Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Astronomy

Supervisor

Sica, Robert J.

Abstract

Water vapor plays a crucially important role in many atmospheric processes. However, it is poorly characterized in much of the atmosphere. Vibrational Raman-scattering Lidar has excellent spatial and temporal resolution, but requires an external calibration to correct for instrumental biases. Microwave Radiometers have poorer resolution, but can be calibrated absolutely and can be used to calibrate the Lidar system. I have implemented a new technique, incorporating both instruments to generate a calibrated water vapor mixing ratio profile. This integrated retrieval uses an inverse method which includes a combined forward model, integrating radiative transfer equations (Schroeder and Westwater 1991) and lidar equations (Sica and Haefele 2016) to account for both radiometer and lidar components. The retrieval uses lidar signal measurements from the RAman Lidar for Meteorological Observations (RALMO) and brightness temperatures from a RPG-HATPRO-G2 microwave radiometer, both located at the MeteoSwiss station in Payerne, Switzerland. The integrated retrieval is tested on synthetically-generated measurements, as well as real measurements from Payerne for clear day and nighttime observations. The performance of this retrieval is compared to the radiosonde-calibrated lidar retrieval technique of Sica and Haefele 2016 and Hicks-Jalali et al 2019, in which lidar constants are determined through a radiosonde-derived calibration factor. The integrated retrieval retrieves this factor directly, which is determined to be within 10\% of the radiosonde-derived value for most nighttime retrievals. Additionally, the uncertainties associated with the integrated method-retrieved factors are around 1.5\%, as opposed to approximately 5\% for the radiosonde-calibrated method. Integrated retrievals over 24-hour periods show diurnal shifts in the calibration factor, which are shown to vary seasonally in parallel with high background count rates in the daytime. For the retrieval of water vapor mixing ratio, the results from the two methods are similar, with retrieved humidity profiles determined with confidence extending into the upper troposphere for clear nights. The integrated retrieval also has the advantage of a lower total systematic uncertainty over the entire effective range of the retrieval, particularly in the lower troposphere. This method is thereby demonstrated to be a viable alternative to water vapor retrievals via radiosonde-calibrated lidar, with the potential to be incorporated into routine operation at the Payerne meteorological site.

Summary for Lay Audience

Water vapor plays an essential role in many atmospheric processes. However, it is difficult to measure it precisely, particularly at the level of clouds in the atmosphere. One instrument for measuring atmospheric water vapor is lidar, which emits laser pulses into the sky, with the returning pulses indicating the abundance of atmospheric constituents, such as water vapor, at various heights. Although they sample the atmosphere on very fine time and height scales, they need to be calibrated against another instrument in order for the result to be physically meaningful. Most commonly, this other instrument is a radiosonde (a weather balloon equipped with sensors for measuring temperature and humidity as it rises) which is launched while the lidar is measuring. In this thesis, I use an alternative instrument, a microwave radiometer measuring the intensity of radiation in the atmosphere, to calibrate the lidar. Although they operate with a poorer height scale than lidars, radiometers can be calibrated internally. This thesis introduces a technique which takes in raw measurements from the two instruments and uses them to simultaneously find the amount of water vapor at different altitudes in the atmosphere. It uses measurements from the RAman Lidar for Meteorological Observations (RALMO) and a RPG-HATPRO (Humidity And Temperature PROfiler), both located at the MeteoSwiss weather station in Payerne, Switzerland. Compared to the atmospheric water vapor determined by the radiosonde-corrected lidar, my method shows similar results for nighttime observations and with better accuracy. This new method is shown to be a viable alternative to water vapor retrievals via radiosonde-calibrated lidar, with the potential to be incorporated into routine operation at the Payerne meteorological site as well as other sites in the future.

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