A document by Jennifer K McWhorter. Click on the document to view its contents.

Airborne Doppler radar observations of the wind field in the tropical cyclone boundary layer (TCBL) during the landfall of Hurricane Ida (2021) are examined here. Asymmetries in tangential and radial flow are governed by TC motion and vertical wind shear prior to landfall, while frictional effects dominate the asymmetry location during landfall. Strong TCBL inflow on the offshore-flow side of Ida occurs during landfall, while the location of the peak tangential wind at the top of the TCBL during this period is located on the onshore-flow side. A comparison of these observations with a numerical simulation of TC landfall shows many consistencies with the modeling study, though there are some notable differences that may be related to differences in the characteristics of the land surface between the simulation and the observations here.

Entrainment of dry moat air with low equivalent potential temperature laterally into the eyewall and rainbands is a unique turbulent process in the inner-core region of a tropical cyclone (TC). By analyzing in-situ aircraft measurements collected by the reconnaissance flights that penetrated the eyewalls and rainbands of Hurricanes Rita (2005), Patricia (2015), Harvey (2017), and Michael (2018), as well as numerical simulations of Hurricanes Patricia (2015) and Michael (2018), we show that the moat air entrained into the eyewall and rainbands meets the instability criterion, and therefore, sinks unstably as a convective downdraft. The resultant positive buoyancy fluxes are an important source for the turbulent kinetic energy (TKE) in the eyewall and rainband clouds. This mechanism of TKE generation via lateral entrainment instability should be included in the TKE-type turbulent mixing schemes for a better representation of turbulent transport processes in numerical forecasts of TCs.

This study demonstrates a link between coastal downwelling and tropical cyclone (TC) intensification. We show that coastal downwelling increases air-sea enthalpy (heat, moisture) fluxes ahead of TCs as they approach land, creating conditions conducive to intensification even in the presence of typically inhibiting factors like strong vertical wind shear. The study uses a coupled TC model (HWRF-B) and buoy observations to demonstrate that coastal downwelling developed as three TCs in 2020 approached land. Results show downwelling maintained warmer sea-surface temperatures over the ocean shelf, enhancing air-sea temperature/humidity contrasts. We found that in such cases resulting air-sea enthalpy fluxes can replenish the boundary-layer even when cool, dry air intrudes, as in sheared storms and storms approaching continental land-masses. The resulting warm, moist air is advected into the TC inner core, enhancing convective development, thus providing energy for TC intensification. These results indicate coastal downwelling can be important in forecasting TC intensity change before landfall.

Our prior analysis of a coupled tropical cyclone (TC) air-sea model (HWRF-B) revealed three cases in the 2020 Atlantic hurricane season where coastal downwelling - developing ahead of a TC - contributed to the intensification of each TC as it approached landfall. Coastal downwelling is a specific oceanographic mechanism: we briefly consider other mechanisms potentially contributing to this “shelf effect”, including coastal trapped waves and near-surface advection. We then present preliminary results from a further analysis of HWRF-B forecasts from the 2017-2020 Atlantic and east Pacific hurricane seasons, evaluating the prevalence of the shelf effect for TCs interacting with continental and insular coasts in these two ocean basins. We explore mechanisms linking shelf SST sustenance with inner core convective development and storm intensification, using both a detailed energy budget and Lagrangian tracking experiments within the boundary layers of case study storms. Finally, we briefly discuss the implications these findings hold for evaluating new coupled TC forecast modeling systems currently being developed as a part of the Hurricane Forecast Improvement Program (HFIP).