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Journal Club: 20-11-2017

Article Title: North Atlantic Oscillation amplifies orographic precipitation and river flow in upland Britain

Published in the Water Resources Research journal in June 2013. Authors: T. P. Burt and N. J. K. Howden.

This article assessed the relationship between both precipitation and river flow and the North Atlantic Oscillation (NAO) in Britain. For this, the authors got station data from all over the UK mainland and computed the seasonal totals of precipitation and river flow. Afterwards they correlated every season’s total of those variables with the NAO index. Although it was previously known that the NAO has considerable effect on European precipitation, the authors detailed this knowledge for Britain, finding that NAO variations cause large differences in seasonal precipitation and river flow totals. Additionally, because the stations were relatively spread out through the region – including low land and upland areas – the authors found that the influence of the NAO on precipitation and river flow is higher on upland areas, an effect they named “double orographic enhancement”, and which has relevant implication to water management in the region (both for flood and drought situations).

Projections of precipitation(a)/flow change (b) [%] per NAO deviation from the neutral conditions. The x axis contains the winter precipitation/flow total, which is correlated with altitude (proxy for altitude, as demonstrated previously in the paper). The y axis contains the percentage of enhancement. The (c) and (d) plots were estimated for the highest (blue) and lowest (red) NAO index on record, based on the regression coefficients found previously in the paper for the rate of change of precipitation/flow per unit change of the NAO index, for every precipitation/flow total.  

 The authors also performed similar preliminary analysis on other areas of the world both influenced by large scale patterns and with orographic arrangements (Pacific North West of the USA and Sri Lanka, both influenced by the El Nino Southern Oscillation). This was done to confirm that the “double orographic enhancement” effect was also present in other locations facing a similar set of conditions as Britain (high interannual variability of precipitation due to large scale patterns and high spatial variability of precipitation due to wind direction, orography and rain-shadow effects).

 

Conclusions:

The North Atlantic Oscillation has considerable influence on interannual variability of precipitation in Britain, an effect which is amplified with altitude.

Relevancy for our research:

To some extent, understanding precipitation variability is understanding solar radiation variability (as both depend highly on cloudiness), which is one of the focus of our group. This work, although not directly related to our research, has given insights on how the impact of a large scale variability pattern such as the North Atlantic Oscillation on some meteorological variables can vary drastically over small regions, as a result of interaction between atmosphere and topography.

 

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Journal Club: 18-04-2018

Article Title: Application of numerical weather prediction in wind power forecasting: Assessment of the diurnal cycle

This paper investigated the performance of different NWP models at forecasting the diurnal pattern of wind speeds at hub height in Germany. Analysis of data from German TSOs found that large biases tend to occur in the early morning. Output from 4 deterministic NWP models and a regional ensemble were compared to observations from masts in Germany/Netherlands/North Sea.

Forecast and observed wind speeds at different heights at Falkenberg mast, June 2015, for (a) ICON global, (b) ICON EU, (c) COSMO-EU and (d) COSMO-DE models. Red and grey shading represent positive and negative biases respectively at a height of 98m.

COSMO-EU and COSMO-DE were found to have larger negative biases overnight when forecasting a day ahead (larger area of grey shading above). This suggests that the nocturnal boundary layer in these models does not stabilise sufficiently allowing faster wind speeds to develop away from the surface. All models also produce positive biases in the early morning, indicating that there is a timing issue with the transition out of the nocturnal layer as convective activity begins to occur.

Ensemble bias for different lead times at (a) onshore and (b) offshore locations. Black line represents ensemble mean.

Additionally, analysis of ensemble output found a difference in performance of the model over land and over sea. Onshore sites exhibited a strong diurnal cycle in mean error (left plot) while the bias is more steady for offshore sites (right)

Applications to our research:

  • Use Eirgrid data to search for similar patterns in Ireland.
  • Good ideas for evaluating model performance.
  • Potentially use techniquees to identify some model deficiencies in WRF and/or HARMONIE forecasts.
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EMS2017: Variations in wind-solar correlations across multiple datasets

At EMS2017 in DCU, Dublin in September 2017 I presented a poster titled: Variations in wind-solar correlations across multiple datasets. This was part of the Energy Meteorology session (OSA2.6).  This examined the accuracy of 2 reanalysis datasets  (ERA-Interim and MERRA2) at replicating the correlation relationship between 10m wind speed and incoming shortwave radiation (SW). It was found that both reanalysis tend to overestimate the strength of negative correlations compared to values calculated from observations. Additionally when the data was binned by wind direction, it was found that correlation values are seen to vary with different wind directions. The final part of the poster looked at mast data in the Netherlands to investigate whether the correlation values vary with height. It was found that correlations vary more with height in observations compared to both reanalyses.