Abstract
Abstract. This paper investigates the impact and potential use of the cut-cell vertical discretisation for forecasts covering five days and climate simulations. A first indication of the usefulness of this new method is obtained by a set of five-day forecasts, covering January 1989 with six forecasts. The model area was chosen to include much of Asia, the Himalayas and Australia. The cut-cell model LMZ (Lokal Modell with z-coordinates) provides a much more accurate representation of mountains on model forecasts than the terrain-following coordinate used for comparison. Therefore we are in particular interested in potential forecast improvements in the target area downwind of the Himalayas, over southeastern China, Korea and Japan. The LMZ has previously been tested extensively for one-day forecasts on a European area. Following indications of a reduced temperature error for the short forecasts, this paper investigates the model error for five days in an area influenced by strong orography. The forecasts indicated a strong impact of the cut-cell discretisation on forecast quality. The cut-cell model is available only for an older (2003) version of the model LM (Lokal Modell). It was compared using a control model differing by the use of the terrain-following coordinate only. The cut-cell model improved the precipitation forecasts of this old control model everywhere by a large margin. An improved, more transferable version of the terrain-following model LM has been developed since then under the name CLM (Climate version of the Lokal Modell). The CLM has been used and tested in all climates, while the LM was used for small areas in higher latitudes. The precipitation forecasts of the cut-cell model were compared also to the CLM. As the cut-cell model LMZ did not incorporate the developments for CLM since 2003, the precipitation forecast of the CLM was not improved in all aspects. However, for the target area downstream of the Himalayas, the cut-cell model considerably improved the prediction of the monthly precipitation forecast even in comparison with the modern CLM version. The cut-cell discretisation seems to improve in particular the localisation of precipitation, while the improvements leading from LM to CLM had a positive effect mainly on amplitude.
Highlights
The cut-cell approach has recently been investigated in a number of two-dimensional test models
It was shown that the impact of this scheme for five-day forecasts is considerable
As shown for shorter forecasts in Stal06, the vertical velocities and precipitation are more realistic for the LMZ model, when compared to the control model LM
Summary
The cut-cell approach has recently been investigated in a number of two-dimensional test models (see Steppeler et al, 2002; Dobler, 2005; Lock, 2008; Yamazaki and Satomura, 2008, 2010; and Walko and Avissar, 2008). The question arises, if for longer forecasts the cut-cell discretisation has a stronger impact on forecasts This question will have to be answered using a large number of consecutive forecasts and an up to date model physics scheme. A set of five-day forecasts is produced covering January 1989 with six forecasts This is a further step towards a test of cut cells for long-time forecasts and climate runs. Lateral boundary values from ERA-interim data are used Such model set-up is often used for model performance evaluation in climate impact studies. It includes the Himalayas and a large area downwind, including southeastern China, Korea and Japan In this target area we expect a strong impact of the cut-cell discretisation
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