Predicting Heat Waves
October 28, 2013

Learning To Predict Deadly Heat Waves

Brett Smith for - Your Universe Online

New research published on Sunday in the journal Nature Geoscience could give meteorologists the ability to predict deadly summer heat waves more than two weeks ahead of time.

In the study, scientists at the National Center for Atmospheric Research (NCAR) used a 12,000-year simulation of atmospheric patterns over the Northern Hemisphere to identify a distinctive "wavenumber-5" pattern that significantly increased the odds of a major heat waves subsequently building over the United States.

"It may be useful to monitor the atmosphere, looking for this pattern, if we find that it precedes heat waves in a predictable way," said study author Haiyan Teng, NCAR scientist. "This gives us a potential source to predict heat waves beyond the typical range of weather forecasts."

According to the scientists, the wavenumber-5 pattern is a sequence of five alternating high- and low-pressure systems that form a loop around the northern mid-latitudes, several miles above the Earth’s surface. This pattern often produces slow-moving weather features, increasing the chances for stagnant conditions often linked to prolonged heat spells.

Initially, the NCAR scientists examined data from atmospheric records dating back to 1948. They were particularly interested in summertime events in the United States in which highs hit the top 2.5 percent of weather readings for that date across approximately 10 percent or more of the contiguous United States. The researchers could only find 17 such events that met such criteria - not enough to establish a reliable signal amid other atmospheric phenomena.

The group then turned to an idealized computer simulation of the atmosphere that covered a 12,000-year span. The simulation had been created a couple of years before using the NCAR-based Community Earth System Model.

After analyzing over 5,900 simulated heat waves, the team noted that heat waves tended to be heralded by a wavenumber-5 pattern. Neither caused by oceanic conditions nor the heating of Earth's surface, the atmospheric pattern was associated with a phenomenon known as a Rossby wave train that loops the Northern Hemisphere along the jet stream.

In the model, the 20 days preceding a heat wave were marked by the five ridges and five troughs that make up a wavenumber-5 pattern, propagating very slowly westward around the globe and moving against the flow of the jet stream. After the formation of the atmospheric pattern, a high-pressure ridge drifted from the North Atlantic into the United States, cutting precipitation and cuing the onset of a heat wave.

The more the wavenumber-5 patterns were amplified in the simulation, the more the heat waves became likely to form 15 days later. In some cases, the odds of a heat wave were more than quadruple what would be expected at random.

The researchers concluded their study by going back to the actual US heat wave data starting at 1948. They saw that some historical heat waves were marked by a large-scale exchange pattern that indicated a wavenumber-5 event.

Teng said she and her team will continue searching for other circulation patterns that may help predict extreme weather events.

"There may be sources of predictability that we are not yet aware of," she said. "This brings us hope that the likelihood of extreme weather events that are damaging to society can be predicted further in advance."