The Life Cycle from Tropical Storms to Extra-Tropical

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  • An extra-tropical storm captured by satellite (credit: NOAA).

    Storm tracks from 1985-2005 across oceanic basins (credit: NOAA).

    Hurricane Charley captured by satellite (credit: Dundee Satellite Receiving Station, Dundee University, UK).

  • The Life Cycle from Tropical Storms to Extra-Tropical
    01.09.2016 14:49



    Of great interest to forecasters here at MeteoGroup is the presence of tropical storms that develop thousands of miles away in the Atlantic. Aside from our natural curiosity of these systems, we are also interested in seeing how, if any, they will have an effect on our weather.

     

    Tropical Storm overview

    Tropical storms develop in latitudes approximately 30 °N to 30 °S (i.e. the Tropics), regions where the annual solar radiation to the Earth exceed the total outgoing solar radiation. The waters of the Tropics are very warm and these act as a driver for storm development. The warm ocean heats the air above it and this in turn causes air to rise, cool and condense, thus clouds are formed. As these clouds gain so much energy from the heat sink that is the Tropical Ocean, they can grow until they become large thunderstorms. These thunderstorms often develop a closed circulation of their own – hence why we see an ‘eye’ in some storms. If a storm grows large enough, it can gain hurricane status – the strongest storms are often hurricanes and these are graded according to the Saffir-Simpson scale. 

    Tropical Storm locations

    Tropical storms have several well-known generation areas across the globe and there is a reasonably good record of storm tracks across the Atlantic, Pacific and Indian Ocean basins. For us here in the UK, the Atlantic basin tends to attract the most attention. Storms can be generated off the coast of Africa and move towards Central America or the southern United States, or else they can form in the Gulf of Mexico or Caribbean Sea and then move north towards the US or Mexico.

    Storm tracks

    Figure 1 shows the typical storm track in each ocean basin. As the storms in the Atlantic basin tend to curve north into the Mid-Atlantic, they lose the characteristics that defined them as being tropical. One significant difference in the atmosphere/environment is the north-south temperature contrast that develops (aka baroclinicity) and the movement away from warm water to somewhat cooler waters. The storms then begin to develop ‘frontal’ zones – defined boundaries between relatively warm and cold airmasses and start to develop into mid-latitude depressions. 

    Extra-tropical storms and their influence on UK weather

    The course the storm takes by moving away from tropical or sub-tropical latitudes is one of the key processes of energy transfer within the atmosphere. A huge amount of moisture and heat is carried poleward. This energy transfer can often result in some of the most damaging weather. In the case of the UK, it is the jet stream that can act as an attractor for new extra-tropical storms as they move towards the North Atlantic before being swept by upper atmospheric forcing and brought eastward to impact on Western Europe. 

    Hurricane Charley – 30 years on

    It is just 30 years since Hurricane Charley affected the UK. In August 1986, a low pressure trough in the Gulf of Mexico and moved north along the Eastern US coast, initially as a tropical depression, but it gained hurricane status by 17 August, near the North Carolina coast. The northward movement of Charley along the coast and away from tropical environs meant that the transition to extra-tropical cyclone occurred on 21 August.

    As the storm moved across the Atlantic and towards Ireland and the UK, several weather warnings were issued for the potential of severe disruption. The storm led to severe disruption and damage. Daily rainfall records were broken across Wales and resulted in local flooding. In Ireland, surface water runoff on the Wicklow Mountains also led to coastal erosion, whilst parts of the capital, Dublin, suffered from flooding. Several fatalities were also noted as a result of the storms impact.