Many of us watched tropical cyclone Irene’s approach with enough anxiety to last a lifetime. After all, a hurricane riding up the East Coast is maybe the nightmare scenario for forecasters, emergency managers, and residents alike. But Irene, thankfully, didn’t quite live up to some of its expectations.
Now that we are a couple of days removed from its landfall, everything we learn from this experience need not be a retrospective analysis of how to deal with a potential disaster. It might prove equally rewarding to more closely examine how much of a disaster we were up against in the first place.
Almost without recognition of, or respect for, the lack of certainty science could realistically offer about where the storm could go or how strong it could be, the threat of Irene captured our attention. And by the time she made it to the Bahamas, the frenzy had irreversibly begun.
But Irene presented symptoms of less-than-ideal health at almost every stage of its journey. For most of its life, the storm had been situated near, and at times surrounded by, a layer very dry air centered near 10,000 feet altitude that came from the African desert (called a Saharan Air Layer [SAL]). Ingestion of the SAL may have been at least partly responsible for its inability to undergo a period of rapid intensification.
As Irene approached the Bahamas, dry-air driven gust fronts –not on the scale of individual thunderstorms, but on the scale of the outer bands themselves- were clearly noted in the visible satellite imagery. The presence of outward-flowing gust fronts like these can be a sign that part of the circulation is trying to brake the inward and upward spin-up process. Rather than thriving atop the 85°F water that lay underneath the entire circulation, Irene may have instead been choking on the SAL.
NASA Earth Observatory animation of Hurricane Irene by Robert Simmon using imagery by the NASA GSFC GOES Project.
Irene also battled a high-altitude southwesterly shearing flow as it approached the United States. While not particularly strong, on the order of 10-20 knots, the shear was apparently influential enough to tilt the vortex in a way that at least forbid much deepening. As Irene moved northward off the Florida Coast, effects of the shear were ostensibly documented in the satellite imagery by the sharp cutoff in the outflow clouds on the storm’s western periphery.
Hurricane Irene as seen from NOAA’s newest satellite GOES-15, courtesy University of Wisconsin Space Science and Engineering Center
Even without regard to the appearance of a conflicted storm in an imperfect environment, evidence began to mount as Irene got closer that it was perhaps not the storm we feared (at least wind-wise). Reports from NOAA’s buoy network and various other observation platforms suggested Irene’s winds at low altitudes were surprisingly weak.
The National Hurricane Center (NHC) duly and repeatedly noted that the maximum surface winds, as measured by the highly respected SFMR (Stepped Frequency Microwave Radiometer – a machine that directly measures surface foam motion and bypasses entirely the complexity of flight-level to surface reduction algorithms) instrumentation, were consistent with a weaker hurricane. Though flight-level winds were still strong, there apparently was a nearly impermeable boundary somewhere between the winds a couple of thousand feet up and those near the surface. Despite the remarkably low sea-level pressure associated with the storm at its North Carolina landfall (near 952 mb), winds near the surface were apparently well insulated from the faster flow aloft.
It should be noted that the minimum pressure in a tropical cyclone does not directly relate to its maximum wind. It’s the change in pressure with distance (the pressure gradient) that drives air-flow accelerations. If the change in pressure is spread out over a large distance, the maximum wind speed attained by the acceleration is less than if that same pressure difference were much tighter. This partly explains why, with Irene, there really wasn’t much of a wind speed difference between that observed at the coast and that several counties inland. Maximum sustained winds were not far from 40 mph everywhere between Ocean City, MD and Washington, D.C.
The meteorological data Irene presented us prior to landfall gave us clues in real-time that perhaps the storm was becoming less of a violent wind maker and more of a heavy rain maker. I’m not so sure how surprising this should have been. Not only have many of us seen this kind of devolution happen before with land-falling tropical cyclones, this behavior is generally understood with only a little technical background.
We know that when tropical cyclones encroach upon the U.S. mainland north of the Florida Peninsula, they often lose symmetry as frictional effects and the ingestion of continental air ruin their maritime tropical pedigree. At these latitudes, in close proximity to land, weather conditions on the inland sides of these systems are often more like windy nor’easters than true hurricanes or tropical storms. And though Irene’s tropical storm radii, as measured for strictly marine conditions, technically extended well inland, tropical-storm winds didn’t. I emphasized in previous posts in the days before landfall that the westward extension of “bad” weather (in terms of wind) would quickly shrink as the storm moved north.
It seemed prudent at the time to expect that the Outer Banks could (not would) receive hurricane force winds, and that the Atlantic Coast further north to New Jersey should receive tropical storm conditions. And as I wrote on the Friday morning before the storm:
…locations as far west as Washington, D.C. are likely to receive wind conditions near or less than tropical-storm strength.
With Irene’s expectations, preparations, and actual impacts still fresh in our minds, I think an expansion of our science into questions regarding how tropical cyclones behave near landfall would prove hugely beneficial. And though my discussion here focused on the wind, further scientific study into rain patterns at landfall could also help us prepare more efficiently. Flooding was indeed a huge problem for many residents from the Carolinas through New York and New England, as spectacularly large amounts of rain filled gauges and watersheds throughout.
Irene’s 10-day life in 15 seconds. Courtesy NOAA’s Environmental Visualization Laboratory.
But because one of the primary, and perhaps most fear-inspiring, measuring sticks we use to judge hurricane impacts is the wind-based Saffir-Simpson Scale, I’m concerned that a lasting impression with Irene is that many of our Atlantic Coast residents experienced a hurricane, as classically defined. Yet the only place where there’s any preliminary evidence of that (as outlined by wind analyses from NOAA’s Hurricane Research Division of the Atlantic Oceanographic and Meteorological Laboratory) is on the Outer Banks, and just barely so. In pursuit of the truth, I imagine our scientists from NHC and the Hurricane Research Division are still busy trying to determine exactly how strong Irene really was.