Wednesday, September 26, 2012

Wild Times in the Gulf of Alaska

As we approach the cool-season, life in the midlatitudes is starting to get interesting.  Today we have an example of an explosively deepening cyclone or bomb over the Gulf of Alaska.  Watch in the loop below how the cyclone deepens from a central pressure of about 1002 mb to less than 960 mb 24 hours.

1200 UTC 25 Sep – 1200 UTC 26 Sep IR imagery and
GFS sea level pressure analysis
The term "bomb" was used in the classic paper by Sanders and Gyakum (1980), which used that label for extratropical cyclones with a central pressure that falls at least 24 mb in 24 hours.

Title and abstract of the seminal paper by
Sanders and Gyakum (1980)
Sanders and Gyakum (1980) showed that bombs in the Northern Hemisphere are most common over oceanic areas, especially along the Gulf Stream in the Atlantic and and from along the Kuroshio Current to the Gulf of Alaska over the Pacific Ocean.  Their analysis was based on a small sample, but more recent climatologies reflect this distribution.

Distribution of bomb events from Sanders and Gyakum (1980)
At the time, bombs were poorly forecast.  Growing up on the east coast, I experienced this first hand as forecasts of nor'easters (cyclones that frequently develop explosively and move up the US east coast) were notoriously bad.  Although annoying for landlubbers, these cyclones are particularly problematic for marine safety and shipping as they can produce hurricane-force winds and high seas.

Major field programs such as GALE (Genesis of Atlantic Lows Experiment) and ERICA (Experiment on Rapidly Intensifying Cyclones over the Atlantic) were held during the 1980s to better understand and forecast this type of cyclogenesis.

Today, bomb forecasts are much better, and this is largely a result of improved numerical model resolution and better assimilation of satellite data over the oceans.  Surprise events are rare and we typically have a good idea of the potential of an explosive cyclogenesis event several days in advance.  Below is the 72 hour GFS forecast valid for 1200 UTC 26 Sep (this morning) and it pretty much nails it.

Such a forecast would have been extremely rare in the 1980s, but is common place today.

Update@1115 MDT:

As shown in the 0000 UTC and recently released 1200 UTC manual surface analysis from the Ocean Prediction Center, the central pressure of this bomb appears to have dropped from about 991 mb to 958 mb in 12 h.  Very impressive!

1 comment:

  1. It is interesting that most of the favored areas of "bomb" development (such as western Pacific, and east coast of the U.S.) are normally associated with strong surface temperature gradients while others (North Pacific and Gulf of Alaska) usually do not have much of a gradient. So perhaps in most areas, these systems get the bulk of their energy from the nearby surface temperature gradient, while for others (like today's example) it could be mostly pre-existing kinetic energy contained in the jet stream?