Our view of Venus is dominated by a thick layer of clouds extending from 48 to 70 kilometers (30 to 43 miles) above the venusian surface. Before the Japanese satellite Akatsuki arrived at Venus in 2015, scientists believed that the clouds primarily followed steady prevailing westward winds, which often approach 80 meters per second (180 miles per hour) near the equator in the venusian cloud layer. However, infrared images from Akatsuki showed a variety of venusian vortices from 100 to 1000 kilometers (60 to 600 miles) across. One possible mechanism for forming these vortices is barotropic instability, which can occur when there is a sufficiently large change in wind speed within a parallel flow (meteorologists refer to this as shear). Once the shear is sufficiently large, vortices form and grow where the shear is largest, mix air with different wind speeds, and eventually fade once the air is sufficiently well mixed.
Using a technique called cloud tracking on Akatsuki images, Takeshi Horinouchi from Hokkaido University and colleagues found a particularly large instance of a venusian vortex caused by barotropic instability. In cloud tracking, images are examined in succession for cloud features, the features in one image are matched with the features in the next image, and the wind speed is determined by the distance the cloud features move divided by the time elapsed between the images. This technique allowed Horinouchi and colleagues to determine that the shear on August 20, 2016, was large enough for barotropic instability to occur and that the air had been sufficiently well mixed by August 25 for barotropic instability to cease. They also identified rotation consistent with a vortex near 15°N, 110°E that was about 1000 kilometers across, larger than previously reported barotropic vortices for both Venus and Earth. The origin of the shear that enabled such instability is unknown, providing an opportunity for further investigation into the large-scale dynamics of the venusian atmosphere. READ MORE