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

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.