During October 2014, Hurricane Gonzalo traveled within 85 km from the location of an underwater glider situated north of Puerto Rico. Observations collected before, during, and after the passage of this hurricane were analyzed to improve our understanding of the upper ocean response to hurricane winds. The main finding in this study is that salinity potentially played an important role on changes observed in the upper ocean; a near‐surface barrier layer likely suppressed the hurricane‐induced upper ocean cooling, leading to smaller than expected temperature changes. Poststorm observations also revealed a partial recovery of the ocean to prestorm conditions 11 days after the hurricane. Comparison with a coupled ocean‐atmosphere hurricane model indicates that model‐observations discrepancies are largely linked to salinity effects described. Results presented in this study emphasize the value of underwater glider observations for improving our knowledge of how the ocean responds to tropical cyclone winds and for tropical cyclone intensification studies and forecasts.

Ensemble Kalman filter (EnKF) assimilation and forecasting experiments are performed for the case of Hurricane Ike (2008), the third most destructive hurricane hitting the USA. Data from two coastal WSR-88D radars are carefully quality controlled before assimilation. In the control assimilation experiment, reflectivity (Z) and radial velocity (V r ) data from two radars are assimilated at 10 min intervals over a 2 h period shortly before Ike made landfall. A 32-member forecast ensemble is initialized by introducing both mesoscale and convective-scale perturbations to the initial National Centers for Environmental Prediction (NCEP) operational global forecast system (GFS) analysis background, and the ensemble spread during the analysis cycles is maintained using multiplicative covariance inflation and posterior additive perturbations. The radar data assimilation results in much improved vortex intensity and structure analysis over the corresponding GFS analysis. Compared with the forecast starting from the GFS analysis, the forecast intensity, track and structure of Ike over a 12 h period are much improved in both deterministic and ensemble forecasts. Assimilation of either V r or Z leads to improvement in the forecasts, with V r data exhibiting much greater impacts than Z data. With the 2 h assimilation window, 30 min assimilation intervals produced results similar to 10 min intervals, while 60 min intervals were found to be too long. The ensemble forecasts starting from the EnKF analyses are found to be mostly better than the corresponding deterministic forecast, especially after ensemble post-processing, such as probability matching for precipitation. Precipitation equitable threat scores were calculated and compared. Copyright c 2012 Royal Meteorological Society Key Words: hurricane forecast; radar data assimilation

The initialization of ocean conditions is essential to coupled tropical cyclone (TC) forecasts. This study investigates the impact of ocean observation assimilation, particularly underwater glider data, on high-resolution coupled TC forecasts. Using the coupled Hurricane Weather Research and Forecasting (HWRF) Model–Hybrid Coordinate Ocean Model (HYCOM) system, numerical experiments are performed by assimilating underwater glider observations alone and with other standard ocean observations for the forecast of Hurricane Gonzalo (2014). The glider observations are able to provide valuable information on subsurface ocean thermal and saline structure, even with their limited spatial coverage along the storm track and the relatively small amount of data assimilated. Through the assimilation of underwater glider observations, the prestorm thermal and saline structures of initial upper-ocean conditions are significantly improved near the location of glider observations, though the impact is localized because of the limited coverage of glider data. The ocean initial conditions are best represented when both the standard ocean observations and the underwater glider data are assimilated together. The barrier layer and the associated sharp density gradient in the upper ocean are successfully represented in the ocean initial conditions only with the use of underwater glider observations. The upper-ocean temperature and salinity forecasts in the first 48 h are improved by assimilating both underwater glider and standard ocean observations. The assimilation of glider observations alone does not make a large impact on the intensity forecast due to their limited coverage along the storm track. The 126-h intensity forecast of Hurricane Gonzalo is improved moderately through assimilating both underwater glider data and standard ocean observations.