DEVELOPMENT OF A COUPLED REGIONAL AIR-SEA MODEL AND ITS NUMERICAL SIMULATION FOR SUMMER MONSOON IN 1998*

A coupled regional air-sea model is developed by using the regional climate model (P-σ RCM) and the regional ocean model (POM),which is used to simulate East Asian monsoon and oceanic elements in East Asian coastal waters.The simulated surface layer oceanic elements are basically consistent with the reality and can reflect the interaction between the monsoon and the surface layer currents.The great difference with the reality is "cold drift" of the simulated surface temperature.The coupled model has certain ability to simulate the atmosphere geopotential height fields,precipitation and low-level southwest wind from May to August in 1998.It can display the process of summer monsoon onset during the third dekad of May and the evolution features after the onset.The differences between the simulation results of the coupled model and that of the single P-σ RCM are shown mainly in the low-level atmosphere and the model internal regions.     

DEVELOPMENT OF A COUPLED REGIONAL AIR-SEA MODEL AND ITS NUMERICAL SIMULATION FOR SUMMER MONSOON IN 1998

A coupled regional air-sea model is developed by using the regional climate model (P-σ RCM)and the regional ocean model (POM),which is used to simulate East Asian monsoon and oceanicelements in East Asian coastal waters.The simulated surface layer oceanic elements are basicallyconsistent with the reality and can reflect the interaction between the monsoon and the surfacelayer currents.The great difference with the reality is “cold drift” of the simulated surfacetemperature.The coupled model has certain ability to simulate the atmosphere geopotential heightfields,precipitation and low-level southwest wind from May to August in 1998.It can display theprocess of summer monsoon onset during the third dekad of May and the evolution features afterthe onset.The differences between the simulation results of the coupled model and that of thesingle P-a RCM are shown mainly in the low-level atmosphere and the model internal regions.     

NUMERICAL SIMULATION OF THE TROPICAL INTRASEASONAL OSCILLATION AND THE EFFECT OF WARM SST

An atmospheric general circulation model is used in a series of three experiments to simulate the intraseasonaloscillation in the tropical atmosphere.Analyses of the model daily data show that various physical variables,from sever-al different regions,exhibit fluctuations with a spectral peak between 30 and 60 days.This represents a 30—60 dayoscillation in the tropical atmosphere and possesses several features which are consistent with observations.These in-clude a horizontal structure dominated by zonal wavenumber 1 and a vertical structure which is predominantlybaroclinic.The effect of warm SST (sea surface temperature) anomalies on the 30—60 day oscillation in the tropical atmos-phere is also simulated by prescribing global SST as observed in 1983.This has the effect of weakening the oscillationwhile at the same time the vertical structure becomes less baroclinic.The importance of cumulus convection to the propagational characteristics of this oscillation is demonstrated by acomparison of results based on different parameterizations for convection.In one case,where the maximum convectionover the Pacific is simulated to be too far east,the simulated 30—60 day oscillation shows evidence of westward propa-gation.In the second case,where the convection maximum is located near the observed position in the western Pacific,there is more clearly evidence of eastward propagation.Both results suggest that the location of maximum convection in the Pacific can have an important influence on thestrength,structure and propagation of the 30—60 day oscillation.     

The climatic effects of the stratospheric volcanic ash

The climatic effects of the stratospheric volcanic ash are simulated. The model we used is a primitive equation model with the P-σ incorporated coordinate system. The model has 5 layers in the atmosphere and 2 layers in the soil. The volcanic ash is introduced to the first (highest) model layer with a fixed optical thickness of 0.1275. Two comparative numerical experiments with and without the volcanic ash are made. Results show that the effects of the stratospheric volcanic ash on the formations of the mean climatic fields are much smaller than those of the land-sea distribution and the large scale topography. However, it does have contributions to the anomalies of the basic climatic states. The direct effect of the volcanic ash is to increase the temperature in the stratosphere. It can also influence the temperature and the height fields of isobaric surfaces, horizontal and vertical motions, precipitation and the surface climate through dynamic and thermodynamic processes in the atmosphere.     

The Climatic Effects of the Stratospheric Volcanic Ash

The climatic effects of the stratospheric volcanic ash are simulated. The model we used is a primitive equation model with the P-σ incorporated coordinate system. The model has 5 layers in the atmosphere and 2 layers in the soil. The volcanic ash is introduced to the first (highest) model layer with a fixed optical thickness of 0.1275. Two comparative numerical experiments with and without the volcanic ash are made. Results show that the effects of the stratospheric volcanic ash on the formations of the mean climatic fields are much smaller than those of the land-sea distribution and the large scale topography. However, it does have contributions to the anomalies of the basic climatic states. The direct effect of the volcanic ash is to increase the temperature in the stratosphere. It can also influence the temperature and the height fields of isobaric surfaces, horizontal and vertical motions, precipitation and the surface climate through dynamic and thermodynamic processes in the atmosphere.     

The Genesis of Tropical Cyclone Bilis (2000) Associated with Cross-equatorial Surges

The purpose of this paper is to explore how a tropical cyclone forms from a pre-existing large-scale depression which has been observed and associated with cross-equatorial surges in the western North Pacific. Tropical cyclone Bilis(2000) was selected as the case to study.The research data used are from the results of the non-hydrostatic mesoscale model(MM5),which has successfully simulated the transformation of a pre-existing weak large-scale tropical depression into a strong tropical storm.The scale separation technique is used to separate the synoptic-scale and sub-synoptic-scale fields from the model output fields. The scale-separated fields show that the pre-existing synoptic-scale tropical depression and the subsynoptic scale tropical cyclone formed later were different scale systems from beginning to end.It is also shown that the pre-existing synoptic-scale tropical depression did not contract to become the tropical cyclone. A series of weak,sub-synoptic-scale low and high pressure systems appeared and disappeared in the synopticscale depression,with one of the low systems near the center of the synoptic-scale depression having deepened to become the tropical cyclone. The roles of the synoptic-scale flow and the sub-synoptic scale disturbances in the formation of the tropical cyclone are investigated by diagnoses of the scale-separated vertical vorticity equation.The results show that the early development of the sub-synoptic scale vortex was fundamentally dependent on the strengthening synoptic-scale environmental depression.The depression was strengthened by cross-equatorial surges,which increased the convergence of the synoptic-scale depression at low levels and triggered the formation of the tropical cyclone.     

DIAGNOSTIC STUDY FOR INTRASEASONAL OSCILLATION OF THE MIDDLE-HIGH LATITUDES IN A LOW RESOLUTION GLOBAL SPECTRAL MODEL

In this paper, five-year simulated data from a low-resolution global spectral model with triangular trunca-lion at wavenumber 10 are analyzed in order to study dynamical features and propagation characteristics ofintraseasonal oxillations over the mid-latitudes and the tropical atmosphere. The simulations show that thereis the 30-50 day periodic oscillation in the low-resolution spectral model without non-seasonal external forcing,and spatial scale of the intraseasonal oscihations is of the globe .Further analysis finds that propagation charac-ters of intraseasonal oscillations over the mid-latitudes and the tropics are different. The 30-50 day oscillationover the tropics exhibits structure of the velocity potential wave with wavenumber 1 in the latitudinal and thecharacter of the traveling wave eastward at speed of 8 longitudes/day. However, the 30-50 day oscillationsin mid-latitude atmosphere exhibit phase and amplitude oscillation of the standing planetary waves and theyare related to transform of teleconnection patterns over the mid-latitudes. The energy is not only transferredbetween the tropics and the middle-high latitudes, but also between different regions over the tropics. Based on the analysis of 5-year band pass filtered data from a 5-layer global spectral model of Jow-ordetwith truncated wavenumber l0,investigation is done of the source of intraseasonal oscillations in the extratropicalmodel atmosphere and its mechanism. Results show that (1) the convective heat transferred eastward alongthe equator serves as the source of the intraseasonal oxillation both in the tropical and the extratropical atmos--phere; (2) the velocity-potential wave of a zonal structure of wavenumber 1 gives rise to oxillation in divergentand convergent wind fields of a dipole-form as seen from the equatorial Indian Ocean to the western Pacificduring its eastward propagation, thus indicating the oscillation in the dipole-form heat soure:e/sink pattertl; (3)the tropical heat-source oscillation is responsible for the variation in phase and intensity of the extratropicalstationary wave train, and the interaction between the oscillating low-frequency inertial gravity and stationaryRossby modes that are probably mechanisms for the oscillations ip the middle-high latitudes.     

Numerical Modelings of the Climatic Effects of the Land-Sea Distribution and Topography

The effects of the land-sea contrast and the topography on the climatic properties are simulated in this paper by use of a p- σ incorporated coordinate system model in a zonal domain. In this paper we firstly discuss the statistical features of the model and find that the capability of the model is stable, with the same land-sea distribution and to-pography seven monthly mean climate states are close to one another, their variance is even less than the initial one. Secondly, we focally discuss the effects of the land-sea contrast and the topography on the modeled climate fields, It is pointed out that the land-sea contrast and the topography influence the atmosphere mainly through the heating ef-fect and the former has larger influences on the simulated large scale climate fields than the latter.     

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.     

Modeling the Tropical Pacific Ocean Using a Regional Coupled Climate Model

A high-resolution tropical Pacific general circulation model (GCM) coupled to a global atmospheric GCM is described in this paper. The atmosphere component is the 5°×4°global general circulation model of the Institute of Atmospheric Physics (IAP) with 9 levels in the vertical direction. The ocean component with a horizontal resolution of 0.5°, is based on a low-resolution model (2°×1°in longitude-latitude).Simulations of the ocean component are first compared with its previous version. Results show that the enhanced ocean horizontal resolution allows an improved ocean state to be simulated; this involves (1) an apparent decrease in errors in the tropical Pacific cold tongue region, which exists in many ocean models,(2) more realistic large-scale flows, and (3) an improved ability to simulate the interannual variability and a reduced root mean square error (RMSE) in a long time integration. In coupling these component models, a monthly "linear-regression" method is employed to correct the model's exchanged flux between the sea and the atmosphere. A 100-year integration conducted with the coupled GCM (CGCM) shows the effectiveness of such a method in reducing climate drift. Results from years 70 to 100 are described.The model produces a reasonably realistic annual cycle of equatorial SST. The large SSTA is confined to the eastern equatorial Pacific with little propagation. Irregular warm and cold events alternate with a broad spectrum of periods between 24 and 50 months, which is very realistic. But the simulated variability is weaker than the observed and is also asymmetric in the sense of the amplitude of the warm and cold events.     

Analysis on the 10-yr Simulation of the Meiyu Precipitation and Its Associated Circulation with the GAMIL Model

The major features of Meiyu precipitation and associated circulation systems simulated by the grid-point atmospheric model of IAP LASG (GAMIL) with Zhang-McFarlane and Tiedtke cumulus parameterization schemes are examined in this paper. The results show that the model with both schemes can reproduce the heavy precipitation center over the Yangtze-Huai River Basin (YHRB) during the Meiyu period. The horizontal and vertical structures of the circulation systems during the Meiyu period are also well simulated,such as the intensive meridional gradients of moisture and μse (pseudo-equivalent temperature), the strong low-level southwesterly flow in the lower troposphere over East China, the location of the westerly jet stream in the upper troposphere, the strong ascending motion in heavy precipitation zone, and compensation downward motion on the northern and southern sides of the heavy precipitation belt. However, obvious discrepancies occur in the simulated temperature field in the mid-lower troposphere,especially with the Zhang-McFarlane scheme. In addition, the simulated Meiyu period (onset and duration) is found to be associated with the temperature difference in the lower atmosphere over the land and ocean, and with the cumulus parameterization schemes. The land-sea thermal contrast (LSTC) simulated by the Zhang-McFarlane scheme increases faster than that in the reanalysis from April to July, and changes from negative to positive at the end of May. Consequently, the simulated Meiyu onset begins in May, one month earlier than the observation. On the other hand, since the LSTC simulated by the Tiedtke scheme is in agreement with the reanalysis during June and July, the simulated Meiyu period is similar to the observation. The different LSTCs simulated by the GAMIL model with the two cumulus parameterization schemes may affect the Meiyu period simulations. Therefore, it is necessary to refine the cumulus parameterization scheme in order to improve the Meiyu precipitation simulation by the GAMIL model.     

一个灵活的海洋——大气耦合环流模式

Based on the National Center for Atmospheric Research (NCAR) Climate System Model version 1(CSM-1), a Flexible coupled General Circulation Model version 0 (FGCM-0) is developed in this study through replacing CSM-1＇s oceanic component model with IAP L30T63 global oceanic general circulation model and some necessary modifications of the other component models. After the coupled model FGCM--0 is spun up for dozens of years, it has been run for 60 years without flux correction. The model does not only show the reasonable long-term mean climatology, but also reproduce a lot of features of the interannual variability of climate, e.g. the ENSO-like events in the tropical Pacific Ocean and the dipole mode pattern in the tropical Indian Ocean. Comparing FGCM-0 with the NCAR CSM-1, some common features are found, e.g. the overestimation of sea ice in the North Pacific and the simulated double ITCZ etc.The further analyses suggest that they may be attributed to errors in the atmospheric model.     

THE THREE-LEVEL GLOBAL ATMOSPHERIC GENERAL CIRCULATION MODEL:FORMULATION AND RESULTS

The“climate draft”often occurs in the coupling process of the atmospheric general circulation model (AGCM) andoceanic general circulation model (OGCM).One of the main methods to overcome the“climate draft”is to simulate theflow and temperature fields in the low-layer correctly.Therefore we designed a three-level AGCM including a planeta-ry boundary layer (PBL) and have run it seven model years to do climate simulation.The results show that the simulatedlower level air flow,surface air temperature and sea-level pressure in January and July,approximate to the climate av-erage fields,especially in Asian monsoon area.The simulated upper level flow and geopotential height are also in betteragreement with the observed fields.Moreover,the two westerly jets over the northern and southern sides of theQinghai-Xizang Plateau in winter,the disappearance of its southern subtropical jet during the seasonal transition fromspring to summer,the establishment of the two easterly jets near the equator and over the subtropical region during theseasonal transition,are also simulated well.In the mainland of China,the seasonal abrupt shift of the rainfall belt,suchas the Meiyu belt in South China during April to May,which jumps to the Changjiang River region in June,again jumpsback to the north China in July,and rapidly withdraws to the south in August,are simulated very well.Now we arecoupling this model to a global six-level OGCM and nesting a fine mesh (1°×1.25°)regional climate model over Chinaarea with it.     

A THEORETICAL STUDY ON THE MULTIPLE EQUILIBRIA OF TROPICAL ATMOSPHERE AND THEIR RELATION TO THE ONSET OF THE SUMMER MONSOON

Multiple equilibria and their stability in tropical atmosphere are investigated through β-plane barotropic models with consideration of heating and dissipation. We have derived the solutions of the model equations corresponding to the multiple equilibria or the steady flows first, and then establish the criteria for the stability of steady flow by use of the Liapunov direct Method. When these criteria are applied to the solutions of equilibria obtained, stable flows, which are closely related to the different patterns of quasi-stationary circulation in the tropical region, are found. The configurations of these stable flows and the shift between two of them as season changes provide quite reasonable explanations to many fundamental problems of tropical circulation features such as the catastrophe mechanism of the onset and the break-active cycle of the Asian summer monsoon. It follows that the onset or the abrupt transition of the Asian summer monsoon could be attributed to the multiple equilibrium property of the tropical circulation resulted from the advective nonlinearity, which provide another explanation among others.     

FIO-ESM v2.0 Outputs for the CMIP6 Global Monsoons Model Intercomparison Project Experiments

Three tiers of experiments in the Global Monsoons Model Intercomparison Project(GMMIP), one of the endorsed model intercomparison projects of phase 6 of the Coupled Model Intercomparison Project(CMIP6), are implemented by the First Institute of Oceanography Earth System Model version 2(FIO-ESM v2.0), following the GMMIP protocols.Evaluation of global mean surface air temperature from 1870 to 2014 and climatological precipitation(1979–2014) in tier-1 shows that the atmosphere model of FIO-ESM v2.0 can reproduce the basic observed atmospheric features. In tier-2, the internal variability is captured by the coupled model, with the SST restoring to the model climatology plus the observed anomalies in the tropical Pacific and North Atlantic. Simulation of the Northern Hemisphere summer monsoon circulation is significantly improved by the SST restoration in the North Atlantic. In tier-3, five orographic perturbation experiments are conducted covering the period 1979–2014 by modifying the surface elevation or vertical heating in the prescribed region. In particular, the strength of the South Asian summer monsoon is reduced by removing the topography or thermal forcing above 500 m over the Asian continent. Monthly and daily simulated outputs of FIO-ESM v2.0 are provided through the Earth System Grid Federation(ESGF) node to contribute to a better understanding of the global monsoon system.     

A Further Inquiry on the Mechanism of 30-60 Day Oscillation in the Tropical Atmosphere

In a simple semi-geostropic model on the equatorial β-plane, the theoretical analysis on the 30-60 day oscillation in the tropical atmosphere is further discussed based on the wave-CISK mechanism. The convection heat-ing can excite the CISK-Kelvin wave and CISK-Rossby wave in the tropical atmosphere and they are all the low-frequency modes which drive the activities of 30-60 day oscillation in the tropics. The most favorable conditions to excite the CISK-Kelvin wave and CISK-Rossby wave are indicated: There is convection heating but not very strong in the atmosphere and there is weaker disturbance in the lower troposphere.The influences of vertical shearing of basic flow in the troposphere on the 30-60 day oscillation in the tropics are also discussed.     

IMPACTS OF THE TROPICAL PACIFIC COUPLED PROCESS ON THE INTERANNUAL VARIABILITY IN THE INDIAN OCEAN

The basic features of climatology and interannual variations of tropical Pacific and Indian Oceans were analyzed using a coupled general circulation model (CGCM), which was constituted with an intermediate 2.5-layer ocean model and atmosphere model ECHAM4. The CGCM well captures the spatial and temporal structure of the Pacific El Ni?o-Southern Oscillation (ENSO) and the variability features in the tropical Indian Ocean. The influence of Pacific air-sea coupled process on the Indian Ocean variability was investigated carefully by conducting numerical experiments. Results show that the occurrence frequency of positive/negative Indian Ocean Dipole (IOD) event will decrease/increase with the presence/absence of the coupled process in the Pacific Ocean. Further analysis demonstrated that the air-sea coupled process in the Pacific Ocean affects the IOD variability mainly by influencing the zonal gradient of thermocline via modulating the background sea surface wind.     

STOCHASTIC SIMULATION OF TROPICAL CYCLONE TRACKS IN THE NORTHWEST PACIFIC REGION WITH CLASSIFICATION MODEL

Accurate simulation of tropical cyclone tracks is a prerequisite for tropical cyclone risk assessment. Against the spatial characteristics of tropical cyclone tracks in the Northwest Pacific region, stochastic simulation method based on classification model is used to simulate tropical cyclone tracks in this region. Such simulation includes the classification method, the genesis model, the traveling model, and the lysis model. Tropical cyclone tracks in the Northwest Pacific region are classified into five categories on the basis of its movement characteristics and steering positions. In the genesis model, Gaussian kernel probability density functions with the biased cross validation method are used to simulate the annual occurrence number and genesis positions. The traveling model is established on the basis of the mean and mean square error of the historical 6 h latitude and longitude displacements. The termination probability is used as the discrimination standard in the lysis model. Then, this stochastic simulation method of tropical cyclone tracks is applied and qualitatively evaluated with different diagnostics. Results show that the tropical cyclone tracks in Northwest Pacific can be satisfactorily simulated with this classification model.     

Evaluation of Summer Monsoon Clouds over the Tibetan Plateau Simulated in the ACCESS Model Using Satellite Products

Cloud distribution characteristics over the Tibetan Plateau in the summer monsoon period simulated by the Australian Community Climate and Earth System Simulator(ACCESS) model are evaluated using COSP [the CFMIP(Cloud Feedback Model Intercomparison Project) Observation Simulator Package]. The results show that the ACCESS model simulates less cumulus cloud at atmospheric middle levels when compared with observations from CALIPSO and CloudSat, but more ice cloud at high levels and drizzle drops at low levels. The model also has seasonal biases after the onset of the summer monsoon in May. While observations show that the prevalent high cloud at 9–10 km in spring shifts downward to 7–9 km,the modeled maximum cloud fractions move upward to 12–15 km. The reason for this model deficiency is investigated by comparing model dynamical and thermodynamical fields with those of ERA-Interim. It is found that the lifting effect of the Tibetan Plateau in the ACCESS model is stronger than in ERA-Interim, which means that the vertical velocity in the ACCESS model is stronger and more water vapor is transported to the upper levels of the atmosphere, resulting in more high-level ice clouds and less middle-level cumulus cloud over the Tibetan Plateau. The modeled radiation fields and precipitation are also evaluated against the relevant satellite observations.