Search

Abstract Tropical cyclone (TC) intensification is influenced by environmental conditions, inner‐core dynamics, and interactions with upper‐ocean layers. Rapid intensification (RI) is a significant threat that is difficult to predict, prompting multiple institutions to collaborate. However, the accuracy still needs further improvements. It is well‐known that a warm upper ocean is conducive to RI, but the role of salinity stratification in this process is not well understood, particularly under different TC translation speeds. This study reveals that rapidly intensifying TCs are related to large salinity stratification, especially when TC moves slowly. To develop a predictive model, several machine learning (ML) algorithms are used, with the most appropriate parameters and weights for each algorithm being determined. Our final ML model, which incorporates salinity stratification as a predictor and TC translation speed as a weight parameter, demonstrates superior performance across various predictive metrics, including the probability of detection (POD), false alarm ratio (FAR), and Peirce Skill Score (PSS) over the Western North Pacific during 2004–2022 compared to the model without these two factors. The most significant enhancement is observed for intense RI episodes. The improvements are up to 14% for both in POD; 7% and 13% in FAR; and 19% and 16% in PSS for 12.75 and 15.3 m s−1 RI thresholds, respectively. These results highlight the importance of including salinity stratification as a new predictor and TC translation speed as a weighted parameter using ML techniques in RI prediction models.

Internal solitary waves (ISWs) propagating in polar seas are affected by the sea ice at upper boundary of seas and thus exhibit complex evolution characteristics. Herein, spatiotemporal changes in the wave element, flow field, and energy of ISWs beneath an ice keel model were investigated to examine the evolution of ISWs. For this purpose, laboratory experiments were conducted using dye-tracing labeling, conductivity probes, Schlieren technology, and particle image velocimetry. The results show that ice keel causes an increase in the thickness of the pycnocline and even the occurrence of breaking and internal surging of ISW. Additionally, the waveform becomes narrower or wider at different positions, and wave amplitude and speed decrease, with a maximum reduction 30%–40%. Furthermore, the ice keel strengthens the shear of the ISW-induced flow field, generating vortices and mixing. The energy of ISWs undergoes internal conversion majorly at the front slope of the ice keel, while energy dissipation occurs largely at the back slope, with dissipation rates as high as 60%.

Abstract Tropical cyclone (TC) rainfall is an important component of global precipitation. Based on a 22-year satellite-based record of observed precipitation from 1998 to 2019, we find that composite rainfall rates averaged within 500 km of TCs have increased. However, the background precipitation rate in the TC-affected regions has also increased. Therefore we find, by subtracting the background increase in precipitation rate from the TC precipitation composite, that TC precipitation rates--relative to the background--have decreased, and that TC-related extreme precipitation rates near the TC center have also decreased. Besides TC intensity and environmental sea surface temperature, the TC-induced sea surface cooling, typically formed like a cold wake feature, is indicated to be a potentially important contributor to TC rainfall changes. The cold wake diminishes the upward transport of surface enthalpy flux and leads to a decrease of moisture in the boundary layer, being unfavorable for the rainfall. The relative decreasing rainfall trend of TCs to the background is therefore attributed to the strengthening sea surface cooling induced by TCs. The TC-ocean interaction is suggested to be fully resolved in state-of-the-art global and regional climate models in order to improve TC rainfall simulations and projections.

Abstract Although the cold wake of tropical cyclones (TCs) is recognized to be typically rightward biased relative to TC track (in the North Hemisphere), it remains unclear to what extent the rightward bias should be for specified TCs. Based on numerical simulations and observational statistics, this study shows that the distance of cold wake rightward bias is closely related to latitude, translation speed and radius of maximum wind (RMW) of a TC, but is independent from maximum surface wind and radial wind profile outside the RMW, namely TC intensity and size. A semi‐empirical equation is derived to represent the rightward bias extent of cold wakes. For TCs with slower translation speed, smaller RMW, or at higher latitudes, the cold wakes are less asymmetric relative to the TC track. Atmospheric simulations also show that the TC tends to be weaker and more asymmetric as the cold wake is closer to TC center.

The cold wake caused by a tropical cyclone (TC) extends for hundreds of kilometers and persists for several weeks, thus influencing the surface response for any subsequent TCs that might pass over it. It is commonly accepted that sea-surface temperature (SST) cooling, as produced by a single TC, occurs primarily through vertical mixing. However, when there are sequential TCs, the earlier TC can dramatically change the thermal structure of the upper ocean, which may influence the subsequent development of a latter-occurring TC (LTC). Therefore, the contribution of horizontal advection and vertical mixing to SST-cooling during the passage of LTCs is of great interest. Using a 19-year-long observational dataset and the heat budget analysis of an idealized numerical simulation, the SST change during the passage of sequential TCs is investigated. The results demonstrate that, on average, the SST cooling caused by the LTC shows an overall decreasing trend with enhanced lingering wakes. Budget analysis of the model simulations suggests that an earlier TC can suppress the vertical mixing induced by an LTC mainly through an alteration of dynamics within the deepened mixed layer and that the contribution of vertical mixing to the SST cooling is weaker due to the intensification of the earlier TC. The weakened vertical mixing dominates the decreased SST cooling induced by an LTC. In contrast, the cold wake generated by an earlier TC can produce more cold water on the right side of the TC’s track, which contributes to stronger horizontal advection upon the arrival of the LTC. In general, the effects of the earlier TC can suppress the sea-surface thermal response to an LTC. If the contribution of the horizontal advection to SST cooling is neglected, the SST cooling induced by an LTC could be reduced by about 40%. As for the response of the sub-surface water to the passage of an LTC, the weakened warm anomaly induced by vertical mixing and the enhanced cooling anomaly caused by the vertical advection explain the reduced tendency for the mixed layer to deepen. As a result, the tendency for the mixed layer depth (MLD) to increase is suppressed during the passage of an LTC. These results highlight the importance of optimally depicting cold wakes in numerical simulations to improve the prediction of the upper ocean’s response to sequential TCs.

A satellite altimeter measures sea surface height (SSH) along the nadir track. Multiple satellite altimeters have been in orbit, and the measurements been merged for mapping mesoscale eddies of ~100 km in size in the oceans. The capability of the mapped SSH for resolving mesoscale eddies depends on mapping algorithms. A two-dimensional variational (2DVAR) algorithm was implemented to generate mapped SSH at a grid size of 1/12° in the South China Sea. A range of comparisons were performed between the mapped SSH and the commonly used AVISO (Archiving, Validation, and Interpretation of Satellite Oceanographic satellite data) mapped SSH data product at a grid size of 1/8° and 1/4°. The effective resolution, which represents the spatial scale that the data can resolve, was examined. The effective resolution of the mapped SSH using the 2DVAR algorithm is approximately 100 km, while it is 250 km with the 1/8° and 1/4° AVISO data products. The difference in the effective resolution results from the difference in the background state and thus the background error. The result suggests that the effective resolution of the mapped data could be increased by choosing a background state so that the associated errors could have a smaller decorrelation length scale.

The impact of tropical cyclones on the characteristics of barrier layer (BL) thickness is investigated in this study in combination with observations and numerical simulations. Statistical results based on Simple Ocean Data Assimilation reanalysis (SODA) data and Argo float measurements reveal a significant effect of TCs on the BL characteristics. The BL thickness increases remarkably with the TC approaching, reaches its maximum 1–5 days after the TC passage, and recovers to its pre-storm state within 30 days on average. The peak increase in BL thickness is slightly rightward biased relative to the TC track, reaching ∼1.31 m (26.93%) relative to the pre-storm state as revealed by SODA data. The changes in BL thickness show a strong correlation with TC characteristics in terms of intensity and translation speed, tending to be more significant with more intense or slower-moving TCs. The thickening of the isothermal layer on the right of TC tracks and shallowing of the mixed layer act together to determine the BL development. The three-dimensional Price–Weller–Pinkel (3DPWP) ocean model is applied to explore the influence of precipitation on BL thickness. The results show that the precipitation tends to decrease the mixed layer depth while the vertical mixing of TCs deepens the isothermal layer depth, both of which act together to increase the BL thickness.

Abstract To explore the effects of air‐sea interaction on the eyewall replacement cycle (ERC) of tropical cyclones (TCs), an ERC of Typhoon Sinlaku (2008) was reproduced using the high‐resolution coupled ocean‐atmosphere‐wave‐sediment transport (COAWST) model. The numerical simulations were evaluated with a wide range of observational datasets. Moreover, the importance of the ocean to ERC is discussed by comparing six sensitive experiments, that is, three uncoupled experiments with different time‐invariant sea surface temperatures, an ocean‐atmosphere coupled experiment, an ocean‐wave coupled experiment, and an ocean‐atmosphere‐wave fully coupled experiment. The results show that Sinlaku simulated by the fully coupled experiment was highly consistent with the observations, and the evolution of ERC varies from those only simulated by the atmospheric models in previous studies. When the ocean and waves were both considered, the lifetime of the ERC was significantly prolonged, even though the simulated Sinlaku was weakened by the cooling of the sea; the asymmetric distribution of the concentric eyewalls (CEs) which caused by the non‐uniform energy exchanges was also prominent. By contrast, without ocean coupling, the simulated secondary eyewall, composed of axisymmetric convections, exhibited false enhancement and fake inward contraction, and the duration of the ERC was also shorter than that actually observed. In conclusion, the results highlight the significance of ocean coupling for the numerical simulation of the ERC of a TC, including survival time and asymmetric structure.

Dust particles in the atmosphere play an important role in air pollution, climate change, and biogeochemical cycles. Some of the dominant sources of dust in mid-latitude regions are in Asia. An intense dust storm engulfed Northern China at the beginning of May 2017, and PM10 mass concentrations of 1500–2000 μg m−3 were measured near the dust source region. We combined numerical simulations, air quality monitoring data, and satellite retrievals to investigate dust emission and transport during this event. We found that the event was closely related to cold front activity, characterized by increased wind speed, which increased dust emission. We improved the dust scheme using a local dust size distribution to better simulate the dust emission flux. We found that accurate parametrization of the dust size distribution was important to effectively simulate both dust emission and ambient particle concentration. We showed that using a local dust size distribution substantially improved the accuracy of the simulation, allowing both the spatial distribution of pollution caused by the dust storm and temporal variability in the pollution to be captured.

This study explored the influence of choosing a nonhydrostatic dynamical core or a hydrostatic dynamical core in the weather research and forecasting (WRF) model on the intensity and structure of simulated tropical cyclones (TCs). A comparison of cloud-resolving simulations using each core revealed significant differences in the TC simulations. In comparison with the nonhydrostatic simulation, the hydrostatic simulation produced a stronger and larger TC, associated with stronger convective activity. A budget analysis of the vertical momentum equation was conducted to investigate the underlying mechanisms. Although the hydrostatic dynamical core was used, the vertical motion was not in strict hydrostatic balance because of the existence of the vertical perturbation pressure gradient force, local buoyancy force, water loading, and sum of the Coriolis and diffusion effects. The contribution of the enhanced vertical perturbation pressure gradient force was found to be more important for stronger upward acceleration in the eyewall in the hydrostatic simulation than in the nonhydrostatic simulation. This is because it leads to intensified convection in the eyewall that releases more latent heat, which induces a larger low-level radial pressure gradient and inflow motion, and eventually leads to a stronger storm.

This study systematically evaluates the accuracy, trends, and error sources for western North Pacific tropical cyclone intensity forecasts between 2005 and 2018. The study uses homogeneous samples from tropical cyclone (TC) intensity official forecasts issued by the China Meteorological Administration (CMA), Joint Typhoon Warning Center (JTWC), and Regional Specialized Meteorological Center Tokyo-Typhoon Center (RSMC-Tokyo). The TC intensity forecast accuracy performances are as follows: 24 - 48 h, JTWC > RSMC-Tokyo > CMA; 72 h, JTWC > CMA > RSMC-Tokyo; and 96 - 120 h, JTWC > CMA. Improvements in TC intensity forecasting are marginal but steady for all three centers. The 24 - 72 h improvement rate is approximately 1 - 2 % yr-1. The improvement rates are statistically significant at the 95 % level for almost half of the verification times from 0 - 120 h. The three centers tend to overestimate weak TCs over the northern South China Sea, but strong TCs are sometimes underestimated over the area east of the Philippines. The three centers generally have higher skill scores associated with forecasting of rapid weakening (RW) events than rapid intensification (RI) events. Overall, the three centers are not skillful in forecasting RI events more than three days in advance. Fortunately, RW events could be forecasted five days in advance with an accuracy order of CMA > RSMC-Tokyo > JTWC. [ABSTRACT FROM AUTHOR]

In Part II of this study, the influence of an oceanic cold-core eddy on the atmospheric boundary layer structures of Typhoon Francisco (2013) is investigated, as well as a comparison with the cold wake effect. Results show that the eddy induces shallower mixed-layer depth and forms stable boundary layer above and near it. The changes of these features shift from northwest to southeast across the storm eye, following the translation of Francisco over the eddy. Nonetheless, the decrease in mixed-layer depth and formation of stable boundary layer caused by the cold wake are located at right rear of the storm. The sensible heat fluxes at the lowest atmospheric model level are mostly downward over the sea surface cooling region. Due to their different relative locations with Francisco, the diabatic heating in the northwest quadrant of the storm can be more effectively inhibited by the cold-core eddy than by the cold wake. The asymmetric characteristics of surface tangential wind are less sensitive to sea surface cooling than those of surface radial wind, implying a change in surface inflow angle. Different from previous studies, the surface inflow angle is found to be reduced especially above the cold-core eddy and cold wake region. An analysis of radial wind tendency budget indicates that the decrease in radial pressure gradient is dominant in changing the acceleration rate of surface radial wind, rather than the decrease in the Coriolis and centrifugal forces, and therefore more outward surface flow is induced by both the cold-core eddy and cold wake. [ABSTRACT FROM AUTHOR]

This study aims to further verify the evaporation duct (ED) model in a coastal region as well as the need for model localization based on the measurements from the Air-Sea Interaction Tower (ASIT). The ducts tended to be more frequent and pronounced in coastal offshore winds than in onshore winds, especially in case of combining unstable conditions (100% and 9.5 m/8.7 m). They occurred with the lowest frequency (56.4%) and thickness (4.9 m/3.5 m) in stable onshore winds. The ED model with ψ functions of Edson04 (or Fairall03) and SHEBA07 delivered the best overall performance in terms of predicting local evaporation ducts. Its considerable advantages over the COARE 3.0 model and BYC model underscore the importance of localization to a given region. The overall performance of the localized ED model was encouraging but it encountered problems in case of extreme ducts under stable offshore conditions, when the available improvements to it failed. The reinforcement of a stable marine inner-boundary layer (MIBL) to a preexisting evaporation duct was considered the main reason for this and, hence, the future solution. [ABSTRACT FROM AUTHOR]

Official forecasts of tropical cyclone (TC) tracks issued by the China Meteorological Administration (CMA); the Regional Specialized Meteorological Centre in Tokyo, Japan; and the Joint Typhoon Warning Center (JTWC) were used to evaluate the accuracies, biases, and trends of TC track forecasts during 2005-14 over the western North Pacific. Overall, the JTWC demonstrated the best forecasting performance. However, the CMA showed the most significant rate of improvement. Two main zones were discovered in the regional distribution of forecast errors: a low-latitude zone that comprises the South China Sea and the sea region east of the Philippines, and a midlatitude zone comprising the southern Sea of Japan and the sea region east of Japan. When TCs moved into the former zone, there were both translational speed and direction biases in the forecast tracks, whereas slow biases were predominated in the latter zone. Twelve synoptic flow patterns of TCs with the largest error have been identified based on the steering flow theory. Among them, the most two common pattern are the pattern with the combination of cyclonic circulations, subtropical ridges, and midlatitude troughs (CRT, 26 TCs), and the pattern of the TCs' track that cannot be explained by steering flow (NSF, 6 TCs). In the CRT pattern, TCs move northwestward forced by the cyclonic circulations and the subtropical ridges and then turn poleward and eastward under the influence of the midlatitude troughs. In the NSF pattern, storms embedded in the southwest flow by the cyclonic circulation and the steering flow suggest TCs should turn to the right and move northeastward but instead TCs persisted in moving northwestward. [ABSTRACT FROM AUTHOR]

The impact of mesoscale ocean eddies on tropical cyclone intensities is investigated based on a combination of observations and atmosphere-ocean coupling simulations. A statistical analysis reveals that the tropical cyclone-eddy interactions occur at very high frequencies; over 90% of the recorded tropical cyclones over the western North Pacific have encountered ocean eddies from 2002 to 2011. The chances of confronting a cold core eddy (CCE) are slightly larger than confronting a warm core eddy (WCE). The observational sea surface temperature data have statistically evidenced that CCEs tend to promote the sea surface temperature decrease caused by tropical cyclones while WCEs tend to restrain such ocean responses. The roles of CCEs are statistically more significant than those of WCEs in modulating the sea surface temperature response. It is therefore proposed that CCEs should be paid no less attention than WCEs during the TC-ocean interaction process. The CCE-induced changes in sea surface temperature decreases are observed to be more remarkable for more intense and slower-moving tropical cyclones and for thinner depth of mixed layers. A set of numerical experiments reveal that the effects of ocean eddies are positively related to their strengths and storm intensities, and the eddy feedback is less pronounced when the eddy is located at one side of storm tracks than right below the tropical cyclone center. The eddy-induced moisture disequilibrium sooner vanishes after the departure of tropical cyclones. The intensity recoveries last for 1-2 days because of the dependence of surface enthalpy fluxes on surface winds. [ABSTRACT FROM AUTHOR]