16 May 1990 The following changes were implemented
in the forecast model cycle 35:
- A reduction of the run-off of convective rain, increasing the amount
of water available to wet the soil. The implied change of energy balance
at the surface increases the latent heat flux at the expense of the
sensible heat flux, giving a decrease in the surface and near-surface
air temperature. This should significantly reduce the warm bias of
the 2m temperature over continental areas during the day.
- Modifications to the treatment of snow covered surfaces:
- the thermal budget of the snow is modified to take into account
the effect of shade from vegetation;
- the albedo of the snow is no longer only dependent on snow depth;
the new formulation also takes into account masking by vegetation,
the effects of temperature and the presence of ice dew.
The overall effect is to decrease the albedo of the
snow covered areas.
- A modification to the model pressure-gradient calculation
and a change in the calculation of pressure level geopotential heights
by the model post-processing and in the first-guess for the analysis.
- Modifications to the convection scheme, mainly to the treatment
of cloud processes at detrainment levels for convective clouds, to
prevent a spurious moistening at cloud tops which was noticeable over
the subtropical oceans in connection with shallow convection.
21 March 1990 An error in the handling of the
climate fields of deepsoil temperature and deep-soil wetness was corrected
(these fields had been static since August 1989, instead of being updated
at the beginning of every month).
5 June 1990 The following changes were implemented
in the forecast model cycle 36:
- The parametrisation of surface fluxes at low wind speed over sea
was modified by replacing the transfer co-efficients for heat and
moisture in unstable conditions (free convection limit). The change
has had a considerable impact on the latent heat flux which, over
the Western Pacific (warm pool), can increase by up to 25 W/m2.
Synoptically it primarily affects the tropical flow which becomes
more realistic at lower levels (e.g. improved monsoon flow) as well
as at upper levels (reduced zonal men wind error);
- The formulation for the convective cloud cover was modified to account
for non-precipitating shallow cumuli. The effects of this change are
an increase of total cloud cover (most noticeable in the trades and
over the continents in daytime), a reduction of continental precipitation,
and enhancement of evaporation over subtropical oceans (by 5-10 W/m2).
