MAUSAM, 71, 4 (October 2020), 649-660 551.515.2 : 551.509.32 A forecasting approach of tropical cyclone genesis based on thresholds of multi-physical parameters and its verification using ECMWF model data WEN FENG#, *, LI ZHU**, WAIKIN WONG***, CHUNWING CHOY*** and JUNFENG MIAO# #College of Atmospheric Sciences, Nanjing University of Information Science and Technology, Nanjing 210 044, China *Hainan Meteorological Observatory, Haikou 510203, China **Taizhou Meteorological Observatory, Taizhou 225300, China ***Hong Kong Observatory, Hong Kong 99907, China (Received 31 January 2020, Accepted 15 September 2020) e mail : [email protected] सार — हालांिक काफी अययन ने सािबत िकया है िक उणकिटबंधीय चक्रवात (टीसी) के माग र् और तीता का पूवार्नमानु संख्यामक मौसम पूवार्नुमान (NWP) मॉडल से प्रा जानकारी पर बहुत अिधक िनभरर् करता है, कु छ शोध से पता चला है िक पिमी उरी प्रशांत महासागर (WNP) बेिसन म NWP मॉडल उणकिटबंधीय चक्रवात की उपि का िकतना अछा पूवार्नमानु लगाते ह। NWP मॉडल द्वारा WNP बेिसन म उणकिटबंधीय चक्रवात उपि पूवार्नमानु की िवशेषताओं को समझन े के िलए, यह अययन ऐितहािसक आंकड़ े का उपयोग करते हुए उणकिटबंधीय चक्रवात के बनने की पहचान हेतु मापदंड का एक सेट प्रा करता है तथा 2013 और 2015 के बीच ECMWF मॉडल आंकड़ े के आधार पर इसे सयािपत करता है - पिरणाम बताते ह िक प्रभाव सीमा मानदंड के आधार पर अपनाए गए प्रितशतता मूय का एगोिरदम के प्रदशनर् पर महवपूण र् प्रभाव पड़ता है। एक िविश अंतराल पर प्रभाव सीमा का उिचत समायोजन प्रभावी प से टीसी की उपि के पूवार्नुमान को बेहतर बना सकता है। उदाहरण के िलए, WNP बेिसन म पूवार्नमानु पिरणाम 850 hPa तर पर सापेिक्षक भ्रिमलता छोटे पिरवतनर् के िलए सबसे अिधक संवेदनशील ह। अनुकू लतम प्रभाव सीमा संयोजन योजना के पूवार्नमानु परीक्षण के पिरणाम बताते ह िक इसके प्रदशनर् का काल 24 से 48 घंटे के बीच होता है और उणकिटबंधीय चक्रवात के बनने के 12-72 घंटे पहले िहट दर से संबंिधत होता है। 24 घंटे से कम के लीड समय के िलए, िहट दर मूल प से थोड़ी कमी के साथ उच तर पर 0.7 से ऊपर रखी गई थी। उसके बाद, प्रदशनर् 48 घंटे से आगे िथर होने से पहले तेजी से िगरता है। इसके अलावा, ईसीएमडयूएफ मॉडल म उणकिटबंधीय चक्रवात की उपि के पूवार्नमानु का प्रदशनर् साल-दर-साल और िविभन डयूएनपी क्षेत्र म भी िभन िभन रहा है। यह दिक्षण चीन सागर (SCS) की तुलना म िफलीपींस के पूव र् म बेहतर प्रदशनर् करता है। दसरीू ओर, उच गलत चेतावनी (FA) दर दिक्षण चीन सागर के मय भाग म िफलीपींस और पिमी उरी प्रशांत महासागर (WNP) के मय भाग के आसपास के जल क्षेत्र म पाए जाते ह। ECMWF के प्रदशर्न म िविभन बेिसन म महवपूण र् िवसंगित भी देखी जा सकती है। एक उणकिटबंधीय चक्रवात की उपि से पहले 24 घंट के भीतर अटलांिटक बेिसन के िलए िदए गए पूवार्नमानु ,पिमी उरी प्रशांत महासागर (WNP) बेिसन के िलए पूवार्नमानु से बेहतर होता है। ABSTRACT. While considerable studies have proved that the track and intensity forecasts of tropical cyclone (TC) relied heavily on output from numerical weather prediction (NWP) model, few researches investigated how well NWP models forecast TC genesis in the western North Pacific (WNP) basin. In order to understand the characteristics of TC genesis forecast in WNP basin by NWP models, this study derives a set of criteria to identify the formation of TC using historical data and verifies it based on ECMWF model data between 2013 and 2015. The results show that the percentile values adopted as the criteria thresholds have a significant impact on the performance of algorithm based on the criteria. A reasonable adjustment of threshold in a specific interval can effectively improve the TC genesis prediction. For example, in the WNP basin the forecast results are most sensitive to small changes in the relative vorticity on the 850 hPa level. The results of forecast test of the optimal threshold combination scheme indicate that the turning point of performance lies between 24 and 48 hours with regard to the hit rate in the 12-72 hours prior to the formation of TC. For lead time less than 24 hours, the hit rate was basically maintained at a high level above 0.7 with a small decrease. After that, the performance drops sharply before stabilizing beyond 48 hours. In addition, the performance of the TC genesis prediction in ECMWF model varies significantly from year to year and also in different WNP regions. It performs better to the east of the Philippines than over the South China Sea (SCS). On the other hand, high false alarm (FA) rates are found in the central parts of the SCS up to the waters around the Philippines and the central part of the WNP. The (649) 650 MAUSAM, 71, 4 (October 2020) significant discrepancy in ECMWF’s performance can also be observed between different basins. Within the 24 hours before the genesis of a TC, the forecasts for the WNP basin verify better than those for the Atlantic basin. Key words – Tropical cyclone, Genesis forecast criteria, Verification, NWP model product. 1. Introduction A clear definition of a model-generated TC is a major key to evaluating the performance of TC genesis The western North Pacific (WNP) is the most active forecast by NWP models. Although the theory of tropical area of tropical cyclones in the world. On average, about cyclogenesis is controversial, the conditions of tropical 33 TCs are generated here each year, accounting for about cyclone generation are widely accepted. In the late 1940s, one-third of global total. More than 80% of them would Palmén (1948) and Riehl (1948) proposed that TC eventually develop from tropical depression into tropical generation needs to meet certain environmental conditions. storms or above (Frank and Young, 2007; Feng et al., Gray (1968) then conducted a systematic climatological 2012; Oropeza and Raga, 2015). Under the influence of study of the genesis and development of TCs worldwide, weather systems such as subtropical ridge and monsoon and pointed out that the frequency of TC occurrence is trough, most of the TCs forming in the WNP would make related to six factors. Based on climatological factors landfall over China, Japan, the Philippines, Vietnam, and discussed in Gray (1968), Tory and Frank (2010) proposed Korean Peninsula, bringing severe economic loss and high five necessary conditions for TC genesis: Sea surface casualties to the regions. Operational TC forecasts mainly temperature higher than 26.5 °C with a mixing layer of rely on outputs from NWP models. However, each model about 50 m; A relatively deep layer of conditional has its unique characteristics, strengths and weaknesses in instability; An increasing positive vorticity at low levels; capturing the genesis and intensification of TCs. Large-scale ascent, a humid mid-troposphere with organized deep convection, and weak to moderate vertical It is still a great challenge for NWP models to wind shear. provide an accurate forecast of TC genesis, because of the scientific hypotheses behind tropical cyclogenesis remain On the basis of TC genesis climatology, previous actively debated. Starting from Riehl (1950), many studies (Bengtsson et al., 1995; Vitart and Anderson, 2001; theories like CISK (the conditional instability of the Oouchi et al., 2006) referred to thresholds of winds at 10m second kind) (Ooyama, 1964; Charney and Eliassen, or at 850 hPa level, relative vorticity at 850 hPa, average 1964), WISHE (wind-induced surface heat exchange), temperature of low to mid-tropospheric levels (700-300 (Emanuel, 1986) “top down” (Ritchie and Holland, 1997; hPa) obtained in NWP models to define a model generated Simpson et al., 1997), “bottom up” (Enagonio and TC. Based on this notion, Tory et al. (2013) replaced the Montgomery, 2001), VHTs (vortical hot towers) relative vorticity by absolute vorticity and added Okubo- (Hendricks et al., 2004) and “pouch” (Dunkerton et al., Weiss parameter and environmental parameters to identify 2009; Wang et al., 2012; Wang, 2014) proposed by many genesis in NWP models. Majumdar and Torn (2014) researchers, aimed at the understanding of TC genesis. further improved Tory’s formalism and set three thresholds However, the theories could not completely explain how to estimate TC genesis location: (i) layer- cumulus convection organizes, or a tropical weak averaged 700-850 hPa relative vorticity computed over a disturbance develops into a large-scale TC with organized disk of radius 200 km; (ii) 200-850 hPa thickness deep convection and a closed surface wind circulation difference computed over a disk of radius 200 km; pattern about a well-defined center (Rajasree et al., 2016). (iii) the local minimum of mean sea level pressure (MSLP) within 5° in latitude or longitude. The values of the above Limited by theoretical studies, grid distances and parameters were collected at the time when a TC formed calculation speeds, it is difficult for numerical models to and thresholds can be obtained using different quantile of perfectly describe the generation process of tropical the collected samples.