A New Asymmetry Parameter for Tropical Cyclones

The Deviation Angle Variance (DAV) technique is an objective method to quantify the circular symmetry of tropical cyclone (TC) cloud top in infrared satellite images. The performance of the DAV technique in the presence of fluctuations is examined in this study. By using idealized TC cloud structure and HURSAT dataset, it is found that the DAV technique is very sensitive to fluctuations and would struggle to distinguish symmetric and asymmetric TC cloud structure. An alternative method to quantify asymmetry is proposed – GASYM, which is originated from astronomy. Unlike the DAV technique, where local gradient, which is sensitive to fluctuations, plays an important role, GASYM is calculated by comparing the original image and the image rotated by 180 degrees. It is shown that GASYM is much less sensitive to fluctuations.

Moreover, the DAV technique requires a predefined radius of calculation (ROC). The problem of the use of ROC is that the size of the TC cloud cluster could be larger or smaller than the ROC such that the calculation would not be accurate. A new approach is proposed in this study – cluster identification (CI) approach. This approach identifies TC cloud clusters in the HURSAT infrared images using density based spatial clustering, such that calculation can be performed on the TC cloud cluster alone without a predefined ROC.

We propose the use of GASYM with CI as a new asymmetry parameter for TC. We will also discuss the relationship between TC asymmetry and intensity.