Improvements to the representation of orography in the Met Office Unified Model

Abstract

SUMMARYThree 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 de nition 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 neargrid-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 Of ce Uni ed Model. A series of numerical weather-prediction experiments demonstrates that the introduction of the new SSO scheme is the most signi cant change. In particular, signi cant 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 signi cant degradations in skill at upper levels, in the southern hemisphere or for summertime trials.

Stratospheric gravity waves revealed in NWP model forecasts

“…It became clear some years later that a substantial component of the missing surface drag was actually associated with mesoscale mountain barriers and flow-blocking dynamics rather than gravity wave dynamics (e.g. Webster et al, 2003). Nevertheless, orographic gravity wave drag is an important contributor to the momentum budget and our later results help to quantify this.…”

“…In the MetUM, orographic gravity waves are parametrized by the Webster et al (2003) scheme. In addition, the MetUM employs the Warner-McIntyre 'ultra-simple' spectral gravity wave parametrization scheme to represent the effects of non-orographic waves (Warner and McIntyre, 1999;Warner et al, 2005;Scaife et al 2000Scaife et al , 2002.…”

Mean and extreme precipitation over European river basins better simulated in a 25 km AGCM

“…HadGEM3-GA6.0 was the closest available model configuration to HadGEM3-GA3.0 at the time the sensitivity experiment was conducted. Differences between GA6.0 and GA3.0 include small adaptations to the semi-implicit semiLagrangian dynamical core from "New Dynamics" (Davies et al, 2005) to ENDGame (Even Newer Dynamics for General atmospheric modelling of the environment; Wood et al, 2014) and the new "5A" subgrid orographic drag parameterisation (Vosper, 2015;Wells, 2015) replacing the previous "4A" scheme (Webster et al, 2003).…”

A high‐resolution modelling case study of a severe weather event over New Zealand

“…The UM configuration used here is similar to that described in Davies et al (2005) and Webster et al (2003), and thus, employs an off-centred semiimplicit, semi-Lagrangian, two-time-level dynamical core. In all simulations, 38 levels are employed in the vertical with the model lid placed at about 39 km, and the highest vertical resolution concentrated near the ground such that 12 levels span the lowest 2 km of the atmosphere.…”

“…In all simulations, 38 levels are employed in the vertical with the model lid placed at about 39 km, and the highest vertical resolution concentrated near the ground such that 12 levels span the lowest 2 km of the atmosphere. The large-scale cloud and precipitation scheme is described in Wilson and Ballard (1999) and includes a prognostic treatment of ice microphysics, whilst the convection scheme is based on Gregory and Rowntree (1990) but, as described in Webster et al (2003), significant modifications to the basic scheme have been made.…”