The influence on the summer flow over Asia of both the orographic and thermal forcing of the Tibetan Plateau is investigated using a sequence of idealised experiments with a global primitive equation model. The zonally averaged flow is prescribed and both realistic and idealised orography and heating are used. There is some similarity between the responses to the two forcings when applied separately. The upper tropospheric Tibetan anticyclone is predominantly forced by the heating but also weakly by the orography. Below this, both forcings tend to give air descending in an equatorward anticyclonic circulation down the isentropes to the west and rising in a similar poleward circulation to the east. However the heating-only response has a strong ascending southwesterly flow that is guided around the south and south-east of the orography when it is included. On the northern side, the westerly flow over the orography gives ascent on the upslope and descent on the downslope. It is found that heating over the Plateau leads to a potential vorticity (PV) minimum and that if it is sufficiently strong the flow is unstable, producing a quasi-biweekly oscillation. During this oscillation the Tibetan anticyclone changes between a single centre over the southwestern side of the Plateau and a split/double structure with centres over China and the Middle East. These characteristics are similar to observed variability in the region. Associated with this quasi-biweekly oscillation are significant variations in the strength of the ascent over the Plateau and the Rossby wave pattern over the North Pacific. The origin of the variability is instability associated with the zonally extended potential vorticity PV minimum on a θ-surface, as proposed by Hsu and Plumb (2000). This minimum is due to the tendency to reduce the PV above the heating over the Plateau and to advection by the consequent anticyclone of high PV around from the east and low PV to the west. The deep convection to the south and southeast of the Plateau tends to suppress the quasi-biweekly oscillation because the low PV produced above it acts to reduce the meridional PV gradient reversal. The occurrence of the oscillation depends on the relative magnitude of the heating in the two regions.
The Asian monsoon is the most significant component of the global climate system. During recent two decades, a more and more efforts have been made to study the Asian monsoon. A substantial achievement has been made in basic physical processes, predictability and prediction since the MONEX of 1978-1979. The major advance in our new understanding of the variability of the Asian summer monsoon has been highlighted in this paper. The present paper is structured with four parts. The first part is the introduction, indicating the new regional division of the Asian monsoon system and significant events of the history of the Asian monsoon research. The second part discusses the annual cycle and seasonal march of the Asian monsoon as the mean state, with a special emphasis on the onset, propagation, active-break cycle and withdrawal of the Asian summer monsoon. The process and mechanism of the earliest onset of the Asian summer monsoon which takes place in the near-equatorial East Indian ocean-central and southern Indochina Peninsula have been well documented. The third part deals with the multiple scale variability of the Asian summer monsoon, including the intraseasonal, interannual and inter-decadal variability. Their dominant modes such as 10-20 day and 30-60 day oscillations for the intraseasonal variability, the Tropospheric Biennial Oscillation (TBO), the Indian Ocean Dippole Mode (IODM) and teleconnection patterns for interannual variability and the 60-year oscillation for the inter-decadal variability, as well as related SST-monsoon relationship and land-monsoon relationship have been discussed in more details. The fourth part is the conclusion, summarizing the major findings and proposing future work.
This paper has discussed the land-atmosphere interaction associated with the Asian monsoon climate and its variability. The snow cover and soil moisture are internal factors of the climate system panicularly in the seasonal to interannual time scales, but are suggested to play some important roles in changing large-scale surface energy and water balance, which in turn affect the monsoon circulation and precipitation. From the modeling as well as observational studies it has been suggested that the snow cover is most likely to influence atmosphere essentially through the albedo effect particularly in lower latitudes and on the Tibetan Plateau. The GCM experiments have strongly suggested that soil moisture anomalies efficiently affect the atmosphere under dry or semi-arid condition. Vegetation has also been noted as an important variable for the formation of moist monsoon flow over the continent, in addition to the dynamical and thermo-dynamical effect of the Tibetan Plateau. Through the GCM and RCM experiments the anthropogenically-induced change of land coverluse including the deforestation has proved to be a great impact on regional precipitation and water cycle of the monsoon region by changing characteristics of vegetation control in energy and water cycle, which in turn affect atmospheric boundary layer (ABL) and cloud/precipitation processes in regional-scale. The discussion has also been made on how the land surface changes could induce change of precipitation through feedback processes of moisture convergence and in-situ evapo-transpiration processes. The critical role of moisture amount near surface and in the ABL is emphasized, to induce positive feedback to change of precipitation over land in the monsoon region.
The histories of numerical weather prediction and atmospheric predictability research are briefly reviewed in this article in celebration of the 125-year anniversary of the foundation of the Japan Meteorological Society. The development of numerical weather prediction in the 20th century has been intimately related to the progress of dynamic meteorology as stated in Section 1, including the development of the quasi-geostrophic system that is a basic tool to describe large-scale balanced flow approximately and the discovery of chaos that is the key concept of atmospheric predictability. In the 1990s, the rapid advancement of computer technology brought a regime shift in the predictability research from fundamental theoretical works with simple nonlinear dynamical systems (Section 2) to practical applied works with operational numerical weather prediction models (Section 3). Ensemble forecasts became in operations in major forecast centers at the end of the 20th century. Some current challenges in the atmospheric predictability research under THORPEX (THe Observing system Research and Predictability EXperiment) program are summarized in Section 4, such as targeted observations, new data-assimilation techniques, and interactive grand global ensemble forecasts.
The multi-scale features of the Meiyu-Baiu front (MBF) and associated precipitation systems are described based on the recent observational studies for the intense rainfalls period in July of 1991. The MBF extends from the Yangtze River basin to the Japan Islands along the northwestern rim of the westward protruding North Pacific subtropical anticyclone. A blocking ridge develops over Primorskiy Kray of Russia, while a cold upper low develops over Mongolia during this period. The low-leveljet stream, nearly moist neutral stratification, and a strong gradient of the specific humidity characterize the MBF. The thermal gradient in the Meiyu front is weak, whereas the relatively large thermal gradient in the Baiu front is sustained between the tropical maritime airmass and the polar maritimeairmass. The strong meridional convergence of the moisture flux sustains the large precipitation in the MBF. The differential advection of the equivalent potential temperature generates convective instability against the stabilizing effect of the cumulus convection, and sustains moist neutral stratification and intense precipitation. The northward ageostrophic winds from the northwestern rim of the westward protruding subtropical anticyclone causes strong low-level convergence in the MBF. The short-wave trough that propagates along the southern rim of the upper cold low couples with the short-wave trough in the MBF. The coupling of the troughs causes the development of the subsynoptic-scale depression (SD) and the associated subsynoptic-scale cloud system. Successively with the development of the SD, a series of the meso-α-scale cloud systems are formed along the long trailing portions of the SD. Thus the ”cloud system family“ with a length of ∼2000 km, which consists of a subsynoptic-scale cloud system and a few meso-α-scale cloud systems aligned along the trailing portion of the SD, is formed along the MBF. Features and the development process of the subsynoptic- and meso-α-scale cloud systems are studied in detail.
Progress in understanding the general circulation of the atmosphere during the past 25 years is reviewed. The relationships of eddy generation, propagation and dissipation to eddy momentum fluxes and mean zonal winds are now sufficiently understood that intuitive reasoning about momentum based on firm theoretical foundations is possible. Variability in the zonal-flow can now be understood as a process of eddy, zonal-flow interaction. The interaction of tropical overturning circulations driven by latent heating with extratropical wave-driven jets is becoming a fruitfu1 and interesting area of study. Gravity waves have emerged as an important factor in the momentum budget of the general circulation and are now included in weather and climate models in parameterized form. Stationary planetary waves can largely be explained with linear theory.
The author's view on tropical cyclones as the conditional instability of the second kind (CISK) is presented. Many theoretical and numerical studies of tropical cyclones have discussed the original CISK of Ooyama (1964, 1969) and Charney and Eliassen (1964) in which frictional convergence plays an important role. In this paper, the author emphasizes that this CISK is applied primarily to the eyewall circulation in intense tropical cyclones, and describes other types of CISK, which are appropriate to explain many of tropical disturbances and tropical depressions, and the formation and early development stages of tropical cyclones. In addition, the importance of resolving mesoscale organized convection in coarse-resolution numerical models is emphasized.
An overview is provided of the current understanding of transport in the middle atmosphere. Over the past quarter century this subject has evolved from a basic recognition of the Brewer-Dobson circulation to a detailed appreciation of many key features of transport such as the stratospheric surf zone, mixing barriers, and the dynamics of filamentation. Whilst the elegant theoretical framework for middle atmosphere transport that emerged roughly twenty years ago never fu1filled its promise, usefu1 phenomenological models have been developed together with innovative diagnostic methods. These advances were made possible by the advent of plentiful satellite and aircraft observations of long-lived chemical species together with developments in data assimilation and numerical modeling, and have been driven in large measure by the problem of stratospheric ozone depletion. This review is primarily focused on the stratosphere, where both the interest and the knowledge are the greatest, but a few remarks are also made on the mesosphere.
Major advances in our understanding of the dynamics of Earth's thermosphere (≈ 90-500 km) during the past 25 years are reviewed. Since the thermosphere is primarily an externally-forced system, a broad overview of the energy input, conversion and transport mechanisms in the ionosphere-thermosphere system is first provided. This serves as background and context for the non-specialist. Then, several broad areas of progress are in turn discussed in some detail: (i) the role of solar thermal tides in imposing significant longitudinal variability in the lower thermosphere (≈ 100-150 km), and affecting the zonal mean circulation at these altitudes; (ii) the zonal mean circulation of the thermosphere, the changes in O and N2 relative densities that accompany it, and the competing roles of solar radiative heating and Joule (ohmic) heating in determining the overall structure of this circulation; (iii) polar and auroral thermosphere dynamics, and connections to relevant magnetosphere and ionosphere processes; and (iv) the global response to geomagnetic disturbances, i.e., relatively sudden injections of energy and momentum from the magnetosphere. The paper concludes with a personal assessment of future research directions and scientific questions that remain to be addressed in forthcoming decades.
With certain limitations, atmospheric radars generally called MST (mesosphere, stratosphere, and troposphere) radars or ST (stratosphere and troposphere) radars are capable of continuously monitoring three-dimensional winds, waves, turbulence, and atmospheric stability over the wide altitude range 1-100 km in the Earth's atmosphere. In particular, direct measurement of venical wind velocity over such a wide attitude range is possible only with MST radars. Their time resolution of about 1 min and attitude resolution of 75-150 m are unequalled by conventional instruments (e.g., rawinsondes and rocketsondes), making it possible for MST radars to quantitatively investigate the small-scale atmospheric gravity waves that are considered to play important roles in the dynamics of the Earth's atmosphere. It is also noted that the vertical flux of horizontal momentum can be measured with high accuracy by MST radars. MST radars in the VHF band have the capability to discriminate echoes from clear air and precipitation particles, while microwave meteorological radars generally detect only precipitation echoes. In the last three decades, this excellent capability has been used extensively to study various dynamical disturbances in the Earth's atmosphere, developing new frontiers of atmospheric research on, primarily, mesoscale and micro-scale phenomena. In the present paper, these advances are reviewed briefly.
The evolution of global atmospheric model dynamical cores from the first developments in the early 1960s to present day is reviewed. Numerical methods for atmospheric models are not straightforward because of the so-called pole problem. The early approaches include methods based on composite meshes, on quasi-homogeneous grids such as spherical geodesic and cubed sphere, on reduced grids, and on a latitude-longitude grid with short time steps near the pole, none of which were entirely successful. This resulted in the dominance of the spectral transform method after it was introduced. Semi-Lagrangian semi-implicit methods were developed which yielded significant computational savings and became dominant in Numerical Weather Prediction. The need for improved physical propenies in climate modeling led to developments in shape preserving and conservative methods. Today the numerical methods development community is extremely active with emphasis placed on methods with desirable physical properties, especially conservation and shape preservation, while retaining the accuracy and efficiency gained in the past. Much of the development is based on quasi-uniform grids. Although the need for better physical properties is emphasized in this paper, another driving force is the need to develop schemes which are capable of running efficiently on computers with thousands of processors and distributed memory. Test cases for dynamical core evaluation are also briefly reviewed. These range from well defined deterministic tests to longer term statistical tests with both idealized forcing and complete parameterization packages but simple geometries. Finally some aspects of coupling dynamical cores to parameterization suites are discussed.
This paper reviews nonhydrostatic atmospheric models for research and NWP. Classification of nonhydrostatic atmospheric models and numerical methods to treat sound waves are described with their relative advantages. The current operational nonhydrostatic NWP models at various forecast centers and community nonhydrostatic models for research are reviewed. Brief history and development of the JMA nonhydrostatic model, a community mesoscale model for research and NWP in Japan, is introduced. Current status and near future plans of the operational nonhydrostatic mesoscale model at JMA are presented.
One of the most promising methods to test the representation of cloud processes used in climate models is to use observations together with cloud resolving models (CRMs). CRMs use more sophisticated and realistic representations of cloud microphysical processes, and they can reasonably well resolve the time evolution, structure, and life cycles of clouds and cloud systems (with sizes ranging fromabout 2-200 km). CRMs also allow for explicit interaction between clouds, outgoing longwave (cooling)and incoming solar (heating) radiation, and ocean and land surface processes. Observations are requiredto initialize CRMs and to validate their results. This paper provides a briefdiscussion and review of the main characteristics of CRMs as well as some of their major applications. These include the use of CRMs to improve our understanding oft (1) convective organization, (2) cloud temperature and water vapor budgets, and convective momentum transport, (3) diurnal variation of precipitation processes, (4) radiative-convective quasi-equilibrium states, (5) cloud-chemistry interaction, (6) aerosol-precipitation interaction, and (7) improving moist processes in large-scale models. In addition, current and future developments and applications of CRMs will be presented.
Data assimilation is a methodology for estimating accurately the state of a time-evolving complex system like the atmosphere from observational data and a numerical model of the system. It has become an indispensable tool for meteorological researches as well as for numerical weather prediction, as represented by extensive use of reanalysis datasets for research purposes. New advances of data assimilation methods emerged from the 1980s. This review paper presents the theoretical background and implementation of two advanced data assimilation methods: four-dimensional variational assimilation (4D-Var) and ensemble Kalman filtering (EnKF), which currently draw much attention in the meteorological community. Recent research results in Japan on those methods are reviewed, especially on mesoscale applications of 4D-Var and tests of the local ensemble transform Kalman filter (LETKF). Comparison of 4D-Var and EnKF is also briefly discussed. An outline of the mesoscale 4D-Var system of the Japan Meteorological Agency, which is the first operational 4D-Var for a mesoscale model, is given in Appendix.
This paper reviews the progress made in urban meteorology over the past few decades. The focus is on the impact of urban surfaces on the overlying atmosphere along the conventional meteorological frameworks. Section 1 details the difliculties in generalizing urban surfaces in a meteorological sense because of surface diversity, and considers whether conventional similarity law is applicable. Section 2 describes the characteristics of urban surfaces as the bottom boundary of the atmosphere and includes a discussion of land surface parameters and the resultant surface energy partitioning. Section 3 explains characteristics of the urban atmosphere, including temperature fields, local circulations and rainfall. Section 4 describes recent progress in numerical modeling and promising new technologies, thus revealing a possible future direction for urban meteorological studies.
Based upon the results obtained from coupled ocean-atmosphere models of various complexities, this review explores the role of ocean in global warming. It shows that ocean can play a major role in delaying global warming and shaping its geographical distribution. It is very encouraging that many features of simulated change of the climate system have begun to agree with observation. However, it has been difficult to confirm the apparent agreement because the density and frequency of the observation are insufficient in many oceanic region of the world, in particular, in the Circumpolar Ocean of the Southern Hemisphere. It is therefore essential to intensify our effort to monitor not only at the surface but also in the subsurface layers of oceans.