Observed sub-hectometer-scale low level jets in surface-layer velocity profiles of landfalling typhoons

This paper extends previous studies concerning the structure of wind in tropical cyclones (TCs) approaching land mass over coastal waters. New sets of field measurements from wind observation towers and Doppler SODARS at levels between the land surface and 100 m height are presented. The measured mean (over 10-min periods) of the horizontal velocity U(r) shows the usual eye, eyewall and outer vortex structure. But very significantly they also exhibit that the mean vertical profile, U(z), does not increase monotonically as in a usual surface boundary layer in the backside eyewall regions. Rather, it is in the form of a jet, with a maximum velocity, U-LLJ, at a height z(LLJ), lying between 40 and 60 m. The data also show that the mean velocity gradient, defined by the ratio U-LLJ/U(10), is typically smaller in the front side of a tropical cyclone in the direction of its motion as compared to the backside. This asymmetric distribution does not vary significantly with the radius. The mechanisms that result in the asymmetric distribution of the low-level jet are complex. One possible source is the downward transportation of convective turbulence. In the backside of a TC, the downflow is dominated, and it transports high velocity flow downward and enhances the mean momentum of wind flow at lower heights. Another factor may be the influence of the swell translation in different azimuth of a TC structure. These mechanisms need further investigation based on high resolution observations through a collaborative multiplatform measurement involving, e.g., Doppler SODAR, conventional sensors and numerical simulations.