ECMWF Operational Data Research Articles

Accounting for residual errors in atmosphere–ocean background models applied in satellite gravimetry

The Atmosphere and Ocean non-tidal De-aliasing Level-1B (AOD1B) product is widely used in precise orbit determination and satellite gravimetry to correct for transient effects of atmosphere–ocean mass variability that would otherwise alias into monthly mean global gravity fields. The most recent release is based on the global ERA5 reanalysis and ECMWF operational data together with simulations from the general ocean circulation model MPIOM consistently forced with fields from the corresponding atmospheric dataset. As background models are inevitably imperfect, residual errors will consequently propagate into the resulting geodetic products. Accounting for uncertainties of the background model data in a statistical sense, however, has been shown before to be a useful approach to mitigate the impact of residual errors leading to temporal aliasing artefacts. In light of the changes made in the new release RL07 of AOD1B, previous uncertainty assessments are deemed too pessimistic and thus need to be revisited. We here present an analysis of the residual errors in AOD1B RL07 based on ensemble statistics derived from different atmospheric reanalyses, including ERA5, MERRA2 and JRA55. For the oceans, we investigate the impact of both the forced and intrinsic variability through differences in MPIOM simulation experiments. The atmospheric and oceanic information is then combined to produce a new time-series of true errors, called AOe07, which is applicable in combination with AOD1B RL07. AOe07 is further complemented by a new spatial error variance–covariance matrix. Results from gravity field recovery simulation experiments for the planned Mass-Change and Geosciences International Constellation (MAGIC) based on GFZ’s EPOS software demonstrate improvements that can be expected from rigorously implementing the newly available stochastic information from AOD1B RL07 into the gravity field estimation process.

Non‐Tidal Background Modeling for Satellite Gravimetry Based on Operational ECWMF and ERA5 Reanalysis Data: AOD1B RL07

AbstractThe Atmosphere and Ocean De‐Aliasing Level‐1B (AOD1B) product provides a priori information about temporal variations in the Earth's gravity field induced by non‐tidal circulation processes in atmosphere and ocean. It is routinely applied as a background model in the Gravity Recovery and Climate Experiment (GRACE)/GRACE Follow‐On (GRACE‐FO) satellite gravimetry data processing. We here present three new datasets in preparation for the upcoming release RL07 of AOD1B, that are based on either the global ERA5 reanalysis or the ECMWF operational data together with simulations from the Max‐Planck‐Institute for Meteorology general circulation model forced consistently with the fields of the same atmospheric data set. The oceanic simulations newly include an updated bathymetry around Antarctica including cavities under the ice shelves, the explicit implementation of the feedback effects of self‐attraction and loading to ocean dynamics as well as a refined harmonic tidal analysis. Comparison to the current release of AOD1B in terms of GRACE‐FO K‐band range‐acceleration pre‐fit residuals, LRI line‐of‐sight gravity differences and band‐pass filtered altimetry data reveals an overall improvement in the representation of the high‐frequency mass variability. Potential benefits of enhancing the temporal resolution remain inconclusive so that the upcoming release 07 will be sampled again every 3 hr.

Exploring the structure of time‐correlated model errors in the ECMWF data assimilation system

AbstractModel errors are increasingly seen as a fundamental performance limiter in both Numerical Weather Prediction and Climate Prediction simulations run with state‐of‐the‐art Earth‐system digital twins. This has motivated recent efforts aimed at estimating and correcting the systematic, predictable components of model error in a consistent data assimilation framework. While encouraging results have been obtained with a careful examination of the spatial aspects of the model error estimates, less attention has been devoted to the time correlation aspects of model errors and their impact on the assimilation cycle. In this work we employ a Lagged Analysis Increment Covariance (LAIC) diagnostic to gain insight in the temporal evolution of systematic model errors in the ECMWF operational data assimilation system, evaluate the effectiveness of the current weak constraint 4D‐Var algorithm in reducing these types of errors and, based on these findings, start exploring new ideas for the development of model error estimation and correction strategies in data assimilation.

Anthropogenic effects on the distribution of minor chemical constituents in the mesosphere/lower thermosphere – A model study

We investigate the influence that rising concentrations of methane, nitrous oxide and carbon dioxide have had upon the chemistry of the mesosphere since 1961. Calculations were performed using our global 3D-model LIMA (Leibniz-Institute Middle Atmosphere), designed for the investigation of the MLT-region (Mesosphere-Lower Thermosphere) and particularly for the extended mesopause region. LIMA utilizes observed tropospheric and lower stratospheric temperature and horizontal wind data up to 35km altitude by assimilating ECMWF/ERA-40 and ECMWF operational data. Real Lyman-α flux values are employed to determine the variable water vapor dissociation rate. Three different calculations were carried out and analyzed: (1) use of the same annual variation of the model dynamics in the chemical transport model (CTM) for all years according to the dynamics of the solar minimum year 1964 and employment of a realistic growth of the anthropogenic gases; (2) use of constant concentrations of the anthropogenic constituents at the lower border, but employment of the varying model dynamics; (3) the so-called realistic case, which considers both the long-term increase in the anthropogenic minor constituents and the varying dynamics according to LIMA calculations. The analysis of these three cases shows that the dynamics are able to counteract the impact of anthropogenic growth of minor constituents in the upper mesosphere-mesopause-lower thermosphere region in middle to high latitudes in summer. The water vapor mixing ratio increases due to rising methane concentration. The reason for this lies in a positive feedback process of autocatalytic water vapor production. The change in concentration of the minor constituents impacts both the cooling rate and the chemical heating rate. We present the relative and absolute deviations between the solar activity minimum years 1964 and 2008 for the most important minor constituents. We discuss the long-term behavior, particularly of water vapor, with regard to the impact on the NLC-region.

Longitudinally Dependent Ozone Increase in the Antarctic Polar Vortex Revealed by Balloon and Satellite Observations

Abstract The horizontal structure of processes causing increases in ozone in the Antarctic polar vortex was examined using data measured in 2003 from an ozonesonde observation campaign at Syowa Station (39.6°E, 69.0°S) and from the Improved Limb Atmospheric Spectrometer II (ILAS-II) onboard the Advanced Earth Observing Satellite II. The ILAS-II data are daily and distributed uniformly at 14 points in the zonal direction, mostly at polar latitudes. The Antarctic ozone hole that developed in 2003 was one of the largest recorded. The period of focus in this study is 26 September through 24 October, when a strong polar vortex was situated in the stratosphere. An ozone mixing ratio contour (1.0 ppmv) moved downward near a height of 20 km during the period of focus. This increase in ozone is likely to result from downward transport of ozone-rich air originating from lower latitudes by Brewer–Dobson circulation. First, the descent rate of the mixing ratio contour was estimated by taking the geometric height as the vertical coordinate for the deep vortex interior around 20 km. A significant longitudinal dependence was observed. An analysis using ECMWF operational data shows that this dependence can be approximately explained by longitudinally dependent vertical movements of the isentropes caused by a zonal wavenumber-1 quasi-stationary planetary wave with amplitude and phases varying on a seasonal time scale. Next, the descent rate was calculated around 500 K (around 20 km) by taking the potential temperature (isentrope) as the vertical coordinate. The longitudinal dependence was still present using this coordinate, meaning that the ozone mixing ratio and its increase are not constant on the isentropic layer even in the interior of the polar vortex. A backward trajectory analysis showed that air parcels with large ozone mixing ratios were mostly transported from the polar vortex boundary region. This result suggests that lateral transport/mixing is important even before the breakup of the polar vortex. Results from a tracer–tracer correlation analysis of O3 and long-lived constituent N2O were also consistent with this inference. The contribution of lateral mixing to the increase in ozone was estimated at about 17% ± 4% that of the Brewer–Dobson circulation around 20 km, using the calculated descent rates. The results of this study also imply that Lagrangian downward motions in the vortex interior are not correctly estimated without accounting for lateral mixing, even if the polar vortex is dynamically stable.

Life cycle of the QBO‐modulated 11‐year solar cycle signals in the Northern Hemispheric winter

AbstractThis paper provides some insights on the quasi‐biennial oscillation (QBO) modulated 11‐year solar cycle (11‐yr SC) signals in Northern Hemisphere (NH) winter temperature and zonal wind. Daily ERA‐40 Reanalysis and ECMWF Operational data for the period of 1958–2006 were used to examine the seasonal evolution of the QBO‐solar cycle relationship at various pressure levels up to the stratopause. The results show that the solar signals in the NH winter extratropics are indeed QBO‐phase dependent, moving poleward and downward as winter progresses with a faster descent rate under westerly QBO than under easterly QBO. In the stratosphere, the signals are highly significant in late January to early March and have a life span of ∼30–50 days. Under westerly QBO, the stratospheric solar signals clearly lead and connect to those in the troposphere in late March and early April where they have a life span of ∼10 days. As the structure changes considerably from the upper stratosphere to the lower troposphere, the exact month when the maximum solar signals occur depends largely on the altitude chosen.For the low‐latitude stratosphere, our analysis supports a vertical double‐peaked structure of positive signature of the 11‐yr SC in temperature, and demonstrates that this structure is further modulated by the QBO. These solar signals have a longer life span (∼3–4 months) in comparison to those in the extratropics. The solar signals in the lower stratosphere are stronger in early winter but weaker in late winter, while the reverse holds in the upper stratosphere. Copyright © 2009 Royal Meteorological Society

Global Characteristics of Medium-Scale Tropopausal Waves Observed in ECMWF Operational Data

Abstract Global characteristics and seasonal variation of medium-scale (zonal wavelengths of 2000–3000 km) waves observed around the midlatitude tropopause are examined using 6-hourly European Centre for Medium-Range Weather Forecasts operational data over four years (1990–93), covering both hemispheres. Medium-scale waves and synoptic-scale waves are extracted using time filters and their characteristics are compared. Hovmoller diagrams indicate the existence of medium-scale waves in the Southern Hemisphere as well as in the Northern Hemisphere. The zonal wavelengths of medium-scale waves are slightly larger in the Southern Hemisphere than in the Northern Hemisphere. Medium-scale waves are mostly active in three regions: the North Atlantic in winter (Dec–Jan–Feb), the North Pacific in spring (Mar–Apr–May, MAM), and the south Indian Ocean in autumn (MAM). These regions are roughly corresponding to and slightly downstream of storm tracks where synoptic waves are dominant. Significant differences in seasona...

北大西洋上に卓越する中間規模東進波

An analysis is made, with the aid of ECMWF operational data, of medium-scale, eastward-traveling waves in the mid-latitude upper troposphere over the north Atlantic area as a counterpart of the similar analysis of Sato et al. (1993) for the medium-scale waves over Japan. It is shown that the waves dominate in wintertime (January and February of 1990) over the north Atlantic centered around 50°N, with the typical wavelength of 2400 km, a period of 33 hours, and hence an eastward phase velocity of about 20 m s -1 . In view of their vertical structure and relationship to the background zonal flow, it is concluded that the wave character obtained in this analysis is the same in essence as that shown by Sato et al. (1993).

A statistical study of the structure, saturation and sources of inertio-gravity waves in the lower stratosphere observed with the MU radar

Using wind data monitored by the MU radar at Shigaraki, Japan (35°N, 136°E) in the three years of 1986–1988, a statistical analysis is made of inertio-gravity waves (IGWs) with short vertical wavelengths (⩽ 4 km) and long ground-based periods (⩾ 10 h) which are dominant in the lower stratosphere in all seasons. The intensity of the IGWs is at its peak in winter when a strong subtropical westerly wind jet is situated over Japan. The zonal component of vertical momentum flux associated with the IGWs in winter is mostly negative and the magnitude decreases with altitude, suggesting a strong interaction between IGWs and the background wind field. The parameters of the wave structure are estimated by a hodograph analysis, taking into consideration the effect of vertical shear of the background wind that is not negligible in winter. The obtained parameters, such as the vertical and horizontal energy propagation directions, vertical and horizontal wavelengths, and intrinsic period, are compared between two periods with a strong subtropical jet including winter and with a weak jet including summer. The degree of saturation of IGWs, defined as the ratio of amplitude of the horizontal perturbation velocity parallel to the horizontal wavenumber vector to the intrinsic horizontal phase velocity, is also statistically and directly examined using the results of wave parameter estimation. It is shown that the degree of saturation is larger in winter than in summer and that there are few oversaturated IGWs even in winter. Moreover, in order to examine the generation mechanisms of the IGWs, an analysis is made from two points of view: geostrophic adjustment of the westerly wind jet and the topographic effect. The meridional propagation direction of IGWs is examined in a section of latitude and altitude relative to the jet axis using ECMWF operational data. Most of the IGWs observed in the 12–18 km height region (above the ground) in winter propagate meridionally toward the jet axis, indicating that geostrophic adjustment at least just as the jet axis is not the main generation mechanism of the IGWs. On the other hand, the characteristics of intensive IGWs propagating westward relative to the background wind in the 18–22 km height region in winter are in good accord with mountain waves excited in strong westerly winds near the surface.

Data assimilation: The 1984/85 revisions of the ECMWF mass and wind analysis

A comprehensive evaluation of the ECMWF operational data assimilation system has been made, based largely on statistics of its performance over a long period of time. the evaluation has identified many deficiencies, covering such aspects of the analysis as the optimum interpolation statistics, the data selection algorithms, and the quality control criteria applied within the analysis. The observation and forecast error variances were seriously in error in the stratosphere and in the tropics. A more faithful model of the forecast error spectrum is provided by the Bessel function expansion than by the Gaussian model. the vertical structure of the height error is assumed to be composed of two components: a ‘barotropic’ part related to a horizontally constant mode and a ‘baroclinic’ structure for the horizontally varying modes. the wind field errors also have a large-scale component. the extratropical vertical correlation remained similar to the earlier correlations, but the tropical height and wind structures changed dramatically. Reasonable rejection limits were defined from the distributions of the observation departures from the first guess, as were observation errors for satellite and aircraft wind reports and Australian pseudo-observations. A comprehensive and homogeneous data selection algorithm has been devised. However, it has been found necessary to exclude land surface wind reports from the analysis process as the wind departures from a first-guess field commonly contradict the surface pressure departures due to difficulties in generating a reasonable 10 m first-guess wind. the omission of surface winds alleviated the data selection problem in data-dense regions. The representation of the vertical correlations was changed to a continuous representation in the vertical which enabled a multivariate interpolation from data on standard pressure levels directly to model coordinates. the earlier procedure used a multivariate interpolation to constant pressure surfaces in the model grid, followed by a univariate interpolation to model surfaces in the vertical. The results of these improvements, in a series of tests covering in total some 30 days of assimilations, are also described. In summary they yield an analysis and a subsequent first guess which are in closer agreement with observations. the analysis scheme yields more balanced fields in the sense that the subsequent initialization produces more modest changes. the major impact on forecasts, in particular in the short range, is also judged to be positive. These improvements have been incorporated in the ECMWF operational data assimilation system. They have also been incorporated in the reassimilation of the final FGGE level IIb data set at ECMWF.

Data Assimilation: the 1984/85 Revisions of the Ecmwf Mass and Wind Analysis

AbstractA comprehensive evaluation of the ECMWF operational data assimilation system has been made, based largely on statistics of its performance over a long period of time. the evaluation has identified many deficiencies, covering such aspects of the analysis as the optimum interpolation statistics, the data selection algorithms, and the quality control criteria applied within the analysis.The observation and forecast error variances were seriously in error in the stratosphere and in the tropics. A more faithful model of the forecast error spectrum is provided by the Bessel function expansion than by the Gaussian model. the vertical structure of the height error is assumed to be composed of two components: a ‘barotropic’ part related to a horizontally constant mode and a ‘baroclinic’ structure for the horizontally varying modes. the wind field errors also have a large‐scale component. the extratropical vertical correlation remained similar to the earlier correlations, but the tropical height and wind structures changed dramatically.Reasonable rejection limits were defined from the distributions of the observation departures from the first guess, as were observation errors for satellite and aircraft wind reports and Australian pseudo‐observations.A comprehensive and homogeneous data selection algorithm has been devised. However, it has been found necessary to exclude land surface wind reports from the analysis process as the wind departures from a first‐guess field commonly contradict the surface pressure departures due to difficulties in generating a reasonable 10 m first‐guess wind. the omission of surface winds alleviated the data selection problem in data‐dense regions.The representation of the vertical correlations was changed to a continuous representation in the vertical which enabled a multivariate interpolation from data on standard pressure levels directly to model coordinates. the earlier procedure used a multivariate interpolation to constant pressure surfaces in the model grid, followed by a univariate interpolation to model surfaces in the vertical.The results of these improvements, in a series of tests covering in total some 30 days of assimilations, are also described. In summary they yield an analysis and a subsequent first guess which are in closer agreement with observations. the analysis scheme yields more balanced fields in the sense that the subsequent initialization produces more modest changes. the major impact on forecasts, in particular in the short range, is also judged to be positive.These improvements have been incorporated in the ECMWF operational data assimilation system. They have also been incorporated in the reassimilation of the final FGGE level IIb data set at ECMWF.