Abstract
SST-forced tropical-channel simulations are used to quantify the control of shortwave (SW) parameterization on the mean tropical climate compared to other major model settings (convection, boundary layer turbulence, vertical and horizontal resolutions), and to pinpoint the physical mechanisms whereby this control manifests. Analyses focus on the spatial distribution and magnitude of the net SW radiation budget at the surface (SWnet_SFC), latent heat fluxes, and rainfall at the annual timescale. The model skill and sensitivity to the tested settings are quantified relative to observations and using an ensemble approach. Persistent biases include overestimated SWnet_SFC and too intense hydrological cycle. However, model skill is mainly controlled by SW parameterization, especially the magnitude of SWnet_SFC and rainfall and both the spatial distribution and magnitude of latent heat fluxes over ocean. On the other hand, the spatial distribution of continental rainfall (SWnet_SFC) is mainly influenced by convection parameterization and horizontal resolution (boundary layer parameterization and orography). Physical understanding of the control of SW parameterization is addressed by analyzing the thermal structure of the atmosphere and conducting sensitivity experiments to O3 absorption and SW scattering coefficient. SW parameterization shapes the stability of the atmosphere in two different ways according to whether surface is coupled to atmosphere or not, while O3 absorption has minor effects in our simulations. Over SST-prescribed regions, increasing the amount of SW absorption warms the atmosphere only because surface temperatures are fixed, resulting in increased atmospheric stability. Over land–atmosphere coupled regions, increasing SW absorption warms both atmospheric and surface temperatures, leading to a shift towards a warmer state and a more intense hydrological cycle. This turns in reversal model behavior between land and sea points, with the SW scheme that simulates greatest SW absorption producing the most (less) intense hydrological cycle over land (sea) points. This demonstrates strong limitations for simulating land/sea contrasts in SST-forced simulations.
Highlights
State-of-the-art global and regional climate models (GCMs and RCMs; see Table 1 for acronyms) used for coordinated projects such as the Climate Model Intercomparison ProjectPhase 5 (CMIP5) and the Coordinated Regional Downscaling Experiment (CORDEX)struggle in simulating tropical climate
While a large body of literature focuses on sensitivity and uncertainties induced by convection (CU), planetary boundary layer (PBL), and microphysics (MP) parameterizations in the tropics, the influence of shortwave (SW) and longwave (LW) radiation parameterizations remains poorly documented
We propose to fill these gaps through analyzing multi-physics and multi-resolution tropicalchannel simulations done with the Weather Research and Forecasting (WRF) model forced with prescribed sea surface temperatures (SSTs)
Summary
This is evidenced by large model biases and intermodel spread in simulating the radiative budget of the Earth system (e.g., Kothe et al 2010; Wang and Su 2013; Li et al 2013; Wild et al 2013, 2015; Pessacg et al 2014). The primary atmospheric reason involves difficulty in accounting for sub-grid processes in GCMs and RCMs. the choice of physical parameterizations induces large uncertainties in simulations (e.g., Flaounas et al 2011; Pohl et al 2011; Crétat et al 2012; Hourdin et al.2013; Lim et al 2015; Raktham et al 2015), and the physical package performing best at a given resolution does not necessarily perform better at higher resolution (e.g., Wehner et al.2014). The new “McRad” package outperforms the previous radiation package for most parameters and temporal scales, mainly because of improved cloud–radiation interactions
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