Sensibilità del modello meteorologico WRF nell'identificazione dell'altezza del top delle nubi e applicazione all'esperimento EUSO-SPB1

The knowledge of cloud properties is fundamental to understand in which way these atmospheric structures interact, reflecting, absorbing and emitting radiation. Cloud Top Height (CTH) and Cloud Top Pressure (CTP) are between the most difficult properties to predict. Usually they are retrieved by satellite measurements, see radiative and stereo-vision techniques. In this thesis another method that allows a cloud coverage estimation also over area not covered by satellite flights is proposed, based on Numerical Weather Prediction (NWP) models. \par CTH and CTP are derived in this work using the Weather Research and Forecasting (WRF) Model, as an NWP system, attending two different methods: the first one starts from a model output field, the cloud fraction, and the second one computes the optical depth. A top-bottom directed algorithm is used to identify, for each grid point, at which level these two quantities overcome a fixed threshold, determining a cloud presence. Simulated CTH and CTP are then compared with MODIS measures to validate the results. \par A scene over the Central Atlantic Ocean for the $2^{nd}$ of April 2014 has been selected, characterized by non-homogeneous cloud coverage and covered by MODIS satellite flight. To identify the better CTH prediction, different simulations have been performed, taking different vertical levels numbers, long-wave and short-wave radiation schemes and planetary boundary layer parametrisations. Every simulation has a fixed horizontal resolution, namely 9 km, and grid points number 250x250 or 100x90.\par The use of some statistical quantities allows to estimatethe simulations goodness.Between the different WRF parametrisations tested,the Goddard radiation scheme, the Mellor-Yamada-Nakanishi and Niino level 2.5 PBL scheme and 50 vertical levels are indicated as the better choices to obtain an accurate CTH retrieval.\par The validated method has been applied to the trajectory followed byEUSO-SPB1 (Extreme Universe Space Observatory-Super Pressure Balloon), an experiment of the JEM-EUSO program launched in Wanaka on the $24^{th}$ of April 2017 and landed thirteen days after in the South Pacific Ocean. Simulated CTH along the trajectory is useful for a correct reconstruction of the air showers produced by cosmic rays passing through the Earth's atmosphere and of their energy, as the intensity of these luminous phenomena is strictly linked on atmospheric conditions. These results have been used by the JEM-EUSO collaboration to estimate the exposure of EUSO-SPB1 to Ultra-High Energy Cosmic Rays all along the flight trajectory.