Assimilation of spaceborne Doppler wind lidar observations in a mesoscale modelŠavli, Matic (Avtor) Žagar, Nedjeljka (Mentor) Aeolus satellitehorizontal line-of-sight windslimited-area modellingensemble Kalman filter data assimilationA continuous improvement of weather prediction is the most important activity of the most of meteorological research. Numerical Weather Prediction (NWP) is the initial value problem, in addition dependent on the quality of numerical model. The NWP improvements rely substantially on the quality atmospheric observations. They are needed in the process of data assimilation that prepares initial conditions for the model forecast. The lack of observations of wind profiles is currently the main shortcoming of the Global Observing System (GOS). The wind information is crucial in the tropics and for small-scale processes in the extra-tropics. On 22 August 2018, a long awaited ESA’s mission, the Aeolus satellite has been launched, which marks the beginning of the new era of measuring winds using lidars from space. Aeolus will measure the so-called horizontal line-of-sight (HLOS) winds below about 30 km. This is the wind component measured in the direction of the pointing lidar and projected horizontally. The line of sight is defined by the azimuth angle from the north which is in the midlatitudes around 60o. Winds are retrieved from the light scattered on the air molecules (Rayleigh winds) and on the air particles such as aerosols and cloud particulates (Mie winds). The HLOS Aeolus winds are expected to improve the forecast skill in global models. The potential of HLOS winds in limited area models (LAMs), the main objective of this thesis, has not been yet addressed. As LAMs simulate small-scale processes, their initialization requires higher resolution observations compared to global models. Even though the Aeolus data with its default horizontal resolution of 90 km can not provide many profiles for the use in a LAM domain, they may be valuable due to the lack of wind profiles. In addition, it is possible to increase the HLOS horizontal resolution at the expense of the data accuracy. The main goal of the thesis is to assess the potential of the HLOS winds in comparison to the zonal and meridional wind components and the full wind information in a LAM domain over Europe and northern Atlantic. As a single HLOS observation contains some information on both the zonal and meridional wind components, its impact in the assimilation will project on both components depending on the azimuth and data assimilation modelling, especially the covariances of the background errors which define the spreading of observed information in the model space. The impact of HLOS profiles in a LAM was addressed using the ensemble data assimilation that provides flow-dependent background error covariance. A novel system built for the thesis is based on the 50-member ensemble using the Weather Research and Forecasting (WRF) model and the Ensemble Adjustment Kalman Filter (EAKF), nested in the state-of-the-art operational ensemble prediction system of the European Centre for medium-Range Weather Forecasts (ECMWF). The flow-dependent representation of the background-error covariances has been shown crucial for the assimilation of HLOS. This was demonstrated on the case of a cold front in the North Atlantic. It was also shown that the assimilation of HLOS winds in special cases with the EAKF may be more useful than the assimilation of full wind vector. An average potential of HLOS winds was investigated using a series of Observing System Simulation Experiments (OSSEs) that compared the impact of simulated HLOS data with the impact of full wind and its two wind components as well as temperature observations. Results show that the impact of HLOS winds is linearly distributed between the zonal and meridional wind components as defined by the applied azimuth of 30◦ from the zonal direction. The multivariate coupling has been found on average weak. Despite a weak multivariate impact, the HLOS winds have been shown promising as they provide better analysis in the zonal wind component compared to the case when only meridional winds are assimilated, and a better impact on the meridional wind compared to the assimilation of the zonal wind component only. The impact of increased resolution of Aeolus observations was addressed using sensitivity experiments with the Aeolus simulator and a global high resolution (T3999) 10-day forecast of ECMWF coupled with the CALIPSO satellite observations of optical properties of the atmosphere. It is found that the Mie winds are less sensitive on the changes in the accumulation length used to prepare a single HLOS profile then the Rayleigh winds. In particular, the Mie wind observation error is found rather constant with amplitude 1-1.2 ms−1 for the range of the accumulation lengths between 30 km and 90 km. These results suggest a significant tuning potential of the Aeolus retrieval for the need of weather prediction with high-resolution LAMs.20182018-10-05 07:45:01Doktorsko delo/naloga104358sl