The Effects of Environmental Wind Shear Direction on Tropical Cyclone Boundary Layer Thermodynamics and Intensity Change from Multiple Observational Datasets

Abstract

Abstract The relationship between deep-layer environmental wind shear direction and tropical cyclone (TC) boundary layer thermodynamic structures is explored in multiple independent databases. Analyses derived from the tropical cyclone buoy database (TCBD) show that when TCs experience northerly component shear, the 10-m equivalent potential temperature θe tends to be more symmetric than when shear has a southerly component. The primary asymmetry in θe in TCs experiencing southerly component shear is radially outward from 2 times the radius of maximum wind speed, with the left-of-shear quadrants having lower θe by 4–6 K than the right-of-shear quadrants. As with the TCBD, an asymmetric distribution of 10-m θe for TCs experiencing southerly component shear and a symmetric distribution of 10-m θe for TCs experiencing northerly component shear was found using composite observations from dropsondes. These analyses show that differences in the degree of symmetry near the sea surface extend through the depth of the boundary layer. Additionally, mean dropsonde profiles illustrate that TCs experiencing northerly component shear are more potentially unstable between 500- and 1000-m altitude, signaling a more favorable environment for the development of surface-based convection in rainband regions. Analyses from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) database show that subsequent strengthening for TCs in the Atlantic Ocean basin preferentially occurs in northerly component deep-layer environmental wind shear environments whereas subsequent weakening preferentially occurs in southerly component wind shear environments, which further illustrates that the asymmetric distribution of boundary layer thermodynamics is unfavorable for TC intensification. These differences emphasize the impact of deep-layer wind shear direction on TC intensity changes that likely result from the superposition of large-scale advection with the shear-relative asymmetries in TC structure. Significance Statement This research investigates how the direction of the winds surrounding the storm impacts the strength of a tropical cyclone. Analyses from this study illustrate that when the winds come from the south the atmospheric boundary layer has a cool and dry side along with a warm and moist side. When the large-scale winds come from the north, temperature and moisture conditions are more uniform throughout the boundary layer. Consequently, results from tropical cyclone climatology show that winds observed to come from the north favor subsequent intensification. These relationships illustrate that tropical cyclone structure and intensity are directly influenced by their surrounding environments and that knowledge of the wind environment could help to improve future forecasts of tropical cyclone intensity change.