Characteristics of Atmospheric Humidity Derived From Reanalyses and Stable Isotopic Measurements From Space

Brown, Derek Philip

Graduate Thesis Or Dissertation

Characteristics of Atmospheric Humidity Derived From Reanalyses and Stable Isotopic Measurements From Space Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/df65v788h

Abstract

This study identifies the large-scale processes that balance regional relative humidity (H), and utilizes satellite measurements of HDO/H2O to characterize moisture processes that influence large-scale humidity. Using the MERRA reanalysis, dynamical and thermodynamical processes that balance zonal mean H are presented. The controls on H vary regionally, with eddy heat and moisture divergence being most influential in the extratropics. Condensation and eddy moisture convergence in midlatitudes, and subsidence and heat divergence in the NH subtropics, have increased from 1979-2004. While H has remained in balance, the strength of the compensating regional controls are changing in response to large-scale circulation shifts. The distribution of HDO/H2O in water vapor, measured from the Tropospheric Emission Spectrometer, is analyzed to quantify influences from advection, convection, condensation, vapor recycling, and evapotranspiration. The analysis focuses on monsoonal regions, where strong hydrological coupling between the land surface and atmosphere provides an ideal test bed for the new dataset. Wet-minus-dry season differences in δD values over the Asian, South American, and North Australian regions are near-zero, negative, and positive, respectively, due to seasonal variations in the characteristics and strength of convection and subsidence. A global Lagrangian mass budget model, constrained by H2O and HDO/H2O measurements, was constructed to give estimates of mixing and loss rates of moisture, fractional increases in humidity due to local moistening, the humidity and isotopic composition of regional source waters, and post-condensational exchange. The source water results are compared to expectations from simple mixing and dehydration models in order to gain new insight into the nature of the exchange processes (e.g., convective detrainment or direct mixing). Further insight is given by the sensitivity of the effective isotopic fractionation in the Lagrangian model to the conditions during condensation. Reversible moist adiabatic processes (i.e., cloud evaporation) are shown to moisten the dry subtropics, while rainfall evaporation is found to provide local moistening in the tropics and summertime subtropics. This study shows that compensating processes may preclude proper interpretation of small changes in mean humidity. Enhanced characterization of the processes that underlie the humidity budget is required for accounting for changes in atmospheric hydrology with climate shifts.

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