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  • 101. van Meijgaard, E
    et al.
    Andrae, Ulf
    SMHI, Forskningsavdelningen, Meteorologi.
    Rockel, B
    Comparison of model predicted cloud parameters and surface radiative fluxes with observations on the 100 km scale2001Inngår i: Meteorology and atmospheric physics (Print), ISSN 0177-7971, E-ISSN 1436-5065, Vol. 77, nr 1-4, s. 109-130Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Cloud parameters and surface radiative fluxes predicted by regional atmospheric models are directly compared with observations for a 10-day period in late summer 1995 characterized by predominantly large-scale synoptic conditions. Observations of total cloud cover and Vertical cloud structure are inferred from measurements with a groundbased network of Lidar ceilometers and IR-radiometers and from satellite observations on a 100 kilometer scale. Groundbased observations show that at altitudes below 3 km, implying liquid water clouds, there is a considerable portion of optically non-opaque clouds. Vertical distributions of cloud temperatures simultaneously inferred from the groundbased infrared radiometer network and from satellite can only be reconciled if the occurrence of optically thin cloud structures at mid- and high tropospheric levels is assumed to be frequent. Results of three regional atmospheric models, i.e. the GKSS-REMO, SMHI-HIRLAM. and KNMI-RACMO, are quantitatively compared with the observations. The main finding is that all models predict too much cloud amount at low altitude below 900 hPa, which is then compensated by an underestimation of cloud amount around 800 hPa. This is likely to be related with the finding that all models tend to underestimate the planetary boundary layer height. All models overpredict the high-level cloud amount albeit it is difficult to quantify to what extent due to the frequent presence of optically thin clouds. Whereas reasonably alike in cloud parameters, the models differ considerably in radiative fluxes. One model links a well matching incoming solar radiation to a radiatively transparent atmosphere over a too cool surface, another model underpredicts incoming solar radiation at the surface due to a too strong cloud feedback to radiation, the last model represents all surface radiative fluxes quite well on average: but underestimates the sensitivity of atmospheric transmissivity to cloud amount.

  • 102. Wolters, L
    et al.
    Cats, G
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    Wilhelmsson, T
    Data-parallel numerical methods in a weather forecast model1995Inngår i: Applied Numerical Mathematics, ISSN 0168-9274, E-ISSN 1873-5460, Vol. 19, nr 1-2, s. 159-171Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 103. Zaplotnik, Ziga
    et al.
    Zagar, Nedjeljka
    Gustafsson, Nils
    SMHI, Forskningsavdelningen, Meteorologi.
    An intermediate-complexity model for four-dimensional variational data assimilation including moist processes2018Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 144, nr 715, s. 1772-1787Artikkel i tidsskrift (Fagfellevurdert)
  • 104. Zilitinkevich, S S
    et al.
    Perov, Veniamin
    SMHI, Forskningsavdelningen, Meteorologi.
    King, J C
    Near-surface turbulent fluxes in stable stratification: Calculation techniques for use in general-circulation models2002Inngår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 128, nr 583, s. 1571-1587Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Practically oriented flux-calculation techniques based on correction functions to the neutral drag and heat/mass transfer coefficients are further developed. In the traditional formulation, the correction functions depend only on the bulk Richardson number. However, data from measurements of turbulent fluxes and mean profiles in stable stratification over different sites exhibit too strong variability in this type of dependencies. Indirect evidence from climate and weather prediction modelling also shows that the traditional flux-calculation technique is not sufficiently advanced. It is conceivable that other mechanisms besides the surface-layer stratification and, therefore, other arguments besides the bulk Richardson number must be considered. The proposed technique includes a newly discovered effect of the static stability in the free atmosphere on the surface-layer scaling and accounts for the general essential difference between the roughness lengths for momentum and scalars. Besides bulk Richardson number, recommended correction functions depend oil one more stability parameter. involving the Brunt-Vaisala frequency in the free atmosphere, and on the roughness lengths.

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