Hurricane Sandy was a wild beast, with a powerful high pressure system, a hurricane, a nor’easter and a high tide all meeting simultaneously to cause the highest flood in NYC history.  As a result the peak flood height was poorly forecast by federal and academic scientists.  One of our main pursuits at Stevens Institute’s Davidson Laboratory (aka Center for Maritime Systems) has thus been “capturing Sandy” — creating a reproduction that is as accurate as possible.

Click to see an animation of Hurricane Sandy modeled wind and pressure driven storm surge (color shading) from Hatteras to Nova Scotia. Arrows are wind velocity vectors (see the scale arrow for a 33 m/s or 74 mph hurricane strength wind), contours are isobars – lines of constant atmospheric pressure. An inset panel shows the four day time-history of modeled surge near NYC’s shoreline at Sandy Hook.

In order to do this, we needed to find the best representation available of Sandy’s winds and pressure, and accurately simulate Sandy’s storm surge using our ocean model.   We also then needed to compare the resulting flood heights around the region to verify that they are accurate (they are, to an average across several tide gauge stations of 15 cm rms error for the animation above, 16 cm rms error for the animation below).  

Above is an animation of Sandy’s winds, barometric pressure, and storm surge, which is just the wind- and pressure-driven water elevation.  Credits for our ocean modeling also go out to my colleagues Alan Blumberg and Nickitas Georgas.  It is based on the Stevens Northwest Atlantic Predictions (SNAP) model grid that we built last fall, as forced by atmospheric model forecast data from the NOAA National Center for Environmental Prediction Global Forecast System (GFS).  I also have made a 3-minute version with voice-over descriptions of what is occurring.

Note how the winds blew from the northeast across the coastal Atlantic Ocean during the three days leading up to Sandy’s landfall, and this started to pile water up against the mid-Atlantic coast.  This is due to Earth’s rotation (and the “Coriolis Effect”), which causes the net flux of water to move to the right of the wind direction.  As Sandy itself approached the New Jersey coast, higher atmospheric pressure (black lines) pushing down outside the circular center of the storm helped force water to rise under the center of the storm, where the pressure is lower. This is the “inverse barometer” effect.

Next up is an animation of Sandy’s winds, pressure and total water elevation, which is the storm surge plus the tides and plus anything else such as rainfall or river-driven freshwater flooding.  This surge modeling is on a nested smaller grid that receives inputs from the larger scale grid at its boundaries.  The smaller grid is the NYHOPS grid, used for our regular regional forecasts of storm surges and ocean conditions.  The simulation is based partly on GFS atmospheric forcing, but mainly on the best forecast we could find – a Rutgers WRF model forecast (by Greg Seroka and Louis Bowers).

Click to see an animation of water elevation (color shading) in the New York City, New Jersey and Long Island region. This simulation includes both storm surge plus tides.  (NOTE: Time is in GMT here, four hours later times than EDT. Fixing that …)  Also shown are wind velocity vectors (arrows) and white lines are isobars. The right panel shows a zoom to the NYC region. The axes on the top right show water elevation at The Battery over four days.  The methods used for this simulation are very similar to a recent peer-reviewed published paper.

Now that we have created simulations with good accuracy, we can show people a new perspective on what happened, and we can also do experiments on Sandy to better understand hurricane storm surges and better forecast them in the future.  Beyond that we can even use the storm surge modeling to test out adaptations that reduce or prevent flooding, as we’re doing for the federal HUD’s Rebuild by Design competition.