Predictability of orographic drag for realistic atmospheric profiles

Current parametrizations of sub-grid scale orographic drag are deterministic in nature. This makes sense since it is well known that the drag is predictable for steady flow over small hills (where linear theory is valid). However, even for steady flow over small hills, fine-scale vertical structure in the atmospheric profiles and/or non-linear effects can have a significant effect on the drag. For example, partial internal wave reflection can cause constructive or destructive interference, leading to a high or low drag state respectively. Because of the sensitivity of the drag to the details of the profiles it may be more appropriate to develop stochastic parametrizations of orographic drag.

We will present ongoing work to assess whether a stochastic, or partially-stochastic, approach should be adopted. We compare results from linear and non-linear numerical models of steady flow over orography for idealised and realistic atmospheric profiles to investigate the effect of fine-scale vertical structure and non-linear enhancement. Additionally we compare results from the two models for simulations conducted from radiosonde soundings to those using vertical profiles from Met Office analysis data (valid at the same time) to investigate whether the vertical structure of the atmosphere in the forecast model is sufficiently accurate to justify using a deterministic parametrization.