Cloud Clusters and Tropical Cyclogenesis: Large Scale Environment of Developing and Nondeveloping Systems

A disturbance with a persistent area of deep convection is a well-known prerequisite for the initial formation of a tropical cyclone (TC)—TC genesis. Duration or longevity is considered to be an indication of a degree of convective organization which is a prerequisite for TC genesis. “Persistent” can be quantified using a fully automated, objective cloud cluster tracking method. The method tracks convective systems with < 208-K infrared cloud top temperatures in time. Based on 140 western North Pacific storms during July-October 2003-2010, TC-genesis occurs after the development of a 208-K cloud cluster lasting for at least 8 hours (8-hr cluster), which is determined to be a necessary criterion for TC genesis. 15% of the 2852 candidate 8-hr clusters (excluding active TC stages) are identified as pre-TC clusters, and 85% are nondeveloping clusters.

8-hr clusters are classified as incipient clusters, precursor clusters, and nondeveloping clusters. The incipient cloud cluster is directly associated with the initial formation of the TC core convection and rainbands. Incipient clusters can be tracked back for over 24 hours prior to best track classification in some cases, but generally for < 12 hours. 105 incipient clusters could be identified (this is less than the number of storms because some storms initially formed outside the study region). Precursors to TC genesis can also be identified for most cases. Precursor clusters are 8-hr clusters which are related to the coherent, track-able, pre-existing vorticity maxima, but dissipated before TC genesis and could not be traced to the cyclone core convection or rainbands. 330 precursors were identified, up to several days prior to best track classification as a TC. The occurrence of distinct precursor and incipient clusters is related to distinct convective “burst” cycles that are often observed leading up to TC genesis. Precursors and incipient clusters are hypothesized to be play distinct roles in TC genesis. Finally, nondeveloping clusters are 8-hr clusters which are neither related to a TC nor a precursor cluster, but apparently possess the minimum degree of convective organization associated with TC genesis.

The large-scale environment of the incipient clusters, precursors, and nondeveloping 8-hr clusters is distinguishable in terms of the sample distributions. The large-scale environment is determined from GFS final model analyses as well as satellite TPW and SST estimates. Not surprisingly, incipient and precursor clusters occur in environments with, on average, higher relative vorticity, stronger low-level convergence, lower vertical wind shear, and higher moisture content compared with nondevelopers. The environments of incipient clusters share some similar characteristics with the early (<24 hour) life of TCs. However, the distributions are not entirely separable. Most frequently, pre-TC clusters occur in a large scale environment that is marginally favorable for TC genesis, as determined by linear discriminant analysis, and many nondevelopers also occur in the similar environments. This is true even at short lead times of < 24 hours, though in that case there are more individual developing cases with extremely favorable environments. To test the generality of these results, similar analysis is currently underway for the Atlantic basin using six years of data from 2004-2005 and 2008-2011.