A new subgrid-scale orographic drag parametrization: Its formulation and testing

A scheme for the representation of subgrid-scale orography (SSO) in numerical weather prediction and climate models is presented. The new scheme arose in part from a desire to represent nonlinear low-level mountain drag effects not currently parametrized. An important feature of the scheme is that it deals explicitly with a low-level flow which is 'blocked', when the effective height of the subgrid-scale orography is sufficiently high. In this new scheme, it is assumed that, for this 'blocked' flow, separation occurs at the mountain flanks, resulting in a form drag. This drag is parametrized on model levels which are intersected by the SSO, and provides a dynamically based replacement for envelope orography. The upper part of the low-level flow goes over the orography and generates gravity waves. At the model resolutions considered (T106 and T213) it is assumed that the length scales characteristic of the SSO are sufficiently small for the Coriolis force to be neglected. The various parameters of the scheme are adjusted using an off-line procedure in which the scheme is used to estimate the mountain drag and the momentum profiles above the Pyrenees; and these estimates are validated with the PYREX data. Forecasts using T106 and T213 resolutions with this new scheme, and with mean orography, show that the forecast mountain drag consistently reproduces the drag measured during PYREX whenever the flow component normal to the ridge is large. Isentropic flow diagnostics, further, show that the new scheme has a realistic impact on the flow dynamics, reinforcing the low-level wake observed in mesoscale analyses of the flow. With this new scheme and a mean orography, the ECMWF model outperformed, in forecast skill, a version of the model which had an envelope orography and the old gravity-wave-drag scheme, while no longer suffering any disadvantages of envelope orography. The proposed low-level drag parametrization should also be relevant at model horizontal resolutions much higher than T213.