A combination of observational and numerical analyses is used to investigate hurricane boundary layer (BL) structure in the context of intensity change. These analyses refer to hurricanes in three modes of intensity change: intensifying (IN), steady-state (SS), and weakening (WE). Observations from global positioning system (GPS) dropwindsondes launched in Atlantic tropical cyclones between 1998 and 2015 are collected for compositing based on intensity change. After quality control and sorting, 3,091 dropwindsondes were composited---1,086 were released into IN hurricanes, 1,042 were released during WE phases, and the remaining 963 releases were associated with steady-state storms. In non-intensifying hurricanes, lower-tropospheric tangential winds were stronger than IN storms outside the radius of maximum winds (RMW), which suggests greater inertial stability I² outside the RMW in non-intensifying hurricanes. The BL radial inflow is of similar thickness across the three composites, and all composite groups have an inflow maximum situated at the RMW. Non-intensifying hurricanes are associated with stronger near-surface inflow outside the eyewall region, which implies more frictionally forced ascent out of the BL at radii outside the RMW. At greater radii, inflow layer Θₑ is relatively low in the WE composite, suggesting enhanced subsidence or downdrafts at those radii. High-resolution numerical case studies of Hurricane Irma in 2017 and Hurricane Earl in 2010 are used to check results found in the composite analysis and highlight BL azimuthal structure. The Weather Research and Forecasting Model for Advanced Research (WRF-ARW) is employed for these full-physics simulations. Irma's strong tangential winds were relatively confined to the RMW, leading to weak I² outside the eyewall. Aside from land interactions, Irma tended to steadily intensify, with an inflow maximum at the RMW and BL ascent isolated inward of the RMW. A brief WE period in Irma was associated with shear- and motion-induced asymmetry, whereby drier air was able to descend into the BL inflow near the RMW. Hurricane Earl had a broader tangential wind field, with high I² outside the eyewall. Earl's strong BL inflow spread over a large radial band, which was associated with widespread BL convergence and shallow ascent outside the RMW. During a prolonged and progressive decay in Earl's intensity, two regions of BL convergence became apparent: one inward of the RMW, and the other well outside the RMW. Descent of low-enthalpy air into the BL near the RMW occurred during Earl's WE phases. Despite shear and storm motion of comparable magnitude to Irma, asymmetries were more pronounced in Earl's BL. Earl's decline in intensity was also associated with strong low-level outflow in the upshear-right quadrant, which may have led to kinematic and thermodynamic evolution that promoted an outer region of BL convergence, as well as an inner-eyewall collapse and coincident secondary eyewall formation.
