Multi-Scale Sensitivity and Predictability of Hurricane Joaquin (2015)

Hurricane Joaquin (2015) was a strong category 4 hurricane (maximum winds of 135 kts) that developed from an upper-level low over the western Atlantic and was noteworthy because of its large impact in the Bahamas, as well as the sinking of the cargo ship El Farro and loss of her 33 crew members. Joaquin initially moved southwest towards the Bahamas and rapidly intensified before sharply turning northeastward. Nearly all operational model forecasts failed to provide an accurate prediction of the rapid intensification and track, even at short lead times. As a result, the National Hurricane Center forecasted landfall in the mid-Atlantic, while in reality the storm moved well offshore.

In this study, we utilize two adjoint modeling systems, the Navy’s COAMPS and the NASA GEOS-5, to investigate the role of initial condition errors that may have led to the relatively poor track and intensity predictions of Hurricane Joaquin. Adjoint models can provide valuable insight into the practical limitations of our ability to predict the path of tropical cyclones and their strength. An adjoint model can be used for the efficient and rigorous computation of numerical weather forecast sensitivity to changes in the initial state. The adjoint sensitivity diagnostics illustrate complex influences on the evolution of Joaquin that occur over a wide range of spatial scales. The sensitivity results highlight the importance of an upper-level trough to the northeast that provided the steering flow for the poorly-predicted southwesterly movement of the hurricane in its early phase. The steering flow and hurricane track are found to be very sensitive to relatively small changes in the initial state to the east-northeast of the hurricane. Additionally, the intensity prediction of Hurricane Joaquin is found to be very sensitive to the initial state moisture including highly structured regions around the storm and in remote regions as well. Sensitivity of Hurricane Joaquin’s track and intensity forecasts to Hurricane Marty’s outflow in the Eastern Pacific will be explored and illustrated using the COAMPS adjoint.

Hurricane Joaquin was observed in four NASA WB-57 research flights during the ONR Tropical Cyclone Intensity (TCI) experiment. Dropwindsondes deployed from the HDSS (High Definition Sounding System), and remotely sensed observations from HIRAD (Hurricane Imaging Radiometer) were onboard the NASA WB-57. Dropwindsondes deployed during Hurricane Joaquin reveal strong spatial gradients in winds and thermodynamic properties with a rich spectrum of structure in the horizontal and vertical. The dropsondes that were deployed in regions of large initial state sensitivity are used to characterize the atmospheric properties of these sensitive regions. We will also quantify the impact of TCI dropsondes on COAMPS forecasts for select forecasts of Hurricane Joaquin.