The role of the large-scale environment in determining North Atlantic tropical cyclone size

Several of the most destructive tropical cyclones (TCs) within the historical record have been particularly devastating due to their large size [e.g., TC Sandy (2012)]. In spite of the heightened damage potential associated with large TCs, our current comprehension of the factors responsible for determining TC size remains incomplete. While previous studies have suggested that large TCs may require a relatively moist environment or larger angular momentum imports, a comprehensive study of the factors governing TC size is warranted given the potential benefits in a forecasting setting. Building upon the foundation of prior work, the present study will examine which large-scale environmental factors are responsible for large North Atlantic TCs.

We hypothesize that the occurrence of large TCs is dependent on three environmental factors. The first of these factors is the formation of a TC from a large precursor disturbance, which can serve as a source of potential vorticity (PV) for the TC. The second factor is a moist lower-and-middle troposphere to provide a favorable atmospheric environment for the diabatic generation of PV at large radii. Lastly, a reduction in the inertial stability of the upper-tropospheric environment may cause the TC-induced diabatic warming to spread to greater radii yielding a larger TC. To investigate this hypothesis, storm-relative composites of small, medium, and large North Atlantic TCs are constructed from the NCEP Climate Forecast System Reanalysis to investigate the processes responsible for determining the size of TCs. In this study, we consider small, medium, and large TCs to be in the first, third, and fifth quintile of the Extended Best-Track radius of 34-kt surface wind speed for North Atlantic TCs from 1989 through 2012. Each Extended Best-Track data point is only used if aircraft reconnaissance observations are available within 12-h prior. PV budgets will also be computed from the storm-relative composites to determine the processes responsible for increases in PV at outer radii as the TC expands.