Clouds are extremely variable in form and size in the Earth's atmosphere. Thus the physical cloud parameters, for example the optical thickness, albedo, and the bottom/top heights from near the ground up to the tropopause, are highly variable. Clouds cover about 66 % of the Earth. The value depends on the used optical thickness threshold . Clouds are composed of liquid particles (T>0 ∘C), ice (T≈<-38 ∘C), or a mixed phase for intermediate temperatures .

Clouds also impact the Earth's radiative balance and play a major, yet still relatively uncertain, role in the changing energy budget . The overall cloud radiative effect depends on cloud type and top height, which is a combination of greenhouse warming (longwave) and reflective cooling (shortwave). The knowledge of global cloud characteristics is essential in different fields of numerical analyses, for instance, weather prediction, circulation, or climate change models.

Optically thick clouds scatter a high percentage of the incoming visible light back to space due to their high albedo. A cloud radiative effect of about -50 Wm-2 is expected due to the enhancement of the annual global albedo. Additionally, clouds absorb and emit infrared (terrestrial) radiation, which strongly depends on cloud temperature. The longwave effect is annually and globally averaged on the order of 30 Wm-2 . Cloud properties and their development in a changing climate will determine the net effect.

The light scattering ability of cloud droplets in the visible and near infrared is only weakly wavelength dependent in comparison to light scattered by molecules. Furthermore, light inside a cloud will encounter enhanced path lengths as it is scattered multiple times at the water or ice particles inside the cloud, and light absorption by trace gases in the clouds will be enhanced.

Tropical clouds can reach heights of roughly 17 km. Either deep convective clouds or the more common cirrus clouds are observed in the tropical tropopause layer (TTL) above 14 km . Cirrus clouds are mainly formed in the upper troposphere (above 8 km) by condensation nuclei such as mineral dust and metallic particles via heterogeneous freezing. Larger biological particles are removed by deposition before reaching these altitudes. Cirrus clouds are either produced by the uplift of humid air or by a convective blow-off from deep convection. They are made of small ice crystals due to the low temperatures at these heights (<-30 ∘C) .

In 84 % of the measurements of Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), cirrus layers have a vertical extension of less than 1.5 km at heights between 13 and 16 km . Only 1.5 % of the cirrus are more than 3 km thick. Cirrus layers near the tropopause can be as thin as 0.5 km and have a horizontal extent up to a few thousand kilometres as detected by Lidar measurements made on the space shuttle .

Cirrus clouds are categorized as (a) subvisual, i.e. optically very thin with cloud optical depths (COD) τN< 0.03, (b) thin (0.03<τN<0.3), or (c) opaque (τN>0.3) . Subvisual clouds are mainly found in the tropics with higher occurrence frequencies over oceans and during the night. Thin cirrus clouds are more frequent at night and are found mainly over equatorial land masses and the western Pacific. Moreover, opaque cirrus clouds are mainly found during the day over oceans . Cirrus cloud occurrence frequencies (COFs) are also coupled to the dynamic circulation patterns. reported that TTL thin cirrus COFs are partly driven by the quasi-biennial oscillation (QBO) and the upwelling in the tropical branch of the Brewer–Dobson circulation (BDC). For example, a reduced BDC upwelling and a warm QBO westerly phase increase the TTL temperature and reduce the relative humidity, which in turn leads to a COF reduction.

High-altitude, optically thin clouds are nearly transparent for the incoming shortwave solar radiation, but they partially absorb Earth's outgoing longwave radiation. As the cloud particles are much colder than the surface, they re-emit comparatively less radiation towards space. The net effect is a warming of the atmosphere below the cloud (“cloud greenhouse forcing”). Low clouds (zct< 3 km), however, are the main contributors to an enhancement of the global albedo. found that nearly half of all clouds over non-polar oceans have low top heights of zct< 3 km. These clouds have an horizontal extent on the order of 2 km. Approximatively 40 % of all clouds are low-level (above 680 hPa, below 3.2 km), 15 % are mid-level, and 45 % high-level  (below 440 hPa, above 6.5 km) clouds .

Different methods exist to measure cloud properties from space . Nadir-viewing, passive instruments like GOME or SCIAMACHY are specifically designed to derive trace gas columns . However, they can also retrieve cloud top heights (CTHs) using the strongest absorption band of molecular oxygen (i.e. O2 A band: 755–775 nm) and infer global trends from the concatenation of a measurement record of 17+ years . The cloud height accuracy of these methods is within a few hundred metres. However, these retrievals are mostly restricted to cloud optical depths larger than 5. A global COD of 3.9 ± 0.3 is retrieved from the International Satellite Cloud Climatology Project (ISCCP) D2 data . Thus not all clouds are detectable from passive nadir observations, like cirrus and cirrostratus clouds with a mean COD of τN=2.2). These clouds have a global annual mean cloud occurrence frequency of 16.7 % as measured by the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the CALIPSO satellite .

A number of satellite missions are specially designed for the detection of cloud properties. For example, CALIOP uses the laser wavelengths of 532 and 1064 nm to measure physical cloud tops with high accuracy of about 60 m in the troposphere . Thin clouds with an optical thickness of about 0.01 are still detectable with this technique. reported an average global cloud amount of about 68 % for clouds with optical depths higher than 0.1. This increases to 73 % when also taking the subvisible cirrus clouds into account .

Optically thin clouds can also be observed by passive sensors in limb or occultation geometry. For instance, the Stratospheric Aerosol Gas Experiment II (SAGE) instrument aboard the NASA Earth Radiation Budget Satellite was able to detect very thin clouds with COD τN>0.03. SAGE detected a high global cloud amount of 95 % . A long light path of about 200 km around the tangent point increases the number of possible cloud measurements. The Optical Spectrograph and Infrared Imaging System (OSIRIS) on-board Odin is a limb-viewing instrument that can accurately measure thin cirrus top heights and COF above 12 km using the 1530 nm IR channel and the optical thickness of subvisual cirrus clouds at 750 nm . The High Resolution Dynamics Limb Sounder (HIRDLS) on-board the AURA spacecraft detects cirrus clouds, deep convective clouds, and polar stratospheric clouds (PSCs) using infrared radiances at 12.1 µm . Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on ENVISAT detected similar cloud parameters using bands in the mid-wavelength infrared and thermal infrared regions .

Clouds in the instruments' field of view influence the retrieval accuracy of atmospheric trace gas concentrations from limb measurements. The cloud-contaminated part of the measurement profile should be omitted in the retrieval algorithm to minimize trace gas retrieval errors . As a consequence, we have developed the SCIAMACHY cloud detection algorithm (SCODA). This retrieval scheme uses a colour ratio method to detect cloud top heights in limb-viewing geometry. It was implemented in the SCIAMACHY level 2 operational processor (version 5.04) to improve the limb trace gas retrievals towards the troposphere. It is also used for scientific profile retrievals of upper tropospheric/lower stratospheric water vapour and trace gases like nitrogen dioxide, bromine oxide, and tropospheric ozone .

In this study limb observations from the SCIAMACHY spectrometer are used to retrieve cloud top heights using the SCODA colour index ratio approach. We investigate the sensitivity of the method to detect clouds in limb view for different tropospheric cloud types using the radiative transfer model SCIATRAN. We study the latitudinal and seasonal variations of cloud heights. Optically thin clouds throughout the free troposphere were detected and even very thin volcanic aerosol layers in the upper troposphere/lower stratosphere (UT/LS) region. Height-resolved cloud occurrence frequencies are derived from the cloud top height data set to study global cloud field distributions. The method is verified by comparing to other satellite data and literature.

The paper is structured as follows. Section  provides information on the data used for this study. The retrieval method to compute cloud top heights from the limb-scattered profiles is described in Sect. . The influence of the limb path and the instrument's vertical field of view on the results is analysed in Sect. . Section  presents model simulations performed with the radiative transfer model SCIATRAN to investigate the robustness of the method. SCIAMACHY retrieval results are discussed in Sect. . The retrieval method is validated with SCIAMACHY nadir data and MIPAS limb data in Sect. . Section  explains the influence of volcanic aerosols on retrieval results and conclusions are given in Sect. .

SCIAMACHY provides measurements of the limb-scattered light integrated along the line of sight. Choosing only wavelengths where trace gas absorptions and emissions can be neglected, for instance around 750 and 1090 nm, the detected radiation is dominated by the scattering processes of molecules, aerosols, and cloud particles . Rayleigh theory describes scattering on air molecules, which is strongly wavelength dependent. Scattering on aerosols and cloud particles, however, is only weakly wavelength dependent and can be approximated by Mie theory. Water and ice clouds show similar scattering characteristics for the chosen wavelengths. Only above 1400 nm, the water and ice absorption have different and non-negligible spectral dependencies , which can be used to discriminate phases or to distinguish between cloud and aerosol particle.

Dividing the spectrally averaged radiances of two wavelength bands (750–751, 1088–1092 nm) by each other, we can differentiate between measurements where particles are situated in the FOV and those not affected by particles. This is called the colour index approach. Colour is defined here as the ratio between radiances from two different wavelength bands of the same viewing LOS or tangent height. The technique was previously applied to SCIAMACHY limb measurements for the detection of polar stratospheric clouds and the determination of the cloud top height of a hurricane cell . This work is a generalization of these previous studies with respect to global particle (cloud) detection and the use of SCIAMACHY tangent heights from the Earth's surface up to 30 km.

The cloud top height zct (km) is determined using radiance profiles in different wavelength regions. The height-dependent colour index CI(zth) is first calculated for all tangent heights zth [km] from the ratio of two limb-scattered radiance intensities I at different wavelengths λ: CI(zth)=Ih(zth,λh)Il(zth,λl), where h denotes the high and l the low wavelength band. The radiances are spectrally averaged for the two bands 750 to 751 nm (l) and 1088 to 1092 nm (h). The signal-to-noise ratios of SCIAMACHY are roughly 2000 at 15 km tangent height (Bovensmann et al., 1999) for both the low and the high band, so that noisy spectra have a negligible effect on the retrievals in general. The colour index ratio (CIR) Θ [-] can then be defined as Θ(zth)=CI(zth)CI(zth+Δzth), where Δzth is the step width between two adjacent tangent heights zth,i and zth,i+1. The tangent height grid of SCIAMACHY limb observations, for example the spacing between two adjacent altitude levels, is about 3.3 km wide. This also determines the retrieval accuracy. The step width (Fig. , red line) is rather constant throughout the mission lifetime.

The function Θ(zth) peaks at the tangent height where the cloud top zct is within the field of view along the line of sight. SCIAMACHY has undersampled the atmosphere in limb view. Cloud tops up to 0.6 km above the FOV are thus not detectable at the next higher tangent height.

If Θ exceeds the predefined constant threshold, the cloud top is allocated to that tangent height. A threshold of 1.4 is chosen to reduce false detections due to aerosols in the lower troposphere (see Sect. ). used 1.3 for the detection of lower stratospheric PSCs. A radiance calibration is not necessary due to the use of ratios, where all multiplicative and height-independent errors cancel out. However, additive errors such as stray light can still influence the retrieval results .

Figure  shows the retrieved profiles of (a) the cloud colour index and (b) the colour index ratio for a SCIAMACHY limb state (29 October 2006, 10:16:10.63 UTC) over Europe with four independent profiles in across-track direction of the flight path. For visualization purposes the colour index is normalized by the highest tangent height. A sharp CI decrease at 12 km is visible for all four measurements. Superimposed is a cloud-free profile (red line) from the same orbit, which exhibits a low surface CI value and a slow decrease with height. Clouds are detected when the CIR Θ peak exceeds the threshold of 1.4 (dashed green line). Depending on the atmospheric conditions, Θ(zth) is an unknown function of the cloud optical thickness (COT), scattering geometry, albedo, aerosol loading, and signal-to-noise ratio of the measurements. The sensitivity of Θ to the scattering geometry and optical characteristics will be discussed in Sect. .

Θ(zth) can peak more than once in a vertical profile, but only the highest cloud top detected is stored with a corrected quality flag. The double peaks occur in roughly 10 % of all retrievals, suggesting that two or more distinct cloud layers are detected. This happens when, for example, a high thin cloud lies above a low thick cloud. It is not possible to separate the cloud layers when the highest cloud is optically thick. The limb-viewing geometry is not well suited to deal with layered clouds as the position of a cloud along the light path is not known (see also Appendix ).

Simulating radiances in cloudy atmospheres is difficult because 1-D radiative transfer models like SCIATRAN treat clouds only as layers with a defined vertical thickness and general stratified physical properties (optical thickness, effective radius of particles, phase function, cloud phase). This is of course a strong simplification of the complex 3-D structure of a cloud, which cannot be resolved with these kind of models.

The software package SCIATRAN version 3.1.28 is used to study the strengths and limitations of the fast and simple CIR retrieval method. The CIR sensitivity is tested for typical atmospheric cloud scenarios like low optically thick or high subvisible clouds. Therefore the influence of cloud parameters (for example, optical thickness τ, top height, layer thickness) and geolocation (sun zenith angle, sun azimuth angle) on the colour index ratio is investigated. The forward model includes a cloud model and has been described and validated by . The radiance accuracy is generally better than 1 % compared to other radiative transfer models.

ENVISAT moved from the north to south along the orbit on the day side, with varying sun zenith angle (SZA) and relative sun azimuth angle (SAA) combinations at the limb tangent point. Because of the sun-synchronous orbit with a descending node crossing time of 10:00, only certain combinations of these parameters were possible. The largest sun zenith angles were found near the poles and minima in the tropics (about 26∘). The relative azimuth angle was generally lower towards the northern high latitudes (SAA about 22∘) and largest at southern high latitudes (SAA about 157∘).

Light scattering on aerosols and cloud particles is largest in the north due to stronger forward scattering at low azimuth angles. Minimum scattering occurs in tropical regions for SAAs around 90∘. Table  shows typical combinations of the SCIAMACHY limb measurement geometry for different latitudes of orbit 24381 from 29 October 2006.

The following parameters are chosen for the tests of CIR sensitivity in a cloudy atmosphere for different cloud top/bottom heights, optical thickness, and geometry. A spherical atmosphere with refraction is modelled with the radiative transfer model SCIATRAN . The scalar discrete ordinate technique is used to solve the radiative transfer equation, where absorption of the trace gases O3, NO2, and SO2 is taken into account. The surface is modelled as a Lambertian reflector with a constant albedo as=0.1, which is roughly twice as high as the albedo of water surfaces. The tangent heights are placed from 0 to 30 km with a step width of 3 km. Radiances are modelled for the wavelengths 750.0 to 751.0 nm with 0.2 nm step width and 1088.0 to 1092.0 nm with 0.8 nm step width, corresponding to the SCIAMACHY channels 4 and 6. The SZA varies between 20 and 82∘ and the SAA between 20 and 160∘ to cover the range of values of the SCIAMACHY limb measurements (see Table ).

We simulated limb radiances for four typical cloud scenarios to find the limitations of the cloud retrieval method with respect to cloud height and optical thickness. The maximum colour index ratios for one retrieval height are shown as function of sun azimuth and sun zenith angles in Fig. . Two water cloud layers with (a) the vertical extents of 2–3 km (lowest cloud case) and (b) 5–6 km (general water cloud case) are modelled. The spectrally dependent optical thicknesses are τN,wc=10.0 and 0.1 at 500 nm respectively. Two cirrus cloud layers made of hexagonal ice crystals with 100 µm height and 50 µm side length are simulated. We assume (c) an optical thickness τN,ic=1.0 for the layer 8–10 km (general ice cloud case) and (d) 0.01 for 14–15 km (subvisual ice cloud case).

The value of CIR is mainly driven by geometry, retrieval height, and cloud and aerosol optical thickness. Thus, we choose combinations of rather low COTs and high aerosol optical thicknesses (AOTs) for all cases except (a). The effective radius of the water droplets is 10 µm , which is in the range of values from literature . The particle size has a negligible effect on the CIR of less than 1 % when varying, for instance, the water droplet size between 10 and 50 µm. In general, ice crystals can vary substantially in size from about 10 to 2000 µm and exhibit a variety of different shapes . However, the CIR is not sensitive to the size of the ice crystals. The CIR varies by less than 1 % when the crystal size is doubled.

The influence of aerosol contamination on the cloud retrieval is taken into account for all cases. We use the LOWTRAN aerosol parametrization defining the height-dependent extinction coefficient and single scattering albedo. The aerosol optical thicknesses at 790 nm are chosen to be in range with global means over water and land from measurements of MODIS (Moderate Resolution Imaging Spectroradiometer). MODIS measured regional annual means between 0.04 and 0.34 at 550 nm . We selected three AOTs τN,a=0.07 (clean case a), 0.14 (oceanic case b–d), and 0.25 (polluted land case c) at 790 nm, which is in the range of global long-term mean values (see , Fig. 7.14). The aerosol phase functions are calculated using the single parameter Henyey–Greenstein analytical formula with an asymmetry factor of 0.772 (0.0–4.0 km), 0.669 (4.0–10.0 km), and 0.657 (above 10.0 km) . The AOT defined at 790 nm corresponds to a larger AOT at 550 nm. For example, τN,a=0.25 at 790 nm is roughly equal to 0.4 at 550 nm.

The step size for the calculations is 5∘ in SZA and 10∘ in SAA direction. The black dots depict tangent point SAA/SZA combinations along a typical SCIAMACHY orbit for the west and east profiles with azimuth angle index 0 and 3. Cloud tops are in general detectable for all SZA/SAA pairs for a CIR threshold of 1.4. When the retrieved cloud height differs from the simulated one, the CIR is not plotted (white areas).

In panel (a) we simulate a low, thick water cloud (2–3 km, τN,wc=10.0) in a clean area (τN,a=0.07), which is found mostly over the oceans . Cloud tops are detected at 3 km tangent height for most of the geometry combinations except for high SZA. If the modelled aerosol loading is set to the global oceanic mean of 0.14, CTHs at 3 km would not be detectable anymore. Because most of the aerosol particles are in the lower layer with an optical depth of roughly 0.125 between the ground and 5 km, the CIR is reduced and the retrieved CTH shifted to the next retrieval tangent height of 6 km.

Low CIR values near the threshold are calculated in sideways (tropics) and backward direction (southern high latitudes) of the scattered light beam (panel b). The cloud is between 4 and 5 km with low COT τN,ic=0.1 and an oceanic global mean τN,a=0.14. High CIR values are found where the water droplet phase function has a peak in forward direction. The CIR of a simulated cloud increases when the cloud is shifted upwards in the model. Thus the threshold is mainly chosen to sort out wrong cloud top heights due to high aerosol loadings in the lower altitudes. Cloud tops are correctly retrieved above 5 km for moderate AOT τN,a≤0.14. Clouds at the lowest possible SCIAMACHY tangent height between 2 and 3 km are only retrievable for very low aerosol loading (τN,a≤0.07). The lowest AOTs are found in clean areas over oceans, especially in the Southern Hemisphere.

Cloud tops consist of ice or mixed phase particles at heights above 6 km . Thus only ice particle clouds are simulated in the upper troposphere. A CIR map for an ice cloud at 8–10 km and τN,ic=1.0 over polluted land (τN,a=0.25) is shown in panel (c). The retrieved CIR is well above the threshold for all cases. High, subvisible ice clouds (14–15 km, τN,ic=0.01, τN,a=0.14) are always detectable with the CIR method (d). Ice and water phase functions differ in complexity and asymmetry (see , Fig. 9). Thus we retrieve a more asymmetric SAA dependence for the ice CIR with lowest values around 140∘. The water phase function has a relatively broad minimum around 100∘. This explains the CIR difference for water (a, b) and ice (c, d) particles between 60 and 120∘, where CIR is more sensitive to ice particles. High CIRs are found for large SAA/SZA values, making the method also very sensitive to polar stratospheric clouds as already shown by .

The strong CIR dependence on tangent height is already observable in Fig.  (Fig. S1 in the Supplement). For instance, the retrieved CIR nearly doubles from about 1.6 (TH =5 km) to 3.1 (TH = 18 km) when moving a water cloud (τN,wc=1.0) with a vertical thickness of 1 km through the troposphere. The retrieval sensitivity is high enough at lower tangent heights. Under normal aerosol conditions (AOT < 0.14) clouds are detectable for COTs τN,wc≥0.1 at 6 km retrieval height and τN,wc≥0.005 in the upper troposphere at 15 km. Sub-visible ice clouds in the upper troposphere are already detectable for cloud optical thicknesses τN,ic≥0.003 (SZA = 25∘, SAA = 83∘, cloud layer 15–16 km). The influence of background aerosols and refractive tangent height changes are negligible at these heights. However, the cloud retrieval can occasionally be obstructed by enhanced levels of aerosols from volcanic activity in the upper troposphere when optical thicknesses of clouds and aerosols have the same order of magnitude. The COT limits have been calculated for a spherical shell cloud layer and have to be interpreted as theoretical lower limits. In reality, we expect lower CIR values for very thin cirrus cloud fields that have limited vertical and horizontal extents.

The SCIAMACHY level 1C tangent heights are geometric ones where refraction is not taken into account. The real tangent height is lowered due to atmospheric refraction. The effect is negligible above 22 km, where differences between geometric and refracted tangent heights are less than 100 m. However, the light path at zth= 6 km has a refracted tangent height of about 4.9 km. Furthermore, the vertical field of view increases. For the geometric VFOV, we calculate the lower edge at 4.7 km and the upper edge at 7.3 km. The corresponding refracted VFOV edges are 3.4 and 6.4 km respectively. The refractive VFOV is thus about 3 km wide. The lower VFOV edge at the geometric tangent height 2.3 km above the ground already is below the surface. Thus it is possible to detect clouds down to the Earth's sea surface. However, the colour index ratio at the lowest heights is already near the threshold and the CIR is very sensitive to aerosol contaminations, which increase towards the surface.

SCIATRAN was already used for cloud sensitivity studies on the limb ozone retrieval. concluded that only tangent heights above the cloud top should be used for ozone profile retrievals in order to reduce ozone retrieval errors. The same limb measurement mode was used here to derive cloud top heights in order to minimize differences in time and geolocation with respect to trace gas profile retrievals. The cloud top height retrieval model SCODA was developed and then implemented in October 2010 into the SCIAMACHY level 2 version 5 operational processor to support the retrieval of trace gas profiles in limb view.

SCODA retrieval results are presented in this chapter. We calculated annual means of cloud top heights and height-dependent occurrence frequencies for the troposphere. The limb COF is defined as the number of measurements flagged as cloudy within one grid cell and atmospheric layer divided by the number of all measurements for that grid cell.

The comparison of limb measurements with nadir retrieved cloud parameters is no simple task due to long light paths as discussed in Sect.  and also addressed by . We first compare cloud top heights retrieved from the two viewing geometries nadir/limb of SCIAMACHY and interpret the differences. We then validate our results with collocated limb CTH from MIPAS in the second subsection.

The influence of aerosols on the colour index ratio Θ has already been studied by . High aerosol loading in the upper troposphere/lower stratosphere can lead to false polar stratospheric cloud detection. This is also true for the retrieval of cloud top heights throughout the troposphere.

The lower the cloud, the more an aerosol layer can disturb the retrieval, as the CIR Θ is strongly height dependent and aerosols tend to reduce the CIR. Aerosols do not exhibit a sharp top height. Because of lower CIR values towards the surface, the extra scattering due to aerosols have a relatively high impact on the CIR. Nevertheless, we detect CTHs at tangent heights below 5 km mainly in oceanic areas and with a high percentage towards the southern polar regions (see Fig. a). As shown in the height-dependent COF distributions, aerosols also influence the detection over desert regions. Furthermore, the intrusion of aerosol particles into the stratosphere is detectable even for very low aerosol optical thicknesses in the UT/LS region, because the threshold τN for subvisual cirrus applies to aerosols as well.

There were three main events during the life cycle of SCIAMACHY, where volcanoes in the Northern Hemisphere ejected a moderate amount of material into the lower stratosphere. The particles are falsely detected as clouds in the lower stratosphere over several months on a semi-global scale. The following eruptions have been observed: Kasatochi (52.1∘ N, 175.3∘ W) starting on 7 August 2008, Sarychev Peak (Matua Island: 48.1∘ N, 153.2∘ E) on 12 June 2009, and Nabro (Eritrea: 13.4∘ N, 41.7∘ E) on 13 June 2011. Mainly for these three events, MIPAS has observed elevated levels of sulfur in the lower stratosphere .

The emitted volcanic sulfur dioxide was transported into the UT/LS region. Over time it was oxidized with water to sulfuric acid and then condensed into larger droplets. The mechanism is slow and takes weeks for the transformation. The sulfuric acid droplets were advected around the globe. Due to the increasing particle diameter, they are detectable in the UT/LS region with the CIR method when the optical depth is high enough.

The EARLINET lidar site found aerosol optical depths at 532 nm between 0.004 and 0.025 in 2008 for heights above 5 km and up to 0.07 in 2009 . measured optical depths up to 0.008 in July 2011 at a wavelength of 780 nm and aerosol layer top heights of 19 km. OSIRIS measured similar values. Also the aerosol layer was still detectable in the middle of September 2011 . At the same time in June 2011, the Puyehue–Cordón Caulle (PCC) volcano (40.6∘ S, 72.1∘ W) emitted large amounts of mainly ash up to heights of 13 km (see e.g. the global volcanism program website).

The main components ejected in the Kasatochi and Sarychev event are sulfate (more than 50 %) and carbonaceous material (less than 43 %) . The percentages changed over time in the UT/LS region and estimated a residence time of 45 ± 22 days for both volcanoes using measurements from the CARIBIC (Civil Aircraft for Regular Investigation of the atmosphere Based on an Instrument Container) platform. Residence times in the UT/LS region are the longest for aerosols with particle diameters around 1 µm. A large spread of residence times from 9 to 62 days was reported in other studies .

The influence of stratospheric particles on the limb CTH detection is already observable in Fig. , where the red dots depict the latitude and estimated start of the breakout from the three volcanoes described above. We now take a closer look at the impact of the three big eruptions and their temporal evolution on the CTH retrieval results. The measurements are sometimes dominated by aerosol particles in the lower stratosphere. Figure  shows the impact of volcanic aerosols can be transported into the lower stratosphere at latitudes between 30 and 70∘ N. The lowest tangent height layer between 12.5 and 15.5 km (red line) is situated in the UT/LS region for this latitude band. While the layer is at the upper edge of the troposphere for latitudes near 30∘ N, it is already in the stratosphere towards 70∘ N. Under normal conditions, the largest occurrence frequencies occur in summer seasons with maxima of nearly 5 % in the lowest layer. The year 2010 is an example of such an undisturbed year in contrast to the other years. Thus only a small amount of particles are observed in the lowest layer, while the two highest layers are completely free of particles for most of the year. Only small areas above 18.8 km in the wintertime Arctic PSC seasons are filled with particles.

Most of the volcanic particles are detected in the layer between 12.5 and 15.5 km. Up to 30 % of the area is covered with aerosols in October 2008. The maximum coverage is about 48 % in September 2009 and 17 % in August/September 2011. The second peak in September 2009 might be due to sedimentation of particles from the layer 15.5 to 18.8 km, where a peak occurrence rate 29 % is seen in August. However, still a considerable amount of aerosols are observed in the intermediate layer with maxima of 19 % in September 2008, 29 % in August 2009, and 23 % in August 2011. A significant occurrence frequency maximum for the layer 18.8–22.2 km is only observed for September 2009 (10 %) with lower values in the months before and after September. Overall, about 45 % of all cloud measurements are located in the lower stratosphere in October 2008, 82 % in August 2009, and 40 % in July 2011.

Elevated aerosol levels are observed for about 4 months in 2008, 7 months in 2009, and 5 months in 2011. The time spans for aerosol layer detection in 2008/2009 are in line with measurements from the Infrared Atmospheric Sounding Interferometer of H2SO4 and OSIRIS for both volcano eruptions in 2008/2009 . Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) measurements of SO2 and sulfate aerosols showed a Sarychev particle lifetime of about 7 months . ACE-FTS detected the plume between 8.5 and 17.5 km with a maximum around 12 km at higher latitudes (55–70∘ N).

While the volcanoes of Kasatochi and Sarychev Peak are at similar latitudes in the Northern Hemisphere, the Nabro volcano is in the tropics and the particles had to be advected into the northern region over time. This might explain why we detect higher amounts (23 %) of particles in the layer 15.5 to 18.8 km (August 2011) than in the layer below, as particles are already vertically transported into the lower tropical stratosphere, then move northwards, and descend again.

The PCC eruption starting in June 2011 is only barely visible in the time series of Fig.  with only a slight increase in mean CTH. However, a similar CIR reduction in the Southern Hemisphere is observed after the eruption as for the other volcanoes. The thin aerosol layers in the UT/LS region lead to lower CIRs compared to clouds and can thus be identified (see Fig. S5).

In this study, we refined the colour ratio method that was first used for the detection of PSCs in limb-viewing geometry. SCIAMACHY limb measurements were used to investigate the global cloud top height and occurrence distributions for the lifetime of the instrument. The use of only one threshold for all atmospheric situations is simplistic in nature but sufficient for the majority of cases, except for the lowest clouds.

For tangent heights between 2 and 5 km, the influence of aerosols and viewing geometry is not negligible. The retrieved cloud top height can be wrong, because a shift to the next tangent height is possible for moderate aerosol loadings in the troposphere. Nonetheless, we are able to detect low clouds within this altitude range in clean areas, mainly over southern oceans. This is also supported by comparisons with SACURA nadir COF measurements.

SCIATRAN model studies have shown that the method is very sensitive for a wide range of cloud optical properties and cloud top heights. The vertical accuracy of the CTH retrieval is limited to the tangent height step size of about 3.3 km. Also, the spatial resolution is rather coarse. The horizontal field of view can be up to 800 km along-track and 240 km across-track, which promotes smearing effects with respect to the location of the cloud field.

Still we found good agreement, comparing our results with co-located MIPAS data. MIPAS has a narrow 30 km across-track FOV and a tangent height step size of 1.5 km. SCIAMACHY CTHs are usually lower by about 1 km. This can be attributed to differences in the tangent height step size and that MIPAS tends to overestimate the CTH . MIPAS also has an 1 km offset compared to other satellite measurements . Furthermore, sensitivity differences of the methods for thin clouds and small aerosol particles lead to larger discrepancies up to 5 km, as seen in the Nabro case in November 2011.

Aerosols affect the CTH retrieval not only at low tangent heights in polluted areas but also after intrusions of larger amounts of volcanic sulfuric acid droplets and ash particles into the UT/LS region. We detected large aerosol-contaminated areas for long periods of several months mainly in the Northern Hemisphere in 2008 and 2009. After the Sarychev volcano eruption in 2009, the northern stratosphere was filled with particles from the poles down to the extra tropics for nearly 7 months. Particles are observed up to heights of about 22 km.

We analysed monthly-sampled cloud top altitudes measured by SCIAMACHY for the two different observational geometries limb/nadir. The CTH differences can partly be attributed to the different sensitivities of the viewing geometries. To this end, a limb/nadir matching technique proves to be advantageous in filling the representation gaps for specific cloud types/regimes, which are only observable in one of both viewing geometries. SCIAMACHY was the only instrument that could simultaneously make use of the two viewing geometries.

The cloud top height detection method first used to detect PSCs was the baseline of the SCODA algorithm. It is now operationally used to improve the operational limb trace gas retrievals towards the tropopause. Here we have shown the importance of the method for cloud and aerosol studies. The use of wavelength pairs in the near IR will possibly enables us to distinguish between clouds and aerosols in the upper troposphere. Scientific studies are currently ongoing. Although SCIAMACHY and MIPAS, with their respective sensitivities, have proven to be of great value for UT/LS studies, new missions with these types of limb-viewing instruments are currently not planned.