Science Briefs

Storms of This Century... and the Next

What did you think the last time the weather forecast called for windy and rainy conditions? Perhaps you grumbled a bit, grabbed your umbrella, and went about your business, accepting that stormy days are a part of life. On the other hand, there have probably been times when you've given storms a lot more attention, perhaps in early 1997, when 12 inches of rain fell on the Ohio River Valley causing massive flooding and dozens of deaths, or in early 1993, when tens of millions of people on the U.S. East Coast were in a blizzard called by some the "Storm of the Century". Maybe you've even considered the situation when too few storms come our way, such as the drought of 1988; that year the lack of storms and associated rainfall caused economic hardships for farmers and drove up the price of many crops. Whether you think about it often, or just when you read about them in the newspaper, the presence or absence of storms influences your everyday life.


Average frequency of winter (Dec-Feb) storms from 1957-1989, as simulated by the GISS Global Climate Model (above) and as observed (below).

Storms are not entirely the fickle phenomena they might appear to be. They tend to follow specific pathways, which are related to flow patterns in Earth's atmospheric circulation. The United States lies below one such pathway that encircles the mid-latitudes of the Northern Hemisphere. Storms in this path are commonly referred to as extratropical cyclones and the pathway they follow is known as a storm track. Precipitation dropped from extratropical cyclones is critical for our well-being, but has that always been the case, and will it be true in the future if global climate changes dramatically?

At the Goddard Institute for Space Studies, Mark Chandler and students from the Institute on Climate and Planets used meteorological records from 1957 to 1989 to develop a climatology (a long-term history) of Northern Hemisphere, mid-latitude storm tracks. Just as the weather map on the evening news shows the location of "lows" indicating the approximate position of storms, records of global sea level pressure helped identify the low pressure centers of extratropical cyclones. Computers were used to analyze series of sea level pressure datasets and to track the low pressure systems as they moved from one 12-hour period to the next. By tracking all of the lows for many days or months we can plot the average storm tracks over any given period. The work was compiled to form an on-line atlas of the seasonally averaged Northern Hemisphere storms for the period 1957-1989. (Note: The atlas was subsequently revised and expanded The current version includes data for both hemispheres and covers the perios 1961-1998.) The atlas includes information and images of storm position, frequency of occurrence, and intensity.

To predict changes in the future positions, frequency, or strength of storms we use a global climate model (GCM) to simulate Earth's atmosphere. Collecting the global sea level pressure generated by the GCM allows us to "track" storms that the model produces in the same manner that we tracked the storms from observed data. While the GCM does not provide a perfect reproduction of the Earth's climate, advances in our understanding of the climate system and in computer technology allow us to simulate the Northern Hemisphere extratropical storm tracks with a fair degree of accuracy (see figure). The GCM gives us a tool we can use to simulate various climate change scenarios, such as increasing greenhouse gases, before they occur. It also allows us to calculate the potential effect of global climate change on the extratropical storms that keep our rivers flowing and our plants growing.


Please address all inquiries about this research to Dr. Mark Chandler.