An Overview of CMIP5 and CMIP6 Simulated Cloud Ice, Radiation Fields, Surface Wind Stress, Sea Surface Temperatures and Precipitation over Tropical and Subtropical Oceans

AbstractWe highlight the impacts of the falling ice radiative effects (FIREs) on the biases of ice water path (IWP), radiation, circulation and precipitation from fully-coupled CMIP5 and CMIP6 present day runs over the tropical Pacific and Atlantic oceans. Most global climate models do not consider “falling ice radiative effects” (FIREs). We examine two groups of models with (SON) and without (NOS) FIREs in CMIP5 and CMIP6. Without FIREs, models produce an underestimate of total IWP from up to 80—85%. These models generate overestimated upward longwave radiation (RLUT) by 2—10 W m2 over the convectively active regions in the Pacific and Atlantic oceans. This leads to low-level divergence of anomalous flows over the convective zones, resulting in weaker surface trade winds stress compared to QuikSCAT. This leads to underestimate of RLUT of 4—20 W m2, overestimated reflected shortwave at the top of the atmosphere (RSUT) and an underestimate of downward shortwave at the surface (RSDS) with values of 12—20 W m2 over the trade wind regions against CERES. With FIREs, the biases of radiation fields are reduced by magnitudes of 12—20 W m2 over the entire trade wind regions. The associated surface wind stress biases reduced and the sea surface temperatures biases are reduced with values of 0.5—1.5 K with FIREs in CMIP6 models but no apparent changes in CMIP5 models. The biased of 0.2—3 mm d1 in precipitation is also reduced with FIREs in CMIP6 models but not in CMIP5 models. Without FIREs, there is very small difference is found between CMIP5 and CMIP6 models. With FIREs, models indicates significant improvement against those without FIREs. The above-mentioned findings are generally consistent with our previous sensitivity studies by turning on and off FIREs using CESM1-CAM5 global climate model and ECMWF IFS model. The four main points of the article:Main point #1: The impacts of falling ice radiative effects (FIREs) over the tropical oceans from CMIP5 and CMIP6 models are examinedMain point #2: Lack of FIREs in CMIP5 and CMIP6 models have larger biases in radiation, SST and precipitationMain point #3: Including FIREs in CMIP5 and CMIP6 models reduce the biases in radiation, precipitation and SSTs over the trade wind regionsMain point #4: There is a slightly improvement found from all ensemble means of CMIP5 to CMIP6 models 1. IntroductionTo improve the representations in coupling of clouds, convection, precipitation and their effects on radiation is the highest priority goal in the previous Phase 5 of the Coupled Model Intercomparison Project [CMIP5: Taylor et al., 2012] as well as in the current phase 6 [CMIP6: Eyring et al., 2016]. One of the uncertainties is the representation of precipitation masses, and their interactions with radiation in the general circulation models (GCMs). Most GCMs do not consider the radiative effects of precipitating ice (snow or falling ice), denoted as FIREs, in CMIP5 [Li et al., 2012; 2013; 2014a,b; Waliser et al., 2009; 2011] and also in CMIP6 (from up to date for 11 out of 25 GCMs]. From observationally-based offline radiative transfer study by Waliser et al. [2011] and a series of sensitivity studies using a fully-coupled NCAR-DOE CESM1-CAM1 climate system [Li et al., 2012; 2013; 2014a,b], they found that ignoring FIREs results in biases of 5-10 W m-2 in the top-of-the-atmosphere (TOA) emitted longwave flux (RLUT) leading to more unstable LW radiatively gradient (Figure S3) over the Inter-tropical Convergence Zone/South Pacific Convergence Zone (ITCZ/SPCZ). This is associated with overestimate TOA reflective shortwave radiation (RSUT) and underestimate downwelling surface shortwave (RSDS) over the trade wind regions. As shown in the highlighted in the Supplementary Information (SI), ignoring the FIREs could contribute to systematic errors over the Pacific Ocean. The total ice water path (TIWP) could be biased low up to 85% against CloudSat-CALIPSO estimate (Figure S1b), with excessive downward shortwave (SW) radiation at the surface (RSDS: Figure S1c), weaker reflected SW (S1a) and excessive TOA outgoing longwave (LW) radiation (Figure S1d) in the convective zones over ITCZ/SPCZ. The leads to the excessive upper-level ascending motions aloft and descending motions below middle-low levels required by the cumulus parameterization, leading to grid-box anomalous outflows shown in Figure S4 and producing anomalous weaker trade winds and surface wind stress (Figures S4 and S5). As a result, this leads to anomalous advection of low-level moist and warm air leading to excessive precipitation and warmer sea surface temperatures (SSTs) over trade wind regions. For more details, see Supplementary Information (SI).Up to date, there are more fully-coupled GCMs considering FIREs in CMIP6 models (11 out of 22 models) than those in CMIP5 models. This provides an opportunity to compare the differences in the simulation fields between models with FIREs in CMIP6 (CM6SON) and without FIREs (CM6NOS) against in CMIP5 models (CM5NOS and CM5SON). The highlights of this comparison can provide a gross degree of the impacts of the FIREs in CMIP5 and CMIP6 models for future in depth research in the radiations, precipitation and SSTs. It should be, however, pointed out that the differences between CMIP5 and CMIP6 models as well as in their NOS and SON model groups involve many other indirect changes not only related to the FIREs but also to model improvements as well as resolutions. This study is to highlight radiation-circulation-precipitation coupling between two groups of inclusion of FIREs in CMIP5 and CMIP6 and without FIREs, over tropical oceans including Pacific and Atlantic oceans between the latitudinal belts of 60o S and 60o N. This study will be served as a base reference for future in depth research for radiation, SSTs and circulation.We present the differences and systematic biases between CMIP5 models without FIREs (CM5NOS), with FIREs (CM5SON) and without FIREs in CMIP6 (CM6NOS) models and with FIREs (CM6SON). The shortwave and longwave radiative fluxes at the surface and TOA from CERES-EBAF (2000—2005), and surface wind stress from QuikSCAT (2000—2005) are used for reference. We will also examine changes in total precipitation and sea surface temperatures (SSTs) against ERSST data for annual time scale. In section 2, we describe the general information of CMIP5 and CMIP6 models, and observational data used in this study are briefly described in section 3. In section 4, we illustrate and discuss the patterns of radiation fields, 10m winds, precipitation and column integrated water vapor content against observations, and section 5 provides a summary and conclusion.