A Fine Line between Storms and Sunshine
A temperature difference of only 1 degree C, which falls within the error of
most weather observations, can spell the difference between potentially
severe storms and no rain at all. Similarly small discrepancies in moisture
can have the same effect. These and other aspects of storm development have
been analyzed by UCAR's Andrew Crook, who presented his findings at the 18th
Conference on Severe Local Storms, conducted by the American Meteorological
Society in San Francisco.
Using computer models of the atmosphere, Crook conducted forward-sensitivity experiments. These bring a set of slightly varying scenarios forward in time to see how a given weather situation might evolve. Wind, temperature, and moisture values were derived from surface, radar, and radiosonde (weather balloon) observations. On average, these data had temperature errors of around 1 degree C and errors in liquid water content of around 1 gram per kilogram (g/kg). The latter is about 7 to 10% of the total atmospheric moisture present on a typical warm, humid day.
Crook found that a temperature decrease of 1 degree C was enough to shut off storm development entirely, while a 1 degree C increase led to a fourfold increase in rainfall. Similarly, rainfall dropped by 80% when liquid water content was lowered by 1 g/kg, while it more than doubled for a 1 g/kg increase. Crook's findings reinforce the difficulty in forecasting where and when thunderstorms might develop on days when conditions are borderline. However, knowing the possible impact of observational error should help forecasters better quantify the uncertainty in a forecast.
Auto-nowcaster Goes on Line in Memphis
An automated thunderstorm forecasting tool developed by NCAR's Research
Applications Program is undergoing its first major real-world test this
summer. The auto-nowcaster, which spots incipient storms and predicts their
development up to a half-hour in advance, will be tested beginning in June
at the Federal Aviation Administration's Integrated Terminal Weather Support
Site near Memphis, Tennessee. The FAA is supporting the project.
Collaborators on the Memphis test include the Massachusetts Institute of
Technology's Lincoln Laboratories and the National Severe Storms Laboratory
of the National Oceanic and Atmospheric Administration (NOAA).
A cumulus cloud can blossom into a thunderstorm in as little as 10 to 20 minutes. Although operational computer forecast models have proven useful in predicting large-scale weather developments 12 to 48 hours in advance, they do not have the resolution to make accurate forecasts on the thunderstorm scale. With help from new techniques for analyzing Doppler radar data, the auto-nowcaster looks for gust fronts and other lines of converging air on which storms might be induced to form. (These boundaries cannot be simulated directly by larger-scale computer models.) Other parts of the auto-nowcaster examine whether atmospheric conditions are sufficient to support storms once formed and how storm motion might evolve over time.
"It has been a major team effort to get the auto-nowcaster ready for field testing," says project manager Jim Wilson, "and we are very anxious to get feedback from this summer's test. We expect the auto-nowcaster will be issuing thunderstorm advisories on its own in the near future."
Planes, Mobile Radars Helping to Analyze Chemistry of Thunderstorms Airplanes and ground-based vehicles will be probing intense Colorado thunderstorms in one of the nation's largest storm-related field programs of this summer. The project is aimed at documenting the chemical, dynamical, and electrical interchange between thunderstorms and their environments.
Entitled "Deep Convection and the Composition of the Upper Troposphere and Lower Stratosphere," the field program will take place in northeast Colorado during late June and July. It is one of three parts of STERAO, the Stratosphere-Troposphere Experiments: Radiation, Aerosols, and Ozone. STERAO is a multiyear study of the chemistry and dynamics of the upper troposphere (the atmosphere's lowest 15 kilometers, where our weather is shaped) and the lower stratosphere (the sensitive zone between 15 and 45 kilometers where the earth's protective ozone layer resides).
Water vapor and nitrogen are of particular interest in STERAO. Thunderstorms bring vast amounts of water vapor from the lower to the upper troposphere, but the exact trajectories are uncertain. Lightning is a significant source of active nitrogen, which can lead to the production of ozone, but the nitrogen's sources and sinks are not yet fully understood.
A high-altitude WB-57F aircraft, recently acquired by the National Science Foundation and operated by NCAR, is expected to make its research debut at STERAO. Also in the plans are a P-3 "hurricane hunter" aircraft from NOAA and a Citation operated by the University of North Dakota.
Researchers and technicians from the NOAA Aeronomy Laboratory and several other research centers and universities will collaborate with NCAR on this summer's field program. Operations will be based at NCAR's Research Aviation Facility a few miles from Boulder, at Buckley Air Force Base east of Denver, and at an operations center for radar and ground-based teams near Greeley, where the Colorado State University CHILL (University of Chicago/Illinois State Water Survey) radar is operated.
Among the instruments probing the Colorado storms will be
* two mobile Doppler radars that can gather data from within several kilometers of severe storms
* a lightning interferometer from France that will make unique three-dimensional observations of lightning channels
* a variety of devices for air sampling and analysis aboard the aircraft to assess the chemical make-up of air in and near the storms at both high and low altitudes.
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