Resolution and Dynamical Core Dependence of Atmospheric River Frequency in Global Model Simulations

Abstract

This study examines the sensitivity of atmospheric river (AR) frequency simulated by a global model with different grid resolutions and dynamical cores. Analysis is performed on aquaplanet simulations using version 4 of the Community Atmosphere Model (CAM4) at 240-, 120-, 60-, and 30-km model resolutions, each with the Model for Prediction Across Scales (MPAS) and High-Order Methods Modeling Environment (HOMME) dynamical cores. The frequency of AR events decreases with model resolution and the HOMME dynamical core produces more AR events than MPAS. Comparing the frequencies determined using absolute and percentile thresholds of large-scale conditions used to define an AR, model sensitivity is found to be related to the overall sensitivity of subtropical westerlies, atmospheric precipitable water content and profile, and to a lesser extent extratropical Rossby wave activity to model resolution and dynamical core. Real-world simulations using MPAS at 120- and 30-km grid resolutions also exhibit a decrease of AR frequency with increasing resolution over the southern east Pacific, but the difference is smaller over the northern east Pacific. This interhemispheric difference is related to the enhancement of convection in the tropics with increased resolution. This anomalous convection sets off Rossby wave patterns that weaken the subtropical westerlies over the southern east Pacific but has relatively little effect on those over the northern east Pacific. In comparison to the NCEP-2 reanalysis, MPAS real-world simulations are found to underestimate AR frequencies at both resolutions likely because of their climatologically drier subtropics and poleward-shifted jets. This study highlights the important links between model climatology of large-scale conditions and extremes.

Atmospheric rivers do not explain UK summer extreme rainfall

“…The 20CR results, however, show an increase in the threshold value for summer compared to winter, differing by up to 10%. The dependency of the results on model resolution are similar to Hagos et al [] who found a decrease in the frequency of AR events with model resolution. The causes of the differences observed here are outside the scope of the present study but merits further investigation.…”

Dynamical and thermodynamical modulations on future changes of landfalling atmospheric rivers over western North America

“…This is because the poleward shift of the mean circulation structure will shift the origins and pathways of the AR to higher latitudes, as ARs have been associated with wave breaking in the storm track [ Ryoo et al ., ]. As a consequence the ARs may be weakened by the reduced water vapor available from the cooler surface that provides the moisture for ARs near their origins and/or along their pathways [ Hagos et al ., ], although other factors such as weakening in wind speed may also play important roles. Despite model consistency in projecting negative impact of circulation changes on AR days, further analysis in Text S6 in the SI shows that uncertainty in projecting the meridional shift of the subtropical high in the eastern basin of the Pacific during winter contributes to considerable intermodel spread in the change of the AR days near California coast.…”

Atmospheric rivers in 20 year weather and climate simulations: A multimodel, global evaluation

“…For example, cold biases in simulated surface temperatures can be associated with weakened IVT and too few ARs [ Lavers et al , ]. Also, the presence/lack of sea ice in a coupled model may affect the location of atmospheric jets [ Deser et al , ], which in turn affects AR frequencies [ Hagos et al , ]. Atmosphere‐only models are largely used for weather forecasts and short‐term predictions, while coupled models are important tools for long‐term climate predictions and projections and are not designed to replicate the day‐to‐day weather.…”

“…The considerable but nonmonotonic relationship between AR frequency biases and native model resolutions is similar to what Payne and Magnusdottir [] found in the northeastern Pacific for 28 CMIP5 models. The relationship is somewhat consistent with the sensitivity experiments in Hagos et al [], although they found that the sensitivity of AR frequency to model resolution was notable only in the southeastern Pacific. Biases in landfalling AR frequencies in the west coasts of North America and Europe have been largely attributed to biases in the near‐surface subtropical jet locations [ Gao et al , , ].…”