Modelling centres collaborate on orographic drag

Annelize van Niekerk (UK Met Office)
Irina Sandu (ECMWF)

  

A new modelling exercise, launched in autumn 2018 and jointly led by the UK Met Office and ECMWF, focuses on the representation of mountain effects on atmospheric flow. The exercise involves major numerical weather prediction and climate centres and is endorsed by the Global Atmospheric System Studies Panel of the Global Energy and Water Exchanges project and the Working Group for Numerical Experimentation (WGNE) of the World Meteorological Organization.

Motivation

Mountains play a vital role in the predictability of the atmosphere from weather to climate timescales. While large-scale mountain ranges (in the order of 100 km or more) are well resolved in models used for climate projection, seasonal forecasting and numerical weather prediction, smaller-scale mountain ranges (in the order of 5 km to 100 km) are generally unresolved. These small-scale mountains can generate gravity waves that grow in amplitude as they propagate vertically, decelerating the flow in the stratosphere when they break. Small-scale mountains also deflect the flow near the surface, therefore acting as a drag on the atmosphere both locally and remotely. It is now widely acknowledged that these unresolved orographic drag processes significantly impact key aspects of the large-scale atmospheric circulation. As a result, they are approximated in models through parametrizations that have, thus far, relied mostly on idealised modelling, linear theory and empirical relations. Owing to the difficulties in directly measuring gravity wave momentum fluxes and the drag that arises from non-linear interactions with orography near the surface, there are very few constraints on the magnitude and spatial distribution of simulated orographic drag processes. Consequently, the accuracy of orographic drag parametrizations is highly uncertain. This was also highlighted by the recent WGNE Drag project, which showed that, while the total parametrized surface stress was roughly similar across models, the magnitude of the contributing components varied greatly across models at similar resolutions.

Motivated by these findings, the new modelling exercise, called COnstraining Orographic DRag Effects (COORDE), proposes a model comparison that seeks to quantify the impact of resolved orography and parametrized orographic drag on the simulated circulation over complex mountainous regions. The main regions of interest are the Middle Eastern mountainous region and the Himalayas.

Nine modelling centres are participating: ECMWF, Environment Canada, the German national meteorological service (Deutscher Wetterdienst), the Japan Meteorological Agency, the Korea Institute of Atmospheric Prediction Systems, Météo-France, the US National Oceanic and Atmospheric Administration, the US National Center for Atmospheric Research, and the UK Met Office. The first model output was received at the end of January 2019 and results are currently being analysed.

Modelling framework

COORDE uses a novel modelling framework that was proposed in a recent study (Van Niekerk et al., J. Adv. Model Earth Sy., doi:10.1029/2018MS001417). In this study, kilometre-scale simulations over some of the most complex mountain chains on Earth (the Himalayas and Middle East mountains) were used to assess the ability of existing low-level blocking and gravity wave drag parametrizations to reproduce the explicitly resolved impacts on the flow. One question the study aimed to address is: to what extent do the circulation impacts of the parametrized orographic drag mimic those obtained when explicitly resolving the orography in high-resolution (kilometre-scale) simulations over such complex mountain ranges? To answer this question, short-range forecasts were produced with various configurations of the UK Met Office Unified Model and ECMWF’s Integrated Forecasting System. The impact of resolved orography on the circulation was deduced by calculating the difference between high-resolution simulations with a high-resolution (4 km to 9 km) and a low-resolution (125 km to 150 km) orography. This was then compared with the impact of parametrized orographic drag, deduced from global low-resolution (125 km to 150 km) simulations with and without parametrized orographic drag.

High- and low-resolution orography. High-resolution orography (left) and low-resolution orography (right) in the Middle East region, used in the high-resolution (4 km) Met Office Unified Model experiments. (Adapted from Van Niekerk et al., J. Adv. Model Earth Sy., doi:10.1029/2018MS001417, under the CC BY-NC-ND 4.0 licence)

At resolutions ranging from tens to hundreds of kilometres, relevant for predictions from a few weeks ahead to climate timescales, errors in simulated winds were shown to be due not only to the orographic drag parametrizations but also to the way the resolved dynamics interacts with the parametrized drag. This highlights the importance of physics–dynamics interactions. COORDE now aims to:

  • expose differences in orographic drag parametrization formulation between models
  • gain a better understanding of the impacts of different orographic drag parametrizations on modelled circulation
  • use high-resolution simulations to quantify drag from small-scale orography, typically unresolved in models used for integrations from monthly to climate timescales, to evaluate orographic drag parametrizations
  • investigate the interaction between the parametrized orographic drag and the resolved dynamics across models.

Further details about COORDE can be found at: https://osf.io/37bsy.