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The upper ocean mixed‐layer depth (MLD) has been recognized as a crucial factor in understanding the impact of climate change on tropical cyclone (TC) intensity, while the specific MLD variations in the western North Pacific (WNP) basin have received comparatively little attention. Using two ocean reanalysis datasets and the output of 28 CMIP6 models, the climatological mean MLD and its trend in the TC active region are examined. Compared to the ORAS4 and EN4 datasets, the spatial pattern of the climatological mean MLD can be well simulated in the multi‐model ensemble (MME). The mean MLD decreases with increasing latitude and maintains the maximum in the equatorial region. The spatial patterns of MLD long‐term (1958–2017) and short‐term (1979–2017) trends are deepening in the TC active region, with the relatively large positive trends occurring to the southeastern WNP primarily between 5°N and 15°N. However, the simulated trend in the MME is dominant with shoaling MLD, and the shoaling of the MLD is also projected under the SSP5‐8.5 scenario. It is suggested that the bias on the simulated shoaling of the MLD in CMIP6 models may result from the overestimated enhancement of the upper‐ocean stratification. Thus, there is a discrepancy between the observed and simulated MLD trends in the WNP basin, which may have implications for TC intensity change under global warming scenarios. [ABSTRACT FROM AUTHOR]

The North Pacific monsoon trough (NPMT) is an imperative large‐scale circulation pattern influencing tropical cyclone (TC) activity in the western North Pacific (WNP), thus its future change has great implications for the WNP TC activity. Here future change in the NPMT and its uncertainty are examined by using 35 climate models from Phase six of Coupled Model Intercomparison Project (CMIP6). Due to the El Niño‐like sea surface temperature (SST) pattern, the multi‐model ensemble (MME) mean projects an eastward extension of the NPMT in the latter half of the 21st century. However, considerable inter‐model uncertainty exists in the projection, which is associated with the diversity in the zonal SST gradient across the tropical Pacific. This diversity in the projected SST is largely rooted in the simulated precipitation biases in the tropical Pacific in the historical experiments of CMIP6 models, which can modulate future Pacific SST distribution through the shortwave‐SST feedback. This linkage between biases in historical climate and future climate allows us to constrain the projection by using observational data sets. Emergent constraint using observational precipitation data set reduces the projection uncertainty by 48% and projects an eastward extension of the north Pacific NPMT by 6.2°. This eastward extension of the NPMT indicates an eastward migration of TC genesis location, and elongated TC track and thus strengthened TC intensity, suggesting an increased TC‐related disaster for residents near the WNP. Plain Language Summary: Tropical cyclones (TCs) in the western North Pacific (WNP) greatly threaten the residents in the Pacific coastal regions due to the TC‐related enormous disasters. How TCs will change in a warmer climate is of great scientific and societal concern. However, current state‐of‐the‐art climate models are still insufficient to resolve TCs. Here we take an alternative way to examine the future change in a large‐scale circulation important for TC activity (i.e., North Pacific monsoon trough (NPMT)) to infer the future WNP TC activity. It is found that large inter‐model uncertainty exists in the projected NPMT, which is linked to simulated precipitation biases of climate models. After constraining the bias by using observational data, the projection shows a robust eastward extension of the NPMT in warming scenarios, indicating an increased TC‐related disaster for residents near the WNP in the future. The results highlight the crucial role of climate models' skills in simulating the current climate in the future climate projection and have important implications for projecting future TC activity. Key Points: The large inter‐model uncertainty in projecting the North Pacific monsoon trough is linked to the diverse zonal Pacific SST gradientThis diversity is rooted in precipitation biases of CIMP6 models, which can modulate future Pacific SST through the shortwave‐SST feedbackEmergent constraint projects an eastward extension of the North Pacific monsoon trough and reduces the projection uncertainty by 48% [ABSTRACT FROM AUTHOR]

Attaining skillful decadal predictions for the western North Pacific tropical cyclone (TC) emerges as a formidable challenge, mainly stemming from the limited prediction skills of Pacific Decadal Variability (PDV) within the state‐of‐the‐art models. Assessing sixth Coupled Model Intercomparison Project models' retrospective predictions finds that the predictability of PDV transcends the expectations set by raw forecasts featured by constrained temporal skills and low signal‐to‐noise ratio. Employing a refined approach, we selectively identify the models that capture the diverse phases of PDV and subsequently adjust their variances. This tailored approach yields a compelling concordance between the predicted PDV and observation in phases and variances. Anchored in the heightened prediction skill of PDV, we establish a sophisticated statistical model adept at predicting the latitude of TC's lifetime maximum intensity (LMI). The near‐term prediction indicates a sustained poleward migration of LMI latitude by 1.53° during 2020–2027, increasing subtropical East Asia's TC‐related disaster vulnerability in the coming decade. Plain Language Summary: Tropical cyclones (TCs) account for one‐third of the deaths and economic losses from weather‐, climate‐ and water‐related disasters. This ratio tends to be much greater in the East Asia region due to its neighborhood to the busiest TC basin in the world‐the western North Pacific (WNP). However, it is particularly challenging to make skillful decadal predictions for the WNP TC activity due to the limited prediction skill of Pacific Decadal Variability (PDV), the foremost driver for the TC decadal variability. Here, we find that the capacity of the current state‐of‐the‐art climate model predicting PDV can be substantially improved. The improved prediction indicates that the latitude of the TCs' lifetime maximum intensity will migrate poleward in 2020–2027, making subtropical East Asia more vulnerable to TC disasters. The finding has profound implications for disaster prevention and mitigation strategies in subtropical East Asia, offering pivotal insights that can significantly enhance preparedness measures. Key Points: Near‐term prediction of tropical cyclone (TC) activity depends on Models' skills to predict Pacific Decadal Variability (PDV), which is limitedWe substantially improved PDV's hindcast skill and the refined approach predicts a PDV negative phase in the near‐term (2020–2027)The predicted PDV phase implies a poleward shift of location of TC's lifetime maximum intensity, increasing subtropical East Asia's TC‐related risk [ABSTRACT FROM AUTHOR]

Many studies have indicated that convective bursts (CBs) are closely related to tropical cyclone (TC) intensification, but few studies have been conducted on the mechanisms that control the formation and evolution of CBs. In this study, the 1‐min output data of a simulated TC are used to understand the convective extreme updrafts of CBs in the TC eyewall. Three different reference states including the local square‐area mean (LAM), the lower wavenumber‐components mean (WNM), and the arc‐area mean (ArcM) are used to calculate the driving forces of convective extreme updrafts. Contrary to the WNM and ArcM reference states, the LAM reference state struggles to capture realistic basic state structures. The LAM reference state can be modified through the inclusion of the mean hydrometeor mixing ratio in the basic state to produce physically consistent forcing in relation to the simulated convective extreme updrafts. The simulated convective extreme updrafts in the TC eyewall exhibit two peaks at middle and upper levels, respectively, since the effect of hydrometeor loading, decelerates the air parcels between the updraft maxima. While the positive buoyancy makes air parcels in the CBs accelerate at middle levels, in agreement with previous studies, it is found that, at the upper levels, both the positive buoyancy and the upward vertical perturbation pressure gradient force accelerate the air parcels. This study suggests that the vertical perturbation pressure gradient force also plays an important role in the formation of CBs in the TC eyewall. Plain Language Summary: This study examines the characteristics and driving forces of convective extreme updrafts of convective bursts (CBs) in the tropical cyclone (TC) eyewall using the 1‐min simulated data. The convective extreme updrafts in the TC eyewall peak at middle and upper levels. The driving forces of convective extreme updrafts are calculated using three different reference states. Results show that, in agreement with previous studies, air parcels in the CBs accelerate up by large positive buoyancy at middle levels. We also found that the air parcels accelerate again by both the positive buoyancy and the upward vertical perturbation pressure gradient force at the upper levels. This study highlights the important role of the upward vertical perturbation pressure gradient force in the formation of CBs in the TC eyewall. Key Points: Extreme updrafts associated with convective bursts in the tropical cyclone eyewall peak at the middle and upper levels, respectivelyBoth the buoyancy and the vertical perturbation pressure gradient force accelerate eyewall air parcels at upper levels in convective burstsThree methods with different reference states perform similarly in assessing driving forces for convective eyewall updrafts [ABSTRACT FROM AUTHOR]

Pacific Meridional Mode (PMM) is known to be significantly correlated with tropical cyclone (TC) genesis over the western North Pacific (WNP), while the stability of their relationship remains unknown. Here we found that their relationship is nonstationary, which depends on the phase of Pacific Decadal Oscillation (PDO). During the PDO warm phases, the PMM‐emanated cyclonic circulation and ascending motion can propagate to the entire WNP due to the enhanced background convection. In contrast, during the PDO cold phases, the PMM‐resulted cyclonic circulation and ascending motion are confined to the eastern WNP, while the compensated descending motion prevails in the western WNP. Accordingly, the PMM‐induced consistent (inconsistent) changes in large‐scale conditions across the western and eastern WNP act to strengthen (weaken) the relationship between the PMM and WNP TC genesis during the PDO warm (cold) phases. The result provides further guidance for improving seasonal prediction of TC genesis. Plain Language Summary: Billions of people in the Pacific islands and Asian coastal regions are subject to enormous tropical cyclone (TC) induced disasters. The Pacific Meridional Mode (PMM), a seasonally evolving mode of coupled climate variability, has a prominent impact on TC genesis in the western North Pacific (WNP) and is usually used as an important predictor for seasonal forecasting of TC genesis. However, stability in the relationship between PMM and TC genesis remains unclear. Here we found that their relationship is nonstationary and depends on the phase of the Pacific Decadal Oscillation (PDO), a decadal fluctuation of the Pacific Ocean. The result highlights the crucial role of PDO in modulating the relationship between the PMM and WNP TC genesis and thus provides further guidance for seasonal forecasting scheme of TC genesis. Key Points: The relationship between the Pacific Meridional Mode (PMM) and tropical cyclone genesis over the western North Pacific is nonstationaryThe nonstationary relationship stems from the diverse atmospheric responses to PMM that depend on the phase of Pacific Decadal Oscillation [ABSTRACT FROM AUTHOR]

Pacific Meridional Mode (PMM) has been found to be significantly correlated with tropical cyclone (TC) genesis in the western North Pacific (WNP), while the seasonality in their relationship remains unknown. Here we found that their relationship experiences remarkable seasonality, with a significant positive correlation in January–April and August–December but an insignificant relationship in May–July. This seasonality stems from the diverse responses of large‐scale conditions to the different magnitude of the PMM‐related sea surface temperature (SST) warming that is dependent on the strength of trade wind in the three seasons. In January–July, strong trade wind facilitates great PMM SST warming that stimulates large‐scale ascending motion in the eastern WNP but compensated descending motion in the western WNP, favoring TC genesis in January–April because its main genesis region is in the eastern WNP. However, the main genesis region extends to the whole WNP in May–July, the inconsistent large‐scale condition anomalies across the western and eastern WNP thus lead to the insignificant relationship in May–July. In August–December, the PMM‐related SST warming and the associated ascending motion fade away due to the much‐weakened trade wind, and the favorable large‐scale ascending motion over the whole WNP is linked to the PMM‐related SST cooling induced descending motion through a modification of the Walker circulation. The results highlight the season‐dependent mechanism of PMM influencing TC genesis and have some insights for improving seasonal forecasting. Plain Language Summary: As the most active tropical cyclone (TC) basin, the western North Pacific (WNP) breeds about one‐third of the global TC genesis number, bringing about enormous disasters to the Pacific islands and Asian coastal regions. Thus, understanding the climate variabilities in TC genesis over the WNP is of prodigious societal and scientific concerns. The Pacific Meridional Mode (PMM), a seasonally evolving mode of coupled climate variability, is significantly correlated with TC genesis in the WNP. However, seasonality in the correlation between the PMM and WNP TC genesis is still unknown. It is found that their relationship is season‐dependent, with a significant positive correlation in January–April and August–December but insignificant in May–July. This seasonality originates from the diverse responses of large‐scale conditions to the different magnitude of the PMM‐related sea surface temperature (SST) warming in the three seasons, which depends on the strength of trade wind. The results highlight the season‐dependent mechanism of PMM influencing TC genesis and have important implications for understanding the variations of TC genesis in the WNP. Key Points: The Pacific Meridional Mode (PMM) is significantly correlated with the western North Pacific tropical cyclone genesis in January–April and August–December but not in May–JulyThis season‐dependent influence stems from the magnitude of the PMM‐related sea surface temperature warming depending on the trade wind strength [ABSTRACT FROM AUTHOR]

China is one of the countries suffering the most tropical cyclone (TC) disasters, while changes in translation speed of TCs landfalling in China remain unclear. Here we found that the translation speed of landfalling TCs in south China during the boreal summer has significantly decreased by 0.3 m s−1decade−1 during 1979–2019, which is largely driven by a decadal deceleration in steering flow in the late 1990s. The slowdown in steering flow is found to be a balance of the opposite effects of the negative phase of the Pacific Decadal Oscillation and sea surface temperature (SST) warming in the Indian Ocean since the late 1990s. Given the recent Indian Ocean SST warming has been viewed as a footprint of external forcing, the results suggest that the effects of external forcing and natural variability on TC translation speed can offset each other. Plain Language Summary: Understanding changes in tropical cyclone (TC) translation speed is of great social and scientific concerns due to the enormous TC‐induced disasters. From a view of a specific site, slower TC motion indicates a longer linger time and thus greater TC‐induced disasters. China is one of the countries suffering the most TC disasters, while its change in translation speed of landfalling TCs remains unclear. Here we found that the translation speed of landfalling TCs in south China during the boreal summer has significantly decreased by 1.2 m s−1 during 1979–2019, which is driven by the deceleration in the TC embedded steering flow. The steering flow shows a southwestward anomaly in south China, which acts to decelerate the climatological northeastward steering and eventually slow the landfalling TC motion. This slowdown is found to be a balance of the opposite effects of the negative phase of the Pacific Decadal Oscillation and sea surface temperature (SST) warming in the Indian Ocean since the late 1990s. Given the recent Indian Ocean SST warming has been viewed as a footprint of climate change, the results have important implications for understanding the roles of external and natural variability in changes in TC translation speed. Key Points: Translation speed of landfalling tropical cyclones in south China has experienced a significant slowdown by 1.2 m s−1 during 1979–2019The slowdown is a balance of the opposite effects of the negative phase of the Pacific Decadal Oscillation and the Indian Ocean sea surface temperature warmingThe results suggest that the effects of climate change and natural variability on tropical cyclone translation speed can offset each other [ABSTRACT FROM AUTHOR]

Previous studies demonstrated that the tropical cyclone (TC) boundary layer (TCBL) contains small‐scale coherent structures, such as roll vortices and tornado‐scale vortices (TSVs), and they play important roles in energy transport and intensity changes in TCs. However, little is known about how horizontal resolutions can affect storm‐scale and fine‐scale structures in the TCBL with the grid spacing decreasing from the turbulent gray zone (100 m–1 km) to the large‐eddy scale (<100 m). In this study, numerical experiments with the large eddy simulation technique are used to investigate the effect of the model horizontal resolution on the simulated TC‐scale and fine‐scale structures in the TCBL. The simulated TC tends to have a shallower BL, a lower BL jet associated with stronger near‐surface vertical wind shear in the TCBL when the grid spacing decreases from 333 to 111 m or 37 m. When the grid spacing decreases to 111 m or 37 m, the fine‐scale coherent structures and near‐surface wind streaks associated with roll vortices and TSVs can be simulated, and the characteristics of the simulated turbulent kinetic energy in the TCBL are in good agreement with observational studies. With the grid spacing of 111 and 37 m, no significant differences in TC intensity are found in terms of instantaneous maximum and azimuthal‐mean maximum wind speeds. This study suggests that simulations with the horizontal grid spacing of 100 m or less can simulate the small coherent structures in the TCBL and their effect on TC intensity. Plain Language Summary: Numerical experiments with large‐eddy simulation are conducted to investigate the effect of the model horizontal resolution on the simulated storm‐scale and fine‐scale structures in the tropical cyclone boundary layer with the grid spacing decreasing from the turbulent gray zone (100 m–1 km) to the large‐eddy scale (<100 m).Storm‐scale and fine‐scale structures can be well simulated when the horizontal resolution is about or less than 100 m. No significant differences in the simulated storm‐scale, fine‐scale structures, and intensity of near‐surface winds are found when the horizontal grid spacing decreases from 111 to 37 m. Key Points: Numerical simulations with the horizontal grid spacing decreasing from the turbulent gray zone to large‐eddy scale are performedStorm‐scale and fine‐scale structures can be well simulated when the horizontal resolution is about or less than 100 mNo significant differences in storm‐scale, fine‐scale structures are found when the horizontal grid spacing decreases from 111 to 37 m [ABSTRACT FROM AUTHOR]

Dangerous turbulences were observed in the eyewall of category 5 Hurricane Hugo (1989) and the surface pressure associated with the turbulence was 8 hPa lower than the mean pressure in the eye. In this study, it is demonstrated that small scale, negative pressure perturbations in the tropical cyclone eyewall are associated with the tornado‐scale vortex (TSV) simulated with the large‐eddy simulation technique in a semi‐idealized numerical experiment. The negative pressure perturbation associated with TSVs is nonhydrostatic, dynamically resulting from the shearing effect. As a result, the negative pressure perturbation is linearly correlated with the vertical component of relative vorticity and maximum wind perturbation. Since the TSV is prevalent near the inner edge of the tropical cyclone eyewall, it is suggested that caution should be taken to estimate the central pressure of tropical cyclones with the extremely low pressure observed in the eyewall. Plain Language Summary: A National Oceanic and Atmospheric Administration (NOAA) research aircraft encountered dangerous turbulences in the eyewall of category 5 Hurricane Hugo (1989) and the observed surface pressure associated with the turbulence sharply decreased to 916 hPa in the eyewall, 8 hPa lower than the mean pressure of 924 hPa in the eye. This study demonstrates that the small‐scale pressure perturbation in the eyewall are associated with the tornado‐scale vortex. The negative pressure perturbation is linearly correlated with the vertical component of relative vorticity and maximum wind perturbation. Since the perturbation is prevalent near the inner edge of the tropical cyclone eyewall, it is suggested that the extremely low pressure observed in the eyewall is not necessarily associated with the tropical cyclone intensity. Key Points: Caution should be taken to estimate the central pressure of tropical cyclones with the extremely low pressure observed in the eyewall.Small scale, negative pressure perturbations are associated with the tornado‐scale vortex in the tropical cyclone eyewall.The negative pressure perturbations associated with tornado‐scale vortices (TSVs) is nonhydrostatic, dynamically resulting from the shearing effect. [ABSTRACT FROM AUTHOR]

While the Atlantic basin experienced the busiest hurricane season in 2020, the typhoon activity over the western North Pacific (WNP) was also record‐setting with no tropical cyclone (TC) formation in July 2020, which is the first time in available historical records. The unprecedented absence of TC formation is consistent with the extremely unfavorable large‐scale conditions that are linked to an anomalous anticyclonic circulation over the WNP, which results mainly from sea surface temperature (SST) anomalies across the tropical oceans. Numerical experiments suggest that the tropical Indian Ocean SST anomalies, which were warmest in July 2020 since 1965, play a dominant role in the anomalous WNP anticyclonic circulation, while the increased zonal SST gradient across the tropical Pacific and the Atlantic SST warming also have important contributions. This study suggests that the configuration of SST anomalies across the tropical basins is quite important to the WNP TC activity. Plain Language Summary: About four tropical cyclones (TCs) form every July in the western North Pacific (WNP), exerting significant societal and economic impacts on the Asian islands and coastal regions. However, no TCs formed in July 2020, which had never happened in historical records. The unprecedented absence of TC formation indicates its possible unique driving factors. It is found that the unprecedented absence is mainly linked to the extremely unfavorable large‐scale conditions that are associated with an anomalous anticyclonic circulation over the WNP, which was driven by trans‐basin sea surface temperature anomalies across the tropical oceans. The tropical Indian Ocean experienced the highest SST in July 2020 since 1965, thus we argue that the record‐breaking Indian Ocean SST, coupled with tropical SST over the Pacific and the Atlantic, is a plausible cause for the unprecedented absence of typhoon formation in July 2020. These results suggest a crucial role of sea surface temperature distribution across the tropical oceans in WNP TC formation and thus can provide some instructive implications for understanding variations in the WNP TC activity. Key Points: No typhoon occurred in July 2020, which is the first time in historical recordsThe unprecedented absence of typhoon is linked to the extremely unfavorable large‐scale conditionsThe unprecedentedly unfavorable large‐scale conditions are mainly driven by sea surface temperature anomalies across the tropical oceans [ABSTRACT FROM AUTHOR]

A reliable shape–slope (μ‐Λ) relationship for polarimetric retrieval in tropical cyclones (TCs) is unavailable due to the lack of raindrop size distribution (RSD) measurements within the TC eyewall. This study presents an analysis of the convective RSDs in the eyewall and spiral rainbands, based on ~26 hr of measurement from 17 Thies disdrometers during the landfall of Typhoon Lekima (2019) in eastern China. A μ‐Λ relationship for polarimetric retrieval in the eyewall is derived, which is different from those in spiral rainbands of Lekima. The average raindrop diameter parameter Dm (concentration parameter log10Nw) is found to generally decrease (increase) radially from the TC center, but note that most rain samples with intense rain rate >50 mm hr−1 occur in the convection‐dominated portion of inner rainbands. To explain why, an equilibrium RSD is proposed as a constraint on the growth of Dm and log10Nw with increasing rain rate. Key Points: A different convective RSD along with shape‐slope (μ‐Λ) relationship is found in the eyewall and various spiral rainbands of LekimaThe average raindrop diameter Dm generally decreases radially from the TC center, whereas the average concentration log10Nw increasesMost rain samples with an intense rain rate >50 mm hr−1 are concentrated in the convection‐dominated portion of inner rainbands [ABSTRACT FROM AUTHOR]

Unlike previous studies from a single disdrometer, this study investigates and compares the raindrop size distribution (RSD) characteristics of convective inner‐rainband rain (CIR) and convective outer‐rainband rain (COR) in Typhoon Maria (2018), simultaneously captured for the first time by a newly established observational network including nine disdrometers in the northeastern Fujian Province of China. It is shown that the radar reflectivity for the CIR increases sharply with decreasing height below the melting layer, whereas it remains nearly unchanged for the COR. This suggests the dominance of collision‐coalescence process in the CIR, that is, the collection of small raindrops by larger ones as they fall. Thus, the surface‐level CIR generally has far lower concentration of small raindrops and larger mean raindrop diameter than those previously found in the COR. Close to the tropical cyclone (TC) eyewall, it is found for the first time that although the raindrops are relatively large, the raindrop concentration is too low to yield a high rain rate. In contrast, high rain rates are concentrated at a distance of about 1.5–2.5 times the radius of maximum wind from the TC center, where there are appropriate normalized concentrations (log10Nw) and corresponding raindrop diameters. In addition, this study demonstrates differences between the CIR and COR in terms of the RSD evolution with increasing rain rate and the radar reflectivity–rain rate (Z‐R) and shape‐slope (μ‐Λ) relationships, confirming the existence of different rain microphysics in various rain regions of a TC. Key Points: The convective inner‐rainband rain has far lower concentration of small raindrops than has the convective outer‐rainband rainThe closer the raindrops are to the TC center, the larger the mean raindrop diameter, but the lower the mean raindrop concentrationHigh rain rates in TC rainbands may concentrate at a region of about 1.5–2.5 times the radius of maximum wind from the TC center [ABSTRACT FROM AUTHOR]

The microphysical characteristics of rain may vary in different rain regions of a tropical cyclone (TC), but few studies have demonstrated the differences in raindrop size distributions (RSDs) of convective rain in different rainbands of a specific TC. This study examines the RSD characteristics and evolution of convective rain within outer rainbands and a coastal‐front‐like rainband associated with Typhoon Fitow, based on observational data from a disdrometer at Shibo station in Shanghai, China, during 6–7 October 2013. Considering the fast passage of convective TC rainbands over the disdrometer and the low rain rate of stratiform rain in the outer area, this study proposes a modified rain‐type classification method based on the disdrometer data. This study indicates that convective outer‐rainband rain (ORR) and coastal‐front rain (CFR) have different rain parameters, three parameters of the gamma model, radar reflectivity–rain rate (Z‐R), and shape–slope (μ‐Λ) relationships. The convective ORR has higher concentrations at all drop sizes than the convective CFR as well as larger spectral width, leading to the greater rainfall rate. The different Z‐R relationships suggest the necessity of a variable relationship for quantitative precipitation estimation (QPE) in different rain regions of the TC. This study also demonstrates for the first time that the RSD evolution with increasing rain rate is different in various convective rainbands associated with Fitow, suggesting that different microphysical parameterization schemes may be required for different rainbands in TC models. Key Points: variable RSDs in different rainbands associated with Typhoon Fitowdifferent Z‐R relationships in different rain regions outwards from the TC centerdifferent evolution of RSDs in different convective TC rainbands with increasing rain rate [ABSTRACT FROM AUTHOR]

The Three Gorges Dam (TGD) is the largest hydropower project in the world. The resulting Three Gorges Reservoir (TGR) extends 660 km along the waterway of the Yangtze River in surrounded by complicated terrain. The dam and its impounded reservoir has triggered large‐scale land use and land cover change, which can potentially impact the environment and climate significantly. In this study, the impact of the TGD on regional‐scale circulation is explored through high‐resolution simulation using the weather research and forecast model. It is found that the construction of the TGD has little influence on the near‐surface temperature in the TGR region even when extreme hot weather occurs in the region. The numerical experiments performed in this study indicate that the construction of the TGD can alter the atmospheric circulation on the regional scale when the weak southeasterly wind in the lower troposphere prevails in the region. Due to the blocking effects of the TGD and the expanding water surface area of the TGR, the low‐level southeasterly flow is turned into northeasterly and southeasterly flows before reaching the dam, leading to an anomalous cyclone over 112–115°E, 33–34°N, and an anomalous anticyclone over 110–112°E, 28–29°N. It is also found that the anomalous cyclone can evidently enhance precipitation locally under some special atmospheric conditions. This study suggests that the impact of the TGD can be amplified due to the complicated terrain in the surrounding areas of the TGD. Key Points: TGD has little influence on the near‐surface temperatureTGD can alter the atmospheric circulation on the regional scale when the weak southeasterly wind prevails in the lower troposphereThe anomalous cyclone can evidently enhance local precipitation under some special atmospheric conditions [ABSTRACT FROM AUTHOR]

Previous studies found that tropical cyclone (TC) formation is generally suppressed over the western North Pacific (WNP) following strong El Niño events. The 2015/2016 event is identified as one of the three major El Niño events since 1950. However, a climatological average of 26 named TCs occurred over the WNP in 2016. The plausible causes for this inconsistency are investigated in this study. By examining the historical records, we also found that 28 named TCs occurred over the WNP following 1991/1992 El Niño. For most strong El Niño cases, the suppressed TC formation in the ensuing early season (January–June) can persist to the peak season and lead to the negative TC frequency anomalies. However, TC formation turns to be active during August–October in 1992 and 2016, offsetting the suppressed TC formation in the early season and thus resulting in the climatological annual TC counts. It is found that anomalous sea surface temperature (SST) cooling over the north Indian Ocean and SST warming over the tropical North Pacific contribute to the enhanced TC formation in 1992 by stimulating an anomalous cyclonic circulation over the WNP, while the tri‐polar SST pattern across the tropical Indo‐western Pacific Ocean and the related convergence zone over 130°–160°E are responsible for the enhanced TC formation in 2016. The results indicate the crucial role of SST evolution over the north Indian Ocean and tropical Pacific in TC formation following strong El Niño events, which has important implication for the seasonal forecasting of TC activity over the WNP. Time series of normalized NDJ Niño3.4 index (bars) and TC counts (green lines) of (a) January–December, (b) January–June, (c) July–September over 1970–2016. The filled bars denote the years with Niño3.4 index greater than one standard deviation. [ABSTRACT FROM AUTHOR]

Over the past decade extensive studies have been undertaken to understand the increasing trend in the proportion of intense tropical cyclones (categories 4 and 5 on the Saffir‐Simpson scale). The trend has been found globally and in some individual basins since the late 1970s. This study quantifies the contributions of various factors that control the proportion of intense typhoons. It is demonstrated that the increase of the proportion of intense typhoons during 1980–2015 is consistent with the corresponding changes in the ocean/atmosphere environment. The proportion change resulted from the temporal variations of the environmental parameters (sea surface temperature, ocean mixed layer depth, outflow temperature, and vertical wind shear), as well as the shifts of tropical cyclone prevailing tracks. The nonuniform spatial distribution of environmental parameters makes the shifts of tropical cyclone prevailing tracks contribute at least half the increase of the proportion of intense typhoons. The deepening of the ocean mixed layer resulting from the temporal variations and track shifts plays a dominant role in the observed increase of the proportion of intense typhoons. Although the maximum potential intensity theory and numerical modeling project an increase of tropical cyclone intensity in a warming climate, the effects of the temporal change of the ocean mixed layer depth and the prevailing track change were not taken into account in the projection. This study suggests that the increase of the proportion of intense typhoons in the western North Pacific basin could be larger than the projection in previous studies. Plain Language Summary: The increasing trend in the proportion of intense tropical cyclones (categories 4 and 5 on the Saffir‐Simpson scale) has been found globally and in some individual basins since the late 1970s. Although extensive studies have been undertaken over the past decade, its attribution still is a subject of controversy. This study quantifies the contributions of various factors that control the proportion of intense typhoons in the western North Pacific and found that the increase of the proportion of intense typhoons during 1980–2015 is consistent with the corresponding changes in the ocean/atmosphere environment. The proportion change resulted from the temporal variations of the environmental parameters (sea surface temperature, ocean mixed layer depth, outflow temperature, and vertical wind shear), as well as the shifts of tropical cyclone prevailing tracks. The deepening of the ocean mixed layer resulting from the temporal variations and track shifts plays a dominant role in the observed increase of the proportion of intense typhoons. This study suggests that the tropical cyclone intensification could be larger than the projection since the change of the ocean mixed layer was not taken into account in previous studies. Key Points: The increase of the proportion of intense typhoons is consistent with the corresponding changes in the ocean/atmosphere environmentThe deepening of the ocean mixed layer plays a dominant role in the observed increase of the proportion of intense typhoonsTropical cyclone intensification in the western North Pacific basin could be larger than the projection in previous studies [ABSTRACT FROM AUTHOR]

Abstract: Most tropical cyclones (TCs) in the western North Pacific basin form, intensify, and move in the monsoon trough (MT), and its future changes have significant implications for TC activity. In this study, future changes in MT and its possible impacts on TC activity are investigated by using outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and a downscaling system. The reliability of CMIP5 models is first examined by using their historical runs. Based on the constructed ensemble of 17 models with relatively high skill, the MT is found to intensify and extend eastward in the warming scenarios. This is also true for the ensemble mean of all of 36 available models. However, considerable intermodel uncertainty exists in the projection, which is related to the diversity in the projected sea surface temperature (SST) gradient over the equatorial North Pacific. The diversity in the SST pattern can generate different configurations of large‐scale circulations, and thus acts as an important source of uncertainty in projecting the TC activity in warming scenarios. The impacts of the MT change on TC activity are further explored by using a downscaling system. It is found that the intensification and eastward migration of the MT tend to shift TC formation locations eastward, increase TC influences in subtropical Asia, and decrease TC influences in south Asia. In addition, TCs intensify by a rate of 8.3% and 26.2% in terms of lifetime peak intensity and intense TC number for 1 K SST warming over the western North Pacific. [ABSTRACT FROM AUTHOR]

ABSTRACT: The convectively coupled equatorial Rossby waves (ERWs) how to quantitatively modulate the western North Pacific (WNP) tropical cyclones (TCs) are investigated systematically from observations in this study. Our results show that ERWs can exert a distinct modulation on the WNP TC genesis by altering large‐scale conditions and thus significantly impact on the TC tracks and basin‐wide TC intensity. During the convectively active ERW phase, more TCs and larger daily genesis rate (DGR) can be observed, while fewer TCs and lower DGR are found during the convectively inactive ERW phase. Such changes are mainly attributed to changes of large‐scale parameters modulated by ERWs over the WNP. Budget terms of the total anomalous observed genesis potential index (GPI) indicate that the mid‐level relative humidity and low‐level vorticity are the two largest contributors to ERWs‐composited GPI anomalies. Similar results are further obtained by analyses of the pattern correlations between the total anomalous GPI associated with ERWs and the anomalous GPI with varying environmental factors. Moreover, more TCs with north‐westward tracks and more intense TCs over the WNP can be generally found during the convectively active phases of ERWs in contrast to that during the convectively inactive phases. This study enhances our understanding of intra‐seasonal variability of WNP TC activity and has a substantial implication for its sub‐seasonal prediction. [ABSTRACT FROM AUTHOR]

Hybrid membranes containing multi-walled carbon nanotubes (MWCNTs) were initially prepared to separate benzene/cyclohexane mixtures. Subsequently, MWCNT surfaces were chemically modified using two methods to change the surface polarity of the MWCNTs and improve the distribution thereof in Poly(methylmethacrylate) (PMMA). This change consequently enhanced the separation performance of hybrid membranes with MWCNTs. Raman spectroscopy was used to characterize the structure of the pristine MWCNTs and the modified MWCNTs. The morphology and distribution of the MWCNTs in PMMA were investigated by transmission electron microscopy. The results showed that the addition of MWCNTs clearly improved the separation performance of the hybrid membranes. Surface modification introduced polar groups onto the MWCNT surface, which significantly improved the distribution of MWCNTs in the PMMA membranes and the performance of hybrid membranes. MWCNTs with higher surface polarity also increased the amount of MWCNTs distributed homogeneously in PMMA. Aminated MWCNTs (MWCNT-NH2) showed the highest surface polarity. Thus, the content of MWCNT-NH2 well distributed in PMMA was the highest among the three types of MWCNTs. The highest separation factor for the hybrid membranes with 1.0 wt% MWCNT-NH2 was about seven times that of membranes containing pristine MWCNTs. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The article reports on the preparation of a reverse osmosis (RO) composite membrane with extra-thin separation layer by the interfacial polymerization (IP) of methaphenylene diamine (MPD)with trimesoyl chloride (TMC) on the surface of polysulfone (PS) support membrane. The purpose was to achieve high permeability and selectivity in composite membranes that are used in RO desalination processes. It demonstrated a simple method of improving the flux of thin film composite RO membranes by 150%.

Upward trends in the power dissipation index (PDI) in the North Atlantic (NA) and western North Pacific (WNP) basins and increases in the number and proportion of intense hurricanes (categories 4 and 5) in all tropical cyclone basins have been reported in recent studies. These changes have been arguably viewed as evidence of the responses of tropical cyclone intensity to the increasing tropical sea surface temperature (SST) over the past 30 years. Using the historical best-track datasets from 1975 to 2004, how the annual frequency, lifetime and intensity of tropical cyclones contribute to the changes in the annual accumulated PDI is examined. As the SST warmed in the NA, WNP and eastern North Pacific (ENP) basins over the past 30 years, the annual accumulated PDI trended upward significantly only in the NA basin, where the decreased vertical wind shear and warming ocean surface may have allowed more storms to form and to form earlier or dissipate later, increasing the lifetime and annual frequency of tropical cyclones. The moderate increase in the annual accumulated PDI in the WNP basin was primarily due to the significant increase in the average intensity. There are no significant trends in the accumulated PDI, average intensity, average lifetime, and annual frequency in the ENP basin. Copyright © 2007 Royal Meteorological Society [ABSTRACT FROM AUTHOR]