A Parameterization of Turbulent-Scale and Mesoscale Orographic Drag in a Global Atmospheric Model

Abstract

The effect of drag due to subgrid orography with turbulent scales, the so-called turbulent orographic form drag, is introduced by revising the surface exchange coefficient for momentum, to alleviate the overall positive bias in wind speed in the near-surface and low-level troposphere over land. Also, the excessive flow-blocking drag, responsible for negative biases in low-level wind speed over highly complex terrains, is abated by modifying the height of the blocked layer, which is achieved without a significant deficiency of total surface drag because of additional turbulent orographic form drag. The impact of the revised subgrid-scale orography parameterization is investigated on the medium-range forecast (January and July 2016) and seasonal ensemble prediction (June–August 2013 and December 2013 to February 2014) regarding stress partitioning and skill scores. With the revised subgrid-scale orography schemes, total surface stress and its partitioning resemble those of the ERA-Interim. Meanwhile, the turbulent (mesoscale orographic) stress is enhanced (reduced) due to the additional turbulent orographic form drag (weakened flow-blocking drag). For the medium-range forecast, systematic biases in near-surface and low-level wind speed are satisfactorily rectified by adding turbulent orographic form drag (reducing flow-blocking drag) in the regions with positive biases across the land (negative biases over highly complex terrains) against reanalysis, as well as observation data. Seasonal simulation is also improved because the weak polar night jet and the corresponding warm pole biases are alleviated by the reductions of the orographic gravity wave and resolved wave drag, which are driven by the revised low-level drag.