Tropical Cyclone Development Research Articles

Latitudinal dependence of the dry air effect on tropical cyclone development

The impacts of dry air on tropical cyclone (TC) development at different latitudes with no mean flows are investigated with idealized simulations. It is found that the effective radius of the dry air is sensitive to its vertical distribution and the background earth rotation. The effect of low-level dry-air layer in inhibiting TC development decreases with increasing latitude. At lower latitudes, the greater boundary layer gradient wind imbalance results in a strong low-level inflow, and the dry air can easily penetrate into the TC inner-core region. The intruding dry air inhibits the inner-core deep convection and leads to marked asymmetric convective structure, which significantly suppresses TC development. In contrast, at higher latitudes, the dry air gets moistened before reaching the TC inner-core region due to a weaker radial inflow but can suppress the development of the outer spiral rainbands. The suppressed outer spiral rainbands lead to a weaker barrier effect to the boundary layer inflow and help TC development. Furthermore, the lower the altitude of dry-air layer resides, the greater the impact on TC intensification. The low-level pathway associated with the boundary layer inflow plays an important role on how dry-air layer acts on a TC without considering the mean flow effects. Through examining the climatological distribution of the moisture field, we expect that the intrusion of dry air can be more frequent in the North Atlantic area and therefore has more effects on TC development than in the western North Pacific.

Analysis of the Meteorological Condition of Tropical Cyclone Cempaka and Its Effect on Heavy Rainfall in Java Island

Indonesia, literally tropical cyclone free area, still gains some influence from the surrounding cyclones, such as high-intensity precipitation which causes flooding. Therefore, cyclones monitoring is important to observe its growth, movement and intensity. Using ECMWF ERA-Interim data, Himawari-8 channel infrared 1 (IR1), and GSMaP satellite data, this study attempts to spatially analyze the evolution of Cempaka tropical cyclones (occurred on November 2017), propagation cyclone and its effect to rainfall over Java Island based on the brightness temperature of the cloud. The results show that the Cempaka tropical cyclone has a three-day life span and moves from the southern ocean of Central Java to the northeast (East Java) and the peak occurs on November 27th, 2017 at 19:00 LT to 23:00 LT. Compared to rainfall data, the intensity of rainfall at 7 areas closed to Cempaka tropical cyclone began to increase ever since the cyclone peak and continue to rise despite the cyclone was slowly decaying. With this result, we hope in the future we can anticipate the impact of any tropical cyclones that arise near Indonesia.

The Characteristics of Spatial and Temporal Variations in the PBL during the Landfall of Tropical Cyclones across East China

AbstractThe planetary boundary layer (PBL) controls the exchange of momentum and energy between the ground surface and the free troposphere, but few studies have been involved in the connection of the PBL with the development and extinction of tropical cyclones (TCs). Studies on the PBL usually need high-resolution soundings in the lowest troposphere that are otherwise quite rare with traditional technology. Here, 1-s resolution L-band radiosonde data are acquired to study the variations in PBL characteristics associated with the development of TCs in eastern China. The strong variations in the vertical profiles of temperature, relative humidity, and wind speed in the PBL during the landfall of a TC are revealed. In addition, four typical methods, including the virtual potential temperature method, Holzworth method, bulk Richardson number method, and potential temperature gradient method, are applied to estimate the PBL height (PBLH). The results indicate that the PBLHs derived by these methods vary by several hundred meters, which may be related to their different definitions of kinetic or thermodynamic theories. Furthermore, the PBLH was found to display a slight upward tendency during the landfall of TC.

Evolution of an Axisymmetric Tropical Cyclone before Reaching Slantwise Moist Neutrality

Abstract In a previous study, the authors showed that the intensification process of a numerically simulated axisymmetric tropical cyclone (TC) can be divided into two periods denoted by “phase I” and “phase II.” The intensification process in phase II can be qualitatively described by Emanuel’s intensification theory in which the angular momentum (M) and saturated entropy (s*) surfaces are congruent in the TC interior. During phase I, however, the M and s* surfaces evolve from nearly orthogonal to almost congruent, and thus, the intensifying simulated TC has a different physical character as compared to that found in phase II. The present work uses a numerical simulation to investigate the evolution of an axisymmetric TC during phase I. The present results show that sporadic, deep convective annular rings play an important role in the simulated axisymmetric TC evolution in phase I. The convergence in low-level radial (Ekman) inflow in the boundary layer of the TC vortex, together with the increase of near-surface s* produced by sea surface fluxes, leads to episodes of convective rings around the TC center. These convective rings transport larger values of s* and M from the lower troposphere upward to the tropopause; the locally large values of M associated with the convective rings cause a radially outward bias in the upper-level radial velocity and an inward bias in the low-level radial velocity. Through a repetition of this process, the pattern (i.e., phase II) gradually emerges. The role of internal gravity waves related to the episodes of convection and the TC intensification process during phase I is also discussed.

Impact of Assimilating Himawari‐8‐Derived Layered Precipitable Water With Varying Cumulus and Microphysics Parameterization Schemes on the Simulation of Typhoon Hato

AbstractUnderstanding moisture information ahead of tropical cyclone (TC) convection is very important for predicting TC track, intensity, and precipitation. The advanced Himawari imager onboard the Japanese Himawari‐8/‐9 satellite can provide high spatial and temporal resolution moisture information. Three‐layered precipitable water (LPW) with its three water vapor absorption infrared bands can be assimilated to generate better understanding and prediction of TC evolution. The impacts of LPW assimilation in the Weather Research and Forecasting model with nine combinations of physical parameterization schemes, including three cumulus parameterization (CP) and three microphysics parameterization (MP) schemes on TC prediction, have been comprehensively analyzed using Typhoon Hato as a case study. The results indicate that LPW assimilation reduces the average track error and speed up TC movement by better adjustment of the atmospheric circulation fields via changing the vertical structure of moisture and thermal profile. The track forecasts retain sensitivity to CP schemes after LPW assimilation. Also, LPW assimilation improves TC intensity prediction because the latent heat release process is accurately adjusted. It has been revealed that LPW assimilation can weaken the intensity sensitivity to MP schemes more than to CP schemes. Skill scores were used to evaluate precipitation forecasts after Hato's landfall. The results indicate that heavy precipitation forecasts are more sensitive to the choice of MP schemes. After LPW assimilation, the equitable threat scores among different results become similar and all forecast skills are increased. In addition, group statistic results with different initial time show the same conclusions.

Detecting Intensity Evolution of the Western North Pacific Super Typhoons in 2016 Using the Deviation Angle Variance Technique with FY Data

This paper analyzes the complete lifecycle of super typhoons in 2016 in the western North Pacific (WNP) using the deviation angle variance technique (DAV-T). Based on the infrared images from Fengyun (FY) satellites, the DAV-T enables quantification of the axisymmetry of tropical cyclones (TCs) by using the DAV values; and thus, it helps improve the capability of TC intensity estimation. Case analyses of Super Typhoons Lionrock and Meranti were performed to explore the distribution characteristics of the DAV values at the various stages of TC evolution. The results show that the minimum DAV values (i.e., map minimum values: MMVs) gradually decreased and their locations constantly approached the circulation center with enhancement of the TC organization; however, when a ring or disk structure was formed around a TC, significant changes in MMV locations were no longer observed. Nonetheless, when large-scale non-closed deep convective cloud clusters appeared at the early stage or the dissipation stage of the typhoon, the axisymmetry of the TC was poor and the MMV locations tended to lie in the most convective region rather than in the TC circulation center. Overall, the MMVs and their locations, respectively, exhibited a strong correlation with the TC intensity and circulation center, and the correlation increased as the TCs became stronger. Combined with the China Meteorological Administration BestTrack dataset (CMA-BestTrack), statistical analysis of all research samples reveals that the correlation coefficient between the MMVs and maximum surface wind speeds (Vmax) was–0.80; the root mean square error (RMSE) of relative distance between the MMV locations and TC centers was 140.3 km; and especially, when the samples below the tropical depression (TD) intensity were removed, the RMSE of the relative distance decreased dramatically to 95.0 km. The value and location of the MMVs could be used as important indicators for estimating TC intensity and center.

Tropopause Evolution in a Rapidly Intensifying Tropical Cyclone: A Static Stability Budget Analysis in an Idealized Axisymmetric Framework

Abstract Upper-level static stability (N2) variations can influence the evolution of the transverse circulation and potential vorticity in intensifying tropical cyclones (TCs). This paper examines these variations during the rapid intensification (RI) of a simulated TC. Over the eye, N2 near the tropopause decreases and the cold-point tropopause rises by up to 4 km at the storm center. Outside of the eye, N2 increases considerably just above the cold-point tropopause and the tropopause remains near its initial level. A budget analysis reveals that the advection terms, which include differential advection of potential temperature θ and direct advection of N2, are important throughout the upper troposphere and lower stratosphere. These terms are particularly pronounced within the eye, where they destabilize the layer near and above the cold-point tropopause. Outside of the eye, a radial–vertical circulation develops during RI, with strong outflow below the tropopause and weak inflow above. Differential advection of θ near the outflow jet provides forcing for stabilization below the outflow maximum and destabilization above. Turbulence induced by vertical wind shear on the flanks of the outflow maximum also modifies the vertical stability profile. Meanwhile, radiative cooling tendencies at the top of the cirrus canopy generally act to destabilize the upper troposphere and stabilize the lower stratosphere. The results suggest that turbulence and radiation, alongside differential advection, play fundamental roles in the upper-level N2 evolution of TCs. These N2 tendencies could have implications for both the TC diurnal cycle and the tropopause-layer potential vorticity evolution in TCs.

Role of the South China Sea Summer Upwelling in Tropical Cyclone Intensity

Tropical cyclones (TCs) can affect the thermal structure in the upper ocean by mixing. In turn, upper-ocean thermal structure also affects the evolution of TCs. Here based on satellite data, in situ temperature and salinity observations and the best-track data of the U.S. Joint Typhoon Warning Center, combined with an ocean mixed layer model, the role of the pre-existing summer upwelling of the northern South China Sea (NSCS) in TCs self-induced sea surface cooling was explored. The modeling results showed that for a given atmospheric thermodynamic condition, TCs self-induced sea surface cooling is quite different when they pass over the regions with pre-existing upwelling and without upwelling. The amplitude of TCs self-induced cooling is larger by more than 50% in the region with pre-existing upwelling than that without. For example, for a slow-moving typhoon with translation speed of 4 m/s and wind speed of 45 m/s, TC self-induced surface cooling is 2.5°C when they pass over the upwelling region, but only 1.5°C when they pass over the region without upwelling. The results suggest that upwelling of the NSCS could amplify TCs self-induced cooling and play a negative role in TCs intensification before they made landfall in Southern China.

Physical modeling of magneto-hydrodynamic processes of powerful tropical cyclones evolution

Physical modeling of magneto-hydrodynamic processes of powerful tropical cyclones evolution

Experiments on Using WRF Model data Assimilation of Coupled 3DVAR – LETKF in Predicting the Geneses of Tropical Cyclones in the Vietnamese East Sea

Abstract:In this research, several experiments using WRF to research the geneses of tropical cyclonesby data assimilation of coupled 3DVAR (3-dimensional variation) – LETKF (Local Ensemble Transform Kalman filter) have been processed. The analysis field after data assimilation procedure with observational information from synop, METAR, ships, soundings,... database produced from 36h before the recorded tropical cyclogeneses from recorded database with 72-days forecasts. The resulted analysis sea level pressure (SLP) fields were compared to the inital fields from control forecast (CTL), which shown the anomalous high pressure of +0.4mb in the area surrounding the actual genesis locations. The SLP and wind fields at 10-meter level in the WUTIP 2013 case has been using as an example of the divergence and diversity of multiple members’ tropical cyclone development.
 Key words: tropical cyclones, tropical cyclogenesis, WRF, data assimilation, 3DVAR, LETKF

Internal Dynamical Control on Tropical Cyclone Intensity Variability – A Review

ABSTRACT The current understanding of internal dynamical processes responsible for tropical cyclone intensity variability is reviewed. These internal dynamic processes are small in horizontal scale and are constrained to characteristics of the vortex itself and less dependent on the environmental and oceanic conditions. Important internal dynamical processes for tropical cyclone evolution are: (i) spiral rainband dynamics, (ii) asymmetric deep convection, (iii) eye/eyewall mixing, and (iv) eyewall replacement cycles. It is shown that while there exists a reasonable understanding of these internal processes in idealized frameworks, there still remains a large gap in our understanding of these processes in the real atmosphere. Additionally, further research is warranted on the interplay of these internal processes with each other as well as their interaction with the environment.

On the Use of a System-Scale Ascent/Descent Diagnostic for Short-Term Forecasting of Tropical Cyclone Development, Intensification and Decay

ABSTRACT The Brisbane Tropical Cyclone Warning Centre has used two forms of a thermal advection diagnostic to identify relatively large areas of isentropic ascent and descent for many years. When the thermodynamic conditions are favourable the ascent regions are correlated with significant outbreaks of convection that produce heavy rainfall. The diagnostic is based on the relationship between geostrophic winds that turn with height and flow perpendicular to thickness contours. As the relationship is also valid for the more general case of gradient winds, the diagnostic, in theory, should be useful for most heavy-rain-bearing tropical systems. A climatology of rainfall rate with one form of the diagnostic is presented at two Queensland locations (one tropical and one subtropical) that demonstrates a clear relationship between the isentropic ascent wind distribution and heavy to extreme rainfall.The diagnostics applied to numerical weather prediction models are valuable forecast tools as they identify heavy rainfall threat regions within which the extreme rain is likely to fall, whereas the rainfall from the same models is often under predicted or has large location errors. Applied to tropical lows and tropical cyclones the diagnostics have been used successfully to forecast tropical cyclone formation and rapid intensification and decay. Examples of such intensification and decay from around the world are presented, as well as a climatology of the diagnostic applied to intensifying tropical cyclones in the Australian region.

The impact of climate model sea surface temperature biases on tropical cyclone simulations

Sea surface temperature (SST) patterns both local to and remote from tropical cyclone (TC) development regions are important drivers of the variability of TC activity. Therefore, reliable simulations and predictions of TC activity depend on a realistic representation of tropical SST. Nevertheless, severe SST biases are common to the current generation of global climate models, especially in the tropical Pacific and Atlantic. These biases are strongly positive in the southeastern tropical basins, and negative, but weaker, in the northwestern tropical basins. To investigate the impact of the tropical SST biases on simulated TC activity, an atmospheric-only tropical channel model was used to conduct several sets of ensemble simulations. The simulations suggest an underrepresentation in Atlantic TC activity caused by the Atlantic cold bias alone, and an overrepresentation in Eastern North Pacific (ENP) TC activity due to the Atlantic cold bias and Pacific warm bias jointly. While the local impact of SST biases on TC activity is generally induced by the local anomalous SST and the associated changes in atmospheric conditions, the remote impact of the Atlantic bias on the ENP TCs is strongly driven by the change in topographically forced regional circulation. Moreover, an eastward shift in Western North Pacific TCs was generated by the Pacific SST biases, even though basin-wide TC activity indicators change insignificantly. The results indicate the importance of considering SST bias effects on simulated TC activity in climate model studies and highlight key regions where reducing SST biases could potentially improve TC representation in climate models.

The Role of WISHE in Secondary Eyewall Formation

Abstract Numerical simulations are conducted to examine the role of the wind-induced surface heat exchange (WISHE) mechanism in secondary eyewall formation (SEF). The control experiment exhibits a coherent secondary eyewall structure as quantified by various parameters (e.g., the azimuthal-mean tangential wind and vertical velocity). Prior to SEF, an area characterized by increasing diabatic heating, enhanced inertial stability, and increasing supergradient winds at the top of the boundary layer is observed outside the eyewall. While these characteristics offer the possibility of both balanced and unbalanced dynamical pathways to SEF, the focus of this study is to evaluate the impact of WISHE. To examine the sensitivity of SEF to WISHE, the surface wind used for the calculation of surface heat fluxes is capped at several designated values and at different radial intervals. When the heat fluxes are moderately suppressed around and outside the SEF region observed in the control experiment, sensitivity experiments show that the formation of the outer eyewall is delayed, and the intensity of both eyewalls is weaker. When the heat fluxes are strongly suppressed in the same region, SEF does not occur. In contrast, suppressing the surface heat fluxes in the storm’s inner-core region has limited effect on the evolution of the outer eyewall. This study provides important physical insight into SEF, indicating that WISHE plays a crucial role in SEF and tropical cyclone evolution.

Lifetime Evolution of Outer Tropical Cyclone Size and Structure as Diagnosed from Reanalysis and Climate Model Data

The present study examines the lifetime evolution of outer tropical cyclone (TC) size and structure in the North Atlantic (NA) and western North Pacific (WNP). The metric for outer TC size is the radius at which the azimuthal-mean 10-m azimuthal wind equals 8 m s−1 ( r8) derived from the NCEP Climate Forecast System Reanalysis (CFSR) and GFDL High-Resolution Forecast-Oriented Low Ocean Resolution model (HiFLOR). Radial profiles of the azimuthal-mean 10-m azimuthal wind are also analyzed to demonstrate that the results are robust across a broad range of wind radii. The analysis shows that most TCs in both basins are characterized by 1) minimum lifetime r8 at genesis, 2) subsequent substantial increases in r8 as the TC wind field expands, 3) peak r8 values occurring near or after the midpoint of the TC lifetime, and 4) nontrivial decreases in r8 and outer winds during the latter part of the TC lifetime. Compared to the NA, WNP TCs are systematically larger up until the end of their lifetime, exhibit r8 growth and decay rates that are larger in magnitude, and are characterized by an earlier onset of lifetime maximum r8 near their lifetime midpoint. In both basins, the TCs exhibiting the largest r8 increases are the longest lived, especially those that traverse the longest distances (i.e., recurving TCs). Finally, analysis of TCs undergoing extratropical transition (ET) shows that NA TCs exhibit negligible changes in r8 during ET, while WNP ET cases either show r8 decreases (CFSR) or negligible changes in r8 (HiFLOR).

Sample Optimization of Ensemble Forecast to Simulate a Tropical Cyclone Using the Observed Track

Ensemble forecasting is widely used in numerical weather prediction. However, the ensemble may not satisfy a perfect Gaussian probability distribution because of a limited number of members, with some members significantly deviating from the true atmospheric state. Such outliers (belonging to low probability events) may downgrade the accuracy of an ensemble forecast. In this study, the observed track of a tropical cyclone (TC) is used to restrict the probability distribution of samples by investigating the evolution of TCs. Unlike data assimilation, the method we employed uses observational data. By restricting the probability distribution, ensemble spread could be decreased through sample optimization. In addition, the prediction results showed that track and intensity errors could be reduced by sample optimization. When the vertical structures of TCs considered in this study were compared, different thermal structures were found. This difference may have been caused by sample optimization, which may affect intensity and track. Nevertheless, it should be noted that the replacement of a large number of inferior samples may inhibit the improvement of simulated results.

Effects of surface waves and sea spray on air–sea fluxes during the passage of Typhoon Hagupit

Air–sea exchange plays a vital role in the development and maintenance of tropical cyclones (TCs). Although studies have suggested the dependence of air–sea fluxes on surface waves and sea spray, how these processes modify those fluxes under TC conditions have not been sufficiently investigated based on in-situ observations. Using continuous meteorological and surface wave data from a moored buoy in the northern South China Sea, this study examines the effects of surface waves and sea spray on air–sea fluxes during the passage of Typhoon Hagupit. The mooring was within about 40 km of the center of Hagupit. Surface waves could increase momentum flux to the ocean by about 15%, and sea spray enhanced both sensible and latent heat fluxes to the atmosphere, causing Hagupit to absorb 500 W/m2 more heat flux from the ocean. These results have powerful implications for understanding TC–ocean interaction and improving TC intensity forecasting.

Tropical Cyclone Boundary Layer Rolls in Synthetic Aperture Radar Imagery

AbstractMarine atmospheric boundary layer (MABL) roll plays an important role in the turbulent exchange of momentum, sensible heat, and moisture throughout MABL of tropical cyclone (TC). Hence, rolls are believed to be closely related to TC's development, intensification, and decay processes. Spaceborne synthetic aperture radar (SAR) provides a unique capability to image the sea surface imprints of quasi‐linear streaks induced by the MABL rolls within a TC. In this study, sixteen SAR images, including three images acquired during three major hurricanes: Irma, Jose, and Maria in the 2017 Atlantic hurricane season, were utilized to systematically map the distribution and wavelength of MABL rolls under the wide range of TC intensities. The images were acquired by SAR onboard RADARSAT‐1/2, ENVISAT, and SENTINEL‐1 satellites. Our findings are in agreement with the previous one case study of Hurricane Katrina (2005), showing the roll wavelengths are between 600 and 1,600 m. We also find that there exist roll imprints in eyewall and rainbands, although the boundary layer heights are shallower there. Besides, the spatial distribution of roll wavelengths is asymmetrical. The roll wavelengths are found to be the shortest around the storm center, increase and then decrease with distance from storm center, reaching the peak values in the range of , where is defined as the physical location to TC centers normalized by the radius of maximum wind. These MABL roll characteristics cannot be derived using conventional aircraft and land‐based Doppler radar observations.

Effects of Asymmetric Secondary Eyewall on Tropical Cyclone Evolution in Hurricane Ike (2008)

AbstractThe secondary eyewall plays an important role in tropical cyclone evolution and intensification and is routinely assumed to be axisymmetric. A unique opportunity to investigate the characteristics of the secondary eyewall in two dimensions is provided by the high spatial resolution (about 1 km) sea surface winds that were observed by spaceborne synthetic aperture radar over Hurricane Ike (2008). Here we extract the asymmetric characteristics using our Symmetric Hurricane Estimates for Winds model and analyze the related hurricane evolution by comparisons with aircraft measurements. Compared to the classic eyewall replacement cycle theory, our investigation finds that the primary eyewall did not weaken and the secondary eyewall did not shrink over a period of more than 30 hr. We suggest that the reason for this persistence is that a boundary layer inflow pathway is provided by the relatively low winds in the asymmetric secondary eyewall area, as observed by synthetic aperture radar.

Impacts of Assimilating Vertical Velocity, Latent Heating, or Hydrometeor Water Contents Retrieved From a Single Reflectivity Data Set

AbstractAssimilation of observation data in cloudy regions has been challenging due to the unknown properties of clouds such as cloud depth or cloud drop size distributions (DSD). Attempts to assimilate data in cloudy regions generally assume a DSD, but most assimilation systems fail to maintain consistency between models and the observation data, as each has its own set of assumptions. This study tries to retain the consistency between the forecast model and the retrieved data by developing a Bayesian retrieval scheme that uses the forecast model itself for the a priori database. Through the retrieval algorithm, vertical profiles of three variables related to the development of tropical cyclones, including vertical velocity (VV), latent heating (LH), and hydrometeor water contents (HYDRO), are derived from the same reflectivity observation. Each retrieved variable is assimilated in the data assimilation system using a flow‐dependent forecast error covariance matrix. The simulations are compared to evaluate the respective impact of each variable in the assimilation system. In this study, three assimilation experiments were conducted for two hurricane cases captured by the Global Precipitation Measurement satellite: Hurricane Pali (2016) and Hurricane Jimena (2015). Analyses from these two hurricane cases suggest that assimilating LH and HYDRO have similar impacts on the assimilation system while VV has less of an impact than the other two variables. Using these analyses as an initial condition for the forecast model reveals that the assimilations of retrieved LH and HYDRO were able to improve the track forecast as well.