1990 summary of changes

16 May 1990 The following changes were implemented in the forecast model cycle 35:
1.    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.
2.    Modifications to the treatment of snow covered surfaces:
o    the thermal budget of the snow is modified to take into account the effect of shade from vegetation;
o    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.
3.    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.
4.    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:


1.    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);
2.    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).

Cycle changes

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Prior to 2006 information on cycle upgrades has been migrated from the old website to the new website "as is". The information has not been reviewed during the migration.