Molecules of water with an extra neutron (called isotopologues) behave slightly differently in the atmosphere due to their extra mass, and we explore here whether that behavior can be exploited to learn about the dynamics of rain clouds.
The atmospheric-science community has long hoped that we could learn about rain clouds by studying the concentrations of water isotopologues in the atmosphere, but we show here that this is not possible: the sensitivity of the concentrations is too small to be a useful indicator of important convective parameters, like the entrainment rate or precipitation efficiency.
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.