Precipitation Efficiency, Water Cycling, and Length Scales of Moisture Transport

February 06, 2025

Adriana Bailey

Hosted by Christine Chiu

Abstract

The global atmospheric circulation transports energy from low latitudes towards the poles. Outside the tropics, about half the energy transport is in the form of latent heat, creating a tight link between the water cycle and climate. As climate changes, it is expected that hydrological connections between latitudes will follow suit. However, understanding these changes is complicated by the fact that characteristic length scales of moisture transport are determined by patterns of evaporation and precipitation, and these exchange processes are difficult to measure and simulate accurately.

In this presentation, I show that evaluating poleward moisture transport on surfaces of constant moist entropy allows us to defi ne moisture length scales in terms of an integrated bulk precipitation efficiency, or drying ratio. This metric can be mapped directly to observable quantities. As warming causes the efficiency with which water cycles through the atmosphere to decrease, moisture length scales increase over much of the globe. This response is consistent across climate models and diverse radiative forcing experiments.

The polar regions are an important exception to this global pattern. There, both simulations and climate proxy records suggest that contributions from local evaporation become increasingly important in response to warming. Especially in the Arctic, the combination of sea ice loss and increasing energy input result in a much larger integrated evaporative flux.

The second part of this presentation leaves the broad view behind and examines up close the extreme evaporative conditions that will ultimately drive the shift toward more “local” moisture at the highest latitudes. The Cold Air Outbreak Experiment in the Sub-Arctic Region, or CAESAR, was an airborne research mission that took place February - April 2024 and that targeted the intense weather conditions that give rise to strong large-scale ocean evaporation events. With diverse participation from students and scientists from the US and abroad, CAESAR fl ew eight research flights between northern Sweden and the Greenland coast. During these flights, the research aircraft porpoised through mixed-phase clouds and sampled just a couple hundred meters above the choppy polar waters. The mission gives us an unprecedented view of the thermodynamics that regulate moisture exchange between the air and sea and an opportunity to investigate the role of microphysics in setting the precipitation efficiency of the atmosphere on a storm-by-storm basis.