THE IMPACT OF RADIATIVE PROCESSES ON THE ONSET OF SUMMER MONSOON——NUMERICAL EXPERIMENTS

Two numerical experiments were performed by using ECMWF limited area model in order to assess theimpact of radiative processes on the onset of summer monsoon.The control experiment includes all model physicsand the other experiment is exclusive of radiative processes.The simulated results show that the radiativeprocesses enhanced the monsoon circulation and the associated vertical circulation over South Asia.Especiallyover the Qinghai-Xizang Plateau,the build-up of the plateau monsoon depression and the beginning of the plat-eau rainy season are highly dependent upon the radiative processes.Through the radiation-cloud interaction,the “moist processes” were enhanced over land.More latent heatwas released.The feedback of the physical processes caused the strengthening of the monsoon circulation oversouthern Asia.     

A MESO-α SCALE STUDY OF MEIYU FRONT HEAVY RAIN——PART I:OBSERVATIONAL STUDIES

Jn this paper, the data collected during the Mesoscale Weather Experiments in East China are utilized to study the meso-a scale rain-bands of meiyu front heavy rain, its structural features as well as the mechanism of its development. It has been revealed that the precipitation band during the meiyu season is in the shape of ribbon, which is parallel to the surface quasi-stationary front. Sometimes two meso-a scale rain-bands are present. The meso-a scale rain-band is associated with meso-a scale convergence line. As shown by the two dimentional disturbance circulation, calculated through band-pass filtering, the single rain-band is quite different from the double rain-bands. The former is, to some extent, akin to the frontogenetical circulation in the vicinity of the high- and low-level frontal zones; the latter features roller-like circulations at middle and low-levels with their axes parallel to the rain-bands while at higher levels they run in the opposite direction. This kind of disturbance may be     

THE INTRASEASONAL OSCILLATION AND ITS INTERANNUAL VARIABILITY—A SIMULATION STUDY

The atmospheric intraseasonal oscillation(ISO)and its interannual variability are simulatedby the atmospheric general circulation model,which was developed at the Institute of AtmosphericPhysics.Two numerical experiments were performed,corresponding to the AMIP-Ⅰ and AMIP-Ⅱsimulations,respectively.The model reasonably reproduces the major aspects of the intraseasonaloscillation,including the propagating property and the seasonal differences in the tropics,thewavenumber structure of ISO in the globe,and the global coincidence in the interannual variationof ISO.Comparison of the results between the two experiments suggests that improvement of theboundary forcing or considering the air-sea interaction may help to improve the simulation on theISO and its interannual variability.     

A GLOBAL MODEL WITH OVERLAPPING MERCATOR AND STEREOGRAPHIC GRIDS

A global nine-layer primitive equation model is developed to investigate the dynamic and thermodynamic influences of plateaus and high mountains on the atmospheric circulation. Besides topography, the effects of solar radiation, longwave radiation, large-scale condensation, cumulus convection and surface fluxes of heat momentum are also included in the model. In order that the finite-difference approximations represent fairly accurately the circulation in both higher and lower latitudes, we use Mercator projection in lower latitudes and Stereographic projection in higher latitudes.The simulated sea level pressure, wind field, precipitation distribution and vertical circulation in summer and winter are given, respectively, and they are compared with the observations.     

A CASE STUDY ON THE RELATIONSHIP BETWEEN A PRECEDING LA NINA EVENT AND EAST ASIAN SUMMER ATMOSPHERIC CIRCULATION*

This study examined the relationship between mature phase of the cold event in 1999 and the East Asian summer atmosphere circulation in 2000.The cold event reaches its mature phase in the autumn and winter of 1999,which is the strongest La Nina episode in recent 11 years.There is a clear anomalous pattern of the atmosphere circulation around East Asia in the summer of 2000, i.e.the negative anomaly centers around the Cherski Mountains and 20°N,170°E at 500 hPa,the main body of the subtropical high keeps in further northern position than usual and the negative anomaly of precipitation located in southern central China.This pattern is thought as the response to the preceding strong cold event in autumn of 1999.It is also identified that the response of the East Asian atmosphere circulation in summer of 2000 to the strong La Nina event in 1999 belongs to the top rank in recent 43 years.On the other hand,the inactive blocking anticyclone around East Asia in summer of 2000 is associated with the positive SST anomaly and the 850 hPa temperature anomaly around the Bering Sea simultaneously.Nevertheless,although the impact on the summer atmosphere circulation around East Asia from La Nina events could not rank with that from E1 Nino,the impact could not be neglected especially in a strong La Nina case.     

ODD AND EVEN SYMMETRY OF ATMOSPHERIC CIRCULATION—THEORETICAL BASIS AND CLIMATIC CHARACTERISTICS

Analysis is done of the distribution of odd and even symmetric components of circulations on a global basis in termsof observations and technique for the odd/even symmetry,indicating that climatic features of the component patternsand their temporal evolution are able to reveal their influence of land-sea discrepancy in the Northern and SouthernHemispheres,the time scale of atmospheric response to radiation heating,circulation waveform structure and seasonaladjustment of global circulation.     

A NUMERICAL EXPERIMENT OF CYCLOGENESIS AND THE DEVELOPMENT OF DISTURBANCES

A numerical experiment has been carried out to study the mechanism of cyclogenesis and the development of disturbances. First, an idealized field of temperature and pressure is designed, which is very similar to the actual synoptic situation, consisting of the jet-like zonal circulation with some nonzonal waves superimposed on it. Prediction is made by using a six-level splitting primitive model with the idealized field as an initial one. The results show that if a disturbance like a frontal zone in the lower troposphere is superimposed on the zonal circulation, a frontal cyclone will quickly develop and then gradually become an occluded one. Its life cycle is similar to that of the actual frontal cyclone on the synoptic map. However, if there is a disturbance superimposed on the zonal circulation in the middle troposphere, the cyclone with weaker intensity will be slowly formed near the surface. Finally, if the initial disturbance is located at the high-level, a situation like a cut-off low rather than     

INTERDECADAL VARIABILITY IN A MODEL ATMOSPHERE

By using the simulation results of an AGCM, which had been run from 1945 to 1993 forced by COADS SST, the interdecadal variability of the model atmosphere was investigated and compared with that of NCEP reanalysis data. It was found that, interdecadal variability exists significantly in both the tropical Pacific wind fields and the mid-high latitude atmospheric circulation of the model atmosphere. The tendency of time variation and spatial distributions of the interdecadal variability of the model atmosphere are basically consistent with observation. Relative to the mid-high latitude atmospheric circulation, the simulation of tropical Pacific wind is more satisfying, which suggests that anomalous variation of SST is still the main factor for the interdecadal variability of tropical Pacific wind. It might have more significant influence on the tropical wind than on the mid-high latitude atmosphere. However, there is still obvious difference between the simulation and observation. They could be attributed to both the simulation capability of the model and absence of other factors in the model which are important for the interdecadal climate variation.     

Simulation of Tropical Cyclogenesis in Association with Large-Scale Cyclonic Circulation over the Western North Pacific

The aim of this study is to examine the difference in tropical cyclone(TC) formation in different cyclonic circulation locations using a mesoscale model on a beta plane. A weak initial vortex is imposed at different positions in a cyclonic circulation. Numerical experiments indicate that the tropical disturbances located in the center and northeastern parts of the cyclonic circulation are favorable to TC formation, while those located in the south of the cyclonic circulation are unfavorable. Since the asymmetric circulation induced by the beta effect peaks in the northeastern quadrant of the vortex, when the initial vortex is placed in the southern part of the cyclonic circulation, the vortex begins to develop in the south due to the effect of the westerly wind of the cyclonic circulation. The westerly wind of the cyclonic circulation gradually decreases and the vortex is contributed mainly by the beta effect afterwards. Thus, establishment of the convection–circulation–moisture positive feedback is delayed, unfavorable to TC rapid development. On the contrary, when the initial vortex is placed in the northern part of the cyclonic circulation, the superposition of the beta gyres and easterly wind of the cyclonic circulation induces stronger wavenumber-1 wind in the northeastern part of the vortex. The greater asymmetric wind is closely associated with the symmetric wind through energy conversion, thus accelerating a positive feedback and facilitating vortex development into a stronger TC. Meanwhile, when the initial vortex is placed in the center and eastern parts of the cyclonic circulation, the vortex develops a little slower than when it is placed in the northern part, but stronger than when placed in the southern part.     

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 hascentral 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 partitionedinto 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 contributionsfrom 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 ascentof air and large-scale heating would work.The cumulus heating and the transfer of momentum and heatin the area of cumulus play an important role during the explosively deepening stage.Thermal advection isthe initial triggering condition for large-scale heating and the conditional instability for the convection ofcumulus.     

THE MEANDER OF KUROSHIO AND OSCILLATION IN A COUPLED OCEAN-ATMOSPHERE MODEL

A numerical experiment of an asynchronous coupled ocean-atmosphere model has been described in this paper.Atwo-layer global atmosphere general circulation model(OSU/IAP-AGCM)and a two-layer North Pacific Oceangeneral circulation model(NPOGCM)developed by Liu et al.(1992)are used in numerical experiment.The sea surfacetemperature anomaly(SSTA)corresponding to the meander of the Kuroshio is treated as the initial perturbation in thePacific Ocean and the abnormal phenomena caused by the disturbance and the interaction between atmosphere andocean,have been studied.The numerical experiment showed that the SST anomaly in the North Pacific could induce a new 30—60 dayoscillation through the coupling between atmosphere and ocean and the interaction between the meander of theKuroshio and atmosphere circulation is a positive feedback process.     

Numerical modellings of properties of the summer quasi-stationary circulation systems and their monthly variations

An ocean-atmosphere and land-air coupled numerical model system is used to study the basic properties and the monthly time variations of the summer quasi-stationary circulation systems. It is found that either at the upper or at the lower levels of the atmosphere, the circulation patterns have a two-wave structure in the zonal direction at the mid and high latitudes of the Northern Hemisphere. Such a structure of circulation is totally matchable to that of the land-sea distribution there. It is proved, hence, that the land-sea distributive pattern is the fundamental cause for the summer quasi-stationary circulation pattern. The topography in the globe is the secondary factor for circulation systems. The circulation centres of the quasi-stationary systems are always located in certain areas due to the thermodynamic contrast between land and sea.From the time evolutions of the circulation systems it is seen that the change is larger at the beginning period of the time integration, it is because of using t     

WAVE BOUNDARY BETWEEN MIDDLE-AND-LOW AND MIDDLE-AND-HIGH LATITUDE CIRCULATIONS, AND SEASONAL TRANSFORMATION OF NORTHERN-HEMISPHERE MEAN CIRCULATION

The definition for stability D of standing waves given in this paper is used to denote the relative magnitude of annual variation of the waves. Analysis of the temporal (seasonal) and spatial (meridional) changes of the monthly mean circulation at 500 and 100 hPa shows its temporal and spatial demarcation and the bourdary between the middle-and-low latitude circulation and the middle-and-high latitude circulation in the wave field. Based on the annual march of D and the position of the boundary, a discussion is made of the seasonal transformation of the Northern-Hemisphere mean circulation and the pattern of its development.     

Interdecadal Enhancement of the Walker Circulation over the Tropical Pacific in the Late 1990s

The Walker circulation is one of the major components of the large-scale tropical atmospheric circulation and variations in its strength are critical to equatorial Pacific Ocean circulation.It has been argued in the literature that during the 20th century the Walker circulation weakened,and that this weakening was attributable to anthropogenic climate change.By using updated observations,we show that there has been a rapid interdecadal enhancement of the Walker circulation since the late 1990s.Associated with this enhancement is enhanced precipitation in the tropical western Pacific,anomalous westerlies in the upper troposphere,descent in the central and eastern tropical Pacific,and anomalous surface easterlies in the western and central tropical Pacific.The characteristics of associated oceanic changes are a strengthened thermocline slope and an enhanced zonal SST gradient across the tropical Pacific.Many characteristics of these changes are similar to those associated with the mid-1970s climate shift with an opposite sign.We also show that the interdecadal variability of the Walker circulation in the tropical Pacific is inversely correlated to the interdecadal variability of the zonal circulation in the tropical Atlantic.An enhancement of the Walker circulation in the tropical Pacific is associated with a weakening zonal circulation in the tropical Atlantic and vise versa,implying an inter-Atlantic-Pacific connection of the zonal overturning circulation variation.Whether these recent changes will be sustained is not yet clear,but our research highlights the importance of understanding the interdecadal variability,as well as the long-term trends,that influence tropical circulation.     

IMPACT OF HUMIDITY DATA ON THE PREDICTION OF ONSET VORTEX WITH A LIMITED AREA MODEL

A limited area model has been applied to study the impact of satellite-derived relative humidity data on the predic-tion of onset vortex of monsoon 1979.The results show that inclusion of satellite-derived relative humidity data im-proved the prediction of track of the cyclonic circulation and the rainfall rates in the region of the vortex.     

A Coupling Experiment of an Atmosphere and an Ocean Model with a Monthly Anomaly Exchange Scheme

A nine-layer spectral atmospheric general circulation model is coupled to a twenty-layer global oceanic general circulation model with the “prediction-correction” monthly anomaly exchange scheme which has been proposed at the Institute of Atmospheric Physics (IAP). A forty-year integration of the coupled model shows that the CGCM is fairly successful in keeping a reasonable pattern of the modelled SST although most of the Pacific become warmer than those given by the uncoupled ocean model. The model tends to reach a more realistic state than the uncoupled one in terms of downward surface heat flux into ocean particularly in the equatorial Pacific region. Also, the model is capable to simulate interannual variability of sea surface temperature in tropical region.     

ABRUPT SEASONAL CHANGE OF ZONAL CIRCULATION OVER THE TIBETAN PLATEAU

The abrupt changes of zonal circulation in the Tibetan Plateau (TP) region and their likely causes are derived from National Centers for Environmental Prediction and the National Center for Atmospheric Research reanalysis data. The zonal circulation over the TP abruptly changed in summer (31st pentad) and winter (59th pentad). The switch from summer to winter circulation is characterized by a sudden northward shift of the westerlies and the zero-velocity curve and disappearance of the westerly jet. The winter–summer switch is characterized by the reverse pattern. Therefore, the circulation conversion between summer and winter can be judged from the position of the zero-velocity curve. Curves located north of 20 °N indicate summer circulation over the TP and vice versa. The abrupt change of zonal circulation is mainly caused by the thermodynamic effect of the TP. In June, this effect causes a huge monsoon circulation cell extending from the TP to low latitudes. Consequently, the westerlies jump to the north as easterlies develop. This process, which is enhanced by the strong northerly in Coriolis, establishes the summer circulation. In October, the Hadley cell recurs as the thermal effects of the TP diminish, the westerlies rush southward, and the winter circulation is established.     

Transient Characteristics of Residual Meridional Circulation during Stratospheric Sudden Warming

The residual meridional circulation derived from the transformed Eulerian-mean thermodynamic equation and continuity equation can be separated into two parts,the slowly varying diabatic circulation and the transient circulation,as demonstrated by others.We calculated and composite-analyzed the transient and diabatic circulation for 14 stratospheric sudden warming(SSW) events from 1979-2002 by using the daily ECMWF reanalysis data.Specifically,the transient residual meridional circulation was calculated both with and without inclusion of the eddy heat transport term in the transformed Eulerian-mean thermodynamic equation to investigate the importance of the eddy heat transport term.The results showed that calculations of transient residual meridional circulation present rapid variations during SSWs,with or without inclusion of the eddy heat transport term.Although the patterns of transient residual meridional circulation with the eddy heat transport term were similar to that without the eddy heat transport term during SSW,the magnitudes in the upper stratosphere and high-latitude regions differed.As for the diabatic circulation,its daily variations were small during SSW events,and its patterns were in agreement with its monthly average.     

A Diagnostic Analysis of Winter Atmospheric Circulation during the 1982-1983 ENSO Event

In this paper, the winter atmospheric circulation, the convection along the equator and their variations of 1982 and 1983 are investigated. It is suggested that there was a well organized three dimensional structure of anomalies of the atmospheric circulations during 1982 winter which may be related to the variations of the convection in the equatorial region.     

CMIP5 simulated change in the intensity of the Hadley and Walker circulations from the perspective of velocity potential

Based on the simulations of 31 global models in CMIP5, the performance of the models in simulating the Hadley and Walker circulations is evaluated. In addition, their change in intensity by the end of the 21 st century(2080–2099) under the RCP4.5 and RCP8.5 scenarios, relative to 1986–2005, is analyzed from the perspective of 200 h Pa velocity potential.Validation shows good performance of the individual CMIP5 models and the multi-model ensemble mean(MME) in reproducing the meridional(zonal) structure and magnitude of Hadley(Walker) circulation. The MME can also capture the observed strengthening tendency of the winter Hadley circulation and weakening tendency of the Walker circulation. Such secular trends can be simulated by 39% and 74% of the models, respectively. The MME projection indicates that the winter Hadley circulation and the Walker circulation will weaken under both scenarios by the end of the 21 st century. The weakening amplitude is larger under RCP8.5 than RCP4.5, due to stronger external forcing. The majority of the CMIP5 models show the same projection as the MME. However, for the summer Hadley circulation, the MME shows little change under RCP4.5 and large intermodel spread is apparent. Around half of the models project an increase, and the other half project a decrease. Under the RCP8.5 scenario, the MME and 65% of the models project a weakening of the summer southern Hadley circulation.