Scientific Online Letters on the Atmosphere (SOLA) is an online, open-access letter journal and was established to rapidly publish scientific discoveries, new findings, and advances of understanding in meteorology and atmospheric sciences as well as in related interdisciplinary areas. SOLA is committed to pursue the followings: rapid review cycle; rapid publication; and wide coverage in atmospheric sciences. SOLA aims to make final decision for the submitted papers within two months from the date of submission. After acceptance, the papers will appear on J-STAGE (https://www.jstage.jst.go.jp/browse/sola/) within one month. To cover wide areas in atmospheric sciences, the Editorial Committee of SOLA consists of 8 steering board members and 35 editors who specialize various fields in atmospheric sciences. SOLA was established in 2005 by the Meteorological Society of Japan. About 40 to 50 papers have been published each year, and in 2016 there were totally 67 papers in the regular volume and the special volume on The First Asian Conference on Meteorology.
The Editorial Committee of SOLA gives The SOLA Award to outstanding paper(s) published each year starting from 2010. The award winning papers are as follows: Ito et al. (2010), Inoue et al. (2011), Endo (2011), Iwasaki and Mochizuki (2012), Moteki and Manda (2013), Miyama and Hasegawa (2014), and Masuda and Ishioka (2015).
Among the 35 editors, 10 editors join the SOLA committee from the international research community outside Japan. SOLA welcomes submission from the international community in atmospheric sciences. We hope that SOLA would be a platform to publish researches in meteorology and atmospheric sciences including their related interdisciplinary fields.
The Editorial Committee of Scientific Online Letters on the Atmosphere (SOLA) gives The SOLA Award to outstanding paper(s) published each year. I am pleased to announce that The SOLA Award in 2016 is going to be presented to the paper by Dr. Toru Adachi et al., entitled with “Rapid volumetric growth of misocyclone and vault-like structure in horizontal shear observed by phased array weather radar” (Adachi et al. 2016).
Misocyclones are vertically oriented vortices with the horizontal scales from 40 m to 4 km and have a potential for wind disasters. This study investigates the rapid evolution of misocyclones that developed in the northern part of Osaka on 25 August 2013 by conducting high-temporal resolution observations with a phased array weather radar (PAWR). The PAWR observation showed that the top altitude of the misocyclones, developed in a horizontally sheared environment, was shown to rapidly grow from 1.2 km to 1.8 km height in two minutes and that the misocyclones gained a vault-like structure in the next two minutes. The analysis indicated that shear instability plays a role in the evolution of the misocyclones through stretching of vertically oriented vortices.
This study is highly evaluated in demonstrating an observational evidence that the structure of misocyclones rapidly changes within a few minutes. Such a rapid growth was captured for the first time with a PAWR that can observe three-dimensional structure of precipitation and wind at the 30-second interval. Because misocyclones have a potential for wind disasters, it is important to understand the evolution of misocyclones and the resulting wind gusts. Furthermore, misocyclones sometimes play a role in initiating strong convection, and therefore, monitoring misocyclones at high-temporal resolutions should contribute to advancing now-casting of severe weather. Therefore, the Editorial Committee of SOLA highly evaluates the excellence of the authors’ study.
With a goal of real-time, high-resolution, short-term prediction of heavy rainfall systems, the SCALE-LETKF was developed implementing the local ensemble transform Kalman filter with the Scalable Computing for Advanced Library and Environment-Regional Model (SCALE-RM). The system has been running in near real time experimentally since May 2015, configured for weather analyses and forecasts at 18-km resolution for a 5760 × 4320 km area around Japan. Among the data for more than one year, the near-real-time forecasts and the 3-km resolution downscaling simulations are demonstrated for a selected case of the September 2015 Kanto-Tohoku heavy rainfall associated with Typhoon Etau (2015). The typhoon track was successfully analyzed and predicted by the system, and the line-shaped rainband producing heavy rainfall can be reasonably forecasted by the downscaling simulation from the near-real-time data.
Future changes in precipitation extremes in East Asia are investigated using large ensemble simulations of about 100 members by a 60-km mesh atmospheric general circulation model (AGCM) for the present climate and 4 K warmer climates, employing six different sea surface temperature (SST) anomaly patterns projected by state-of-the-art climate models. The high-resolution AGCM demonstrates good performance for reproducing precipitation extremes such as annual maximum 1-day precipitation total (Rx1d). Under the warmer climates, Rx1d are robustly projected to increase throughout East Asia. However, there is large range of their quantitative estimates, arising from internal variability and uncertainty in future SST patterns. Over land regions such as inland China, internal variability is the major source of the uncertainty in climatological-mean Rx1d change. However, over oceanic regions including Japan, Korea and coastal China, uncertainty in the SST patterns contributes greatly to the uncertainty in Rx1d through modulation of tropical cyclone activity, suggesting large regional variations in the relative importance of the two sources of uncertainty.
This study evaluates the performance of the Non-hydrostatic Regional Climate Model (NHRCM) in simulating the present climate over Southeast Asia to determine its applicability in downscaling climate projections in the region. Simulations from 1989 to 2008 are conducted over the region at 25-km resolution using boundary conditions from the ECMWF ERA-Interim dataset. The topographic effect on rainfall is well represented in NHRCM but can result in wet (dry) biases in the windward (leeward) side of mountains. NHRCM is able to reduce the overestimated rainfall in ERA-Interim, particularly over eastern Philippines and in the Maritime Continent, with improvements in spatial patterns. Both seasonality and daily distribution of rainfall are represented in most regions. On the other hand, there is a tendency to underestimate the number of wet days, especially during the respective wet season of the subregion, and to overestimate daily rainfall intensity. NHRCM also has an overall cold model bias, which reduces the warm bias in ERA-Interim, except for some parts of Indochina during boreal winter and spring. These results indicate the improved representation of present climate in Southeast Asia using NHRCM, and its potential applicability in downscaling climate projections to increase projected climate scenarios for the region.
The impact of the radiosonde observations of cold surge over the Philippine Sea on the tropical region and the Southern Hemisphere has been investigated by the assimilation of radiosonde data obtained during the R/V Hakuho Maru cruise KH–12–6 in late December 2012. After assimilating the observation data, the modified surface winds of the cold surge were generally stronger than those before the assimilation. In addition, cyclonic rotations around the 4 developing tropical cyclones in the Northern and Southern Hemispheres were more intensified. Furthermore, the analysis errors over the Indian Ocean and the Pacific Ocean in the Northern and Southern Hemispheres were reduced by 1 to 10%.
The impacts of the additional radiosonde observations in the cold surge immediately propagated up to the updraft region near the equator and to the mid-latitude downdraft regions through the local Hadley circulation. After the impact spread in the lower troposphere, large impacts were deepened around the tropical cyclones and depressions within 2 days. The propagation process of the additional observation impact over the Philippine Sea suggested that the cold surge could affect large-scale circulation, including typhoons and tropical depressions in the tropics and the mid-latitude regions.
Since around 2013, the globally averaged sea surface temperature has rapidly warmed up and reached its highest on record. During this time, there was an intensifying El Niño event that caused positive temperature anomalies in the tropical Pacific Ocean. Compared with the conditions observed in 1997/98, when the previous highest record was marked associated with strong El Niño event, there were notable differences detected in the recent conditions. In the tropical Pacific, remarkable warming near sea surface associated with strong El Niño event in 2015/16 started from significantly warmed conditions along with positive temperature anomaly redistributed from the western part since early 2014, resulting in positive anomalies in the central to eastern part remaining for more than two years, much longer than 1997/98 event. In addition, substantial warming was observed in the North Pacific around 2013 and contribution of the North Pacific region to the global averaged SST anomaly marked significantly large value and was comparable to that of the tropical Pacific.
An inverse modeling system for estimating Asian dust emissions was developed by combining the GEOS-Chem chemical transport model with the Green's function method. We applied the system to two heavy dust storms that occurred in 2014 (10-25 March and 24 May to 5 June), using surface-based polarization optical particle counter (POPC) observations at Fukuoka. Validation by independent observation datasets, including POPC measurements and PM10 observations at Seoul, showed that the use of a posteriori dust emissions improved overestimations in the a priori simulation and achieved much better agreement with observations. Satellite observations, surface synoptic observations, and modeled wind fields indicated that the major dust source region differed between the two dust storms; the major dust outbreak of one storm occurred in the northeastern Gobi Desert, whereas that of the other occurred in the southern Gobi Desert. The a posteriori dust emissions successfully reproduced this difference. Thus, the inverse modeling system developed in this study was able to improve the estimation of not only the intensity but also the geographical distribution of dust emissions.
This study examined the impact of an active phase of the Madden–Julian Oscillation (MJO) on a torrential rain event that occurred on the western coast of Sumatra Island on 12 December 2015, using surface meteorological observations, meteorological radar observations, and balloon sounding data obtained from the pre-Years of the Maritime Continent field campaign. Strong MJO activity took place in mid-December 2015 into January 2016. Radar observations revealed that a convergence and convective cloud merger of mesoscale convective systems from an eastward propagating MJO and westward moving diurnal convection over the western coast of the island was the immediate cause of the torrential rain. An investigation of the occurrence of convection over the island showed that both westward moving diurnal convection from the mountains and eastward propagating convection from the Indian Ocean occurred on 12 December, because the westerly winds in the lower troposphere associated with the MJO were only just initiated and were weak on the day. The results suggest that the leading edge of the MJO westerly wind bursts provided favorable conditions for an active phase of the MJO to work with the westward moving diurnal convection and cause torrential rain on the western coast of Sumatra Island.
The observation operators in the local ensemble transform Kalman filter (LETKF) were improved to enable use of observations in the vicinity of the poles in the data assimilation system composed of the atmospheric general circulation model for the Earth Simulator (AFES) and the LETKF. The improved observation operators allow to assimilate the observations located south (north) of southernmost (northernmost) Gaussian grid latitudes. An algorithm for searching the nearest observations from an analyzed grid for error covariance localization was also modified to efficiently assimilate observations near the poles.
The new algorithms were incorporated into the LETKF, and the impacts of routine radiosonde observations at the South Pole during the periods of July 2012 and January 2013 were assessed. The radiosonde observations suppressed an artificial expansion of the analysis ensemble spread which occasionally caused numerical instability in the upper troposphere and the lower stratosphere over the Antarctic regions. The analysis was also improved in the Antarctic regions.
Heavy dust storms that occurred between 13 and 23 March 2014 were selected for analysis as a case study of dust and sand storm events by the Tripartite Environment Ministers Meeting. During this period, two dust and anthropogenic aerosol events were observed and analyzed. The GEOS-Chem chemical transport model, which includes dust-acid uptake processes, successfully reproduced the aerosol variations and explained the Asian-scale dust-pollutant transport processes. Our results confirmed the importance of coarse-mode dust-nitrate as evidence of ‘polluted dust’ pollution. The model analysis showed that the formation of dust-nitrate occurred over the Yellow Sea and East China Sea before arriving in Japan. We showed that more than 40% of nitrate exists in dust-nitrate when air mass arrived in Japan.
A delayed response of the winter North Atlantic oscillation (NAO) to the 11-year solar cycle has been observed and modeled in recent studies. However, the mechanisms creating this 2-4-year delay to the solar cycle have still not been well-understood. This study examines the effects of the 11-year solar cycle and the resulting modulation in the strength of the winter stratospheric polar vortex. A coupled atmosphere–ocean general circulation model is used to simulate these effects by introducing a mechanistic forcing in the stratosphere. The intensified stratospheric polar vortex is shown to induce positive and negative ocean temperature anomalies in the North Atlantic Ocean. The positive ocean temperature anomaly migrated northward and was amplified when it approached an oceanic frontal zone approximately 3 years after the forcing became maximum. This delayed ocean response is similar to that observed. The result of this study supports a previous hypothesis that suggests that the 11-year solar cycle signals on the Earth's surface are produced through a downward penetration of the changes in the stratospheric circulation. Furthermore, the spatial structure of the signal is modulated by its interaction with the ocean circulation.
Heat island intensity approaches the maximum during urban Phase A period, which constitutes a few hours around sunset when radiative cooling primarily determines the temperature decrease. To evaluate the thermal inertia of urban and rural canopy layer, we suggested a local effective thermal inertia (LETI) that corresponds to a canopy layer thermal inertia, excluding advection effects caused by such as heat island circulation, and estimated the value by observing a 30-min temperature variation and radiation flux when clouds appeared during night. The urban area LETI is approximately two times higher than the rural value. LETI has a close relationship to the cooling rate in Phase A and is an important value for urban canopy layer thermal inertia.
The estimation accuracy of ensemble forecast errors has a key influence on the assimilation results of all ensemble-based schemes. The ensemble transform Kalman filter (ETKF) assimilation scheme can assimilate nonlinear observations without using the adjoint of a dynamical model; however, the initially estimated ensemble forecast errors must be further adjusted. In this paper, the estimation of forecast error is improved using a self-generated analysis and a corresponding iterative procedure is then established for the ETKF with nonlinear observation operators. The improved assimilation scheme is validated using the Lorenz-96 model with a nonlinear and spatially correlated observation system as a test bed. The experiment results demonstrate that the improved ETKF assimilation scheme can effectively reduce the analysis error.
A shallow water model is developed on the regular hexagonal mesh by combining the hexagonal B1-grid and B2-grid schemes. The new scheme called as the hexagonal synchronized B-grid (SB-grid) scheme in this work allows avoiding a computational mode problem of the ZM-grid scheme. It is known that the problem is caused by the mismatch of degrees of freedoms of the prognostic variables. The SB-grid uses the same variable arrangement as the ZM-grid, placing fluid depths and fluid velocities at the centers and corners of hexagonal cells, respectively, but the nonlinear terms of the momentum equation are discretized using wider spatial stencils than those of the ZM-grid. This change results in the inhibition of extra interactions in the velocity fields that enhances a computational mode in the ZM-grid. Geostrophic adjustment tests on a regular hexagonal mesh confirm that the SB-grid shallow water model behaves almost equivalently to the Z-grid model, and the computational mode problem is certainly settled down.
A wind gust of F0 scale occurred in Shonai town, Yamagata Prefecture, Japan, on 4 December 2015. It damaged some houses in a narrow valley 24 km from the coast of the Japan Sea. Around that time, one of many isolated convective clouds greater than 6 km in height traveled over the area. Observation of two Doppler radars with high resolution in time and space showed that a cyclonic vortex generated at the south edge of the convective cloud after landfall from the Japan Sea rapidly developed with increasing vorticity as it moved inland. The vortex traveled across a mountain, and its southern part near its center subsequently passed over the damaged area. The radius and vorticity of the vortex were 0.64 km (misoscale) and 0.048 s−1, respectively. A hook-shaped echo, a vault-shaped echo, and a couplet of maximum and minimum Doppler velocities, which indicate the existence of a vertically oriented vortex tube, are clearly shown. This study discusses the development of the winter misocyclone observed by the radars in terms of the relationship with the behavior of the parent cloud and the possible topographical influence of the mountain and valley on the vortex structure.
Localized propagation delay signals associated with line-aligned convective cells were detected by the Synthetic Aperture Radar Interferometry (InSAR) technique on 25 August 2010 in Niigata prefecture. The maximum amplitude of the signal reached up to 22.5 cm, which was approximately equivalent to 29 mm anomaly in precipitable water vapor (PWV). The nationwide radar rainfall intensity captured the spatial distribution of hydrometeors on both land and sea, which was similar to that of the InSAR-derived water vapor field, suggesting that the convective cells were initiated on the Japan Sea to the west-southwest of the observation area. A numerical weather model (NWM) simulation with the grid spacing of 2.5 km reproduced line-aligned convective cells with 3 cm smaller maximum amplitude to that in InSAR. A NWM simulation that assimilates Global Navigation Satellite System (GNSS)-derived PWV data for four-dimensional variational assimilation enhanced the water vapor flux convergence at the surface, which improved the amplitude of the localized delay signals. The advantage of the unique water vapor observation by InSAR enabled us to assess the meso-gamma scale NWM reproducibility in terms of water vapor, which is one of the fundamental prognostic parameter for NWMs.
The North Pacific shows dominant monthly-mean large-scale atmospheric circulation anomalies, even after removing the variabilities of the Northern Hemisphere annular mode and the El Niño/Southern Oscillation. This work examines the precursors and their development for these residual anomalous circulations by applying objective reanalysis data to empirical orthogonal function (EOF) analysis. The first EOF mode (EOF1) features a monopole, while a dipole characterizes the second EOF mode (EOF2) over the North Pacific. Transient eddies (TEs) interactively induce the two EOF modes.
The precursors of EOF1 and EOF2 are detected in the anomalies of one month earlier; these are the systematic zonal bands detected in surface temperature and baroclinic instability (BI) in the lower troposphere over Eurasia and North America. The BI anomalies further extend into the central North Pacific at different latitudes between EOF1 and EOF2. Coherent zonal bands also appear in the geopotential height anomalies of upper troposphere. Such zonal bands reflect the ovalization of Arctic circumpolar circulation with different continent-ward shifts of the longer axis. The direction of this shift determines which EOF mode develops by the following month through the guided activation of TEs at a designated latitude over the North Pacific.
Ulaanbaatar, the capital city of Mongolia, is subject to high levels of atmospheric pollution during the winter, which severely effects the health of the exposed population. Using lidar and ground level meteorological observations, we studied the temporal variation of the PM2.5 and the structure of the atmospheric boundary layer (ABL) during the 2010 heating season. The concentration of PM2.5 increased after the air temperatures sharply decreased during two cold waves occurring 8-10 and 21-25 October. The surface air temperatures first dropped below 0°C because of the cold wave beginning on 10 October, which prompted the households in the ger (traditional Mongolian dwelling) districts to start combusting coal for heating, resulting in increased PM2.5 concentrations. Meanwhile, the maximum ABL height continuously decreased from summer to winter and dropped below 800 m after the second cold wave, when the weather was influenced by a Siberian high. The stable atmospheric conditions and surface inversion layer in winter resulted in low wind velocities (< 2 m s−1), especially at night. Consequently, because of both the meteorological and topographical conditions, air pollutants remained at the urban surface level, which resulted in high concentrations of PM2.5 in winter.
Monitoring particulate matter is essential to alert the public about health risks. The Terra/MODIS true color image clearly captured a yellow band over Hokkaido prefecture in Japan on 7 March 2016. We investigated whether this event was an Asian dust (Kosa) transport or not with the ground-based observations in Sapporo and Takikawa in Hokkaido and NASA's MERRA-2 re-analysis data. The timing of increased particle number concentrations (PNCs; greater than 0.5 μm) was clearly measured by a low-cost aerosol sensor at Sapporo and Takikawa in the early afternoon. For this particle size range, the PNC by this aerosol sensor had greater agreement with another commercial instrument for the 1-hourly mean data. The lidar data at Takikawa and NASA's AERONET at Sapporo also implied the increased dust particles (i.e., dominance of non-spherical and coarse particles, respectively), which supported that the PNC increase was due to the dust transport. The hourly PM2.5 data in Sapporo significantly increased in the evening rather than around the noon to early afternoon. We concluded that this event was judged as an Asian dust (Kosa) event in Hokkaido starting from the early afternoon, which was, however, not reported by Japan Meteorological Agency (JMA) based on their visible observations.
The meridional structures of tropical sea surface temperature (SST) play an important role in impacting the variations of Hadley circulation (HC), and the response amplitudes of the HC to different meridional tropical SST structures show contrasting differences. Using the simulations of phase 5 of the Coupled Model Intercomparison Project (CMIP5), the performance of the state-of-art models in reproducing the response contrast of the HC to different SST meridional structures during the seasonal cycle is evaluated in this study. The result indicates that the models show high skills in capturing the climatological features of annual mean HC and tropical SST. Moreover, the leading variabilities of HC and tropical SST during the seasonal cycle are well reproduced for both the equatorially asymmetric and symmetric variations. Furthermore, most of the models display good agreement with the observations in depicting the responses contrast of the HC to different SST meridional structures. These results indicate that the current CMIP5 models show high capability in capturing the response of HC to tropical SST during the seasonal cycle, and provide confidence for further detecting the future variations of HC.
Two heavy dust storms that occurred between 24-26 May 2014 in Mongolia and Inner Mongolia, China were responsible for long-lasting dust episodes observed from 26 May to 2 June 2014 in Fukuoka, Japan. During this period, dust and anthropogenic pollutants were transported simultaneously to Fukuoka and Korea and remained there for almost 8 days. We successfully observed fine and coarse aerosol time variations (1-hour intervals) in Fukuoka. The GEOS-Chem chemical transport model, which includes dust-acid uptake processes, successfully reproduced the aerosol variations and explained the Asian-scale dust-pollutant transport and transformation processes. Model sensitivity analyses with and without dust-acid uptake processes showed that the formation of dust-nitrate occurred over the Yellow Sea and East China Sea before arriving in Japan. The model sensitivity analysis showed that less than 5% of the coarse dust-nitrate originated from NOx emissions from Japan, and large amounts of dust-nitrate originated outside of Japan.
To overcome the limitation of low network density and sparse distribution of meteorological stations, spatial interpolation is being performed for estimating meteorological variables that are not geographically covered by existing observation network. While there are several readily available spatial interpolation techniques, it is still difficult to determine which one best estimates actual observation. Considering the stimulus for disaster risk reduction, hydrological, agricultural, and other applications of interpolated data, this study compared six interpolation techniques (Inverse Distance Weighted (IDW), Completely Regularized Spline (CRS), Tension Spline (TS), Ordinary Kriging (OK), Universal Kriging (UK), and ANUSPLIN) that have been recommended in tropical maritime region. Validation results comparing historical monthly and interpolated rainfall data from 1981-2010 in 65 stations in the Philippines show that OK has the best performance among the aforementioned techniques followed by ANUSPLIN and TS. Ultimately, this study is a contribution to the existing inadequate literatures that have documented and evaluated interpolation techniques that can be used in archipelagic regions with prominent climate variability.
This study analyzes a Rossby wave-breaking event east of Japan that enhanced the convective activities over the subtropical western North Pacific Ocean. In August 2016, Rossby-wave packets in the upper troposphere above Eurasia reached over and around the seas east of Japan. The wave-breaking event accompanied the amplification of a blocking ridge and the southward intrusion of upper-level high-potential vorticity (PV) south of the ridge. The high PV (i.e., the enhanced mid-Pacific trough) promoted upward motion and enhancement of convective activities over the subtropical western North Pacific Ocean through a quasi-geostrophic balance. In the lower troposphere, large-scale cyclonic circulation anomalies, including tropical disturbances, were observed south and southeast of Japan, and the anomalies caused significant wet climate conditions in the eastern and northern parts of the country. A linear baroclinic model experiment indicates that the lower-level cyclonic circulation anomalies were the Rossby-wave responses to heating anomalies associated with the enhanced convective activities. These results suggest the existence of dynamic interaction between extratropical and tropical circulation over the western North Pacific Ocean and its influence on boreal summer climate in Japan.
The impact of the length of the evolutionary window (EW) on the estimation of the predictability limit of the Lorenz-63 model using the nonlinear local Lyapunov exponent (NLLE) method is studied. The structure of the initial errors and error growth dynamics are analyzed. It is found that there exists an optimal EW, at which the estimated predictability limit is closest to its theoretical value. With a shorter EW, the predictability limit is underestimated, while at longer EWs it is overestimated. The optimal EW is approximately equal to the decorrelation time of the system. A preliminary explanation for this link, based on the loss of information from the initial state, is given.
Size distributions of tropical convective systems in regional numerical atmospheric models are analyzed over a 2.5 × 105 km2 domain using different model grid spacing and parameterization schemes. The 5- and 20-km-resolution experiments are configured with a cumulus parameterization scheme, whereas the 2- and 4-km-resolution experiments are not. Precipitation systems are defined by either synthetic satellite infrared images, surface rain rates, or vertical winds at 600 hPa. The size distributions of systems defined by shallower clouds, lower rain rates, and weaker updrafts follow power laws, whereas those defined by deep clouds, higher rain rates, and stronger updrafts show lognormality. The cloud size distribution of the 5-km-resolution experiment is most similar to that of the real geostationary satellite observations. Generally, the largest system size becomes larger in the 5- and 20-km-resolution experiments, implying that the cumulus parameterization may have an impact on that scale. Exceptionally, all the model-simulated size distributions of heavy rain areas agree well at the largest scale. Lower-resolution experiments tend to underestimate the number of small-scale systems when compared with higher-resolution experiments. The size distributions also capture a temporal modulation of precipitation during the 2007 Jakarta flood event; small-scale intense precipitation systems increase during the period.
A super high-resolution simulation of the 6 May 2012 Tsukuba supercell tornado with a horizontal grid spacing of 10 m is conducted to investigate its fine-scale structure under realistic environmental conditions including surface friction. The simulated tornado repeatedly exhibits evolutions from one-cell to two-cell vortex, and subsequently to a multiple-vortex structure, where the vortex structure is sensitive to a swirl ratio. Subvortices in the multiple-vortex structure are located on the immediate inside of the radius of the maximum tangential wind speed, and cyclonically rotate around the tornado center with a slower speed less than half of the maximum tangential wind speed. The subvortices have a feature of a suction vortex accompanied by strong horizontal convergence and strong updraft near the surface. Although a superposition of the swirling winds associated with the subvortices and the parent tornado vortex causes locally intensified winds, the maximum horizontal and upward winds over the tornado's lifetime occur at the stage of shrinking of the vortex radius right before a transition to a multiple-vortex structure.
Eurasia experienced severe cold weather in December 2012. Particularly strong cold spells occurred in two regions: one over eastern Eurasia in early December, and the other in central to western Eurasia in mid-December. These extreme events were produced in association with blocking phenomena triggered by reflection or downward propagation of planetary waves from the stratosphere.
This event was unusual in that dominant component of upward and downward propagating wave is different: zonal wavenumber 2 component is dominant in the upward propagation; due to the amplification of zonal wavenumber 1 component in the upper stratosphere, zonal wavenumber 1 component becomes important in the subsequent downward propagation in the polar region. Identification of the tropospheric impact of the downward propagation of planetary waves is usually not easy. In the present case, however, the difference in the dominant zonal wavenumber between the upward and downward propagation facilitates to separate upward and downward propagation processes, and to isolate the amplification process of planetary waves in the upper stratosphere.
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