Advances in simulating atmospheric variability with the ECMWF model are presented that stem from revisions of the convection and diffusion parametrizations. The revisions concern in particular the introduction of a variable convective adjustment time‐scale, a convective entrainment rate proportional to the environmental relative humidity, as well as free tropospheric diffusion coefficients for heat and momentum based on Monin–Obukhov functional dependencies.The forecasting system is evaluated against analyses and observations using high‐resolution medium‐range deterministic and ensemble forecasts, monthly and seasonal integrations, and decadal integrations with coupled atmosphere‐ocean models. The results show a significantly higher and more realistic level of model activity in terms of the amplitude of tropical and extratropical mesoscale, synoptic and planetary perturbations. Importantly, with the higher variability and reduced bias not only the probabilistic scores are improved, but also the midlatitude deterministic scores in the short and medium ranges. Furthermore, for the first time the model is able to represent a realistic spectrum of convectively coupled equatorial Kelvin and Rossby waves, and maintains a realistic amplitude of the Madden–Julian oscillation (MJO) during monthly forecasts. However, the propagation speed of the MJO is slower than observed. The higher tropical tropospheric wave activity also results in better stratospheric temperatures and winds through the deposition of momentum.The partitioning between convective and resolved precipitation is unaffected by the model changes with roughly 62% of the total global precipitation being of the convective type. Finally, the changes in convection and diffusion parametrizations resulted in a larger spread of the ensemble forecasts, which allowed the amplitude of the initial perturbations in the ensemble prediction system to decrease by 30%. Copyright © 2008 Royal Meteorological Society

A key aim of climate monitoring is to place the current conditions of climate variables, such as surface air temperature and precipitation, in their historical context. In Europe, a leading provider of this information is the Copernicus Climate Change Service (C3S), which is implemented by the European Centre for Medium‐Range Weather Forecasts (ECMWF) on behalf of the European Commission. To undertake this activity in close‐to‐real time, C3S predominantly uses the ECMWF ERA5 reanalysis, and ERA5 precipitation is widely used as a proxy for observed precipitation for hydrological monitoring. However, for there to be confidence in the ERA5 data, it is essential to evaluate its ability to capture observed precipitation. In this study, we therefore evaluate ERA5 precipitation against observations globally to inform C3S monitoring activities and the broader climate science community. Using 24 hr precipitation observations at 5,637 stations from 2001 to 2020, results show that the smallest ERA5 random errors occur in the winter Extratropics and the largest ERA5 errors are in the Tropics. The errors grow in the summer Extratropics and the errors in the Tropics move with the intertropical convergence zone. These findings are mirrored in the stable equitable error in probability space (SEEPS) score, with the SEEPS signifying that ERA5 is more able to discriminate between different precipitation events in the Extratropics. In general, an ERA5 wet bias is also found. The assessment of annual maximum 1 day (RX1) precipitation accumulations and four extreme events shows that ERA5 cannot model the highest observed precipitation totals but that it can generally capture their locations and patterns. Furthermore, an evaluation of monthly precipitation corroborated that ERA5 is more skilful in the Extratropics. These results imply that users can have confidence in ERA5 precipitation in extratropical regions, and therefore it is recommended that ERA5 is mostly used for extratropical precipitation monitoring.

The summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and anticyclones over the oceans. In winter, the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography. Here, the mechanics of the summer and winter lower-tropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations. By prescribing in the model the heatings associated with several of the world's monsoons, it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the Kelvin wave response to the monsoon heating over the continent to the west. A poleward-flowing low-level jet into a monsoon (such as the Great Plains jet) is required for Sverdrup vorticity balance. This jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region. The low-level jet into North America induced ... Abstract The summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and anticyclones over the oceans. In winter, the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography. Here, the mechanics of the summer and winter lower-tropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations. By prescribing in the model the heatings associated with several of the world's monsoons, it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the Kelvin wave response to the monsoon heating over the continent to the west. A poleward-flowing low-level jet into a monsoon (such as the Great Plains jet) is required for Sverdrup vorticity balance. This jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region. The low-level jet into North America induced ...

Trustworthy probabilistic projections of regional climate are essential for society to plan for future climate change, and yet, by the nonlinear nature of climate, finite computational models of climate are inherently deficient in their ability to simulate regional climatic variability with complete accuracy. How can we determine whether specific regional climate projections may be untrustworthy in the light of such generic deficiencies? A calibration method is proposed whose basis lies in the emerging notion of seamless prediction. Specifically, calibrations of ensemble-based climate change probabilities are derived from analyses of the statistical reliability of ensemble-based forecast probabilities on seasonal time scales. The method is demonstrated by calibrating probabilistic projections from the multimodel ensembles used in the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), based on reliability analyses from the seasonal forecast Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) dataset. The focus in this paper is on climate change projections of regional precipitation, though the method is more general. © 2008 American Meteorological Society

A time-dependent primitive equation model with specified zonal flow, mountains, and diabatic heating is described. The model is used to investigate the effects of mountains, diabatic heating, and the nonlinear interactions of their responses in the June–August large-scale global circulation. Particular emphasis is placed on the Asian summer monsoon, a dominant feature of the global circulation at this time. The prescribed heating is calculated as a residual in the time-mean thermodynamic equation from ECMWF data, and the model employs a linear surface drag that is enhanced over the land. The integration is initiated with an observed June–August zonal-mean flow that is maintained throughout the integration. A smoothed earth topography is raised from days 0 to 5 with hydrostatic adjustments made to temperature and surface pressure. When the integration is continued in the absence of diabatic heating, an almost steady-state flow pattern exists until about day 20 when instability begins to dominate. ... Abstract A time-dependent primitive equation model with specified zonal flow, mountains, and diabatic heating is described. The model is used to investigate the effects of mountains, diabatic heating, and the nonlinear interactions of their responses in the June–August large-scale global circulation. Particular emphasis is placed on the Asian summer monsoon, a dominant feature of the global circulation at this time. The prescribed heating is calculated as a residual in the time-mean thermodynamic equation from ECMWF data, and the model employs a linear surface drag that is enhanced over the land. The integration is initiated with an observed June–August zonal-mean flow that is maintained throughout the integration. A smoothed earth topography is raised from days 0 to 5 with hydrostatic adjustments made to temperature and surface pressure. When the integration is continued in the absence of diabatic heating, an almost steady-state flow pattern exists until about day 20 when instability begins to dominate. ...