- New forecasting method seeks to improve on traditional persistence-style models
- Siberian snowfall alters atmospheric energy dynamic over the North Pole
- Colder, wetter winter predicted for US in 2018
Is the rotating air over the North Pole like a spinning top or more like a boulder in a stream?
Known as the Polar Vortex (PV), this ribbon of air high in the stratosphere twirls around the polar cap, moving steadily east to west. It traps cold air over the polar region while much milder air remains in the lower latitudes.
But when conditions are right, energy emanating from the temperature contrasts between continents and the ocean can cause the PV to meander like a top. As it is buffeted by these waves, it begins to wobble, becoming further susceptible to energy waves rising off the land/sea contrast.
A weakened PV diverts the normally clear flow from its strong circular pattern, sending the ribbon of air further afield as it moves around the obstruction in its wake — much like a boulder in a stream.
When this happens, the normal counterclockwise motion can start moving in a north to south direction. Thus disrupted, the spinning PV moves the cold air normally trapped over the pole down into lower latitudes.
And that’s your winter forecast for 2017-2018, says Cohen.
Let it snow
How does he know this? Cohen and colleagues at AER are pioneering a new technique that takes a cue from snowfall seen by satellites. What they’ve seen this fall tells them we’ve got a doozy on the way.
As Cohen explains, the advance of snow cover across Eurasia and Siberia in the fall leads to the formation of a high-pressure system. This dome of cold air starts to expand beyond Siberia during winter, and because there are high topographical barriers to the south and to the east, it spreads either west towards Europe or over the North Pole into North America.
Siberian snowfalls this past autumn have been particularly heavy, Cohen notes, and that reflective blanket has amplified the energy exchange that results from the roiling atmospheric waves.
Siberia is the refrigerator of the Northern Hemisphere – air masses are chilled and then exported to other regions of the hemisphere. ~Judah Cohen
Armed with this real-time observational data, Cohen’s 2017/2018 winter model calls for below normal temperatures and above normal precipitation across the north and eastern US. The southwestern and southcentral US can expect above normal temperatures and below normal precipitation.
Despite his model’s past accuracy, Cohen’s work is not without its detractors. He grounds his forecasts in observations, statistical analysis, and significance testing, but he admits the evidence provided by computer modeling is mixed, at best.
“Models that run freely with no prescribed forcing do not simulate or exhibit the relationships that I describe. However, with models that are forced with fixed highs and low snow cover, then the relationship which I described is apparent.”
Cohen’s work may have cast him as a bit of a maverick in meteorological circles, but he’s not deterred. Weather has been a fascination since he was a child.
“When it started snowing in class I would jump out of my seat and run to the window, (something that did not endear me with my teachers),” Cohen recalls.
“For me, knowing the weather was not enough – I also wanted to understand what causes the different weather. I have always enjoyed the challenge of forecasting the weather, though I do admit it can be frustrating and humbling at times.”
Personal satisfaction aside, Cohen sees his work as a great benefit for humanity. Persistence models still dominate the meteorological community he notes, but he wants to include more of the dynamical influences on weather to add precision to forecasting. For Cohen, an improved weather forecasts means an improved society.
“Better long-range forecasts allow businesses, financial markets, utilities, insurance companies, the military, governments and even individual citizens to prepare for favorable and adverse weather on their operations.”