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.

Nonhydrostatic model with height as the vertical coordinate.
Full equations are used with virtually no approximations.
The new model is suitable for running at very high resolution.

CharneyPhilips gridstaggering 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 Cgrid staggering in the horizontal, i.e. the
u component is eastwest, staggered from temperatures and the
v component is northsouth staggered. This brings improved geostrophic
adjustment, no gridsplitting, better coupling with the physics
and the data assimilation, less noise and better stability.

Two timelevel, semiLagrangian advection and semiimplicit
time stepping. This brings: greater accuracy, efficiency (with
a long timestep), shape preservation and conservation; reduced
filtering; better coupling with the physics and the data assimilation;
less noise and better stability.

EdwardsSlingo radiation scheme with nonspherical 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 correlatedk 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 largescale cloud.

Largescale precipitation includes prognostic ice microphysics.
The new scheme employs a moredetailed 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 largescale cloud scheme. The standard
Smith largescale 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 subgrid 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 subgrid 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 incloud
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 boundarylayer
type diagnosis adopted in the Lock et al. (2000)
boundarylayer scheme. Convective cloud base is defined at the
local condensation level (and boundarylayer 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 fluxgradient 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 nongradient
term associated with the transport (using a mass flux approximation).
There are also new cloudbase closures for thermodynamics and
momentum transport. The thermodynamic closure for shallow convection
follows Grant (2001) in relating the cloudbase
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 largescale 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 boundarylayer scheme which is nonlocal in unstable regimes.
Explanation: The vertical diffusion coefficients are specified
functions of height over a diagnosed mixedlayer depth that are
scaled by both the surface and cloudtop turbulence forcing. It
also includes an explicit parametrization of entrainment at the
boundarylayer 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 Richardsonbased scheme
that relates fluxes to the local gradients within the layer).
It is numerically more robust.

Gravitywave drag scheme which includes flow blocking. Strictly,
the new parametrization is best described as a subgrid orography
scheme. It consists of a gravitywave drag (GWD) bit (due to flow
over) and a nonGWD bit (the flowblocking bit that is due to
flow around).
The new subgridscale orography (SSO) scheme uses a simplified
GWD scheme and includes a flowblocking scheme. The new scheme
is thus more robust and applies much more drag at low levels.

GLOBE orography dataset. The US Navy 10minute orography
data have been replaced with 30 arcsecond (~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 sixthorder lowpass 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
Clough, S.A., Kneizys, F.X. and Davies, R.W., 1989:
Line shape and the water vapor continuum, Atmos Res, 23, 229241.
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, 183203.
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,
2,0512,057.
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, 1,7221,762.
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, 116.
Grant, A.L.M., 2001: Cloudbase fluxes in the cumuluscapped
boundary layer. Quart J R Meteorol Soc, 127, 407421.
Grant, A.L.M. and Brown, A.R., 1999: A similarity
hypothesis for shallowcumulus transports. Quart J R Meteorol Soc,
125, 19131936.
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, 10,06310,079.
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 singlecolumn model tests. Mon Weather Rev,
128, 3,1873,199.
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, 1,6071,636.
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Background to the
Development of the New Dynamics
