Determining water sources in the boundary layer from tall tower profiles of water vapor and surface water isotope ratios after a snowstorm in Colorado
- ISSN: 16807316
- DOI: 10.5194/acp-13-1607-2013
The D/H isotope ratio is used to attribute boundary layer humidity\nchanges to the set of contributing fluxes for a case following a\nsnowstorm in which a snow pack of about 10 cm vanished. Profiles of H2O\nand CO2 mixing ratio, D/H isotope ratio, and several thermodynamic\nproperties were measured from the surface to 300m every 15 min during\nfour winter days near Boulder, Colorado. Coeval analysis of the D/H\nratios and CO2 concentrations find these two variables to be\ncomplementary with the former being sensitive to daytime surface fluxes\nand the latter particularly indicative of nocturnal surface sources.\nTogether they capture evidence for strong vertical mixing during the\nday, weaker mixing by turbulent bursts and low level jets within the\nnocturnal stable boundary layer during the night, and frost formation in\nthe morning. The profiles are generally not well described with a\ngradient mixing line analysis because D/H ratios of the end members\n(i.e., surface fluxes and the free troposphere) evolve throughout the\nday which leads to large uncertainties in the estimate of the D/H ratio\nof surface water flux. A mass balance model is constructed for the snow\npack, and constrained with observations to provide an optimal estimate\nof the partitioning of the surface water flux into contributions from\nsublimation, evaporation of melt water in the snow and evaporation from\nponds. Results show that while vapor measurements are important in\nconstraining surface fluxes, measurements of the source reservoirs (soil\nwater, snow pack and standing liquid) offer stronger constraint on the\nsurface water balance. Measurements of surface water are therefore\nessential in developing observational programs that seek to use isotopic\ndata for flux attribution.