Prediction and Diagnosis of Typhoon Morakot (2009) Using the Naval Research Laboratory's Mesoscale Tropical Cyclone Model

Numerical simulations of Typhoon Morakot (2009) were performed using the Naval Research Laboratory¡¦s Coupled Ocean/Atmosphere Mesoscale Prediction System for Tropical Cyclones (COAMPS-TC). COAMPS-TC was run in real-time in 2009 in the western North Pacific Ocean basin, and simulations of Morakot were executed during its life cycle, from formation through landfall in Taiwan. In this work, an evaluation of the model¡¦s performance is presented. The COAMPS-TC average track errors were small and very close to those of the consensus. Further, the intensity errors were small; the mean absolute intensity error at the 48 h lead time was 9 kt. Particular focus was placed on the 72-h simulation beginning on 1200 UTC 6 August, encompassing the time frame before, during and after landfall. COAMPS-TC was able to predict the structure of Morakot reasonably well before and after landfall, capturing a large asymmetric tropical cyclone with the precipitation shield shifted to the south of its center. Qualitatively, the precipitation forecast was consistent with observations from the Taiwan rain gauge network, as the model was able to predict two maxima, in both the northern and southern portions of the central mountain range. However, the accumulated precipitation maximum in the southern portion of the central mountain range was underpredicted by approximately 50%. The underprediction in precipitation by COAMPS-TC in southern Taiwan was due to four factors: (i) the premature dissipation of tropical storm Goni causing errors in the large-scale flow and moisture pattern after landfall, (ii) inaccuracies in the spatial location and timing of convective and stratiform precipitation as Morakot interacted with land and the southwest monsoon flow, (iii) a simulated track that moved slightly too slow prior to landfall and slightly too fast after landfall, and (iv) a horizontal resolution (5-km) that may be too coarse to resolve the interaction of convection with the complex topography

A View of Tropical Cyclones from Above: The Tropical Cyclone Intensity Experiment

Tropical cyclone (TC) outflow and its relationship to TC intensity change and structure were investigated in the Office of Naval Research Tropical Cyclone Intensity (TCI) field program during 2015 using dropsondes deployed from the innovative new High-Definition Sounding System (HDSS) and remotely sensed observations from the Hurricane Imaging Radiometer (HIRAD), both on board the NASA WB-57 that flew in the lower stratosphere. Three noteworthy hurricanes were intensively observed with unprecedented horizontal resolution: Joaquin in the Atlantic and Marty and Patricia in the eastern North Pacific. Nearly 800 dropsondes were deployed from the WB-57 flight level of ∼60,000 ft (∼18 km), recording atmospheric conditions from the lower stratosphere to the surface, while HIRAD measured the surface winds in a 50-km-wide swath with a horizontal resolution of 2 km. Dropsonde transects with 4–10-km spacing through the inner cores of Hurricanes Patricia, Joaquin, and Marty depict the large horizontal and vertical gradients in winds and thermodynamic properties. An innovative technique utilizing GPS positions of the HDSS reveals the vortex tilt in detail not possible before. In four TCI flights over Joaquin, systematic measurements of a major hurricane’s outflow layer were made at high spatial resolution for the first time. Dropsondes deployed at 4-km intervals as the WB-57 flew over the center of Hurricane Patricia reveal in unprecedented detail the inner-core structure and upper-tropospheric outflow associated with this historic hurricane. Analyses and numerical modeling studies are in progress to understand and predict the complex factors that influenced Joaquin’s and Patricia’s unusual intensity changes

Saturation of Ocean Surface Wave Slopes Observed During Hurricanes

Abstract Drifting buoy observations of ocean surface waves in hurricanes are combined with modeled surface wind speeds. The observations include targeted aerial deployments into Hurricane Ian (2022) and opportunistic measurements from the Sofar Ocean Spotter global network in Hurricane Fiona (2022). Analysis focuses on the slope of the waves, as quantified by the spectral mean square slope. At low‐to‐moderate wind speeds (15 m s−1), slopes continue to increase, but at a reduced rate. At extreme winds (>30 m s−1), slopes asymptote. The mean square slopes are directly related to the wave spectral shapes, which over the resolved frequency range (0.03–0.5 Hz) are characterized by an equilibrium tail (f−4) at moderate winds and a saturation tail (f−5) at higher winds. The asymptotic behavior of wave slope as a function of wind speed could contribute to the reduction of surface drag at high wind speeds

A view of tropical cyclones from above: The Tropical Cyclone Intensity Experiment

The article of record as published may be found at https://doi.org/10.1175/BAMS-D-16-0055.1High-resolution observations of Hurricanes Patricia, Joaquin, and Marty in 2015 provide new insight into tropical cyclone structure and intensity change as part of the Tropical Cyclone Intensity field program