title: Investigation of precipitation and its retrieval using ground-based remote sensing techniques
Creators: name: Wang, Wenyue
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 530 Physics
subject: 550 Earth sciences &amp; geology
subject: 620 Engineering

description: Precipitation plays an irreplaceable role in many aspects of the climate system, including the global water cycle, climate change, and weather prediction. However, the current mainstream technologies for obtaining precipitation information have its inherent limitations. The potential of ground-based microwave radiometers for rain parameter retrieval is complementary to these techniques. The main aim of my thesis is thus to optimally estimate precipitation for microwave radiometers and further apply these retrieval methods to monitor precipitation systems in synergy with other remote sensing or in situ techniques.
This thesis is divided in three parts. Part i summarizes the formation process and categorization of precipitation in Chapter 1. The virga phenomenon, the inter-relations of aerosols, clouds, and precipitation, and the precursors of rainfall are also introduced.
Part ii covers the principles of radiative transfer and tropospheric microwave radiometers in Chapter 2 and presents the datasets used and the methodology for rain rate retrieval and data analysis in Chapter 3.
Part iii comprises six of my studies that have been published or submitted to peer-reviewed journals. Firstly, based on the physical characteristics of raindrops in the atmosphere that affect microwave radiation signals, I detected rainfall with high accuracy and high time resolution from rain-contaminated microwave radiometer data using Gradient Boosted Decision Tree (GBDT) algorithms in Chapter 4. The rain type classification from micro rain radar (MRR) is used as the target labels to train this model. Secondly, I investigated an optical depth based physical method in Chapter 5 and developed two machine learning based methods in Chapter 6, to retrieve rain rates from the tropospheric microwave radiometers. By comparing with rain rates measured by rain gauges or MRR, all these methods perform excellently. Then, I assessed the performance of indoor and outdoor microwave radiometers for brightness temperature and atmospheric water measurements, and explored the source of deviation in brightness temperature using GBDT by comparing the importance of various factors on the biases in Chapter 7. The innovative device setting of the indoor microwave radiometer effectively avoids the water film on the radome due to rain. Finally, as applications for rain estimation, I investigated the characteristics of atmospheric parameters observed from ground-based microwave radiometer and weather station using a superposed epoch analysis method before, during, and after rain events over the Swiss Plateau, deducing the temporal evolution of rain events and identifying possible rainfall precursors in Chapter 8. Moreover, I monitored the inter-relations between various factors, such as aerosols, clouds, and meteorological variables, and precipitation systems using ground-based remote sensing and in situ instruments in Granada including a microwave radiometer, ceilometer, cloud radar, nephelometer, and weather station in Chapter 9. I analyzed the potential reasons for the predominant rain type, the main rain intensity class, and the occurrence of the virga phenomenon over southern Spain.