A Lagrangian Trajectory Analysis of Azimuthally Asymmetric Equivalent Potential Temperature in the Outer Core of Sheared Tropical Cyclones

In this study,the characteristics of azimuthally asymmetric equivalent potential temperature(θ_(e))distributions in the outer core of tropical cyclones(TCs)encountering weak and strong vertical wind shear are examined using a Lagrangian trajectory method.Evaporatively forced downdrafts in the outer rainbands can transport low-entropy air downward,resulting in the lowestθ_(e)in the downshear-left boundary layer.Quantitative estimations ofθ_(e)recovery indicate that air parcels,especially those originating from the downshear-left outer core,can gradually revive from a low entropy state through surface enthalpy fluxes as the parcels move cyclonically.As a result,the maximumθ_(e)is observed in the downshear-right quadrant of a highly sheared TC.The trajectory analyses also indicate that parcels that move upward in the outer rainbands and those that travel through the inner core due to shear make a dominant contribution to the midlevel enhancement ofθ_(e)in the downshear-left outer core.In particular,the former plays a leading role in suchθ_(e)enhancements,while the latter plays a secondary role.As a result,moist potential stability occurs in the middle-to-lower troposphere in the downshear-left outer core.

Characteristics of the Outer Rainband Stratiform Sector in Numerically Simulated Tropical Cyclones:Lower-Layer Shear versus Upper-Layer Shear

Idealized numerical simulations are conducted in this study to comparatively investigate the characteristics of the stratiform sector in the outer rainbands of tropical cyclones(TCs)in lower-and upper-layer vertical wind shear(VWS)with moderate magnitude.Consistent with the results in previous studies,the outer rainband stratiform sector of the TCs simulated in both experiments is generally located downshear left.Upper-layer VWS tends to produce stronger asymmetric outflow at upper levels in the downshear-left quadrant than lower-layer shear.This stronger asymmetric outflow transports more water vapor radially outward from the inner core to the outer core at upper levels in the downshear-left quadrant in the upper-layer shear experiment.More depositional growth of both graupel and cloud ice thus occurs downshear left in upper layers in the outer core,yielding more diabatic heating and stronger upward motions,particularly in the stratiformdominated part of the stratiform sector in the upper-layer shear experiment.Resultingly,a better-organized stratiform sector in the outer rainbands is found in the upper-layer VWS experiment than in the lower-layer VWS experiment.The diabatic heating associated with the stratiform sector produces strong midlevel outflow on the radially inward side of,and weak midlevel inflow on the radially outward side of,the heating core,with lower-level inflow beneath the midlevel outflow and upper-level inflow above.The upper-layer VWS tends to produce a deeper asymmetric inflow layer in the outer rainband stratiform sector,with more significant lower-level inflow and tangential jets in the upper-layer VWS experiment.

The Characteristics of Atmospheric Ice Nuclei Measured at Different Altitudes in the Huangshan Mountains in Southeast China

The concentration of ice nuclei （IN） and the relationship with aerosol particles were measured and analyzed using three 5-L mixing cloud chambers and a static diffusion cloud chamber at three altitudes in the Huangshan Mountains in Southeast China from May to September 2011.The results showed that the mean total number concentration of IN on the highest peak of the Huangshan Mountains at an activation temperature （Ta） of-20℃C was 16.6 L-1.When the supersaturation with respect to water （Sw） and with respect to ice （Si） were set to 5％,the average number concentrations of IN measured at an activation temperature of-20℃C by the static diffusion cloud chamber were 0.89 and 0.105 L-1,respectively.A comparison of the concentrations of IN at three different altitudes showed that the concentration of IN at the foot of the mountains was higher than at the peak.A further calculation of the correlation between IN and the concentrations of aerosol particles of different size ranges showed that the IN concentration was well correlated with the concentration of aerosol particles in the size range of 1.2-20 μtm.It was also found that the IN concentration varied with meteorological conditions,such as wind speed,with higher IN concentrations often observed on days with strong wind.An analysis of the backward trajectories of air masses showed that low IN concentrations were often related to air masses travelling along southwest pathways,while higher IN concentrations were usually related to those transported along northeast pathways.

Decadal Variations of Intense Tropical Cyclones over the Western North Pacific during 1948–2010

Using Joint Warning Typhoon Center （JTWC） best track data during the period 1948-2010, decadal and interdecadal changes of annual category 4 and 5 tropical cyclone （TC） frequency in the western North Pacific basin were examined. By allowing all of the observed TCs in the JTWC dataset to move along the observed TC tracks in a TC intensity model, the annual category 4 and 5 TC frequency was simulated. The results agreed well with observations when the TC intensity prior to 1973 was adjusted based on time-dependent biases due to changes in measurement and reporting practices. The simulated and adjusted time series showed significant decadal （12-18 years） variability, while the interdecadal （18-32 years） variability was found to be statistically insignificant. Numerical simulations indicated that changes in TC tracks are the most important factor for the decadal variability in the category 4 and 5 TC frequency in the western North Pacific basin, while a combined effect of changes in SST and vertical wind shear also contributes to the decadal variability. Further analysis suggested that the active phase of category 4 and 5 TCs is closely associated with an eastward shift in the TC formation locations, which allows more TCs to follow a longer journey, favoring the development of category 4 and 5 TCs. The active phase corresponds with the SST warming over the tropical central and eastern Pacific and the eastward extension of the monsoon trough, thus leading to the eastward shift in TC formation locations.

Influence of Future Tropical Cyclone Track Changes on Their Basin-Wide Intensity over the Western North Pacific: Downscaled CMIP5 Projections

The possible changes of tropical cyclone（TC） tracks and their influence on the future basin-wide intensity of TCs over the western North Pacific（WNP） are examined based on the projected large-scale environments derived from a selection of CMIP5（Coupled Model Intercomparison Project Phase 5） models. Specific attention is paid to the performance of the CMIP5 climate models in simulating the large-scale environment for TC development over the WNP. A downscaling system including individual models for simulating the TC track and intensity is used to select the CMIP5 models and to simulate the TC activity in the future.The assessment of the future track and intensity changes of TCs is based on the projected large-scale environment in the21 st century from a selection of nine CMIP5 climate models under the Representative Concentration Pathway 4.5（RCP4.5）scenario. Due to changes in mean steering flows, the influence of TCs over the South China Sea area is projected to decrease,with an increasing number of TCs taking a northwestward track. Changes in prevailing tracks and their contribution to basin-wide intensity change show considerable inter-model variability. The influences of changes in prevailing track make a marked contribution to TC intensity change in some models, tending to counteract the effect of SST warming. This study suggests that attention should be paid to the simulated large-scale environment when assessing the future changes in regional TC activity based on climate models. In addition, the change in prevailing tracks should be considered when assessing future TC intensity change.

Buoyancy of convective-scale updrafts in the outer cores of sheared tropical cyclones

通常认为局地浮力与对流活动息息相关。本文统计分析了不同强度垂直风切变影响下热带气旋外核区对流尺度上升运动的浮力特征。研究发现,弱至中等强度的垂直风切变环境下,顺切变象限的浮力大于逆切变象限的浮力,顺切变象限的总浮力随风切变的增大而减小。而在极端强度的风切变影响下,大部分总浮力为负值。热力浮力、动力浮力与上升运动垂直质量输送没有明显相关性,因此总浮力和垂直质量输送也无显著相关性。上述结果加深了我们对热带气旋外核区对流发生发展过程的物理认识。

NUMERICAL SIMULATION OF SSTA IMPACTS OVER THE GLOBAL OCEAN ON THE ANOMALOUS CIRCULATION OVER EURASIA IN JANUARY 2008

In this paper,we discussed the features of atmospheric circulations over Eurasia as a response to sea surface temperature anomalies (SSTAs) over the tropical Indian Ocean,the equatorial Pacific,Kuroshio and the North Atlantic.Our results are shown as follows: (1) CAM3.0,driven by the combined SSTAs over the four oceanic regions,can simulate well the features of anomalous atmospheric circulations over Eurasia in January 2008,indicating that the effects of the SSTAs over these four regions were one of the key causes of the anomalous systems over Eurasia.(2) The SSTAs over each key region contributed to the intensification of blocking over the Urals Mountains and a main East Asian trough.However,the influence of the SSTAs over individual oceanic regions differed from one another in other aspects.The SSTAs over the North Atlantic had an impact on the 500-hPa anomalous height (Z500A) over the middle-high latitudes and had a somewhat smaller effect over the low latitudes.For the warm SSTAs over Kuroshio,the subtropical high was much stronger,spread farther north than usual,and had an anomalous easterly that dominated the northwest Pacific Ocean.The warm SSTAs over the tropical Indian Ocean could have caused a negative Z500A from West Asia to Middle Asia,a remarkably anomalous southwesterly from the Indian Ocean to the south of China and an anomalous anticyclone circulation over the South China Sea-Philippine Sea region.Because of the La Nia event,the winter monsoon was stronger than normal,with an anomalously cooler northerly over the southeastern coastal areas of China.(3) The combined effects of the SSTAs over the four key regions were likely more important to the atmospheric circulation anomalies of January 2008 over Eurasia than the effects of individual or partly combined SSTAs.This unique SSTA distribution possibly led to the circulation anomalies over Eurasia in January 2008,especially the atmospheric circulation anomalies over the subtropics,which were more similar to those of the winter El Ni?o events than to the circulation anomalies following La Nia.

Revisiting the response of western North Pacific tropical cyclone intensity change to vertical wind shear in different directions

不同方向垂直风切变对热带气旋强度变化的影响存在明显差异,但对热带气旋无维强度变化的影响却差异较小.研究发现不同方向垂直风切变对无维强度变化的影响的细微差异与热带气旋周围的高层西风槽,倒槽,低层季风槽等天气系统配置有关.这一结果表明,在研究不同方向切变对热带气旋强度变化的影响时,除了下垫面热力作用外,还应考虑与切变相关的天气尺度系统的热,动力作用.

Idealized Numerical Simulations of Tropical Cyclone Formation Associated with Monsoon Gyres

Monsoon gyres have been identified as one of the important large-scale circulation patterns associated with tropical cyclone （TC） formation in the western North Pacific.A recent observational analysis indicated that most TCs form near the center of monsoon gyres or at the northeast end of the enhanced low-level southwesterly flows on the southeast-east periphery of monsoon gyres.In the present reported study,idealized numerical experiments were conducted to examine the tropical cyclogenesis associated with Rossby wave energy dispersion with an initial idealized monsoon gyre.The numerical simulations showed that the development of the low-level enhanced southwesterly flows on the southeasteast periphery of monsoon gyres can be induced by Rossby wave energy dispersion.Mesoscale convective systems emerged from the northeast end of the enhanced southwesterly flows with mid-level maximum relative vorticity.The simulated TC formed in the northeast of the monsoon gyre and moved westward towards the center of the monsoon gyre.The numerical experiment with a relatively smaller sized initial monsoon gyre showed the TC forming near the center of the initial monsoon gyre.The results of the present study suggest that Rossby wave energy dispersion can play an important role in TC formation in the presence of monsoon gyres.

Effects of Doubled Carbon Dioxide on Rainfall Responses to Large-Scale Forcing:A Two-Dimensional Cloud-Resolving Modeling Study

ABSTRACT Rainfall responses to doubled atmospheric carbon dioxide concentration were investigated through the analysis of two pairs of two-dimensional cloud-resolving model sensitivity experiments. One pair of experiments simulated pre-summer heavy rainfall over southern China around the summer solstice, whereas the other pair of experiments simulated tropical rainfall around the winter solstice. The analysis of the time and model domain mean heat budget revealed that the enhanced local atmospheric warming was associated with doubled carbon dioxide through the weakened infrared radiative cooling during the summer solstice. The weakened mean pre-summer rainfall corresponded to the weakened mean infrared radiative cooling. Doubled carbon dioxide increased the mean tropical atmospheric warming via the enhanced mean latent heat in correspondence with the strengthened mean infrared radiative cooling during the winter solstice. The enhanced mean tropical rainfall was associated with the increased mean latent heat.

A Relocation-based Initialization Scheme to Improve Track-forecasting of Tropical Cyclones

A relocation procedure to initialize tropical cyclones was developed to improve the representation of the initial conditions and the track forecast for Panasonic Weather Solutions Tropical Operational Forecasts. This scheme separates the vortex perturbation and environment field from the first guess, then relocates the initial vortex perturbations to Lhe observed position by merging them with the environment field. The relationships of wind vector components with stream function and velocity potential are used for separating the vortex disturbance from first guess. For the separation of scalars, a low-pass Barnes filter is employed. The irregular-shaped relocation area corresponding to the specific initial conditions is determined by mapping the edge of the vortex radius in 36 directions.Then, the non-vortex perturbations in the relocation area are removed by a two-pass Barnes filter to retain the vortex perturbations, while the variable fields outside the perimeter of the modified vortex are kept ide.ntical to the original first guess. The potential impacts of this scheme on track forecasts were examined for three hurricane cases in the 2011-12 hurricane season. The experimental results demonstrate that the initialization scheme is able to effectively separate the vortex field from the environment field and maintain a relatively balanced and accurate relocated first guess. As the initial track error is reduced, the following track forecasts are considerably improved. The 72-h average track forecast error was redu,~ed by 32.6% for the cold-start cases, and by 38.4% when using the full-cycling data assimilation because of the accumulatedL improvements from the initialization scheme.

Geometric Characteristics of Tropical Cyclone Eyes before Landfall in South China Based on Ground-Based Radar Observations

The geometric characteristics of tropical cyclone（TC） eyes before landfall in South China are examined using groundbased radar reflectivity. It is found that the median and mean eye area decrease with TC intensity, except for the severe typhoon category, and the eye size increases with height. The increasing rate of eye size is relatively greater in upper layers.Moreover, the ratio of eye size change in the vertical direction does not correlate with TC intensity. No relationship is presented between the ratio of eye size change in the vertical direction and the vertical wind shear. No relationship between the vertical change in eye size and the eye size at a certain level is found, inconsistent with other studies. No relationship exists between the vertical change in eye size and the intensity tendency. The eye roundness values range mainly from 0.5 to 0.7, and more intense TCs generally have eyes that are more circular.

ANALYSIS OF EFFECT OF ENVIRONMENTAL POTENTIAL VORTICITY LOCATED AT SOUTH EDGE OF SOUTH ASIA HIGH ON INTENSIFICATION OF TROPICAL CYCLONES

The NCEP 1°×1°reanalysis of June-to-September dataset between 2002 to 2009 is used in this study to conduct statistical analysis of the relationship between the environmental potential vorticity(PV)on 150 hPa located at the south edge of South Asia High(SAH)and TCs making landfall.The results show that 23 of the TCs are affected by the PV on 150 hPa located at the south edge of SAH between 2002 to2009,and three TCs'center pressure decline after the high-value environmental PV moves to the center of the TCs.These three TCs are Senlaku(0216),Bilis(0604)and Linfa(0903).Through diagnostic analysis from the viewpoint of isolines,we determined the relationship between the intensification of these TCs and the PV anomaly at high levels;the isentropic surface is close to the high level’s PV anomaly under the influence of the 150-hPa PV anomaly,leading to the decline of isentropic surfaces on both sides of the PV anomaly.Then the warm core of the middle and high levels of the TC strengthens and PV increases at the middle level,and both of them are beneficial to the reinforcement of the cyclonic vorticity in the low level.As a result,the center pressure of the TC declines.According to Wu’s theory of Slantwise Vorticity Development(SVD),the incline of the isentropic surfaces leads to the development of vertical vorticity,contributing to the vertical motion and the release of the latent heat.Then the warm core of the TC strengthens and the TC strengthens,too.Otherwise,piecewise PV inversion also shows that the high-level PV influences the mid-level more than the low level.

Prediction of the trajectory of the manned spacecraft SHENZHOU-7 deploying a parachute based on a fine wind field

Given that horizontal wind plays an important role in predicting the trajectory of the manned spacecraft SHENZHOU-7 when employing a parachute, the China Meteorological Administration conducted an experiment involving high-resolution wind ob servation, analysis and prediction between September 21 and 28, 2008. In this work, an algorithm for tracking a spacecraft de ploying a parachute is studied. High-resolution wind profiles obtained from a numerical weather model, upper-atmosphere soundings and mobile incoherent Doppler wind lidar are compared. Forward and backward trajectory predictions based on various wind profiles, as well as their differences, are presented. In addition, the trajectory of SHENZHOU-7 is predicted using different wind profiles, and the predicted parachute-opening and landing points are compared with the observed points. Results indicate that a high-resolution numerical weather model and fine observation data can offer more-detailed wind information for the prediction of spacecraft trajectories and can thus help in the editing and sending of flight commands, consequently increas ing the accuracy and reliability of landing on an assigned spot and reducing the search area and rescue time.

A numerical study of convective-scale downdrafts in the outer core of tropical cyclones in vertically varying environmental flows

The characteristics of convective-scale downdrafts in the outer core of tropical cyclones in the lower-and upper-layer vertical wind shear(VWS)are investigated based on two high-resolution idealized numerical experiments.Four types of outer-core downdrafts,originating from the lower troposphere,the midtroposphere,the upper level,and the tropopause,respectively,are found.The downdrafts originating from the lower and mid troposphere can penetrate down near the surface,and those originating from the tropopause in upper-layer VWS tend to penetrate more downward than in lower-layer VWS.Downdrafts tend to be located in the more upwind portion of the downshear-right quadrant in lower-layer VWS than in upper-layer VWS.The frequency of downdrafts outside and upwind of the parent updraft increases with the increasing downdraft top height.Vertical momentum budgets indicate that downward-oriented buoyancy due to the evaporational cooling of rainwater and precipitation drag mainly contribute to the occurrence of low-level downdrafts,and the midlevel and upper-level downdrafts originate due to precipitation drag and are strengthened by the downward-oriented,buoyancy-induced perturbation pressure gradient.The processes governing the downdrafts from the tropopause are different between the two experiments.More icy-type particles are produced and transported outward at upper levels in the lower-layer shear experiment,resulting in larger downward-oriented buoyancy due to the sublimational cooling of icy-type particles and contributing to the development/maintenance of the downdraft from the tropopause in that experiment.However,the downwardoriented perturbation pressure gradient leads to the development/maintenance of the downdraft from the tropopause in the upper-layer shear experiment.

Relationship of tropical-cyclone-induced remote precipitation with tropical cyclones and the subtropical high

This study concerns the precipitation induced by a tropical cyclone （TC） before the TC arrives, which will be referred to as TC remote precipitation （TRP）. Based on the distribution characteristics of the non-rotational wind and the divergent-wind vertical circulation related to TC, the subtropical high, and TRP of 45 TRP events during June, July, and August of 2000-2009, the relationships among these three entities （TC, subtropical high, and TRP） can be categorized into four patterns. The first pattern accounts for the highest proportion of the TRP events （59%）, and a conceptual model is then provided for this pattern. The primary characteristics of this model are as follows： TC, the subtropical high, and TRP can interact with each other through the divergent-wind secondary circulation at both sides of the ridge line of the subtropical high （between the subtropical high and TC, and between the subtropical high and TRP）. At the upper level （150 or 200 hPa）, the northward non-rotational wind from the TC converged toward the subtropical high ridge line and subsided, and at 950 hPa the divergent wind from the ridge line of the subtropical high converged toward TC; these constructed the secondary circulation between TC and the subtropical high. In the meantime, the southward non- rotational wind at the upper level （150 or 200hPa） from TRP and the divergent wind at 950hPa from the subtropical high ridge line toward TRP constructed the secondary circulation between TRP and the subtropical high. As TC and TRP interacted with each other, the subtropical high ridge line was usually under the down- draft area of the whole atmosphere. The other three patterns are different from the first pattern mainly in terms of the intensity and position of the non-rotational-wind secondary circulation. The numerical simulation of the Beijing 7.21 rainstorm confirmed the relationship amongTC, the subtropical high, and TRP, indicating that when the interaction weakened, the TRP also weakened.

Tropical Cyclone Tilts Under Vertically Varying Background Flows:Preliminary Results Based upon TCM4 Simulations

The characteristics of tropical cyclone(TC) tilts under vertically varying background flows were preliminarily examined in this study based on numerical simulations with the Tropical Cyclone Model version 4(TCM4).The tilt magnitudes presented a linearly decreasing tendency in the simulation with the environmental wind speed vertically varying throughout the troposphere and in the simulation with the vertical wind shear concentrated in the lower troposphere,while the vortex tilt showed a linearly increasing tendency in magnitude in the simulation where the vertical shear was concentrated in the upper troposphere.The change in tilt magnitude was found to be related to the evolution of the penetration depth near the eyewall.When the shear was concentrated in the lower troposphere,the vortex tended to tilt downshear right during the early integration and underwent more precession processes.When the shear was concentrated in the upper troposphere,the vortex rapidly tilted downshear left during the early simulation and vortex precession was less frequently observed.The storms simulated in all experiments were finally in downshear-left tilt equilibrium.

Re-examining Tropical Cyclone Fullness Using Aircraft Reconnaissance Data

We use FLIGHT+ aircraft reconnaissance data for tropical cyclones(TCs) in the North Atlantic and Eastern Pacific from 1997 to 2015 to re-examine TC fullness(TCF) characteristics at the flight level.The results show a strong positive correlation between the flight-level TCF and the intensity of TCs,with the flight-level TCF increasing much more rapidly than the near-surface TCF with increasing intensity of the TCs.The tangential wind in small-TCF hurricanes is statistically significantly stronger near the eye center than that in large-TCF hurricanes.Large-TCF hurricanes have a ring-like vorticity structure.No significant correlation is observed between the flight-level TCF and the comparative extent of the vorticityskirt region occupied in the outer core skirt.The proportion of the rapid filamentation zone in the outer core skirt increases with increasing flight-level TCF.The differences in entropy between the radius of the maximum wind and the outer boundary of the outer core skirt also increase with increasing flight-level TCF.