When Cyclone Shaheen struck northern Oman in October 2021, the sea wasn't the only factor controlling the storm's behavior. The mountains rising behind the coast were also part of the equation, but in a different way. While warmer waters could give the cyclone more power and moisture, the terrain could alter its path and redistribute its rainfall between the coast and the highlands.
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This picture is provided by a new study that used Cyclone Shaheen as a case study to understand what happens when the conditions surrounding a tropical cyclone approaching Oman change. It showed that an increase in sea surface temperature in the simulation led to an intensification of the cyclone and an increase in rainfall, while a change in terrain elevation had a greater impact on its path, the movement of air within it, and the areas receiving rainfall.
Badriya Al-Maawali, a researcher in the Physics Department at Sultan Qaboos University and the General Directorate of Meteorology in Oman, and the lead author of the study, says that the team’s main message is that simulating a tropical cyclone approaching Oman can be heavily influenced by both ocean conditions and the region’s complex topography, but each factor operates through different mechanisms.
Warmer sea and a stronger hurricane
To determine the extent of this effect, the researchers virtually rebooted the hurricane in a series of experiments, each time changing some of the surrounding conditions, such as sea surface temperature, topography elevation, and sea surface roughness, and then observed what happened to the storm's strength, path, rainfall, and internal structure.
the strongest response occurred when the team raised the sea surface temperature by two degrees Celsius. In this experiment, the warmer water provided the cyclone with more energy and moisture, which boosted cloud activity and increased its strength. The researcher explained that the average wind speed in the simulation increased by about 2.1 meters per second compared to the original experiment, while the average sea surface pressure decreased by about 3.2 hectopascals, both indicators of a stronger cyclone.
The impact wasn't limited to wind strength; the area receiving significant rainfall also expanded, and the main region affected by the cyclone experienced increased precipitation. Regarding coastal communities in Oman, the study's lead author states that a key finding is that cyclone intensity and rainfall can be sensitive to the ocean conditions the storm encounters before making landfall.
But when the researchers moved from the sea to the land, the story changed. Reducing the terrain height in the model essentially altered the hurricane's path and reduced the mountain-enhanced rainfall, and it also reorganized the vertical air movement within the storm, from the flow of air and moisture near the surface to the rising and spreading air in the upper atmosphere.
This finding is particularly significant in Oman, where mountain ranges rise near parts of the coast. When moist air collides with these highlands, it is forced upwards, which helps condense water vapor and produce more rainfall. Thus, mountains not only determine the terrain over which a cyclone passes, but can also help identify the areas that receive the heaviest rainfall.
In contrast, the researchers found that the actual changes in sea surface roughness had a smaller impact on hurricane strength and rainfall distribution compared to water temperature or topography, according to the researcher.
Badriya adds that the results confirm the importance of accurately representing the sea and topography in models of cyclones affecting Oman, as sea surface temperature was more influential on the strength of the cyclone and the amount of rainfall, while topography played a greater role in determining its path, vertical movement.
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