TY - JOUR
T1 - What can we learn about orographic drag parametrisation from high-resolution models? A case study over the Rocky Mountains
AU - Vosper, Simon B.
AU - van Niekerk, Annelize
AU - Elvidge, Andrew
AU - Sandu, Irina
AU - Beljaars, Anton
PY - 2020/1/1
Y1 - 2020/1/1
N2 - Comprehensive high-resolution numerical weather prediction models provide a virtual laboratory for modelling the atmospheric flow over complex mountain ranges. In this study, global and regional simulations with horizontal grid spacing ranging from 2 to 32 km, focused over the northern Rocky Mountains, are used to assess the orographic blocking and gravity wave drag parametrisations employed in the Met Office Unified Model (UM) and the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (IFS). The total, resolved and parametrised drag components in coarse-resolution simulations are compared with those in high-resolution simulations, in which the orographic drag processes are better resolved. The total surface stresses and gravity wave momentum fluxes in the free atmosphere of the global 16 km UM and IFS simulations are shown to compare well with 2 km regional simulations in terms of variability and mean. While the total gravity wave momentum flux is somewhat underestimated by both global models, its vertical distribution is well captured. The “seamlessness” of the parametrisation scheme is then assessed by comparing the total orographic stress – and its components – across several horizontal resolutions of the UM. The surface stress remains relatively constant across resolutions, such that the reduction in resolved orographic stress at coarser resolutions is compensated for by an almost equivalent increase in parametrised orographic stress. However, the parametrised orographic gravity wave momentum flux in the free atmosphere remains almost constant with resolution, failing to compensate for the lack of resolved flux at coarse resolutions. This leads to an underestimation of the total gravity wave drag at coarser resolutions. Further analysis suggests that this underestimation is due to the monochromatic wave assumption made by the gravity wave drag parametrisation scheme.
AB - Comprehensive high-resolution numerical weather prediction models provide a virtual laboratory for modelling the atmospheric flow over complex mountain ranges. In this study, global and regional simulations with horizontal grid spacing ranging from 2 to 32 km, focused over the northern Rocky Mountains, are used to assess the orographic blocking and gravity wave drag parametrisations employed in the Met Office Unified Model (UM) and the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (IFS). The total, resolved and parametrised drag components in coarse-resolution simulations are compared with those in high-resolution simulations, in which the orographic drag processes are better resolved. The total surface stresses and gravity wave momentum fluxes in the free atmosphere of the global 16 km UM and IFS simulations are shown to compare well with 2 km regional simulations in terms of variability and mean. While the total gravity wave momentum flux is somewhat underestimated by both global models, its vertical distribution is well captured. The “seamlessness” of the parametrisation scheme is then assessed by comparing the total orographic stress – and its components – across several horizontal resolutions of the UM. The surface stress remains relatively constant across resolutions, such that the reduction in resolved orographic stress at coarser resolutions is compensated for by an almost equivalent increase in parametrised orographic stress. However, the parametrised orographic gravity wave momentum flux in the free atmosphere remains almost constant with resolution, failing to compensate for the lack of resolved flux at coarse resolutions. This leads to an underestimation of the total gravity wave drag at coarser resolutions. Further analysis suggests that this underestimation is due to the monochromatic wave assumption made by the gravity wave drag parametrisation scheme.
KW - gravity wave drag
KW - high-resolution modelling
KW - mountains
KW - numerical weather prediction
KW - orographic drag
UR - http://www.scopus.com/inward/record.url?scp=85078671556&partnerID=8YFLogxK
U2 - 10.1002/qj.3720
DO - 10.1002/qj.3720
M3 - Article
AN - SCOPUS:85078671556
SN - 0035-9009
VL - 146
SP - 979
EP - 995
JO - Quarterly Journal of the Royal Meteorological Society
JF - Quarterly Journal of the Royal Meteorological Society
IS - 727
ER -