Observed Turbulent Dissipation Rate in a Landfalling Tropical Cyclone Boundary Layer

Abstract

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 second- and the third-order structure functions) are compared for the calculation of ε. The third-order structure function computes the smallest ε among the three methods, but shows the largest uncertainty. The second-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 O(10−1) m2 s−3, much greater than typical atmospheric boundary layer values as well as oceanic TCBL values. The value of 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. Significance Statement Dissipation in a turbulent flow refers to the conversion of kinetic energy to heat through molecular viscosity, and is of key dynamic and thermal–dynamic importance in the tropical cyclone boundary layer (TCBL). Past observations of ε in the TCBL have been rare. Few existing ones are based on surface and flight measurements that are either constrained in height or limited in time. This study utilizes turbulence measurements taken from a 356-m meteorological tower to study dissipation of a landfalling TC. The tall tower platform offers a valuable turbulence dataset that extends beyond the surface layer of the TCBL. The observations are used to improve the physical understanding of dissipation, and to evaluate diagnostic ε models for numerical weather prediction.