Multiplatform observations of boundary layer structure in the outer rainbands of landfalling typhoons

Abstract

This paper analyzes data collected from a new set of observational platforms in the coastal area of China, which consist of a mobile observation system, meteorological tower, automatic weather station, and Doppler radars, to investigate the mean and turbulent boundary layer structure and evolution during the landfall of typhoons. An example of these data is provided from Typhoon Morakot (2009). Vertical profiles of wind velocities and thermodynamic parameters from the observed data allow us to identify different boundary layer structures during and after landfall. These structures, sampled in regions of the outer core, are stratified into periods where convection is occurring (termed "convective") and periods where convection has recently (<2 h) occurred (termed "postconvective"). Data analyses show that the thermodynamic mixed-layer depth and inflow layer depth are higher during the convective period than the postconvective period. The mixed-layer depth is found to be within the strong inflow layer, but the height of the maximum tangential wind speed is above the inflow layer during both periods, contrary to recent observational studies of the boundary-layer structure of tropical cyclones over water. High-frequency wind data show that momentum flux, turbulent kinetic energy (TKE) and integral length scales of wind velocities are all much larger during the convective period than the postconvective period. The results suggest that convective downdrafts may play an important role in modulating turbulent flux, TKE, vertical mixing, and boundary layer recovery processes.

Transition of the Hurricane Boundary Layer during the Landfall of Hurricane Irene (2011)

“…When the areaaveraged reflectivity was at a local maximum, the normalized winds often maximized aloft. This result is similar to the results of Ming et al (2014), who showed that downward turbulent fluxes were often maximized on the exterior of rainbands, leading to the downward transport of high momentum. Retrieved vertical velocity was also explored, but did not show significant trends in an area-averaged sense.…”

“…9). Since the perturbations exist primarily on gradients in reflectivity where downdrafts are anticipated, we expect that the downward flux of high momentum as in Ming et al (2014) is likely an important mechanism for the local enhancements to the near-surface winds. It has also been observed that gust factors are higher in rainbands (e.g., Schroeder et al 2009;Giammanco et al 2016), to which this analysis lends support.…”

“…Observations over the land surface are limited by aircraft flight safety concerns while the sparse nature of profiling systems over land limit where vertical profiles of the boundary layer may be retrieved. Nonetheless, a few observational studies have been performed (Morrison et al 2005;Lorsolo et al 2008;Giammanco et al 2013;Hirth et al 2012;Ming et al 2014). Hirth et al (2012) found that an internal boundary layer (IBL; cf.…”

“…However, the authors acknowledged that VAD retrievals are limited in assessing HBL characteristics across large spatial regions and emphasized the need for spatially contiguous observations from, for example, dual-Doppler analyses across the coastal region. Ming et al (2014) documented fluctuations in HBL structure with the passage of outer rainbands in Typhoon Morakot (2009). Downdrafts associated with rainbands tended to reduce the height of the tangential wind maximum.…”

Direct Measurements of Momentum Flux and Dissipative Heating in the Surface Layer of Tropical Cyclones During Landfalls

“…As expected, both towers observed the increase of the momentum flux and TKE with the wind speed, consistent with previous observational studies (e.g., Black et al, 2007;French et al, 2007;Ming et al, 2014;Potter et al, 2015;Tang et al, 2015). The DH measured by both towers is found to increase with the wind speed as well, in agreement with previous aircraft observations given by Zhang (2010) and tower observations given by Zhang, Zhu, et al (2011).…”