Technical Note on a Track-pattern-based Model for Predicting Seasonal Tropical Cyclone Activity over the Western North Pacific

Recently, the National Typhoon Center （NTC） at the Korea Meteorological Administration launched a track-pattern-based model that predicts the horizontal distribution of tropical cyclone （TC） track density from June to October. This model is the first approach to target seasonal TC track clusters covering the entire western North Pacific （WNP） basin, and may represent a milestone for seasonal TC forecasting, using a simple statistical method that can be applied at weather operation centers. In this note, we describe the procedure of the track-pattern-based model with brief technical background to provide practical information on the use and operation of the model. The model comprises three major steps. First, long-term data of WNP TC tracks reveal seven climatological track clusters. Second, the TC counts for each cluster are predicted using a hybrid statistical-dynamical method, using the seasonal prediction of large-scale environments. Third, the final forecast map of track density is constructed by merging the spatial probabilities of the seven clusters and applying necessary bias corrections. Although the model is developed to issue the seasonal forecast in mid-May, it can be applied to alternative dates and target seasons following the procedure described in this note. Work continues on establishing an automatic system for this model at the NTC.

Two climate factors in May that affect Korean rainfall in September

This study revealed a high positive correlation between rainfall in Korea during September and the trade wind （TW）/Arctic Oscillation （AO） index in May that combines two climate factors, low-level TWs and the AO. This correlation was identified on the basis of the difference in the 850 hPa streamline analysis between the positive and negative phases selected using the combined TW/AO index. In May, the spatial pattern of the anomalous pressure systems is similar to that in the positive AO phase. These anomalous pressure sys- tems continue in June to August （JJA） and September, but the overall spatial distribution shifts a little to the south. Particularly in September, a huge anomalous anticyclone centered over the southeast seas of Japan strengthens in most of the western north Pacific region and supplies a large volume of warm and humid air to the region near Korea. This characteristic is confirmed by the facts that during the positive TW/AO phase, the subtropical western north Pacific high （SWNPH） is more developed to the north and that the continu- ous positioning of the upper troposphere jet over Korea from May to September strengthens the anomalous upward flow, bringing warm and humid air to all layers. These factors contribute to increasing September rainfall in Korea during the positive TW/AO phase. Because the SWNPH develops more to the north in the positive phase, tropical cyclones tend to make landfall in Korea frequently, which also plays a positive role in increasing September rainfall in Korea.The above features are also reflected by the differences in average rainfall between the six years that had the highest May Nifio 3.4 indices （El Niflo phase） and the six years that had the lowest May Niflo 3.4 indices （La Nifia phase）.

Third assessment on impacts of climate change on tropical cyclones in the Typhoon Committee Region——Part Ⅰ:Observed changes, detection and attribution

Published findings on climate change impacts on tropical cyclones(TCs)in the ESCAP/WMO Typhoon Committee Region are assessed.We focus on observed TC changes in the western North Pacific(WNP)basin,including frequency,intensity,precipitation,track pattern,and storm surge.Results from an updated survey of impacts of past TC activity on various Members of the Typhoon Committee are also reported.Existing TC datasets continue to show substantial interdecadal variations in basin-wide TC frequency and intensity in the WNP.There has been encouraging progress in improving the consensus between different datasets concerning intensity trends.A statistically significant northwestward shift in WNP TC tracks since the 1980s has been documented.There is low-to-medium confidence in a detectable poleward shift since the 1940s in the average latitude where TCs reach their peak intensity in the WNP.A worsening of storm inundation levels is believed to be occurring due to sea level rise-due in part to anthropogenic influence-assuming all other factors equal.However,we are not aware that any TC climate change signal has been convincingly detected in WNP sea level extremes data.We also consider detection and attribution of observed changes based on an alternative Type II error avoidance perspective.

Third assessment on impacts of climate change on tropical cyclones in the Typhoon Committee Region-Part Ⅱ:Future projections

This paper assesses published findings on projections of future tropical cyclone(TC)activity in the ESCAP/WMO Typhoon Committee Region under climate change scenarios.This assessment also estimates the projected changes of key TC metrics for a 2℃anthropogenic global warming scenario for the western North Pacific(WNP)following the approach of a WMO Task Team,together with other reported findings for this region.For projections of TC genesis/frequency,most models suggest a reduction of TC frequency,but an increase in the proportion of very intense TCs over the WNP in the future.However,some individual studies project an increase in WNP TC frequency.Most studies agree on a projected increase of WNP TC intensity over the 21 st century.All available projections for TC related precipitation in the WNP indicate an increase in TC related precipitation rate in a warmer climate.Anthropogenic warming may also lead to changes in TC prevailing tracks.A further increase in storm surge risk may result from increases in TC intensity.The most confident aspect of forced anthropogenic change in TC inundation risk derives from the highly confident expectation of further sea level rise,which we expect will exacerbate storm inundation risk in coastal regions,assuming all other factors equal.

SEASONAL TROPICAL CYCLONE FORECASTING

This paper summarizes the forecast methods,outputs and skill offered by twelve agencies for seasonal tropical cyclone(TC)activity around the world.These agencies use a variety of techniques ranging from statistical models to dynamical models to predict basinwide activity and regional activity.In addition,several dynamical and hybrid statistical/dynamical models now predict TC track density as well as landfall likelihood.Realtime Atlantic seasonal hurricane forecasts have shown low skill in April modest skill in June and good skill in August at predicting basinwide TC activity when evaluated over 2003-2018.Real-time western North Pacific seasonal TC forecasts have shown good skill by July for basinwide intense typhoon numbers and the ACE index when evaluated for 2003-2018.Both hindcasts and real-time forecasts have shown skill for other TC basins.A summary of recent research into forecasting TC activity beyond seasonal(e.g.,multi-year)timescales is included.Recommendations for future areas of research are also discussed.

Recent progress on the seasonal tropical cyclone predictions over the western North Pacific from 2014 to 2020

This study summarized the procedure for the seasonal predictions of tropical cyclones(TCs)over the western North Pacific(WNP),which is currently operating at the Korea Meteorological Administration(KMA),Republic of Korea.The methodology was briefly described,and its prediction accuracy was verified.Seasonal predictions were produced by synthesizing spatiotemporal evolutions of various climate factors such as El Ni no–Southern Oscillation(ENSO),monsoon activity,and Madden–Julian Oscillation(MJO),using four models:a statistical,a dynamical,and two statistical–dynamical models.The KMA forecaster predicted the number of TCs over the WNP based on the results of the four models and season to season climate variations.The seasonal prediction of TCs is announced through the press twice a year,for the summer on May and fall on August.The present results showed low accuracy during the period 2014–2020.To advance forecast skill,a set of recommendations are suggested.

TROPICAL CYCLOGENESIS DETECTION IN NWP AT KMA

In this study,a tropical cyclogenesis detection system,Tropical Cyclone Analysis&Forecast(TCAF),was evaluated with an operational numerical model of the Korea Meteorological Administration(KMA).The tracking performance was compared with the result with the ECMWF model input field(TCAF-ECMWF).In order to improve the performance,different tracking time at an interval of 6 hours were investigated.The lowest false alarm rate and the highest hit rate(correct detection)were achieved at 06 hour after the initial tracking time.The tracking performance was also tested on two typhoons in 2013,LEEPI(1304)and DANAS(1324).The results showed that the TCAF-ECMWF detected tropical depressions 72 hours before the formation of the typhoon DANAS,which is a 12-hour earlier detection compared with the current performance with the use of KMA’s numerical weather prediction(NWP)model data.So,it is expected that TC genesis detection could be improved by determining an optimal tracking time and by using more accurate NWP model data.