A Forward Model of Total Column Humidity and Water Vapor δD Compared to Subtropical FTIR Observations

In the marine subtropics, constraining the mixing of moisture from the boundary layer into the free troposphere is key to understanding processes that set the humidity profile and maintain low-cloud cover. This is particularly important for diagnosing the response of low-cloud feedbacks within global climate models (GCMs). However, this mixing is difficult to measure and computationally expensive to model. The sensitivity of water vapor isotopic composition to mixing and phase change suggests observations of water vapor isotopologues could yield quantitative estimates of vertical moisture transport. High elevation Fourier-Transform Infrared (FTIR) spectrometers, such as at the Mauna Loa Observatory, Hawaii at 3397 m, typically lie above the marine boundary layer and can retrieve estimates of total column humidity and isotopic composition. We have developed a framework that models the total column moisture and isotopic composition of the free troposphere as an ideal mixing between a subsiding background air mass from a dry and depleted tropospheric source and a moist and enriched boundary-layer source. This mixing is controlled by a variable exponential decay of moisture with height. Modeled points are compared to FTIR observations to estimate vertical mixing as a percent total column boundary layer moisture required; typically 50-90%. The modeling framework is also used to investigate characteristics of the total column moisture-isotopologue space via the relative influences of the various source parameters. Further work seeks to apply the framework to GCM output to provide a unique, quantitative comparison between simulated and observation-estimated mixing.