Spiral Cloud Bands Research Articles

Revisiting the Characteristics of Super Typhoon Saola (2023) Using GPM, Himawari-9 and FY-4B Satellite Data

Typhoon Saola was the ninth typhoon that generated over the Western North Pacific (WNP) in 2023, and it caused severe storm impacts. However, its complex moving track and heavy intensity made it extremely difficult to forecast; therefore, detailed analysis is necessary. In this study, GPM, Himawari-9, and FY-4B satellite data were used to analyze the characteristics of the structure, brightness temperature, and precipitation of the typhoon cloud system. Our results showed that, in the 89 and 183 GHz channels of GPM-1CGMI, the brightness temperature of the typhoon eye was 80–90 K higher than that of the eye wall, and the strong convective areas below 200 K were clearer in these high-frequency channels. GPM-2ADPR estimated heavy rain (over 30 mm/h) area, storm height (5 km), and vertical precipitation rate (30–40 mm/h) more accurately than the GPM-2Aka and GPM-2Aku products. Himawari-9 satellite data showed that the brightness temperature of the eye wall and spiral cloud bands was 180–200 K, the typhoon eye was small and round, and strong convective activities were mostly located in the southwest side of the center. The FY-4B CLP and CLT products showed that, in the mature period of the typhoon, the percentage of supercooled and mixed clouds first stabilized and then rapidly decreased. The trends observed among the three types of ice-phase clouds were characterized by an initial increase, followed by a decrease, and then another increase, with percentages between 10% and 25%, 5% and 15%, and 15% and 30%, respectively.

Deep Learning for Typhoon Intensity Classification Using Satellite Cloud Images

Abstract A tropical cyclone, also known as a typhoon, is one of the most destructive weather phenomena. Its intense cyclonic eddy circulations often cause serious damage to coastal areas. Accurate classification or prediction for typhoon intensity is crucial to disaster warning and mitigation management. But typhoon intensity-related feature extraction is a challenging task as it requires significant preprocessing and human intervention for analysis, and its recognition rate is poor due to various physical factors such as tropical disturbance. In this study, we built a Typhoon-CNNs framework, an automatic classifier for typhoon intensity based on a convolutional neural network (CNN). The Typhoon-CNNs framework utilized a cyclical convolution strategy supplemented with dropout zero-set, which extracted sensitive features of existing spiral cloud bands (SCBs) more effectively and reduces the overfitting phenomenon. To further optimize the performance of Typhoon-CNNs, we also proposed the improved activation function (T-ReLU) and the loss function (CE-FMCE). The improved Typhoon-CNNs was trained and validated using more than 10 000 multiple sensor satellite cloud images from the National Institute of Informatics. The classification accuracy reached to 88.74%. Compared with other deep learning methods, the accuracy of our improved Typhoon-CNNs was 7.43% higher than ResNet50, 10.27% higher than InceptionV3, and 14.71% higher than VGG16. Finally, by visualizing hierarchic feature maps derived from Typhoon-CNNs, we can easily identify the sensitive characteristics such as typhoon eyes, dense-shadowing cloud areas, and SCBs, which facilitate classifying and forecasting typhoon intensity.

Effects of microphysical processes on the rapid intensification of Super-Typhoon Meranti

Typhoon Meranti (2016) underwent extreme rapid intensification (RI) over the western North Pacific (WNP), just before its landfall in Fujian Province, China. In this study, the extreme RI, track, and structures of the inner core and spiral cloud bands of Meranti were simulated, reasonably realistically, using WRF-ARW (the Advanced Research version of the Weather Research and Forecasting model). Firstly, the convective bursts (CBs) and associated microphysical processes were analyzed in detail to evaluate the probable mechanism responsible for the RI of Meranti. The results showed that Meranti experienced RI over the warm ocean with high ocean heat content and decreasing environmental vertical shear. The onset of RI was triggered by CBs, which penetrated the upper troposphere, leading to upper-tropospheric warming and the formation of an upper-level warm core. Diagnosis of the complete Q-vector showed that the CBs of the inner core were jointly triggered by the dynamic forcing of the frontogenesis and the non-adiabatic phase transition process. Further analysis of the heat budget of microphysical processes revealed that the condensation process (accounting for 76.3% of the total heating rate of the warming processes) played a leading role for CBs in the lower and middle troposphere, while the deposition process (accounting for 19% of the total heating rate of the warming processes) was a critical factor of the CBs in the upper level of the troposphere.

Estimation of Tropical Cyclone Intensity from the Satellite Infrared Images of Its Spiral Cloud Bands

The spiral signatures of infrared satellite images of the tropical cyclone Phanfone observed in the Northwest Pacific in August 2002 are analyzed using the hyperbolic-logarithmic model of a streamline in the Rankine vortex. Maximum wind speed Vmax and some other physical characteristics of the cyclone are estimated by identifying hyperbolic-logarithmic spirals (RLS) which “fit” into the geometric boundaries of the signatures. The comparison of temporal variations in Vmax with the data of meteorological services of the USA (JTWC) and Japan (RSMC) revealed satisfactory agreement of the time course of tropical cyclone intensity from the HLS estimates and JTWC data. The absolute estimates of intensity were within an interval between the corresponding JTWC and RSMC data. It seems that if the presented approach is enhanced, the HLS estimation may be a useful contribution to the empirical methods of tropical cyclone intensity determination by remote sensing means.

Genesis and Development Mechanisms of a Polar Mesocyclone over the Japan Sea

Abstract A polar mesocyclone (PMC) observed over the Japan Sea on 30 December 2010 was studied using a nonhydrostatic mesoscale numerical model with a horizontal resolution of 2 km. The numerical simulation successfully reproduced the observed life cycle of the PMC. The results of the numerical simulation suggest that the life cycle of the PMC may be divided into three stages: an early development stage, in which a number of small vortices appear in a shear zone; a late development stage, which is characterized by the merger of vortices and the formation of a few larger vortices; and a mature stage, in which only a single PMC is present. During the early development stage, vortices are generated in the shear zones of strong updrafts in discrete cumulus convection cells. In contrast, during the late development stage, the vortices develop as a result of barotropic instability in the shear zone. A cloud-free eye and spiral cloud bands accompany the mature stage of a simulated PMC. A warm core structure also forms at the center of the PMC on account of adiabatic warming associated with downdrafts. The structures in the PMC during the mature stage resemble those of a tropical cyclone. Sensitivity experiments, in which sensible and latent heat fluxes from the sea surface and condensational heating were switched on/off, demonstrate that condensational heating is critical to the development of the PMC at all stages, and that sensible and latent heat fluxes play secondary roles.

Observational Occurrence of Tropical Cyclone Ducts from GPS Dropsonde Data

AbstractOn the basis of global positioning system dropsonde data, Japan Meteorology Agency Regional Spectral Model analysis data, National Centers for Environmental Prediction reanalysis data, satellite products from the Naval Research Laboratory, and best-track tropical-cyclone (TC) datasets from the Shanghai Typhoon Institute, the statistical characteristics of the ducts induced by TCs (TC ducts) over the western North Pacific Ocean were analyzed for the period from September 2003 to September 2006, and two typical strong-duct cases with remarkable differences in formation cause were analyzed and compared. Of the total of 357 dropsondes, there are 212 cases that show ducting conditions, with an occurrence percentage of ~59%. Of the 212 TC-duct events, profiles with multiple ducting layers make up nearly one-half, with the main type of ducts being elevated ducts; in contrast, weak ducts make up over one-half, resulting in a weak median duct strength and thickness. Ducts formed in the transition zone, especially on the left side of TC tracks, tend to be much stronger and thicker than those formed inside TCs. The former are induced by the interaction between TCs and their surrounding systems, such as the inrush of dry and cold air from the north on the left side of TC tracks. The latter are associated with successive subsidence in the gaps between spiral cloud bands. With increasing TC intensities, the associated ducts inside TCs tend to be much stronger and thicker and to appear at higher altitudes.

A ‘hurricane‐like’ polar low fuelled by sensible heat flux: high‐resolution numerical simulations

AbstractAn unusually deep (961 hPa) hurricane‐like polar low over the Barents Sea during 18–21 December 2002 is studied by a series of fine‐mesh (3 km) experiments using the Weather Research and Forecasting (WRF) model. The simulated polar low was similar to hurricanes and similar previous case‐studies in that it had a clear, calm and warm eye structure surrounded by moist convection organized in spiral cloud bands, and the highest surface wind speeds were found in the eye wall. The proximity to the sea ice and the high surface wind speeds (about 25 m s−1) during the deepening stage triggered extremely high surface sensible and latent heat fluxes at the eye wall of about 1200 and 400 W m−2, respectively. As the polar low moved eastward and weakened, maximum surface sensible and latent heat fluxes dropped to about 600 and 300 W m−2, respectively. Two types of sensitivity experiments were designed to analyse the physical properties of the polar low. Firstly, physical processes such as condensational heating and sensible and/or latent heat fluxes were switched off–on throughout the simulation. In the second type, these processes were turned off–on after the polar low had reached its peak intensity, which minimized the deformation of the polar‐low environment, making it suitable to study the direct effect of physical processes on the mature vortex. The experiments suggest that the deepening stage of the polar low was dominated by baroclinic growth and that upper‐level potential vorticity forcing contributed throughout its life cycle. After the deepening stage, the baroclinicity vanished and the polar low was fuelled by surface sensible heat fluxes while latent heat fluxes played a minor role. Condensational heating was not essential for the energetics of the polar low. Surprisingly, in experiments where condensational heating was turned off throughout the simulation, the polar low intensified. Copyright © 2012 Royal Meteorological Society

The generalized energy equation and instability in the two-layer barotropic vortex

The linear two-layer barotropic primitive equations in cylindrical coordinates are used to derive a generalized energy equation, which is subsequently applied to explain the instability of the spiral wave in the model. In the two-layer model, there are not only the generalized barotropic instability and the super high-speed instability, but also some other new instabilities, which fall into the range of the Kelvin-Helmholtz instability and the generalized baroclinic instability, when the upper and lower basic flows are different. They are perhaps the mechanisms of the generation of spiral cloud bands in tropical cyclones as well.

Effects of baroclinicity on the cloud pattern and structure of polar lows: A high‐resolution numerical experiment

High‐resolution numerical experiments using a three‐dimensional non‐hydrostatic model with horizontal grid size of 5 or 2 km are performed to clarify what determines the structure and cloud pattern of polar lows. An axisymmetric initial vortex is imposed in a baroclinic basic state for which thermal wind balance holds. It is found that when the baroclinicity is absent, a hurricane‐like vortex with spiral cloud bands, a cloud‐free eye, and a warm core develops. When the baroclinicity is strong, on the other hand, a vortex with a comma‐shaped cloud and slightly larger horizontal scale develops. The former appears to develop due to CISK/WISHE mechanism, while the latter due to baroclinic instability modified by latent heating.

The Dynamic Characteristics of Typhoon Vortex Spiral Waves and Its Transportation: Diagnostic Analyses

AbstractBy adopting the moving‐coordinate on the typhoon center, the TBB time‐serial datasets of three target typhoons are analyzed through power‐spectrum analysis to investigate the wave train characteristics of typhoon spiral cloud bands. The results show that the TBB correlation coefficient field between the typhoon eye region and its surrounding areas has a remarkable spiral wave feature. The spiral waves in typhoon‐effected regions also show a conspicuous sub‐synoptic scale and meso‐scale characteristic, and their wave period and phase speed exhibit two peak values, one similar to internal gravity wave of over the 6h time scale and the other to the vortex Rossby wave of about 24h time scale. The high‐ and low‐level streams of typhoon spiral bands have close relation with its persistence and development. Besides, the TBB time departures show waves similar to the vortex Rossby wave train, and the wave path is similar to the Rossby wave transportation path too. Finally, a structure model of typhoon vortex spiral waves is put forward.

Generation of Moving Spiral Bands in Tropical Cyclones

Abstract A theory based on the shallow water equation is developed to explain the generation of moving spiral-banded structures in tropical cyclones. The theory proposes that fluctuation of the vorticity distribution in the compact core region can act as a source, generating gravity waves that produce the banded structures. This mechanism is usually called the Lighthill radiation mechanism. A numerical experiment using the Fifth Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) code is used to test this theory. The moving bands in the simulated tropical cyclone show good agreement with the theoretically calculated gravity waves that emerge from a core region with an elliptical distribution of vorticity. Results of the simulated cyclone also illustrate that spiral cloud bands are triggered by the propagating gravity waves.

Studies on the Initialization and Simulation of a Mature Hurricane Using a Variational Bogus Data Assimilation Scheme

A bogus data assimilation (BDA) scheme is presented and used to generate the initial structure of a tropical cyclone for hurricane prediction. It was tested on Hurricane Felix (1995) in the Atlantic Ocean during its mature stage. The Pennsylvania State University‐National Center for Atmospheric Research nonhydrostatic Mesoscale Model version 5 was used for both the data assimilation and prediction. It was found that a dynamically and physically consistent initial condition describing the dynamic and thermodynamic structure of a hurricane vortex can be generated by fitting the forecast model to a specified bogus surface low based on a few observed and estimated parameters. Through forecast model constraint, BDA is able to recover many of the structural features of a mature hurricane including a warm-core vortex with winds swirling in and out of the vortex center in the lower and upper troposphere, respectively; the eyewall; the saturated ascent around the eye and descent or weak ascent in the eye; and the spiral cloud bands and rainbands. Satellite and radar data, if available, can be incorporated into the BDA procedure. It was shown that satellite-derived water vapor winds have an added value for BDA—they can generate a more realistic initial vortex. As a result of BDA using both a bogus surface low and satellite water vapor wind data, dramatic improvements occurred in the hurricane prediction of Felix. First of all, the initial fields of model variables describing the BDA initial vortex are well adapted to the forecast model. Second, the intensity forecast was greatly improved. The mean error of the central sea level pressure during the entire 72-h forecast period reduced from 25.9 hPa without BDA to less than 2.1 hPa with BDA. Third, the model captured the structures of the storm reasonably well. In particular, the model reproduced the ring of maximum winds, the eye, the eyewall, and the spiral cloud bands. Finally, improvement in the track prediction was also observed. The 24-, 48-, and 72-h forecast track errors with BDA were 76, 76, and 84 km, respectively, compared to the track errors of 93, 170, and 193 km without BDA.

An Arctic Hurricane over the Bering Sea

Abstract A mesoscale “arctic hurricane” developed over the western Bering Sea on 7 March 1977 and traveled eastward parallel to the ice edge along a zone of large sea surface temperature gradient. Satellite imagery reveals spiral cloud bands of unusual symmetry and mesoscale dimension associated with the mature stage of the low. The track of the low pressure center passed over the rawinsonde station at St. Paul Island where time series of surface data show a pronounced maximum in equivalent potential temperature at the core of the low. The storm made landfall with surface winds >30 m s−1; at Cape Newenham, Alaska, on 9 March and rapidly dissipated thereafter. Synoptic analyses show that the arctic hurricane formed at the leading edge of an outflow of arctic air that originated over the ice and passed over the open water of the western Bering 5u. In the mid- and upper troposphere a large cold-core low dominated the Bering Sea region. Quasi-geostrophic analysis at 0000 UTC 7 March 1977 reveals conditions co...

A simple model of spiral cloud bands in large-scale atmospheric vortices

Eine einfache Methode, die auf der kombinierten Benutzung der ω-Gleichung und der Satelliten-wolkenbilder beruht, wird in dieser Arbeit gezeigt. Namentlich wird der Fall einer okkludierten extratropischen Zyklone studiert.