ECMWF Model Research Articles

Mesoscale modelling of water vapour in the tropical UTLS: two case studies from the HIBISCUS campaign

Abstract. In this study, we evaluate the ability of the BRAMS (Brazilian Regional Atmospheric Modeling System) mesoscale model compared to ECMWF global analysis to simulate the observed vertical variations of water vapour in the tropical upper troposphere and lower stratosphere (UTLS). The observations are balloon-borne measurements of water vapour mixing ratio and temperature from micro-SDLA (Tunable Diode Laser Spectrometer) instrument. Data from two balloon flights performed during the 2004 HIBISCUS field campaign are used to compare with the mesoscale simulations and to the ECMWF analysis. The observations exhibit fine scale vertical structures of water vapour of a few hundred meters height. The ECMWF vertical resolution (~1 km) is too coarse to capture these vertical structures in the UTLS. With a vertical resolution similar to ECMWF, the mesoscale model performs better than ECMWF analysis for water vapour in the upper troposphere and similarly or slightly worse for temperature. The BRAMS model with 250 m vertical resolution is able to capture more of the observed fine scale vertical variations of water vapour compared to runs with a coarser vertical resolution. This is mainly related to: (i) the enhanced vertical resolution in the UTLS and (ii) to the more detailed microphysical parameterization providing ice supersaturations as in the observations. In near saturated or supersaturated layers, the mesoscale model predicted relative humidity with respect to ice saturation is close to observations provided that the temperature profile is realistic. For temperature, the ECMWF analysis gives good results partly attributed to data assimilation. The analysis of the mesoscale model results showed that the vertical variations of the water vapour profile depends on the dynamics in unsaturated layer while the microphysical processes play a major role in saturated/supersaturated layers. In the lower stratosphere, the ECMWF model and the BRAMS model give very similar water vapour profiles that are significantly drier than micro-SDLA measurements. This similarity comes from the fact that BRAMS is initialised using ECMWF analysis and that no mesoscale process acts in the stratosphere leading to no modification of the BRAMS results with respect to ECMWF analysis.

Profiling the Lower Troposphere over the Ocean with Infrared Hyperspectral Measurements of the Marine-Atmosphere Emitted Radiance Interferometer

Abstract Measurements of the spectra of infrared emission from the atmosphere were taken by a Marine-Atmospheric Emitted Radiance Interferometer (M-AERI) deployed on the NOAA ship Ronald H. Brown during the Aerosol and Ocean Science Expedition (AEROSE) in the tropical Atlantic Ocean from 29 February to 26 March 2004. The spectra are used to retrieve profiles of temperature and humidity in the lower troposphere up to a height of 3000 m. The M-AERI retrievals of the atmospheric structure require an initial guess profile. In this work, retrievals obtained from four separate initializations are compared, using 1) radiosondes launched from the Ronald H. Brown, 2) NOAA/NWS/NCEP model reanalyses, 3) ECMWF model analyses, and 4) ECMWF model forecasts. The performance of the M-AERI retrievals for all four first-guess sources is then evaluated against the radiosonde measurements. The M-AERI retrievals initialized using radiosondes reproduce the radiosonde profiles quite well and capture much of the observed vertical structure as should be expected. Of the retrievals initialized with model fields, those obtained using the ECMWF data yielded results closest to the radiosonde observations and enabled detection of the Saharan air layer (SAL) evident during AEROSE. However, the NCEP reanalysis, as well as the corresponding retrievals, failed to detect the SAL. These results demonstrate the ability of the M-AERI profile retrievals to identify the anomalous humidity distributions in the lower troposphere, but underscore the need for suitable vertical resolution in the first-guess profile used in the retrievals under such conditions.

Evaluation of ECMWF cloud type simulations at the ARM Southern Great Plains site using a new cloud type climatology

A new method to derive a cloud type climatology is applied to cloud observations over the Southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) site and to ECMWF model forecasts, in order to compare model and radar derived cloud type statistics and identify the major deficiencies in model cloud vertical distribution. The results indicate that cirrus and to a lesser extent middle level clouds are the major cloud types missing in the model simulations and that they are missing mostly as parts of multi‐type rather than single‐type structures. Boundary layer clouds are simulated at approximately the right amounts in the annual mean statistics, but this result comes from the model simulating too little boundary layer cloud in the winter and too much in the summer. Overall, the model forecasts miss about 11% of the total cloud amount, and most of the missing cloud occurs at time periods when multiple cloud types are present in the observations.

Estimation of the Impact of Sampling Errors in the VOS Observations on Air–Sea Fluxes. Part II: Impact on Trends and Interannual Variability

Abstract Using the same approach as in Part I, here it is shown how sampling problems in voluntary observing ship (VOS) data affect conclusions about interannual variations and secular changes of surface heat fluxes. The largest uncertainties in linear trend estimates are found in relatively poorly sampled regions like the high-latitude North Atlantic and North Pacific as well as the Southern Ocean, where trends can locally show opposite signs when computed from the regularly sampled and undersampled data. Spatial patterns of shorter-period interannual variability, quantified through the EOF analysis, also show remarkable differences between the regularly sampled and undersampled flux datasets in the Labrador Sea and northwest Pacific. In particular, it is shown that in the Labrador Sea region, in contrast to regularly sampled NCEP–NCAR reanalysis fluxes, VOS-like sampled NCEP–NCAR reanalysis fluxes neither show significant interannual variability nor significant trends. These regions, although quite localized covering small parts of the globe, play a crucial role for the coupled atmosphere–ocean system. In the Labrador Sea, for instance, interannual and decadal-scale changes of the surface net heat fluxes are known to affect oceanic convection and, thus, the meridional overturning circulation of the Atlantic Ocean. From a discussion of current atmospheric data assimilation systems it is argued that in poorly sampled regions reanalysis products are superior to VOS-based products for studying interannual and interdecadal variations of atmosphere–ocean interaction. In well-sampled regions, on the other hand, conclusions about surface heat flux variations are relatively insensitive to the choice of the flux products used (VOS versus reanalysis data). The results are confirmed for two different datasets, that is, ECMWF 40-yr Re-Analysis (ERA-40) data and seasonal integrations with a recent version of the ECMWF model in which no actual data were assimilated.

Numerical simulation of severe convective phenomena over Croatian and Hungarian territory

Squall lines and supercells cause severe weather and huge damages in the territory of Croatia and Hungary. These long living events can be recognised by radar very well, but the problem of early warning, especially successful numerical forecast of these phenomena, has not yet been solved in this region. Two case studies are presented here in which dynamical modelling approach gives promising results: a squall line preceding a cold front and a single supercell generated because of a prefrontal instability. The numerical simulation is performed using the PSU/NCAR meso-scale model MM5, with horizontal resolution of 3 km. Lateral boundary conditions are taken from the ECMWF model. The moist processes are resolved by Reisner mixed-phase explicit moisture scheme and for the radiation scheme a rapid radiative transfer model is applied. The analysis nudging technique is applied for the first two hours of the model run. The results of the simulation are very promising. The MM5 model reconstructed the appearance of the convective phenomena and showed the development of thunderstorm into the supercell phase. The model results give very detailed insight into wind changes showing the rotation of supercells, clearly distinguish warm core of the cell and give rather good precipitation estimate. The successful simulation of convective phenomena by a high-resolution MM5 model showed that even smaller scale conditions are contained in synoptic scale patterns, represented in this case by the ECMWF model.

Improving Week-2 Forecasts with Multimodel Reforecast Ensembles

Abstract It has recently been demonstrated that model output statistics (MOS) computed from a long retrospective dataset of ensemble “reforecasts” from a single model can significantly improve the skill of probabilistic week-2 forecasts (with the same model). In this study the technique is extended to a multimodel reforecast dataset consisting of forecasts from ECMWF and NCEP global models. Even though the ECMWF model is more advanced than the version of the NCEP model used (it has more than double the horizontal resolution and is about five years newer), the forecasts produced by the multimodel MOS technique are more skillful than those produced by the MOS technique applied to either the NCEP or ECMWF forecasts alone. These results demonstrate that the MOS reforecast approach yields benefits for week-2 forecasts that are just as large for high-resolution state-of-the-art models as they are for relatively low resolution out-of-date models. Furthermore, operational forecast centers can benefit by sharing both retrospective reforecast datasets and real-time forecasts.

On the Use of QuikSCAT Scatterometer Measurements of Surface Winds for Marine Weather Prediction

Abstract The value of Quick Scatterometer (QuikSCAT) measurements of 10-m ocean vector winds for marine weather prediction is investigated from two Northern Hemisphere case studies. The first of these focuses on an intense cyclone with hurricane-force winds that occurred over the extratropical western North Pacific on 10 January 2005. The second is a 17 February 2005 example that is typical of sea surface temperature influence on low-level winds in moderate wind conditions in the vicinity of the Gulf Stream in the western North Atlantic. In both cases, the analyses of 10-m winds from the NCEP and ECMWF global numerical weather prediction models considerably underestimated the spatial variability of the wind field on scales smaller than 1000 km compared with the structure determined from QuikSCAT observations. The NCEP and ECMWF models both assimilate QuikSCAT observations. While the accuracies of the 10-m wind analyses from these models measurably improved after implementation of the QuikSCAT data assimilation, the information content in the QuikSCAT data is underutilized by the numerical models. QuikSCAT data are available in near–real time in the NOAA/NCEP Advanced Weather Interactive Processing System (N-AWIPS) and are used extensively in manual analyses of surface winds. The high resolution of the QuikSCAT data is routinely utilized by forecasters at the NOAA/NCEP Ocean Prediction Center, Tropical Prediction Center, and other NOAA weather forecast offices to improve the accuracies of wind warnings in marine forecasts.

Sensitivity of the Tropospheric Circulation to Changes in the Strength of the Stratospheric Polar Vortex

Abstract The sensitivity of the wintertime tropospheric circulation to changes in the strength of the Northern Hemisphere stratospheric polar vortex is studied using one of the latest versions of the ECMWF model. Three sets of experiments were carried out: one control integration and two integrations in which the strength of the stratospheric polar vortex has been gradually reduced and increased, respectively, during the course of the integration. The strength of the polar vortex is changed by applying a forcing to the model tendencies in the stratosphere only. The forcing has been obtained using the adjoint technique. It is shown that, in the ECMWF model, changes in the strength of the polar vortex in the middle and lower stratosphere have a significant and slightly delayed (on the order of days) impact on the tropospheric circulation. The tropospheric response shows some resemblance to the North Atlantic Oscillation (NAO), though the centers of action are slightly shifted toward the east compared to those of the NAO. Furthermore, a separate comparison of the response to a weak and strong vortex forcing suggests that to first order the tropospheric response is linear within a range of realistic stratospheric perturbations. From the results presented, it is argued that extended-range forecasts in the European area particularly benefit from the stratosphere–troposphere link.

Validation of mesoscale low-level winds obtained by dynamical downscaling of ERA40 over complex terrain

The mesoscale numerical weather prediction model ALADIN has been applied for downscaling ERA40 data onto a 10 km grid covering the complex terrain of Slovenia. The modelled wind field is compared with the time-series of observations at 11 stations. In addition to traditional scores (root-mean-square error, mean absolute error, anomaly correlation), a frequency-domain comparison is carried out in order to explore aspects of the mesoscale model performance other than that depicted by the conventional statistics. The verification period is the Special Observing Period of the Mesoscale Alpine Program (MAP-SOP), for which ECMWF reanalyses including MAP-SOP observations are available every 3 hr on a ∼40 km grid. Traditional scores indicate that the downscaling has been successful. Scores are little dependent on the nesting strategy (direct versus two-step nesting), in spite of a ratio of horizontal resolutions between ERA40 and ALADIN as large as 12. The model performs best at mountaintop stations, characterized by over 80% of their spectral power in motions with longer than diurnal periods. A majority of stations is, however, located in the complex terrain where around 40% of the spectral wind power is contained in the subdiurnal frequency range. This part of the spectrum is significantly underestimated by the model, indicating that the downscaling is predominantly a dynamical adjustment to the new terrain. At the same time, the MAP-SOP reanalyses of the ECMWF model include relatively more power in the subdiurnal frequency range than ALADIN. However, these subdiurnal oscillations do not agree with observations and their removal improves conventional scores for the MAP-SOP wind data. It is suggested that a frequency-domain comparison is a useful complement to the conventional statistics and it enables a more physical insight into a mesoscale model performance.

An Assessment of the Sea Surface Temperature Influence on Surface Wind Stress in Numerical Weather Prediction and Climate Models

Abstract The ability of six climate models to capture the observed coupling between SST and surface wind stress in the vicinity of strong midlatitude SST fronts is analyzed. The analysis emphasizes air–sea interactions associated with ocean meanders in the eastward extensions of major western boundary current systems such as the Gulf Stream, Kuroshio, and Agulhas Current. Satellite observations of wind stress from the SeaWinds scatterometer on NASA’s Quick Scatterometer and SST from the Advanced Microwave Scanning Radiometer clearly indicate the influence of SST on surface wind stress on scales smaller than about 30° longitude × 10° latitude. Spatially high-pass-filtered SST and wind stress variations are linearly related, with higher SST associated with higher wind stress. The influence of SST on wind stress is also clearly identifiable in the ECMWF operational forecast model, having a grid resolution of 0.35° × 0.35° (T511). However, the coupling coefficient between wind stress and SST, as indicated by the slope of the linear least squares fit, is only half as strong as for satellite observations. The ability to simulate realistic air–sea interactions is present to varying degrees in the coupled climate models examined. The Model for Interdisciplinary Research on Climate 3.2 (MIROC3.2) high-resolution version (HIRES) (1.1° × 1.1°, T106) and the NCAR Community Climate System Model 3.0 (1.4° × 1.4°, T85) are the highest-resolution models considered and produce the most realistic air–sea coupling associated with midlatitude current systems. Coupling coefficients between SST and wind stress in MIROC3.2_HIRES and the NCAR model are at least comparable to those in the ECMWF operational model. The spatial scales of midlatitude SST variations and SST-induced wind perturbations in MIROC3.2_HIRES are comparable to those of satellite observations. The spatial scales of SST variability in the NCAR model are larger than those in the ECMWF model and satellite observations, and hence the spatial scales of SST-induced perturbations in the wind fields are larger. It is found that the ability of climate models to simulate air–sea interactions degrades with decreasing grid resolution. SST anomalies in the GFDL Climate Model 2.0 (CM2.0) (2.0° × 2.5°), Met Office Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) (2.5° × 3.8°), and MIROC3.2 medium-resolution version (MEDRES) (2.8° × 2.8°, T42) have larger spatial scales and are more geographically confined than in the higher-resolution models. The GISS Model E20/Russell (4.0° × 5.0°) is unable to resolve the midlatitude ocean eddies that produce prominent air–sea interaction. Notably, MIROC3.2_MEDRES exhibits much weaker coupling between wind stress and SST than does the higher vertical and horizontal resolution version of the same model. GFDL CM2.0 and Met Office HadCM3 exhibit a linear relationship between SST and wind stress. However, coupling coefficients for the Met Office model are significantly weaker than in the GFDL and higher-resolution models. In addition to model grid resolution (both vertical and horizontal), deficiencies in the parameterization of boundary layer processes may be responsible for some of these differences in air–sea coupling between models and observations.

Passive microwave radiometer channel selection basedoncloudandprecipitation information content

AbstractThe information content of microwave frequencies between 5 and 200 GHz for rain, snow and cloud water retrievals over ocean and land surfaces was evaluated using optimal estimation theory. The study was based on large datasets representative of summer and winter meteorological conditions over North America, Europe, Central Africa, South America and the Atlantic obtained from short‐range forecasts with the operational ECMWF model. The information content of rain, snow and cloud water was traded off against the uncertainties due to the natural variability of other variables that microwave observations are sensitive to. These are surface emissivity, land surface skin temperature, atmospheric temperature and moisture. The estimation of the underlying error statistics was based on ECMWF model forecast error statistics.The results suggest that a number of frequency bands are most suited for the retrieval of(i) rain over oceans: 15–18, 35–40, 80, 145, 118.75±10–14 GHz; rain over land: 85–100, 135–140 GHz,(ii) snow over land and oceans: 95–100, 140–150, 187 GHz,(iii) clouds over oceans: 40, 80–85 GHz; clouds over land: 90–100, 135–140 GHz.For radiometers designed for global and multi‐season applications, several channels in all of the above frequency ranges would be desirable for optimizing channel usage in hydrometeor retrievals depending on the observed situation. Copyright © 2006 Royal Meteorological Society.

GCM sensitivity experiments with locally modified land surface properties over tropical South America

Ensembles of 1-year-long experiments with a relatively high-resolution ECMWF model were conducted in order to investigate the impact of modified land surface properties on local, regional and large-scale atmospheric circulations. The modifications consisted of changes to land cover and increased albedo over the northern part of South America. In many respects the experimental design resembles the setting of classical deforestation experiments. The local model response to imposed modifications, which includes a reduction in precipitation as well as in evaporation and an increase in surface temperature, was found to be stronger in dry (July–September, JAS) than in wet (January–March, JFM) season, and in the ensemble with higher albedo value. Local drying is discussed in terms of locally generated overturning that resembles a direct thermal circulation. The effects of this circulation seem to be dominant over the reduction in large-scale moisture supply from the adjacent ocean. On large scales, changes to the Pacific branch of the Walker circulation lead, through modified divergent flow, to a tropics-wide impact on precipitation. In addition to South America, the largest changes are seen in the south Pacific convergence zone in JFM, while the impact on the Atlantic inter-tropical convergence zone is stronger in JAS. In the extratropics, there is little change in precipitation. In the upper troposphere, a distinctive teleconnection wave-pattern could be seen in the Pacific/North American region during JFM. A notable feature in the upper-air model response in JAS is a wave train extending from South America, over the northern Atlantic into Europe. With regard to the interaction between the land surface response and model systematic errors, our results suggest that the erroneous shift of the downward branch of the Pacific/South American Walker circulation is likely to be a cause, rather than a consequence, of the rainfall deficit over South America in the model climatology.

Validation of mesoscale low-level winds obtained by dynamical downscaling of ERA40 over complex terrain

The mesoscale numerical weather prediction modelALADINhas been applied for downscalingERA40data onto a 10 km grid covering the complex terrain of Slovenia. The modelled wind field is compared with the time-series of observations at 11 stations. In addition to traditional scores (root-mean-square error, mean absolute error, anomaly correlation), a frequency-domain comparison is carried out in order to explore aspects of the mesoscale model performance other than that depicted by the conventional statistics. The verification period is the Special Observing Period of the Mesoscale Alpine Program (MAP-SOP), for which ECMWF reanalyses including MAP-SOP observations are available every 3 hr on a ∼40 km grid.Traditional scores indicate that the downscaling has been successful. Scores are little dependent on the nesting strategy (direct versus two-step nesting), in spite of a ratio of horizontal resolutions between ERA40 and ALADIN as large as 12. The model performs best at mountaintop stations, characterized by over 80% of their spectral power in motions with longer than diurnal periods. A majority of stations is, however, located in the complex terrain where around 40% of the spectral wind power is contained in the subdiurnal frequency range. This part of the spectrum is significantly underestimated by the model, indicating that the downscaling is predominantly a dynamical adjustment to the new terrain. At the same time, the MAP-SOP reanalyses of the ECMWF model include relatively more power in the subdiurnal frequency range than ALADIN. However, these subdiurnal oscillations do not agree with observations and their removal improves conventional scores for the MAP-SOP wind data.It is suggested that a frequency-domain comparison is a useful complement to the conventional statistics and it enables a more physical insight into a mesoscale model performance.

Influence of a stochastic parameterization on the frequency of occurrence of North Pacific weather regimes in the ECMWF model

One common problem of many atmospheric circulation models is the overestimation of the mean westerly winds in the mid‐latitude North Pacific. This westerly wind bias is also a prominent feature in a recent version of the ECMWF model. Here we use the ECMWF model to investigate whether the use of stochastic parameterizations help reduce this error, using the concept of weather regimes. The focus is on the winter season when the atmospheric regime structure is most pronounced. It is shown that the operational version of the ECMWF stochastic physics scheme has little impact on the frequency of occurrence of North Pacific weather regimes. A recently developed scheme, however, which is based on combining a cellular automaton with a stochastic backscatter component, leads to substantial improvements in the simulation of the frequency of occurrence of North Pacific weather regimes and therefore a reduction of the westerly wind bias.

A prognostic scheme of sea surface skin temperature for modeling and data assimilation

A prognostic scheme is derived for the computation of sea surface skin temperature in weather forecasting, four‐dimensional data assimilation, and ocean‐atmosphere coupled modeling. This scheme is then tested using the in situ data over tropical and midlatitude oceans. By implementing this scheme into the ECMWF model, the diurnal variation of sea surface temperature as measured by the geostationary satellite can also be reproduced.

Investigation of the sensitivity of the ECMWF radiation scheme to input parameters using the adjoint technique

AbstractThe potential of the adjoint technique for studying the sensitivity of a physical parametrization scheme to its input parameters is investigated. Compared with the standard evaluation approach, where the sensitivity of all the outputs is obtained by modifying one given input, the adjoint method offers a complementary and very efficient approach for sensitivity studies. The paper focuses on an estimation of the sensitivity of the ECMWF radiation schemes to changes in temperature, humidity, aerosols and cloud properties. The sensitivity of the topof‐atmosphere radiation fluxes in clear‐ and cloudy‐sky conditions, computed globally by the ECMWF model for the different seasons, is discussed in this study. This allows us to investigate spatial and temporal patterns in the sensitivity variations. The evaluation of the Jacobian of radiation fluxes by the adjoint technique provides an insight on the extent to which the various radiation fluxes are sensitive to the meteorological variables and confirms results usually obtained by more computationally intensive traditional methods. In addition, such a sensitivity study gives some indications on the importance and efficiency of particular types of observations in modifying the initial conditions of a model. Copyright © 2005 Royal Meteorological Society.

Verification of Precipitation Forecasts from Two Limited-Area Models over Italy and Comparison with ECMWF Forecasts Using a Resampling Technique

Abstract This paper presents the first systematic limited area model (LAM) precipitation verification work over Italy. A resampling technique was used to provide skill score results along with confidence intervals. Two years of data were used, starting in October 2000. Two operational LAMs have been considered, the Limited Area Model Bologna (LAMBO) operating at the Agenzia Regionale Prevenzione e Ambiente-Servizio Meteorologico Regionale (ARPA-SMR) of the Emilia–Romagna region, and the QUADRICS Bologna Limited Area Model (QBOLAM) running at the Agenzia per la Protezione dell’Ambiente e per i Servizi Tecnici (APAT). A 24-h forecast skill score comparison was first performed on the native 0.1° high-resolution grids, using a Barnes scheme to produce the observed 24-h accumulated rainfall analysis. Two nonparametric skill scores were used: the equitable threat score (ETS) and the Hanssen and Kuipers score (HK). Frequency biases (BIA) were also calculated. LAM forecasts were also remapped on a lower-resolution grid (0.5°), using a nearest-neighbor average method; this remapping allowed for comparison with ECMWF model forecasts, and for LAM intercomparisons at lower resolution, with the advantage of reducing the skill score sensitivity to small displacements errors. LAM skill scores depend on the resolution of the verification grid, with an increase when they are verified on a lower-resolution grid. The selected LAMs have a higher BIA compared to ECMWF, showing a tendency to overforecast precipitation, especially along mountain ranges, possibly due to undesired effects from the large-scale and/or convective precipitation parameterizations. Lower ECMWF BIA accounts for skill score differences. LAMBO precipitation forecasts during winter (adjusted for BIA differences) have less misses than ECMWF over the islands of Sardinia and Sicily. Higher-resolution orography definitely adds value to LAM forecasts.

Some aspects of systematic error in the ECMWF model

AbstractFirst, systematic errors of short‐range and medium‐range Z500 forecasts are described along with their changes since the early 1980s. Then systematic cloud error will be described. Finally, the capability of the ECMWF model to simulate the Madden‐and‐Julian Oscillation is assessed. Copyright © 2005 Royal Meteorological Society

Systematic errors of the atmospheric circulation in the ECMWF forecasting system

The aim of this study is to document some of the key systematic errors of the atmospheric circulation as simulated by the ECMWF model during the winter season (December–March). Two major topics are addressed. First, key systematic circulation errors are described and it is shown how they grow from the short range all the way to the extended range. Second, it is shown how systematic circulation errors changed during the last two decades. Results are presented for 500 hPa geopotential height fields, eddy kinetic energy, synoptic variability, and atmospheric blocking. A concise summary is somewhat hampered by the fact that systematic error characteristics significantly depend on the parameter, region, and the forecast range being considered. In general, though, it has been found that systematic circulation errors have been reduced considerably in recent years, particularly from the 1980s to the early 1990s. Model improvements are primarily manifest through a reduction of the rate at which systematic errors grow; their spatial structure, however, remains more or less unchanged for most of the parameters. On the other hand, and somewhat surprisingly, it has been found that for some parameters and regions systematic circulation errors have slightly deteriorated since the mid-1990s. Moreover, it turns out that during the first few days of the integration the spatial structure of systematic circulation errors undergoes considerable changes; in the medium range and beyond their spatial structure remains virtually unchanged. The most prominent systematic circulation errors identified in this study are: the development of a large anticyclonic bias in the central North Pacific in the medium-range and extended-range, the underestimation of kinetic energy of the transient eddies, a pronounced underestimation of synoptic activity in high latitudes, and the underestimation of atmospheric blocking in the extended range. In the medium range over Europe, however, it is shown that recent cycles of the ECMWF model are able to produce realistic blocking frequencies. Copyright © 2005 Royal Meteorological Society

A fast radiative-transfer model for the assimilation of infrared limb radiances from MIPAS

A new fast radiative-transfer model to compute emitted infrared limb radiances for the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) has been developed and validated. The model, referred to as RTMIPAS, can simulate apodized radiances for all channels of the high spectral resolution MIPAS instrument in the 685–2000 cm−1 wave-number region. RTMIPAS is part of a wider effort to develop the capability to assimilate infrared limb radiances into the ECMWF model. The model uses a linear regression scheme to parametrize the effective layer optical depths, and it can simulate the effect of variable water vapour and ozone; for other gases included in the model, a fixed climatological profile is assumed. The development of the model has involved the calculation of an accurate line-by-line transmittance database, the selection of suitable predictors for the gases and the viewing geometry modelled, and the generation of regression coefficients for 43205 MIPAS channels. RTMIPAS can reproduce line-by-line radiances to an accuracy that is below the noise level of the instrument for most spectral points and tangent heights, while offering significantly more rapid radiance calculations compared with currently available radiative-transfer models. The comparison of RTMIPAS transmittances with line-by-line model equivalents indicates that the accuracy of the RTMIPAS transmittance model is comparable to that of similar regression-based radiative-transfer models for nadir-viewing geometry. Copyright © 2005 Royal Meteorological Society