Accounting for moist processes in a sub‐grid orographic drag scheme

Abstract

AbstractA method for estimating the effective moist stability within a sub‐grid orographic drag scheme is described. It accounts for latent heat effects during low‐level saturated sub‐grid orographic ascent, with the lifting condensation level determining how much of the assumed sub‐grid ascents are saturated. The scheme has been verified for two cases with moist south to southwesterly low‐level flow impinging on mountain ranges of the USA and India, within both low‐resolution global simulations and high‐resolution limited‐area model simulations with degraded orography. It produces significant improvements to the model wind fields in both regions. In particular, the amount of low‐level orographic blocking predicted by the orographic drag scheme is reduced, particularly at low resolution, as are the associated large negative near‐surface wind biases over the mountains. Precipitation is shifted slightly downstream. A suite of 24 five‐day global NWP forecasts at N320 resolution was used to demonstrate that these results are robust, and that similar effects are seen over a number of mountain ranges. Over India, the convection scheme is very active in the global simulations, and has a larger effect on the winds at all levels than the orographic drag scheme. However the two schemes interact. Reducing the sub‐grid low‐level orographic blocking drag resulted in a reduction in the convergence occurring as the westerly flow decelerates towards the mountains. This decreased the amount of convection occurring over the sea upstream and along the coast, reducing the associated negative low‐level wind bias there, as well as reducing the negative wind biases over land.