| Περίληψη: | The overarching goal of this thesis is to fill existing gaps in our understanding on the role of aerosols in atmospheric electricity, with focus on the effect of aerosols on lightning activity modulation. Towards this goal, satellite-based remote sensing synergies have been utilized to examine the full three-dimensional (3D) structure of aerosols during both fair weather and lightning-active atmospheric conditions. In addition, satellite-based active remote sensing was used to investigate the effect of the aerosol type on lightning activity. Advanced methods and synergies have been developed, especially in focus of desert dust aerosols, targeting the discrimination of the pure-dust component from the total aerosol space-borne observations.
For this analysis, lightning activity observations were provided by ZEUS network, which is operated and maintained by the National Observatory of Athens (NOA) in Greece. For the horizontal distribution of aerosols, Aerosol Optical Depth (AOD) observations were provided by the Moderate Resolution Imaging Spectrometer (MODIS), onboard Aqua sun-synchronous satellite. To derive the 3D distribution of aerosols over extensive geographical domains, active remote sensing techniques utilizing “light detection and ranging” (lidar) space-borne sensors were required. During the course of the thesis, the only available spaceborne lidar systems were the (i) Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and the (ii) Cloud Aerosol Transport System (CATS) onboard the International Space Station (ISS).
In the first part of the thesis, satellite-based passive and active remote sensing observations were utilized synergistically with ZEUS lightning activity observations, towards depicting the full 3D structure of aerosols during lightning-active atmospheric conditions. Cloud to Ground lightning activity observations were provided by ZEUS, AOD at 550 nm by MODIS Aqua, and the vertical aerosol structure by CALIOP, on board CALIPSO. Satellite-based aerosol and ground-based lightning activity observations were studied over the broader Mediterranean Sea for a ten year period (2005 – 2014). Overall, the results revealed the importance of aerosols in lightning activity modulation.
Regarding the horizontal distribution of aerosols and lightning activity, the mean AOD of the days with lightning activity was found higher than the mean seasonal AOD in 90% of cases. Furthermore, it was shown that the number of lightning strikes is increasing with increasing AOD. The most pronounced increase was observed during summertime and for AOD values up to 0.4. In addition, time series showed similar temporal behavior between AOD seasonal anomalies and the number of days with lightning activity. Both the spatial and temporal analysis showed that the lightning activity is correlated to AOD, a characteristic which is consistent for all seasons.
In addition to the horizontal distribution of aerosols, CALIOP observations were utilized in order to provide the vertical distribution of aerosols during lightning active days and, in addition, to study the effect of different aerosol types on lightning activity. The results indicated that the lightning activity is enhanced during days characterized by higher AOD values, compared to days with no lightning. Correlation coefficients of R = 0.73, between CALIPSO AOD values and the number of lightning strikes detected by ZEUS, and of R = 0.93 between European Centre for Medium-Range Weather Forecasts (ECMWF) Convective Available Potential Energy (CAPE) and lightning activity were obtained. The analysis of extinction coefficient values at 532 nm indicated that an altitudinal range exists, between 1.1 km and 2.9 km, where the extinction coefficients of lightning-active and non-lightning-active cases are statistically significantly different. Finally based on the CALIPSO aerosol subtype classification, is was shown that polluted dust aerosols are more frequently observed during non-lightning-active days, while dust and smoke aerosols are more abundant in the atmosphere during lightning-active days.
The second part of this thesis focused on dust aerosols. The complexity of the effect of dust on lightning activity modulation provoked the need to decouple the pure dust component from the total aerosol load. In addition, the need to focus on dust aerosols originated from the dominance of these particles over the complex broader Mediterranean Sea region, the domain that was initially selected to study the relationship between aerosols and lightning activity. In terms of understanding the effect of dust aerosols, advanced remote sensing methods, established under the European Aerosol Research Lidar Network (EARLINET), were implemented. These methodologies were applied towards the decoupling of the pure-dust component in any diverge layer, external mixture of different aerosol components. The pure-dust methodology was applied and validated against the AErosol RObotic NETwork (AERONET) and accordingly applied over South and East Asia. This region was selected due to its intensive anthropogenic aerosol emissions along with the presence of desert dust particles. Using this domain, we managed to distinguish desert dust from the total aerosol load and the anthropogenic component, avoiding at the same time the marine component that is ubiquitous present in the Mediterranean Sea. Sea salt particles are large enough to be distinguished from the dust particles and South and East Asia served as a first step for the final discrimination of dust over the Mediterranean.
The results provided the horizontal and vertical distribution of dust aerosols over South and East Asia along with the seasonal transition of dust transport pathways. Persistent high DAOD (Dust Aerosol Optical Depth) values, of the order of 0.6, were observed over the arid and semi-arid desert regions of Taklimakan and Gobi. In addition, it was shown that the dust aerosol transport (range, height and intensity) is subject to high seasonality, with the highest values during spring for northern China (Taklimakan and Gobi deserts) and during summer over the Indian subcontinent (Thar Desert). In addition, the CALIPSO AOD was decomposed into dust and non-dust aerosol components. This revealed the non-dust AOD over the highly industrialized and densely populated regions of South and East Asia, where the non-dust aerosols yield AOD values of the order of 0.5.
Towards depicting the role of aerosols in atmospheric electricity, satellite-based active remote sensing observations, had to be utilized. The use of space-borne lidar sensors was imperative in order to examine the full 3D structure of aerosols during both fair weather and lightning-active atmosphere. To fulfil our objectives we utilized the validated CALIPSO dataset which is representative for specific case studies but also capable of providing 3D climatological averages. For revealing the diurnal aerosol variations though we needed to utilize CATS, which does not follow a polar orbit. Unfortunately, the CATS products were not validated, thus in order to use the spaceborne lidar datasets a validation of the CATS lidar sensor had to be performed. Thus we proceeded with a thorough validation of the representativeness and quality of CATS observations, for the first time, employing the EARLINET ground-based lidar measurements, which is also the third and final part of this thesis. The results showed that the distribution of the absolute differences between CATS and EARLINET is characterized by a mean value of -1.518e-04 km-1sr-1, a median value of -9.94e-05 km-1sr-1 and a standard deviation of 6.634e-04 km-1sr-1. Overall, the study demonstrated the good performance of CATS, especially during nighttime conditions.
|
|---|
