Monitoring tropical cyclones from space

As the North Atlantic Hurricane season officially starts today (1 June) we take a look at how you can see tropical cyclones forming and follow where they are moving using satellite imagery.

Tropical cyclone is the generic term for a low pressure system over tropical or sub-tropical waters, with organised convection (i.e. thunderstorm activity) and winds at low levels circulating either anti-clockwise (in the northern hemisphere) or clockwise (in the southern hemisphere).

At its very early and weak stages it is called a Tropical Depression. When the winds reach 39 mph (62.8 km/h) it is called a Tropical Storm. If the wind should reach 74 mph (119km/h) or more is called a Hurricane in the Atlantic and the north-east Pacific, and a Typhoon in the north-west Pacific. In other parts of the world, such as the Indian Ocean and South Pacific the term Cyclonic Storm or Tropical Cyclone is used.

A tropical cyclone has:

• air flowing into the system at low levels in the atmosphere and outwards at high levels;
• an eye wall;
• organised bands of rain;
• a large shield of cirrus cloud.

Spotting hurricanes in imagery

Satellite information is very important for tracking and determining intensity trends of hurricanes and other tropical storms. When a hurricane is well offshore and out of effective radar range, forecasters use satellite imagery to continuously track the storm’s movement and development. The imagery gives information about the top of the storm. Satellites can also give information about the winds speeds over the ocean surface.

Forecasters are able to use satellite imagery over the west of Africa to spot the initial formation of the storms, before they even reach the classification of Tropical Depression.

The ASCAT instrument on Metop measures surface wind speeds and directions over the ocean. This is crucial for monitoring the formation and development of the storms and is used to pinpoint the storm centre.

Hurricane forecasters use both visible and infrared satellite imagery to track the motion and cloud patterns of hurricanes and infrared to monitor cloud-top temperatures.

Being able to see when a tropical cyclone is forming, and to continuously track where it is heading, means more timely warnings can be issued and mitigating actions taken. Forecasters also pay attention to the sea surface temperature. As a cyclone travels over water, if it passes over warmer water it will pick up energy and intensify, and travelling over cooler water will weaken it. The temperature of the sea surface is monitored from satellites. For a cyclone to form the sea surface temperature needs to be at least 26 °C.

Image 1, shows Hurricane Igor in the Caribbean on September 14, 2010, as seen from EUMETSAT’s Metop satellite. The rain bands cannot be seen directly as they are covered by the cirrus shield. After languishing for several days as a tropical storm, Igor rapidly developed into a category 4 hurricane.

Image 2, shows Tropical Cyclone Funso, off Mozambique on January 25, 2012. The wind flags are from the ASCAT instrument and the image is from MSG. The strongest winds are the flags with small triangles (at least 50knts). The underlying image is the IR channel, the colours show the coldest clouds (Blue is -30 °C, Red is -70 °C). The eye of the storm has less cold cloud and lighter winds.

Atlantic Hurricane season forecast

Months before the season starts forecasters at the National Hurricane Center in Miami are already using satellite data to determine how many hurricanes there are likely to be. Each April, all the satellite data about both atmospheric and ocean conditions are fed into a computer model of the global climate; those conditions are the starting points for simulation of the season (from June to November).The simulations are then combined with other statistical estimates based on current climate conditions to produce the outlook – for 2012 they predict around 12 named storms in the North Atlantic and they have already had two pre-season Tropical Storms.

- Eumetsat