A Parameter for Quantifying the Macroscale Asymmetry of Tropical Cyclone Cloud Clusters

Kelvin Sai-Cheong Ng,Yongqiang Zong,Man Hoi Lee

doi:10.1175/jtech-d-19-0160.1

Kelvin Sai-Cheong Ng, Yongqiang Zong + Show 1 more

Open Access

https://doi.org/10.1175/jtech-d-19-0160.1

Copy DOI

Abstract

AbstractA parameter to quantify macroscale (i.e., systemwide) asymmetry of tropical cyclones (TC) in infrared satellite images, galaxy asymmetry (GASYM), which is adopted from astronomy, is described. In addition, an alternative approach to identify TC cloud clusters that is based on a density-based spatial clustering algorithm, cluster identification (CI), is presented in this study. Although a commonly used approach in TC study, the predefined radius of calculation (ROC), can be used to identify the TC region in the calculation of GASYM, this approach is not optimal because the size of the TC cloud cluster is often unknown in the calculation. The area specified by the ROC often includes pixels that do not belong to the TC cloud cluster and excludes pixels that belong to the TC cloud cluster. The CI approach addresses this issue by identifying TC cloud clusters of any size with any shape, because it depends solely on the threshold brightness temperature that corresponds to the upper bound of the brightness temperature of the specific cloud types. This study shows that the CI approach can be integrated into the GASYM calculation as an objective measure of TC symmetry. Although GASYM-CI and intensity are correlated, the relationship between GASYM-CI and intensity depends on the size of the TC cloud cluster. Comparison between GASYM and an existing objective method to quantify symmetry of TCs, the deviation angle variance technique, is also presented.

Highlights

One of the key observations about tropical cyclones (TCs) is that intense TCs tend to have circularly symmetric cloud tops in satellite images (e.g., Dvorak 1975)
In this study we focus on quantifying the macroscale asymmetry of TC cloud cluster (TCCC) by adapting a parameter from astronomy, called the galaxy asymmetry (GASYM) parameter, which is widely used for examining asymmetry in the study of galaxy morphology (Conselice 1997; Conselice et al 2000)
It is shown that while macroscale asymmetry is negatively correlated to intensity, it does not explain all variance of intensity

Summary

Introduction

One of the key observations about tropical cyclones (TCs) is that intense TCs tend to have circularly symmetric cloud tops in satellite images (e.g., Dvorak 1975). The key idea of the DAV technique is to construct the distribution of deviation angle (DA), i.e., angular deviation of the brightness temperature gradient vectors for the pixels in the IR satellite image from the idealized circularly symmetric image (i.e., from the direction pointing radially from the center of the TC) within a predefined radius of calculation (ROC). The use of predefined ROC is a common practice in the study of TCs because it is difficult to determine the size of TCCCs using satellite images; in particular TCs could be surrounded by random unorganized tropical cloud clusters and random deep convections. There are three objectives in this study: 1) the introduction of a parameter to quantify macroscale asymmetry of TC using IR images (i.e., GASYM), 2) an investigation of the relationship between macroscale asymmetry of TCCC and intensity, and 3) the introduction of an approach to identify TCCCs without predefined ROC. Since immature and weak TCs are usually highly asymmetric, the impact on the current analysis is negligible

Macroscale asymmetry of TCCC and TC intensity

Comparison of DAV and GASYM

Findings

Discussion

Conclusions and remarks