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

Abstract

The hurricane boundary layer (HBL) has been observed in great detail through aircraft investigations of tropical cyclones over the open ocean, but the coastal transition of the HBL has been less frequently observed. During the landfall of Hurricane Irene (2011), research and operational aircraft over water sampled the open ocean HBL simultaneously with ground-based research and operational Doppler radars onshore. The location of the radars afforded 13 hours of dual-Doppler analysis over the coastal region. Thus, the HBL from the coastal waterways, through the coastal transition, and onshore was observed in great detail for the first time. Three regimes of HBL structure were found. The outer bands were characterized by temporal perturbations of the HBL structure with attendant low-level wind maxima in the vicinity of rainbands. The inner core, in contrast, did not produce such perturbations, but did see a reduction of the height of the maximum wind and a more jet-like HBL wind profile. In the eyewall, a tangential wind maximum was observed within the HBL over water as in past studies and above the HBL onshore. However, the transition of the tangential wind maximum through the coastal transition showed that the maximum continued to reside in the HBL through 5 km inland, which has not been observed previously. It is shown that the adjustment of the HBL to the coastal surface roughness discontinuity does not immediately mix out the residual high momentum jet aloft. Thus, communities closest to the coast are likely to experience the strongest winds onshore prior to the complete adjustment of the HBL.

Scrambling and Reorientation of Classical Atmospheric Boundary Layer Turbulence in Hurricane Winds

“…In the considered landfall case, structures within the darker region experience about 2 times less wind loads than structures that extend up to the lighter regions in Figure 3b. These results are consistent with recent observational findings from dropsonde and radar data (Alford et al., 2020). One can estimate this reduction in the mean wind speed via a damped‐oscillator approach for the ABL introduced in Momen and Bou‐Zeid (2016, 2017a).…”

Observations of Boundary Layer Wind and Turbulence of a Landfalling Tropical Cyclone

“…The wind fetch over land for the SMT is more than 40 km relative to the location of maximum radar reflectivity, although the distance of the SMT from the nearest coastline is ~ 9 km in the west. Previous studies showed that the transition of the TCBL across the coastal region occurred mainly in the first 5 km over land (Alford et al 2020b), so the observations here represent the inland surface condition. The underlying of SMT is tropical shrubs.…”

“…Although several observational studies have investigated the mean boundary layer structure of landfalling TCs (Lorsolo et al 2008, Hirth et al 2012, Ming et al 2014, Ming and Zhang 2018, Alford et al 2019, Alford et al 2020a, how the mean and turbulence structures interact remain to be understood. This study presents collocated wind and flux observations by a high (350 m) tower in the boundary layer of Typhoon Mangkhut (1822).…”

Tropical Cyclone Outer Size Impacts the Number and Location of Tornadoes

“…This metric has been used in prior TC tornado studies of convective‐scale environments (Schenkel et al., 2021). We choose 925 hPa because it is typically the isobaric level with the strongest winds (Franklin et al., 2003; Alford et al., 2020). Operational wind field metrics are not used here since they are: 1) defined at a 10‐m height resulting in artificial decreases as TCs move inland due to friction that can differ with the above surface winds (Hlywiak & Nolan, 2021; Chen & Chavas, 2020) and 2) may not encircle the TC radii where most tornadoes occur (Kimball & Mulekar, 2004; Demuth et al., 2006).…”