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Key changes


A major upgrade to the Met Office Global Numerical Weather Prediction model was implemented on 7 August 2002. The package of changes was under trial for over a year and is known as 'New Dynamics'. This document details some of the key changes that are part of the New Dynamics package.

  • Non-hydrostatic model with height as the vertical co-ordinate. Full equations are used with virtually no approximations. The new model is suitable for running at very high resolution.

  • Charney-Philips grid-staggering in the vertical, i.e. potential temperature is on the same levels as the vertical velocity, including top and bottom boundaries where the vertical velocity is zero. This gives an improved thermal wind balance, no computational mode, better coupling with the physics and the data assimilation, less noise and better stability.

  • Arakawa C-grid staggering in the horizontal, i.e. the u component is east-west, staggered from temperatures and the v component is north-south staggered. This brings improved geostrophic adjustment, no grid-splitting, better coupling with the physics and the data assimilation, less noise and better stability.

  • Two time-level, semi-Lagrangian advection and semi-implicit time stepping. This brings: greater accuracy, efficiency (with a long time-step), shape preservation and conservation; reduced filtering; better coupling with the physics and the data assimilation; less noise and better stability.

  • Edwards-Slingo radiation scheme with non-spherical ice spectral files. Ice crystals are modelled as planar polycrystals with sizes related to the temperature (Kristjansson et al., 2000).
    Gaseous transmission is treated by using correlated-k methods (Cusack et al., 1999) with six bands in the short wave, nine in the long wave (Cusack et al. have eight in the long wave, but we split one of these in HadAM4 and this configuration has gone into the New Dynamics model). The CKD continuum model is used (Clough, 1989). Fractional cloud is treated as in Geleyn and Hollingsworth (1979) with convective cloud distinguished from large-scale cloud.

  • Large-scale precipitation includes prognostic ice microphysics. The new scheme employs a more-detailed representation of the microphysics occurring within clouds. Water is contained in vapour, liquid, ice and rain categories, with physically based parametrization of transfers between the categories. The ice content becomes a prognostic variable within the model, rather than one diagnosed from a cloud scheme (Wilson and Ballard, 1999).

  • Vertical gradient area large-scale cloud scheme. The standard Smith large-scale cloud scheme returns a cloud volume fraction which is assumed to take up the entire vertical depth of the gridbox and is therefore equal to the cloud area fraction. The vertical gradient method performs the standard Smith cloud calculation at three heights per gridbox (on the grid level and equispaced above and below it), using interpolation of input data according to the estimated sub-grid vertical profiles. Weighted means are then used to calculate the volume data for the gridbox, while the area cloud fraction is taken to be the maximum sub-grid value. This modification allows the area cloud fraction to exceed the volume fraction and, hence, the radiation scheme, which uses area cloud, can respond to larger cloud area coverage and smaller in-cloud liquid water paths than the standard scheme would produce.

  • Convection with convective available potential energy (CAPE) closure, momentum transports and convective anvils. Diagnosis of deep and shallow convection are included, based on the boundary-layer type diagnosis adopted in the Lock et al. (2000) boundary-layer scheme. Convective cloud base is defined at the local condensation level (and boundary-layer scheme prevented from operating above this, so it no longer overlaps with convection scheme).
    There is a new parametrization for convective momentum transports, based on a flux-gradient relationship. This is obtained from the stress budget by parametrizing the terms (by analogy with scalar flux budgets) such that there is a gradient term associated with the mean wind shear (involving an eddy viscosity) and a non-gradient term associated with the transport (using a mass flux approximation).

    There are also new cloud-base closures for thermodynamics and momentum transport. The thermodynamic closure for shallow convection follows Grant (2001) in relating the cloud-base mass flux to a convective velocity scale. For deep convection, the thermodynamic closure is based on the reduction to zero of CAPE over a given timescale (based on Fritsch and Chappell, 1980). These closures replace the standard buoyancy closure, which has been found to be both noisy and unreliable. The momentum transport closure for deep and shallow convection is based on the assumption that large-scale horizontal pressure gradients should be continuous across cloud base.
    Parametrized entrainment and detrainment rates for shallow convection are obtained (Grant and Brown, 1999) using similarity theory by assuming that the entrainment rate is related to the rate of production of turbulent kinetic energy (TKE).

  • A boundary-layer scheme which is non-local in unstable regimes. Explanation: The vertical diffusion coefficients are specified functions of height over a diagnosed mixed-layer depth that are scaled by both the surface and cloud-top turbulence forcing. It also includes an explicit parametrization of entrainment at the boundary-layer top.

    Rationale: There is more direct physical coupling between the turbulence forcing of unstable boundary layers and the transports generated within them (rather than the Richardson-based scheme that relates fluxes to the local gradients within the layer). It is numerically more robust.

  • Gravity-wave drag scheme which includes flow blocking. Strictly, the new parametrization is best described as a sub-grid orography scheme. It consists of a gravity-wave drag (GWD) bit (due to flow over) and a non-GWD bit (the flow-blocking bit that is due to flow around).

    The new sub-gridscale orography (SSO) scheme uses a simplified GWD scheme and includes a flow-blocking scheme. The new scheme is thus more robust and applies much more drag at low levels.

  • GLOBE orography dataset. The US Navy 10-minute orography data have been replaced with 30 arc-second (~1 km) GLOBE orography data, averaged to 10 minutes. The US Navy data have been used for over 30 years, but they are known to have many deficiencies. The GLOBE data set is currently the best orography data set that is freely available and is far superior to the Navy data set. Before it is used in the model, the data are filtered using a sixth-order low-pass implicit tangent filter, constrained so that the filtering is isotropic in real space.

  • The MOSES (Met Office Surface Exchange Scheme) surface hydrology and soil model scheme. This is already running in the operational model (Cox et al., 1999).

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Background to the Development of the New Dynamics

References

Clough, S.A., Kneizys, F.X. and Davies, R.W., 1989: Line shape and the water vapor continuum, Atmos Res, 23, 229-241.

Cox, P.M., Betts, R.A., Bunton, C.B., Essery, R.L.H., Rowntree, P.R. and Smith, J., 1999: The impact of new land surface physics on the GCM simulation of climate and climate sensitivity. Clim Dyn, 15, 183-203.

Cusack, S. Edwards, J.M. and Crowther, J.M., 1999: Investigating k distribution methods for parameterizing gaseous absorption in the Hadley Centre Climate Model. J Geophys Res (Atmos), 104, 2,051-2,057.

Fritsch, J.M. and Chappell, C.F., 1980: Numerical prediction of convectively driven mesoscale pressure systems. Part I. Convective parameterization. Part II. Mesoscale model. J Atmos Sci, 37, 1,722-1,762.

Geleyn, J.-F. and Hollingsworth, A., 1979: An economical analytical method for the computation of the interaction between scattering and line absorption of radiation.Beitr Phys Atmos, 52, 1-16.

Grant, A.L.M., 2001: Cloud-base fluxes in the cumulus-capped boundary layer. Quart J R Meteorol Soc, 127, 407-421.

Grant, A.L.M. and Brown, A.R., 1999: A similarity hypothesis for shallow-cumulus transports. Quart J R Meteorol Soc, 125, 1913-1936.

Kristjansson, J.E., Edwards, J.M. and Mitchell, D.L., 2000: Impact of a new scheme for optical properties of ice crystals on climates of two GCMs. J Geophys Res (Atmos), 105, 10,063-10,079.

Lock, A.P., Brown, A.R., Bush, M.R., Martin, G.M. and Smith, R.N.B., 2000: A new boundary layer mixing scheme. Part I. Scheme description and single-column model tests. Mon Weather Rev, 128, 3,187-3,199.

Wilson, D.R. and Ballard, S.P., 1999: A microphysically based precipitation scheme for the UK Meteorological Office Unified Model. Quart J R Meteorol Soc, 125, 1,607-1,636.

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Background to the Development of the New Dynamics

 

 
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