Case study of high waves in the South Pacific generated by Tropical Cyclone Harold in 2020

This study highlighted a high wave case by severe tropical cyclone Harold and conducted a simulation with a newly developed wave forecasting system for the South Pacific based on the Japan Meteorological Agency third generation wave model(JMA MRI-III)using the National Center for Environment Prediction Global Forecast System(GFS)winds.Harold was a very intense tropical cyclone(TC)and very high waves up to 10 m affected parts of Vanuatu and Fiji.The model results were reasonable and verified against observations of orbital satellites and a wave buoy at Komave in Fiji.The statistical verifications were carefully analysed.The Root Mean Squared Error(RSME),Scatter Index(SI),Bias and R2 are all showing very impressive results.The new wave forecasting system is the first high resolution operational model at Fiji Meteorological Service(FMS),which covers the whole Fiji area.The system will provide guidance to FMS in preparing marine alerts and warning better and more confidence in providing the marine forecast accurately.

Assessing global ensemble systems’forecasts of tropical cyclone genesis in differing environmental flow regimes in the western North Pacific

The forecast probability of tropical cyclone(TC)genesis in the western North Pacific from 2017 to 2020 was investigated using global ensembles from the Japan Meteorological Agency(JMA),the European Centre for Medium-Range Weather Forecasts(ECMWF),the U.S.National Centers for Environmental Prediction(NCEP),and the Met Office in the United Kingdom(UKMO).The time of TC genesis was defined as the time the TCs were first recorded in the best-track data(Case 1)and as the time they reached the intensity of a Tropical Storm(Case 2).The results in Case 1 showed that differences between the forecast probability based on each global ensemble were large,even for a 1-day forecast,and that mean probability were from 18%to 74%.The forecasts based on the NCEP had a large frequency bias and overpredicted TC genesis events.The results indicated that the representation of genesis events differed greatly between global ensembles.The effectiveness of multiple ensembles was investigated.The results from the threat score and the false alarm ratio indicated that multiple ensembles had skillful forecasts.When the forecast probability was examined for environmental patterns of synoptic low-level flow,the mean 5-day forecast probability was highest for the pattern in the confluence region.The results also showed that the forecast probability was much larger in Case 2 than in Case 1.In all global ensembles,the mean probability with a lead time of up to 1-week was below 10%for both Case 1 and 2.This result indicates that even with today's operational forecasting systems,it is difficult to regularly predict TC genesis events with a 1-week lead time with high confidence.These results provide a better understanding of TC genesis forecast products in each global ensemble and will be useful information when multiple-ensemble products are created.

A GLOBAL VIEW OF THE LANDFALL CHARACTERISTICS OF TROPICAL CYCLONES

This report presents a climatology of the landfall characteristics of tropical cyclones(TCs) by country. Landfalling TCs can produce strong winds, storm surges and severe flooding that may result in loss of life and widespread damages. Although historically many governments have estimated the frequency of TC landfall for their own country, less work has been undertaken from a global viewpoint. This report addresses that gap by presenting a comparative assessment that may assist global efforts toward disaster risk reduction through improved understanding of the relative level of exposure of countries to TCs. The various aspects of the climatology of TC landfalls for each country are provided. Using the definition of a TC landfall as the intersection of the TC track with a coastline, eleven countries were detected as having an average of at least one landfall per year. China has the highest rate of TC landfalls. The Philippines ranks second and Japan third. A lot of countries that face the North Western Pacific were included in TC-Landfall countries, which is attributed to the high rate of TC formation over the basin.

Tropical cyclogenesis:Controlling factors and physical mechanisms

In this review,advances in the understanding of the controlling factors and physical mechanisms of tropical cyclogenesis(TCG)are summarized from recent(2018–2022)research on TCG,as presented in the Tenth International Workshop on Tropical Cyclones(IWTC-10).Observational,theoretical,and numerical modeling studies published in recent years have advanced our knowledge on the influence of large-scale environmental factors on TCG.Furthermore,studies have shown clearly that appropriate convective coupling with tropical equatorial waves enhances the development chances of TCG.More recently,illuminating research has been carried out on analyzing the mechanisms by which oscillations and teleconnections(El Ni˜no Southern Oscillation(ENSO)in particular)modulate TCG globally,in association with changes in the sea surface temperature(SST).In addition to this,recent research has diligently addressed different aspects of TCG.Multiple studies have reported the applicability of unified theories and physical mechanisms of TCG in different ocean basins.Recently,research has been carried out on TCG under different flow pattern regimes,dry air intrusion,importance of marsupial pouch,genesis of Medicanes,wind shear,convection and vertical structure.Furthermore,studies have discussed the possibility of near equatorial TCG provided that there is enough supply of background vertical vorticity and relatively low vertical wind shear.Progress has been made to understand the role of climate change on global and regional TCG.However,there are still significant gaps which need to be addressed in order to better understand TCG prediction.