Consistency and Homogeneity of Atmospheric Energy, Moisture, and Mass Budgets in ERA5

Abstract

This study uses advanced numerical and diagnostic methods are used to evaluate the atmospheric energy budget with the fifth generation European Re-Analysis (ERA5) in combination with observed and reconstructed top-of-the-atmosphere (TOA) energy fluxes for the period 1985–2018. We assess the meridional as well as ocean-to-land energy transport and perform internal consistency checks using mass-balanced data. Furthermore, the moisture and mass budgets in ERA5 are examined and compared with previous budget evaluations using ERA-Interim as well as observation-based estimates. Results show that peak annual mean meridional atmospheric energy transports in ERA5 (4.58±0.07 PW in the northern hemisphere) are weaker compared to ERA-Interim (4.74±0.09 PW), where the higher spatial and temporal resolution of ERA5 can be excluded as possible reason. The ocean-to-land energy transport in ERA5 is reliable at least from 2000 onwards (∼2.5 PW) such that the imbalance between net TOA fluxes and lateral energy fluxes over land are on the order of ∼1W m-2. Spin-up/-down effects as revealed from inconsistencies between analyses and forecasts are generally smaller and temporally less variable in ERA5 compared to ERA-Interim. Evaluation of the moisture budget shows that the ocean-to-land moisture transport and parameterized freshwater fluxes agree well in ERA5, while there are large inconsistencies in ERA-Interim. Overall, the quality of the budgets derived from ERA5 is demonstrably better than estimates from ERA-Interim. Still some particularly sensitive budget quantities (e.g., precipitation, evaporation, and ocean-land energy transport) show apparent inhomogeneities, especially in the late 1990s, which warrant further investigation and need to be considered in studies of interannual variability and trends.

Evaluating Twenty‐Year Trends in Earth's Energy Flows From Observations and Reanalyses

“…Trends over the Gulf Stream are particularly noteworthy, as all three results show large negative trends, implying increased surface‐to‐atmosphere heat flux. In this region, the climatological mean $${F}_{S}$$ is also strongly negative since the atmosphere is supplied energy from warm water masses transporting energy poleward (Mayer et al., 2021; Trenberth & Fasullo, 2017). We also find large negative trends for Inferred (Figure 4a) and ERA5 forecasts (Figure 4b) over the East Pacific off of North and South America and a line of positive trends along the equatorial Pacific stretching from the Maritime Continent to Central America.…”

“…The $$\nabla \cdot {F}_{A}$$ term is computed from hourly ERA5 analyses of atmospheric wind, temperature, and humidity profiles using an improved budget formulation that treats lateral and vertical moisture enthalpy fluxes in a consistent manner (Mayer et al., 2017) and ensures mass consistency following J. Mayer et al. (2021). Maps of $$\nabla \cdot {F}_{A}$$ are smoothed using a tapered filter truncating at T42.…”

“…For ERA5, we use short‐term forecasts and subtract $${F}_{\text{TOA}}$$ and $${F}_{S}$$ (Equation ). Neglecting the effect of assimilation increments in this estimate will lead to differences with the divergence estimate based on analyzed state quantities (J. Mayer et al., 2021). The short‐term ERA5 forecasts are averaged over the first 12 hours of the forecasts and initialized from analyses that are constrained by observations and in that sense are still influenced by observations.…”

“…The limited number of datasets used enables a more focused assessment of the impact data assimilation in reanalysis on trends. In addition, to our knowledge, ECMWF data are the only source that have been used to calculate the divergence of lateral atmospheric energy transports using the most recent methodological advances (Mayer et al., 2021). The datasets used in the analysis are described in Section 2.…”

Evaluation of Regional Surface Energy Budget Over Ocean Derived From Satellites

“…The net surface flux F net,t averaged over the global open water is 2.1 Wm −2 , whereas its F net,c counterpart is 16.3 Wm −2 . Mayer et al (2021) who computed net surface energy flux over ocean with ECMWF ERA5 (Hersbach et al, 2018) by the transport approach report F net,t of 1.6 Wm −2 . Josey et al (2013) who computed net surface energy flux over ocean with OAFlux turbulent fluxes (Yu and Weller, 2007) and International Satellite Cloud Climatology Project (ISCCP) irradiances (Zhang et al, 2004) by the component approach report F net,c of 29 Wm −2 .…”