Cyclone Size Research Articles

Development of PM10 and PM2.5 cyclones for small sampling ports at stationary sources: Numerical and experimental study

Air pollution caused by particulate matter (PM) has become a serious issue, and significant research has focused on managing large stationary emission sources, i.e., the primary sources of PM. Currently, the U.S. Environmental Protection Agency (EPA) Method 201A and ISO 23210 are predominantly employed to measure the PM emissions at large stationary sources. Method 201A is designated as a standard test method in Korea, but it is difficult to measure PM10 and PM2.5 simultaneously owing to the size of the full-set cyclone. In large stationary emission sources, the use of a serial connection of PM10 and PM2.5 cyclones is unsuitable for measurements at conventional sampling ports featuring diameters of approximately 100 mm. Therefore, in this study, PM10 and PM2.5 cyclones were developed to replace the cyclones currently used in Method 201A. The developed cyclones featured a cutoff diameter, which was confirmed by numerical and experimental analyses that were close to Method 201A. Moreover, there was an increase in the stiffness of collection efficiency. The hook adaptor, which is a key accessory used in Method 201A, was found to be applicable to the newly developed cyclones. This alternative method will help reduce the measurement time by simultaneously measuring TSP, PM10, and PM2.5 and eliminates the costs of installing or refurbishing additional sampling ports at existing large stationary sources.

How well is outer tropical cyclone size represented in the ERA5 reanalysis dataset?

The outer tropical cyclone (TC) size was extracted from the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation global atmospheric reanalysis (ERA5), the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), and the ECMWF Interim reanalysis (ERA-I), as well as a hybrid reanalysis-observation dataset, the Cross Calibrated Multi-Platform (CCMP) ocean surface wind vector product. Results are compared with the QuikSCAT Tropical Cyclone Radial Structure Dataset (QSCAT-R). Several size metrics were applied including the radii of 6–15 m s−1 winds. The capability of ERA5 in describing TC outer wind fields was evaluated, and improvements in the representation of outer size assessed.The ERA5 reanalysis can resolve stronger TC winds, which leads to more valid outer size samples, especially for the radii of 12 and 15 m s−1 winds, and for the cases with outer sizes of <300 km. This makes the distribution characteristics of the outer size in the ERA5 reanalysis closer to the observation. These improvements of ERA5 are more obvious in the eastern North Pacific basin where TCs are generally of smaller size. However, ERA5 still underestimates the outer size, similarly to its predecessor ERA-I. The size error in ERA5 becomes more negative with increasing observed outer size, with this tendency also occurring in previous reanalysis datasets. Nevertheless, among the size metrics examined, the radius of 9 m s−1 winds is regarded as the optimal size metric for examining the extent of outer winds, thus providing an effective reference for constructing an objective, multi-decadal dataset of outer size using the ERA5 reanalysis.

Effects of perturbation type on tropical cyclone size over tropical North Western Pacific and Atlantic

In authors’ previous studies, the role of distinctive mean states in the western North Pacific (WNP) and North Atlantic (NA) in affecting tropical cyclone (TC) size was investigated. In this study, the effect of different synoptic scale perturbation types in the two basins on TC size is further investigated. Numerical model experiments with an initial synoptic wave train (SWT) and easterly wave (EW) environmental condition show that the former (latter) leads to the development of a larger (smaller) TC at an equilibrium state. The physical mechanism responsible for the difference is revealed. Compared with the EW, the surface wind speed of the SWT is larger. Therefore, the SWT generates more moisture in the outer region than EW through surface evaporation process. This favors the development of stronger convection in the outer region. The enhanced convection leads to greater diabatic heating, which lower the local sea level pressure (SLP), which changes surface radial pressure gradients in the inner-core and outer-core of the vortex. On the one hand, the falling of the SLP decreases the surface radial pressure gradient in the inner-core. As a result, the radial wind is weakened and then the radius of maximum wind (RMW) extends outwards after initial contraction. On the other hand, the lower SLP increases the radial pressure gradient in the outer region, strengthening the radial wind outside. The wind convergence favors stronger development of convection in the outer region. Through this positive feedback, inflow in the outer region is further strengthened. This accelerates local tangential wind and thus enlarges the TC size.

C-band Doppler weather radar observations during the passage of tropical cyclone ‘Ockhi’

Recently, a C-band polarimetric Doppler weather radar (DWR) was installed and commissioned at Thumba (8.5° N, 77° E), a west coastal station in the southern peninsula of India known as ‘Gateway of Indian Summer Monsoon’, for monitoring severe weather events and tropical cyclones. The DWR operates in the frequency range of 5.6 to 5.65 GHz with a peak transmitting power of 250 kW and employs state-of-the-art technology for both transmitter and receiver subsystems. Apart from providing regular base products such as reflectivity (Zh), radial velocity (V) and spectral width (σ), the DWR being equipped with dual polarization capability provides additional products such as differential reflectivity (Zdr), differential phase shift (ϕdp) and correlation coefficient (ρhv), which are used to study the microphysical properties of rain bearing clouds. The ‘Ockhi’ cyclone was the most recent one from 29th November to 2nd December 2017, which caused large devastation in terms of both economic and human life in Kerala coastal places. The C-band polarimetric DWR provides an opportunity to investigate the spatial and vertical structure of ‘Ockhi’ cyclone over the south-west coast of Kerala. The cyclone ‘Ockhi’ developed near the west of Srilanka and later advanced and stretched out near south-east of Kanyakumari through the Arabian Sea and impacted the coastal places of Kerala on 1st December 2017. During its formative stage, the eyewall was not clearly visible. The cyclone eye was clearly seen at the well-developed stage of cyclone, when it was in the vicinity of DWR range. The overall spatial extent of cyclone is ~ 1000 km as seen in INSAT-3D images. The maximum surface wind speed was observed to be around 30 m/s near to the Kerala coast. The outer fringes of the cyclone were located above the Trivandrum and caused heavy rainfall on 30 November 2017 around 17 h. The C-band DWR has captured the various stages of cyclone ‘Ockhi’ and its primary bands. Observations of C-band polarimetric DWR indicates that the vertical cross section of reflectivity along the cyclone eye showed that the size of cyclone is around 20 km and the precipitating cloud top heights reaching as high as 10–12 km. The cyclone’s outer rainbands are slanted outward and were more intense than the inner rainbands. The area surrounding the cyclone eye is the eyewall, characterized by very strong wind and torrential rainfall. The large differential reflectivity values ranging from 2 to 3 dB are found in the eyewall and rainbands regions. The outer rainband is characterized by large Zdr values with high reflectivity above the melting layer. The different values of Zdr are observed in the eyewall and outer rainband regions, which suggest that different microphysical characteristics prevail. As time progress, the cyclone system gradually intensified and the eyewall became more intense compared to the outer ones. The present study utilizes the C-band polarimetric DWR observations to characterize the spatial and vertical structure of cyclone ‘Ockhi’ and provides the ample information on the polarimetric signatures of cyclone structure to improve the better understating on the microphysical characteristics of tropical cyclones.

Global climatology of rainfall rates and lifetime accumulated rainfall in tropical cyclones: Influence of cyclone basin, cyclone intensity and cyclone size

AbstractSeventeen years of 3‐hr, 0.25° resolution, precipitation data from the Tropical Rainfall Measurement Mission (TRMM) multi‐satellite precipitation analysis (TMPA) product are used to develop a global climatology of precipitation in tropical cyclones (TCs). Due to very large SDs for rainfall in each stratification by intensity class or cyclone basin, our methodology concentrates on frequency distributions and percentage representation in different rainfall rate categories. The stratifications reveal that the TC rainfall climatologies are dependent on three inter‐related factors: TC intensity, TC size and TC basin. The interdependence of the three is examined. The distributions of TC intensity classes in the different basins are not a significant contributor to the fact that certain basins (Northwest Pacific, North Atlantic) have higher TC rainfall rates than other basins (Northeast Pacific, South Indian). In contrast to this, the distributions of TC size classes between basins are a significant contributor to why some basins are wetter than others. A climatology is also presented of lifetime accumulated rainfall (LAR) in TCs. The record LAR belongs to hurricane Ivan in 2004, with 300 km3 of rain over the 0–350 km radius, and 432 km3 over 0–500 km. The largest LAR values occur almost exclusively in two cyclone basins: The Northwest Pacific and the North Atlantic. Not unexpectedly, LAR is determined primarily by TC duration, which accounts for around 70% of the variance. Examination of the full 17‐year dataset reveals a decreasing trend in both median and extreme TC rainfall rates in all basins except the Northeast Pacific. However, mechanisms responsible for this decrease are yet to be identified and may be primarily due to the sample size or data inhomogeneities. The difference between the trends and those expected from physical principles is a concern which we hope will be taken up by other investigators.

Influence of model resolution on bomb cyclones revealed by HighResMIP-PRIMAVERA simulations

Bomb cyclones are explosively intensifying extratropical cyclones that can cause severe damage to life and property. However, the poor ability of coarse-resolution climate models to simulate bomb cyclones, including underestimation of the frequency of bomb cyclones, remains a problem. In this study, the dependence of bomb cyclone characteristics on horizontal resolution from 135 to 18 km is investigated by analyzing the outputs of HighResMIP historical simulations of atmospheric general circulation models and four reanalysis datasets. Robust resolution dependence of bomb cyclone characteristics is identified for both the models and the reanalyses. Finer horizontal resolution significantly increases the frequency of bomb cyclones and reduces their average horizontal size. A regression analysis indicates that bomb cyclone frequency is roughly doubled from 140 km to 25 km resolution. The overall increase in bomb cyclone number is associated with a large increase in small bomb cyclones and a moderate decrease in large ones. Bomb cyclones in higher-resolution models are also accompanied by a higher maximum wind speed and more extreme wind events, which is probably related to the increased pressure gradients due to the smaller size of the bomb cyclones. These results imply that high-resolution models should be used for evaluating the impacts of bomb cyclones.

Revisiting the Relationship Between Tropical Cyclone Size and Intensity Over the Western North Pacific

AbstractThis study revisits the nonlinear relationship between tropical cyclone (TC) size and intensity over the western North Pacific (WNP). Using best track data from the Japan Meteorological Agency from 2007–2019, we find that the statistical turning point on a fitted size‐intensity curve only exists for TCs with greater lifetime peak intensities (e.g., typhoons). The track and evolution type are two factors determining the statistical size‐intensity relationship. The nonlinear size‐intensity relationship is only observed for recurving typhoons reaching their peak intensities and sizes at different times, due to a nonmonotonic size change during the weakening stage. The slight size decrease in the initial weakening stage can be linked to a TC weakening‐induced shrinkage balanced by a baroclinic expansion when recurving typhoons move into higher latitudes. Finally, different evolution types for WNP typhoons can be determined by the ratio of intensity/size to its theoretical maximum.

Size and Structure of Dry and Moist Reversible Tropical Cyclones

AbstractThe size and structure of tropical cyclones (TCs) are investigated using idealized numerical simulations. Three simulations are conducted: a pure dry TC (DRY), a moist reversible TC (REV) with fallout of hydrometeors in the atmosphere disallowed, and a typical TC (CTL). It was found that the width of the eyewall ascent region and the radius of maximum windrmare much larger in DRY and REV than those in CTL. This is closely related to the deep inflow layer (~4 km) in DRY and REV associated with a different entropy restoration mechanism under the subsidence region. With the wide ascents, the close link betweenrmand the outer radius in DRY and REV can be well predicted by the Emanuel and Rotunno (ER11) model. The magnitude of subsidence, mainly controlled by the vertical gradient of entropy in the mid- and upper troposphere, is nearly one order greater in DRY and REV than that in CTL. This study demonstrates that the falling nature of hydrometeors poses a strong constraint on the size and structure of real world TCs via the entropy distribution in the subsidence region. The wide ascent, self-stratification in the outflow, and decently reproduced wind profile in DRY and REV suggest that DRY and REV behave like a prototype of the ER11 model with CTL being an extreme type.

Ensemble Based Diagnosis of the Track Errors of Super Typhoon Mangkhut (2018)

Super Typhoon Mangkhut (2018) was the most high-impact typhoon in 2018 because of its long lifespan and significant intensity. The operational track forecasts in the short-to-medium range (deterministic and probabilistic forecast) showed a great uncertainty and the forecast landing points varied with different lead times. This study applied ensembles of high-resolution ECMWF forecasts to investigate the major factors and mechanisms of the bias production of the Mangkhut forecast track. The ensembles with the largest track bias were analyzed to examine the possible bias associated factors. The results suggested that environmental steering flows were the main cause for the erroneous southward track error with a variance contribution of 72%. The tropical cyclone (TC) size difference and the interaction of the TC with the subtropical high (SH) were other two key factors that contributed to the track error. Particularly, larger TCs may have led to a stronger erosion of the southern part of the SH, and thus induced significant changes in the large-scale environment and eventually resulted in an additional northward movement of TC.

Tropical Cyclone Size Change under Ocean Warming and Associated Responses of Tropical Cyclone Destructiveness: Idealized Experiments

The power dissipation index (PDI), which is defined as the sum of the cube of tropical cyclone (TC) maximum wind speed during TC lifetime, is widely used to estimate the TC destructive potential. However, due to the lack of high-resolution observations, little attention has been paid to the contribution of TC size change to TC destructive potential in response to ocean warming. In this study, sensitivity experiments are performed by using the high-resolution Weather Research and Forecasting (WRF) model to investigate the responses of TC size and TC destructive potential to prescribed sea surface temperature (SST) increase under the present climate condition. The results show that TC size increases with the ocean warming. Possible reasons for TC size change are investigated with a focus on the outer air–sea moisture difference (ASMD). As SST increases, ASMD in the outer zone of the TC is larger than that in the inner zone, which increases the surface entropy flux (SEF) of the outer zone. This change in the radial distribution of SEF causes the increase of tangential wind in the outer zone, which further increases SEF, resulting in a positive feedback between outer-zone SEF and outer-zone tangential wind. This feedback leads to the increase of the ra-dius of gale-force wind, leading to the expansion of TC size. Moreover, to estimate the contribution of TC size change to TC destructiveness, we calculate TC size-dependent destructive potential (PDS) as the storm size information is available in the model outputs, as well as PDI that does not consider the effect of TC size change. We find that PDS increases exponentially as SST increases from 1 to 4°C, while PDI increases linearly; hence the former is soon much greater than the latter. This suggests that the growth effect of TC size cannot be ignored in estimating destruc-tiveness under ocean warming.

Monthly variation and spatial distribution of quadrant tropical cyclone size in the Western North Pacific

AbstractWith the tropical cyclone (TC) size parameter defined as the radius of 17‐m·s−1 oceanic surface wind, 225 TC cases were recorded in the western North Pacific during 2000–2009 based on the QuikSCAT near‐surface wind vector database and the best‐track dataset. In accordance with the symmetry index (the ratio of minimum and maximum quadrant sizes), the TCs were classified into symmetric and asymmetric structures. The asymmetric TCs were divided into four types: the northeast, southeast, southwest, and northwest. The spatio‐temporal characteristics of these TC types were further investigated. The monthly variation and the spatial distribution of maximum quadrant TC size exhibited significant differences among the four types. By contrast, the quadrant size of the symmetric TCs from June to November showed few changes (2.6°–2.7° latitude). It was found that the TC lifetime is an important factor affecting the quadrant TC size because of its close relationship with the activities of the western Pacific subtropical high. In addition, the climatological mean circulation also has a notable influence on the quadrant TC size through superposition of prominent background wind. Symmetric TCs are more likely to occur in the oceanic region where the background low‐level wind is the weakest.

Influence of Tropical Cyclone Intensity and Size on Storm Surge in the Northern East China Sea

Typhoon storm surge research has always been very important and worthy of attention. Less is studied about the impact of tropical cyclone size (TC size) on storm surge, especially in semi-enclosed areas such as the northern East China Sea (NECS). Observational data for Typhoon Winnie (TY9711) and Typhoon Damrey (TY1210) from satellite and tide stations, as well as simulation results from a finite-volume coastal ocean model (FVCOM), were developed to study the effect of TC size on storm surge. Using the maximum wind speed (MXW) to represent the intensity of the tropical cyclone and seven-level wind circle range (R7) to represent the size of the tropical cyclone, an ideal simulation test was conducted. The results indicate that the highest storm surge occurs when the MXW is 40–45 m/s, that storm surge does not undergo significant change with the RWM except for the area near the center of typhoon and that the peak surge values are approximately a linear function of R7. Therefore, the TC size should be considered when estimating storm surge, particularly when predicting marine-economic effects and assessing the risk.

A Study of the Intensity of Tropical Cyclone Idai Using Dual-Polarization Sentinel-1 Data

Monitoring the intensity and size of a tropical cyclone (TC) is a challenging task, and is important for reducing losses of lives and property. In this study, we use Idai, one of the deadliest TCs on record in the Southern Hemisphere, as an example. Dual-polarization synthetic aperture radar (SAR) measurements from the Copernicus Sentinel-1 mission are used to examine the TC structure and intensity. The wind speed is estimated and compared using well known C-band model functions based on calibrated cross-polarization SAR images. Because of the relatively high noise floor of the Sentinel-1 data, wind speeds under 20 m/s from cross-polarization models are ignored and replaced by low to moderate wind speeds retrieved from co-polarization radar signals. Wind fields retrieved from the co- and cross-polarization model results are then merged together to estimate the TC size and the TC fullness scale, a concept related to the wind structure of a storm. Idai has a very strong wind speed and fullness structure, indicating that it was indeed a very intense storm. The approach demonstrates that open and freely available Sentinel-1 SAR data is a unique dataset to estimate the potential destructiveness of similar natural disasters like Idai.

Relationship Between the Change in Size of Tropical Cyclones and Spatial Patterns of Precipitation

AbstractThe relationship between the size change of tropical cyclones (TCs) and radial distribution of precipitation was investigated for TCs occuring over the western North Pacific Ocean with satellite observation data. The size of a TC is defined as the radial extent of 15 m s−1 winds (R15). The spatial distribution of precipitation was investigated by extracting two subperiods, which are the size‐increasing period and the size‐stationary period, as determined from R15 evolution. The results show a contrast that during the size‐increasing period, precipitation falls both inside and outside the R15, whereas during the size‐stationary period, it is concentrated near the center of the TC and falls less around and outside R15. This contrast is also confirmed statistically by composite analysis using 64 TC cases. Further analysis reveals that this contrast does not depend on the intensity or the change in intensity of the TC, which indicates that the area of precipitation related to the intensity or intensity change of TCs is different from that related to the size change of TCs. Finally, we examined the radial distribution of inertial stability in TCs and found that precipitation during the size‐stationary period occurs in a region with a higher inertial stability than that which occurs during the size‐increasing period.

The Orthogonal Experiments for Radial Jet Cyclone

Radial jet cyclone (RJC) is a new type of dust removal equipment based on the improvement of traditional cyclone which greatly improves the separation efficiency of small size particles and inevitably increases the pressure drop. In order to explore the comprehensive influence of the structure size and process parameters of the radial jet cyclone on its separation efficiency, this paper selected five main influencing factors: blade helix angle, blade length, vortex finder depth, airflow velocity and particle concentration to design a set of orthogonal experiments. Through analysis, the experimental results show that the flow velocity has the greatest influence on the pressure drop, followed by the blade helix angle, and the other three factors have a smaller influence. The insertion depth of the riser has the greatest influence on the separation efficiency. While the smaller the vortex finder depth is, the higher the separation efficiency. The other influencing factors are blade helix angle, airflow velocity, blade length and dust concentration.

Impact of Dry Midlevel Air on the Tropical Cyclone Outer Circulation

AbstractThe impact of dry midlevel air on the outer circulation of tropical cyclones is investigated in idealized simulations with and without a moist envelope protecting the inner core. It is found that a dry midlevel layer away from the cyclone center can broaden the outer primary circulation and thus the overall destructive potential at both developing and mature stages. The midlevel outer drying enhances the horizontal gradient of latent heating in the rainbands and drives the expansion of the outer circulation. The moist convection at large radii is suppressed rapidly after the midlevel air is dried in the outer rainbands. An enhanced horizontal gradient of latent heating initiates a radial–vertical overturning circulation anomaly in the rainbands. This anomalous overturning circulation accelerates the radial inflow of the main secondary circulation, increases the angular momentum import, and thus increases the cyclone size. The dry air, mixed into the boundary layer from the midtroposphere, is “recharged” by high enthalpy fluxes because of the increased thermodynamical disequilibrium above the sea surface. This recharge process protects the eyewall convection from the environmental dry-air ventilation. The proposed mechanism may explain the continuous expansion in the tropical cyclone outer circulation after maturity, as found in observations.

Challenges and Future Directions in Ocean Wave Modeling — A Review

An increasing number of instances in extreme weather events over the global oceans have deepened the concerns on the impact of climate change. The frequency of extreme weather events is also seen to increase attributes to climate change across the globe. In the Indian context, there has been about 285 reported flooding events over the period from 1950 to 2017 that affected nearly 850 million people with many causalities. As the global oceans become stormier, the effects are seen in rising sea level and infrastructural facilities. Major flooding events are caused by tropical cyclone-induced storm surge and associated breaking waves. These extreme weather events coupled with sea level rise have serious repercussions on the coastal vulnerability. Also recently, there is an upsurge in the intensity and tropical cyclone size that forms over the North Indian Ocean region that brought attention among the scientific community. The worst possible scenario of extreme water level can occur when the time of storm surge occurrence coincides with the astronomical high water. This review paper provides a comprehensive overview on the research developments and efforts made in ocean wave modeling in particular for the Indian seas. As per the Fifth Assessment Report of Intergovernmental Panel on Climate Change (IPCC), the role and influence of ocean surface gravity wave in the climate system are considered to be very important. At present, numerical models are widely used and that can be used to hindcast and forecast the wave characteristics over both regional and global ocean basin scales. A detailed overview on the observational techniques is listed along with the historical perspective and recent developments in wind-wave modeling for the Indian seas. Recent developments in computing technology and advanced numerical techniques have made it possible to solve the complex problems in coastal science and engineering using state-of-the-art numerical models providing realistic estimates, cost effective and having immense potential in operational weather centers. This review also deals with some of the important issues and future directions in wind-wave modeling studies such as improvements required in momentum transfer, bottom dissipation, and rain–wave interaction effects that require detailed understanding and concentrated efforts.

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).

Conditions associated with rain field size for tropical cyclones landfalling over the Eastern United States

This study analyzes the relationships between the size of tropical cyclone (TC) rainfall fields and storm attributes and surrounding environmental conditions. A Geographic Information System analysis is employed to measure the area and averaged-quadrant extent of rain fields associated with 70 U.S. landfalling TCs as detected by satellites at a three-hour resolution during 1998–2015. Statistical models are developed to predict the area and extent of TC rainfall fields before and after landfall. The conditions examined include storm intensity, motion, storm location, total precipitable water (TPW), upper-tropospheric divergence, and deep-layer wind shear. The spatial pattern of the area and the quadrant-averaged extent exhibits large variability. The total area and rear extents are much larger over the southeastern Gulf, especially for the right-rear extent, which is approximately 300 km away from the center. As the area and rear extents quickly decay, rainfall in the forward quadrants can still extend farther than 200–300 km from the storm center during and after landfall. The average rainfall duration is 30 (22) hours on the right (left) side of the storm track. The statistical models reveal that TPW within 400 km of the storm center and upper-tropospheric divergence are the strongest predictors of both overall coverage and extent in all quadrants. A deep-layer vertical wind shear with a strong westerly component correlates well with a larger extent in the forward quadrants of the storm. Storm intensity has more influence on rainfall over the ocean, while the distance to the coastline has more influence over land.

Enlarging Rainfall Area of Tropical Cyclones by Atmospheric Aerosols

AbstractThe size of a tropical cyclone (TC), measured by the area of either rainfall or wind, is an important indicator for the potential damage by TC. Modeling studies suggested that aerosols tend to enhance rainfall in the outer rainbands, which enlarges the eyewall radius and expands the extent of rainfall area. However, no observational evidence has yet been reported. Using TC rainfall area and aerosol optical depth (AOD) data, we find that aerosols have a distinguishable footprint in the TC size. Other dynamical factors for TC size, such as relative SST and Coriolis parameter, are also quantified and discussed. We show that, on average, TC rainfall size increases 9–20 km for each 0.1 increase of AOD in the western North Pacific. This finding implies that anthropogenic aerosol pollution can increase not only TC rainfall rate, but also TC rainfall area, resulting in potentially more destructive flooding affecting larger areas.