|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 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).|
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.
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.|
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.
Hart, R. E., 2003: A cyclone phase space derived from thermal wind and thermal asymmetry. Mon. Wea. Rev, 131, 585–616.
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.
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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.
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