New techniques for accurate measurement of water and water isotopes: Insights into the mechanisms that control the humidity of the upper troposphere and lower stratosphere

Elucidating the mechanisms that control stratospheric humidity is essential if global climate models are to accurately predict how changes in the boundary layer will effect ozone loss in the stratosphere. One method of testing which dehydration mechanisms are prominent in the tropical upper troposphere is using the isotopic ratio of water vapor, as the ratio records the dehydration history of the air parcel. Measurements of the isotopic ratio of water vapor in the overworld stratosphere, over the continental United States and Gulf of Mexico, show air that is enriched in HDO compared with previous remote measurements of tropical stratospheric air. It is concluded that the cause of this enrichment is evaporation of lofted ice as a result of deep convection that penetrates as high as 430 K. Based on a simple mixing model, it is shown that as much as 40% of water vapor in the midlatitude overworld is the result of convective ice lofting. During the recent CR-AVE mission, tropical profiles of HDO and H2O show that the tropical stratosphere over Costa Rica in wintertime is also enriched in HDO. However, measurements in the lower part of the TTL are consistent with convective air following a Rayleigh profile and detraining at 350 K and then rising without further depletion. It is proposed that the stratospheric air over Costa Rica is heavily influenced by middleworld air from the midlatitudes and is not the result of slow ascent in the tropics.; The measurements used in this thesis were obtained using a new instrument that leverages advances in electronic design and laser development with the sensitivity obtained from cavity enhanced absorption spectroscopy to make accurate measurements of water isotopes. The Harvard Integrated Cavity Output Spectroscopy (ICOS) instrument uses a high-finesse optical cavity to produce kilometer pathlengths in a meter sized cell. The theory and application of ICOS as a tracer instrument are laid out in the context of making accurate measurements traceable to laboratory standards. Laboratory calibrations with two different water addition systems as well as cross-calibration with other water and water isotope instruments yield an accurate determination of molecular line strengths and line widths and a robust method for testing the ICOS fitting algorithm. Comparisons with other water and water isotope instruments were made during the AVE-WIIF campaign. ICOS shows good agreement in both H2O and HDO when compared to other instruments. However, a small bias is detected at low mixing ratios and pressures below 100 mbar. The cause of this bias is described as well as possible solutions.; Accurate remote sensing measurements are also important for understanding the role of convection and other dehydration mechanisms. Comparisons made during the CRYSTAL-FACE mission use in situ ice water content (IWC) from the Harvard Total Water and Water Vapor instruments with remote measurements of radar reflectivities from the Cloud Radar System (CRS). A cloud model is used to assess the sampling error caused by comparing measurements that sample air parcels that are not spatially nor temporally collocated. The conclusion from the model, which is confirmed by the CRYSTAL-FACE data, is that measurements must be made within 2 kilometers of each other. Limiting the comparisons to times when in situ and remote measurements were within 2 kilometers of each other, IWC derived from CRS measurements is within 15% of the IWC measured in situ.