Tropical Cloud Cluster Environments and Their Importance for Tropical Cyclone Formation

Abstract

This study uses a nonhierarchical cluster analysis to identify the major environmental circulation patterns associated with tropical cloud cluster (TCC) formation in the western North Pacific. All TCCs that formed in July–October 1981–2009 are examined based on their 850-hPa wind field around TCC centers. Eight types of environmental circulation patterns are identified. Of these, four are related to monsoon systems (trough, confluence, north of trough, and south of trough), three are related to easterly systems (low-latitude zone, west of subtropical high, and southwest of subtropical high), and one is associated with low-latitude cross-equatorial flow. The genesis potential index (GPI) is analyzed to compare how favorable the environmental conditions are for tropical cyclone (TC) formation when TCCs form. Excluding three cluster types with the GPI lower than the climatology of all samples, TCCs formed in monsoon environments have larger sizes, lower brightness temperatures, longer lifetimes, and higher GPIs than those of TCCs formed in easterly environments. However, for TCCs formed in easterly environments, the average GPI for those TCCs that later develop into TCs (developing TCCs) is higher than that for other TCCs (nondeveloping TCCs). This difference is nonsignificant for TCCs formed in monsoon environments. Conversely, the average magnitudes of GPI are similar for developing TCCs, regardless of whether TCCs form in easterly or monsoon environments. In summary, the probability of a TCC to develop into a TC is more sensitive to the environmental conditions for TCCs formed in easterly environments than those formed in monsoon environments.

Landfalling tropical cyclone characteristics and their multi‐timescale variability connected to monsoon and easterly formation environments over the western North Pacific

“…Details of the environmental classification of TC formations are given by Teng et al . (2019; 2020b). They identified five main environments for the WNP TC formation: monsoon trough, monsoon confluence, north of monsoon trough, and easterly systems west and southwest of the subtropical high.…”

“…In the WNP, TCs typically form in monsoon, easterly, and their interaction environments with favourable environmental conditions (Ritchie and Holland, 1999; Chen et al ., 2008; Lee et al ., 2008; Yoshida and Ishikawa, 2013; Teng et al ., 2019). Within these environments, the TC formation processes, defined as the development of tropical cloud clusters (TCCs) into TCs, have different thermodynamic and dynamic characteristics and variabilities.…”

Tropical Cyclone Formation within Strong Northeasterly Environments in the South China Sea

“…Strong northeasterly flow over the SCS provides different contributions to TC formation, depending on its strength, direction, and timing. The environment with the strong northeasterly flow is not a common circulation pattern for TC formation in the WNP [5,[31][32][33]; thus, the interaction between northeasterly flow and pre-TC disturbance is still unclear. To increase our understanding of TC formation and improve the forecast skill of TC formation in the SCS, this study explores and quantifies the importance of strong northeasterly flow for TC formation in the SCS using numerical experiments.…”

“…During the peak typhoon season (July−October), the monsoon trough provides a strong southwesterly flow with high moisture in the SCS, favoring the formation of tropical cloud clusters and TCs [4,5]. However, the probability density function (PDF) of TC formation number in the SCS during the peak typhoon season is lower than that in the WNP [6].…”

A Contrast of Recent Changing Tendencies in Genesis Productivity of Tropical Cloud Clusters over the Western North Pacific in May and October

“…The proportion of TCCs that can evolve into TCs could be quantified by using the genesis productivity (GP) (the ratio of TC numbers to TCC numbers) [3,4]. It is of great significance to reveal the difference between developing and non-developing TCCs for understanding the formation mechanism of TCs and the tropical cyclogenesis prediction [5][6][7][8][9][10].…”