Global spectra of energy and enstrophy and their fluxes during July 1979

Transient and stationary spectra of kinetic energy (KE), available potential energy (APE) and enstrophy (EN), and their spectral fluxes as a function of the two-dimensional wavenumbern were computed for July 1979. Triangular truncation at zonal wavenumber 42 was used for computation. The slopes of various spectra in the wavenumber range 14≤n≤25 were obtained by fitting a straight line in log-log scale by the least square method. The transientKE, APE andEN spectra in the lower (upper) troposphere had slopes −2·21 (−2·30), −2·65 (−2·64) and −0·36 (−0·46), respectively. The effect of stationary and divergent motion on the slope values was investigated. The possible correlation between the slope and percentage of transient component in the combined energy and enstrophy was examined to identify the transient motion of the atmosphere with the two-dimensional homogeneous isotropic turbulence. The vertically averaged slope of kinetic energy and enstrophy in the lower (upper) troposphere was close to the value at 700 (200) hPa level. The spectral fluxes of kinetic energy and enstrophy in the wavenumber range 14≤n≤25 satisfied, to a very rough approximation, the criteria of inertial subrange. The stationary fluxes were small. The estimated stationary-transient component of flux was larger, comparable and less than the corresponding transient flux of APE, KE and EN. Representative levels for computation of energy and enstrophy spectra and their fluxes in the lower and upper troposphere were identified.     

Upper and lower tropospheric energetics of standing and transient eddies in wave number domain during summer monsoon of 1991

Kinetic energy exchange equations (Saltzman 1957) in wave number domain are partitioned into standing, transient and standing-transient components following Murakami (1978, 1981). These components are computed for the 1991 summer monsoon using dailyu andv grid point data at 2.5° latitude-longitude interval between the equator and 40°N at 200 hPa and 850 hPa levels for the period June through August. The data are obtained from NCMRWF, New Delhi. The study shows that at 200 hPa wave number 1 over Region 3 (30°N to 40°N), wave number 2 over Region 2 (15°N to 30°N) and wave number 3 over Region 1 (equator to 15°N) dominate the spectrum of transport of momentum and wave to zonal mean flow interaction. Wave number 1 over Region 1 and Region 3 and wave number 2 over Region 2 are the major sources of kinetic energy to other waves via wave-to-wave interaction. At 850 hPa wave number 1 over Region 3 has maximum contribution in the spectrum of transport of momentum and kinetic energy and more than 90% of its contribution is from the standing component. This indicates that standing wave number 1 over Region 3 plays a very important role in the dynamics of monsoon circulation of the lower troposphere. The study further shows that although the circulation patterns at 200 hPa and 850 hPa levels are opposite in character, a number of energy processes exhibit a similar character at these levels. For example, (i) transport of momentum by most of the waves is northward, (ii) small scale eddies intensify northward, (iii) eddies are sources of kinetic energy to zonal mean flow over Region 1 and (iv) standing eddies are sources of kinetic energy to transient eddies. Besides the above similarities some contrasting energy processes are also observed. Over Region 2 and Region 3 standing and transient eddies are sources of kinetic energy to zonal mean flow at 200 hPa, while at 850 hPa the direction of exchange of kinetic energy is opposite i.e. zonal mean flow is a source of kinetic energy to standing as well as transient eddies. L(n) interaction indicates that at 200 hPa waves over R2 maintain waves over R1, while at 850 hPa waves over R1 maintain waves over R2. It has been found that the north-south gradient of zonal mean of zonal wind is the deciding factor of wave to zonal mean flow interaction.     

Some studies of the growth of monsoon disturbances

The structure of the monsoon depression and the observed flow features prior to and at the time of monsoon depression formation (composite of 15 depressions) are examined. The composite monsoon depression (transient eddy) has a scale of 25° longitude and extends up to 300 mb and has the greatest intensity at 700 mb. It shows north-north-east to south-south-west tilt in the lower levels indicating that it may draw upon zonal kinetic energy for its growth. The disturbance has lower temperatures to its west and tilts westwards with height indicating that eddy available potential energy is not converted from zonal available potential energy by large scale advection. There appears to be a reduction of vertical shear at the time of formation of monsoon depressions and this possibly aids cumulus convection. The profiles of potential vorticity indicate extremes (i) in the upper troposphere and (ii) at several midtropospheric levels in the region of the monsoon trough indicating the possibility of combined barotropic-baroclinic instability. Using multi-level quasi-geostrophic model and employing the eigen-value technique it is shown that the monsoon zonal current is notbaroclinically unstable. A barotropic stability analysis is also done for monsoon zonal current in the lower and middle tropospheres. It yields very slowly growing unstable modes at lower tropospheric levels with wave lengths of 2500 km and 5000 km.     

Has influence of extratropical waves in modulating Indian summer monsoon rainfall (ISMR) increased?

In the paper, influence of extratropical circulation features on Indian Summer Monsoon Rainfall (ISMR) is examined. Energetics of extratropics, north of Indian subcontinent for deficient and non-deficient ISMR years, during two periods 1951–1978 and 1979–2005, are calculated and critically analyzed. It is observed that for the period 1951–1978, only two out of the 10 energetics parameters, viz., the zonal available potential energy (high) and conversion of zonal kinetic energy to eddy kinetic energy (low) differed significantly in JJA months of the deficient years from that of the non-deficient years. However, during the 1979–2005 period, as many as six out of the 10 energetics parameters, viz., eddy available potential energy, zonal available potential energy, eddy kinetic energy, generation of zonal available potential energy, conversion of zonal available potential energy to zonal kinetic energy and conversion of zonal kinetic energy to eddy kinetic energy differed significantly in JJA months of the deficient years from that of the non-deficient years. These results confirm growing influence of the transient stationary waves in deficient years after the climate shift year, 1979. Analysis of energetics parameters of the pre-monsoon season of the two periods also reveals similar results. This suggests that forcings apparently responsible for energetics in JJA months of the deficient years of the later period were present even before the advent of Indian summer monsoon season.     

Intra-seasonal variations of kinetic energy of lower tropospheric zonal waves during northern summer monsoon

Space spectral analysis of zonal (u) and meridional (v) components of wind and time spectral analysis of kinetic energy of zonal waves at 850 hPa during monsoon 1991 (1st June 1991 to 31st August 1991) for the global belt between equator and 40°N are investigated. Space spectral analysis shows that long waves (wavenumbers 1 and 2) dominate the energetics of Region 1 (equator to 20°N) while over Region 2 (20°N to 40°N) the kinetic energy of short waves (wavenumbers 3 to 10) is more than kinetic energy of long waves. It has been found that kinetic energy of long waves is dominated by zonal component while both (zonal and meridional) the components of wind have almost equal contribution in the kinetic energy of short waves. Temporal variations of kinetic energy of wavenumber 2 over Region 1 and Region 2 are almost identical. The correlation matrix of different time series shows that (i) wavenumber 2 over Regions 1 and 2 might have the same energy source and (ii) there is a possibility of an exchange of kinetic energy between wavenumber 1 over Region 1 and short waves over Region 2. Wave to wave interactions indicate that short waves over Region 2 are the common source of kinetic energy to wavenumber 2 over Regions 1 and 2 and wavenumber 1 over Region 1. Time spectral analysis of kinetic energy of zonal waves indicates that wavenumber 1 is dominated by 30–45 day and bi-weekly oscillations while short waves are dominated by weekly and bi-weekly oscillations. The correlation matrix, wave to wave interaction and time spectral analysis together suggest that short period oscillations of kinetic energy of wavenumber 1 might be one of the factors causing dominant weekly (5–9 day) and bi-weekly (10–18 day) oscillations in the kinetic energy of short waves.     

Sensitivity of the tropical nonlinear stationary Kelvin and Rossby waves to the vertical structure of heating

Idealized experiments using linear (LM) and nonlinear (NM) multilevel global spectral models have been carried out to investigate and understand the impact of nonlinearities on the stationary wave response in the tropical atmosphere and its sensitivity to the vertical profile of heating. It is found that nonlinearities exert a dominant influence on the low-latitude stationary Kelvin and Rossby waves particularly in the vicinity of the forcing region. Our study shows that nonlinear effects on the upper tropospheric response produce prominent eastward displacement of the anticyclonic vorticity and horizontal shifts of the maximum equilibrium divergence relative to the prescribed heating. These changes due to nonlinear terms are found to be quite sensitive to the vertical structure of diabatic heating. The strongest nonlinear effects are found to occur when the vertical level under consideration is strongly forced from below. Detailed vorticity budget calculations indicate that stronger nonlinear contributions from stretching and horizontal advection of relative vorticity favour the generation of upper tropospheric anticyclonic circulation and its eastward displacement. Larger vertical advection and twisting terms appear to oppose the generation of upper tropospheric anticyclonic vorticity. It is found that the nonlinear terms which affect the vorticity generation in the upper levels are crucially controlled by the vertical profile of heating. The mid-tropospheric response due to deep convective heating in the NM is characterized by anomalous equatorial westerlies in the low-latitude Rossby regime and exhibits prominent ageostrophic motions. Such nonlinear effects appear probably because of a vertical shift of the low level circulation anomalies in the NM. In the case of shallow convective heating the occurrence of anomalous zonal flows and ageostrophic motions in the low latitude regions of the NM takes place near the level of the maximum heating. Our study shows that large heating amplitudes and small vertical gradient of heating at a given vertical level together favour generation of anomalous zonal flows and ageostrophic motions in the near equatorial regions. These anomalous basic flows in the low-latitudes have implications on the propagation of transients from the tropics to midlatitudes. Non-linear effects on the lower tropospheric stationary waves are prominently seen in the case of strong low level heating which produces a large strengthening of the lower tropospheric cyclonic anomalies that exhibit distinct eastward shifts in the NM relative to the LM.     

Case Comparing Study of Downstream Circulation Development during Typhoon Extratropical Transition

The interaction between extratropical transition process and the mid-latitude jet system stimulates the downstream development. In this paper, three typhoon cases were selected to study their downstream development mechanism through the analysis of the eddy kinetic energy budget and the idealized simulations. The results of Chen’s work to the Pacific region were examined. The results were consistent with the results of Chen’s Atlantic hurricane Case. ET downstream at the upper levels generated first eddies, and the disturbances triggered the low level eddy development. Then the upper and the lower coupled and formed a deep cyclone system throughout the whole troposphere. The ageostrophic geopotential flux promoted the formation and development of the downstream ridge from the typhoon. Vertical ageostrophic geopotential flux transferred energy from upper downward that convergence happened in lower, which stimulated the lower-level cyclone development. Simulation results showed that, in the process of ET, TC outflow transported low potential vorticity to mid-latitude jet, which enhanced the PV gradient and the baroclinic. Then, it is inspired the Rossby wave in the jet and propagated downstream. The formation of downstream ridge-trough couple and development of the further wave was the spread to the downstream through the Rossby wave.     

A solution for the northern hemisphere climatic zonation during a glacial maximum

The same model previously used to deduce an acceptable first order picture of the present zonally averaged macroclimate is now solved for the climatic response to the “glacial” surface boundary conditions that prevailed at 18,000 BP in the northern hemisphere. The equilibrium solution obtained gives the distributions with latitude of the mean temperature, wind, humidity, precipitation, evaporation, heat balance, transient baroclinic eddy statistics (i.e., kinetic energy of the meridional wind and meridional flux of heat, momentum, and water vapor), and the energy integrals. In general terms, the solution shows the glacial atmosphere to be colder and drier than at present, with an intensified polar front, stronger mean zonal and poloidal winds, more intense transient baroclinic eddies (storms) transporting heat, momentum and water vapor poleward at higher rates, and reduced precipitation and evaporation. Also evident is an equatorward shift of the climatic zones (as delineated by the mean surface zonal winds, the poloidal motion, and the difference between mean evaporation and precipitation), particularly in higher latitudes. Other properties of the solution, such as the effect of zonal wind changes on the length of the day, are discussed.     

Moist baroclinic instability and the growth of monsoon depressions— linear and nonlinear studies

The objective of this study was two-fold: the first to investigate the role of moist convection and nongeostrophic effects on the growth of the monsoon depressions using a linearized multi-level moist primitive equation (PE) model and quasi-geostrophic (QG) model with only vertical shear. The second was to study the nonlinear evolution, growth, movement and detailed energetics of the monsoon depressions using a nonlinear moist global spectral model. Our linear studies using both models revealed lower as well as upper tropospheric growing modes. For the lower tropospheric modes the shorter scales were found to grow faster. While the PE model showed faster growth rate for shorter scales, as compared to longer scales, the QG model showed less tendency for scale selection. The shorter scales in PE model had phase speeds ranging from 4 to — 1 ms−1 and in QG model from 8 to — 4 ms−1. The nongeostrophic effects were found to be, in general, important. One of the lower tropospheric modes with wavelength 2500 km was found to have many features similar to the observed monsoon depression of the Bay of Bengal. In the upper troposphere the PE model showed much faster growth rates compared to the QG model. Also the fastest growing mode with a doubling time of 2.5 days had a scale of 6000 km. This was shorter than the scale predicted in the QG model. This mode had many characteristics similar to the observed features of the monsoon upper tropospheric easterly waves. Using a nonlinear global spectral model, we simulated the monsoon depression around 21°N starting from an antisymmetric heating distribution (with respect to the equator) and with a specific vertical structure with and without basic flows. The model was integrated for a period of five days incorporating a simple form of cumulus heating. The simulated model disturbance showed a pronounced growth and a westward movement in the presence of cumulus heating. The detailed energetics calculations revealed that the baroclinic energy exchange is the primary energy exchange process and cumulus heating is the driving force for the generation of available potential energy.     

Stability of a stationary Rossby wave embedded in barotropic monsoon zonal flow

Barotropic stability of a stationary Rossby wave of wavelength 30° longitude superposed on the uniform monsoon zonal flow has been examined. The wave is unstable to perturbations and the growth rate depends on the meridional scale. These perturbations grow by drawing on the kinetic energy of the stationary Rossby wave.     

Flow and contaminant dispersion analysis around a model building using non-linear eddy viscosity model and large eddy simulation

In the present paper, the analysis of concentration and flow fields around a model building was performed using two different approaches in turbulence modeling. In the first approach, the non-linear model of Ehrhard and Moussiopoulos was employed as one of the best cubic non-linear eddy viscosity models, and large eddy simulation was utilized in the second approach. The obtained results suggest that although the non-linear model has the ability to predict the anisotropic normal components of Reynolds stress tensor, due to time-averaged nature of its governing equations, it is incapable of estimating the correct values of Reynolds stress components and turbulent characteristics in the wake region of the model building. So the predicted turbulence kinetic energy values by the non-linear model are 45% smaller than those of large eddy simulation approach in the wake region behind the model building. Also different predictions of the wake region structure by the non-linear model and large eddy simulation approach revealed that elongation of iso-surface for \(\left\langle K \right\rangle\) = 1.5 in non-linear model is 56% more than that of large eddy simulation approach. The large eddy simulation approach shows much consistent behavior to the physics governing the flow compared to the non-linear model. Also good agreement observed between the results obtained through this approach and the experimental data. However, the disadvantage of large eddy simulation approach is the high computational costs.     

Energetics of lower tropospheric planetary waves over mid latitudes: Precursor for Indian summer monsoon

Based on NCEP/NCAR reanalysis data, kinetic energy and momentum transport of waves 0 to 10 at 850 hPa level are computed from monthly mean zonal (u) and meridional (v) components of wind from equator to 90?N. Fourier technique is used to resolve the wind field into a spectrum of waves. Correlation analysis between All India Seasonal Monsoon Rainfall (AISMR) and energetics of the waves indicates that effective kinetic energy of waves 1, 3 and 4 around 37.5?N in February has significant correlation (99.9%) with the subsequent AISMR. A simple linear regression equation between the effective kinetic energy of these three waves and AISMR is developed. Out of 47 years’ (1958–2004) data, 32 years (1958–1989) are utilized for developing the regression model and the remaining 15 years (1990–2004) are considered for its verification. Predicted AISMR is in close agreement with observed AISMR. The regression equation based on the dynamics of the planetary waves is thus useful for Long Range Forecasting (LRF) of AISMR. Apart from the regression equation, the study provides qualitative predictors. The scatter diagram between AISMR and effective kinetic energy of waves 1, 3 and 4 around 37.5?N indicates that if the kinetic energy is more (less) than 5m²s^-2, the subsequent monsoon will be good (weak). Stream function fields indicate that high latitude trough axis along 40?E (70?E) leads to a good (weak) monsoon over India.     

平流层-对流层交换研究进展

平流层与对流层之间的物质输送和混合（STE）是控制自然和人为排放的化学痕量物质对大气成分影响的一个重要过程。STE可以影响温室气体在上对流层和下平流层中的垂直分布，进而影响气候。要预报全球气候变化就必须了解平流层与对流层之间动力、化学及辐射的耦合。从 STE研究的尺度问题，热带和中纬度地区STE研究以及我国STE研究现状进行了评述。STE具有多种尺度和形式，热带外平流层由波强迫驱动的全球尺度环流，可以诊断长时间尺度的STE，它不能充分描述短时间尺度过程。热带外低平流层环流不能简单地描述为纬向平均，要正确描述痕量气体的分布必须包含纬向非对称的天气尺度过程。热带地区的滴漏管理论提供了一个新的诊断 STE框架。目前对中纬度地区对流层顶折叠和切断低压的研究是比较充分的。     

The formation of radio-pulsar spectra

It is shown that scattering of electromagnetic waves by Langmuir waves taking into account the electrical drift motion of the particles is the most efficient nonlinear process contributing to a radio pulsar’s spectrum. If an inertial interval exists, stationary spectra with spectral indices of ?1.5 or ?1 can be formed, depending on the wave excitation mechanism. The obtained spectra are in satisfactory agreement with observational data.     

Space observations of Transient Luminous Events and associated emissions in the upper atmosphere above thunderstorm areas

Sprites, jets and elves called Transient Luminous Events (TLE), observed in the middle and upper atmosphere above thunderstorms, are the manifestation of intense energy exchanges between the troposphere, stratosphere and mesosphere. Different types of luminous emissions have been identified by ground-based observations, showing the complexity of these phenomena. Space missions showed that transient emissions in the Earth atmosphere are very broad including Radio Frequency (RF), IR to FUV radiations and X-gamma ray emissions called Terrestrial Gamma Flashes (TGF) with energies reaching 30 Mev. However, there are no global observations of these events together. This paper reviews space observations performed up to now and emphasizes the challenges of the future space missions in global measurements of all possible emissions together for the understanding of the physical mechanisms at the origin of these phenomena and their effects on the Earth environment.     

不同饱和度土层分界面上剪切波的反射与透射

基于多相孔隙介质弹性理论,给出了非饱和土中不同弹性波的传播方程。根据分界面上的边界条件,建立了各势函数波幅值之间的关系式,讨论了入射剪切波在不同饱和度土层分界面上的反射与透射问题。在无限空间非饱和土体中存在3种压缩波和1种剪切波,因此,当剪切波传播到不同饱和度的非饱和土层分界面上将分别在上、下土层激发产生4种反射波和4种透射波。推导出不同反射波和透射波的振幅比例系数和能量比例系数的理论表达式,并且在此基础上进行数值分析。在数值算例中分别研究了各反射波与透射波的能量比例系数（即能量反射率和能量透射率）受入射频率、入射角度以及上、下土层土体饱和度变化的影响情况。计算结果表明：各能量反射率和能量透射率不仅与入射角和入射频率有关,而且其受上、下土层饱和度变化的影响也同样不能忽视。     

激光测速粒子对复杂流动的响应研究——Ⅰ颗粒非恒定运动数学模型及其数值方法

流体激光测速的精度与示踪粒子的跟随特性即流体中异质粒子的非恒定运动特性密切相关。首先对粒子非恒定运动方程进行了探讨,着重考虑了在高颗粒雷诺数时该方程的修正问题,简要分析了该方程的数学属性,并构造了这类方程的数值计算方法。分析表明,高颗粒雷诺数下的粒子非恒定运动方程为非线性奇异积分方程,而当颗粒雷诺数小于1时,则线性化为第二类渥尔特拉(Volterra)积分方程。以几种均匀流中球形小颗粒的非恒定运动为算例,计算结果与其解析解及有关实验数据的比较表明,数值方法具有良好的计算精度。     

非线性波浪作用下海底管线-海床动力响应分析

确定波浪荷载作用下海底埋置管线和海床的响应是海底管线设计中的关键问题。目前大多数研究只是考虑了管线、海床在线性推进波作用下的响应,并没有考虑管线与海床之间的相互作用效应。采用接触摩擦理论,考虑管线与海床之间的相互作用效应,基于有限元方法研究了非线性波浪作用下海底埋置管线和多孔海床相互作用问题。数值计算结果表明,在计算中如果忽略波浪非线性项,既有可能低估海底管线内应力及管线周围海床中孔隙水压力,也有可能高估海底管线内应力及管线周围海床中孔隙水压力。     

波流边界层水沙运动数值模拟——I:水动力模拟

波流边界层水动力模拟对研究波流相互作用和泥沙运动具有重要的理论意义和实践价值。开发了波流边界层1DV垂向一维水动力数值模型,可用于模拟漩涡沙波床面和平底床面水动力特征。模型的构建基于边界层控制方程,平底床面采用k-ε模型,沙波床面采用双层模型,提出了漩涡层和紊动扩散层交界面紊动动能和紊动耗散率表达式。试验资料验证表明,模型较好地模拟了波浪-水流-床面共同作用下的边界层水动力特征,包括波周期内不同相位流速分布、紊动动能、剪切应力等以及波致时均流速分布和波流相互作用下的时均流速分布等。根据所建模型,讨论了不同床面和波流组合条件下的水动力特征。该模型可为研究波流边界层内水动力特征提供工具。     

20世纪全球变暖的冰冻圈证据

20世纪80年代以来，地面气象观测结果表明全球气温在明显升高，而探空资料和卫星遥感资却表明低层大气在降温，这使得人们对于全球变暖问题提出了质疑。通过对近100多年来冰冻圈各组成要素（冰川、积雪、冻土、海冰等）变化的综合分析，并结合全球不同地区的冰芯记录，证明了20世纪全球气候是在变暖，指出开展冰冻圈（尤其是冰盖、积雪、冻土和海冰）与大气／海洋之间相互作用（影响和响应）过程研究的必要性和迫切性。