Hurricane Irma has caused severe damage and flooding over the Caribbean in the past few days. With the highest hurricane category, 5, and wind speeds up to some 290 km/h, Irma is among the most powerful tropical storms to have appeared in the Atlantic since the beginning of systematic weather recording. Irma developed end of August near the Cape Verde Islands and started moving westward into the Caribbean. It is now heading toward Florida.
According to calculations made by the DLR Institute of Space Operations and Astronaut Training, DLR’s TET satellite crossed the path of the storm on 7 September 2017 at 19:20 UTC, making it possible to record the eye of the hurricane. DLR’s TET satellite is one of two satellites in the FireBIRD mission and it has been in orbit since 2012. The FireBIRD mission was developed at the DLR Institute of Optical Sensor Systems and the satellites were constructed to primarily record high-temperature events. However, with the help of the thermal camera employed, other phenomena can also be monitored.
An analysis of the data at DLR’s Earth Observation Center (EOC) shows the dimensions of Hurricane Irma and its thermal pattern: in the ca. 50 km wide eye of the storm, high temperatures in the air layer at sea level underneath could be detected, while the adjacent cloud cover has a very low temperature because of its altitude (image at left). These clouds are located in the troposphere and are significantly colder than the ocean.
In the TET image the two light blue areas north of the centre and separated by about 270 km caught the special interest of the atmospheric researchers at EOC. They are probably sections of concentric circles surrounding the eye of the hurricane. First analyses suggest they might come from atmospheric gravity waves of the type generated by storm systems. Analogous to the concentric waves of water that spread out from a stone thrown into the water, the storm’s strong convection currents lead to the formation of such gravity waves. A special feature is TET’s high horizontal resolution of 150 m, which makes it possible to detect even the collapse of gravity waves. This can be seen in the ribbed structures visible at the gravity wave front. Gravity waves transport energy through the atmosphere, sometimes over long distances, contributing to its distribution. In this way a hurricane not only affects the earth’s surface and the lower level of the atmosphere, the troposphere, but also locations far above and distant.