Core Of Tropical Cyclones Research Articles

Long Memory Impact of Ocean Mesoscale Temperature Anomalies on Tropical Cyclone Size

AbstractMesoscale ocean temperature anomalies modify a tropical cyclone (TC). Through a modeling study we show that, while the maximum wind speed is rapidly restored after the TC passes a warm‐ or cold‐ (eddy size) sea surface temperature (SST) anomaly, the storm size changes are more significant and persistent. The radius of gale force winds and integrated kinetic energy (IKE) can change by more than 10% per degree and this endures several days after crossing an SST anomaly. These properties have a long memory of the impact from the ocean fluxes and depend on the integrated history of SST exposure. They are found to be directly proportional to the storm total precipitation. Accurate continuous forecast of the SST along the track may therefore be of central importance to improving predictions of size and IKE, while instantaneous local SST near the TC core is more important for the forecast of maximum wind speed.

Tropical Cyclone Surface Pressure Field Estimation Using Satellite Passive Microwave Observations over the Oceans

AbstractThe real time and accurate estimation of tropical cyclone (TC) surface pressure fields is highly desired in TC forecasting and analysis. Using satellite passive microwave observations over the oceans, the retrieval of TC surface pressure fields was investigated in this study, and a new retrieval algorithm was developed. This algorithm uses brightness temperatures (TBs) and TB anomalies together to accurately estimate TC surface pressure fields. Considering the relative superiority of 118‐GHz radiometers in spatial resolution, this study not only used the measurements of 60‐GHz radiometer Advanced Technology Microwave Sounder (ATMS) but also the 118‐GHz radiometer Microwave Humidity and Temperature Sounder (MWHTS). The TC measurements from July 2015 to September 2018 and the corresponding Global Data Assimilation System (GDAS) Final (FNL) analysis data were used to develop and test the retrieval algorithm. The testing results show that the proposed method can estimate the surface pressure fields of a TC's core area (within a 2° radius from the center) with an accuracy of better than 4.0, 5.0, and 8.0 hPa for tropical depressions and tropical storms, typhoons or hurricanes, and severe typhoons or major hurricanes, respectively. In addition, an in situ comparison demonstrates that the accuracy of the retrieved central pressure is better than 6.0, 8.0, and 10.0 hPa for tropical depressions and tropical storms, typhoons or hurricanes, and severe typhoons or major hurricanes, respectively.

Elliptical Structures of Gravity Waves Produced by Typhoon Soudelor in 2015 near Taiwan

Tropical cyclones (TCs) are complex sources of atmospheric gravity waves (GWs). In this study, the Weather Research and Forecasting Model was used to model TC Soudelor (2015) and the induced elliptical structures of GWs in the upper troposphere (UT) and lower stratosphere (LS) prior to its landfall over Taiwan. Conventional, spectral and wavelet analyses exhibit dominant GWs with horizontal and vertical wavelengths, and periods of 16–700 km, 1.5–5 km, and 1–20 h, respectively. The wave number one (WN1) wind asymmetry generated mesoscale inertia GWs with dominant horizontal wavelengths of 100–300 km, vertical wavelengths of 1.5–2.5 km (3.5 km) and westward (eastward) propagation at the rear of the TC in the UT (LS). It was also revealed to be an active source of GWs. The two warm anomalies of the TC core induced two quasi-diurnal GWs and an intermediate GW mode with a 10-h period. The time evolution of dominant periods could be indicative of changes in TC dynamics. The FormoSat-3/COSMIC (Formosa Satellite Mission-3/Constellation Observing System for Meteorology, Ionosphere, and Climate) dataset confirmed the presence of GWs with dominant vertical wavelengths of about 3.5 km in the UT and LS.

Response of tropical cyclone Phailin (2013) in the Bay of Bengal to climate perturbations

Tropical cyclones in the Bay of Bengal (BoB) are the major source of loss of properties and lives in the adjacent coastal regions during post-monsoon (October–November) season. This study explores the impact of large-scale environmental change on tropical cyclone (TC) Phailin (2013) in the Bay of Bengal. An ensemble simulation with a regional atmospheric model using different physics and initial conditions is able to capture the observed track of Phailin. The cyclone intensity, however, is underestimated due to the coarse resolution (10 km) of the model. To explore Phailin’s response to a warming climate, a global climate model change signal is added to the cyclone environment using the pseudo-global warming approach, and the ensemble is run again. It is found that changes in the track of Phailin would be small, except around the time of landfall, when Phailin shifts southward in its track. The cyclone becomes more intense and larger, but the translation speed remains nearly unchanged. An analysis of physical processes indicates a deeper TC core due to enhanced latent heating. The vertical distribution of temperature and wind indicates that the eye of Phailin would have a greater vertical extent in the future climate. The expanded size is found to be due to an increase in environmental humidity and temperature in the lower-troposphere leading to an increase in environmental convective available potential energy. The translation speed remains nearly unchanged due to insignificant changes in the steering flow. The result suggests that Phailin, under a warmer climate, may have higher damage potential (~ 22%) compared to the present at the time of landfall.

Revisiting Azimuthally Asymmetric Moist Instability in the Outer Core of Sheared Tropical Cyclones

Abstract This study revisits the characteristics and physical processes of the azimuthally asymmetric distribution of moist instability in the outer core of vertically sheared tropical cyclones (TCs) using a numerical model. The results indicate that a downshear–upshear contrast in outer-core conditional instability occurs in the weakly sheared TCs, while an enhanced downshear-left–downshear-right difference is found in strongly sheared storms. Specifically, lower (higher) conditional instability arises downshear left (right) in the strongly sheared TCs. Downward transports of low-entropy air by convective and mesoscale downdrafts in principal rainbands reduce the equivalent potential temperature (θe) in the downshear-left boundary layer, contributing to lower convective available potential energy. Positive horizontal advection of both potential temperature and water vapor by the asymmetric outflow leads to a midlevel maximum of θe in the same quadrant. Hence, a positive θe vertical gradient (thus potential stability) is present in the downshear-left outer core. In the downshear-right quadrant, a lack of convective downdrafts, together with surface fluxes, leads to higher θe in the boundary layer. A dry intrusion is found at the middle to upper levels in the downshear-right outer core, and significant negative horizontal advection of water vapor produces low θe near the midtroposphere. A negative vertical gradient of θe (thus potential instability) in the outer core arises below the downshear-right midtroposphere. The presence of azimuthally asymmetric moist instability is expected to play an important role in fostering and maintaining azimuthally asymmetric convective activity in the outer core of TCs.

Study of upper ocean parameters during passage of tropical cyclones over Indian seas

ABSTRACTThe primary energy supply for tropical cyclones is the upward latent heat fluxes that are directly related to Sea Surface Temperatures (SST). The strong winds induce a negative SST anomaly in the tropical cyclone wake. This is usually referred to as the ‘cold wake’. Many studies have suggested that the cold wake results in a significant reduction of upward latent heat fluxes that supply energy to the tropical cyclone, and hence provides a negative feedback on its intensity. The cold-wake feedback on the intensity of tropical cyclones is a strong motivation to understand the oceanic response to tropical cyclones. A recent study of re-examining the mechanisms controlling the cold wake has shown the importance of vertical Ekman pumping. Under the core of tropical cyclones (typically over a disk of 100 km radius), vertical pumping is responsible for a cooling of the entire water column, while surface heat fluxes and vertical mixing both contribute to the surface cooling during the cyclone passage. Farther away from the cyclone core, vertical mixing generally overwhelms the effect of vertical pumping, and also the effect of heat fluxes in the case of strong tropical cyclones. The study examines the Ekman pumping during the passage of few cyclonic storms in the Arabian Sea (AS) and Bay of Bengal (BOB) during last decade using different datasets such as Satellite (QuikScat) and reanalysis (ERA-Interim) data and calls up the value for satellite deliverables. Surface currents and subsurface temperature structure is examined further and for different Ekman pumping velocity (EPV), variable subsurface response is illustrated. The changes in ocean surface temperature, sea level and heat content obtained from the remote sensing data products, in the wake of cyclone are also reported and thus the usefulness of satellite products to study the coupled tropical system is demonstrated.

CONDITIONAL NONLINEAR OPTIMAL PERTURBATIONS: APPLICATION IN TROPICAL CYCLONE FORECASTS

Considering the feature of tropical cyclones (TCs) that strong positive vorticity exists in the lower layers of troposphere, this study proposed to use vorticity at 850 hPa as cost function to find the conditional nonlinear optimal perturbation (CNOP), which was largely different from those previous studies using total energy of perturbed forecast variables. The CNOP was obtained by an ensemble-based approach. All of the sensitive areas determined by CNOP with vorticity at 850 hPa as cost function for the three cases were located over the TC core region and its vicinity. The impact of the CNOP-based adaptive observations on TC forecasts was evaluated with three cases via observational system simulation experiments (OSSEs). Results showed obvious improvements in TC intensity or track forecasts due to the CNOP-based adaptive observations, which were related to the main error source of the verification area, i.e., intensity error or location error.

Examining the contribution of surface sensible heat flux induced sensible heating to tropical cyclone intensification from the balance dynamics theory

Observations and numerical simulations demonstrate that surface sensible heat fluxes (SHX) can account for 20%–30% of surface enthalpy fluxes in the inner core of tropical cyclones (TCs). The SHX have effects of warming the near surface air directly. However, the SHX are found to have no distinct contribution to the TC intensification. The possible reason for the minor role of SHX is investigated in this study by resolving the Sawyer-Eliassen equation and computing the tangential wind tendency equation with the SHX-induced sensible heating as the forcing source. Results reveal that the SHX-induced sensible heating merely forces very weak radial inflow near the surface and downward motion in the eyewall region, even by elevating it to larger magnitudes than eyewall heating. A decomposition of diabatic heating into horizontal and vertical gradient parts in the Sawyer-Eliassen equation suggests that the horizontal gradient of diabatic heating is essential for driving the TC response, while the influence of vertical gradient of diabatic heating is almost negligible. It implies that the processes that can alter radial distributions of diabatic heating in TC systems should be paid more attention. Although the SHX-induced sensible heating possesses relatively large vertical gradient, it changes smoothly in the radial direction and produces quite small horizontal gradient. Hence the SHX-induced sensible heating cannot contribute to the intensification of the TC remarkably even by enhancing it to very large values. It is therefore proposed that the insignificant contribution of SHX is not simply attributed to its small fraction to total surface enthalpy fluxes, but is because the SHX are inefficient in driving the spin up of the TC. It should be noted that the conclusions based on balanced dynamics can only reveal the direct contribution of SHX-induced heating to the intensification.

Diagnosing Conditions Associated with Large Intensity Forecast Errors in the Hurricane Weather Research and Forecasting (HWRF) Model

Abstract Understanding and forecasting tropical cyclone (TC) intensity change continues to be a paramount challenge for the research and operational communities, partly because of inherent systematic biases contained in model guidance, which can be difficult to diagnose. The purpose of this paper is to present a method to identify such systematic biases by comparing forecasts characterized by large intensity errors with analog forecasts that exhibit small intensity errors. The methodology is applied to the 2015 version of the Hurricane Weather Research and Forecasting (HWRF) Model retrospective forecasts in the North Atlantic (NATL) and eastern North Pacific (EPAC) basins during 2011–14. Forecasts with large 24-h intensity errors are defined to be in the top 15% of all cases in the distribution that underforecast intensity. These forecasts are compared to analog forecasts taken from the bottom 50% of the error distribution. Analog forecasts are identified by finding the case that has 0–24-h intensity and wind shear magnitude time series that are similar to the large intensity error forecasts. Composite differences of the large and small intensity error forecasts reveal that the EPAC large error forecasts have weaker reflectivity and vertical motion near the TC inner core from 3 h onward. Results over the NATL are less clear, with the significant differences between the large and small error forecasts occurring radially outward from the TC core. Though applied to TCs, this analog methodology could be useful for diagnosing systematic model biases in other applications.

Probing the core of tropical cyclones

A recently launched network of small satellites will help meteorologists measure the intensity and structure of a storm’s strongest but most obscured region.

Estimating Tropical Cyclone Integrated Kinetic Energy with the CYGNSS Satellite Constellation

AbstractThe Cyclone Global Navigation Satellite System (CYGNSS) constellation is designed to provide observations of surface wind speed in and near the inner core of tropical cyclones with high temporal resolution throughout the storm’s life cycle. A method is developed for estimating tropical cyclone integrated kinetic energy (IKE) using CYGNSS observations. IKE is calculated for each geographically based quadrant out to an estimate of the 34-kt (1 kt = 0.51 m s−1) wind radius. The CYGNSS-IKE estimator is tested and its performance is characterized using simulated CYGNSS observations with realistic measurement errors. CYGNSS-IKE performance improves for stronger, more organized storms and with increasing number of observations over the extent of the 34-kt radius. Known sampling information can be used for quality control. While CYGNSS-IKE is calculated for individual geographic quadrants, using a total-IKE—a sum over all quadrants—improves performance. CYGNSS-IKE should be of interest to operational and research meteorologists, insurance companies, and others interested in the destructive potential of tropical cyclones developing in data-sparse regions, which will now be covered by CYGNSS. The CYGNSS-IKE product will be available for the 2017 Atlantic Ocean hurricane season.

AMSR2 all-sky radiance assimilation and its impact on the analysis and forecast of Hurricane Sandy with a limited-area data assimilation system

A method to assimilate all-sky radiances from the Advanced Microwave Scanning Radiometer 2 (AMSR2) was developed within the Weather Research and Forecasting (WRF) model's data assimilation (WRFDA) system. The four essential elements are: (1) extending the community radiative transform model's (CRTM) interface to include hydrometeor profiles; (2) using total water Qt as the moisture control variable; (3) using a warm-rain physics scheme for partitioning the Qt increment into individual increments of water vapour, cloud liquid water and rain; and (4) adopting a symmetric observation error model for all-sky radiance assimilation.Compared to a benchmark experiment with no AMSR2 data, the impact of assimilating clear-sky or all-sky AMSR2 radiances on the analysis and forecast of Hurricane Sandy (2012) was assessed through analysis/forecast cycling experiments using WRF and WRFDA's three-dimensional variational (3DVAR) data assimilation scheme. With more cloud/precipitation-affected data being assimilated around tropical cyclone (TC) core areas in the all-sky AMSR2 assimilation experiment, better analyses were obtained in terms of the TC's central sea level pressure (CSLP), warm-core structure and cloud distribution. Substantial (>20 %) error reduction in track and CSLP forecasts was achieved from both clear-sky and all-sky AMSR2 assimilation experiments, and this improvement was consistent from the analysis time to 72-h forecasts. Moreover, the all-sky assimilation experiment consistently yielded better track and CSLP forecasts than the clear-sky did for all forecast lead times, due to a better analysis in the TC core areas. Positive forecast impact from assimilating AMSR2 radiances is also seen when verified against the European Center for Medium-Range Weather Forecasts (ECMWF) analysis and the Stage IV precipitation analysis, with an overall larger positive impact from the all-sky assimilation experiment.

Evaluation of Atmosphere and Ocean Initial Condition Uncertainty and Stochastic Exchange Coefficients on Ensemble Tropical Cyclone Intensity Forecasts

Tropical cyclone (TC) intensity forecasts are impacted by errors in atmosphere and ocean initial conditions and the model formulation, which motivates using an ensemble approach. This study evaluates the impact of uncertainty in atmospheric and oceanic initial conditions, as well as stochastic representations of the drag Cd and enthalphy Ck exchange coefficients on ensemble Advanced Hurricane WRF (AHW) TC intensity forecasts of multiple Atlantic TCs from 2008 to 2011. Each ensemble experiment is characterized by different combinations of either deterministic or ensemble atmospheric and/or oceanic initial conditions, as well as fixed or stochastic representations of Cd or Ck. Among those experiments with a single uncertainty source, atmospheric uncertainty produces the largest standard deviation in TC intensity. While ocean uncertainty leads to continuous growth in ensemble standard deviation, the ensemble standard deviation in the experiments with Cd and Ck uncertainty levels off by 48 h. Combining atmospheric and oceanic uncertainty leads to larger intensity standard deviation than atmosphere or ocean uncertainty alone and preferentially adds variability outside of the TC core. By contrast, combining Cd or Ck uncertainty with any other source leads to negligible increases in standard deviation, which is mainly due to the lack of spatial correlation in the exchange coefficient perturbations. All of the ensemble experiments are deficient in ensemble standard deviation; however, the experiments with combinations of uncertainty sources generally have an ensemble standard deviation closer to the ensemble-mean errors.

Tropical Cyclone–Relative Environmental Helicity and the Pathways to Intensification in Shear

Abstract Tropical cyclone–relative environmental helicity (TCREH) is a measure of how the wind vector changes direction with height, and it has been shown to modulate the rate at which tropical cyclones (TCs) develop both in idealized simulations and in reanalysis data. The channels through which this modulation occurs remain less clear. This study aims to identify the mechanisms that lead to the observed variations in intensification rate. Results suggest that the difference in intensification rate between TCs embedded in positive versus negative TCREH primarily results from the position of convection and associated latent heat fluxes relative to the wind shear vector. When TCREH is positive, convection is more readily advected upshear and air parcels that experience larger fluxes are more frequently ingested into the TC core. Trajectories computed from high-resolution simulations demonstrate the recovery of equivalent potential temperature downwind of convection, latent heat flux near the TC core, and parcel routes through updrafts in convection. Differences in trajectory characteristics between TCs embedded in positive versus negative TCREH are presented. Contoured frequency-by-altitude diagrams (CFADs) show that convection is distributed differently around TCs embedded in environments characterized by positive versus negative TCREH. They also show that the nature of the most intense convection differs only slightly between cases of positive and negative TCREH. The results of this study emphasize the fact that significant variability in TC-intensification rate results from vertical variations in the environmental wind direction, even when the 850–200-hPa wind shear vector remains unchanged.

Secondary Circulation of Tropical Cyclones in Vertical Wind Shear: Lagrangian Diagnostic and Pathways of Environmental Interaction

Abstract This study introduces a Lagrangian diagnostic of the secondary circulation of tropical cyclones (TCs), here defined by those trajectories that contribute to latent heat release in the region of high inertial stability of the TC core. This definition accounts for prominent asymmetries and transient flow features. Trajectories are mapped from the three-dimensional physical space to the (two dimensional) entropy–temperature space. The mass flux vector in this space subsumes the thermodynamic characteristics of the secondary circulation. The Lagrangian diagnostic is then employed to further analyze the impact of vertical wind shear on TCs in previously published idealized numerical experiments. One focus of this analysis is the classification and quantitative depiction of different pathways of environmental interaction based on thermodynamic properties of trajectories at initial and end times. Confirming results from previous work, vertical shear significantly increases the intrusion of low–equivalent potential temperature () air into the eyewall through the frictional inflow layer. In contrast to previous ideas, vertical shear decreases midlevel ventilation in these experiments. Consequently, the difference in eyewall between the no-shear and shear experiments is largest at low levels. Vertical shear, however, significantly increases detrainment from the eyewall and modifies the thermodynamic signature of the outflow layer. Finally, vertical shear promotes the occurrence of a novel class of trajectories that has not been described previously. These trajectories lose entropy at cold temperatures by detraining from the outflow layer and subsequently warm by 10–15 K. Further work is needed to investigate in more detail the relative importance of the different pathways for TC intensity change and to extend this study to real atmospheric TCs.

Sensitivity of the warm core of tropical cyclones to solar radiation

To investigate the impacts of solar radiation on tropical cyclone (TC) warm-core structure (i.e., the magnitude and height), a pair of idealized simulations are conducted by specifying different strengths of solar shortwave radiation. It is found that the TC warm core is highly sensitive to the shortwave radiative effect. For the nighttime storm, a tendency for a more intense warm core is found, with an elevated height compared to its daytime counterpart. As pointed out by previous studies, the radiative cooling during nighttime destabilizes the local and large-scale environment and thus promotes deep moist convection, which enhances the TC’s intensity. Due to the different inertial stabilities, the diabatic heating in the eyewall will force different secondary circulations. For a strong TC with a deeper vertical structure, this promotes a thin upper-level inflow layer. This inflow carries the lower stratospheric air with high potential temperature and descends adiabatically in the eye, resulting in significant upper-level warming. The Sawyer–Eliassen diagnosis further confirms that the height of the maximum temperature anomaly is likely attributable to the balance among the forced secondary circulations.

Shear-Relative Asymmetries in Tropical Cyclone Eyewall Slope

Abstract Recent studies have analyzed the azimuthal mean slope of the tropical cyclone (TC) eyewall. This study looks at the shear-relative azimuthal variability of different metrics of eyewall slope: the 20-dBZ surface, the radius of maximum wind (RMW), and an angular momentum (M) surface passing through the RMW. The data used are Doppler radar composites from the NOAA Hurricane Research Division (HRD). This study examines 34 TCs, with intensities ranging from 3 to 75 m s−1 and shear magnitudes ranging from 0 to 10 m s−1. Calculation of the mean slope in each quadrant for all cases shows that RMW slope has the strongest asymmetry, with downshear slope larger than upshear in 62% of cases. Slopes of momentum surfaces and dBZ surfaces are also greater downshear in some cases (65% for M and 47% for dBZ), but there is more variance than in the RMW slope. The azimuthal phase of maximum slope occurs most often downshear, particularly downshear left, consistent with the depiction of a mean vortex tilt approximately 10° left of shear. Filtering the cases into high and low shear illustrates that the tendency for greater slope downshear is magnified for high-shear cases. In addition, although the dBZ slope shows less shear-relative signal overall, the difference between the dBZ slope and momentum slope is an important factor in distinguishing between strengthening and weakening or steady TCs. Intensifying TCs tend to have dBZ surfaces that are more upright than M surfaces. Further investigation of these results will help to illustrate the ways in which vertical shear can play a role in altering the structure of the TC core region.

On the Warm Core of a Tropical Cyclone Formed near the Tropopause

Abstract On the basis of numerical results of a three-dimensional model diagnosed using balance dynamics, a mechanism by which the upper-level warm core of tropical cyclones (TCs) forms is proposed. The numerical results reveal that an upper-level warm core develops when TCs intensify just prior to reaching the mature stage. Potential temperature budget analysis reveals that for the tendency of potential temperature, the azimuthal-mean component of advection is dominant at the upper level of the eye at the mature stage. Sawyer–Eliassen diagnosis shows that tendencies due to forced flow by diabatic heating and diffusion of tangential wind are dominant in the eye and are negatively correlated to each other. The distributions of the diabatic heating in the simulated TC are not peculiar. Therefore, it is unlikely that the heating distribution itself is the primary cause of the flow from the lower stratosphere. The analyses of forced circulations of idealized vortices show that the upper-level subsidence is enhanced in the eye when the vortex is sufficiently tall to penetrate the statically stable stratosphere. This result is deduced because the stronger inertial stability extends the response to the heating of the lower stratosphere and causes upper-level adiabatic warming. Therefore, the upper-level warm core emerges if angular momentum is transported into the lower stratosphere due to processes such as convective bursts. The present analysis suggests that TCs can be even stronger than that expected by theories in which the TC vortex is confined in the troposphere.

Impact of Typhoons on the Ocean in the Pacific

Tropical cyclones (TCs) change the ocean by mixing deeper water into the surface layers, by the direct air–sea exchange of moisture and heat from the sea surface, and by inducing currents, surface waves, and waves internal to the ocean. In turn, the changed ocean influences the intensity of the TC, primarily through the action of surface waves and of cooler surface temperatures that modify the air–sea fluxes. The Impact of Typhoons on the Ocean in the Pacific (ITOP) program made detailed measurements of three different TCs (i.e., typhoons) and their interaction with the ocean in the western Pacific. ITOP coordinated meteorological and oceanic observations from aircraft and satellites with deployments of autonomous oceanographic instruments from the aircraft and from ships. These platforms and instruments measured typhoon intensity and structure, the underlying ocean structure, and the long-term recovery of the ocean from the storms' effects with a particular emphasis on the cooling of the ocean beneath the storm and the resulting cold wake. Initial results show how different TCs create very different wakes, whose strength and properties depend most heavily on the nondimensional storm speed. The degree to which air–sea fluxes in the TC core were reduced by ocean cooling varied greatly. A warm layer formed over and capped the cold wakes within a few days, but a residual cold subsurface layer persisted for 10–30 days.

Idealized simulations of the effect of Taiwan and Philippines topographies on tropical cyclone tracks

The effects of topography on tropical cyclone (TC) tracks are investigated using the Weather Research and Forecasting model running on a beta plane with actual topography. Two sets of experiments are performed, one focusing on Taiwan topography and the other on the Philippines. In each set, different parts of the terrain are set to be either present or absent and the TCs are inserted at a location southeast of the mountain so that landfall occurs at the central part of the mountain in the no‐terrain (control) cases. Relative to the control case, a pair of terrain‐induced gyres is found in all with‐terrain experiments. This pair of gyres rotates cyclonically around the TC and the gyre‐associated flow near the TC centre causes a northward deflection of the TC track prior to landfall. An examination of the potential vorticity tendencies (PVTs) suggests that, in addition to the PVT produced by this flow, the PVT from diabatic heating cannot be ignored in explaining the TC track during landfall. The asymmetric diabatic heating is significant, which tends to retard the fast displacement of the TC caused by horizontal advection. This diabatic heating distribution results from three different mechanisms: an incursion of dry air from the mountain, low‐level convergence induced by the terrain‐altered wind field in the southwestern part of the TC and the development of convergence cyclonically inward from the eastern side of the mountain to the TC core. However, the first two mechanisms are apparently missing in the Philippine experiments because of the lower height and the moister air over the mountains on Luzon Island.