Urban Impact on Landfalling Tropical Cyclone Precipitation:A Numerical Study of Typhoon Rumbia(2018)

Coastal urban areas are prone to serious disasters caused by landfalling tropical cyclones(TCs). Despite the crucial role of urban forcing in precipitation, how fine-scale urban features impact landfalling TC precipitation remains poorly understood. In this study, high-resolution ensemble simulations of Typhoon Rumbia(2018), which crossed the Yangtze River Delta urban agglomeration, were conducted to analyze the potential urban impact on TC precipitation. Results show that the inner-core rainfall of Rumbia is strengthened by approximately 10% due to the urban impact near the landfall,whereas minor differences in outer-core rainfall are found when the urban impact is excluded. Further diagnostic analyses indicate that low-level upward motion is crucial for precipitation evolution, as both co-vary during landfall. Moreover, the frictionally induced upward motion plays a decisive role in enhancing the rainfall when the urban impacts are included.Urban surface friction can decelerate the tangential wind and therefore destroy the gradient balance and strengthen the radial wind within the boundary layer and thus can enhance upward motion. This study demonstrates that urban surface friction and related physical processes make the most significant contribution to landfalling TC rainfall enhancement.

An Overview of Research and Forecasting on Rainfall Associated with Landfalling Tropical Cyclones

The ability to forecast heavy rainfall associated with landfalling tropical cyclones （LTCs） can be improved with a better understanding of the mechanism of rainfall rates and distributions of LTCs. Research in the area of LTCs has shown that associated heavy rainfall is related closely to mechanisms such as moisture transport, extratropical transition （ET）, interaction with monsoon surge, land surface processes or topographic effects, mesoscale convective system activities within the LTC, and boundary layer energy transfer etc.. LTCs interacting with environmental weather systems, especially the westerly trough and mei-yu front, could change the rainfall rate and distribution associated with these mid-latitude weather systems. Recently improved technologies have contributed to advancements within the areas of quantitative precipitation estimation （QPE） and quantitative precipitation forecasting （QPF）. More specifically, progress has been due primarily to remote sensing observations and mesoscale numerical models which incorporate advanced assimilation techniques. Such progress may provide the tools necessary to improve rainfall forecasting techniques associated with LTCs in the future.

Impact of the Monsoonal Surge on Extreme Rainfall of Landfalling Tropical Cyclones

A comparative analysis and quantitative diagnosis has been conducted of extreme rainfall associated with landfalling tropical cyclones(ERLTC)and non-extreme rainfall(NERLTC)using the dynamic composite analysis method.Reanalysis data and the tropical cyclone precipitation dataset derived from the objective synoptic analysis technique were used.Results show that the vertically integrated water vapor transport(Q_(vt))during the ERLTC is significantly higher than that during the NERLTC.The Q_(vt)reaches a peak 1−2 days before the occurrence of the ERLTC and then decreases rapidly.There is a stronger convergence for both the Q_(vt)and the horizontal wind field during the ERLTC.The Q_(vt)convergence and the wind field convergence are mainly confined to the lower troposphere.The water vapor budget on the four boundaries of the tropical cyclone indicates that water vapor is input through all four boundaries before the occurrence of the ERLTC,whereas water vapor is output continuously from the northern boundary before the occurrence of the NERLTC.The water vapor inflow on both the western and southern boundaries of the ERLTC exceeds that during the NERLTC,mainly as a result of the different intensities of the southwest monsoonal surge in the surrounding environmental field.Within the background of the East Asian summer monsoon,the low-level jet accompanying the southwest monsoonal surge can increase the inflow of water vapor at both the western and southern boundaries during the ERLTC and therefore could enhance the convergence of the horizontal wind field and the water vapor flux,thereby resulting in the ERLTC.On the other hand,the southwest monsoonal surge decreases the zonal mean steering flow,which leads to a slower translation speed for the tropical cyclone associated with the ERLTC.Furthermore,a dynamic monsoon surge index(DMSI)defined here can be simply linked with the ERLTC and could be used as a new predictor for future operational forecasting of ERLTC.

STATISTICAL ANALYSIS ON THE INFLUENCE OF THE LANDFALLING STRONG TROPICAL CYCLONES IN THE CATASTROPHIC MIGRATIONS OF NILAPARVATA LUGENS(STL) IN CHINA

In order to clarify the statistical pattern by which landfalling strong tropical cyclones(LSTCs)influenced the catastrophic migrations of rice brown planthopper(BPH),Nilaparvata lugens(stl)in China,the data of the LSTCs in China and the lighting catches of BPH that covered the main Chinese rice-growing regions from 1979 to 2008 were collected and analyzed in this work with the assistance of ArcGIS9.3,a software of geographic information system.The results were as follows:(1)In China,there were 220 strong tropical cyclones that passed the main rice-growing regions and 466 great events of BPH’s immigration in the 30 years from 1979 to 2008.73 of them resulted in the occurrence of BPH’s catastrphic migration(CM)events directly and 147 of them produced indirect effect on the migrations.(2)The number of the LSTCs was variable in different years during 1979 to 2008 and their influence was not the same in the BPH’s northward and southward migrations in the years.In the 30 years,the LSTCs brought more obvious influence on the migrations in 1980,1981,2005,2006 and 2007.The influence was the most obvious in2007 and all of the 7 LSTCs produced remarkable impact on the CMs of BPH’s populations.The effect of the LSTCs on the northward immigration of BPH’s populations was the most serious in 2006 and the influence on the southward immigration was the most remarkable in 2005.(3)In these years,the most of LSTCs occurred in July,August and September and great events of BPH's immigration occurred most frequently in the same months.The LSTCs played a more important role on the CM of BPH’s populations in the three months than in other months.(4)The analysis on the spatial distribution of the LSTCs and BPH’s immigration events for the different provinces showed that the BPH’s migrations in the main rice-growing regions of the Southeastern China were influenced by the LSTCs and the impact was different with the change of their spatial probability distribution during their passages.The most serious influence of the LSTCs on the BPH’s migrations occurred in Guangdong and Fujian provinces.(5)The statistical results indicated that a suitable insect source is an indispensable condition of the CMs of BPH when a LSTC influenced a rice-growing region.

Possible Linkage Between Tropical Indian Ocean SST Anomalies and the Date of First and Last Tropical Cyclones Landfalling in the Chinese Mainland

Bases on the NCEP/NCAR reanalysis products,Had ISST dataset,and data of tropical cyclone(TC)landfalling in the Chinese mainland during 1960-2019,the possible impacts of Indian Ocean Dipole(IOD)mode and Indian Ocean basin(IOB)mode on the last-TC-landfall date(LLD)and first-TC-landfall date(FLD),respectively,are investigated in this study.The LLD is in significantly negative correlation with autumn IOD on the interannual timescale and their association is independent of El Nino-Southern Oscillation(ENSO).The LLD tends to be earlier when the IOD is positive while becomes later when the IOD is negative.An anomalous lower-level anticyclone is located around the Philippines during October-November,resulting from the change of Walker circulation over the tropical Indo-west Pacific Ocean forced by sea surface temperature(SST)anomalies related to a positive IOD event.The Philippines anticyclone anomaly suppresses TC formation there and prevents TCs from landfalling in the Chinese mainland due to the anomalous westerly steering flows over southeast China during October-November,agreeing well with the earlier LLD.However,the robust connection between spring IOB and FLD depends on ENSO episodes in the preceding winter.There is an anticyclonic anomaly around the Philippines caused by the tropical SST anomalies through modulating the Walker circulation during May-June when the IOB is warming in the El Nino decaying phase.Correspondingly,the TC genesis is less frequent near the Philippines and the mid-level steering flows associated with the expanded western Pacific subtropical high are disadvantageous for TCs moving towards southeast China and making landfall during May-June,in accordance with the later FLD.By contrast,cooling IOB condition in spring of a La Nina decaying year and negative IOD cases during autumn could produce a completely reversed atmospheric circulation response,leading to an earlier FLD and a later LLD over the Chinese mainland,respectively.

Importance of Air-Sea Coupling in Simulating Tropical Cyclone Intensity at Landfall

An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones(TCs)along the South China coast is upgraded by including ocean and wave models.A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea.Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size.In particular,interaction with the ocean does not necessarily imply a weakening of the TC,with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC.These results suggest that to simulate the evolution of TC structure more accurately,it is essential to use an air-sea coupled model instead of an atmosphere-only model.

On the Improvement of the DSAEF_LTP Model to Heavy Precipitation Simulation of Landfalling Tropical Cyclones over China in 2018

In this study,the Dynamical-Statistical-Analog Ensemble Forecast model(DSAEF_LTP model)for landfalling tropical cyclone(LTC)precipitation was employed to simulate the precipitation of 10 LTCs that occurred over China in 2018.With similarity region scheme(SRS)parameter values added and TC intensity introduced to the generalized initial value(GIV),four groups of precipitation simulation experiments were designed to verify the forecasting ability of the improved model for more TC samples.Results show that the simulation ability of the DSAEF_LTP model can be optimized regardless of whether adding SRS values only,or introducing TC intensity into GIV,while the experiment with both the two improvements shows a more prominent advantage in simulating the heavier precipitation of LTCs.Compared with four NWP models(i.e.,ECMWF,GFS,GRAPES and SMS-WARMS),the overall forecasting performance of the DSAEF_LTP model achieves a better result in simulating precipitation at the thresholds over 250 mm and performs slightly better than NWP models at the thresholds over 100 mm.

Growing Threat of Rapidly-Intensifying Tropical Cyclones in East Asia

This study examines the long-term change in the threat of landfalling tropical cyclones(TCs) in East Asia over the period 1975–2020 with a focus on rapidly intensifying(RI) TCs. The increase in the annual number of RI-TCs over the western North Pacific and the northwestward shift of their genesis location lead to an increasing trend in the annual number of landfalling RI-TCs along the coast of East Asia. The annual power dissipation index(PDI), a measure of the destructive potential of RI-TCs at landfall, also shows a significant increasing trend due to increases in the annual frequency and mean landfall intensity of landfalling RI-TCs. The increase in mean landfall intensity is related to a higher lifetime maximum intensity(LMI) and the LMI location of the landfalling RI-TCs being closer to the coast. The increase in the annual PDI of East Asia is mainly associated with landfalling TCs in the southern(the Philippines, South China, and Vietnam) and northern parts(Japan and the Korean Peninsula) of East Asia due to long-term changes in vertical wind shear and TC heat potential. The former leads to a northwestward shift of favorable environments for TC genesis and intensification, resulting in the northwestward shift in the genesis, RI, and LMI locations of RI-TCs. The latter provides more heat energy from the ocean for TC intensification, increasing its chances to undergo RI.

Experiments with the Improved Dynamical-Statistical-Analog Ensemble Forecast Model for Landfalling Typhoon Precipitation over South China

In recent work,three physical factors of the Dynamical-Statistical-Analog Ensemble Forecast Model for Landfalling Typhoon Precipitation(DSAEF_LTP model)have been introduced,namely,tropical cyclone(TC)track,TC landfall season,and TC intensity.In the present study,we set out to test the forecasting performance of the improved model with new similarity regions and ensemble forecast schemes added.Four experiments associated with the prediction of accumulated precipitation were conducted based on 47 landfalling TCs that occurred over South China during 2004-2018.The first experiment was designed as the DSAEF_LTP model with TC track,TC landfall season,and intensity(DSAEF_LTP-1).The other three experiments were based on the first experiment,but with new ensemble forecast schemes added(DSAEF_LTP-2),new similarity regions added(DSAEF_LTP-3),and both added(DSAEF_LTP-4),respectively.Results showed that,after new similarity regions added into the model(DSAEF_LTP-3),the forecasting performance of the DSAEF_LTP model for heavy rainfall(accumulated precipitation≥250 mm and≥100 mm)improved,and the sum of the threat score(TS250+TS100)increased by 4.44%.Although the forecasting performance of DSAEF_LTP-2 was the same as that of DSAEF_LTP-1,the forecasting performance was significantly improved and better than that of DSAEF_LTP-3 when the new ensemble schemes and similarity regions were added simultaneously(DSAEF_LTP-4),with the TS increasing by 25.36%.Moreover,the forecasting performance of the four experiments was compared with four operational numerical weather prediction models,and the comparison indicated that the DSAEF_LTP model showed advantages in predicting heavy rainfall.Finally,some issues associated with the experimental results and future improvements of the DSAEF_LTP model were discussed.

An application of the LTP＿DSEF model to heavy precipitation forecasts of landfalling tropical cyclones over China in 2018

Recently, a track-similarity-based Dynamical-Statistical Ensemble Forecast(LTP_DSEF) model has been developed in an attempt to predict heavy rainfall from Landfalling Tropical cyclones(LTCs). In this study, the LTP_DSEF model is applied to predicting heavy precipitation associated with 10 LTCs occurring over China in 2018. The best forecast scheme of the model with optimized parameters is obtained after testing 3452 different schemes for the 10 LTCs. Then, its performance is compared to that of three operational dynamical models. Results show that the LTP_DSEF model has advantages over the three dynamical models in predicting heavy precipitation accumulated after landfall, especially for rainfall amounts greater than 250 mm. The model also provides superior or slightly inferior heavy rainfall forecast performance for individual LTCs compared to the three dynamical models. In particular, the LTP_DSEF model can predict heavy rainfall with valuable threat scores associated with certain LTCs, which is not possible with the three dynamical models. Moreover, the model can reasonably capture the distribution of heavier accumulated rainfall, albeit with widespread coverage compared to observations. The preliminary results suggest that the LTP_DSEF model can provide useful forecast guidance for heavy accumulated rainfall of LTCs despite its limited variables included in the model.

Large-scale characteristics of landfalling tropical cyclones with abrupt intensity change

Data from the China Meteorological Administration and ERA-Interim are used to examine the environmental characteristics of landfalling tropical cyclones(TCs)with abrupt intensity change.The results show that,of all 657 landfalling TCs during 1979-2017,71%,70%and 65%of all landfalling TDs,TSs and TYs,respectively,intensify.Of all the 16595 samples,4.0%and 0.2%of typhoons and tropical storms,respectively,experience over-water rapid intensification(RI)process during their life cycle.Meanwhile,4.5%and 0.6%of typhoons and tropial storms,respectively,undergo overwater rapid decay(RD).These two kinds of cases,i.e.,RI and RD,are used to analyze their associated large-scale conditions.Comparisons show that the RI cases are generally on the south side of the strong western Pacific subtropical high(WPSH);warm sea surface temperatures(SSTs)and sufficient water vapor fluxes existing in RI samples is a dominant feature that is conducive to the development of TCs.Also,the moderate low-level relative vorticity is favorable for TC intensification.On the contrary,the RD TCs are located on the west side of the WPSH;significant decreasing SSTs and low-level water vapor transport may synergistically contribute to RD.Simultaneously,low-level relative vorticity seems to be unfavorable for the development of TCs.

Characteristics of extreme rainfall and rainbands evolution of Super Typhoon Lekima (2019) during its landfall

As one of the most devastating tropical cyclones over the western North Pacific Ocean,Super Typhoon Lekima(2019)has caused a wide range of heavy rainfall in China.Based on the CMA Multi-source merged Precipitation Analysis System(CMPAS)-hourly data set,both the temporal and spatial distribution of extreme rainfall is analyzed.It is found that the heavy rainfall associated with Lekima includes three main episodes with peaks at 3,14 and 24 h after landfall,respectively.The first two rainfall episodes are related to the symmetric outburst of the inner rainband and the persistence of outer rainband.The third rainfall episode is caused by the influence of cold,dry air from higher latitudes and the peripheral circulation of the warm moist tropical storm.The averaged rainrate of inner rainbands underwent an obvious outburst within 6 h after landfall.The asymmetric component of the inner rainbands experienced a transport from North(West)quadrant to East(South)quadrant after landfall which was related to the storm motion other than the Vertical Wind Shear(VWS).Meanwhile the outer rainband in the vicinity of three times of the Radius of Maximum Wind(RMW)was active over a 12-h period since the decay of the inner rainband.The asymmetric component of the outer rainband experienced two significant cyclonical migrations in the northern semicircle.