The effects of coastal topography and coastal location in the distribution of boun dary layer winds in the inner core of mature tropical cyclones are examined using a high-resolution multi-level model. In these numerical simulations, the evolution of the tropical cyclone boundary layer (TCBL) is studied in storm-relative coordinates, and in lieu of an actual steering current moving the model vortex, the position of the land-sea interface was shifted through the grid domain at a constant speed with separate surface boundary conditions specified over the land and ocean areas. It is shown that the presence of a coastal boundary produces land-induced asymmetries (along with an internal boundary layer) due to the asymmetric structure of surface drag. This land-induced asymmetry is found in both the azimuthal and radial wind field at landfall. For a moving storm, nonlinear advective interactions between storm-induced asymmetries and land-induced asymmetries can generate a low-level vorticity band ahead of the hurricane. When the storm motion vector has a component that is perpendicular to the coastal boundary, the interaction between this band and the mean vortex leads to a temporary weakening and re-intensification cycle. Furthermore, it is shown that the relative magnitude of the land-induced asymmetry depends upon the terrain slope and the terrain height such that the land-induced asymmetry dominates over the motion-induced asymmetry for elevated terrain. These results underscore the specific differences in boundary layer evolution and intensity evolution for hurricanes interacting with complex topographical features.

Sea surface temperature (SST) varies significantly in the presence of tropical cyclones (TCs). Using fixed SST throughout the integration of high resolution TC models is general practice in research and operational endeavor over the North Indian Ocean. The present study is to assess the impact of updating realistic SST in TC lifetime on track, intensity and rainfall of TCs. The Hurricane Weather Research Forecast (HWRF) model of single domain with 9km resolution is used. A total of 31 forecast cases are considered from 6 TCs during 2007-16 with unique track and intensity characteristics. A set of two numerical experiments are done without (CNTL) and with 6-hourly SST update (SST) in TC lifetime.
Mean track and intensity errors show that there is an improvement of 3–41% in track during 12–120h forecast length for SST run. The SST runs improve landfall position and time prediction by 20% and 33% respectively. The cross track error of SST run is comparable (44km) with average errors available for this basin (34km); and along track errors are improved by 60% as compared to CNTL as well as average errors of the basin. The model is biased to overestimate a weaker TC and underestimate a stronger TC, however, the bias is reduced in SST run by 5–51%. The analyses of wind, enthalpy flux and warm core structures provide insight for realistic intensity prediction of SST run unlike CNTL. Rainfall intensity and radial distribution is also improved in SST run. Thus, this study highlights the significance of ocean coupling with TC models to advance forecast guidance.

This paper reviews the major achievements of the Working Group on Meteorology (WGM) of ESCAP/WMO Typhoon Committee since its establishment in 2004, especially in tropical cyclone observational research and scientific experiments, tropical cyclone monitoring and forecasting technologies, seasonal prediction and climate change assessment for the past decade. The progress illustrates the great value of the Committee and WGM in monitoring and forecasting of tropical cyclones in the region and the improvement of disaster prevention and reduction capabilities.

Hurricane Michael was intensifying as it made landfall devastating areas of the Florida Panhandle including the small town of Mexico Beach. The structure of the hurricane is examined using radar wind data made available from aircraft reconnaissance missions. This showed a dominant warm air advection configuration (winds turning in direction in an anticyclonic fashion with height) around the core of the hurricane. Conventional radiosonde data was also used to study the warm air advection environment east of a deep layered tough system which Michael moved into and which appeared to favour such strong intensification. The structure of this deep trough is also examined and compared with a situation where Hurricane Dennis in 2005 weakened as it approached the coast in much the same region. It appears that the thermal structure of the upper trough at low to middle levels is critical to whether the hurricane intensifies or weakens with the presence of strong cold air advection associated with weakening.

Hurricanes Lane intensified in the Central Pacific with data from reconnaissance aircraft indicating it reached category 5 intensity on the Saffir-Simpson Hurricane Wind Scale while moving closer to Hawaii. The cyclone weakened as it moved closer to the Hawaiian Islands with its closest approach at 1500UTC 25 August 2018 when 175km south southwest of Honolulu with a central pressure of 995hPa. The impact on Hawaii was mainly record rainfall and the structure of this weather system is examined here to show how the winds turning anticyclonically with height in the lower to middle level troposphere play a crucial role in the intensification of Hurricane Lane to category 5 intensity and the generation of extreme rainfall.