Spatial Heterodyne Observations of Atmospheric Water Vapour from a High Altitude Aircraft
The atmosphere of the Earth is divided into layers, and the boundaries of these layers are positioned at altitudes where inversions in the atmospheric temperature lapse rate occur. The content of the present work is concerned with water vapour within the upper troposphere - lower stratosphere (UTLS) which is a region of the atmosphere within five kilometres of the tropopause - the boundary between the troposphere and stratosphere. The behaviour of this region is very important in the formation of both the troposphere and stratosphere with water vapour being a principle factor. While broad characteristics of the UTLS, and the role of water vapour within, are well understood questions do remain. To get a better understanding of the answers to these questions there is a need for a continuous high resolution scientific data set of UTLS constituents like water vapour. The Spatial Heterodyne Observation of Water (SHOW) is a scientific instrument designed to measure atmospheric water vapour. The premise of the SHOW measurement technique is Spatial Heterodyne Spectroscopy (SHS) which performs a frequency decomposition on input light to create an interferogram. SHOW measures the atmosphere by observing the UTLS region in the limb. The measurements of SHOW can be combined with a radiative transfer model, a SHS instrument model, and an inversion technique to infer the water content of the observed atmosphere. Due to the nature of the SHS technique, SHOW has the ability to obtain measurements at a high spatial and altitude resolution. This gives SHOW, and SHS technology, the potential to obtain the desired high resolution data sets of the UTLS. However, SHS based instruments, including SHOW, are largely unproven for the application of atmospheric research like measuring water vapour. As a demonstration to validate SHOW, and SHS technology, as applicable to atmospheric science, SHOW was deployed on NASA's ER-2 high altitude science aircraft in July of 2017 for a scientific campaign. The goal of this campaign was to determine the abilities of SHOW with the desired results being to measure atmospheric water vapour to within 1 ppm, with a vertical resolution of less than five hundred meters, and with high spatial sampling. To provide a comparison for the assessment of SHOW, a Vaisala RS41 radiosonde was launched from the Jet Propulsion Laboratory facility located close to Table Mountain. This radiosonde was launched in-situ with some SHOW measurements taken during the engineering flight on July 18th, 2017 and measured the atmospheric water vapour within the region above the facility The SHOW data which corresponded to the radiosonde measurements was analyzed and found that the measurements between the two instruments agreed largely within the goal of 1 ppm. Furthermore, SHOW was able to do so at a vertical resolution of two hundred and fifty meters and achieved a spatial resolution of 0.005 to $0.01 degrees in latitude (roughly 500 m to 1000 m) along a north-south flight track when deployed on the ER-2 platform. These results lend strong supporting evidence that SHOW is capable of providing the desired high resolution UTLS water vapour data set and should be considered for further development and deployment in the future. Furthermore, SHS based instruments should be considered viable atmospheric instruments, and should potentially be used to measure other atmospheric constituents within the UTLS.
SHOW, ER-2, SHS, UTLS, Water
Master of Science (M.Sc.)
Physics and Engineering Physics