Tuesday, January 22, 2013

More 'Sandy-like' storms? What do we do about them?

Truly remarkable storm occurred over the West Pacific last week that reminded me that there are storms like the landfalling hurricane Sandy traveling across the oceans on a more frequent basis than we may want to hear about.  It had a very low pressure of 932 mb at a latitude of only 40 deg north as it went explosive deepening just off the coast of Japan.  This pressure is actually lower than Sandy's 936 mb value as it transitioned toward an extratropical state.

An Ocean Prediction Center analysis of the west Pacific cyclone at near peak intensity.

The wind field that was hurricane force over a wide area, mostly west and south of the low center in a famously typical area for rapidly deepening extratropical cyclones.

An ECMWF 3 hour forecast of the 10 m winds around the west Pacific storm (courtesy of  the Weather Underground).

With the intense, wide wind field, the NOAA's WaveWatch model forecasted a significant wave height of 63'!  This is in itself an amazingly bold forecast considering that the highest scientifically measured significant wave height as of 2006 was 61' off the coast of Scotland by a British Oceanographic Vessel RRS Discovery (see Holliday et al. 2006).  Since the significant wave height represents the 66th percentile of wave heights, a wave exceeding 100' may not even have been considered a rogue wave (more than 4 standard deviations above the mean significant wave height).

The NOAA WaveWatch model 48 hr forecast from 2013-01-13 - 06 UTC courtesy of the Ocean Prediction Center.
The analysis later on had a more 'modest' significant wave height of 49'.
The Ocean Prediction Center analysis of significant wave height

This even came about by phasing of two shortwave troughs, much like what happens often in the eastern US.

500 mb (left) heights and anomalies and sea-level pressure (right) from 2013-01-13 - 00 UTC to 2013-01-15 18 UTC (courtesy of Penn State e-wall).  The west Pacific storm evolves in the upper-left of each panel.

The IR imagery confirms the  spectacular coil so often associated with the intense oceanic cyclones.  An old technique of using this kind of imagery for estimating the central pressure of oceanic cyclones comes to my mind.  Called the Smigielsky, Mogil and Burt technique (SMB; Smigielsky and Mogil, 1995), it's empirical in a similar way to the concept of the Dvorak technique for estimating tropical cyclone strength. The SMB applies a workflow where once a baroclinic developing system is established, one compares the pattern of the mid- and high level clouds.  The more spiraling exhibited by the storm, the deeper the central pressure is likely to be.  Comparing this imagery and the technique's flowchart for north Pacific storms seems to show a degree of spiral that exceeds the lowest pressure option.

A GMS infrared satellite loop of the west Pacific cyclone (courtesy of the Penn State e-wall).
The SMB flowchart for estimating central pressure of extratropical cyclones in the west Pacific.

Why was it so reminiscent of 'Sandy'?  Both of these storms were warm core at their respective lowest pressures.  At its peak intensity, the west Pacific storm showed a region of higher 1000-500 mb thickness as seen in the analysis and forecasts of the GFS, ECMWF and even Canadian models.  Here is an example from the ECMWF model analysis near the time of the storm's lowest pressure.  The thickness represents the mean temperature from the 1000 to 500 mb layer and it shows a bubble surrounding the surface low.  Outside, the thickness gradient shows a ridge indicating the occluded front.
ECMWF sealevel pressure and 1000-500 mb thickness 33 hour forecast from 2013-01-16 00 UTC (courtesy of the Weather Underground).  
This structure is not new.  It has been well documented going back decades.  One of the most incredibly intense extratropical cyclones in recorded history at relatively low latitudes was captured within the domain of project ERICA (Experiment of the Rapidly Intensifying Cyclones over the Atlantic).  The storm's central pressure dropped to below 930 mb at only 40 deg N latitude!  And it also exhibited a warm core as analyzed by Neiman and Shapiro (1993) that closely resembles the west Pacific storm.  The authors noted that this storm was the most intense extratropical cyclone that they recollected in this part of the western Atlantic, south of Newfoundland.  I believe their assessment holds to this day even if Sandy was considered extratropical.

An analysis of an intense oceanic extratropical storm in project ERICA at 1989-01-05 00 UTC.  On the left is 850 mb temperature analysis with plotted wind observations (left) while on the right is the analysis of se-level pressure, fronts and observations.  See Neiman and Shapiro (1993) for more details.

Even though Sandy wasn't extratropical,  her structure also agrees quite well with the west Pacific storm.   The warm bubble surrounding the surface center of Sandy shows up embedded within a cold front to the south and a semblence of a warm frontal baroclinic zone on the other side.  The only differences here are that the warm bubble around Sandy has a higher thickness value (570 dam vs 540) and the cold/warm frontal bands were rotated counter clockwise about 90 deg.

Comparison of the ECMWF 33 hr sea-level pressure and 1000-500 mb thickness forecast (yes it's mislabeled) of the west Pacific storm (upper-left) to the 30 hr GFS forecast of Sandy (upper right) and the 114 hr GFS forecast of Sandy (lower right).  The shading in the ECMWF forecast represents 6 hour QPF.

How did these similarities come about? This question can be answered by viewing the structure of these storms in the form of a cyclone phase space diagram conceived of by Hart (2003).  In the picture below the west Pacific storm produced its warm core quite a bit differently.  The phase space diagram shows the cyclone began as a traditional deep cold core extratropical baroclinic system (point A lower left) as the thermal wind indicated the typical increasing values with height from near the surface to 300 mb (hPa) around the surface low center.  As the storm exploded in intensity, it entrained warm air from its warm sector and wrapped it around the center, similar to that described by Shapiro and Keyser (1990).  They have a classic conceptual model of what's been named the 'warm seclusion'.  Schultz and Vaughan (2011) later explained that this kind of process should occur with occlusions by wrapping up the air along the warm front around the surface low (see below).  At it's lowest pressure, the west Pacific storm evolved into a deep warm core system (lower left) with a high degree of symmetry (upper left).  Sandy, on the other hand, began as a traditional tropical cyclone warm core, symmetric system (point A lower right) and then evolved into a slightly asymmetric, deep warm core system nearing landfall in New Jersey.    But by the time the respective cyclones were at their lowest pressure, their thermal structures are not very different.  If anything, the west Pacific storm had a slightly more symmetric warm core than Sandy, if you can believe it.  But the vagaries of model analysis and forecasts suggest being careful in interpreting these differences too literally.  The bottom line is that both storms were deep warm core within 500 km of the low-level centers.

A cyclone phase space diagram of the west Pacific extratropical storm (left panels) and hurricane Sandy (right panels).  The top row represents the degree of thermal symmetry exhibited by the cyclones and the lower panels represent the depth of the thermal core (warm or cold).  A detailed explanation of these diagrams is available at Hart (2003) or here.

A conceptual model of the Shapiro and Keyser (1990) warm seclusion process modified by Schultz and Vaughan (2011).  The top row represents sea-level pressure and fronts in four stages temporally separated by 6-24 hours.  The bottom row of contours represent the lower tropospheric temperatures for each stage.
The most intense winds typically are located along the exterior of the warm core, just outside of the bent back occlusion rearward and right of the surface low.  This is the most dangerous area that both Sandy, the west Pacific storm, and the project ERICA storm shared.   Most of the time, the east coasts of continents are spared this area and don't experience what these parts of the storms can offer in terms of wind damage, coastal flooding, and general suffering.  The west coasts of continents do experience these on occasion though perhaps not the worst either since many times systems have finished bombing out by the time they hit the west coast, especially that of North America.  Yet some big storms have hit Europe such as the great Braer storm of 1993, arguably the lowest nontropical/nontornadic sea level pressure on record (<911 mb).  This storm actually helped to disperse an oil spell east of the Shetland islands.  Of course it was responsible for grounding the ship that spilled the oil in the first place.  The west coast of Alaska can get nailed by intense Aleutian lows that may have exhibit warm seclusions and were responsible for severe coastal flooding in western Alaska as noted by Christopher Burt in his blog post.   And there was the great Columbus day blowdown in the Pacific Northwest caused by an intensifying 962 mb low.   But imagine a scenario where the ERICA storm of 1989 or the west Pacific storm last week was caught in a blocking situation where they turned the worst of their winds toward their respective coasts in regions unaccustomed to them (in other words, the eastern coasts of major continents).  Despite their completely extratropical origins, I cannot imagine any difference in impact from that of Sandy.

Several blog posts have stressed that hurricane warnings should have been maintained for Sandy as a result of it's maintaining a warm core right up till landfall (see Norcross, 2012, and others).  Perhaps that may have been the solution that would've offered the least trouble with Sandy.  But perhaps it's not the best general solution.  As we've seen, Sandy was not so different than other intense extratropical cyclones. The only difference is that Sandy turned westward.  In some near future event, a more traditional extratropical storm may turn in a similar direction, with a similar sealevel pressure and wind field.  Certainly in such a situation nobody I know would think hurricane warnings would be the answer.  But the end result would be the same.  And how different would the result be from a more traditional hurricane?  Given similar winds and pressure distribution, I don't think the impacts to people would be any different either.  Yes, deep convective hurricanes have the capability of generating much stronger pressure gradients and higher winds, but a building or structure doesn't care what generated the wind, only that the wind is not exerting a potentially damaging force.  Likewise the sea doesn't care whether it's a hurricane or extratropical storm when the water's being pushed upward toward the coast.  Perhaps it's time for warnings to reflect this view.

Now upon finishing this reflection, I see the latest numerical guidance shows three sub-950 mb cyclones expected to form in the north Atlantic in the next five days, two of them are forecasted to fall below 940 mb, one below 930 mb.  All of them will have warm seclusions and likely exhibit warm core behavior.

GFS forecasts of intense north Atlantic lows this week.

The ECMWF forecast of a north Atlantic low on 2013-01-26 with a warm seclusion, hurricane force winds, near calm eye and a sea level pressure of < 930 mb.

Brian Norcross, 2012:

Stu Ostro, 2012:

Recount of record-breaking oceanic cyclones by Christopher Burt

Recount of the Braer storm of 1993

the great Columbus day blow of 1962:

Doswell, C. A., III, 1984: A kinematic analysis of frontogenesis associated with a nondivergent vortex. J. Atmos. Sci., 41, 1242–1248.

HartR. E., 2003A cyclone phase space derived from thermal wind and thermal asymmetry. Mon. Wea. Rev131585616.

Hart, Robert E., Jenni L. Evans, Clark Evans, 2006: Synoptic Composites of the Extratropical Transition Life Cycle of North Atlantic Tropical Cyclones: Factors Determining Posttransition Evolution. Mon. Wea. Rev.134, 553–578.

Holliday, NP, MJ Yelland, RW Pascal, VR Swail, PK Taylor, CR Griffiths, and EC Kent (2006). Were extreme waves in the Rockall Trough the largest ever recorded?Geophysical Research Letters, Vol. 33,  DOI: 10.1029/2005GL025238

Neiman, Paul J., M. A. Shapiro, 1993: The Life Cycle of an Extratropical Marine Cyclone. Part I: Frontal-Cyclone Evolution and Thermodynamic Air-Sea Interaction. Mon. Wea. Rev.121, 2153–2176.

Schultz, David M., Geraint Vaughan, 2011: Occluded Fronts and the Occlusion Process: A Fresh Look at Conventional Wisdom. Bull. Amer. Meteor. Soc.92, 443–466.

Shapiro, M. A., and D. Keyser, 1990: Fronts, jet streams and the tropopause. Extratropical Cyclones: The Erik Palmén Memorial Volume, C. W. Newton and E. O. Holopainen, Eds., Amer. Meteor. Soc., 167–191. 

Smigielski, F. J., and H. M. Mogil, 1995: A systematic satellite approach for estimating central surface pres- sures of mid-latitude cold season oceanic cyclones. Tellus, 47A, 876–891.