The water isotopologues H218O and HDO have provided valuable information about temperatures and rain rates in past climates and the sources of boundary layer air in the current climate, among other uses. Despite substantial interest and effort, however, studies of water isotopologues have not taught us anything new about moist convection in the free troposphere. Why is that?
Perhaps, the sensitivity of free-tropospheric isotopologues to the most important convective parameters is simply too weak to be of any use. To explore this hypothesis, we build an analytical model of radiative-convective equilibrium that includes HDO. By varying three fundamental convective parameters -- the entrainment rate, the precipitation efficiency, and the distance that evaporating condensates fall before they evaporate -- we show that the HDO profile is, indeed, quite insensitive to these parameters. The upshot is that measuring HDO does not provide any more useful information than we already get by measuring relative humidity.
For an observed relative humidity of 61%, the full range of possible delta-D profiles for all plausible values of the fractional entrainment rate, precipitation efficiency, and distance that evaporating condensates fall before they evaporate. Note that the full range of possible values is small compared to the uncertainty from the combination of observational uncertainty and the uncertainty in other poorly understood convective processes.