In Situ Observations of the Diurnal Variation in the Boundary Layer of Mature Hurricanes

Recent studies have suggested that the structure of tropical cyclones (TCs), especially the upper‐level clouds as indicated by satellite infrared brightness temperatures and precipitation, fluctuates with the diurnal cycle. The diurnal cycle of the low‐level structure, including the boundary layer, has not yet been investigated with observations. This study analyzes data from 2242 GPS dropsondes collected in mature hurricanes to investigate the diurnal variation of the mean boundary layer structure. A composite analysis is conducted to compare the kinematic and thermodynamic structure during nighttime (0–6 local time) vs in the afternoon (12–18 local time). The composites show that much stronger inflow occurs during nighttime and the moist entropy is also larger than that in the daytime. Grouping the dropsonde data into 6‐h time windows relative to the local time shows a clear diurnal signal of boundary layer inflow. The amplitude of the diurnal signal is largest at a radius of 250–500 km. Plain Language Summary The upper‐level clouds that we see in satellite images of tropical cyclones (also known as hurricanes) are often seen to expand and contract over the course of each day. These expansions are associated with a pulse of thunderstorms and rain that travel hundreds of kilometers away from the storm center. Although this daily cycle at upper levels of the atmosphere is well established, it remains unknown whether there are similar changes in winds and moisture near the surface. This study uses observations from hundreds of dropsondes – instruments on parachutes that are dropped out of airplanes – to determine whether there are similar daily changes in the hurricane winds at low altitudes. These winds are indeed shown to have a daily pattern, with stronger inflow (wind flowing toward the storm center) and increased moisture occurring in the overnight hours as compared to the rest of the day. These periods of increased inflow and moisture precede the outward moving bands of thunderstorms, and then diminish as the bands steadily move outward to larger distances. This study could help us better understand how the TC diurnal cycle affects the low‐level structure of storms. Key Points Dropsonde data from mature hurricanes are composited to study the diurnal variation of the boundary layer structure Both the inflow speed and moist entropy values are greater in the nighttime boundary layer than in the daytime boundary layer The diurnal cycle of tropical cyclone