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Harvey & Irma, Part One: Attribution, Precipitation, & Flooding












The U.S. and Caribbean at night with Katia, Irma, and Jose (left to right). Image from NASA.

The NOAA CPO Modeling, Analysis, Predictions, and Projections (MAPP) program hosted a webinar on research on the topic of Harvey and Irma: Attribution, Precipitation, and Flooding on Wednesday, September 27, 2017. The announcement is provided below.

Date/Time Title & Presenters
September 27, 2017
12:00 PM – 2:00 PM ET
Harvey & Irma, Part One: Attribution, Precipitation, & Flooding
Speakers and Topics Adam Sobel (Columbia University)
Event attribution: the NAS 2016 report and comments on Harvey

Tom Knutson and Sarah Kapnick (NOAA OAR Geophysical Fluid Dynamics Laboratory)
Hurricanes Harvey and Irma: Did climate change play a role?

John Nielsen-Gammon (Texas A&M University)
Harvey: An exceptional event

Amir AghaKouchak (University of California, Irvine)
Compounding effects of ocean and fluvial flooding in a warming climate

Youlong Xia (NOAA NWS/NCEP Environmental Modeling Center)
Using the operational and the experimental NLDAS monitoring systems to investigate the impact of Hurricane Harvey and Irma on flooding

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Rainfall from Tropical Cyclone Harvey as of August 28, 2017. Image from NOAA/NWS


Adam Sobel – I will briefly summarize key points from the National Academy report Attribution of Extreme Weather Events in the Context of Climate Change published in 2016. I will argue that in general, we should no longer repeat the old chestnut “one can’t attribute any single event to climate change,” at least not without qualification. One can never attribute any single event *exclusively* to climate change, as there are always multiple causes, but attribution science provides a basis for more probabilistic statements quantifying the role of climate change as one of them. I will then make a few remarks on Hurricanes Harvey and Irma. As no formal attribution studies have yet been done on these events – and in fact doing them is probably just at the edge of what is possible with today’s methods – one cannot justify strong or precise statements at present. However, based on our broader understanding of similar events and their relations to climate, there are some things that can be said.

Tom Knutson and Sarah Kapnick – Hurricane Irma was an exceptionally intense storm.  It likely set a global record for the satellite era as the longest duration that a tropical cyclone has maintained surface wind speeds of at least 185 mph. There is indirect evidence that the severity of some of Irma’s impacts, specifically storm surge levels and rainfall rates, were increased by anthropogenic climate change through increased sea level and atmospheric moisture. However,  increased hurricane surge levels and hurricane rainfall rates have not been clearly detected in observed climate data. Scientists expect that Atlantic hurricanes will become more intense on average as the climate continues to warm, but there is no clear observational evidence to date for this connection.  Climate change is expected to increase the global frequency of intense storms similar to Irma in the future, but quantifying the effect remains difficult, and there is only low confidence for such an increase in the Atlantic basin. 

John Nielsen-Gammon (Co-author: Brent McRoberts) – The real-time multi-sensor daily rainfall estimates produced by NOAA’s River Forecast Centers are used to place the extent and severity of heavy rainfall associated with Harvey into historical context.  Over a wide range of sizes, ranging from smaller than Rhode Island to larger than Pennsylvania, the average rainfall produced by Harvey over multiple days appears to have been unprecedented among measured storms in the continental United States.  Some records, particularly at longer durations, were broken by more than 50%.  Harvey has potentially important implications for the design standards necessary for critical infrastructure to survive extreme rainfall.

Amir AghaKouchak – Common flood hazard assessment practices typically account for one driver at a time (e.g., either fluvial flooding only or ocean flooding only), whereas coastal areas are at risk for flooding from multiple drivers (e.g., extreme coastal high tide, storm surge, and river flow). Here, we propose a bivariate flood hazard assessment approach that accounts for compound flooding from river flow and coastal water level, and we show that a univariate approach may not appropriately characterize the flood hazard if there are compounding effects.  Sea level rise (SLR), a well-documented and urgent aspect of anthropogenic global warming, threatens population and assets located in low-lying coastal regions and can exacerbate coastal flooding. Using copulas and bivariate dependence analysis, we also quantify the increases in failure probabilities for 2030 and 2050 caused by SLR under representative concentration pathways 4.5 and 8.5. The increase in failure probability is shown to be strongly affected by compounding effects of both ocean and terrestrial drivers. The proposed failure probability method offers an innovative tool for assessing compounding flood hazards in a warming climate.

Youlong Xia (Co-authors: Michael Ek, David Mocko, and Christa Peters-Lidard) – In a three-week span during the 2017 Atlantic Basin hurricane season, Hurricane Harvey made landfall in Texas (25August – 02 September 2017) and Irma made landfall in Florida (10-13 September 2017) bringing huge effects on the United States society and economy by damaging powerlines, houses, and buildings, as well as causing catastrophic flooding.  Harvey was the wettest tropical hurricane on record in the contiguous United States and it brought more than 1000 mm (~40 inches) of rain in many regions of Texas and Louisiana during a four-day period, leading to over 70 fatalities people died and at least $70 billion economic losses recorded to date. Hurricane Irma was the most powerful hurricane ever recorded in the open Atlantic Ocean, making landfall in Florida on 10 September. The North American Land Data Assimilation System (NLDAS), developed by multi-institutions and continuously sponsored by NOAA/CPO/MAPP project, is a widely-used monitoring tool for drought and flood. The NLDAS monitor, in particular its experimental realtime system recently jointly developed by NCEP/EMC and NASA/GSFC/HSL, is providing some guidance for the occurrence, persistence, and recovery of floods in the regions affected by these storms. The NLDAS system is also used to post-diagnose conditions, and to investigate its capabilities and weaknesses, using Harvey and Irma as two typical examples. In this presentation, NLDAS precipitation, soil moisture for different soil layers, evapotranspiration, and total runoff/streamflow was analyzed for the whole period from 20 August – 20 September 2017. Soil moisture conditions before hurricane landfall and total runoff/streamflow after landfall are addressed. The results show that there is a significant benefit to monitor floods when the realtime system is used in comparison to using the near-realtime operational system. The four-day delay of the current operational system is not sufficient to timely monitor flooding cases caused by hurricane and other significant rain events. This suggests the importance and need of a truly realtime NLDAS system upgrade for operational flood monitoring tasks.

***Any opinions, findings, and conclusions or recommendations expressed during the webinars do not necessarily reflect the views of the National Oceanic and Atmospheric Administration.

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