Development and Application of a Tropical Cyclone Environmental Wind Climatology

Recent idealized modeling studies have revealed aspects of the environmental flow around a Tropical Cyclone (TC) other than the deep-layer (200-850-hPa) vertical wind shear that may influence TC intensity and structural evolution. In order to better understand the range of environmental flows TCs experience in nature, this study utilizes reanalysis and best track datasets to construct a climatology of the mean environmental wind profiles to which TCs are exposed. This climatology provides distributional information about the characteristics of sheared environments around TCs, permitting more extensive statistical analysis of the relationship between TC evolution and the environmental flow. Particular attention is given to joint distributions of two derived parameters describing the height at which vertical wind shear is maximized, and the depth of the layer in which significant vertical wind shear exists. These two parameters were shown in an idealized modeling study to strongly influence TC structural and intensity change, and the newly developed wind climatology now enables an assessment of the likelihood of a real TC encountering each shear height and depth examined in that study. It is found that vertical wind shear is most likely to occur in shallow layers in the upper troposphere, even after filtering out circulations associated with the reanalyzed TC such as the storm-induced outflow. There is also a sizeable population of environments with shallow layers of vertical wind shear maximized in the lower and middle troposphere, which tend to correspond to cases near low-level easterly jets in the deep tropics. The distributions of shear height and depth are largely insensitive to the choice of reanalysis dataset. Quantitative methods of analyzing the relationship between the derived shear height and depth parameters and TC intensity change are also explored.