Improved Ice Content, Radiation, Precipitation and Low-level Circulation over the Tropical Pacific from ECMWF ERA-Interim to ERA5

Abstract This study evaluates changes in simulated Pacific climate between two ECMWF re-analyses; (ERA) Interim (ERAI) and the newest ERA5. Changes in the Integrated Forecasting System (IFS) and possibly sea surface temperature result in greatly reduced discrepancies in ERA5’s ice water path (IWP), radiative fluxes and precipitation relative to satellite-based products. IWP shows the largest percentage change, increasing by over 300% from ERAI to ERA5, due to inclusion of radiatively active snow. ERAI to ERA5 changes in high-cloud fraction generally anticorrelate as expected with outgoing longwave radiation, with ERA5’s having smaller longwave discrepancy versus CERES observations compared with ERAI. Reflected shortwave discrepancies are similarly reduced from ERAI to ERA5, which appears to be due to changes in both cloud fraction and optical depth. Finally, ERA5 also reduces a longstanding excess precipitation discrepancy relative to the GPCP observational product in the southern trade wind region between the Southern Pacific and intertropical convergence zones. This appears to be related to cooler prescribed sea surface temperatures reducing local moisture supply via suppressing net latent heat flux. Compared with GPCP and CERES, ERA5 shows similar geographic patterns of discrepancies to ERAI in terms of precipitation and top-of-atmosphere radiation, but their magnitudes are greatly reduced.The three main points of the article:Main point #1: Satellite-based products are used to examine the changes between ERAI and ERA5 over the tropical and subtropical Pacific OceanMain point #2: ERA5 generally reduces discrepancies relative to the observations across all fields, compared to ERAIMain point #3: The largest fractional change is in total ice water path, cloud fraction and SSTs which may contribute to the improved radiation and circulation produced by ERA5 Key words: ERA Interim, ERA5, Tropical Pacific climate, Ice water content, Radiation, Precipitation, Low-level circulation 1. IntroductionRecently, the European Centre for Medium-range Weather Forecasts (ECMWF) released its fifth generation of reanalysis or ERA5 [Hersbach and Dee, 2016], following the previous generation ERA-Interim (ERAI). The ERA5 Integrated Forecasting System (IFS) and its data assimilation system have major modifications including higher spatial and temporal resolutions, a cloud microphysics scheme with multiple hydrometeor species, and an improved radiation scheme [Urraca et al., 2018] with treatments of subgrid cloud heterogeneity and overlap, and allowing radiatively interactive falling snow [Forbes et al., 2011; Copernicus Climate Change Service, 2017]. This study considers ERA5’s representation of the Pacific climate from 60 °S—60 °N, which contains the Intertropical and Southern Pacific Convergence Zones (ITCZ, SPCZ), plus the Indo-Pacific Warm Pool, regions of intense convective activity that efficiently transport heat to the free troposphere. They contribute to driving the Hadley and Walker Circulations, which are the mean meridional and longitudinal overturning circulations, respectively, and in the western Pacific changes in tropospheric heating have been linked to remote changes in atmospheric stability in the eastern Pacific, and therefore substantial changes in low-level clouds, to an extent that may dominate short-term changes in global net TOA radiative fluxes [Gregory and Andrews, 2006; Mauritsen, 2006; Zhou et al., 2016].ERAI showed a consistent pattern of discrepancies in top-of-atmosphere (TOA) radiation relative to the Clouds and Earth’s Radiant Energy System (CERES) satellite-based product, with too much outgoing TOA longwave in many Pacific convective regions, too much reflected shortwave in the trade wind regions, and too little in the coastal stratocumulus decks. This study investigates whether changes in Pacific climate from ERAI to ERA5 reduce these discrepancies, and our primary motivation was the inclusion of radiatively interactive falling ice (snow) in ERA5, which we previously investigated in short-term forecasts using offline runs of the ERAI IFS [Li et al., 2014b].The reanalyses assimilate large amounts of observational data, but with fewer direct surface-based or radiosonde measurements over ocean than over land, ocean surface and near-surface properties will be more sensitive to changes in model physical parameterizations and their imperfections than those over land. Over ocean they assimilate satellite-observed radiances and retrieved atmospheric wind vectors, but we restrict our analysis to non-assimilated data to provide an independent test.It is challenging to disentangle the factors that cause changes between the IFSs used in the ERAI and ERA5 reanalyses [Hersbach et al., 2020]. Initial evaluation studies have noted that ERA5 shows more consistent Lagrangian transport during the Asian monsoon [Legras and Bucci, 2020], better conservation of potential temperature at high altitudes [Hoffmann et al., 2019] and small biases over the tropical tropopause [Tegtmeier et al., 2020] where there was an increase in the number of IFS levels. Satellite and ground-based radiometers also show lower radiative flux bias magnitudes [Urraca et al., 2018], and when a land surface model was evaluated with ERA5 and ERAI input, discrepancies were reduced relative to eight observed surface properties [Albergel et al., 2018]. This analysis considers changes in ice water, radiation, precipitation, cloud fraction and near-surface properties over the tropical Pacific Ocean from ERAI to ERA5. We were particularly interested in whether changes from ERAI to ERA5 over the Pacific are consistent with our previously reported effects of including falling ice radiative effects (FIREs) [e.g., Li et al., 2014b, 2015, 2016]). The ERAI IFS and approximately half of Coupled Model Intercomparison Project phase 6 (CMIP6) models [Li et al., 2020a] do not include falling ice (i.e. snow) mass in their radiative transfer calculations [Li et al., 2012, 2013, 2014a, 2014b; Waliser et al., 2009, 2011]. We previously found that adding FIREs to the ERAI IFS reduced discrepancies in TOA radiation relative to the CERES satellite-based products [Li et al., 2014b] and similar mean-state changes were seen in controlled simulations with the CESM1-CAM5 and MIROC6 fully-coupled GCMs [Li et al., 2015, 2016; Chen et al., 2018; Michibata et al., 2019].Our analysis is limited to 2007—2010 when all satellite products are available, but we find that substantial reductions in the magnitude of discrepancies relative to satellite-based products for ice water path (IWP), radiation and precipitation do not depend upon the length of the analysis period. Ultimately, while the changes in IWP follow those expected due to the inclusion of radiatively active snow, comparison of ERAI-ERA5 differences in cloud fractions and near-surface properties provide better candidates to explain other changes in TOA and surface radiation and precipitation. It is likely that the implementation of FIREs in ERA5 either did not lead to some of the changes and improvements that may have been expected given the previous results of Li et al., [2014b], Li et al. [2015], Chen et al. [2018] and Michibata et al. [2019] or the impacts of FIREs were outweighed by other changes from ERAI to ERA5. The potential reasoning for this result will be discussed.