El-Niño and La-Niña are believed to change the intensity and frequencies of extreme weather events globally. The present study aims to analyse the impact of El-Niño and La-Niña on the lightning activities of cloud systems and their associated precipitation and thermodynamic indices over the Eastern India regions (Odisha, Jharkhand, and West Bengal) during the pre-monsoon season (March–May). Eastern India receives catastrophic thunderstorm events during the pre-monsoon season. The results suggest that the number of lightning flashes was higher in the El-Niño years than in the La-Niña periods, which helps convective activities to be developed over the study region. The precipitation variations showed similar patterns during El-Niño and La-Niña periods, but the magnitudes were higher in the latter. Results from the analysis of thermodynamic indices show that, during the La-Niña phase, the convective available potential energy (CAPE), convective inhibition (CIN), severe weather threat index (SWEAT), humidity index (HI), and total totals index (TTI) values increased, while the cross total index (CTI) and K index (KI) decreased. In contrast, the vertical total index (VTI) and Boyden index (BI) values showed less significant changes in both El-Niño and La-Niña periods. The anomalies of flash rate densities over most parts of our domain were positive during the El-Niño years and negative during the La-Niña years. Precipitation anomalies had a higher positive magnitude during the La-Niña phase, but had spatial variability similar to the El-Niño phase. The anomalies of most of the thermodynamic indices also showed noticeable differences between El-Niño and La-Niña periods, except for the HI index. El-Niño periods showed higher lightning and increased values of associated thermodynamic indices over eastern India, indicating more pronounced convective systems.

Microphysical properties of precipitating hydrometeors, such as size, concentration, or shape, can be retrieved by vertically-resolved remote-sensing measurements. For a thorough quality assessment, a direct evaluation of those retrievals is required. Surface in-situ observations of precipitation particles are one possible approach. In the framework of the Master’s thesis presented in here, the two-dimensional video disdrometer (2DVD), a ground-based precipitation sensor was applied for this purpose. The 2DVD provides horizontal line scans of two orthogonally aligned cameras so that hydrometeors in a well-defined measurement area are detected. This allows the derivation of single-particle properties as well as precipitation rates, particle number concentrations, or particle size distributions. In this thesis defense, the 2DVD including its measurement and calibration principle, the data processing chain, as well as former 2DVD-related research is presented. Further, the successful instrument evaluation of the 2DVD against other precipitation in-situ sensors and its capabilities to distinguish different ice crystal shapes will be elaborated on. Additionally, case studies about combined 2DVD and remote-sensing observations of cloud seeding experiments will be shown to demonstrate that the 2DVD can contribute to the investigation of clouds and precipitation processes. Data used in this thesis were collected during the PolarCAP / CLOUDLAB campaign near Eriswil, Switzerland between 12/2022 and 02/2023.:1 Introduction 2 Measurement campaign and operating instruments 2.1 Cloudlab and PolarCAP 2.2 Instrument overview 2.3 Two-dimensional video disdrometer (2DVD) 2.3.1 Measurement principle and data processing 2.3.2 Calibration procedure 2.3.3 Former 2DVD research 2.4 HOLIMO 3 2DVD – deducible hydrometeor and precipitation properties 3.1 Properties of single particles 3.2 Precipitation properties 3.2.1 Precipitation rate 3.2.2 Particle size distribution 3.2.3 Relations of particle properties 3.2.4 Particle number concentration 4 Measurement results 4.1 2DVD calibration and evaluation 4.1.1 Calibration procedure on 12 December 2022 4.1.2 Comparison of precipitation rates of different instruments 4.2 Detection of different ice crystal shapes 4.2.1 17 January 2023, 11:30 – 11:35 UTC: irregular crystals and aggregates 4.2.2 17 January 2023, 13:50 – 13:54 UTC: dendrites, strongly rimed particles, irregular crystals 4.2.3 17 January 2023, 14:07 – 14:11 UTC: dendrites 4.2.4 18 January 2023, 09:44 – 09:48 UTC: needles / columns 4.3 Case studies: Precipitation monitoring during cloud seeding experiments 5 Discussion 6 Summary, Conclusions, and Outlook Appendix A 2DVD data processing chain Appendix B Calibration offset between Mira-35 and RPG94 LACROS Appendix C Unrealistic particles not filtered by the 2DVD sno-algorithm Bibliography

For the first time, vertical water vapour profiles were measured in the Central Arctic North of 85°N during the MOSAiC campaign (Multidisciplinary drifting Observatory for the Study of Arctic Climate). Continuous measurements of the Raman lidar PollyXT are used to retrieve high-resolved vertical profiles of the water vapour mixing ratio (WVMR) during the polar night. The collected data are calibrated and evaluated by use of selected clear-sky profiles between 25 October 2019 and 29 February 2020. Three different calibration methods are applied using reference data from radiosonde launches or microwave radiometer (MWR) measurements, respectively. The calibration with the least error results from a linear fit between collocated radiosonde and lidar measurements and delivers a final calibration constant of 15.96 ± 0.37 g/kg for the period from 25 Oct 2019 to 29 Feb 2020. The calibrated WVMR profiles are analysed regarding the vertical distribution of water vapour in the Arctic, its impact on the downward thermal-infrared radiation (DTIR) at the surface, and its relation to the Arctic Oscillation (AO) index as a measure for the general atmospheric circulation. The Arctic atmosphere is very dry during the winter time with WVMR values below 2 g/kg. The vertical water vapour distribution is strongly related to the temperature profile. Layers with higher WVMR values are often capped by temperature inversions. Layers with higher integrated water vapour values (IWV) are located either close to the surface (coupled) or in an elevated layer (decoupled), related to local or advective processes, respectively. The impact of the vertical distributed water vapour on the clear-sky DTIR at the surface was investigated by evaluating the evolution of the air mass at the measurement location over several hours for seven clear-sky cases. The relation between the measured DTIR at the surface and the lidar IWV shows a linear correlation for each case, but with a shift in the radiation values depending on the temperature of the vertical distributed water vapour. The impact of the IWV on the DTIR is determined to be 9.33 − 15.03 W/kg from the example cases. Beside, a linear correlation is found between the temperature of the vertical distributed water vapour and the radiation temperature of the sky, which is derived from the Stefan-Boltzmann’s Law. Both results depict the high impact of the atmospheric water vapour profile on the surface energy budget during clear-sky winter conditions. The influence of the atmospheric circulation on the vertical water vapour distribution in the Arctic is investigated by use of the AO index. While very stable conditions with a weak exchange with lower latitudes are expected during the positive phase of the AO, a stronger meridional transport is related to the negative phase of the AO. The evaluation of 71 randomly selected clear-sky profiles shows differences in the amount and the vertical structure of each WVMR profile between the two phases. Higher WVMR values and layers with higher IWV are observed during the negative AO phase. Nonetheless, a high variability between dry and humid cases is seen during all phases of the AO due to synoptic events. Two main sources for water vapour in the Eastern Central Arctic are identified independent of the AO. These are cyclones on the one hand and the occurrence of a main wind direction from the seas north of Siberia namely Laptev, Kara and Barents Sea on the other hand. In summary, the thesis discusses different calibration methods for the derivation of WVMR profiles from Raman lidar measurements in its first part. In the second part, the thesis gives an overview over the vertical water vapour distribution in the Central Arctic winter and its complex relation to temperature profiles, radiation measurements at the surface and the atmospheric circulation.