As news media fixated on athletic achievements at the Rio Olympics, and of course remained fixated on Donald Trump, different corners of the United States were confronting weather-related hazards — including relentless steamy heat in the East and drought-fueled wildfires all around California, including the explosively spreading “Blue Cut” Fire east of Los Angeles. But nothing has come close to the deadly off-the-chart deluges and flooding in southern Louisiana, which the Red Cross says have produced the country’s worst natural disaster since Hurricane Sandy.
I’ll soon be adding to what I’ve already posted on the mix of bad forest and development policies, indefensible homeowner choices, combustible invasive species and climate change that is amplifying fire danger in the West. (The Eastern heat is, well, August, with some nudge from global warming probably hidden in the data.)
But it’s worth going into more depth on what created the monstrous and deadly dump of rain over Louisiana — with more than 30 inches in some spots in a stretch from August 11, when federal forecasters realized something truly scary was unfolding, until today, with 11 deaths attributed to the storm so far, thousands displaced and tens of thousands of homes and businesses swamped. There’s still flooding in Louisiana, but the super-saturated, slow-motion weather system that triggered that state’s disaster has headed into the Midwest and will eventually reach the Northeast.
First, I encourage you to listen to the latest episode of Warm Regards, the new climate podcast hosted by the blogging meteorologist Eric Holthaus along with the climate-focused ecologist Jacquelyn Gill and yours truly. We focus on this climate context of this storm.
The Times has published an article by Jonah Engel Bromwich assessing the storm’s consistency with climate change projections, but also noting that the South was not among the areas identified in the 2014 National Climate Assessment as most at risk from more downpours.
Ryan M. Maue, a meteorologist for Weatherbell Analytics, offered this post mortem on Tuesday:
An area of low pressure formed over Florida and drifted over land during the week previous to the Louisiana flooding. This area of vorticity served as a focusing mechanism for moisture lift and advection (movement) from the Gulf of Mexico consistently replenishing the available precipitable water. An upper-level divergent anti-cyclone formed over this area of surface low pressure and served as an effective exhaust mechanism for the on-land convection.
Huge rainfall totals occurred over the Big Bend of Florida as well as over the Gulf of Mexico. This freshwater creates a lens on the Gulf surface as well as the cooling effects of coastal upwelling due to wind stress. Compare the SST before and after along the coastline [1-3°C cooling] … some of this is the freshwater run-off from Mississippi River which will increase over the next few days … thus, we see the footprint of the “land-phoon” on the Gulf of Mexico and its climate impacts.
This Twitter item from Maue illustrates the point above, showing the spread of cooler (gray, brown) water:
I also contacted several meteorologists and climate analysts who’ve published studies on factors that shape what happens to Gulf of Mexico storm systems once they push in over land. In this case, it appears that the heat contained in warm, wet swampy terrain of Louisiana south of Baton Rouge may well have offered a last-minute power boost to the storm system.
Roger A. Pielke, Sr., a senior research scientist at the University of C0lorado, Boulder, who has written a batch of papers on the influence of different land surfaces as tropical storms traverse land, sent this thought:
[I]n regions such as southern Louisiana and the Everglades, shallow warm water permits heating to still occur as air moves towards the low pressure center. This mimicking of aspects of the ocean conditions, including the evaporation of water vapor into the system, permits tropical systems to persist longer than they otherwise would. This is what occurred in this Louisiana deluge. That the steering currents were so weak, little vertical shear and a richly humid environmental atmosphere facilitated this extreme rainfall event in one region…. That the Louisiana system also was parked right next to the warm Gulf also helped it sustain itself.
In this context, Maue pointed to previous NASA-funded research by Theresa Andersen and J. Marshall Shepherd making the case that a “brown ocean effect” — evaporation from moist warm soils — can energize tropical systems.
A NASA news release on the 2013 research explained:
Before making landfall, tropical storms gather power from the warm waters of the ocean. Storms in the newly defined category derive their energy instead from the evaporation of abundant soil moisture – a phenomenon that Andersen and Shepherd call the “brown ocean.”
“The land essentially mimics the moisture-rich environment of the ocean, where the storm originated,” Andersen said.
Late on Tuesday, Maue sent a link to a relevant new paper illustrating why it remains difficult (I’d assert a distraction) to find a signal of greenhouse-driven climate change in such storm events, or even patterns of rare heavy downpours. The paper, in the new issue of Journal of Climate, is “The resolution dependence of contiguous U.S. precipitation extremes in response to CO2 forcing” – by Karin van der Wiel of Princeton and colleagues.
Here’s the relevant line from the abstract: “Finally, the observed record and historical model experiments were used to investigate changes in the recent past. In part because of large intrinsic variability, no evidence was found for changes in extreme precipitation attributable to climate change in the available observed record.”
But Maue appropriately stresses what this does and does not mean:
Of course, absence of evidence is NOT evidence of absence. It’s doubtful we will come to a conclusion in real-time on the attribution of this event — or any event — without significant research, better tools/models, and observations. Right now it is easier to shoot holes through others’ arguments on climate change rather than come up with compelling alternative hypotheses.
In a final round of emails, these researchers all agreed on the importance of doing more to track and integrate surface conditions into working models and forecasts for regions where disastrous flooding can occur.
Here’s Shepherd, who’s the director of the atmospheric sciences program at the University of Georgia (and wrote a great Forbes post on why the media were late to this emergency):
I always remind folks not to forget that what falls from the sky is only part of a story like this. Urbanization and associated impervious surface modifies the water cycle. It reduces infiltration into the soil and increases runoff to conveyance systems, lakes, streams, etc. Couple these effects with the geography and topography of the region and it is a bad situation.
It screams for more of an Earth System Modeling approach. The weather models had a pretty good handle on the fact that there would be “lots” of rainfall. However, how can we predict the tipping point where the convergence of rainfall, soil saturation, runoff, etc. triggers the flooding? We are working on some ideas.
For even more, read Jeff Masters at Wunderground and Andrew Freedman’s excellent Mashable coverage: