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  • 1.
    Andersson, Tage
    et al.
    SMHI, Forskningsavdelningen.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    COAST OF DEPARTURE AND COAST OF ARRIVAL - 2 IMPORTANT CONCEPTS FOR THE FORMATION AND STRUCTURE OF CONVECTIVE SNOWBANDS OVER SEAS AND LAKES1994Ingår i: Monthly Weather Review, ISSN 0027-0644, E-ISSN 1520-0493, Vol. 122, nr 6, s. 1036-1049Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A case with intense convective snowbands over the Baltic Sea is examined using the High-Resolution Limited Area Model. The intention is to gain a better insight into the importance of the shape of the cowta, the orography, and the surface roughness on the formation and evolution of the snowbands. Among the factors studied am the shape of the coast from which the air departs and that to which it arrives. These factors are so important that two new concepts-coast of departure and coast of arrival-are introduced.

  • 2.
    Bengtsson,, Lennart
    et al.
    SMHI.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Döös, Bo
    SMHI.
    Söderman, Daniel
    Helsinki University in Finland.
    Moen, Lars
    SMHI, Forskningsavdelningen, Klimatforskning - Rossby Centre.
    Thompson, Thomas
    SMHI.
    Jakobsson, Paul
    SMHI.
    Bleckert, Gunnar
    SMHI.
    Henriksson, Ann-Beate
    SMHI.
    Lindgren, Bo
    SMHI.
    Kållberg, Per
    SMHI, Forskningsavdelningen, Meteorologi.
    The Meteorological Auto Code (MAC) and Numerical Weather Prediction (NWP) at SMHI2016Rapport (Övrigt vetenskapligt)
    Abstract [sv]

    Sverige var ett föregångsland inom numeriska vädderprognoser och den allra första operativa väderprognosen gjordes redan 1954 på det Internationella Meteorologiska Institutet i Stockholm. SMHI kom igång senare, men 1961 startade man ett långsiktigt program för NWP (numerical weather prediction). Projektet växte gradvis under 1960-talet och blev så småningom en central komponent i SMHIs prognostjänst. En utmaning under de tidiga åren var de begränsade dataresurserna med primitiv programvara, och med dagens mått begränsat minnesutrymme och låg beräkningshastighet. För att kompensera dessa brister krävdes både beslutsamhet och ett stort mått av kreativitet. Som en central komponent i arbetet utvecklade NWP-gruppen datorsystemet MAC (Meteorological Auto Code) som här beskrivs i detalj samt också alla de beräkningsprogram som krävdes för prognostjänsten. Detta inkluderade olika prognosmodeller, analys samt program för databehandling och observationskontroll samt produktion av prognosresultaten i grafisk eller digital form.

    Det är vår förhoppning att f´öreliggande artikel skall ge den yngre generationen en inblick i hur det var att syssla med NWP under 1960-talet.

  • 3. Blazica, V.
    et al.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Zagar, N.
    The impact of periodization methods on the kinetic energy spectra for limited-area numerical weather prediction models2015Ingår i: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 8, nr 1, s. 87-97Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The paper deals with the comparison of the most common periodization methods used to obtain spectral fields of limited-area models for numerical weather prediction. The focus is on the impact that the methods have on the spectra of the fields, which are used for verification and tuning of the models. A simplified model is applied with random fields that obey a known kinetic energy spectrum. The periodization methods under consideration are detrending, the discrete cosine transform and the application of an extension zone. For the extension zone, three versions are applied: the Boyd method, the ALADIN method and the HIRLAM method. The results show that detrending and the discrete cosine transform have little impact on the spectra, as does the Boyd method for extension zone. For the ALADIN and HIRLAM methods, the impact depends on the width of the extension zone - the wider the zone, the more artificial energy and the larger impact on the spectra. The width of the extension zone correlates to the modifications in the shape of the spectra as well as to the amplitudes of the additional energy in the spectra.

  • 4.
    Bojarova, Jelena
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Relevance of climatological background error statistics for mesoscale data assimilation2019Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 71, nr 1, artikel-id 1615168Artikel i tidskrift (Refereegranskat)
  • 5. Bojarova, Jelena
    et al.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Johansson, Åke
    SMHI, Forskningsavdelningen, Meteorologi.
    Vignes, Ole
    The ETKF rescaling scheme in HIRLAM2011Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 63, nr 3, s. 385-401Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The ETKF rescaling scheme has been implemented into the HIRLAM forecasting system in order to estimate the uncertainty of the model state. The main purpose is to utilize this uncertainty information for modelling of flow-dependent background error covariances within the framework of a hybrid variational ensemble data assimilation scheme. The effects of rank-deficiency in the ETKF formulation is explained and the need for variance inflation as a way to compensate for these effects is justified. A filter spin-up algorithm is proposed as a refinement of the variance inflation. The proposed spin-up algorithm will also act to prevent ensemble collapse since the ensemble will receive 'fresh blood' in the form of additional perturbation components, generated on the basis of a static background error covariance matrix. The resulting ETKF-based ensemble perturbations are compared with ensemble perturbations based on targeted singular vectors and are shown to have more realistic spectral characteristics.

  • 6.
    Bringfelt, Björn
    et al.
    SMHI, Forskningsavdelningen, Klimatforskning - Rossby Centre.
    Heikinheimo, M
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Perov, Veniamin
    SMHI, Forskningsavdelningen, Meteorologi.
    Lindroth, A
    A new land-surface treatment for HIRLAM - comparisons with NOPEX measurements1999Ingår i: Agricultural and Forest Meteorology, ISSN 0168-1923, E-ISSN 1873-2240, Vol. 98-9, s. 239-256Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In order to improve the accuracy of forecasting near-surface atmospheric variables over a heterogeneous landscape, a framework of subgrid surface types and the ISBA parameterisation scheme for land surfaces have been tested in the operational weather forecast model HIRLAM, using a 5.5 km grid resolution. Surface energy fluxes measured during a single summer day at six fixed sites in the NOPEX area, representing agricultural fields, boreal forests and lakes, were used for verification. Both, in-situ field measurements and the HIRLAM simulation indicated that the Bowen ratio over forests was about twice as large as that of adjacent agricultural fields. This difference could be explained by the more effective turbulent mixing and larger surface resistance associated with the forest, thus making the sensible heat flux relatively large there. The use of initial soil moisture from a routine hydrological model gave improved agreement with measured surface fluxes and radiosonde temperature and humidity profiles compared to initialising from routine HIRLAM surface data. The differences in heat fluxes between the various surface types were also demonstrated by airborne flux measurements flown along a track at a height of ca. 100 m above the terrain. Modelled heat fluxes along the flight track were considerably smoothed due to the grid resolution used, e.g. the effect of a lake in reducing grid-averaged sensible heat flux could only be weakly detected, because the lake surface represented only 10% of the grid area. When the proportion of a contrasting surface type (lake) was altered from 10 to 100%, the surface fluxes calculated for the lake surface were almost unchanged; the results of the comparison did not provide evidence that more complex aggregation schemes for heat fluxes than straightforward area-weighted averaging would be required. The hourly variation of the modelled and simulated heat fluxes during the day studied could not be directly compared, because the simulated cloudiness did not exactly match that observed at the field sites. When the simulated net radiation was replaced with direct measurements, the model-based estimates of sensible and latent heat fluxes were closer to the corresponding field measurements. The divergence of sensible heat flux with height, as inferred from the tower measurements made over the forest, were supported by the aircraft measurements and the HIRLAM simulations. (C) 1999 Elsevier Science B.V. All rights reserved.

  • 7.
    Dahlgren, Per
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Assimilating host model information into a limited area model2012Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 64, artikel-id 15836Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We propose to add an extra source of information to the data-assimilation of the regional HIgh Resolution Limited Area Model (HIRLAM) model, constraining larger scales to the host model providing the lateral boundary conditions. An extra term, J(k), measuring the distance to the large-scale vorticity of the host model, is added to the cost-function of the variational data-assimilation. Vorticity is chosen because it is a good representative of the large-scale flow and because vorticity is a basic control variable of the HIRLAM variational data-assimilation. Furthermore, by choosing only vorticity, the remaining model variables, divergence, temperature, surface pressure and specific humidity will be allowed to adapt to the modified vorticity field in accordance with the internal balance constraints of the regional model. The error characteristics of the J(k) term are described by the horizontal spectral densities and the vertical eigenmodes (eigenvectors and eigenvalues) of the host model vorticity forecast error fields, expressed in the regional model geometry. The vorticity field, provided by the European Centre for Medium-range Weather Forecasts (ECMWF) operational model, was assimilated into the HIRLAM model during an experiment period of 33 d in winter with positive impact on forecast verification statistics for upper air variables and mean sea level pressure.

  • 8.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Control of lateral boundary conditions in four-dimensional variational data assimilation for a limited area model2012Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 64, artikel-id 17518Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The limited area model forecasting problem is a lateral boundary condition (LBC) problem in addition to the initial condition problem. The data assimilation has traditionally been considered as a process for estimation of the initial condition only, while for the limited area data assimilation this estimation may be extended to include also the LBCs, at least during the data assimilation time window when observations are available. A procedure for such a control of the LBCs has been included in the four-dimensional variational data assimilation (4D-Var) scheme for the HIgh Resolution Limited Area Model (HIRLAM) forecasting system. A description of this procedure is provided together with results from idealised as well as real data experiments. The results indicate that control of LBCs may be important with small forecast domains and in particular for weather disturbances moving quickly into and through the forecast domain.

  • 9.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Discussion on '4D-Var or EnKF?'2007Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 59, nr 5, s. 774-777Artikel i tidskrift (Övrigt vetenskapligt)
  • 10.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Statistical issues in weather forecasting2002Ingår i: Scandinavian Journal of Statistics, ISSN 0303-6898, E-ISSN 1467-9469, Vol. 29, nr 2, s. 219-239Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Research and operational applications in weather forecasting are reviewed, with emphasis on statistical issues. It is argued that the deterministic approach has dominated in weather forecasting, although weather forecasting is a probabilistic problem by nature. The reason has been the successful application of numerical weather prediction techniques over the 50 years since the introduction of computers. A gradual change towards utilization of more probabilistic methods has occurred over the last decade; in particular meteorological data assimilation, ensemble forecasting and post-processing of model output have been influenced by ideas from statistics and control theory.

  • 11.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Berre, Loik
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Hörnquist, Sara
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Huang, X Y
    Lindskog, Magnus
    SMHI, Forskningsavdelningen, Meteorologi.
    Navascues, B
    Mogensen, K S
    Thorsteinsson, S
    Three-dimensional variational data assimilation for a limited area model Part I: General formulation and the background error constraint2001Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 53, nr 4, s. 425-446Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A 3-dimensional variational data assimilation (3D-Var) scheme for the HIgh Resolution Limited Area Model (HIRLAM) forecasting system is described. The HIRLAM 3D-Var is based on the minimization of a cost function that consists of one term J(b). which measures the distance between the resulting analysis and a background field, in general a short-range forecast. and another term J(o). which measures the distance between the analysis and the observations. This paper is concerned with the general formulation of the HIRLAM 3D-Var and with Jb. while the companion paper by Lindskog and co-workers is concerned with the handling of observations, including the J(o) term, and with validation of the 3D-Var through extended parallel assimilation and forecast experiments. The 3D-Var minimization requires a pre-conditioning that is achieved by a transformation of the minimization control variable. This change of variable is designed as an operator approximating an inverse square root of the forecast error covariance matrix in the model space. The main transformations are the Subtraction of the geostrophic wind increment, the bi-Fourier transform, and the projection on vertical eigenvectors. The spectral bi-Fourier approach allows one to derive non-separable structure functions in a limited area model. in the form of vertically dependent horizontal spectra and scale-dependent vertical correlations. Statistics have been accumulated from differences between +24 h and +48 h HIRLAM forecasts valid at the same time. Results from single observation impact studies as well as results from assimilation cycles using operational observations are presented. It is shown that the HIRLAM 3D-Var produces assimilation increments in accordance with the applied analysis structure functions, that the fit of the analysis to the observations is in agreement with the assumed error statistics. and that assimilation increments are well balanced. It is also shown that the particular problems associated with the limited area formulation have been solved. These results, together with the results of the companion paper, indicate that the 3D-Var scheme performs significantly better than the statistical interpolation scheme.

  • 12.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Bojarova, J.
    Four-dimensional ensemble variational (4D-En-Var) data assimilation for the HIgh Resolution Limited Area Model (HIRLAM)2014Ingår i: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 21, nr 4, s. 745-762Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A four-dimensional ensemble variational (4D-EnVar) data assimilation has been developed for a limited area model. The integration of tangent linear and adjoint models, as applied in standard 4D-Var, is replaced with the use of an ensemble of non-linear model states to estimate four-dimensional background error covariances over the assimilation time window. The computational costs for 4D-En-Var are therefore significantly reduced in comparison with standard 4D-Var and the scalability of the algorithm is improved. The flow dependency of 4D-En-Var assimilation increments is demonstrated in single simulated observation experiments and compared with corresponding increments from standard 4D-Var and Hybrid 4D-Var ensemble assimilation experiments. Real observation data assimilation experiments carried out over a 6-week period show that 4D-En-Var outperforms standard 4D-Var as well as Hybrid 4D-Var ensemble data assimilation with regard to forecast quality measured by forecast verification scores.

  • 13.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Bojarova, J.
    Vignes, O.
    A hybrid variational ensemble data assimilation for the HIgh Resolution Limted Area Model (HIRLAM)2014Ingår i: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 21, nr 1, s. 303-323Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A hybrid variational ensemble data assimilation has been developed on top of the HIRLAM variational data assimilation. It provides the possibility of applying a flow-dependent background error covariance model during the data assimilation at the same time as full rank characteristics of the variational data assimilation are preserved. The hybrid formulation is based on an augmentation of the assimilation control variable with localised weights to be assigned to a set of ensemble member perturbations (deviations from the ensemble mean). The flow-dependency of the hybrid assimilation is demonstrated in single simulated observation impact studies and the improved performance of the hybrid assimilation in comparison with pure 3-dimensional variational as well as pure ensemble assimilation is also proven in real observation assimilation experiments. The performance of the hybrid assimilation is comparable to the performance of the 4-dimensional variational data assimilation. The sensitivity to various parameters of the hybrid assimilation scheme and the sensitivity to the applied ensemble generation techniques are also examined. In particular, the inclusion of ensemble perturbations with a lagged validity time has been examined with encouraging results.

  • 14.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Huang, S Y
    Sensitivity experiments with the spectral HIRLAM and its adjoint1996Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 48, nr 4, s. 501-517Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The tangent-linear and the adjoint of the spectral High Resolution Limited Area Model (HIRLAM) have been derived as a first step in the development of a 4-dimensional variational data assimilation system for HIRLAM. The adjoint of the spectral HIRLAM was applied successfully to test the sensitivity of short-range forecast errors to initial conditions. These sensitivity experiments were carried out for a particular case study in addition to a full 5-day period. The results of the sensitivity experiments indicate an ability of the adjoint model to improve the assimilation of baroclinically developing systems and this may open possibilities for application of the adjoint model in a ''Poor mans 4-dimensional variational data assimilation'' in advance of the implementation of the full 4-dimensional variational data assimilation.

  • 15.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Huang, Xiang-Yu
    Yang, Xiaohua
    Mogensen, Kristian
    Lindskog, Magnus
    SMHI, Forskningsavdelningen, Meteorologi.
    Vignes, Ole
    Wilhelmsson, Tomas
    Thorsteinsson, Sigurdur
    SMHI, Forskningsavdelningen, Meteorologi.
    Four-dimensional variational data assimilation for a limited area model2012Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 64, artikel-id 14985Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A 4-dimensional variational data assimilation (4D-Var) scheme for the HIgh Resolution Limited Area Model (HIRLAM) forecasting system is described in this article. The innovative approaches to the multi-incremental formulation, the weak digital filter constraint and the semi-Lagrangian time integration are highlighted with some details. The implicit dynamical structure functions are discussed using single observation experiments, and the sensitivity to various parameters of the 4D-Var formulation is illustrated. To assess the meteorological impact of HIRLAM 4D-Var, data assimilation experiments for five periods of 1 month each were performed, using HIRLAM 3D-Var as a reference. It is shown that the HIRLAM 4D-Var consistently out-performs the HIRLAM 3D-Var, in particular for cases with strong mesoscale storm developments. The computational performance of the HIRLAM 4D-Var is also discussed.

  • 16.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Janjić, Tijana
    Schraff, Christoph
    Leuenberger, Daniel
    Weissmann, Martin
    Reich, Hendrik
    Brousseau, Pierre
    Montmerle, Thibaut
    Wattrelot, Eric
    Bučánek, Antonĺn
    Mile, Máté
    Hamdi, Rafiq
    Lindskog, Magnus
    SMHI, Forskningsavdelningen, Meteorologi.
    Barkmeijer, Jan
    Dahlbom, Mats
    Macpherson, Bruce
    Ballard, Sue
    Inverarity, Gordon
    Carley, Jacob
    Alexander, Curtis
    Dowell, David
    Liu, Shun
    Ikuta, Yasutaka
    Fujita, Tadashi
    Survey of data assimilation methods for convective‐scale numerical weather prediction at operational centres2018Ingår i: Quarterly Journal of thte Royal Meteorology Society, ISSN 1350-4827, Vol. 144, nr 711Artikel i tidskrift (Övrigt vetenskapligt)
  • 17.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Källén, Erland
    SMHI, Forskningsavdelningen, Meteorologi.
    Thorsteinsson, Sigurdur
    SMHI, Forskningsavdelningen, Meteorologi.
    Sensitivity of forecast errors to initial and lateral boundary conditions1998Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 50, nr 2, s. 167-185Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The adjoint of a limited area model has been used to study the sensitivity of 12 h forecast errors to initial and lateral boundary conditions. Upper troposphere potential vorticity and mean sea level pressure verification scores for 1 month of operational forecasts from the Swedish Meteorological and Hydrological Institute were used to select 2 cases with particularly poor forecast performance. The sensitivity experiments show that errors in initial data is the most likely explanation for one of the forecast failures, while errors in initial as well as lateral boundary data can explain the 2nd forecast failure. Results from the sensitivity experiments with respect to the lateral boundary conditions indicate that poor quality lateral boundary conditions may be improved by utilizing subsequent downstream observations within the model integration area. This result is of great significance with regard to the possibilities for applying 4-dimensional variational data assimilation (4DVAR) for limited area forecast models. Results from the sensitivity experiments also reveal, however, that the lateral boundary treatment in operational limited area models needs to be improved with regard to the mathematical formulation. It is furthermore shown that modifications to be applied to the lateral boundary conditions need to be determined with appropriate time resolution and that some filtering of these lateral boundary modifications has to be introduced to avoid enhanced high-frequency gravity wave noise in the vicinity of the lateral boundaries.

  • 18.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Lonnberg, P
    Pailleux, J
    Data assimilation for high-resolution limited-area models1997Ingår i: Journal of the Meteorological Society of Japan, ISSN 0026-1165, Vol. 75, nr 1B, s. 367-382Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The present status of development of data assimilation techniques for high resolution limited area models is reviewed and various candidates for a new generation of a data assimilation system are compared. It is concluded that data assimilation based on 3-dimensional or 4-dimensional variational techniques is the most promising approach. Further investigations are needed, however, to find out whether variational data assimilation is feasible over "limited" model integration areas or whether global/hemispheric model integration areas have to be introduced for high resolution limited area data assimilation purposes. The main problem in this connection is the rapid propagation of forecast errors on a global or at least hemispheric atmospheric scale within a time-scale of a few days.

  • 19.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    McDonald, A
    A comparison of the HIRLAM gridpoint and spectral semi-Lagrangian models1996Ingår i: Monthly Weather Review, ISSN 0027-0644, E-ISSN 1520-0493, Vol. 124, nr 9, s. 2008-2022Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A comparison of two semi-Lagrangian limited area models, one spectral, and the other grid point, is described. Forecasts from both models are compared and contrasted, first on a 55-km horizontal mesh and subsequently an a 22-km horizontal mesh. The weaknesses in the respective models exposed by these tests, and the corrections made to overcome them are described. The final models arrived at are shown to be accurate and more efficient than the Eulerian counterpart for the test dataset. It is also found that the spectral model is as accurate as the gridpoint model and is also computationally competitive. It is concluded that with sufficient thought and effort the gridpoint and spectral models can be made to produce equally good forecasts at comparable computer costs. Finally. a reassessment of the relative merits and drawbacks of the spectral and gridpoint schemes is attempted taking into account the fact that the advection terms are integrated by the semi-Lagrangian scheme.

  • 20.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Nyberg, Leif
    SMHI.
    Omstedt, Anders
    SMHI, Forskningsavdelningen, Oceanografi.
    Coupling of a high-resolution atmospheric model and an ocean model for the Baltic Sea1998Ingår i: Monthly Weather Review, ISSN 0027-0644, E-ISSN 1520-0493, Vol. 126, nr 11, s. 2822-2846Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The coupling between a high-resolution weather forecasting model and an ocean model is investigated. It is demonstrated by several case studies that improvements of short-range weather forecasting in the area of the Baltic Sea require an accurate description of the lower boundary condition over sea. The examples are taken from summer situations without sea ice as well as from winter situations with extreme sea ice conditions. It is shown that the sea state variables used in the model influence the weather forecast both directly on the local scale due to the local impact of surface fluxes of latent and sensible heat and on regional and larger scales. The convective snowbands during winters with cold airmass outbreaks over the open water surfaces of the Baltic Sea are extreme examples of the influence of sea state variables on a regional scale, It is furthermore demonstrated that the sea state conditions may change considerably within forecasting periods up to 48 h. This implies the necessary application of ocean models, two-way interactively coupled to the weather forecasting model. The coupling of an advanced 2.5-dimensional ice-ocean model to the operational Swedish Meteorological and Hydrological Institute (SMHI) weather forecasting model HIRLAM is described. The ice-ocean model includes two-dimensional, horizontally resolved ice and storm surge models and a one-dimensional, vertically resolved ocean model applied to 31 Baltic Sea regions. The coupled model system is applied operationally in a data assimilation system at the SMHI. No data assimilation is applied in the operational ocean component: manual modifications to the sea state variables are introduced a few times every winter season. The application of this operational coupled model data assimilation system to the mesoscale reanalysis for the Baltic Sea Experiment (BALTEX) shows that it is necessary to apply data assimilation fur the sea state variables in order to avoid drift of the coupled model system toward less realistic model states. A successful application of a simple assimilation of SST observations is presented. The observed SSTs are first subject to a horizontal filter in order to minimize the effects of observational errors and to restrict the influence to a larger horizontal scale. Then the differences between these filtered temperature observations and the model SSTs are used to construct a modified sensible heat Aux that is applied as a form of a "nudging" term to the ocean model. It turns out that this "nudging" is successful in avoiding the drift away from realistic sea state conditions. The described atmosphere and ocean data assimilation scheme has been applied in a rerun of the BALTEX mesoscale reanalysis for the cold winter 1986/87. The quality of this reanalysis was assessed through the successful simulation of the convective snowbands in January 1987.

  • 21.
    Gustafsson, Nils
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Thorsteinsson, S.
    Stengel, M.
    Holm, E.
    Use of a nonlinear pseudo-relative humidity variable in a multivariate formulation of moisture analysis2011Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 137, nr 657, s. 1004-1018Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a reformulation of the humidity part of the HIRLAM (HIgh-Resolution Limited-Area Model) variational data assimilation. The purpose is to rectify some of the shortcomings of the present formulation which uses specific humidity, q, as an assimilation control variable with homogeneous and static covariances. One problem is that specific humidity forecast errors tend to have a non-Gaussian probability distribution, in particular near saturation and near zero humidity. In addition, the variance of the distribution tends to change in space and time due to the dependency of the water vapour saturation pressure on temperature. A modified pseudo-relative humidity variable has been adapted to the statistical balance background constraint, including the associated moisture balance formulation. Background-error statistics for the new moisture control variable and the moisture-related balances were derived, taking differences between forecasts valid at the same time as a proxy for background forecast errors. The background-error statistics were compared with the corresponding statistics for specific humidity as the moisture assimilation control variable. In connection with the nonlinearity of the change of the variable, it was noted that specified background-error standard deviations were chosen to be substantially reduced for nearly dry and saturated states, which can raise difficulties. The impact of the new moisture assimilation control variable is illustrated with simulated observation experiments as well as data assimilation experiments using real observations, for one summer month and one winter month in a 4D-Var assimilation cycle using two outer loop iterations in the 4D-Var minimization. The impact of the new formulation on forecast verification scores is small and essentially neutral, while using the second outer loop in the old formulation has a small positive impact. Copyright (C) 2011 Royal Meteorological Society

  • 22. HAUGEN, JE
    et al.
    MACHENHAUER, B
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    A Spectrial Limited-Area Formulation with Time-Dependent Boundary-Conditions Applied to the Shallow-Water Equations - Reply1995Ingår i: Monthly Weather Review, ISSN 0027-0644, E-ISSN 1520-0493, Vol. 123, nr 10, s. 3124-3124Artikel i tidskrift (Refereegranskat)
  • 23. Heygster, Georg
    et al.
    Melsheimer, Christian
    Mathew, Nizy
    Toudal, Leif
    Saldo, Roberto
    Andersen, Soren
    Tonboe, Rasmus
    Schyberg, Harald
    Tveter, Frank Thomas
    Thyness, Vibeke
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Landelius, Tomas
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Dahlgren, Per
    SMHI, Forskningsavdelningen, Meteorologi.
    Integrated Observation and Modeling of the Arctic Sea Ice and Atmosphere2009Ingår i: Bulletin of The American Meteorological Society - (BAMS), ISSN 0003-0007, E-ISSN 1520-0477, Vol. 90, nr 3, s. 293-297Artikel i tidskrift (Refereegranskat)
  • 24.
    Lindskog, Magnus
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Mogensen, Kristian S.
    Representation of background error standard deviations in a limited area model data assimilation system2006Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 58, nr 4, s. 430-444Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two different approaches for improving the representation of background error standard deviations have been developed and introduced into the HIRLAM high-resolution limited area model 3-D variational data assimilation scheme. One of the methods utilizes a horizontally varying climatological background error standard deviation field, estimated from a time-series of innovations. The second approach attempts to take temporal and spatial variations of the background error standard deviations into account by applying an Eady instability measure to the background field. The two approaches are described in detail and their functionality is demonstrated. Parallel data assimilation and forecasts experiments indicate a slightly positive impact on average verification scores, and in addition a positive impact is demonstrated for an individual synoptically active case.

  • 25.
    Lindskog, Magnus
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Navascues, B
    Mogensen, K S
    Huang, X Y
    Yang, X
    Andrae, Ulf
    SMHI, Forskningsavdelningen, Meteorologi.
    Berre, Loik
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Thorsteinsson, S
    Rantakokko, J
    Three-dimensional variational data assimilation for a limited area model Part II: Observation handling and assimilation experiments2001Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 53, nr 4, s. 447-468Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A 3-dimensional variational data assimilation (3D-Var) scheme for the HIgh Resolution Limited Area Model (HIRLAM) forecasting system is described. The HIRLAM 3D-Var is based on the minimisation of a cost function that consists of one term, J(b), which measures the distance between the resulting analysis and a background field, in general a short-range forecast, and another term. J(o), which measures the distance between the analysis and the observations. This paper is concerned with J(o) and the handling of observations, while the companion Paper by Gustafsson et al. (2001) is concerned with the general 3D-Var formulation and with the J(b) term. Individual system components. such as the screening of observations and the observation operators, and other issues, such as the parallelisation strategy for the computer code, are described. The functionality of the observation quality control is investigated and the 3D-Var system is validated through data assimilation and forecast experiments. Results from assimilation and forecast experiments indicate that the 3D-Var assimilation system performs significantly better than two currently used HIRLAM systems. which are based on statistical interpolation. The use of all significant level data from multilevel observation reports is shown to be one factor contributing to the superiority of the 3D-Var system. Other contributing factors are most probably the formulation of the analysis as a single global problem, the use of non-separable structure functions and the variational quality control, which accounts for non-Gaussian observation errors.

  • 26.
    Ljungemyr, Patrik
    et al.
    SMHI, Samhälle och säkerhet.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Omstedt, Anders
    SMHI, Forskningsavdelningen, Oceanografi.
    Parameterization of lake thermodynamics in a high-resolution weather forecasting model1996Ingår i: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 48, nr 5, s. 608-621Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A model for the parameterization of lake temperatures and lake ice thicknesses in atmospheric models is presented. The model is verified independently, and it is also tested within the framework of the High Resolution Limited Area Model(HIRLAM), applied operationally for short range weather forecasting at the Swedish Meteorological and Hydrological Institute (SMHI). The lake model is a slab model based upon energy conservation and treats the lakes as well mixed boxes with depths represented by the mean depths. The model is forced by near surface fluxes calculated from total cloudiness, air temperature, air humidity and low-level winds. A data base, describing 92000 Swedish lakes. provides the model with lake mean depths, areal sizes and locations. When the model is used for parameterization of lake effects in the atmospheric model, all the smaller lakes and the fractions of larger lakes within each horizontal grid square of the atmospheric model are parameterized by four model lakes, representing the lake size distribution. The verification of the lake model is done by comparing it with a more advanced, vertically resolved model, including parameterization of turbulent mixing processes, as well as by comparison with observations. A sensitivity test shows great interannual variations of the ice-covered season, which implies that lake models should be used instead of climate data. The results from an experiment with two-way coupling of the lake model to the atmospheric model are verified by comparing forecasted weather parameters with routine meteorological observations. These results show that the impact of lake effects can reach several degrees C in air temperatures close to the surface.

  • 27.
    Meuller, Lars
    et al.
    SMHI.
    Robertson, Lennart
    SMHI, Forskningsavdelningen, Luftmiljö.
    Andersson, E.
    SMHI.
    Gustavsson, Nils
    Meterologi.
    Meso-g scale objective analysis of near surface temperature, humidity and wind, and its application in air pollution modelling1990Rapport (Övrigt vetenskapligt)
  • 28.
    Ridal, Martin
    et al.
    SMHI, Forskningsavdelningen, Meteorologi.
    Lindskog, Magnus
    SMHI, Forskningsavdelningen, Meteorologi.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Haase, Günther
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Optimized advection of radar reflectivities2011Ingår i: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 100, nr 2-3, s. 213-225Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A nowcasting system for generation of short-range precipitation forecasts has been developed at the Swedish Meteorological and Hydrological Institute (SMHI). The methodology consists of utilising a time-series of radar reflectivity composites for deriving an advection field, which will give a better representation of the motion of the precipitation pattern compared to a model wind field. The advection field is derived applying a 4-dimensional variational data assimilation technique. The resulting field is then used for a semi-Lagrangian advection of the latest available reflectivity field forward in time. During the forecast, the advected field is gradually replaced by a numerical weather prediction forecast in order to include the onset of convection and advection into the radar coverage area. In an idealised example with simulated observations the functionality of the method is demonstrated. For a case study of a full scale example the resulting precipitation forecast shows large improvements compared to the operational numerical weather prediction model used at SMHI, especially for forecasts up to three hours, where the largest influence from the radar advection occurs. In an objective validation of the structure, amplitude and location of modelled precipitation, where the forecasts are compared to radar observations, these findings are confirmed. The same validation of model runs over a longer time period also clearly indicates that the amplitude, structure and location of the precipitation patterns are significantly improved as compared to a short-range forecast from the operational forecast model used at SMHI. (C) 2010 Elsevier B.V. All rights reserved.

  • 29. Stengel, M.
    et al.
    Lindskog, Magnus
    SMHI, Forskningsavdelningen, Meteorologi.
    Unden, Per
    SMHI, Forskningsavdelningen, Meteorologi.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    The impact of cloud-affected IR radiances on forecast accuracy of a limited-area NWP model2013Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 139, nr 677, s. 2081-2096Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The impact of cloud-affected satellite radiances on numerical weather prediction (NWP) accuracy is investigated. The NWP model used is the HIgh Resolution Limited Area Model (HIRLAM). Its four-dimensional variational data assimilation (4D-Var) system was used to assimilate cloud-affected infrared (IR) radiances from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI). Cloud parameters are modelled internally in the observation operator and used in the radiative transfer calculations. The interaction between the cloud parameters and the model control vector variables is incorporated in the adjoint version of the observation operator, which is used to derive cloud-affected Jacobians prior to the inner-loop minimization of the cost function. The developed framework supports an extensive usage of satellite observations with spatial coverage extended into cloudy regions, which therefore provides additional analysis increments and supports a more accurate description of the atmospheric state. In extended assimilation and forecast experiments the total number of assimilated satellite observations could be increased by approximately 10%. This was associated with a clear indication of a positive impact of cloud-affected radiances on the moisture and geopotential height fields of the NWP model analysis and forecast accuracy when used on top of clear-sky radiance observations. This is revealed by reduced analysis errors of the total integrated water vapour and by reduced forecast errors in the mid and upper troposphere.

  • 30. Stengel, M.
    et al.
    Lindskog, Magnus
    SMHI, Forskningsavdelningen, Meteorologi.
    Unden, Per
    SMHI, Forskningsavdelningen, Meteorologi.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Bennartz, R.
    An extended observation operator in HIRLAM 4D-VAR for the assimilation of cloud-affected satellite radiances2010Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 136, nr 649, s. 1064-1074Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An extended observation operator for the direct assimilation of cloud-affected infrared satellite radiances in the High Resolution Limited Area Model (HIRLAM) is examined. The operator includes a simplified moist-physics scheme, which enables the diagnosis of cloudiness in itself using background values of temperature, moisture and surface pressure. Subsequently, a radiative transfer model provides simulated cloud-affected radiances to be used as background equivalents to the satellite observations. The observation operator was evaluated by using infrared observations measured by the Spinning Enhanced Visible and Infrared Imager (SEVIRI). An observation-screening procedure, which incorporates SEVIRI cloud-retrieval products, supports an improved selection of usable cloudy scenes, leading to good agreement between the observations and background equivalents. The tangent-linear observation operator was verified against finite differences from its nonlinear formulation. The increments revealed a near-linear behaviour for the selected channels for a large number of cases. The adjoint observation operator was used to derive brightness-temperature sensitivities with respect to temperature and moisture changes in the presence of radiance-affecting clouds. Differences from the clear-sky sensitivities were found in and below clouds. In a four-dimensional variational data assimilation experiment, cloud-affected SEVIRI observations were assimilated, resulting in additional increments in both moisture and wind fields. The corresponding analysis fields revealed a reduced deviation from the observations for the majority of all cloudy scenes and a reduced bias for wind and temperature in the upper troposphere against independent radiosonde observations. Overall, our results highlight the capability of this observation operator in the HIRLAM assimilation system and encourage its application for the extended usage of cloudy satellite observations in numerical weather prediction. Copyright (C) 2010 Royal Meteorological Society

  • 31. Stengel, M.
    et al.
    Undén, Per
    SMHI, Forskningsavdelningen, Meteorologi.
    Lindskog, Magnus
    SMHI, Forskningsavdelningen, Meteorologi.
    Dahlgren, Per
    SMHI, Forskningsavdelningen, Meteorologi.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Bennartz, R.
    Assimilation of SEVIRI infrared radiances with HIRLAM 4D-Var2009Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 135, nr 645, s. 2100-2109Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Four-dimensional variational data assimilation (4D-Var) systems are ideally suited to obtain the best possible initial model state by utilizing information about the dynamical evolution of the. atmospheric state from observations, such as satellite measurements, distributed over a certain period of time. In recent years, 4D-Var systems have been developed for several global and limited-area models. At the same time, spatially and temporally highly resolved satellite observations, as for example performed by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) on board the Meteosat Second Generation satellites, have become available. Here we demonstrate the benefit of a regional NWP model's analyses and forecasts gained by the assimilation of those radiances. The 4D-Var system of the High Resolution Limited Area Model (HIRLAM) has been adjusted to utilize three of SEVIRI's infrared channels (located around 6.2 mu m, 7.3 mu m, and 13.4 mu m, respectively) under clear-sky and low-level cloud conditions. Extended assimilation and forecast experiments show that the main direct impact of assimilated SEVIRI radiances on the atmospheric analysis were additional tropospheric humidity and wind increments. Forecast verification reveals a positive impact for almost all upper-air variables throughout the troposphere. Largest improvements are found for humidity and geopotential height in the middle troposphere. The observations in regions of low-level clouds provide especially beneficial information to the NWP system, which highlights the importance of satellite observations in cloudy areas for further improvements in the accuracy of weather forecasts. Copyright (C) 2009 Royal Meteorological Society

  • 32. Sundstrom, Nils
    et al.
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Kruglyak, Andrey
    Lundberg, Angela
    Field evaluation of a new method for estimation of liquid water content and snow water equivalent of wet snowpacks with GPR2013Ingår i: HYDROLOGY RESEARCH, ISSN 1998-9563, Vol. 44, nr 4, s. 600-613Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Estimates of snow water equivalent (SWE) with ground-penetrating radar can be used to calibrate and validate measurements of SWE over large areas conducted from satellites and aircrafts. However, such radar estimates typically suffer from low accuracy in wet snowpacks due to a built-in assumption of dry snow. To remedy the problem, we suggest determining liquid water content from path-dependent attenuation. We present the results of a field evaluation of this method which demonstrate that, in a wet snowpack between 0.9 and 3 m deep and with about 5 vol% of liquid water, liquid water content is underestimated by about 50% (on average). Nevertheless, the method decreases the mean error in SWE estimates to 16% compared to 34% when the presence of liquid water in snow is ignored and 31% when SWE is determined directly from two-way travel time and calibrated for manually measured snow density.

  • 33. Wolters, L
    et al.
    Cats, G
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Wilhelmsson, T
    Data-parallel numerical methods in a weather forecast model1995Ingår i: Applied Numerical Mathematics, ISSN 0168-9274, E-ISSN 1873-5460, Vol. 19, nr 1-2, s. 159-171Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The results presented in this paper are part of a research project to investigate the possibilities to apply massively parallel architectures for numerical weather forecasting. Within numerical weather forecasting several numerical techniques are used to solve the model equations. This paper compares the performance of implementations on a MasPar system of two techniques, finite difference and spectral, that are adopted in the numerical weather forecasting model HIRLAM. The operational HIRLAM model is based on finite difference methods, while the spectral model is still in a research phase. Also the differences in relative performance of these methods on the MasPar and vector architectures will be discussed.

  • 34. Zaplotnik, Ziga
    et al.
    Zagar, Nedjeljka
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    An intermediate-complexity model for four-dimensional variational data assimilation including moist processes2018Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 144, nr 715, s. 1772-1787Artikel i tidskrift (Refereegranskat)
1 - 34 av 34
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