A. J. Simmons

Barotropic Wave Propagation and Instability, and Atmospheric Teleconnection Patterns

Abstract A global barotropic model, linearized about the 300 mb climatological mean January flow, is perturbed by applying a series of localized forcings distributed throughout the tropics and subtropics. Structures which resemble the observed “Pacific/North American” and “East Atlantic” teleconnection patterns noted by Wallace and Gutzler (1981) tend to recur in the responses. Similar patterns are found to result from the dispersion of isolated initial perturbations placed at a variety of locations in the tropics and midlatitudes. It is shown that these structures are related to the most rapidly growing mode associated with barotropic instability of the zonally-varying climatological basic state. In the absence of damping, this mode has an e-folding time of about a week and a period close to 50 days. In localized regions the instantaneous growth rates can be competitive with those of baroclinic instability. These episodes of rapid local barotropic growth are interspersed with intervals in which the local...

Some aspects of the improvement in skill of numerical weather prediction

Recent verification statistics show a considerable improvement in the accuracy of forecasts from three global numerical weather prediction systems. The improvement amounts to about a 1-day gain in predictability of mean-sea-level pressure and 500 hPa height over the last decade in the northern hemisphere, with a similar gain over the last 3 years in the southern hemisphere. Differences between the initial analyses from the three systems have been substantially reduced. Detailed study of the European Centre for Medium-Range Weather Forecasts verifications shows that identifiable improvements in the data assimilation, model and observing systems have significantly increased the accuracy of both short- and medium-range forecasts, although interannual (flow-dependent) variations in error-growth characteristics complicate the picture. The implied r.m.s. error of 500 hPa height analyses has fallen well below the 10 m level typical of radiosonde measurement error. Intrinsic error-doubling times, computed from the divergence of northern hemisphere forecasts started 1 day apart, exhibit a small overall reduction over the past 10 years at day two and beyond, and a much larger reduction at day one. Error-doubling times for the southern hemisphere have become generally shorter and are now similar to those for the northern hemisphere. One-day forecast errors have been reduced so much in the southern hemisphere that medium-range forecasts for the region have become almost as skilful as those for the northern hemisphere. The approach to saturation of forecast error beyond the 10-day range has been examined for sets of 21-day forecasts. When the systematic (sample-mean) component of the error is subtracted, forecast errors and the differences between successive forecasts both appear to level out near the end of the 21-day range at values close to the limit set by the natural level of variance of the atmosphere for the northern hemisphere. A number of features of the model 500 hPa height fields remain quite realistic at the three-week range. The most obvious discrepancy in mean climate is in the Pacific/North-American sector, and variance is too high in the southern hemisphere. Copyright

The Life Cycles of Some Nonlinear Baroclinic Waves

Abstract Some aspects of the nonlinear behavior of mid-latitude baroclinic waves are investigated by means of a series of integrations of the primitive equations with spherical geometry. Each integration has as initial conditions a balanced zonal flow perturbed by a small-amplitude disturbance of normal-mode form. Results are presented in detail for several zonal flows and perturbations which are confined initially to either zonal wavenumber 6 or zonal wavenumber 9. In each case a disturbance grows by baroclinic instability and develops a structure in some agreement with the usual synoptic picture of an occluding system. Its growth rate at low levels decreases more rapidly than that at higher levels, as found by Gall using a more severely truncated model, and upper-level amplitudes become larger relative to surface values than in the initial linear mode. This is more marked for wavenumber 6 than for wavenumber 9, and differences in linear structure are thus enhanced in the nonlinear regime. Barotropic pro...

Aerosol analysis and forecast in the European Centre for Medium‐Range Weather Forecasts Integrated Forecast System: 2. Data assimilation

[1] This study presents the new aerosol assimilation system, developed at the European Centre for Medium-Range Weather Forecasts, for the Global and regional Earth-system Monitoring using Satellite and in-situ data (GEMS) project. The aerosol modeling and analysis system is fully integrated in the operational four-dimensional assimilation apparatus. Its purpose is to produce aerosol forecasts and reanalyses of aerosol fields using optical depth data from satellite sensors. This paper is the second of a series which describes the GEMS aerosol effort. It focuses on the theoretical architecture and practical implementation of the aerosol assimilation system. It also provides a discussion of the background errors and observations errors for the aerosol fields, and presents a subset of results from the 2-year reanalysis which has been run for 2003 and 2004 using data from the Moderate Resolution Imaging Spectroradiometer on the Aqua and Terra satellites. Independent data sets are used to show that despite some compromises that have been made for feasibility reasons in regards to the choice of control variable and error characteristics, the analysis is very skillful in drawing to the observations and in improving the forecasts of aerosol optical depth.

ERA-20C: An Atmospheric Reanalysis of the Twentieth Century

AbstractThe ECMWF twentieth century reanalysis (ERA-20C; 1900–2010) assimilates surface pressure and marine wind observations. The reanalysis is single-member, and the background errors are spatiotemporally varying, derived from an ensemble. The atmospheric general circulation model uses the same configuration as the control member of the ERA-20CM ensemble, forced by observationally based analyses of sea surface temperature, sea ice cover, atmospheric composition changes, and solar forcing. The resulting climate trend estimations resemble ERA-20CM for temperature and the water cycle. The ERA-20C water cycle features stable precipitation minus evaporation global averages and no spurious jumps or trends. The assimilation of observations adds realism on synoptic time scales as compared to ERA-20CM in regions that are sufficiently well observed. Comparing to nighttime ship observations, ERA-20C air temperatures are 1 K colder. Generally, the synoptic quality of the product and the agreement in terms of climat...

The ECMWF medium-range prediction models development of the numerical formulations and the impact of increased resolution

SummaryAn outline is given of the numerical formulations that have been utilized for medium-range prediction at the European Centre for Medium-Range Weather Forecasts. The sigma-coordinate grid-point model adopted for the first phase of operational forecasting, and the hybrid-coordinate spectral model which replaced it, are described. Particular emphasis is placed on aspects of the time-stepping employed in the spectral model. It is shown how a semi-implicit treatment of the zonal advection of vorticity and specific humidity, and selective use of enhanced horizontal diffusion, enables time-steps to be used which are well over 50% longer than would otherwise be possible in a spectral model with a classical semi-implicit treatment of gravity-wave terms, with negligible computational cost and negligible impact on forecast accuracy, at T 106 resolution at least. Some discussion of the stability of the semi-implicit gravity-wave scheme is also included.The experimental evidence which led to the operational change from the grid-point to the spectral model is summarized, and the sensitivity of forecasts to the horizontal resolution of the spectral model is illustrated. Both the change in method and increases in resolution result in systematic improvements in the details of cyclone development, and they occasionally have major beneficial impact on the evolution of the forecast in the medium range. Conventional objective verification indicates a much smaller mean improvement of T 106 resolution over T 63 than of T 63 over T 42, although the advantage of T 106 is nevertheless clear-cut. This advantage is more than confirmed by synoptic assessment. Scope for further improvement is indicated by first results from global forecasts with a resolution of T 159. Indications of sensitivity to the magnitude of horizontal diffusion and the resolution of the model orography are also given.The spectral model was initially used operationally with a 16-level vertical resolution, and the uppermost level at which prognostic variables were held was 25 mb. The results which justified a subsequent change to 19 levels, with increased stratospheric resolution and a 10 mb top level, are presented. In this case the principal medium-range forecast improvement came about because better initial analyses could be produced when the 19-level model was used in data assimilation.

Aerosol analysis and forecast in the European Centre for Medium-Range Weather Forecasts Integrated Forecast System: Forward modeling

[1] This paper presents the aerosol modeling now part of the ECMWF Integrated Forecasting System (IFS). It includes new prognostic variables for the mass of sea salt, dust, organic matter and black carbon, and sulphate aerosols, interactive with both the dynamics and the physics of the model. It details the various parameterizations used in the IFS to account for the presence of tropospheric aerosols. Details are given of the various formulations and data sets for the sources of the different aerosols and of the parameterizations describing their sinks. Comparisons of monthly mean and daily aerosol quantities like optical depths against satellite and surface observations are presented. The capability of the forecast model to simulate aerosol events is illustrated through comparisons of dust plume events. The ECMWF IFS provides a good description of the horizontal distribution and temporal variability of the main aerosol types. The forecastonly model described here generally gives the total aerosol optical depth within 0.12 of the relevant observations and can therefore provide the background trajectory information for the aerosol assimilation system described in part 2 of this paper.

Toward a Monitoring and Forecasting System For Atmospheric Composition: The GEMS Project

The Global and Regional Earth System Monitoring Using Satellite and In Situ Data (GEMS) project is combining the manifold expertise in atmospheric composition research and numerical weather prediction of 32 European institutes to build a comprehensive monitoring and forecasting system for greenhouse gases, reactive gases, aerosol, and regional air quality. The project is funded by the European Commission as part of the Global Monitoring of Environment and Security (GMES) framework. GEMS has extended the data assimilation system of the European Centre for Medium-Range Weather Forecasts (ECMWF) to include various tracers for which satellite observations exist. A chemical transport model has been coupled to this system to account for the atmospheric chemistry. The GEMS system provides lateral boundary conditions for a set of 10 regional air quality forecast models and global atmospheric fields for use in surface flux inversions for the greenhouse gases. Observations from both in situ and satellite sources are used as input, and the output products will serve users such as policy makers, environmental agencies, the science community, and providers of end-user services for air quality and health. This article provides an overview of GEMS and uses some recent results to illustrate the current status of the project. It is expected that GEMS will grow into a full operational service for the atmospheric component of GMES in the next decade. Part of this transition will be the merge with the Protocol Monitoring for the GMES Service Element: Atmosphere (PROMOTE) GMES project into the Monitoring of Atmospheric Composition and Climate (MACC) project.

The Concept of Essential Climate Variables in Support of Climate Research, Applications, and Policy

Climate research, monitoring, prediction, and related services rely on accurate observations of the atmosphere, land, and ocean, adequately sampled globally and over sufficiently long time periods. The Global Climate Observing System, set up under the auspices of United Nations organizations and the International Council for Science to help ensure the availability of systematic observations of climate, developed the concept of essential climate variables (ECVs). ECV data records are intended to provide reliable, traceable, observation-based evidence for a range of applications, including monitoring, mitigating, adapting to, and attributing climate changes, as well as the empirical basis required to understand past, current, and possible future climate variability. The ECV concept has been broadly adopted worldwide as the guiding basis for observing climate, including by the United Nations Framework Convention on Climate Change (UNFCCC), WMO, and space agencies operating Earth observation satellites. This ...

Implementation of the Semi-Lagrangian Method in a High-Resolution Version of the ECMWF Forecast Model

Abstract In this article the implementation of the semi-Lagrangian method in a high-resolution version of the ECMWF forecast model is examined. Novel aspects include the application of the semi-Lagrangian scheme to a global model using the ECMWF hybrid coordinate in the vertical and its use in a baroclinic spectral model in conjunction with a reduced Gaussian grid in the horizontal. The former Eulerian vorticity-divergence formulation is first converted into a momentum-equation formulation that is considerably more economical, thanks in part to the incorporation of Legendre transform efficiencies that were previously demonstrated for the shallow-water equations. The semi-Lagrangian formulation is presented in detail, together with a discussion of computational aspects that are relevant for executing the high-resolution model efficiently on a modestly parallel supercomputer. The impact of formulation changes is assessed via numerical experiments on a set of 12 independent cases. In particular it is shown t...