Mitigating surface temperature errors using approximate radiation updates

TitleMitigating surface temperature errors using approximate radiation updates
Publication TypeTechnical memorandum
Date Published02/2015
Secondary TitleECMWF Technical Memoranda
AuthorsHogan, R, Bozzo, A

Due to computational expense, the current radiation scheme in the IFS is called infrequently in time (every hour in the high resolution forecast and every three hours in the ensemble system) and on a reduced spatial grid. This can lead to large surface temperature errors at coastal land points due to surface fluxes computed over the ocean being used where the skin temperature and surface albedo are very different. It can also lead to a lag in the diurnal cycle of surface temperature. This memorandum describes a computationally efficient solution to these problems, in which the surface longwave and shortwave fluxes are updated every timestep and gridpoint according to the local skin temperature and albedo. In order that energy is conserved, it is necessary to compute the change to the net flux profile consistent with the changed surface fluxes. The longwave radiation scheme has been modified to compute also the rate of change of the profile of upwelling longwave flux with respect to the value at the surface. Then at each gridpoint and timestep, the upwelling flux and heating-rate profiles are updated using the new value of skin temperature. The computational cost of performing approximate radiation updates is only 2% of the cost of the full radiation scheme, so increases the overall cost of the model by only of order 0.2%. Testing the new scheme by running daily 5-day forecasts over an eight-month period reveals significant improvement in 2-m temperature forecasts at coastal stations compared to observations

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