Quantifying precipitation and cloud re-evaporation: a novel Lagrangian diagnostic evaluated with field observations and moisture tendency outputs from numerical simulations

While first estimates of the importance of below-cloud evaporation for reducing precipitation exist, the impact of this process on the atmospheric water vapour budget and on the downstream dynamics is largely unknown. Previous modeling work has indicated that below-cloud rain evaporation can account for about one-third of the moisture uptakes when a dry intrusion penetrates the subtropical boundary layer, emphasizing the importance of this process for re-moistening the atmosphere. Such internal moisture recycling plays a key role in feeding subsequent storm systems with moisture, particularly in dry regions.

We present an extension to an existing trajectory-based moisture source diagnostic (MSD), incorporating the moisture sources of precipitation and cloud evaporation. The extended MSD identifies increases in specific humidity along Lagrangian trajectories, categorizing the uptakes occurring in the presence of rain or snow as precipitation evaporation and the uptakes occurring in the presence of cloud liquid or ice water as cloud evaporation. In total, the methodology defines six uptake categories based on these hydrometeor types, mixing and the surface evaporation flux.
The extended MSD is evaluated for a 13-day test case in January/February 2018 over the North Atlantic, including three types of airstreams: a dry intrusion, a warm conveyor belt, and an adiabatic flow segment along the jet stream in the mid-latitudes. Physical consistency is then analysed from the model perspective using moisture tendency outputs from the microphysical, convective, and turbulent parameterisations of the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). Since the moisture tendencies indicate which physical processes influenced the moisture budget, comparing them with moisture uptakes from the extended MSD allows verification of whether the MSD identifies these processes in a consistent way within the modeling framework. Potential discrepancies are addressed by defining physically meaningful thresholds for moisture uptake and rainout, constrained by multi-platform observations from the North Atlantic Waveguide, Dry Intrusion, and Downstream Impact Campaign (NAWDIC). Furthermore, different approaches for attributing moisture uptake to the newly introduced source categories are tested. These include methods based on the relative rain, snow, cloud liquid, and cloud ice water contents along the trajectories, as well as approaches that additionally account for Lagrangian changes in hydrometeor contents.

This analysis enables an assessment of the diagnostic’s ability to attribute moisture uptakes to specific processes, even when several act simultaneously. Ultimately, this development provides a necessary framework for quantifying the role of internal recycling processes in the atmosphere and assessing its role for downstream intensification of strongly precipitating airmasses such as in extratropical cyclones or mesoscale convective systems.