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System Assesses Storm Severity

A tornado About 44 000 thunderstorms occur around the world each day —from localized single-cell storms that produce short-lived cloudbursts, to powerful supercells that cause high winds, hail, flash floods and even tornadoes. The challenge for forecasters is to figure out which pose a potential hazard to people and property, and to issue warnings in time to help protect them from harm.

Until fairly recently, the only tools at forecasters’ disposal were first-hand reports from observers and data from weather radars. Unfortunately, because observations are usually reported when storms are almost overhead, they provide less lead time for warnings. Conventional radars, on the other hand, can send microwave signals up to 325 kilometres and measure the time it takes for them to bounce back off rain, snow or ice — thereby enabling meteorologists to determine the type, amount and rate of precipitation in a storm. New Doppler radars provide additional information on the speed precipitation is moving, as well as wind shifts and cyclonic patterns.

Although the curve of the earth’s surface makes it impossible for radars to detect low-altitude storms at their furthest points of reach, their data reveal much about the internal structure of storms less than 150 kilometres away. The unique air circulation and precipitation patterns in a storm indicate whether or not it is a supercell — an immensely powerful kind of storm that is responsible for most of our severe weather, including tornadoes.

RDSS output on a tornadic supercell that occurred southwest of Winnipeg on July 24, 2000. The circle in the upper left-hand corner marks the location of the radar, and the shaded area indicates the extent of the storm — with the red area around the “H” identified as the most dangerous, and the “H” as an area where large hail is occurring.

To assess the strength of the storm’s updraft — and therefore its supercell potential — radar operators have traditionally used the Lemon Technique, sending beams into the storm to take readings of the rainy downdraft from a variety of angles. With these readings, and some complex calculations, they can visualize the downdraft in three dimensions and infer the strength of the updraft. The problem with this technique is that it takes about 15 minutes to assess a single storm — a long time considering how quickly they change. It also means that meteorologists must try to guess which storms, out of a possible choice of dozens, to focus their efforts on.

To speed the process, Environment Canada meteorologists in Winnipeg and experts from Infomagnetics Technologies devised a computer software package, called the Radar Decision Support System (RDSS). Using conventional weather radar data, the state-of-the-art system analyzes the internal structure of every storm in the area and flags those with supercell characteristics as green, yellow or red (most severe) — all in less than five minutes. In addition to tracking the storms and indicating if they’re getting stronger or weaker, it allows forecasters to click on them to see their characteristics in more detail — a feature that has also proven invaluable for post-storm studies.

Since the RDSS was first instituted in Manitoba in 1994, it has undergone numerous improvements, including the addition of step-by-step computational procedures, or algorithms, for identifying the formation of hail and strong winds. The Prairie Storm Prediction Centre now uses the system to monitor all the weather radars in Manitoba, Saskatchewan and Alberta simultaneously, and has seen a significant improvement in the lead time for issuing warnings. One of the many situations in which the RDSS has proven useful was in 1996, when hail as big as baseballs rained down on Winnipeg from a supercell storm that materialized out of clear air in about half an hour. At the first sign of the storm, the RDSS flagged it red, prompting the forecaster to issue a hail warning before any observational reports of severe weather were made.

A presentation on the system attracted great interest from countries participating in an international meteorology workshop held in Sydney, Australia, after the 2000 Olympics. As part of Canada’s National Radar Project, Environment Canada is currently developing a new Unified Radar Processor that will combine RDSS and Doppler data to create an even more effective severe weather assessment system. The new processor may be operational as early as 2002. Over the long term, Environment Canada’s scientists also plan to develop and implement new algorithms to detect severe winter weather, such as heavy snowstorms, blizzards and freezing rain.

Anatomy of a Storm
The churning cauldron of dark clouds we see during a thunderstorm is actually a complex and continously evolving three-dimensional structure. This structure is made up of one or more self-contained systems or “cells” — each with an organized pattern of rising and sinking air that moves moisture between the upper and lower atmosphere, and affects the wind flow around it.

In Canada, most thunderstorms occur during the spring and summer, usually on hot, muggy days, when the air higher up in the atmosphere is cold and dry. In this unstable atmosphere, all it takes is a lifting mechanism — such as heat from the sun and ground, a cold air mass, or a hill, mountain or other obstacle —to cause the warm, moist air near the earth’s surface to rise rapidly. As this warm updraft rises into the atmosphere, it creates a cumulus cloud.

As a thunderstorm develops, these updraft cells grow and reach ever higher. Although the air in the updrafts cools slightly as it reaches greater heights, it still remains warmer than the surrounding environment. As it cools, the moisture inside the updraft condenses, forming rain droplets and ice crystals. Eventually, the abundance of condensation in the cloud becomes too much for the air to hold, and it begins falling as precipitation — marking the transition from a cumulus cloud to a mature cumulonimbus or “thunder” cloud.

The drag of the precipitation as it falls to the earth is one of the factors that causes a downdraft. At first, the downdraft is found only in the middle and lower levels of the cells, but after 15 to 30 minutes, it gradually expands upward and outward to occupy the entire cloud, except for its very summit — smothering the warm updrafts and causing the storm system to collapse. During this dissipation stage, the rainfall gradually stops, and the cold air from the downdraft spreads across the surface of the earth. This is what causes the cold air that is felt after a storm passes.

About 90 per cent of storms fit into this pattern, lasting half an hour to an hour before they decay. Some are single-cell thunderstorms that have a single updraft that forms, grows to maturity, produces a heavy downpour, and then dissipates. More common are multicell storms that consist of successive, separate updraft pulses that maintain a more or less steady state. Multicell storms can have severe effects, and occasionally produce short-lived tornadoes.

Hail can occur during the mature stage of a cell when an updraft of higher than usual intensity carries raindrops into extremely cold areas of the atmosphere, where they freeze and merge into lumps of ice. When the lumps become too heavy to be supported by the updraft, they fall to the ground at high speeds.

In about 10 per cent of cases, an overabundance of moisture in the lower atmosphere feeds an updraft with such vigour that it becomes greatly magnified and begins to rotate — a phenomenon known as a mesocyclone. These supercell storms can maintain an intense, steady state for many hours, and actually exert control over the surrounding environment, rather than being affected by it. Supercells account for most of the extreme weather events we experience, including very large hail and long-lived, damaging tornadoes.

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