Diagnostic Study of an Extreme Explosive Cyclone over the Kuroshio/Kuroshio Extension Region

Explosive cyclones(ECs)occur frequently over the Kuroshio/Kuroshio Extension region.The most rapidly intensified EC over the Kuroshio/Kuroshio Extension region during the 42 years(1979-2020)of cold seasons(October-April)was studied to reveal the variations of the key factors at different explosive-developing stages.This EC had weak low-level baroclinicity,mid-level cyclonic-vorticity advection,and strong low-level water vapor convergence at the initial explosive-developing stage.The low-level baroclinicity and mid-level cyclonic-vorticity advection increased substantially during the maximum-deepening-rate stage.The diagnostic analyses using the Zwack-Okossi equation showed that diabatic heating was the main contributor to the initial rapid intensification of this EC.The cyclonic-vorticity advection and warm-air advection enhanced rapidly in the middle and upper troposphere and contributed to the maximum rapid intensification,whereas the diabatic heating weakened slightly in the mid-low troposphere.The relative contribution of the diabatic heating decreased from the initial explosive-developing stage to the maximum-deepening-rate stage due to the enhancement of other factors(the cyclonic-vorticity advection and warm-air advection).Furthermore,the physical factors contributing to this EC varied with the explosive-developing stage.The non-key factors at the initial explosive-developing stage need attention to forecast the rapid intensification.

A 38-Year Climatology of Explosive Cyclones over the Northern Hemisphere

Explosive cyclones(ECs)over two basins in the Northern Hemisphere(20°-90°N)from January 1979 to December2016 are investigated using ERA-Interim and Optimum Interpolation Sea Surface Temperature(OISST)data.The classical definition of an EC is modified considering not only the rapid drop of the central sea level pressure of the cyclone,but also the strong wind speed at the height of 10 m in which maximum wind speeds greater than 17.2 m s^-1are included.According to the locations of the northern Atlantic and northern Pacific,the whole Northern Hemisphere is divided into the"A region"(20°-90°N,90°W-90°E)and"P region"(20°-90°N,90°E-90°W).Over both the A and P regions,the climatological features of ECs,such as their spatial distribution,intensity,seasonal variation,interannual variation,and moving tracks,are documented.

Structures and Evolutions of Explosive Cyclones over the Northwestern and Northeastern Pacific

In this study, the structures and evolutions of moderate(MO) explosive cyclones(ECs) over the Northwestern Pacific(NWP) and Northeastern Pacific(NEP) are investigated and compared using composite analysis with cyclone-relative coordinates. Final Operational Global Analysis data gathered during the cold seasons(October–April) of the 15 years from 2000 to 2015 are used. The results indicate that MO NWP ECs have strong baroclinicity and abundant latent heat release at low levels and strong upper-level forcing, which favors explosive cyclogenesis. The rapid development of MO NEP ECs results from their interaction with a northern cyclone and a large middle-level advection of cyclonic vorticity. The structural differences between MO NWP ECs and MO NEP ECs are significant. This results from their specific large-scale atmospheric and oceanic environments. MO NWP ECs usually develop rapidly in the east and southeast of the Japan Islands; the intrusion of cold dry air from the East Asian continent leads to strong baroclinicity, and the Kuroshio/Kuroshio Extension provides abundant latent heat release at low levels. The East Asian subtropical westerly jet stream supplies strong upper-level forcing. While MO NEP ECs mainly occur over the NEP, the low-level baroclinicity, upper-level jet stream, and warm ocean currents are relatively weaker. The merged cyclone associated with a strong middle-level trough transports large cyclonic vorticity to MO NEP ECs, which favors their rapid development.

Diagnosis and Numerical Modeling of an Explosive Cyclone over the Northwestern Pacific

An explosive cyclone that took place over the Northwestern Pacific from 12 UTC 18 to 18 UTC 21 November 2007 was investigated.The synoptic situations and structure of this cyclone were documented by using the 1°×1°final analysis data of the National Center for Environmental Prediction.This cyclone developed explosively around 18 UTC 19 and reached its maximum deepening rate(MDR,1.3 Bergeron)around 06 UTC 20 November 2007.At its MDR moment,the surface cyclone center was located in the downstream of the upper-level trough and northern entrance zone of the upper-level jet.The diagnosis using Zwack-Okossi equation suggested that cyclonic-vorticity advection and warm air advection acted to deepen this cyclone,while adiabatic cooling suppressed its development.In an investigation of this cyclone development,numerical sensitivity results obtained by using the Weather and Research Forecasting model showed that the latent heat release in the lower level had less contribution,whereas the surface sensible and latent fluxes played important roles.With a warmer ocean surface,the cyclone tended to intensify.Two topography tests were designed to examine the mountain influences on the development of this cyclone:removing a mountain and doubling the height of a mountain.Results show that the Changbai Mountains suppressed the development of the cyclone by preventing the southern moisture air from invading the inland.Without the moisture air,no latent heat release occurs when this cyclone passes over the Changbai Mountains.

Spatial Distribution and Seasonal Variation of Explosive Cyclones over the North Atlantic

Spatial distribution and seasonal variation of explosive cyclones （ECs） over the North Atlantic from October 2000 to September 2016 are investigated using the reanalysis data of Final Analysis （FNL）, Mean Sea Level Pressure （MSLP） and Optimum Interpolation （OI） Sea Surface Temperature （SST） provided by the National Centers for Environmental Prediction （NCEP）, the European Centre for Medium-Range Weather Forecasts （ECMWF） and the National Oceanic and Atmospheric Administration （NOAA）, respectively. Considering the meridional distribution of ECs and 10-m height wind field associated with the ECs, the definition of EC given by Yoshida and Asuma （2004） is modified. It is found that the ECs occurred mainly in four regions during winter season, namely, North America continent （NAC）, the Northwest Atlantic （NWA）, the North-centzal Atlantic （NCA）, and the Northeast Atlantic （NEA）, depending on the spatial distribution of EC＇s maximum deepening rate of central sea level pressure （SLP）. According to the magnitude of maximum deepening rate, the trend of EC numbers basically decrease with the increase of EC＇s maximum deepening rate over the North Atlantic during the whole time period. Over the North Atlantic basin, for monthly statistics, the NEA, NCA, and NWA cyclones occur mainly in December, from December to March, and from January to February, respectively. NWA, NCA and NEA cyclones in winter are associated with low-level barocliincity, both low-level baroclinicity and upper-level forcing and upper-level forcing, respectively. According to monthly variation, the averaged maximum deepening rate of central SLP firstly increases and then decreases from July to June. Overall, the distribution of ECs＇ tracks is basically in the southwest-northeast direction. During winter circulation stage （from October to May）, the averaged maximum deepening rate of central SLP and the averaged minimum central SLP of ECs decrease, and the averaged explosive-deepening duration of ECs shortens from west to east over the North Atlantic basin. During summer circulation stage （from June to September）, the number of ECs is far less than that of winter circulation. NCA cyclones are the lowest in the average minimum central SLP of ECs, and the longest in the average explosive-deepening duration ofECs. NEA cyclones are the strongest in the average maximum deepening rate of central SLR

The 9–11 November 2013 Explosive Cyclone over the Japan Sea- Okhotsk Sea: Observations and WRF Modeling Analyses

During the period from 9 to 11 November 2013,an explosive cyclone(EC)occurred over the Japan Sea-Okhotsk Sea.This EC initially formed around 18 UTC 9 November over the Japan Sea and developed over the Okhotsk Sea when moving northeastward.It had a minimum sea level pressure of 959.0 hPa,a significant deepening rate of central pressure of 2.9 Bergeron,and a maximum instantaneous wind speed of 42.7 m s−1.This paper aims to investigate the conditions that contributed to the rapid development of this low-pressure system through analyses of both observations and the Weather Research Forecasting(WRF)modeling results.The evolutionary processes of this EC were examined by using Final Analyses(FNL)data,Multi-Functional Transport Satellites-1R(MTSAT-1R)data,upper observation data and surface observation data.WRF-3.5 modeling results were also used to examine the development mechanism of this EC.It is shown that the interaction between upper-level and low-level potential vorticity seemed to be very essential to the rapid development of this EC.

Diagnostic Analyses and Numerical Modeling of Explosive Cyclone over Northwestern Pacific on January 11-13,2012

In this paper,we use FNL grid data obtained from the National Centers for Environmental Prediction(NCEP)to analyze an explosive cyclone(EC)that occurred over the northwestern Pacific Ocean from January 11 to 13,2012.To simulate the EC,we used the Weather Research and Forecasting model(WRFV3.5).The cyclone outbreak occurred east of Japan from January 11 to 12 and weakened near the Kamchatka Peninsula on January 13.The analysis results show a distinct frontal structure,in which the high potential vorticity(PV)of the upper troposphere extends downward to the surface,which can facilitate EC development.A low-level jet stream develops with the EC,which can lead to more distinct convergence.The results of sea surface temperature(SST)sensitivity tests suggest that changes in the SST can affect cyclone intensity,but have little effect on its path.When small changes are made to the SST,the air pressure at the cyclonic center responds more distinctly to an increased SST than a decreased SST.The results of our latent heat release test suggest that diabatic heating processes lead to maximum PV values in the lower troposphere.Latent heat is also one of the important factors influencing EC development.

Characteristics of an Explosive Cyclone over Northeast China Revealed by Satellite Water Vapor Imagery

In this paper,an explosive cyclone(EC)that occurred over Northeast China in the spring of 2016 is studied by using 6.7μm FY satellite water vapor(WV)imagery and NCEP(1°×1°)reanalysis data.Moreover,the evolutions of the upper-level jet stream(ULJ),the vertical motions,and the potential vorticity(PV)are analyzed in detail.Results show that different shapes of the WV image dark zones could reflect different stages of the EC.At the pre-explosion stage,a small dark zone and an S-shaped baroclinic leaf cloud can be found on the WV imagery.Then the dark zone expands and the leaf cloud grows into a comma-shaped cloud at the explosively developing stage.At the post-explosion stage,the dark zone brightens,and the spiral cloud forms.The whole process can be well described by the WV imagery.The dynamic dry band associated with the sinking motion and the ULJ can develop into the dry intrusion later,which is an important signal in forecasting the EC and should be paid attention to when analyzing the WV imagery.Furthermore,the mechanism is also analyzed in detail in this article.EC usually occurs in the left-exit region of the 200-h Pa jet and the region ahead of the 500-h Pa trough where there is significant positive vorticity advection(PVA).When the EC moves onto the sea surface,the decreased friction would favour the development of the EC.The upper-level PVA,the strong convergence at low level,and the divergence at high levels can maintain the strong updraft.Meanwhile,the high PV zone from the upper levels extends downward,approaching the cyclone.Together,they keep the cyclone deepening continuously.

A Diagnostic Study of Explosive Development of Extratropical Cyclone over East Asia and West Pacific Ocean

In this paper, a diagnostic analysis is made for a kind of explosive cyclone ovcr East Asia and the West PacificOcean in cold season, using the level Ⅲ FGGE dataset. The cyclone started developing at 0000 UTC 30 March, 1979.Q vector analysis shows that ageostrophic wind was obvious in cyclone region. The calculation of different kindsof frontogenetical functions indicates that the development of cyclone was closely related to baroclinicity, especially,at lower levels.Isentropic analysis revealed the three-dimensional structure of cyclone development, that is, ascent of southerlywarmer current and descent of northerly colder current existed around the cyclonic center during the developing process of the cyclone and is very favourable to the release of available potential energy and generation of eddy kineticenergy.Not only shear component, but also curvature component of upper level jet contributed to the explosive development of the cyclone.The computation of convergence of moisture flux demonstrated that the moisture probably came from the tropical ocean. The distribution of water vapor supply in this case was very advantageous to the deepening of cyclone, especially, during the well-developing period.Comparison between East Asia Pacific case and North America-Atlantic case (Ogura and Juang, 1990) hasbeen conducted. The common characteristics were that there existed strong baroclinicity in both cases. However, inthe latter case, the latent heat release was of secondary importance and in our case, moisture also played very important role in certain. stages of the cyclogenesis, especially, during well-developing stage when it moved over oceanicsurface.

ANALYSIS OF COMPOSITE DIAGNOSIS OF OCEANIC EXPLOSIVE CYCLONES

Using the data of ECMWF (European Center for Medium-range Weather Forecasts) to undertake composite diagnoses of 16 explosive cyclones occurring at the Atlantic and the Pacific Oceans,it is found that there are a lot of obvious discrepancies on the basic fields between these strong and weak explosive cyclones.The major reasons why the explosive cyclones over the Atlantic are stronger than those over the Pacific Ocean are that the non-zonal upper jet and the low-level warm moist flow over the Atlantic are stronger.The non-zonal upper jet offers stronger divergence,baroclinicity and baroclinic instability fields for explosive cyclones.Anticyclonic curvature at the high level of strong explosive cyclones is easy to make the inertia-gravitational wave developing at the moment of northward transfer of energy and stimulate the cyclones deepening quickly.Warm advection and diabatic heating can cause the upper isobaric surface lifting,as a result,the anticyclone curvature of cyclones enlarges,and wave energy develops easily as well.The most powerful period of the development of explosive cyclones is just the time when the positive vorticity advection center is located over the low vortex.At the upper level,when the distribution of potential vorticity contours changes suddenly from rareness to denseness,and the large values of the potential vorticity both in the west and north sides of cyclones extend downwards together,then cyclones are easy to explosively develop.The formation of strong explosive cyclones is closely related with the non-zonality of upper jet and the anticyclonic curvature.

NUMERICAL TEST OF AN EARLY-SUMMER EXPLOSIVE CYCLONE EVENT IN EAST ASIA

A series of ten numerical tests are carried out using smoothing techniques in the PSU/NACR mesoscale model MM5 initial field in order to study the development reasons of a pre-summer uncommon explosive event which took place in East Asia from 1—2 June.1993.The integration fields are compared with that of original results obtained by non-smoothed initial field.The results show that:(1)After the northern trough is smoothed,its corresponding cold air can not move downward and southward.Only a weak cyclone system forms south of 25°N after 24 h integration. (2)After the southern strong moisture transportation channel is smoothed,the northem trough system can only form a weak trough along the east coast of China after 24 h integration.(3)These two separate low trough systems in the southern and northern jet systems,with strong warm moisture transportation channel and cold air respectively,are both necessary for explosive cyclone development.In such an unfavorable season and location for explosive cyclone to take place,only after these two low troughs merged into a strong low vortex can the surface cyclone he developed explosively.Both the northern trough system and the southern moisture transportation channel are all indispensable for the explosive cyclone development.This explosive cyclone event is the result of the interaction of northern and southern systems.

Historic and Future Perspectives of Storm and Cyclone

In weather sciences,the two specific terms“storm”and“cyclone”frequently appear in literature and usually refer to the violent nature of a number of weather systems characterized by central low pressure,strong winds,large precipitation amounts in the form of rain,freezing rain,or snow,as well as thunder and lightning.But what is the connection between these two specific terms?In this paper,the historic evolutions of the terms“storm”and“cyclone”are reviewed from the perspective of weather science.The earliest recorded storms in world history are also briefly introduced.Then,the origin of the term“meteorological bomb”,which is the nickname of the“explosive cyclone”is introduced.Later,the various definitions of explosive cyclones given by several researchers are discussed.Also,the climatological features of explosive cyclones,as well as the future trends of explosive cyclones under global climate change,are discussed.

Energetics characteristics accounting for the low-level wind’s rapid enhancement associated with an extreme explosive extratropical cyclone over the western North Pacific Ocean

During mid-January 2011,a rarely seen twin-extratropical-cyclone event appeared over the western North Pacific Ocean.One of the twin cyclones developed into an extreme explosive extratropical cyclone(EEC),which was comparable to the intensity of a typhoon.Rotational and divergent wind kinetic energy(KE)analyses were applied to understand the low-level wind’s rapid enhancement associated with the cyclone.It was found that:(i)the total wind KE associated with the EEC showed a remarkable enhancement in the lower troposphere during the cyclone’s maximum development stage,with the maximum/minimum wind acceleration appearing in the southeastern/northwestern quadrant of the EEC;(ii)the rotational wind KE experienced an obvious increase,which corresponded to the total wind KE enhancement,whereas the divergent wind KE,which was much smaller than the rotational wind,mainly featured a decreasing trend;(iii)the rotational wind KE enhancement showed variational features consistent with the horizontal enlargement and upward stretching of the EEC;(iv)the nonorthogonal wind KE enhanced the total wind KE in regions with strong rotational wind,which resulted in the maximum lower-tropospheric maximum wind,whereas in regions with strong divergent wind it mainly reduced the total wind KE;(v)the northward transport of total wind KE and the rotational wind KE production due to the work done by pressure gradient force were dominant factors for the enhancement of winds associated with the EEC,particularly in its southeastern section.In contrast,an overall conversion from rotational wind KE to divergent wind KE decelerated the rotational wind enhancement.

Statistical Characteristics of Austral Summer Cyclones in Southern Ocean

Characteristics of cyclones and explosively developing cyclones (or 'bombs') over the Southern Ocean in austral summer (December, January and February) from 2004 to 2008 are analyzed by using the Final Analysis (FNL) data produced by the National Centers for Environmental Prediction (NCEP) of the United States. Statistical results show that both cyclones and explosively developing cyclones frequently develop in January, and most of them occur within the latitudinal zone between 55°S and 70°S. These cyclones gradually approach the Antarctic Continent from December to February. Generally cyclones and bombs move east-southeastward with some exceptions of northeastward movement. The lifetime of cyclones is around 2-6 d, and the horizontal scale is about 1000 km. Explosive cyclones have the lifetime of about 1 week with the horizontal scale reaching up to 3000 km. Compared with cyclones developed in the Northern Hemisphere, cyclones over the southern ocean have much higher occurrence frequency, lower central pressure and larger horizontal scale, which may be caused by the unique geographical features of the Southern Hemisphere.

ENERGY DISPERSION EFFECTS ON EXPLOSIVE DEVELOPMENT OF MARINE CYCLONES

A dominant role played by energy dispersion in the explosive development of extratropical marine cyclones over the Northwest Pacific has been revealed based on both the eddy energy equations and the energy flux vectors of nonlinear wave packet.At the initial and explosive time. the eddy energy from neighboring upstream systems beyond the radius of Rossby deformation is dispersed into the eddy energy center associated with the cyclone via ageostrophic geopotential fluxes,and results in the rapid increase of eddy kinetic energy and the occurrence of explosive cyclogenesis.When the cyclone begins to decay,its corresponding eddy energy is exported downstream and hence triggers the growth of new perturbation downstream.Through generalizing the energy flux vectors of quasigeostrophic wave packets to the nonlinear forms and making use of the relationship between the energy flux vectors and the total eddy energy,the approximation expressions of the total group velocity and relative group velocity are derived,and then they are used to compute an explosive case.The normalized ageostrophic geopotential fluxes by dividing the volume integral of ageostrophic geopotential fluxes by the integral of the total eddy energy determine the relative group velocity at which the eddy energy is spreading out,and they can be used to evaluate the position of next new disturbance.The nonlinear advective fluxes influence primarily the phase speed and translation of the cyclones.The results in this paper facilitate to expanding the mechanism research on explosive cyclones and have great significance for predicting the explosive intensification and downstream disturbance growth.

A DIAGNOSTIC STUDY OF AN EXPLOSIVELY DEEPENING OCEANIC CYCLONE OVER THE NORTHWEST PACIFIC

The vertical motions and secondary circulation of an explosively deepening oceanic cyclone,which oc- curred over the Northwest Pacific Ocean and was in conjunction with 200 hPa-level jet stream and has central pressure falls of 33.9 hPa/24h,have been computed from seven-level nonlinear balance model and Saw- yer-Eliassen-Shapiro equation for the transverse ageostrophic circulation.The vertical motions are partitioned into contributions from large-scale latent heat release,effect of cumulus heating,thermal advection,differen- tial vorticity advection,etc.,while the secondary circulation stream function is partitioned into contributions from geostrophic deformation,transfer of momentum and heat in the area of cumulus and diabatic heating. The principal results are the following.Large-scale latent heat release is very crucial to the explosive de- velopment of cyclones.If there is enough transfer of moisture,the positive feedback process between ascent of air and large-scale heating would work.The cumulus heating and the transfer of momentum and heat in the area of cumulus play an important role during the explosively deepening stage.Thermal advection is the initial triggering condition for large-scale heating and the conditional instability for the convection of cumulus.

Adjoint Sensitivity Study on Idealized Explosive Cyclogenesis

The adjoint sensitivity related to explosive cyclogenesis in a conditionally unstable atmosphere is investigated in this study.The PSU/NCAR limited-area,nonhydrostatic primitive equation numerical model MM5 and its adjoint system are employed for numerical simulation and adjoint computation,respectively.To ensure the explosive development of a baroclinic wave,the forecast model is initialized with an idealized condition including an idealized two-dimensional baroclinic jet with a balanced three-dimensional moderateamplitude disturbance,derived from a potential vorticity inversion technique.Firstly,the validity period of the tangent linear model for this idealized baroclinic wave case is discussed,considering different initial moisture distributions and a dry condition.Secondly,the 48-h forecast surface pressure center and the vertical component of the relative vorticity of the cyclone are selected as the response functions for adjoint computation in a dry and moist environment,respectively.The preliminary results show that the validity of the tangent linear assumption for this idealized baroclinic wave case can extend to 48 h with intense moist convection,and the validity period can last even longer in the dry adjoint integration.Adjoint sensitivity analysis indicates that the rapid development of the idealized baroclinic wave is sensitive to the initial wind and temperature perturbations around the steering level in the upstream.Moreover,the moist adjoint sensitivity can capture a secondary high sensitivity center in the upper troposphere,which cannot be depicted in the dry adjoint run.

INSTABILITY PROPERTIES OF ATMOSPHERIC DISTURBANCES

There are some basic problems in previous theoretical studies of baroclinic instability.The derived critical baroclinity was considerably lower than the time averaged mean meridional temperature gradient,especially in the lower troposphere.Also,the linear mechanism of baroclinic disturbance development which is noted restricted by the critical baroclinity was not studied sufficiently.The realistic critical baroclinity and disturbance development are discussed in this study.It will be shown that the critical condition of instability and typical time and space scales of disturbances de- pend on three-dimensional structures of atmosphere and sphericity of the earth,other than the horizontal temperature gradient alone.The variant behaviour of atmospheric disturbances depends highly on their specific scales that may be described by the same theoretical model.Thus,there would be no substantial differences in the basic instability mecha- nism of many disturbances including the polar lows and explosive cyclones.