (between 0.6µm and 1.6µm), one in the boundary region between solar and terrestrial radiation at a 20 central wavelength of 3.9µm and the resuming seven channels measure the outgoing infrared radiation at central wavelengths between 6.2µm and 13.4µm. The HRV channel measures reflected sunlight in a much broader wavelength band than the other solar channels of the instrument and covers half of the visible disk of the earth at a much finer spatial resolution. A more detailed description of the satellite and the SE- 25 VIRI instrument is given by Schmetz et al. (2002). (1) from the HRV channel radiance for four subsequent hours. The measured radiance L is tranformed to reflectance R by where F 0 is the solar flux in the channel bandwidth and µ 0 the cosine of the sun zenith angle.

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A major smoke plume emerging from burning Lebanese oil tanks can be identified in Fig. 2 as a dark to black plume spreading inland. Likewise developing convective clouds at the south-eastern edge of the plume can be detected by their high reflectance in the images from 10:00 UTC onward.
An animation of the HRV images from 17 July 2006, which shows the temporal evo-10 lution of the observed clouds, is also supplied online: http://www.atmos-chem-phys-discuss.net/8/549/2008/ acpd-8-549-2008-supplement.zip The area of the smoke plume (south-eastern part) and convective clouds has been highlighted by a white square consisting of 100×100 HRV pixels (corresponding to 15 about 66 km in zonal direction and about 136 km in meridional direction due to the viewing geometry of MSG). With the square corresponding to the same HRV pixels in all four images, one can easily follow the propagation and evolution of the clouds together with the smoke plume. Figure 3 introduces an additional way to visualise the smoke plume and the convective clouds of 17 July 2006. First, a background reference 20 field is generated by selecting the HRV pixels with the lowest reflectance during the period from 01 to 16 July 2006 for 10:00, 11:00, 12:00 and 13:00 UTC respectively. Then the difference in reflectance between the reference field and the corresponding reflectance of 17 July 2006 is calculated according to Eq. (2): (2) 25 By this method, the smoke plume can be discriminated in Fig. 3  EGU ysis reveals the obvious connection between the smoke plume (dark area) and the convective clouds at the edge. Furthermore some convection takes place above the north-western part of the plume, which is not included in the analysis presented here. In Fig. 4 10.8 µm brightness temperatures of the region obtained from SEVIRI channel 9 are shown. The brightness temperatures in the area of the smoke plume are 5 reduced to some extend. This reduction of 10.8 µm brightness temperatures is further analyzed by means of reference field deviations equivalent to the calculation of ∆R from Eq. (2). A reference field (for the highlighted area only) has been generated by calculating the mean pixel brightness temperature (10.8 µm) from all cloud free days (determined as described in the next passage) of the period also used for 10 the reflectance reference field. Just like for the reflectance, the difference between the brightness temperatures of 17 July 2006 and the reference fields is calculated by Eq. (3): ∆T is shown in Fig. 5 for the highlighted area (confined to the interval [−20 K, 0K] for 15 contrast enhancement). Brightness temperatures in smoke-and cloud-free areas are about 1-3 K above the reference values (not shown), while for the smoke plume ∆T reaches negative values of about −10 K. This brightness temperature reduction on 17 July 2006 represents a significant reduction in surface temperature below the smoke plume due to the solar heating of the smoke (and hence a lack of solar irradiation at 20 the surface). The period from which the reference field in Fig. 3 has been generated, has been searched for cloud occurrence in the region of interest. Cloud discrimination follows the method of Rosenfeld and Lensky (1998)  EGU the reflectance-threshold of 0.5 led to accurate discrimination of cloudy pixels. Furthermore only HRV pixels with 10.8 µm brightness temperatures, which are mapped to co-registered 3×3 HRV pixels, below 295 K were selected as cloudy. This brightness temperature threshold has proven its appropriateness for the detection of convective clouds in the region of interest for July 2006, whereas some fog and very shallow 5 cloudfields in the coastal regions to the West remain undetected. Given these two thresholds, convective activity above the considered desert regions in July 2006 can be detected very accurate. Figure 6 shows the number of days with convective clouds for each HRV pixel of the region. Pixels with more than 3 cloudy days in the smoke-free 16-day analysis period 10 of July 2006 have been set to the value of 3 for visualisation purposes. It is obvious, that convection takes place in the highlighted area in the first half of July 2006, but not very often. Cloud development is more likely in the coastal regions to the north with some pixels there being cloudy all 16 days. Most pixels of the highlighted box, which reveal some cloud appearance, show clouds only on one single day (which moreover 15 is not the same day for all pixels). Furthermore, no characteristic pattern of enhanced cloudiness following the topography is apparent within the highlighted box, which would be the case, if surface features played a role in modulating the cloud formation in the region under the observed conditions. Figure 7 shows the percentage of pixels detected as being cloudy within the high- EGU cover decreases significantly after 11:30 UTC, on all smoke free days with convective activity it is just the other way round: significant cloud development normally starts about 11:00 UTC, if at all, and the cloud cover keeps raising to higher values at least until about 13:00-14:00 UTC. Figure 6 supports this later convection development under smoke free conditions with the highest number of pixels with at least one cloudy 5 day being present at 13:00 UTC in the highlighted box. Thus analysis of the time of cloud development also strongly supports that the clouds observed on 17 July 2006 are connected directly to the presence of the smoke plume. Further insights into the clouds forming at the edge of the smoke plume can be gained through Aqua MODIS data from 17 July 2006. Figure 8 shows the scene at 10:50 UTC as seen by MODIS. The image generation scheme used here and its physical interpretation is described in Rosenfeld and Lensky (1998). For cloudy pixels within the area surrounded by the red line in Fig. 8 11 µm brightness temperature (T ) and cloud droplet effective radius r are retrieved as described in Rosenfeld and Lensky (1998). The T -r graph is shown in Fig. 9, brightness temperatures in this graph are 15 in • C and effective radius is in µm. These data first of all reveal extremely small cloud droplet effective radii for all cloud covered pixels, actually being at the lower limit of the retrieval algorithm. This extremely small cloud droplet size indicates highly polluted air supplying a large number of small cloud condensation nuclei (CCN) and thus leading to very small cloud droplets, an effect first described by Twomey (1974). 20 Furthermore, Fig. 9 reveals cloud base temperatures of about 285 K-286 K for the convection along the edge of the smoke plume, while cloud top temperatures, reaching values of about 273 K at the time of the MODIS image acquisition, obviously stay below the freezing level. 25 Formation of convective clouds in relationship with oil fire plumes was described e.g. by Rudich et al. (2003). Those authors detected convective clouds above smoke plumes  Rudich et al. (2003) reveals that smaller convective clouds also were formed at the periphery of the smoke plumes, probably by a mechanism similar to that proposed here. But the clouds mainly investigated in Rudich et al. (2003) 5 developed in the middle of the Kuwait plumes and so were probably formed by a different mechanism. Thus another mechanism for the formation of convective clouds in a smoke-polluted environment is suggested here.

Convective cloud formation by the presence of a smoke plume
From the MODIS scene a reduction of surface brightness temperature under the smoke plume of about 12 K can be inferred, which corresponds to the findings from 10 Fig. 5. This temperature reduction beneath the smoke together with the reduced reflection of solar radiation observed in Fig. 3 indicates strong solar heating within the smoke plume, as stated above. Given this solar heating of the smoke, a strong thermal contrast between the hot air of the smoke plume and the warm smoke free air exists, which induces a general rising motion of the smoky air. Along the line of the highest 15 horizontal temperature gradient the sharp thermal contrast triggers convection, which obviously benefits from the high amount of CCN present in the highly polluted air.
The smoke seems to be darkest around the clouds. This probably indicates the deepening of the smoky layer where convergence occurs and the top of the smoky layer rises (see Fig. 3). The strongest rising is topped by the convective clouds, which 20 are fed by it. As the horizontal temperature gradient is maintained by the solar heating of the smoke even after invocation of the convection and additional warming through the release of latent heat takes place, convective clouds grow regardless of an environment, in which convection without the presence of a smoke plume would be (and actually is) suppressed.
A major smoke plume spreading over middle-east desert regions on 17 July 2006 has been detected and analyzed in the data of the SEVIRI instrument on the geostationary MSG satellite. Imagery of reflected solar radiation at the fine spatial resolution of the HRV channel gives evidence that the spreading smoke induces convective clouds. 5 This conclusion is strongly supported by comparison of HRV reflectance of the day of the smoke event with cloud-free reference reflectance of a smoke-free 16 day period before and by the early cloud formation time compared to cloud formation times on smoke-free days in the region during July 2006. The cloud formation and evolution times are quantitatively determined by regional cloud cover of 17 July 2006 and com-10 pared with the smoke-free period. Furthermore extremely small cloud droplet effective radius can be inferred from analysing MODIS data of the scene. The small droplet size indicates a very large number of cloud condensation nuclei within the observed clouds. This supports the inference that the rising motion within the smoky air produced these clouds. Cloud top and base temperatures are also determined for the MODIS scene. 15 Cloud base temperature is high to the extent that is consistent with the rising air that formed the clouds originating at the boundary layer. This rising motion at the edge of the smoke plume may result from the sharp horizontal thermal contrast between the smoky and the smoke free air masses.
Smoke plumes from biomass burning on the African continent during the dry season 20 have been searched for similar cloud structures, but none have been found so far. Thus we conclude that the smoke plume induced convective clouds observed in this study are a result of the rather special conditions of the presented event. Given these conditions, i.e. soot from burning oil, which is much darker than smoke from forest fires and thus is subject to stronger solar heating, yielding to high absorption of solar 25 radiation within the smoke plume, a strong horizontal temperature gradient at its very edge is suggested to induce the cloud formation. These conclusions give evidence, that absorption of solar radiation by aerosols not 557