Sensitivities of the hydrologic cycle to model physics, grid resolution, and ocean type in the aquaplanet Community Atmosphere Model

Abstract

Precipitation distributions and extremes play a fundamental role in shaping Earth's climate and yet are poorly represented in many global climate models. Here, a suite of idealized Community Atmosphere Model (CAM) aquaplanet simulations is examined to assess the aquaplanet's ability to reproduce hydroclimate statistics of real‐Earth configurations and to investigate sensitivities of precipitation distributions and extremes to model physics, horizontal grid resolution, and ocean type. Little difference in precipitation statistics is found between aquaplanets using time‐constant sea‐surface temperatures and those implementing a slab ocean model with a 50 m mixed‐layer depth. In contrast, CAM version 5.3 (CAM5.3) produces more time mean, zonally averaged precipitation than CAM version 4 (CAM4), while CAM4 generates significantly larger precipitation variance and frequencies of extremely intense precipitation events. The largest model configuration‐based precipitation sensitivities relate to choice of horizontal grid resolution in the selected range 1–2°. Refining grid resolution has significant physics‐dependent effects on tropical precipitation: for CAM4, time mean zonal mean precipitation increases along the Equator and the intertropical convergence zone (ITCZ) narrows, while for CAM5.3 precipitation decreases along the Equator and the twin branches of the ITCZ shift poleward. Increased grid resolution also reduces light precipitation frequencies and enhances extreme precipitation for both CAM4 and CAM5.3 resulting in better alignment with observational estimates. A discussion of the potential implications these hydrologic cycle sensitivities have on the interpretation of precipitation statistics in future climate projections is also presented.

Climate Change Feedbacks in Aquaplanet Experiments With Explicit and Parametrized Convection for Horizontal Resolutions of 2,525 Up to 5 km

“…Experiments with the nonhydrostatic global atmosphere model ICON in an aquaplanet configuration for parametrized and explicit convection at 10 different resolutions and for a +4 K surface warming have been explored. The large‐scale circulation of ICON develops a double ITCZ for parametrized convection and resolutions finer than 316 km and other than in, for example, Williamson () or Benedict et al (), the double ITCZ does not narrow for increasing resolutions. .…”

“…In Williamson () a double structure ITCZ is narrowing for increasing horizontal resolution and an additional sensitivity to the model's time step length is found (see also Williamson, ). In Benedict et al () a double ITCZ narrows or widens for finer resolutions, depending on the model's physics package, and a single ITCZ structure might transition into a double structure or stay as a single ITCZ, depending on the dynamical core as in Landu et al ().…”

Precipitation Characteristics in the Community Atmosphere Model and Their Dependence on Model Physics and Resolution

“…The estimated precipitation F, I, and D are dependent on horizontal resolutions of the input data (Benedict et al, ; Chen & Dai, ). The CAM simulations with varying resolutions provide a good opportunity to further investigate this issue.…”

“…The same deep convection parameterization (Zhang & McFarlane, ) is used in CAM4 and CAM5. Most previous studies using CAM4 and CAM5 utilized the highly idealized aqua‐planet configuration (Benedict et al, ; O'Brien et al, , ; Zarzycki et al, ), rather than the realistic land‐ocean configuration used here.…”

“…Updrafts are strongly correlated with extreme precipitation (Li et al, ), and several studies (e.g., Kopparla et al, ; Wehner et al, ) found that CAM‐simulated extreme daily precipitation amount is overall larger and more realistic in high resolution than in lower‐resolution configurations. Benedict et al () investigated the sensitivities of precipitation A and F and extreme precipitation to model physics, horizontal resolution, and ocean type, using idealized CAM aqua‐planet simulations. They found that precipitation A and F are most sensitive to the choice of horizontal grid spacing, whose impact on precipitation A and F also depends on the choice of model physics packages, especially for tropical precipitation A and F.…”