Atmospheric Rivers in a Hierarchy of High-Resolution Global Atmospheric Models

Abstract:

Atmospheric rivers are long and narrow plumes that carry moisture over land along frontal zones associated with mid-latitude storms. They can account for 90% of the horizontal moisture transport in a given day and are responsible for major flooding, particularly along western coastal regions (western coasts of North America and Europe). It is therefore crucial to well simulate these events in climate models in order to improve predictions and attributions of heavy precipitation and flooding along western coastal regions.

In this study, we investigate the ability of a state-of-the art climate model to represent the location, frequency and structure of atmospheric rivers affecting Western Europe and California.

By making use of the UPSCALE (UK on PRACE: weather resolving Simulations of Climate for globAL Environmental risk) campaign, a traceable hierarchy of global atmospheric simulations (based on the Met Office Unified Model, GA3 formulation), with mesh sizes ranging from 130 km to 25 km, we study the impact of improved representation of small-scale processes on the mean climate, its variability and extremes in order to understand the processes underlying observed improvement with higher resolution. Five-member ensembles of 27-year, atmosphere-only integrations are available at these resolutions, using both present day forcing and a future climate scenario.

Demory et al (2014) have already shown that a relatively coarse resolution limits the model’s ability to simulate moisture transport from ocean to land. This is particularly true at mid-latitude, where the transport is dominated by eddies. Increasing horizontal resolution increases eddy transport of moisture at mid-latitudes. Here, we investigate the climatology of atmospheric rivers, in particular their frequency and associated precipitation, compared to reanalysis products. Some aspects of the relationship between the improved simulation of moisture transport in current climate conditions, and how this impacts changes in atmospheric rivers in the future climate, with much larger atmospheric moisture availability, will also be discussed.