Ice supersaturation in the ECMWF Integrated Forecast System.

TitleIce supersaturation in the ECMWF Integrated Forecast System.
Publication TypeMiscellaneous
Year of Publication2005
AuthorsTompkins, AM, Gierens, K, Rädel, G
Secondary TitleTechnical Memorandum

A simple parameterisation for ice supersaturation is introduced into the ECMWF IFS model, compatible with the cloud scheme that allows partial cloud coverage. It is argued that many existing schemes that explicitly represent the ice nucleation process without prognosing the humidity evolution separately in the clear sky and cloudy portions of a model grid-cell are subject to arti?cial subgrid-scale horizontal water vapour transfer, possibly dominating the scheme's behaviour. Here this is avoided by making the simple but justifiable diagnostic assumption that the nucleation process is fast compared to a GCM timestep, thus supersaturation is only permitted in the clear-sky portion of the gridcell. Results from model integrations using the new scheme are presented. It is shown that the new scheme increases upper-tropospheric humidity, decreases high level cloud cover and to a much lesser extent cloud ice amounts, all as expected from simple arguments. The increases in humidity are expected to be beneficial, since much evidence points to a model dry bias in the upper troposphere. The relative distribution of supersaturation occurrence is found to reproduce the observed climatology derived from in situ aircraft observations very well. It is also demonstrated that the new scheme leads to improved predictions of permanent contrail cloud over Southern England, which indirectly implies upper-tropospheric humidity fields are better represented for this region.