scholarly journals Can a Warm Ocean Feature Cause a Typhoon to Intensify Rapidly?

Atmosphere ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 797
Author(s):  
Leo Oey ◽  
Shimin Huang
Keyword(s):  
Vertical Mixing ◽  
Vertical Wind Shear ◽  
Coupled Model ◽  
Internal Variability ◽  
Intensity Change ◽  
Rapid Intensification ◽  
Surface Cooling ◽  
Translation Speed ◽  
Typhoon Intensity ◽  
Surface Enthalpy

The hypothesis that a warm ocean feature (WOF) such as a warm eddy may cause a passing typhoon to undergo rapid intensification (RI), that is, the storm’s maximum 1-min wind speed at 10-m height increases by more than 15.4 m/s in 1 day, is of interest to forecasters. Testing the hypothesis is a challenge, however. Besides the storm’s internal dynamics, typhoon intensity depends on other environmental factors such as vertical wind shear and storm translation. Here we designed numerical experiments that exclude these other factors, retaining only the WOF’s influence on the storm’s intensity change. We use a storm’s translation speed Uh = 5 m/s when surface cooling is predominantly due to 1D vertical mixing. Observations have shown that the vast majority (70%) of RI events occur in storms that translate between 3 to 7 m/s. We conducted a large ensemble of twin experiments with and without ocean feedback and with and without the WOF to estimate model uncertainty due to internal variability. The results show that the WOF increases surface enthalpy flux and moisture convergence in the storm’s core, resulting in stronger updrafts and intensity. However, the intensification rate is, in general, insufficiently rapid. Consequently, the number of RIs is not statistically significantly different between simulations with and without the WOF. An analytical coupled model supports the numerical findings. Furthermore, it shows that WOF-induced RI can develop only over eddies and ambient waters that are a few °C warmer than presently observed in the ocean.

Assessing the Influence of Convective Downdrafts and Surface Enthalpy Fluxes on Tropical Cyclone Intensity Change in Moderate Vertical Wind Shear

2019 ◽  
Vol 147 (10) ◽  
pp. 3519-3534 ◽  
Author(s):  
Leon T. Nguyen ◽  
Robert Rogers ◽  
Jonathan Zawislak ◽  
Jun A. Zhang
Keyword(s):  
Boundary Layer ◽  
Wind Shear ◽  
Deep Convection ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Observation Time ◽  
Vertical Wind ◽  
Intensity Change ◽  
Conditional Instability ◽  
Surface Enthalpy

Abstract The thermodynamic impacts of downdraft-induced cooling/drying and downstream recovery via surface enthalpy fluxes within tropical cyclones (TCs) were investigated using dropsonde observations collected from 1996 to 2017. This study focused on relatively weak TCs (tropical depression, tropical storm, category 1 hurricane) that were subjected to moderate (4.5–11.0 m s−1) levels of environmental vertical wind shear. The dropsonde data were analyzed in a shear-relative framework and binned according to TC intensity change in the 24 h following the dropsonde observation time, allowing for comparison between storms that underwent different intensity changes. Moisture and temperature asymmetries in the lower troposphere yielded a relative maximum in lower-tropospheric conditional instability in the downshear quadrants and a relative minimum in instability in the upshear quadrants, regardless of intensity change. However, the instability increased as the intensification rate increased, particularly in the downshear quadrants. This was due to increased boundary layer moist entropy relative to the temperature profile above the boundary layer. Additionally, significantly larger surface enthalpy fluxes were observed as the intensification rate increased, particularly in the upshear quadrants. These results suggest that in intensifying storms, enhanced surface enthalpy fluxes in the upshear quadrants allow downdraft-modified boundary layer air to recover moisture and heat more effectively as it is advected cyclonically around the storm. By the time the air reaches the downshear quadrants, the lower-tropospheric conditional instability is enhanced, which is speculated to be more favorable for updraft growth and deep convection.

scholarly journals Key Environmental Factors for Rapid Intensification of the South China Sea Tropical Cyclones

2021 ◽  
Vol 8 ◽  
Author(s):  
Yao Chen ◽  
Si Gao ◽  
Xun Li ◽  
Xinyong Shen
Keyword(s):  
South China Sea ◽  
Tropical Cyclones ◽  
South China ◽  
Vertical Wind Shear ◽  
Onset Time ◽  
The South China Sea ◽  
Rapid Intensification ◽  
The South ◽  
Translation Speed ◽  
China Sea

Forecasting rapid intensification (RI) of the South China Sea (SCS) tropical cyclones (TCs) remains an operational challenge, mainly owing to the incomplete understanding of its physical mechanisms. Based on TC best-track data, atmospheric analysis data, and sea surface temperature data, this study compares temporal evolution characteristics of environmental conditions from the previous 24 h to the onset time for RI and non-RI TCs in the SCS during 2000–2018, and then identifies key factors for RI of the SCS TCs using the box difference index and stepwise regression. A combination of strong divergence in the upper troposphere and strong convergence in the boundary layer, weak deep-layer vertical wind shear, fast storm translation speed, and high TC intensification potential (i.e., maximum potential intensity minus current intensity) north of the storm center at the previous 24 h are favorable for RI of the SCS TCs, and their importance for RI is in descending order. The results may shed light on operational forecasting of rapid intensification of the SCS TCs.

scholarly journals Structural Changes Preceding Rapid Intensification in Tropical Cyclones as Shown in a Large Ensemble of Idealized Simulations

2018 ◽  
Vol 75 (2) ◽  
pp. 555-569 ◽  
Author(s):  
Yoshiaki Miyamoto ◽  
David S. Nolan
Keyword(s):  
Tropical Cyclones ◽  
Structural Changes ◽  
Deep Convection ◽  
Vertical Wind Shear ◽  
Rapid Intensification ◽  
Large Ensemble ◽  
Translation Speed ◽  
Budget Analysis ◽  
Tangential Wind ◽  
Vortex Size

Abstract Structural changes that precede rapid intensification (RI) of tropical cyclones (TCs) are examined in a full-physics model by conducting a large ensemble (270) of idealized TC simulations. The processes leading to RI in a representative case with moderate shear are consistent with previous studies for weakly sheared cases. The most distinct changes are that the vortex tilt and the vortex size begin to decrease more rapidly 6 h before the onset of RI. A vorticity budget analysis for the upper layer around the low-level center reveals that the vertical vorticity is increased by vertical advection, stretching, and tilting terms before RI, whereas the horizontal advection is small. Thus, the upright vortex structure is not achieved through a vortex alignment process but rather is built upward by deep convection. The ensemble simulations are generated by changing the intensity and size of the initial vortex, the magnitude of vertical wind shear, and the translation speed. The ensemble members that show RI are consistent with the control case and many previous studies: before the onset of RI, the intensity gradually increases, the radius of maximum tangential wind (RMW) decreases, the flow structure becomes more symmetric, the vortex tilt decreases, and the radius of maximum convergence approaches the radius of maximum winds. A dimensionless parameter representing a tendency for the formation of the vertically upright structure is considered. The product of this parameter and the local Rossby number is significantly larger for TCs that exhibit RI in the next 24 h.

scholarly journals Satellite Air–Sea Enthalpy Flux and Intensity Change of Tropical Cyclones over the Western North Pacific

2016 ◽  
Vol 55 (2) ◽  
pp. 425-444 ◽  
Author(s):  
Si Gao ◽  
Shunan Zhai ◽  
Long S. Chiu ◽  
Dong Xia
Keyword(s):  
Heat Flux ◽  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
North Pacific Ocean ◽  
Sensible Heat Flux ◽  
Vertical Wind Shear ◽  
Intensity Change ◽  
Rapid Intensification ◽  
Sea Temperature

AbstractAn improved high-resolution satellite enthalpy flux dataset is employed to study the composites of initial (i.e., t = 0 h) latent heat flux (LHF), sensible heat flux (SHF), and their bulk variables associated with four intensity-change categories of tropical cyclones (TCs) over the western North Pacific Ocean—rapidly intensifying (RI), slowly intensifying, neutral, and weakening—in a vertical wind shear–relative coordinate system with horizontal dimensions normalized by the radius of maximum wind. Results show that RI TCs are associated with significantly higher LHF and SHF in all TC environments than non-RI TCs, which are mainly attributable to the air–sea humidity difference (DQ) and the air–sea temperature difference (DT), respectively. Higher DQ and DT are primarily due to significantly higher sea surface temperature (SST) underlying RI TCs, emphasizing the crucial role of SST in supplying more energy to TCs that undergo rapid intensification, in which LHF plays a more important role than SHF. Relative to non-RI TCs, LHF and SHF for RI TCs show a more symmetric pattern. The magnitude and pattern of air–sea enthalpy flux could serve as potential predictors for rapid intensification of TCs.

scholarly journals The Influence of Environments on the Intensity Change of Typhoon Soudelor

Atmosphere ◽  
2021 ◽  
Vol 12 (2) ◽  
pp. 162
Author(s):  
Leo Oey ◽  
Yuchen Lin
Keyword(s):  
Tropical Cyclones ◽  
Wind Shear ◽  
Vertical Wind Shear ◽  
Intensity Variation ◽  
Vertical Wind ◽  
Intensity Change ◽  
Rapid Intensification ◽  
Philippine Sea ◽  
Anomalous Anticyclone ◽  
Upper Level

Previous studies have shown that background oceanic and atmospheric environments can influence not only the formation but also the intensity of tropical cyclones. Typhoon Soudelor in August 2015 is notable in that it underwent two rapid intensifications as the storm passed over the Philippine Sea where the 26 °C isotherm (Z26) was deeper than 100 m and warm eddies abounded. At the same time, prior to the storm’s arrival, an anomalous upper-level anticyclone developed south of Japan and created a weakened vertical wind shear (Vs) environment that extended into the Philippine Sea. This study examines how the rapid intensification of Typhoon Soudelor may be related to the observed variations of Z26, Vs and other environmental fields as the storm crossed over them. A regression analysis indicates that the contribution to Soudelor’s intensity variation from Vs is the largest (62%), followed by Z26 (27%) and others. Further analyses using composites then indicate that the weak vertical wind shear produced by the aforementioned anomalous anticyclone is a robust feature in the western North Pacific during the developing summer of strong El Ninos with Oceanic Nino Index (ONI) > 1.5.

scholarly journals The Association of Typhoon Intensity Increase with Translation Speed Increase in the South China Sea

2020 ◽  
Vol 12 (3) ◽  
pp. 939
Author(s):  
Ya-Ting Chang ◽  
I-I Lin ◽  
Hsiao-Ching Huang ◽  
Yi-Chun Liao ◽  
Chun-Chi Lien
Keyword(s):  
South China Sea ◽  
South China ◽  
Vertical Wind Shear ◽  
The South China Sea ◽  
Intensity Increase ◽  
The South ◽  
Translation Speed ◽  
Speed Increase ◽  
China Sea ◽  
Typhoon Intensity

Tropical cyclone (TC) translation speed is an important parameter. In the context of TC–ocean interaction, faster translation speed can contribute to less TC-induced ocean cooling and thus enables more air–sea enthalpy flux supply to favor TC intensification. In 2018, Kossin published an interesting paper in Nature, reporting a global slow-down of TC translation speed since the 1950s. However, upon close inspection, in the last two decades, TC translation speed actually increased over the western North Pacific (WNP) and neighboring seas. Thus, we are interested to see which sub-region in the WNP and neighboring seas had the largest increase during the last two decades, and whether such increases contribute to TC intensification. Our results found statistically significant translation speed increases (~0.8 ms−1 per decade) over the South China Sea. Ruling out other possible factors that may influence TC intensity (i.e., changes in atmospheric vertical wind shear, pre-TC sea surface temperature or subsurface thermal condition), we suggest, in this research, the possible contribution of TC translation speed increases to the observed TC intensity increases over the South China Sea in the last two decades (1998–2017).

New Probabilistic Forecast Models for the Prediction of Tropical Cyclone Rapid Intensification

2011 ◽  
Vol 26 (5) ◽  
pp. 677-689 ◽  
Author(s):  
Christopher M. Rozoff ◽  
James P. Kossin
Keyword(s):  
Logistic Regression ◽  
North Atlantic ◽  
North Pacific ◽  
North Pacific Ocean ◽  
Vertical Wind Shear ◽  
Intensity Change ◽  
Rapid Intensification ◽  
The North ◽  
Skill Scores ◽  
The North Atlantic

Abstract The National Hurricane Center currently employs a skillful probabilistic rapid intensification index (RII) based on linear discriminant analysis of the environmental and satellite-derived features from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) dataset. Probabilistic prediction of rapid intensity change in tropical cyclones is revisited here using two additional models: one based on logistic regression and the other on a naïve Bayesian framework. Each model incorporates data from the SHIPS dataset over both the North Atlantic and eastern North Pacific Ocean basins to provide the probability of exceeding the standard rapid intensification thresholds [25, 30, and 35 kt (24 h)−1] for 24 h into the future. The optimal SHIPS and satellite-based predictors of rapid intensification differ slightly between each probabilistic model and ocean basin, but each set of optimal predictors incorporates thermodynamic and dynamic aspects of the tropical cyclone’s environment (such as vertical wind shear) and its structure (such as departure from convective axisymmetry). Cross validation shows that both the logistic regression and Bayesian probabilistic models are skillful relative to climatology. Dependent testing indicates both models exhibit forecast skill that generally exceeds the skill of the present operational SHIPS-RII and a simple average of the probabilities provided by the logistic regression, Bayesian, and SHIPS-RII models provides greater skill than any individual model. For the rapid intensification threshold of 25 kt (24 h)−1, the three-member ensemble mean improves the Brier skill scores of the current operational SHIPS-RII by 33% in the North Atlantic and 52% in the eastern North Pacific.

Numerical Investigation of the Upper Ocean Response to the Typhoon Winnie (1997) Using an Air-Sea Coupled Model

2013 ◽  
Vol 726-731 ◽  
pp. 3443-3446
Author(s):  
Li Wen Huang ◽  
Yi Jun Ge
Keyword(s):  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Vertical Mixing ◽  
Coupled Model ◽  
Upper Ocean ◽  
Surface Mixed Layer ◽  
Large Area ◽  
Surface Cooling ◽  
Sea Temperature ◽  
The Impact

In order to investigate the impact of air-sea interaction on the ocean surface mixed layer, Typhoon Winnie (1997) was numerical simulated with a two-way air-sea coupled model MCM (Mesoscale Coupled Model). It shows that the Sea Surface Temperature (SST) decreases over a large area with a magnitude up to 4°C. The sea temperature at 30-50 m depth increases about 1°C corresponding to the surface cooling. Moreover, the oceanic mixed layer depth deepens by 20-30 m. It can be included that these variations in the upper ocean are mainly due to vertical mixing induced by the wind stress of the typhoon.

The implication of outflow structure for the rapid intensification of tropical cyclones under vertical wind shear

2021 ◽  
Author(s):  
Donglei Shi ◽  
Guanghua Chen
Keyword(s):  
Vertical Wind Shear ◽  
Intensity Change ◽  
Rapid Intensification ◽  
Strong Shear ◽  
Thermodynamic Conditions ◽  
Local Shear ◽  
Upper Level ◽  
Shear Building ◽  
Active Convection ◽  
Blocking Mechanism

AbstractThe implication of outflow structure for tropical cyclone (TC) rapid intensification (RI) is investigated via a climatological study using the best-track, reanalysis and infrared brightness temperature data during 1980–2019. Composite analyses are performed in a shear-relative framework for the RI events under different strengths of environmental shear. Results show that for the RI events under moderate (4.5–11 m s-1) or strong (> 11 m s-1) environmental shear the RI onset follows a significant increase of upper-level outflow upshear of the storm, which is intimately linked with the increasing active convection upshear. The intensified outflow blocks the upper-level environmental flow and thus decreases the local shear, building an environment favorable for RI. In contrast, the RI under weak environmental shear (< 4.5 m s-1) is found to be less attributed to this outflow-blocking mechanism. Comparison between the RI and non-RI cases under moderate or strong environmental shear reveals that the RI cases tend to have stronger outflow and convection in the upshear flank than the non-RI cases, confirming the importance of outflow blocking on the occurrence of RI. Statistical analysis further indicates that the 24-h future intensity change under moderate or strong shear is more negatively correlated with the local shear than with the environmental shear, implicating the potential of local shear and upshear outflow as predictors to improve the forecasting of TC intensity change and especially RI. Further analysis suggests that the environmental thermodynamic conditions may play an important role in modulating the upshear convection and thus outflow blocking.

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