Tropical Cyclone Cases Research Articles

A Statistical Perspective on Wind Profiles and Vertical Wind Shear in Tropical Cyclone Environments of the Northern Hemisphere

Abstract A statistical analysis of tropical cyclone (TC) environmental wind profiles is conducted in order to better understand how vertical wind shear influences TC intensity change. The wind profiles are computed from global atmospheric reanalyses around the best track locations of 7554 TC cases in the Northern Hemisphere tropics. Mean wind profiles within each basin exhibit significant differences in the magnitude and direction of vertical wind shear. Comparisons between TC environments and randomly selected “non-TC” environments highlight the synoptic regimes that support TCs in each basin, which are often characterized by weaker deep-layer shear. Because weaker deep-layer shear may not be the only aspect of the environmental flow that makes a TC environment more favorable for TCs, two new parameters are developed to describe the height and depth of vertical shear. Distributions of these parameters indicate that, in both TC and non-TC environments, vertical shear most frequently occurs in shallow layers and in the upper troposphere. Linear correlations between each shear parameter and TC intensity change show that shallow, upper-level shear is slightly more favorable for TC intensification. But these relationships vary by basin and neither parameter independently explains more than 5% of the variance in TC intensity change between 12 and 120 h. As such, the shear height and depth parameters in this study do not appear to be viable predictors for statistical intensity prediction, though similar measures of midtropospheric vertical wind shear may be more important in particularly challenging intensity forecasts.

A Multiple Linear Regression Model for Tropical Cyclone Intensity Estimation from Satellite Infrared Images

An objectively trained model for tropical cyclone intensity estimation from routine satellite infrared images over the Northwestern Pacific Ocean is presented in this paper. The intensity is correlated to some critical signals extracted from the satellite infrared images, by training the 325 tropical cyclone cases from 1996 to 2007 typhoon seasons. To begin with, deviation angles and radial profiles of infrared images are calculated to extract as much potential predicators for intensity as possible. These predicators are examined strictly and included into (or excluded from) the initial predicator pool for regression manually. Then, the “thinned” potential predicators are regressed to the intensity by performing a stepwise regression procedure, according to their accumulated variance contribution rates to the model. Finally, the regressed model is verified using 52 cases from 2008 to 2009 typhoon seasons. The R2 and Root Mean Square Error are 0.77 and 12.01 knot in the independent validation tests, respectively. Analysis results demonstrate that this model performs well for strong typhoons, but produces relatively large errors for weak tropical cyclones.

Response of Tropical Cyclone Tracks to Sea Surface Temperature in the Western North Pacific

Abstract A set of short-term experiments using a regional atmospheric model (RAM) were carried out to investigate the response of tropical cyclone (TC) tracks to sea surface temperature (SST) in the western North Pacific. For 10 selected TC cases occurring during 2002–07, a warm and a cold run are performed with 2 and −2 K added to the SSTs uniformly over the model domain, respectively. The cases can be classified into three groups in terms of recurvature: recurved tracks in the warm and cold runs, a recurved track in the warm run and a nonrecurved track in the cold run, and nonrecurved tracks in both runs. Commonly the warm run produced northward movement of the TC faster than the cold run. The rapid northward migration can be mainly explained by the result that cyclonic circulation to the west of the TC is found in the steering flow in the warm run and it is not in the cold run. The beta effect is also activated under the warm SST environment. For the typical TC cases, a linear baroclinic model experiment is performed to examine how the cyclonic circulation is intensified in the warm run. The stationary linear response to diabatic heating obtained from the RAM experiment reveals that the intensified TC by the warm SST excites the cyclonic circulation in the lower troposphere to the west of the forcing position. The vorticity and thermodynamic equation analysis shows the detailed mechanism. The time scale of the linear response and the teleconnection are also discussed.

On the Sensitivity of Tropical Cyclone Intensification under Upper-Level Trough Forcing

Abstract This study is part of the efforts undertaken to resolve the “bad trough/good trough” issue for tropical cyclone (TC) intensity changes and to improve the prediction of these challenging events. Sensitivity experiments are run at 8-km resolution with vortex bogusing to extend the previous analysis of a real case of TC–trough interaction (Dora in 2007). The initial position and intensity of the TC are modified, leaving the trough unchanged to describe a realistic environment. Simulations are designed to analyze the sensitivity of TC prediction to both the variety of TC–trough configurations and the current uncertainty in model analysis of TC intensity and position. Results show that TC intensification under upper-level forcing is greater for stronger vortices. The timing and geometry of the interaction between the two cyclonic potential vorticity anomalies associated with the cutoff low and the TC also play a major role in storm intensification. The intensification rate increases when the TC (initially located 12° northwest of the trough) is displaced 1° closer. By allowing a gradual deformation and equatorward tilting of the trough, both scenarios foster an extended “inflow channel” of cyclonic vorticity at midlevels toward the vortex inner core. Conversely, unfavorable interaction is found for vortices displaced 3° or 4° east or northeast. Variations in environmental forcing relative to the reference simulation illustrate that the relationship between intensity change and the 850–200-hPa wind shear is not systematic and that the 200-hPa divergence, 335–350-K mean potential vorticity, or 200-hPa relative eddy momentum fluxes may be better predictors of TC intensification during TC–trough interactions.

A Horizontal Index for the Influence of Upper-Level Environmental Flow on Tropical Cyclone Intensity

Abstract A horizontal map of the upper-level forcing index (ULFI) is constructed to show the possible influence of upper-level large-scale environmental flow on the intensity change of tropical cyclones (TCs). The ULFI includes three commonly used diagnostics, that is, 200-hPa eddy flux convergences of both relative (REFC) and planetary angular momentum (PEFC), as well as axisymmetric absolute vorticity as a denominator that rescales the strength of the eddy forcings similar to the outflow-layer inertial stability. A simple procedure is adopted to convert these storm-relative components and the ULFI into Eulerian horizontal maps. Applications of this index map to three selected TC cases clearly demonstrate the process of upper-level TC–environment interaction: when a TC moves into a region of high (low) index, significant upper-level asymmetric forcing is exerted on the TC, leading to the strengthening (weakening) of the TC’s axisymmetric outflow and then possibly its intensity. As such, the horizontal map of ULFI not only provides a quantitative way of estimating the strength of upper-level asymmetric forcing at each grid point, but also serves as an indicator showing where the possible intensity change of a TC would occur under the influence of upper-level environmental flow. The index is thus recommended to be used in future studies of TC–environment interaction.

Dependence of the relationship between the tropical cyclone track and western Pacific subtropical high intensity on initial storm size: A numerical investigation

AbstractA suite of numerical experiments were conducted to investigate the sensitivity of the tropical cyclone (TC) motion—western Pacific subtropical high (WPSH) intensity relationship to initial storm size. Two TC cases, Songda (2004) and Megi (2010), were studied. It was found that with the increase of initial storm size, the main body of the WPSH tends to withdraw eastward and the TC tends to turn northward earlier. The involved physical mechanism was investigated. Rather than the change of the beta effect due to storm size change, it is the change of the geopotential height in the TC outer region that is critical for the different TC tracks between the sensitivity experiments. Due to increase of the initial storm size, the inflow mass flux entering the TC region increases, leading to a significant decrease in 500 hPa geopotential height in the TC outer region after 2–3 day integration. As a result, the simulated intensity of the WPSH over its fringe close to the TC decreases notably when the WPSH edge is within the TC outer region. Such a decrease leads to a break of WPSH. Subsequently, the TC turns northward toward the break of the subtropical high. This further weakens the intensity of the WPSH over the region close to the TC. The result helps us better understand the relationship between the TC track and WPSH intensity. It also indicates that a proper representation of initial storm size is important for realistic prediction of TC track and the change of the WPSH.

Idealized simulations of the effect of Taiwan topography on the tracks of tropical cyclones with different sizes

This is the third part of a study of the effect of topography on tropical cyclone (TC) tracks. In previous parts, idealized simulations were conducted using the Weather Research and Forecasting model to study the underlying mechanisms for the upstream northward deflection of a TC and the effects of local and remote topography. The initial TC size was the same in all these simulations. In this third part, the track deflections of TCs with different sizes are investigated. As in the two earlier parts, the simulations are made on a beta plane with no background flow.For a TC approaching Taiwan, there is an apparent optimal TC size with the largest northward deflection. However, an appreciable track deflection is only found for smaller TCs approaching Taiwan directly. In addition, as might be expected, the onset location of the northward deflection is closer to Taiwan for smaller TCs. Moreover, the contribution from diabatic heating is opposite to that from horizontal advection (HA) in the potential vorticity tendency (PVT) calculation due to the vertical wind shear induced by the terrain‐induced gyres although HA is still the dominant term in the PVT equation.A southward deflection on the eastern side of Taiwan is found in some smaller TC cases. A looping track occurs for a medium‐size TC approaching central Taiwan and a V‐shape track is found for a small‐size TC approaching northern Taiwan. Among the cases, some similarities can be found. First, the gyre flow becomes weak so that the TC is slowed down. Second, an individual cyclonic gyre is found on the southeastern side of Taiwan so that the TC is deflected westward and then southward. Third, another newly generated gyre pair pushes the TC northward after the southward deflection.

A parabolic model of drag coefficient for storm surge simulation in the South China Sea

Drag coefficient (Cd) is an essential metric in the calculation of momentum exchange over the air-sea interface and thus has large impacts on the simulation or forecast of the upper ocean state associated with sea surface winds such as storm surges. Generally, Cd is a function of wind speed. However, the exact relationship between Cd and wind speed is still in dispute, and the widely-used formula that is a linear function of wind speed in an ocean model could lead to large bias at high wind speed. Here we establish a parabolic model of Cd based on storm surge observations and simulation in the South China Sea (SCS) through a number of tropical cyclone cases. Simulation of storm surges for independent Tropical cyclones (TCs) cases indicates that the new parabolic model of Cd outperforms traditional linear models.

A data assimilation technique to account for the nonlinear dependence of scattering microwave observations of precipitation

AbstractSatellite microwave observations of rain, whether from radar or passive radiometers, depend in a very crucial way on the vertical distribution of the condensed water mass and on the types and sizes of the hydrometeors in the volume resolved by the instrument. This crucial dependence is nonlinear, with different types and orders of nonlinearity that are due to differences in the absorption/emission and scattering signatures at the different instrument frequencies. Because it is not monotone as a function of the underlying condensed water mass, the nonlinearity requires great care in its representation in the observation operator, as the inevitable uncertainties in the numerous precipitation variables are not directly convertible into an additive white uncertainty in the forward calculated observations. In particular, when attempting to assimilate such data into a cloud‐permitting model, special care needs to be applied to describe and quantify the expected uncertainty in the observations operator in order not to turn the implicit white additive uncertainty on the input values into complicated biases in the calculated radiances. One approach would be to calculate the means and covariances of the nonlinearly calculated radiances given an a priori joint distribution for the input variables. This would be a very resource‐intensive proposal if performed in real time. We propose a representation of the observation operator based on performing this moment calculation off line, with a dimensionality reduction step to allow for the effective calculation of the observation operator and the associated covariance in real time during the assimilation. The approach is applicable to other remotely sensed observations that depend nonlinearly on model variables, including wind vector fields. The approach has been successfully applied to the case of tropical cyclones, where the organization of the system helps in identifying the dimensionality‐reducing variables.

Applicability of the superensemble to the tropical cyclone track forecasts in the western North Pacific

In this study a superensemble was constructed and assessed to examine its applicability to the tropical cyclone track forecasts in the western North Pacific. The data used for this study were outputs of 20 tropical cyclone forecast models and analyzed tropical cyclone tracks by the Korea Meteorological Administration from 2011 to 2013. The annual mean track errors were analyzed at the 24-, 48-, 72-, 96- and 120-h periods for 2012 and 2013, and the superensemble forecasts showed lower annual errors than the simple mean consensus (using 20 numerical models), ECMWF_TIGG, and GFS. The superensemble track errors for individual tropical cyclone cases were lower than the simple mean consensus over 60% of the total cases, and lower than the best-performing model over 50% of the total cases for the 24-, 48-, and 72-h forecast periods. In the track error distribution, the superensemble had lower density for relatively large errors than the simple mean consensus, and higher density for smaller errors than single models. When the results are combined, the probability of the superensemble yielding lower errors than the simple mean consensus and single models becomes high, which indicates that the superensemble can serve as an objective reference for the tropical cyclone track forecasts.

Understanding the Role of Cloud and Convective Processes in Simulating the Weaker Tropical Cyclones over Indian Seas

This study addresses the problem of incorporating moist processes (resolving the grid scale and parameterizing the subgrid scale) at resolutions of 9 and 3 km with double- and triple-nested domains, respectively, in predicting the track and intensity of four cases of weaker tropical cyclones over the North Indian Ocean. The sensitivity experiments are carried out with three convective parameterization schemes, and the results are evaluated based on the track and intensity of cyclones. The Betts-Miller-Janjic scheme shows the most reasonable representation of track and intensity and therefore is used for all sensitivity experiments related to microphysical schemes in two and three domains. Three sets of microphysics sensitivity experiments are carried out: The first set includes experiments with parameterized moist convection (referred to as the 9-km experiment) in two domains (27 and 9 km). The second and third sets of simulation experiments are carried out at 9 km in two domains (27 and 9 km; referred to as the 9-km-noCP) and at 3 km in three domains (27 km, 9 km and 3 km; referred to as the 3-km experiment), respectively, by resolving the grid-scale convection explicitly with the four bulk microphysical schemes. The explicit moist convection treatment at 9- and 3-km resolution produces a better cyclone simulation than the parameterized convection at 9-km resolution. The latent heat released in the generation of hydrometeors such as snow and graupel in the mid-tropospheric levels appears to influence the heating within the inner core of the cyclone. The comparable and more realistic representation of mid-tropospheric heating is possibly one of the main reasons behind the improvement at 9-km-noCP and 3 km. The stronger vertical advection of moist static energy gives well-organized mesoscale convection within the cyclone environment at 9-km-noCP and 3-km resolution. This study therefore demonstrates the importance of microphysical processes in the simulation of weaker TC over the North Indian Ocean.

Terrain-Induced Turbulence Intensity during Tropical Cyclone Passage as Determined from Airborne, Ground-Based, and Remote Sensing Sources

AbstractLow-level turbulence [rapid headwind fluctuations below 1600 ft (500 m)] poses potential safety hazards to landing/departing aircraft and is capable of disrupting air traffic. Timely, accurate alerts of low-level turbulence require reliable determination of its intensity, quantified by an internationally adopted aircraft-independent metric [cube root of the eddy dissipation rate (EDR1/3)], which cannot be directly measured but only inferred from observational data. In this paper, a large-scale survey of terrain-induced low-level turbulence intensity around the Hong Kong International Airport (HKIA) during tropical cyclone (TC) passage is presented, utilizing EDR1/3 values determined from multiple remote sensing and in situ sources, including the scanning Doppler lidar, the terminal Doppler weather radar (TDWR), a high-resolution anemometer, and the operational Windshear and Turbulence Warning System (WTWS) at HKIA. Over a 18 720-min study period spanning five TC cases between 2010 and 2012, ground-based EDR1/3 was computed using a variety of first-principle and empirical methods and was shown to demonstrate a strong linear correlation with airborne values determined from quick access recorder (QAR) data of over 350 landing flights. Spatiotemporal features as experienced on board aircraft were also reproduced by the lidar- and TDWR-derived profiles. Positive skill could be extracted from threshold-based alerting of low-level turbulence events by considering each ground-based source individually, while a combination of lidar and TDWR alerts demonstrated enhanced performance and hence the potential value of complementary surveillance under clear-air and in rain conditions. This study serves to establish the ability of ground-based instruments in correlating with airborne EDR1/3 and the performance of threshold-based alerting algorithms for turbulence events, contributing toward improvements in turbulence-alerting techniques for the aviation community.

DEVELOPMENT OF AN ADAPTIVE EMPIRICAL MODEL FOR STORM TIDE SIMULATION

The abrupt rise in local sea level due to storm tide caused by an approaching tropical cyclone could cause severe flooding and devastating influence to low-lying regions of a coastal city. Studies show that many empirical offline modeling techniques are useful tools for storm tide simulation, e.g. the Artificial Neural Network (ANN). However, these techniques are non-adaptive and retraining is necessary when new data are available. The present study introduces an adaptive empirical model for improvement. It is a dynamic linear regression model with the harmonic tidal prediction, wind speed, wind direction and atmospheric pressure as the model input parameters. Application of the model to simulate the storm tide variation of forty tropical cyclone cases in Macau gives values of the root-mean-squared error and the coefficient of determination ranging between 0.08~0.20m and 0.82~0.98, respectively. In addition, the proposed model could capture the storm tide maxima of the seventeen flooding cases among the forty with a root-mean-squared error of 0.15m. Simulation of the corresponding peak arrival for the flooding cases is mostly within one hour of the actual one and at most two hours. Therefore, the proposed adaptive model is a promising tool for storm tide simulation.

Influence of Pacific Decadal Oscillation on the relationship between ENSO and tropical cyclone activity in the Bay of Bengal during October–December

The relationship between ENSO and tropical cyclones (TCs) activity in the Bay of Bengal (BoB) during October–December under cold (1950–1974) and warm (1975–2006) phase of Pacific Decadal Oscillation (PDO) is investigated. A statistically significant difference in the formation of total number of TCs and intense TCs (Category-1 and above) between El Nino and La Nina years is observed when the PDO was in warm phase. Our analysis shows that, there is a tendency to form more number of TCs during La Nina years (2.62 TCs per season) than during El Nino years (1.6 TCs per season) under warm phase of PDO. Moreover, the difference is quite high for intense TCs cases, such as, relatively more number of intense TCs forms in the BoB during La Nina years (1.4 TCs per season) compared to El Nino years (0.10 TCs per season) under warm phase of PDO. However, the difference in the formation of total number of TCs and intense TCs between La Nina and El Nino years is not significant under cold phase of PDO. Significant enhancement in low level cyclonic vorticity and mid-troposphere humidity during La Nina years compared to El Nino years when the PDO was in warm phase, rather than the PDO was in cold phase leads to this difference. Our analysis further shows that how the ENSO related teleconnection to the Indian Ocean region differ under warm and cold phase of PDO.

Sensitivity of different convective parameterization schemes on tropical cyclone prediction using a mesoscale model

This study presents an intercomparison of four cumulus parameterization schemes (CPS) in the prediction of three cases of tropical cyclones in the north Indian Ocean. The study makes use of the Weather Research and Forecasting model of Non-hydrostatic Mesoscale Model version with a horizontal resolution of 27 km. The four deep cumulus schemes studied are (a) modified Kain–Fritsch (KF), (b) Betts–Miller–Janjic, (c) Simplified Arakawa–Schubert and (d) Grell-Devenyi Ensemble (GD) schemes. Three cases chosen for the study are unique cases with entirely different characteristics, synoptic/convective conditions and with varying levels of performance of the driving global model forecasts. The objective of the current study is to report the relative performance of the CPSs rather than the accuracy of the forecasts, under different convective conditions as reflected in the initial and boundary conditions. The study shows that generally KF scheme produced near-realistic track, intensification and the associated rainfall patterns and GD performed worst in terms of convective organisation and the sustained intensity. The impact of cumulus parameterization schemes and its performance vary widely among the three cases studied. The standard verification scores and the contribution of grid-scale precipitation towards the total rainfall by the mesoscale model are also compared between the different cases as well as the different cumulus parameterization schemes. The performance evaluation of the tropical cyclone predictions by the mesoscale model is influenced by not only the model physics but also the convective conditions as input into the model .

Can adaptive observations improve tropical cyclone intensity forecasts?

In order to investigate whether adaptive observations can improve tropical cyclone (TC) intensity forecasts, observing system simulation experiments (OSSEs) were conducted for 20 TC cases originating in the western North Pacific during the 2010 season according to the conditional nonlinear optimal perturbation (CNOP) sensitivity, using the fifth version of the PSU/NCAR mesoscale model (MM5) and its 3DVAR assimilation system. A new intensity index was defined as the sum of the number of grid points within an allocated square centered at the corresponding forecast TC central position, that satisfy constraints associated with the Sea Level Pressure (SLP), near-surface horizontal wind speed, and accumulated convective precipitation. The higher the index value is, the more intense the TC is.

Computable general equilibrium analyses of global economic impacts and adaptation for climate change: the case of tropical cyclones

Computable general equilibrium models have been widely used for simulating global warming and evaluating economic damages caused by climate change. However, to date little research has focused on the economic consequences incurred across several industry sectors at a global level. This article uses the evaluation model for environmental damage and adaptation (EMEDA) to simulate direct economic damages caused by tropical cyclones and losses that are offset through growth in other sectors to measure the global economic impacts arising from climate change. Simulated results by EMEDA indicate that: 1) several regions experience economic growth, with four regions offsetting economic damages in the primary industry sector whilst the other regions increase their damages; 2) seven regions show economic growth whilst only North America neutralises damage in their secondary sectors, with the other regions revealing more severe losses; 3) several regions are able to offset their tertiary sector losses yet the other regions show an increase in damages; 4) the equivalent variation in all regions except East Asia decreases as temperature increases.

Physical Mechanisms Underlying Selected Adaptive Sampling Techniques for Tropical Cyclones

Abstract To efficiently and effectively prioritize resources, adaptive observations can be targeted by using some objective criteria to estimate the potential impact an initial condition perturbation (or analysis increment) in a specific region would have on the future forecast. Several objective targeting guidance techniques have been developed, including total-energy singular vectors (TESV), adjoint-derived sensitivity steering vectors (ADSSV), and the ensemble transform Kalman filter (ETKF), all of which were tested during the 2008 The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) and the Office of Naval Research Tropical Cyclone Structure-2008 (TCS-08) field experiments. An intercomparison between these techniques is performed in order to find underlying physical mechanisms in the respective guidance products, based on four tropical cyclone (TC) cases from the T-PARC/TCS-08 field campaigns. It is found that the TESV energy norm and the ADSSV response function are largely indirect measures of the TC track divergence that can be produced by an initial condition perturbation, explaining the strong correlation between these products. The downstream targets routinely chosen by the ETKF guidance system are often not found in the TESV and ADSSV guidance products, and it is found that downstream perturbations can affect the steering of a TC through the development of a Rossby wave in the subtropics that modulates the strength of the nearby subtropical ridge. It is hypothesized that the ubiquitousness of these downstream targets in the ETKF is largely due to the existence of large uncertainties downstream of the TC that are not taken into consideration by either the TESV or ADSSV techniques.

Modeling high-impact weather and climate: lessons from a tropical cyclone perspective

Although the societal impact of a weather event increases with the rarity of the event, our current ability to assess extreme events and their impacts is limited by not only rarity but also by current model fidelity and a lack of understanding and capacity to model the underlying physical processes. This challenge is driving fresh approaches to assess high- impact weather and climate. Recent lessons learned in modeling high-impact weather and climate are presented using the case of tropical cyclones as an illustrative example. Through examples using the Nested Regional Climate Model to dynamically downscale large-scale climate data the need to treat bias in the driving data is illustrated. Domain size, location, and resolution are also shown to be critical and should be adequate to: include relevant regional climatephysicalprocesses;resolvekeyimpactparameters;andaccuratelysimulatetheresponse to changes in external forcing. The notion of sufficient model resolution is introduced together with the added value in combining dynamical and statistical assessments to fill out the parent distribution of high-impact parameters.

A study of the connection between tropical cyclone track and intensity errors in the WRF model

This study examines the dependence of the tropical cyclone (TC) intensity errors on the track errors in the Weather Research and Forecasting (WRF-ARW) model. By using the National Centers for Environmental Prediction global final analysis as the initial and boundary conditions for cloud-resolving simulations of TC cases that have small track errors, it is found that the 2- and 3-day intensity errors in the North Atlantic basin can be reduced to 15 and 19 % when the track errors decrease to 55 and 76 %, respectively, whereas the 1-day intensity error shows no significant reduction despite more than 30 % decrease of the 1-day track error. For the North-Western Pacific basin, the percentage of intensity reduction is somewhat similar with the 2- and 3-day intensity errors improved by about 15 and 19 %, respectively. This suggests that future improvement of the TC track forecast skill in the WRF-ARW model will be beneficial to the intensity forecast. However, the substantially smaller percentages of intensity improvement than those of the track error improvement indicate that ambient environment tends to play a less important role in determining the TC intensity as compared to other factors related to the vortex initialization or physics representations in the WRF-ARW model.