An Analysis of Vertical Wind Shear Characteristics Associated with Rapidly Weakening Tropical Cyclones

Though work in recent decades has improved forecast skill for both tropical cyclone (TC) track and intensity, the rate of improvement for intensity has been slower than that for track. Both external factors and internal processes affect TC intensity change, and more intense TCs often cause greater impacts, thus considerable effort has been directed toward analyzing processes that support intensifying TCs. However, less work has been conducted on the other end of the intensity change spectrum, that of rapid weakening (RW). Environments associated with RW tend to be characterized by vertical wind shear, dry air, and a sharp, decreasing sea surface temperature (SST) gradient. Since these TCs are often not a threat to land, or pose a decreasing threat to land due to their decreasing intensity, such cases are not often observed by aircraft reconnaissance. As a result, these events tend to be examined via large datasets such as satellite and reanalysis data.

Vertical wind shear can impact TCs in multiple ways, such as inducing vortex tilt which can reduce the efficiency of latent heat release within the TC core and lead to dry air intrusions. This shear can be caused by a range of atmospheric features, including upper-level troughs and enhanced trade winds. Past work has shown a range of vertical wind shear magnitudes associated with over-ocean weakening of TCs when that shear is computed between the typical levels of 850 and 200 hPa. As a result, this study analyzes the vertical profiles of wind in rapidly weakening TCs, which are computed over 200-800-km and 0-500-km regions relative to the TC center, to explore wind shear characteristics using the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) ReAnalysis (ERA5). Mean vertical wind profile attributes in the Atlantic Ocean (ATL) and Eastern North Pacific (ENP) will be compared. Aspects such as vertical wind shear magnitude (200-850 hPa), vertical position (upper, middle, lower troposphere), direction, and spatial distribution will be presented.