Sub-cloud Rain Evaporation in the North Atlantic Ocean

Abstract. Sub-cloud rain evaporation in the trade wind region significantly influences boundary layer mass and energy budgets. Parameterizing it is, however, difficult due to the sparsity of well-resolved rain observations and the challenges of sampling short-lived marine cumulus clouds. In this study, rain evaporation is analyzed using a one-dimensional model that simulates both changes in drop size and changes in drop isotopic composition. The model is initialized with raindrop size distributions and water vapor isotope ratios (e.g. δD, δ18O) sampled by the NOAA P3 aircraft during the Atlantic Tradewind Ocean- Atmosphere Mesoscale Interaction Campaign (ATOMIC). Sensitivity tests suggest that the concentration of raindrops (N0), the geometric mean diameter of the drops (Dg) and the width of the raindrop size distribution (σ) significantly control sub- cloud rain evaporation fluxes (Fe). While N0 determines the overall magnitude of Fe, Dg and σ determine its vertical structure. Overall, the model suggests 65 % of rain sampled by the P3 during ATOMIC evaporates into the sub-cloud layer. To assess the representativeness of these results, we leverage the fact that the percentage of rain that evaporates is proportional to the change in the deuterium excess (d=δD-8×δ18O) of the drops between cloud base and the surface. We compare the deuterium excess simulated by the model with surface isotopic observations from the NOAA Research Vessel Ronald H. Brown. We find that the Brown must have sampled in conditions with higher surface relative humidity, larger cloud-base Dg, and larger cloud-base σ than the P3. Overall, our analysis indicates that both thermodynamic and microphysical processes have an important influence on sub-cloud rain evaporation in the trade wind region.