The Highveld region of South Africa contributes substantially to the aerosol loading over southern Africa because of its importance as an industrial, mining and farming base. Aerosols affect climate by absorbing or reflecting incoming solar radiation, and by affecting cloud microphysics, cloud albedo and precipitation. The physical and optical properties of industrial/urban aerosols over the Highveld region of South Africa were analysed during a 32-day winter sampling period (21 May to 21 June) in 2002; a 32-day summer sampling period (21 October to 21 November) in 2002, and a second 32-day winter sampling period (19 May to 19 June) in 2003. Synoptic circulation systems were examined in as far as they affect the horizontal transport of aerosols over the Highveld region. Measurements of aerosol optical thickness (AOT) from the ground to the top of the atmosphere and aerosol size distribution characteristics over the Highveld region were taken using hand-held hazemeters and a CIMEL sun photometer. The AOT observed over the region during the winter 2002 and 2003 sampling periods and during the summer 2002 sampling period indicated high turbidity. In the 2002 winter sampling period, the AOT530nm ranged between 0.05 to 0.7 with an average of 0.14. In the 2002 summer sampling period, the AOT530nm ranged between 0.05 to 0.6, with an average of 0.24. In the 2003 winter sampling period, the AOT500nm ranged between 0.06 to 0.6, with an average of 0.21. The Ångström exponent value had a wide range, 0.8 to 2.4 in the 2002 winter and summer sampling periods and also in the 2003 winter sampling period, indicating that a range of particle sizes was present over the Highveld region. The Ångström exponent values obtained were derived from the influences of Aeolian dust, coarse-mode industrial particles and, to a small extent, fine-mode biomassburning aerosols. Case studies, based on trajectory analysis and meteorology of the sampling area, were made of the aerosols emanating from the township sites during each of the three sampling periods to observe the build-up and dispersion of aerosols at that time.

Cirrus clouds play an important role in radiative transfer, and thus have impact on the energy balance of the atmosphere and the climate of the Earth. Furthermore, they occur often and cover large areas globally at any time. Nevertheless, cirrus clouds are poorly studied, especially in the polar regions. Cirrus clouds are present in a large amount of the 14 years of data produced by the lidar at the Swedish Institude of Space Physics (IRF), but has not been studied to a large extent. A lidar is an active remote sensing instrument using a laser. This master's thesis develops and improves programs for analysis of cirrus clouds from this lidar data. It also performs analysis of six case studies chosen from the available data, and statistics of these six cases. The parameters calculated for each date are the cloud top, base and mean altitude, the geometrical thickness, the depolarisation ratio, the backscatter ratio (BSR), the backscatter coefficient, the extinction coefficient, the optical thickness and the number of cloud layers. No clear correlation between the optical thickness and the cloud top, base or mean altitude was found. There seems to be a weak correlation between increased optical thickness and increased geometrical thickness, which is not unreasonable. The mean cloud layer top altitude was 11.82 km and the mean cloud base was 10.36 km. The mean optical thickness for a cloud layer was 1.46 km, and the average of the cloud layer mean altitude was 11.09 km. It should be noted that the statistical analysis is based on only six cases with a total observation time of no more than 37 hours. A far larger dataset is needed in order to obtain any statistically signicant conclusions. The effect of averaging is studied, and it is concluded that averaging over altitude reduced the noise and facilitated the interpolation more than averaging over time did. Different approaches to obtain the molecular backscatter coefficient are compared, as well as the effect on the simulated molecular signal. Two of these approaches calculate the molecular backscatter coeffcient with input of the temperature and pressure either as continuously measured ground vales from the weather station at IRF or as radiosonde profiles for a specific time. In the other two, the molecular backscatter coeffcient is obtained from ECMWF data and from the standard atmosphere. Differences in the range 12-35% between the methods are found. Different approaches to calculate the backscatter ratio (BSR) are also compared. At cirrus altitudes, the decrease in the signal due to the molecular cloudfree part of the atmosphere is still strong, and finding the top and base separately by comparison with the standard deviation of the signal is proven a better method than interpolating between the point where the signal starts to increase and the point where it reaches the same signal value again. Height-normalising the signal provides a more robust method. For thin cirrus, the signal is not significantly attenuated above the cloud layer, and it is found that a method based on the ratios between the measured signal and the simulated molecular signal at cloud top and base did not produce reliable results for the optical thickness. In addition to analysing data and data processing methods, new data processing tools in MATLAB have been developed and existing functions have been improved. These will be valuable for continued studies with the IRF lidar, for cirrus as well as PSCs and thick and/or low-altitude clouds.

Within this Ph.D. thesis, the optical thickness and particle effective radius of cirrus and deep convective clouds (DCCs) are retrieved using passive remote sensing techniques. For this purpose, airborne and satellite measurements of spectral solar radiation combined with extensive radiative transfer simulations have been conducted. Data analyzed in this study were collected during the ML-CIRRUS and the ACRIDICON-CHUVA campaigns, which aimed to study natural and contrail cirrus over Europe and DCCs over the Amazon rainforest using the German High Altitude and Long Range Research Aircraft (HALO), respectively. During the campaigns, HALO was equipped with a comprehensive set of remote sensing and in situ instruments. In particular flights, closely collocated measurements with the overpasses of the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard of the Aqua satellite were carried out. A cirrus located above liquid water clouds and a DCC topped by an anvil cirrus are investigated. In general, the research framework can be divided into four parts. In the first part, the spectral upward radiances measured by the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer aboard of HALO are compared with those measured by the MODIS. In the second part, a radiance ratio retrieval assuming a vertically homogeneous cloud is applied to obtain the cloud optical thickness and particle effective radius based on the measurements of SMART-Albedometer and MODIS. Multiple near-infrared wavelengths with different absorption characteristics are utilized in the retrieval in order to study the vertical structure of cloud particle sizes. In the third part, the retrieved cloud properties are compared with those derived from the MODIS cloud products. For the cirrus case, the retrieved values of particle effective radius are further compared to in situ data measured by the Cloud Combination Probe (CCP). To allow this comparison, a vertical weighting method is applied. Although the comparison results in a good agreement, retrievals using this conventional technique only provide information on cloud particle sizes from the upper layers, even if spectral measurements have been employed. The retrieved particle effective radius represents a vertically weighted value, where the upper cloud layers are weighted at most. In the fourth part, an extended technique based on Bayesian optimal estimation has been developed to obtain the full vertical profile of particle effective radius. For this purpose, a parameterization assuming the shape of the vertical profile with respect to a vertical coordinate within the cloud is applied. The information content of SMART-Albedometer measurements is analyzed to identify wavelengths that bring the most information pertaining to each retrieval parameter. The new retrieval technique is applied to the cirrus case to infer the profile of particle effective radius as a function of optical thickness. The comparison between the retrieved and the in situ profiles shows a good agreement with a deviation of about 5 % at the cloud top and increases to values of up to 15 % at the cloud base. The new retrieval technique has shown excellent skill in improving the study of the vertical profile of cloud microphysical properties, which can be applied in the future generation of airborne and satellite retrievals based on the measurements of passive remote sensing.:1. Introduction 2. Definitions 3. Measurements 4. Comparison of upward radiance 5. Retrieval of cloud optical thickness and particle effective radius 6. Comparison of cloud optical thickness and particle effective radius 7. Retrieval of the vertical profile of particle effective radius 8. Summary and conclusion

En présence de diffusions multiples, les algorithmes de Monte-Carlo sont trop coûteux pour être employés dans les algorithmes de reconstruction d'images de géométries tridimensionnelles réalistes. Pour des trajectoires de premiers retours, l'approximation de diffusion est communément employée afin de représenter la statistique des chemins aux nombres de Knudsen tendant vers zéro. En formulant des problèmes équivalents sur des trajectoires de premiers passages, l'usage de l'approximation est étendue en un développement théorique. Cette nouvelle formulation assure un bon niveau de précision, sur une large plage de valeurs du nombre de Knudsen en ce qui concerne l'évaluation des moments de la distribution des longueurs des chemins de premier retour. La résolution numérique du modèle formulé est confrontée aux simulations numériques type Monte- Carlo sur des géométries mono-dimensionnelles et un cas tridimensionel ouvrant des perspectives vers une généralisation aux applications réelles. / For multiple scattering, Monte-Carlo algorithms are computationally too demanding for use in image reconstruction of 3D realistic geometries. In the study of first return path, the diffusion approximation is commonly used to represent their statistical behaviour when the Knudsen number tends to zero. With the formulation of equivalent problems for first passage path, the use of the approximation is extended in a theoretical development. The new model provides a good level of accuracy, for a wide distribution of Knudsen numbers when evaluating the moments distribution of the first return paths length. Numerical application of the model is confronted to Monte-Carlo simulations on one dimension geometries and a simple three-dimension case opening perspectives for the generalization to practical applications.

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<p>A study of air mass origin’s effect on marine stratus and stratocumulus clouds has been performed on clouds north of Scandinavia between 2000 and 2004. The aerosol number size distribution of the air masses has been obtained from measurements in northern Finland. A trajectory model has been used to calculate trajectories to and from the measurement stations. The back trajectories were calculated using the measurement site as receptor to make sure the air masses had the right origin, and forward trajectories were calculated from receptor stations to assure adequate flow conditions. Satellite data of microphysical parameters of clouds from the Moderate Resolution Imaging Spectrometer (MODIS) has been downloaded where the trajectories indicated that clouds could be studied, and where the satellite images displayed low-level clouds. The 25 % days with the highest number of aerosol with a diameter over 80 nm (N₈₀) and the 35% with the lowest N₈₀ have been used to represent polluted and clean conditions respectively. After screening trajectories and satellite imagery, 22 cases of clouds with northerly trajectories that had low N₈₀ values (i.e. clean) and 25 southerly cases with high N₈₀ values (i.e. polluted) where identified for further analysis.</p><p>   The average cloud optical thickness (τ) for all polluted pixels was more than twice that of the clean pixels. This can most likely be related to the differences in aerosol concentrations in accordance with the indirect effect, yet some difference in τ caused by different meteorological situations cannot be ruled out. The mean cloud droplet effective radius (a_ef) was for the polluted pixels 11.2 µm and for the clean pixels 15.5 µm, which results in a difference of 4.3 µm and clearly demonstrates the effect that increased aerosol numbers has on clouds. A non-linear relationship between a_ef and N₈₀ has been obtained which indicates that changes in lower values of aerosol numbers affect a_ef more than changes in larger aerosol loads. The results from this study also indicate that there is a larger difference in the microphysical cloud parameters between the polluted and clean cases in spring and autumn than in summer.</p>

A study of air mass origin’s effect on marine stratus and stratocumulus clouds has been performed on clouds north of Scandinavia between 2000 and 2004. The aerosol number size distribution of the air masses has been obtained from measurements in northern Finland. A trajectory model has been used to calculate trajectories to and from the measurement stations. The back trajectories were calculated using the measurement site as receptor to make sure the air masses had the right origin, and forward trajectories were calculated from receptor stations to assure adequate flow conditions. Satellite data of microphysical parameters of clouds from the Moderate Resolution Imaging Spectrometer (MODIS) has been downloaded where the trajectories indicated that clouds could be studied, and where the satellite images displayed low-level clouds. The 25 % days with the highest number of aerosol with a diameter over 80 nm (N80) and the 35% with the lowest N80 have been used to represent polluted and clean conditions respectively. After screening trajectories and satellite imagery, 22 cases of clouds with northerly trajectories that had low N80 values (i.e. clean) and 25 southerly cases with high N80 values (i.e. polluted) where identified for further analysis.    The average cloud optical thickness (τ) for all polluted pixels was more than twice that of the clean pixels. This can most likely be related to the differences in aerosol concentrations in accordance with the indirect effect, yet some difference in τ caused by different meteorological situations cannot be ruled out. The mean cloud droplet effective radius (aef) was for the polluted pixels 11.2 µm and for the clean pixels 15.5 µm, which results in a difference of 4.3 µm and clearly demonstrates the effect that increased aerosol numbers has on clouds. A non-linear relationship between aef and N80 has been obtained which indicates that changes in lower values of aerosol numbers affect aef more than changes in larger aerosol loads. The results from this study also indicate that there is a larger difference in the microphysical cloud parameters between the polluted and clean cases in spring and autumn than in summer.