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Keywords:

  • Unified Model;
  • microphysics;
  • thunderstorm electrification;
  • convective clouds;
  • cold air outbreak;
  • aviation

Abstract

Helicopter-triggered lightning is a phenomenon which affects operations over the North Sea during the winter. It is thought that the presence of the helicopter triggers the majority of lightning strikes, since there is generally little or no natural lightning activity in the area in question prior to or following the strike, and strike rates are much higher than would be expected if due purely to chance. However, there has been little progress to date in the ability to predict triggered lightning strike occurrence with NWP data. Previous attempts have resulted in forecasts which are insufficiently discriminating (i.e. high false alarm rate) to be of practical use.

In this study, previous work on triggered lightning is reviewed and case studies are examined in order to identify common meteorological conditions for helicopter-triggered lightning strikes. Using forecast data from the Met Office Unified Model, an algorithm for triggered lightning risk was produced based on outside air temperature and precipitation rate. Evaluation against past helicopter strike cases has demonstrated that the new algorithm successfully forecasts lightning risk on 80% of occasions when triggered lightning occurred. In addition, the algorithm correctly forecast 8/9 natural lightning strikes which were observed in the operating area during winter 2010–2011. The areas of risk highlighted are usually small, which should allow helicopter operators to plan flights around high risk regions. The information in this study can also be used to inform helicopter operators of the likely conditions in which triggered lightning strikes occur. Copyright © 2012 British Crown Copyright, the Met Office. Published by John Wiley & Sons Ltd.