Gravity wave-like structures observed in onshore typhoon boundary layer of Typhoon Kirogi (2000)

Understanding the typhoon boundary layer (TBL) over land is one of the most important topics in typhoon studies as it directly affects society. In this study, an observational analysis of the typhoon boundary layer (TBL) in the onshore outer rainband region of Typhoon Kirogi (2000) on 7 July 2000 was performed. The data from the Doppler radar for Airport Weather (DRAW), as well as aircraft soundings of wind and temperature (ACARS), provided unique data set that would permit analysis of small-scale structures embedded within the TBL and TBL environmental conditions. High-resolution observations obtained with the DRAW revealed the existence of long-lived, kilometer-scale, coherent streaks within the TBL where the stratiform rain and relatively weak convection were dominated. The horizontal wavelength was approximately 8.4km for the first one hour, then changed 4.2km and persisted for over 2 hours. Differently from hurricane boundary layer rolls, almost aligned with the mean wind direction, the orientation of the streaks in this case was nearly perpendicular to it. It is noteworthy that the streaks occurred with a horizontal wind shear greater than the warning threshold (i.e., 2.5 ms-1km-1) commonly accepted in aviation safety. The profiles from ACARS revealed a strongly stable layer and the wind direction veered significantly throughout the 3 km. The frequency and the phase of surface pressure perturbations are consistent with theoretical relations with gravity waves, suggesting that they were caused by the passage of the gravity waves. These results indicate that the TBL acts as a duct and the gravity waves were trapped vertically and reflected downward.