Every year has its highlights and events in atmospheric science; the past one had more than its share. Here is a condensation of an article I was asked to write for the Encyclopaedia Britannica.
Nobel Prize for atmospheric chemists. The highlight of 1995 in atmospheric sciences was the awarding of the Nobel Prize in chemistry to three atmospheric chemists for identifying chemicals that destroy stratospheric ozone. The awardees, Paul Crutzen (Max Planck Institute for Chemistry in Mainz, Germany), Sherwood Rowland (University of California, Irvine), and Mario Molina (Massachusetts Institute of Technology), demonstrated in 1974 that industrially produced chlorofluorocarbons (CFCs) could be transported into the stratosphere where they produce ozone-destroying reactions. The Royal Swedish Academy of Sciences stated, "the three researchers contributed to our salvation from a global environmental problem that could have catastrophic consequences."
The year also produced encouraging signs that the intensity of the ozone hole over the South Pole is leveling off. The Montreal Protocol, a 1987 international agreement to reduce the global production of CFCs and other ozone-depleting chemicals, may cause a slow healing of the ozone hole to begin early in the next century. Measurements of total ozone in October 1995 indicate values of around 100 Dobson Units (a measure of the thickness of the ozone layer), about the same as in 1994 and slightly higher than the record low of 91 units observed in 1993. Prior to the springtime depletion of ozone, typical values are 300 Dobson Units.
IPCC consensus on climate warming. For many years many atmospheric scientists have predicted that the human-induced increase in carbon dioxide and other radiatively active trace gases would enhance the greenhouse effect and cause a global warming of the earth's surface. Indeed, the global mean temperature has increased by between about 0.3 and 0.6°C over the past 100 years. Recent years have been among the warmest ever recorded, despite the cooling effect of the 1991 Mt. Pinatubo volcanic eruption. Still, the majority of scientists believed that the role of humans was inconclusive, in part because the magnitude of the observed warming was not large compared to natural variability and in part because global climate models did not do well in simulating the observed patterns of regional temperature changes. Now, the Intergovernmental Panel on Climate Change (IPCC), composed of some 2,500 scientists from more than 100 countries, has concluded officially for the first time that "the balance of evidence suggests a discernible human influence on global climate." A major reason for this shift in scientific opinion was the discovery that the models do a better job of replicating the observed regional variations of temperature change if they take into account the cooling effect of human-produced dust and aerosols, which has masked the greenhouse effect on the climates of eastern North America, Europe, and South Asia in recent decades. The IPCC now estimates global warming of 1.0 to 3.5°C over the next 100 years. This is lower than the range estimated in 1990, due primarily to three factors: lower emission scenarios, inclusion of the cooling effect of aerosols, and improvements in modeling the carbon cycle.
A banner year for tropical storms. 1995 witnessed 19 Atlantic tropical storms, 11 of which attained hurricane status, making this the second (next to 1933) most active hurricane season since records began in 1871. The end of one of the most persistent El Ninos in history set the stage. During the El Nino of 1991-95 a persistent pattern of enhanced vertical wind shear and above-normal surface air pressure over the subtropical Atlantic inhibited tropical storm formation. With the demise of El Nino in the spring of 1995, these factors both decreased, creating an environment conducive to development of tropical storms and hurricanes.
Better forecasts. In 1995 the National Weather Service's National Centers for Environmental Prediction (formerly the National Meteorological Center) adopted a new, physically comprehensive hurricane prediction model. The model was developed at NOAA's Geophysical Fluid Dynamics Laboratory and tested successfully during 1993 and 1994. The improved model, and other factors such as better ways of using data in the model, are being transferred to other weather services, including those responsible for forecasting typhoons in the Pacific.
The NWS is nearly two-thirds of the way through the major modernization of its observing and forecast systems. As of December 1995, it had completed 108 of the 118 planned new Weather Forecast Offices and commissioned 84 Doppler radars and 140 Automated Surface Observing Systems (ASOS). The second advanced Geostationary Operational Environmental Satellite, GOES-9, was launched on 23 May and has demonstrated its potential to provide data with unprecedented resolution and quality. GOES-9 will replace GOES-7 at a position of 135°W while GOES-8 operates at 75°W. The data gathered by these satellites, the new Doppler radars, and ASOS will help forecasters provide improved forecasts and warnings of severe weather.
GPS/Meteorology. On 3 April, a Pegasus rocket carried a small satellite, MicroLab-1, into a low-earth orbit. Circling the earth every 100 minutes at an altitude of approximately 750 km and an inclination angle of 70 degrees, the satellite carries a radio receiver for signals from the constellation of 24 GPS (Global Positioning System) satellites. Approximately 500 times each day, as the satellite rises or sets relative to the GPS transmitters, it receives radio signals which are refracted as they pass through the atmosphere. Knowing precisely where the satellites are and when the signals are transmitted and received, it is possible to calculate vertical profiles of atmospheric refractivity, which depends upon temperature and water vapor, and with independent estimates of either variable it is straightforward to compute the other.
The GPS/Meteorology experiment was carried out by UCAR, the University of Arizona, and the Jet Propulsion Laboratory. (See the Spring 1995 UCAR Quarterly.) Preliminary results have shown that accurate profiles of refractivity can be obtained from an altitude of about 50 km down close to the surface. Where water vapor effects are negligible, which is usually the case above an altitude of 5-7 kilometers, temperatures accurate to within 1°C may be obtained. However, for purposes such as climate monitoring it is not necessary to separate temperature and moisture effects; long-term trends of refractivity alone could indicate climate change. It is also possible to incorporate the observed refractivities directly into numerical weather prediction models.
Field programs. In June the largest network of weather instruments ever deployed to study tornado development took place across the southern plains of the United States. (See the Summer 1995 UCAR Quarterly.) During the Verification of the Origin of Rotation in Tornadoes Experiment, the recently completed Electra Doppler Radar (ELDORA) gleaned high-resolution images of unprecedented quality of the formation of a tornado. Developed by scientists at NCAR and the French Centre de Récherche en Physique de l'Environment, ELDORA is a dual-Doppler radar mounted on the tail of NCAR's Electra aircraft.
In November, another experiment to study the formation of severe weather was carried out near Darwin, Australia. The Maritime Continent Thunderstorm Experiment, designed to study the role of seabreeze fronts and other circulations in generating severe thunderstorms, included the first successful research flight of a fully autonomous meteorological aircraft, named the Aerosonde. The Aerosonde flew flights of up to seven hours in duration through seabreeze fronts, over hilly terrain, and into an intense thunderstorm, demonstrating the feasibility of a new observational tool for research and operations.
In November and December, more than 100 scientists from 57 institutions in 10 nations conducted ACE-1, the first of the Aerosol Characterization Experiments. The experiment is described in more detail in a separate article in this issue.
International data exchange. For more than a century, global weather data have been freely shared among scientists and weather forecasters of all countries. However, more than a decade ago, some European meteorological services began to engage in commercial activities to recover some of their operational costs. Concerns about foreign competition led some countries to ask the World Meteorological Organization (WMO) to restrict the international exchange of some meteorological data. Last June the WMO's Twelfth Congress approved a resolution which said, in part, "WMO commits itself to broadening and enhancing the free and unrestricted international exchange of meteorological and related data and products."
Adapted with permission from Encyclopaedia Britannica, Inc.
In this issue...
Other issues of UCAR Quarterly
Edited by Carol Rasmussen,
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 09:13:34 MDT 2000