Q. J. R. Meteorol. Soc. (2003), 129, pp. 1989–2010 doi: 10.1256/qj.02.133 Improvements to the representation of orography in the Met Ofce Unied Model By S. WEBSTER ¤ , A. R. BROWN, D. R. CAMERON and C. P. JONES Met Ofce, UK (Received 14 June 2002; revised 22 November 2002) SUMMARY Three improvements to the representation of orography for use in numerical weather- and climate-prediction models are presented. The rst improvement is to replace the US Navy dataset with a new digitally generated dataset as the denition of the true earth topography. There are large differences on all scales between the two datasets and these lead to large differences in the mean and subgrid-scale elds that are derived from them. The second improvement is to lter the mean and subgrid-scale orography (SSO) elds to remove grid-scale and near- grid-scale features and thus suppress forcing on scales that the model cannot treat well. The third improvement is to implement a new, simple parametrization of the effects of SSO in which the total surface pressure drag is calculated using the analytical expression for linear hydrostatic ow over a two-dimensional ridge in the absence of friction and rotation. The surface pressure drag is partitioned into gravity-wave and blocked-ow components that depend on the Froude number of the ow impinging on the SSO. The new scheme attributes about 70% of the total drag to ow blocking. These improvements have been incorporated into a new version of the Met Ofce Unied Model. A series of numerical weather-prediction experiments demonstrates that the introduction of the new SSO scheme is the most signicant change. In particular, signicant improvements to forecast skill, attributable to the SSO scheme’s ow-blocking drag component, are found at low levels in the northern hemisphere and the Tropics for an extended northern hemisphere wintertime forecast trial. Furthermore, there are no signicant degradations in skill at upper levels, in the southern hemisphere or for summertime trials. KEYWORDS: Flow blocking Gravity-wave drag Parametrization Smoothing 1. I NTRODUCTION The representation of orography has long been known to be crucial to the perfor- mance of global circulation models (GCMs). Palmer et al. (1986) and McFarlane (1987) were the rst to show clearly the benets of augmenting the resolved GCM orography with a represention of the effects of subgrid-scale orography (SSO). This they did by using a gravity-wave-drag (GWD) parametrization that accounted for the drag induced by the breaking or absorption of linear hydrostatic gravity waves excited by air owing over the SSO. Typically, most of the drag applied by these original GWD schemes was exerted in the lower stratosphere. More recently, further improvements in the representation of the effects of SSO have been made by additionally accounting for various tropospheric SSO drag mecha- nisms. Thus, as described in Gregory et al. (1998, hereafter GSM98), the SSO scheme used in the old version of the Met Ofce Unied Model (UM) from January 1995 until August 2002 (hereafter the 95 scheme) includes a representation of tropospheric GWD due to hydraulic jumps and trapped lee waves, whilst at low levels a representation of the impact of SSO on boundary-layer drag is also included. Elsewhere, Lott and Miller (1997, hereafter LM97) have shown that the numerical weather-prediction performance of a similar conguration, but without hydraulic jumps, was signicantly improved by including an explicit low-level ow-blocking scheme. More recently, Scinocca and Mc- Farlane (2000, hereafter SM00) have shown the benet of including a ow-blocking scheme in their global climate simulations. Thus, the absence of a ow-blocking scheme in the 95 scheme described previously appears to be a distinct weakness. One of the ¤ Corresponding author: Met Ofce, London Road, Bracknell, Berkshire RG12 2SZ, UK. e-mail: stuart.webster@metofce.com c ° Crown copyright, 2003. 1989