Digital Holographic Studies of Cloud and Precipitation Microphysics

Abstract

This thesis describes the development and field testing of two in situ cloud observation instruments based on the principle of digital holography. These instruments have been designed to be both low cost and light weight, as suited for observations over wide areas as part of a network of sensors, and for vertical profiling of clouds from an untethered weather balloon. These capabilities are believed, by this author, to be unique to the instruments presented in this work and are intended to address the distinct lack of in situ cloud microphysical observations that are required for improving the understanding of cloud processes, calibration of climate and weather models, and validation of remote sensing observation methods. A major challenge in the development of holographic instruments relates to their autonomous operation under field conditions. Methods are presented to overcome these issues, and aspects of the design process that allow significant reductions in cost and weight, as compared with standard instruments, are described. Automated analysis methods are an essential aspect of a holographic system, particularly for those presented in this work which are intended to be deployed under conditions in which they may be lost. Two automated analysis methods are presented in this work and are tested and optimised using field observations as well as a numerical model of a holographic system that was developed in this work. One of the developed holographic instruments was deployed in a multi-month field campaign in the Australian Snowy Mountains alongside a range of standard instruments. An in-depth analysis of observations from all instruments for a range of different atmospheric events is undertaken, with a particular focus on assessing the performance of the developed holographic instrument. The holographic observations were found to be consistent with those of the other instruments within the overlapping resolution ranges that each were sensitive to. The potential for this instrument to classify atmospheric events was demonstrated using case studies, and holographic observations revealed biases in the microphysical outputs of a reanalysis model. A world-first untethered balloon launch of a holographic microscope into clouds is described. Multiple bands of cloud were identified and the feasibility of this approach was demonstrated. Insights are presented relating to the microphysical structure of a post-drizzling warm stratus cloud, as well as cold clouds at higher altitudes. Microphysical retrievals from an imaging satellite are evaluated using the in situ observations from this launch. The satellite retrievals were found to exhibit distinct biases, consistent with those identified in prior evaluation campaigns.