Rivers in the Sky

... and tropical taps

News reports this month have spoken of record rainfall in normally sunny California, with more than half of the annual average rainfall falling in Los Angeles in just three days. The BBC website reported floods and landslides, including a video of a dramatic helicopter rescue from the Los Angeles River - its normally dry concreted channel the scene of many movie car chases. A family member who lives there confirmed the endless days of rain.

The initial culprits were two successive atmospheric rivers, which are vast streams of water vapour that cross oceans to drop rainfall on coastal hills and mountain ranges. Sometimes called ‘Rivers in the Sky’, they can be several hundred miles wide and thousands of miles long and some transport water faster than the Amazon or Nile. In the Pacific, they are sometimes said to result from a ‘tropical tap’, where the ‘tap’ lies in the tropical waters of the western Pacific, while those that begin near Hawaii are dubbed the ‘Pineapple Express’.

Estimated path of an atmospheric river that reached the UK in November 2009 (from Gimeno et al. 2014; see end of post for full credits)

Interestingly, their discovery was quite recent compared to that of other large scale features of the atmosphere such as tropical cyclones, jet streams, fronts and monsoons. Indeed, it wasn’t until around fifteen years ago that pioneering research brought them to public attention, and I certainly don’t remember them being mentioned when I lived in Los Angeles. They have since been detected along coastal regions in Norway, western South America, South Island in New Zealand, and the eastern USA.

Closer to home, researchers have shown that much of the rainfall during Storm Desmond in 2015 was due to an atmospheric river. This was one of the most severe storms in the UK in recent times, causing extensive flooding in Cumbria, Lancashire and southern Scotland. The Met Office website notes that 341.4mm of rainfall was recorded in 24 hours at Honister Pass – a new UK record – with 405mm recorded in 48 hours at Thirlmere.

As an aside, this led to one of my more bizarre work-related coincidences as I was flying out to southern Africa on the night that flooding began to help give a regional workshop on yes, you guessed it, flood risk management. It was both alarming and fascinating to see events unfold via social media and news reports, some of which we used as examples during the course. I remember one engineer joking that if only there was a way to get all that water to his country, it would provide almost a year’s typical rainfall. Later that month on a trip to Kendal, there was still a pile of rusting cars in a riverside carpark that had taken the full force of the flood.

More recently, atmospheric rivers have been implicated in heavy persistent rain in several more winter storms in the UK. The science has also continued to advance with California leading the way, including developing a formal warning service with an array of instruments ready to detect their arrival on both land and sea. The potential severity is gauged on a five point scale similar to the Fujita scale for tornado intensity and the Saffir-Simpson scale for classifying hurricanes.

Perhaps most impressively of all, routine research flights are made to see how widespread and severe incoming atmospheric rivers are likely to be, using both on board instruments and dropsondes, which fall to earth measuring relative humidities, air temperatures and wind speeds on their way. This approach is modelled on the much longer established flights used to monitor incoming hurricanes in the southeast of the USA.

Even in drier years, more than half of California’s annual average rainfall comes from atmospheric rivers which although causing floods is not all bad, helping to top up reservoirs and snowpack for water supplies in the dry summer months.

Full caption for figure: Composite Integrated Total Column of Water Vapor (IWV) between 00 and 18UTC 19 November 2009 showing an atmospheric river (AR) associated with extreme precipitation events that affected the United Kingdom (UK).Data: ERA-Interim. In Gimeno L, Nieto R,  Vázquez M, Lavers DA (2014), Atmospheric Rivers: A mini review, Frontiers in Earth Science, 5 March 2014 (Creative Commons Attribution Licence)