Observed Turbulent Dissipation Rate in a Landfalling Tropical Cyclone Boundary Layer

Abstract

Based on turbulence measurements from sonic anemometers instrumented at multiple levels on a 356 m-tall meteorological tower located on the south coast of China, an observation study of the turbulent dissipation rate (ε) in a landfalling tropical cyclone boundary layer (TCBL) is conducted. Three indirect methods (i.e., the power spectra, the 2nd- and the 3rd-order structure functions) are compared for the calculation of ε. The 3rd-order structure function computes the smallest ε among the 3 methods, but shows the largest uncertainty. The 2nd-order structure function gives similar ε estimates as the power spectra, and is adopted for its reduced uncertainty. The measured ε in the landfalling TCBL is of O(10−1) m2 s−3, much greater than typical atmospheric boundary layer values as well as oceanic TCBL values. ε is found to scale with the local friction velocity rather than the surface friction velocity, implying a highly localized nature of turbulence. Conventional parameterizations of ε are evaluated against observations. Process-based ε models assuming a local balance between shear production and dissipation prove inadequate, as shear production merely accounts for half of the dissipation away from the surface. In comparison, scaling-based ε models used by planetary boundary layer (PBL) schemes are more advantageous. With both tuning of the model coefficients and adjustment of the dissipation length scale, the performance of an ε model in a widely used PBL scheme is shown to produce similar values to the observations.