Forecasters at the Atlantic High Seas desk at the Ocean Prediction Center (OPC) were challenged by a very large, strong storm that encompassed much of the North Atlantic. This storm was so large, if it was over the U.S., more than 2/3 of the U.S. would have been affected in some way. At maximum intensity, the storm deepened to 948 mb southeast of Greenland and had a very expansive area of storm force winds (greater than 50 knots or 60 mph) and wave heights greater than 35 ft over a large chunk of the North Atlantic!
A forecast concern that was raised during this event was whether the storm would produce hurricane-force conditions at any point. I talked with one of the forecasters (Hugh McRandall – OPC) and we tried to use the stratospheric intrusion as an indicator and I received some supporting AIRS Total Column Ozone Retrievals from Brad Zavodsky (NASA SPoRT) to dissect the system. Note that the red-coloring in the RGB Air Mass image correlates to stratospheric drying between the 500 mb and 300 mb levels and should be associated with increased ozone levels.
The two images above show the initial stratospheric intrusion that was associated with a shearing complex upper-level low pressure system. The yellow outline highlights the strongest signal in the RGB Air Mass image with significant drying noted near the shearing potential vorticity (PV) anomalies. This matches up well with the AIRS Total Column Ozone Retrievals in the second image where ozone levels exceed 350-400 Dobson units (this is fairly significant in terms of ozone concentration and indicates the presence of stratospheric air).
Eleven hours later, the initial PV anomalies and associated stratospheric drying have been sheared and distorted by a stronger impulse moving in from the west near the Canadian maritimes. In the RGB Air Mass image above, there appears to be significant stratospheric drying with the new impulse and evidence that the tropopause fold occurred west of New Foundland. In the AIRS image below this, notice how seemingly low the ozone concentrations are compared to the previous images. There is a thin ribbon of 300+ Dobson units, but the majority of the red-coloring is associated with ozone concentrations between 250-300 Dobson units. This seems rather low for a developing monster storm!
At 05z on 12/13/12, the storm is maturing and expanding in coverage with a very pronounced area of red-coloring associated with the stratospheric drying. At this time, the storm is completing its bombogenesis cycle with a surface pressure approaching the minimum of 948 mb. The AIRS image shows plenty of ozone co-located near or just northwest of the center of the PV anomaly southeast of Greenland. At this time, the forecaster was still debating whether any verifiable hurricane-force winds would show up. I conjectured that the pressure gradient might be to large, although it was possible that hurricane-force wind gusts were showing up in the southwestern to southeastern quadrants.
The ASCAT high resolution image above shows a large swath of storm-force winds in the southwest quadrant of between 40 and 48 knots (46-55 mph) with a few flags between 48 and 56 knots (55-66 mph). Another swath of strong winds exists in the northeast quadrant and although not in this image, there was a very strong zone of wind near the tip of Greenland known as the “Tip Jet”. Jame Kells (OPC) noted that the “Tip Jet” did contain 65+ knot (75 mph) winds in a later OSCAT high resolution pass, therefore the hurricane-force wind advisory verified. Unfortunately for me, I did not have the correct quadrant of this storm. This goes to show that the stratospheric drying noted in the RGB Air Mass product may not always be associated with hurricane-force events, but keep in mind that the scatterometer data cannot verify wind gusts. It is my assumption that the southwest quadrant contained some very strong wind gusts well over 65 knots.
Feel free to comment or start a discussion on this topic as I find it very interesting and look forward to observing more of these events with the RGB Air Mass products.
Thank you for reading!
*SEVIRI RGB Air Mass imagery is created using various bands from Meteosat-9 (EUMETSAT) and is disseminated by NASA SPoRT and CIRA.