Examination of the Combined Effect of Deep-Layer Vertical Shear Direction and Lower-Tropospheric Mean Flow on Tropical Cyclone Intensity and Size Based on the ERA5 Reanalysis

• Published in: Monthly weather review 2021-12, Vol.149 (12), p.4057-4076
• Main Authors: Chen, Buo-Fu, Davis, Christopher A., Kuo, Ying-Hwa
• Format: Article
• Language: English
• Subjects: Amplification; Boundary layers; Cyclones; Direction; Hemispheric laterality; Hurricanes; Northern Hemisphere; Relative humidity; Sea surface; Sea surface temperature; Statistical analysis; Statistical methods; Surface temperature; Tropical climate; Tropical cyclone intensities; Tropical cyclones; Vertical shear; Vertical wind shear; Wind shear
• ISSN: 0027-0644; 1520-0493
• DOI: 10.1175/MWR-D-21-0120.1

Idealized numerical studies have suggested that in addition to vertical wind shear (VWS) magnitude, the VWS profile also affects tropical cyclone (TC) development. A way to further understand the VWS profile’s effect is to examine the interaction between a TC and various shear-relative low-level mean flow (LMF) orientations. This study mainly uses the ERA5 reanalysis to verify that, consistent with idealized simulations, boundary layer processes associated with different shear-relative LMF orientations affect real-world TC’s intensity and size. Based on analyses of 720 TCs from multiple basins during 2004–16, a TC affected by an LMF directed toward downshear-left in the Northern Hemisphere favors intensification, whereas an LMF directed toward upshear-right is favorable for expansion. Furthermore, physical processes associated with shear-relative LMF orientation may also partly explain the relationship between the VWS direction and TC development, as there is a correlation between the two variables. The analysis of reanalysis data provides other new insights. The relationship between shear-relative LMF and intensification is not significantly modified by other factors [inner-core sea surface temperature (SST), VWS magnitude, and relative humidity (RH)]. However, the relationship regarding expansion is partly attributed to environmental SST and RH variations for various LMF orientations. Moreover, SST is critical to the basin-dependent variability of the relationship between the shear-relative LMF and intensification. For Atlantic TCs, the relationship between LMF orientation and intensification is inconsistent with all-basin statistics unless the analysis is restricted to a representative subset of samples associated with generally favorable conditions.