Joint Seminar: Why and how should we reduce uncertainties in surface drag processes?

The flow of the atmosphere is strongly influenced by various features on the Earth’s surface: vegetation, buildings, hills, mountains, ice ridges or ocean waves all slow down or deflect the airflow in a variety of ways. Close to the surface, the air is slowed by friction and deflected by larger obstacles, such as mountains. Atmospheric gravity waves triggered by mountains propagate upward and slow down the flow where they break, generally in the upper troposphere and in the stratosphere. All these processes contribute to the drag exerted by the Earth’s surface on the atmosphere. Despite their importance for the large-scale circulation, to date the representation of drag processes remains a major source of uncertainty in numerical weather prediction (NWP) and climate models. Among the different drag processes the representation of orographic drag is particularly challenging. The need to reduce the uncertainties in the representation of surface drag is well recognized and this has become in recent years one of the foci of both the Working Group on Numerical Experimentation (WGNE) and the Global Atmospheric System Studies (GASS) of the World Meteorological Organization.

This seminar gives an overview of the work done at several modelling centres and universities (ECMWF, MetOffice, Environment Canada, NCAR, University of Reading, University of East Anglia), but also in the framework of WGNE, that helped revive the interest of the community for this important topic. I will discuss how the representation of surface drag processes in NWP and climate models, and the uncertainties related to it affect the large scale circulation and to what extent it can explain known circulation biases at different timescales. A particular focus will be on orographic drag processes, for which I will also discuss how progress can be made by using kilometer-scale simulations of complex mountain ranges that now become feasible. At kilometre-scale resolutions, the orographic low-level blocking and gravity wave processes become largely resolved. This confers an exciting opportunity for constraining the parametrizations used for representing these processes in NWP and climate models. Examples will be shown from a recent study (Van Niekerk et al., 2018) using a hierarchy of numerical simulations to explore the circulation impacts of resolved versus parametrized orographic drag over some of the most complex mountain ranges on Earth, the Himalayas and the Middle East mountains. A protocol based on this study is now used for an inter-comparison exercise in the framework of GASS and WGNE, at which most major NWP and climate centres will participate.




13:30 h


Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg


Irina Sandu, ECMWF


Daniel Klocke

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