The Role of Environmental Moisture on Tropical Cyclone Size and Structure

Tropical cyclone (TC) size is integral in determining the spatial extent of TC impacts and is influenced by environmental wind shear and the overall moisture environment. Since initial TC size is related to future TC size, research focused on understanding the influences of TC size away from land can lead to a more complete understanding of the extent of coastal impacts associated with landfalling TCs. This study considers TCs located in an area of low to moderate wind shear (< 20 knots) located at least 100 km from major land masses. An empirical orthogonal function (EOF) analysis is used to distinguish moisture environments based on the large-scale spatial pattern of total column water vapor (TCWV) surrounding the TC. Using these EOF patterns, four separate categories (groups) are created: (1) Moist_SE with enhanced moisture [TCWV > 60 mm] to the southeast of the TC center, (2) Dry_Extreme with extremely dry air (TCWV ~30-40 mm) surrounding the entire TC, (3) Dry_SE with anomalously dry air (TCWV <40 mm) to the southeast of the TC center, and (4) Dry_NW with moist air to the southeast but anomalously dry air (TCWV <30 mm) to the northwest of the TC center. Principal component (PC) scores indicate the time steps most contributing to the EOF pattern for each group and ultimately determine the time steps included in each group. TC structural differences among the groups are compared using size metrics based on the wind field and shape metrics based on the precipitation field. These metrics are considered at the central timestep identified in the EOF analysis as well as a 48-hour window centered on the central timestep. Results indicate that there are no significant differences in the wind field size, but Moist_SE TCs are the largest in terms of overall precipitation area. This suggests that moisture affects the size of the precipitation field but not the wind field. However, more research is needed to confirm this relationship. Storms in Moist_SE environments are also more intense and younger than TCs in other groups and show signs of inner core organization and subsequent intensification while TCs in the other groups do not. TCs in an extremely dry environment (Dry_Extreme) or with dry air to the southeast of the TC center (Dry_SE) are generally smaller, less closed, less solid, and older than TCs with moisture to the southeast (Moist_SE) and TCs with dry air to the northwest of the TC center (Dry_NW). An additional analysis comparing the same size and shape metrics between TCs experiencing easterly and westerly shear is also completed. The wind shear results of this case study suggest that, while easterly shear is more commonly associated with younger and intensifying TCs regardless of moisture group, an environment with westerly shear is more favorable for intensification of TCs with moisture to the southeast due to the alignment of moisture and upward motion. Future work will include in-depth case studies to gain insight into the physical processes responsible for the observed differences in spatial structure among TCs in different moisture environments.