**Author details**

Yujuan Sun1,2,3, Jiayi Pan1,4,5\* and William Perrie2,3

\*Address all correspondence to: panj@cuhk.edu.hk

1 Institute of Space and Earth Information Science, The Chinese University of Hong Kong, Hong Kong, China

2 Department of Engineering Mathematics and Internetworking, Dalhousie University, Hali‐ fax, Nova Scotia, Canada

3 Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Sco‐ tia, Canada

4 Shenzhen Research Institute, The Chinese University of Hong Kong, Shenzhen, China

5 School of Marine Science, Nanjing University of Information Science and Technology, Nanjing, Jiangsu, China

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Yujuan Sun1,2,3, Jiayi Pan1,4,5\* and William Perrie2,3

\*Address all correspondence to: panj@cuhk.edu.hk

**Author details**

Hong Kong, China

tia, Canada

**References**

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2 Department of Engineering Mathematics and Internetworking, Dalhousie University, Hali‐

3 Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Sco‐

5 School of Marine Science, Nanjing University of Information Science and Technology,

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84 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

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86 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection


### **Planetary‐Scale Low‐Level Circulation and the Unique Development of Hurricane Wilma in 2005 Planetary**‐**Scale Low**‐**Level Circulation and the Unique Development of Hurricane Wilma in 2005**

Jinwoong Yoo, Robert V. Rohli and Jennifer Collins Jinwoong Yoo, Robert V. Rohli and Jennifer Collins

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/64061

#### **Abstract**

Large‐scale atmospheric and oceanic conditions in the western Atlantic basin were analyzed to understand the unique tropical cyclogenesis (TCG) and intensification mechanism of Hurricane Wilma in 2005, the most intense Atlantic basin tropical cyclone (TC) on record. An analysis of 850 hPa circulations depicted in the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data suggests that anomalous development of the 850 hPa circulation pattern triggered by Hurricane Vince (October 8–11, 2005) contributed to the development of a large‐scale low‐level vortex that preceded Wilma's TCG in the eastern Caribbean. In particular, weakened easterly winds in the central tropical Atlantic assisted the unique large‐scale cyclonic circulation over the western Atlantic about a week before Wilma's TCG. The unprecedented rapid intensification of Wilma was investigated considering the interactions between mid‐latitude troughs and large‐ scale low‐level circulations as well as anomalously warm SST conditions. The global Weather Research and Forecasting (WRF) model run for Hurricane Wilma suggests that the role of mid‐latitude systems in TC activity is more important than previously believed and that every in situ large‐ or meso‐scale vortex and circulation component at least in the immediately neighboring region of TCG seems to have a significant influence on TCG processes.

**Keywords:** tropical cyclone genesis, Hurricane Wilma, vortices, WRF, low‐level circu‐ lations

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1. Introduction**

The importance of large‐scale external circulations in tropical cyclogenesis (TCG) seems irrefutable, based on the numerous persuasive previous studies [1–3]. In a composite analysis of tropical cyclones (TCs) that developed from monsoon troughs over the western North Pacific, Briegel and Frank [2] hypothesized that eastward‐propagating subtropical troughs poleward of the location of TCG support TCG by providing upper‐tropospheric vorticity advection, thereby forcing upper‐level divergence and uplift. Briegel and Frank [2] highlighted that successfully developing TCs had 850 hPa southwesterly surges in addition to the monsoonal easterly winds approximately 48–72 hours prior to TCG, potentially triggering the low‐level convergence and deep uplift necessary for TCG. Using a global numerical weather prediction model for TCG in the western North Pacific, Chan and Kwok [4] derived a similar conclusion to Briegel and Frank [2] regarding the general synoptic‐scale features present before a TCG, specifically the relatively important roles of the low‐level trade winds and the southwesterly low‐level wind surge prior to TCG. Interestingly, however, Briegel and Frank [2] also found that nongenesis cases have upper‐level troughs both to the northwest and to the northeast of the genesis region, which complicates the distinction between TCG‐triggering and non‐TCG‐triggering synoptic settings. Data limitations precluded the specification of the source of the southwesterly surge into the genesis location, other than any preexisting TCs, which only occur in about 34% of all the genesis cases [2].

Therefore, there are still some uncertainties with regard to synoptic‐scale features as predictors of TCG. The confusion partly arises from a lack of understanding of the circumstances under which the combination of synoptic‐scale features optimizes TCG. Moreover, use of these synoptic‐scale features as TCG predictors in the Atlantic basin can be problematic because of the differences in basin size and landmass‐ocean distribution. While the monsoon trough is the breeding region of most TCs in the western North Pacific basin, there is apparently no monsoon trough region in the western Atlantic [4].

In an analysis of Tropical Storm Arlene (2005), Yoo et al. [5, 6] found that low‐level vortex dynamics advected temporary low‐level westerly winds from the eastern North Pacific into the western Atlantic which, when combined with orographically enhanced low‐level south‐ easterly winds from Central America, promoted TCG in the western Atlantic basin. Yoo [7] also suggested a potential influence of low‐level wind enhancement in North America on several cases of western Atlantic TCG. Yoo [7] noted that when strong positive potential vorticity (PV) anomalies in the form of mid‐latitude troughs occur with strong low‐level convection over a vast region in the middle‐to‐high latitudes of North America, occasionally an alley of a low‐level wind surge develops from the mid‐latitude trough southward toward the western Atlantic, enhancing the large‐scale low‐level vortex of the developing storm. Yoo [7] also suggested that the enhancement of this low‐level wind surge alley was a harbinger of the intensification of Hurricane Cindy and Hurricane Dennis of 2005. To better understand the relationship between large‐scale geophysical features and TCG mechanisms over the western Atlantic, more case studies of TCG in the western Atlantic are warranted to analyze the characteristics of interactions of such features leading to TCG.

Hurricane Wilma (October 15–25, 2005) was the most intense hurricane recorded in the Atlantic basin, with a minimum central pressure of 882 hPa and an estimated peak sustained wind speed of 160 kt. Wilma caused 23 deaths and US\$20.6 billion damage to the US alone [8]. Despite its record‐breaking anomaly features, with the exception of a cursory analysis in the TC report from the National Hurricane Center (NHC), no study has been conducted to understand the relative importance of the atmospheric features discussed above in Wilma's TCG. This chapter reviews the large‐scale atmospheric conditions from the early development stage of Wilma. The focus of the study is on the relative and collective roles of high‐latitude PV anomaly and low‐level wind surges of various origins.
