Seasonal Prediction of Tropical Cyclones and Storms over the Southwestern Indian Ocean Region Using the Generalized Linear Models

Tropical cyclones (TCs) and storms (TSs) are among the devastating events in the world and southwestern Indian Ocean (SWIO) in particular. The seasonal forecasting TCs and TSs for December to March (DJFM) and November to May (NM) over SWIO were conducted. Dynamic parameters including vertical wind shear, mean zonal steering wind and vorticity at 850 mb were derived from NOAA (NCEP-NCAR) reanalysis 1 wind fields. Thermodynamic parameters including monthly and daily mean Sea Surface Temperature (SST), Outgoing Longwave Radiation (OLR) and equatorial Standard Oscillation Index (SOI) were used. Three types of Poison regression models (i.e. dynamic, thermodynamic and combined models) were developed and validated using the Leave One Out Cross Validation (LOOCV). Moreover, 2 × 2 square matrix contingency tables for model verification were used. The results revealed that, the observed and cross validated DJFM and NM TCs and TSs strongly correlated with each other (p ≤ 0.02) for all model types, with correlations (r) ranging from 0.62 - 0.86 for TCs and 0.52 - 0.87 for TSs, indicating great association between these variables. Assessment of the model skill for all model types of DJFM and NM TCs and TSs frequency revealed high skill scores ranging from 38% - 70% for TCs and 26% - 72% for TSs frequency, respectively. Moreover, results indicated that the dynamic and combined models had higher skill scores than the thermodynamic models. The DJFM and NM selected predictors explained the TCs and TSs variability by the range of 0.45 - 0.65 and 0.37 - 0.66, respectively. However, verification analysis revealed that all models were adequate for predicting the seasonal TCs and TSs, with high bias values ranging from 0.85 - 0.94. Conclusively, the study calls for more studies in TCs and TSs frequency and strengths for enhancing the performance of the March to May (MAM) and December to October (OND) seasonal rainfalls in the East African (EA) and Tanzania in particular.

Trend of Storm Surge Induced by Typical Landfall Super Typhoons During 1975–2021 in the Eastern China Sea

Climate change affects the activity of global and regional tropical cyclones(TCs).Among all TCs,typical super typhoons(STYs)are particularly devastating because they maintain their intensity when landing on the coast and thus cause casualties,economic losses,and environmental damage.Using a 3D tidal model,we reconstructed the typhoon(TY)wind field to simulate the storm surge induced by typical STYs.The TY activity was then analyzed using historical data.Results showed a downtrend of varying degrees in the annual frequency of STYs and TCs in the Western North Pacific(WNP)Basin,with a significant trend change observed for TCs from 1949 to 2021.A large difference in the interannual change in frequency was found between STYs and TCs in the WNP and Eastern China Sea(ECS).Along the coast of EC,the frequency of landfall TCs showed a weak downtrend,and the typical STYs showed reverse micro growth with peak activity in August.Zhejiang,Fujian,and Taiwan were highly vulnerable to the frontal hits of typical STYs.Affected by climate change,the average lifetime maximum intensity(LMI)locations and landfall locations of typical STYs in the ECS basin showed a significant poleward migration trend.In addition,the annual average LMI and accumulated cyclone energy showed an uptrend,indicating the increasing severity of the disaster risk.Affected by the typical STY activity in the ECS,the maximum storm surge area also showed poleward migration,and the coast of North China faced potential growth in high storm surge risks.

Effects of Tropical Storm on Mangroves Forest Structure of Dongzhai Harbor in Hainan Island

The effects of tropical storm on the community structure of Sonneratia caseolaris-Sonneratia apetala(S.caseolaris-S.apetala) artificial mangroves and Ceriops tagal-Rhizophora stylosa(C.tagal-R.stylosa) natural mangroves were analyzed in Dongzhai Harbor in Hainan Island,and the results showed that the average tree height,crown width(CW) in east-west direction(E-W) and north-south direction(N-S) of S.caseolaris-S.apetala artificial mangroves were decreased by 2.8%,14.3% and 12.1% respectively,but the average clear bole height was increased by 60.0% after tropical storm.For C.tagal-R.stylosa natural mangroves,the average tree height and clear bole height were increased by 8.3% and 20.0%,but there was no change in CW(E-W) and CW(N-S).Therefore,tropical storm had greater effects on artificial mangroves than natural mangroves.After tropical storm,tree heights of different species increased in the following sequence:C.tagal>R.stylosa>S.apetala>S.caseolaris,and the sequence of effect degree on CW was C.tagal>R.stylosa>S.caseolaris>S.apetala,while it was C.tagal < R.stylosa < S.caseolaris < S.apetala for clear bole height.Under the effect of tropical storm,the average biomass loss and dry biomass loss of S.caseolaris-S.apetala artificial mangroves were 0.22 and 0.13 t/hm2 respectively,while there was a minimal biomass loss in C.tagal-R.stylosa natural mangroves.On the whole,the wind resistance of natural mangroves was better than artificial mangroves,and that of C.tagal was stronger than R.stylosa,while S.caseolaris was better than S.apetala.

Effects of Vertical Wind Shear on Intensity and Rainfall Asymmetries of Strong Tropical Storm Bilis (2006)

The effects of environmental vertical wind shear （VWS） on the intensity and rainfall asymmetries in Tropical Storm （TS） Bilis （2006） have been analyzed based on TRMM/TMI-estimated surface rainfall data, QuikSCAT wind fields, 850- and 200-hPa winds of the NCEP-NCAR reanalysis, precipitation data at 5-min intervals from automatic weather stations over China's Mainland, and the best track data of TS Bilis （2006）. The results show that the simultaneous and 6-hour-lagged correlation coefficients between VWS and storm intensity （the minimum central sea level pressure） are 0.59145 and 0.57438 （P 〈0.01）, respectively. The averaged VWS was found to be about 11 m s-1 and thus suppressed the intensification of Bilis （2006）. Distribution of precipitation in Bilis （2006） was highly asymmetric. The azimuthally-averaged rainfall rate in the partial eyewall, however, was smaller than that in a major outer rainband. As the storm intensified, the major rainband showed an unusual outward propagation. The VWS had a great impact on the asymmetric distribution of precipitation. Consistent with previous modeling studies, heavy rainfall generally occurred downshear to downshear-left of the VWS vector both near and outside the eyewall, showing a strong wavenumber-one asymmetry, which was amplified as the VWS increased.

NUMERICAL SIMULATION OF THE RELATIONSHIP BETWEEN THE MAINTENANCE AND INCREASE IN HEAVY RAINFALL OF THE LANDING TROPICAL STORM BILIS AND MOISTURE TRANSPORT FROM LOWER LATITUDES

The NCEP/NCAR reanalysis, Japan Meteorological Agency(JMA) tropical cyclone tracks and intensive surface observations are used to diagnose the features of moisture transport of tropical storm Bilis(No. 0604), which is simulated by the WRF(weather research and forecasting) mesoscale numerical model. It is shown that the Bilis was linked with the moisture channel in the lower latitudes after its landing. Meanwhile, the cross-equatorial flows over 80°-100°E and Somali were active and brought abundant water vapor into the tropical storm, facilitating the maintenance of the landing storm with intensified heavy rainfall along its path. The simulation suggested that the decreased water vapor from lower latitudes prevents the maintenance of Bilis and the development of rainfall. While the cross-equatorial flows over 80°-100°E and Somali were in favor of keeping the cyclonic circulation over land. If the moisture supply fro m the Somali jet stream was reduced, the strength and area of heavy rainfall in tropical cyclone would be remarkably weakened. Consequently, the decreased water vapor from lower latitudes can remarkably suppress the deep convection in tropical storm, then Bilis was damped without the persistent energy support and the rainfall was diminished accordingly.

Derivation of Parametric Tropical Cyclone Models for Storm Surge Modeling

In this paper, the parametric tropical cyclone models for storm surge modeling are further developed. Instead of tangential wind speed via cyclostrophic balance and radial wind speed using a simple formulation of defection angle, the analyrical expressions of tangential and radial wind speed distribution are derived from the governing momentum equations based on the general symmetric pressure distribution of Holland and Fujita. The radius of the maximum wind is estimated by tropical cyclone wind structure which is characterized by the radial extent of special wind speed. The shape parameter in the pressure model is estimated by the data of several tropical cyclones that occurred in the East China Sea. Finally, the Fred cyclone （typhoon 199417） is calculated, and comparisons of the measured and calculated air pressures and wind speed are presented.

EFFECTS OF LOW-LATITUDE MONSOON SURGE ON THE INCREASE IN DOWNPOUR FROM TROPICAL STORM BILIS

By using the dataset of CMA-STI Tropical Cyclone Optimal Tracks, NCEP/NCAR reanalysis and intensive surface observations, a study is performed of the influences of a low-latitude monsoon surge on the longer persistence and increase in torrential rains from the landing tropical storm Bilis. Results suggest that the southwest monsoon was anomalously active after Bilis came ashore. The westerly winds in Bilis's south side might give rise to the poleward movement of the SW monsoon, thus enlarging the pressure gradient between Bilis and the anticyclonic circulation to the south with the result of greatly intensified SW monsoon, which fueled plentiful water vapor, heat and momentum into the declining Bilis and allowed its long stay over land instead of erosion and disappearance. Before Bilis's landfall, the 2006 East Asian monsoon surge, characterized by the atmospheric ISO, experienced remarkable northward propagation. After landfall, the strong surge and powerful low frequency vapor convergence were just on the south side of Bilis, resulting in sharply increased rainfall. In addition, a broad belt of high-valued vapor fluxes extended from the eastern Arabian Sea via the Bay of Bengal, Indochina Peninsula and the South China Sea into the south of China. The belt was linked with the SW monsoon surge forming a moist tongue stretching from the Bay of Bengal to the south of China, which supplied continuously abundant vapor for Bilis along with the surge propagating poleward.

OBJECTIVE CLASSIFICATION OF THE TRACKS OF TROPICAL STORMS IN THE BAY OF BENGAL

An objective analysis of tropical cyclone tracks is performed, with which the tracks of 131 tropical storms(TSs) in 1972-2011 are separated into three types that move west-, north- and northwestward, denoted as Types A, B and C, respectively. Type A(21 TSs and 16% of total) has the origin in the southwestern Bay of Bengal, with the TS in a unimodal distribution as its seasonal feature, occurring mainly in autumn; 18 of the 21 TSs(taking up 90%) land mostly on the western Bay coast(west of 85°E); 5% of Type-A TSs attains the wind speed of >42.7 to 48.9 m/s. Type A has little or no effect on Tibet. Type B(74 TSs, 56.6% of the total) has its preferable origin in the central Bay of Bengal, with the TS in a bimodal distribution as its seasonal pattern. This type denotes the travel in the north in spring,with the landfall of 67 of the 74 TSs(accounting for 91%) mainly on the middle coast of the Bay(85° to 95°E), and19% of the TSs reaching the wind velocity of >42.7 to 48.9 m/s, which exert great effect on Tibet and it is this TS track that gives strong precipitation on its way through this region. Type C(36 TSs, 27.5% of the total) has its main origin in the southern part of the bay, and these TSs are formed largely in autumn, moving in the northwest direction,and 23 of the 36 TSs(64%) land mostly on the western Bay coast, lasting for a longer time, with almost no impact upon Tibet.

Diurnal and Seasonal Variations of Rain Rate and Rain Attenuation on Ku-Band Satellite Systems in a Tropical Region: A Synthetic Storm Techniques Approach

In this paper, a time-varying rain characterization and diurnal variation in the Ku-band satellite systems simulated with synthetic storm techniques (SST) over a tropical location in Nigeria have been presented. Three years’ rain rate time-series data measured by a raingauge located inside the Federal University of Technology Akure, Nigeria were utilized for the purpose of this work. The analysis is based on the CDF of one-minute rain rate;time-series simulated annual/seasonal and diurnal rain rate, rain attenuation statistics and fade margins observed over four time intervals: 00:00-06:00, 06:00-12:00, 12:00-18:00 and 18:00-24:00. In addition, comparison was also made between the synthesized values and rain attenuation statistics, at 12.245 GHz for a hypothetical downlink from EUTELSAT W4/W7 satellite in the area. It could be observed that at 99.99% link availability, the fade margin as high as ~20 dB may be required at Ku band uplink frequency bands in this area. We also observed that the communication downlinks working in the early morning and early to late in the evening hours must be compensated with an appropriate Down-Link Power Control (DLPC) for optimum performances during severe atmospheric influences in the region.

Analysis on the Abnormal and Special Pathway of Number 0907 Tropical Storm "Swan"

By the comprehensive analysis on the circulation background,the pathway of tropical storm 'swan' was analyzed.The results showed that 'swan' moved toward the southwest direction.The main reasons were the spin-spin interaction of binary typhoons and the weak environmental wind field in 500 hPa.Moreover,the guidance airflow wasn't obvious,and the northeast guidance airflow in the south of continental high which was in the north of 'swan' strengthened.The asymmetric structure of wind speed near 'swan' center also favored the taxis of shift pathway.In addition,the variations of u component in south-north side and v component in east-west side of 500 hPa tropical storm center had the good prediction on the shift of 'swan'.

SAMPLE OPTIMIZATION OF ENSEMBLE FORECAST TO SIMULATE TROPICAL STORMS(MERBOK, MAWAR, AND GUCHOL) USING THE OBSERVED TRACK

Nowadays,ensemble forecasting is popular in numerical weather prediction(NWP).However,an ensemble may not produce a perfect Gaussian probability distribution due to limited members and the fact that some members significantly deviate from the true atmospheric state.Therefore,event samples with small probabilities may downgrade the accuracy of an ensemble forecast.In this study,the evolution of tropical storms(weak typhoon)was investigated and an observed tropical storm track was used to limit the probability distribution of samples.The ensemble forecast method used pure observation data instead of assimilated data.In addition,the prediction results for three tropical storm systems,Merbok,Mawar,and Guchol,showed that track and intensity errors could be reduced through sample optimization.In the research,the vertical structures of these tropical storms were compared,and the existence of different thermal structures was discovered.One possible reason for structural differences is sample optimization,and it may affect storm intensity and track.

Three-dimensional variational data assimilation of WindSat ocean surface winds for the genesis and forecasting of tropical storm Henri

With available high-resolution ocean surface wind vectors retrieved from the U.S.Naval Research Laboratory's WindSat on Coriolis,the impact of these data on genesis and forecasting of tropical storm Henri is examined using the non-hydrostatic,fifth-generation mesoscale model(MM5) of Pennsylvania State University-National Center for Atmospheric Research plus its newly released three-dimensional variational data assimilation(3DVAR) system.It is shown that the assimilation of the WindSat-retrieved ocean surface wind vectors in the 3DVAR system improves the model initialization fields by introducing a stronger vortex in the lower troposphere.As a result,the model reproduces the storm formation and track reasonably close to the observations.Compared to the experiment without the WindSat surface winds,the WindSat assimilation reduced an error between the model simulated track and observations of more than 80 km and also improved the storm intensity by nearly 2 hPa.It suggests that these data could provide early detection and prediction of tropical storms or hurricanes.

Two parametric tropical cyclone models for storm surge modeling

In this paper,the two parametric tropical cyclone models for storm surge modeling are further developed.The analytical expressions of tangential and radial velocity distribution are derived from the governing momentum equations,based on the general symmetric pressure distribution proposed by Holland and Fujita.On the basis of the data of several tropical cyclones that occurred in East China Ocean,the shape parameter in pressure model is estimated.Finally,the Fred cyclone(typhoon 199417)is calculated,and comparisons of measured and calculated air pressures and wind speed are presented.

Variations in Dissolved Oxygen Induced by a Tropical Storm Within an Anticyclone in the Northern South China Sea

Tropical storms(TSs)can induce sea surface cooling,freshening,and phytoplankton blooms.The dissolved oxygen(DO)concentration response to TSs within an anticyclone is still unclear due to the rarity of in situ observations.In this study,we investi-gate the variations in DO concentration attributed to TS‘Haitang’within an anticyclonic eddy in the northern South China Sea based on Chinese underwater glider data.DO concentrations have a higher value at the edge of eddy than at the core.Influenced by TS,DO concentrations decrease remarkably in the subsurface layer in all three regions(inside,edge,and outside of the anticyclonic eddy).The mean DO concentrations decrease more at the edge of the anticyclone than those inside the anticyclone.The recovery time of DO concentration after TS is around one week at the edge of the anticyclone and is>10 days within the eddy.Our observations show that the DO concentrations decrease above the subsurface chlorophyll a maxima layer.Quantitative analysis shows that variations in DO concentration are dominated by horizontal advection and vertical advection terms in the subsurface layer.

Analysis of Heavy Rain Process in South Liaoning Province Caused by No. 1105 Tropical Storm "Meari"

[ Objective] The aim was to study the heavy rainstorm by tropical storm ＂Meari＂ in Liaoning. [ Method] Based on the ground data, radar data, numerical report data, encrypted automatic station and NCEP reanalysis data, the heavy rain process by tropical storm ＂ Meari＂ was ana- lyzed. The changes of each physical quantity were discussed while the tropical storm moving northward. [ Result] The subtropical high pressure and Mainland high pressure jointed and maintained, forced ＂ Meari＂ turned northwest. The low-level southeast jet by ＂Meari＂ provided full water trans- mission and layer unstable conditions. The heavy rain mainly distributed in the tongue area of high energy, and the total energy was a key index to predict the rainfall area. [ Condusion] The study provided reference for the report business.

What Drives the Decadal Variability of Global Tropical Storm Days from 1965 to 2019?

The tropical storm day(TSD)is a combined measure of genesis and lifespan.It reflects tropical cyclone(TC)overall activity,yet its variability has rarely been studied,especially globally.Here we show that the global total TSDs exhibit pronounced interannual(3-6 years)and decadal(10 years)variations over the past five-to-six decades without a significant trend.The leading modes of the interannual and decadal variability of global TSD feature similar patterns in the western Pacific and Atlantic,but different patterns in the Eastern Pacific and the Southern Indian Ocean.The interannual and decadal leading modes are primarily linked to El Ni?o-Southern Oscillation(ENSO)and Pacific Decadal Oscillation(PDO),respectively.The TSDs-ENSO relationship has been steady during the entire 55-year period,but the TSDs-PDO relationship has experienced a breakdown in the 1980 s.We find that the decadal variation of TSD in the Pacific is associated with the PDO sea surface temperature(SST)anomalies in the tropical eastern Pacific(PDO-E),while that in the Atlantic and the Indian Ocean is associated with the PDO SST anomalies in the western Pacific(PDO-W).However,the PDO-E and PDO-W SST anomalies are poorly coupled in the 1980 s,and this"destructive PDO"pattern results in a breakdown of the TSDs-PDO relationship.The results here have an important implication for seasonal to decadal predictions of global TSD.

The Kinetic Energy Budget and Circulation Characteristics of the Tropical Storm Irma during AMEX Phase Ⅱ

By using the data from observation on the Chinese research vessel Xiang Yang Hong No.5 and other sources during AMEX phase II, the kinetic energy budget and circulation characteristics of the tropical storm Irma were analyzed.Irma formed on the ITCZ of the Southern Hemisphere. During the formative stage of the storm, the SE trades and monsoon westerlies on both sides of the ITCZ strengthened, and more importantly, there was a strong divergent flow in upper troposphere. These contributed to the intensification of Irma. At the time when Irma formed, the Richardson number (Ri) in middle and lower troposphere was much smaller than that prior to and post the formation.When Irma intensified rapidly, the area-averaged kinetic energy in the general flow increased in the whole troposphere . The largest contribution came from kinetic energy generation term, -[v.(?)(?)] .indicates that there existed a strong ageostrophic accetration. As to the generation term , the conversion of available potential energy to kinetic energy, - |ωα|, made the largest contribution. This illustrates the importance of internal sources and of the ensemble effect of cumulus convection to the kinetic energy.To the increase of area-averaged eddy kinetic energy during the rapid intensification of Irma, the most impor tant source in the whole troposphere was the dissipation term - [E'], that should be interpreted as the. feeding of eddy kinetic energy from smaller to larger scale disturbances. Another important source was generation term, - [v' (?)(?)'], in the lower troposphere. Rather small contribution came from the energy conversion from the kinetic energy of area-mean flow to eddy kinetic energy. Therefore, the eddy kinetic energy of the developing tropical disturbance extracted both from smaller an, .arger scale motions. The former was much more important than the latter In addition, the disturbance acting as a generator and exporter, generated and exported eddy kinetic energy to the environmental atmosphere.

A Case Study of Ocean-Atmosphere Interactions during the Passage of an Extra-Tropical Cyclone in the Vicinity of Cape Hatteras, North Carolina

The authors document the interaction of the atmosphere and ocean during the formation and passage of an Extra-Tropical Cyclone, which is a Nor-Easter, winter storm that formed in the southern apex of the Middle Atlantic Bight near Cape Hatteras North Carolina, between February 15 and 18, 1996. While Nor-Easters per se, which have formed along the Atlantic Eastern Seaboard of the United States have been studied for decades, the actual atmospheric-oceanic mechanics and thermodynamics in the formation of a Nor-Easter has never been documented. We report on having done so with in-situ observations and data-based calculations and a numerical model. The in-situ observations were made via a Control Volume consisting of an array of Eulerian Oceanic-Atmospheric Moorings with current meters, temperature and salinity sensors and meteorological towers. We find that Gulf Stream waters were located surrounding the mooring array, and that with the invasion of cold dry atmospheric air, there was a rapid loss of heat from the ocean to the atmosphere via latent and sensible surface heat flux during the cyclogenesis onset of the storm. A unique feature of this storm was that neither satellite nor buoy data showed significant sea surface cooling in the control volume. The findings indicate that storm winds drove warm saline waters from the Gulf Stream across the continental shelf into the control volume, accounting for a 51 cm rise in water level along the coast. This lateral heat advection provided heat to the control volume of 3.4e+18 Joules. On average, the heat loss at the surface of the control volume, via sensible and latent heat fluxes and radiation, was 0.7e+18 Joules, corresponding to a surface heat flux of -600 Watts per Meter2 (W/m2). However, the heat lost by the control volume as latent and sensible heat fluxes was less than the heat it received via lateral heat advection, resulting in the lack of an often-observed sea surface cooling during other winter storms. The serendipitous and detailed observations and calculations reveal a climatological flywheel in this region, documenting the role of ETCs in the global heat balance.

中国沿海地区热带气旋危险性分析

基于1980—2016年的西北太平洋热带气旋尺度资料,采用2种方法分析了热带气旋灾害的危险性。第一种方法根据热带气旋的风圈半径,用GIS生成了每个气旋的影响范围,对于登陆中国沿海的227个热带气旋,用叠加法得到了其影响范围与频次。第二种方法侧重热带气旋引发的风暴潮灾害,用潮位模型估算了热带气旋登陆时的潮位数据,提出了H3指标,该指标用潮位、风速和中心气压3个因子计算潮位点的风暴潮危险性,并根据H3指标得到了海岸线的危险性综合指标。分析结果表明,福建、广东两省受热带气旋的影响最为严重。37 a间,福建省有80%的面积(约9.8万km^(2))受热带气旋影响的频次超过20次;广东省则有70%的面积(约12.6万km2)受热带气旋影响的频次超过20次。受风暴潮危害大的海岸线主要位于广东、广西、福建、浙江四省,危险性指标在30以上的有广东的湛江市、茂名市、江门市、阳江市和珠海市;广西的北海市、防城港市和钦州市;福建的福州市以及浙江的台州市和温州市。

热带海洋的降水区域划分及相关大气背景环境特征

[目的]探究热带海洋上空的大气背景环境差异对降水特征的影响及其内在机制.[方法]使用1998—2013年热带降雨测量任务(TRMM)的逐月降水特征数据,利用聚类分析的方法将热带降水区域划分为5种典型类型,并通过湿静能(MSE)和对流有效位能(CAPE)等诊断变量探讨不同降水类型与大气背景环境之间的关系.[结果]研究发现以西太平洋暖池为代表的强降水区域表现出强烈的垂直运动和水汽背景.相反地,大陆副热带西岸等区域由于下沉异常和相对较干的大气条件,降水较为微弱.其他开放大洋区域的降水受水汽异常和大气稳定性影响,较低的稳定度和更湿的中低层大气有利于形成高空降水.[结论]本研究探讨了热带海洋上空大气背景环境与降水类型之间的关系,指出大尺度气候背景的动力热力作用对热带海洋上空降水类型的关键影响,有助于加深对热带海洋上空降水空间分布的理解.