Impacts of parameterized orographic drag on the Northern Hemisphere winter circulation.
A recent intercomparison exercise proposed by the Working Group for Numerical Experimentation (WGNE) revealed that the parameterized, or unresolved, surface stress in weather forecast models is highly model-dependent, especially over orography. Models of comparable resolution differ over land by as much as 20% in zonal mean total subgrid surface stress (τtot). The wayτtotis partitioned between the different parameterizations is also model-dependent. In this study, we simulated in a particular model an increase inτtotcomparable with the spread found in the WGNE intercomparison. This increase was simulated in two ways, namely by increasing independently the contributions toτtotof the turbulent orographic form drag scheme (TOFD) and of the orographic low-level blocking scheme (BLOCK). Increasing the parameterized orographic drag leads to significant changes in surface pressure, zonal wind and temperature in the Northern Hemisphere during winter both in 10 day weather forecasts and in seasonal integrations. However, the magnitude of these changes in circulation strongly depends on which scheme is modified. In 10 day forecasts, stronger changes are found when the TOFD stress is increased, while on seasonal time scales the effects are of comparable magnitude, although different in detail. At these time scales, the BLOCK scheme affects the lower stratosphere winds through changes in the resolved planetary waves which are associated with surface impacts, while the TOFD effects are mostly limited to the lower troposphere. The partitioning ofτtotbetween the two schemes appears to play an important role at all time scales.
European Centre for Medium-Range Weather Forecasts Reading UK.
Irina
I
Sandu
European Centre for Medium-Range Weather Forecasts Reading UK.
Peter
P
Bechtold
European Centre for Medium-Range Weather Forecasts Reading UK.
Anton
A
Beljaars
European Centre for Medium-Range Weather Forecasts Reading UK.
Alessio
A
Bozzo
University of Reading Reading UK.
Felix
F
Pithan
University of Reading Reading UK.
Theodore G
TG
Shepherd
Environment Canada Dorval Quebec Canada.
Ayrton
A
Zadra
large‐scale circulation; orographic drag
J Adv Model Earth Syst. 2016 Mar;8(1):196-211
27668040