Manifestation of heliophysical disturbances in the tropospheric characteristics

The response of the thermobaric characteristics of the high-latitude troposphere to short-term events attributed to solar activity (solar cosmic rays and geomagnetic storms) has been investigated. The spatial manifestation of these disturbances in the troposphere is shown to be of a “focal” character. It is found that the manifestation is most evident in the cold period and depends on the properties of the underlying surface (land, ocean). The properties of the variations of the troposphere air temperature in the manifestation “foci” on the standard isobaric surfaces, as well as the variations of the altitude profile of temperature and the long-wave radiation flux at the upper boundary of the atmosphere, have been considered. The variations of the heat content of the high-latitude troposphere after solar flares have been analyzed. The variation of the thermobaric field is shown to be accompanied by the rearrangement of circulation forms in moderate and polar latitudes. The revealed properties are completely explained within the mechanism proposed here for the solar activity effect on the climatic characteristics of the troposphere.     

Influence of wave groups on SSC patterns over vortex ripples

Estimates of spatial and temporal variations in suspended sand concentrations (SSC) made with a multi-transducer Acoustic Backscatter Sensor (ABS) under a repeated wave group over a mobile rippled bed in the wave research flume at the National Hydraulics Laboratory in Ottawa, Canada, reveal an number of complex and intriguing patterns. Ensemble averages of 8 nearly identical wave groups provided much more robust estimates of SSC and allowed a detailed examination of the wave group effects. The largest SSC near the bed (< 0.10 m) occurs in phase with the largest waves in the group. Above approximately 0.10 m elevation, SSC lags behind the near bed SSC by as much as 2–3 waves; introducing significant curvature (on a semi-log plot) to the SSC profile. The log linear segments of the SSC profile grow and decay systematically on the scale of the wave group. The range in lengths of log-linear profile segments ( 0.03–0.355 m) suggest that the boundary layer thickness also fluctuates throughout the passage of the wave group. Furthermore, there are significant variations in the patterns of SSC, which occur under the largest and smallest waves in the group. Under the largest waves vertical bands of alternating high and low SSC produce an intra-wave modulation in the upper water column ( 0.075–0.30 m). The equivalent horizontal excursion of these bands scales to the ripple length. Under the smaller waves the intra-wave modulation of the SSC disappears and is replaced by temporally homogenous suspension that expands vertically through several individual wave cycles. The former pattern of homogenous suspension appears to be associated with growth of a boundary layer due to the persistent uni-directional horizontal flow during this part of the group together with the persistence of antecedent bed generated turbulence and vorticity which maintains the suspension. The latter pattern of bands of high and low SSC indicates a strong temporal and spatial constraint on the SSC (phase coupling) induced by the presence of the bedforms which may be enhanced by strong reversals in both flow and vorticity under the large waves in the group.     

Role of basic rheological models in determination of wave attenuation over muddy seabeds

Among rheological models for estimating the rate of dissipation of non-breaking waves in muddy seabeds, those representing viscoelastic and poroelastic behaviors are used widely. In that regard, the dependence of the wave attenuation coefficient derived from basic rheological representations of mud behavior is examined on a cursory basis. For wave attenuation due to viscoelastic muds, results based on a semi-analytical model incorporating the effects of typically thin mud layers are summarized. This and an existing model for poroelastic beds are tested against selective laboratory data. Relying on these tests, it is emphasized that fluid-like mud should be modeled as a viscoelastic fluid medium, and that only non-fluid beds can be modeled as poroelastic media. Mechanisms for energy dissipation depend on bed compactness specified by the solids volume fraction, porosity or density, and on a characteristic Péclet number defined in terms of particle size, permeability and wave frequency. Due to the role of the latter parameter, for simulation of wave attenuation the chosen rheological model for a bed of given compactness must be applicable over the expected range of wave frequency.     

An experimental study of wave packets in travelling ionospheric disturbances

The complex demodulation method was employed to analyse observations of TIDs carried out by means of multifrequency HF Doppler sounding at vertical incidence over Almaty (76°55′E, 43°15′N). Two types of behaviour of the amplitude and phase in the band-pass filtered Doppler shift oscillations were revealed. These were packet-like wave structures and quasi-stochastic TIDs. A study of the phase behaviour inside wave packets showed that most of them had a quasi-monochromatic structure, some of them had oscillations with a period which increased with time, and none had oscillations with a period which decreased with time. Phase jumps between nearby packets were explained as a result of interference between two adjacent packets having similar oscillation periods. A method of calculating the correlation coefficient for the wave packets propagating on a background of quasi-stochastic TIDs was developed. This showed that wave packets (having periods between 7 and 50 min) travelled in the vertical plane practically without any loss of coherence, while quasi-stochastic TIDs had a short coherence length; the velocity of the quasi-stochastic TIDs was noticeably larger than that of the wave packets.     

Studies of the vertical distribution of atmospheric ozone in association with western disturbances over India

A series of ozone soundings were made at New Delhi (77°E 28°N) from 21 to 30 January 1969 and 10 to 22 February 1972 to study the changes in the vertical distribution of atmospheric ozone associated with western disturbances. The sonde used was the Indian ozonesonde made in the Instruments Laboratories at Poona.In February 1972, two western disturbances moved eastwards in quick succession across the western Himalayas, the first between the 11th and 13th and the second between the 13th and 15th. Associated with the first tropospheric trough was a high-speed jet stream with wind speeds reaching 180 knots, when the tropopause descended to 304 mb over Delhi. The second trough had no high-speed jet associated with it and the tropopause was at 227 mb. Ozone maxima were observed at 350, 180 and 125 mb in addition to the main peak at 35 mb in association with the upper tropospheric troughs over Delhi and its neighbourhood. A similar lowering of the tropopause and the influx of ozone in shallow layers was observed during the passage of two upper air troughs in January 1969. The study shows that with the approach of upper tropospheric troughs and the simultaneous lowering of the tropopause there is an increased influx in shallow layers of middle latitude ozone-rich air through breaks in the tropopause, replacing the subropical ozone-poor air over the station.     

Hydrological analysis of extreme rainfall events and severe rainstorms over Uttarakhand,India

ABSTRACT

Following the June 2013 disaster in the Uttarakhand Himalayas, many discussions are ongoing with regard to how climate change is seeking revenge on mankind by endowing us with disasters! The event was mostly linked with the occurrence of an extreme event due to climate change. In view of this, an attempt has been made in this paper to analyse the extreme rainfall events experienced by the Uttarakhand during 1901–2013 using more than 100 stations’ daily rainfall data. The study revealed that during the 113-year period, the highest numbers of extreme events were recorded during the decade 1961–1970, and to some extent in the decade 1981–1990. Thereafter, there is a decrease in extreme rainfall events. The comparative study of extreme events prior to 1901 showed that on 17–18 September 1880, a rainstorm which occurred in close vicinity to Uttarakhand caused serious floods and damage to lives and properties. The extreme rainfall recorded by some stations during this unprecedented rainstorm has not been surpassed to date.     

On cumulus-scale transport of horizontal momentum in monsoon depression over India

A diagnostie method of cumulus parameterization is suggested in which vertical transport of horizontal momentum by cumulus-scale is derived by making use of large-scale vorticity as well as divergence budget equations. Data for composite monsoon depression over India available from our earlier studies used to test the method. As a first approximation, the results are obtained using only the vorticity budget equation.The results show that in the southwest sector of the monsoon depression, which is characterized by maximum cloudiness and precipitation, there is an excess of cyclonic vorticity in the lower troposphere and anticyclonic vorticity in the upper troposphere associated with the large-scale motion. The distribution of eddy vertical transport of horizontal momentum is such that anticyclonic vorticity is generated in the lower troposphere and cyclonic vorticity aloft. Cumulus-scale eddies thus work against the large-scale system and tend to off-set the large-scale imbalance in vorticity.     

Anelasticity of the crust and upper mantle beneath north and central India from the inversion of observed Love and Rayleigh wave attenuation data

The fundamental mode Love and Rayleigh waves generated by earthquakes occurring in Kashmir, Nepal Himalaya, northeast India and Burma and recorded at Hyderabad, New Delhi and Kodaikanal seismic stations are analysed. Love and Rayleigh wave attenuation coefficients are obtained at time periods of 15–100 seconds, using the spectral amplitude of these waves for 23 different paths along northern (across Burma to New Delhi) and central (across Kashmir, Nepal Himalaya and northeast India to Hyderabad and Kodaikanal) India. Love wave attenuation coefficients are found to vary from 0.0003 to 0.0022 km^–1 for northern India and 0.00003 km^–1 to 0.00016 km^–1 for central India. Similarly, Rayleigh wave attenuation coefficients vary from 0.0002 km^–1 to 0.0016 km^–1 for northern India and 0.00001 km^–1 to 0.0009 km^–1 for central India. Backus and Gilbert inversion theory is applied to these surface wave attenuation data to obtainQ ^–1 models for the crust and uppermost mantle beneath northern and central India. Inversion of Love and Rayleigh wave attenuation data shows a highly attenuating zone centred at a depth of 20–80 km with lowQ for northern India. Similarly, inversion of Love and Rayleigh wave attenuation data shows a high attenuation zone below a depth of 100 km. The inferred lowQ value at mid-crustal depth (high attenuating zone) in the model for northern India can be by underthrusting of the Indian plate beneath the Eurasian plate which has caused a low velocity zone at this shallow depth. The gradual increase ofQ ^–1 from shallow to deeper depth shows that the lithosphere-asthenosphere boundary is not sharply defined beneath central India, but rather it represents a gradual transformation, which starts beneath the uppermost mantle. The lithospheric thickness is 100 km beneath central India and below that the asthenosphere shows higher attenuation, a factor of about two greater than that in the lithosphere. The very lowQ can be explained by changes in the chemical constitution taking place in the uppermost mantle.     

Experimental study of a stiff wave barrier in gelatine

Railway induced vibrations and re-radiated noise in buildings can be mitigated by means of wave barriers in the soil. Numerical simulations demonstrate that a stiff wave barrier, consisting of a material that is stiffer than the surrounding medium, can be very effective if the stiffness contrast between the barrier and the medium is sufficiently large. This paper presents results of a lab experiment that has been carried out to validate these findings, using gelatine instead of soil in order to reduce the wavelengths and thus the scale of the test setup. An expanded polystyrene beam is employed as wave barrier, while a non-contact measurement procedure is applied for visualizing the waves in the gelatine, based on reflections of a grid of laser rays. The experimental results are found to be in line with the numerical predictions, confirming the vibration reduction effectiveness of stiff wave barriers.     

Prominent wave disturbances over the region east of Indian Ocean during a Quasi-Biennial Oscillation

Summary The wind and temperature data for three stations, latitudinally separated in the region east of the Indian Ocean, were analysed by spectrum analysis. The study pointed out two types of prominent wave disturbances, one with periodicity of about 5 days and the other of about 8 days. The occurrence of the two types of disturbances is closely associated with the alternating wind regimes of the Quasi-Biennial Oscillation.     

A two-dimensional horizontal wave propagation and mud mass transport model

The present study offers a two-dimensional horizontal wave propagation and morphodynamic model for muddy coasts. The model can be applied on a general three-dimensional bathymetry of a soft muddy coast to calculate wave damping, fluid mud mass transport and resulting bathymetry change under wave actions. The wave propagation model is based on time-dependent mild slope equations including the wave energy dissipation due to the wave-mud interaction of bottom mud layers as well as the combined effects of the wave refraction, diffraction and breaking. The constitutive equations of the visco-elastic–plastic model are adopted for the rheological behavior of fluid mud. The mass transport velocity within the fluid mud layer is calculated combining the Stokes’ drift, the mean Eulerian velocity and the gravity-driven mud flow. The results of the numerical model are compared against a series of conducted wave basin experiments, wave flume experiments and field observations. Comparisons between the computed results with both the field and laboratory data reveal the capability of the proposed model to predict the wave transformation and mud mass transport.     

Stable isotope evidence of dual (Arabian Sea and Bay of Bengal) vapour sources in monsoonal precipitation over north India

High resolution time series data of hydrogen (δD) and oxygen (δ¹⁸O) isotope values of precipitation have been generated for the first time at Kolkata, eastern India where the summer monsoon clouds from Bay of Bengal (BOB) commence their journey over India. Use of a Rayleigh cum two component mixing model and comparison of Kolkata data with the International Atomic Energy Agency (IAEA)–Global Network of Isotopes in Precipitation (GNIP) data base of New Delhi suggest that the precipitation at New Delhi cannot be explained by simple continental effect of a BOB vapour source alone, traveling and raining successively along Kolkata–New Delhi route. It is necessary to invoke an admixture of  20% vapour originating from the Arabian sea with the vapour coming from BOB and finally causing summer monsoon rains at New Delhi. The findings have major implications to the regional water vapour budget over India.     

Arc magmatism in eastern Kumaun Himalaya,India: A study based on geochemistry of granitoid rocks

Petrochemical studies of granitoid rocks from the eastern part of Kumaun region suggest that the leading edge of India represents an active arc during Late Paleoproterozoic times. It has been observed that melt generation for granodiorite rocks from the eastern Almora Nappe and Chhiplakot klippe along with the Askot klippe was caused through a subduction‐related process involving hydrous partial melting of a Paleoproterozoic amphibole‐ and/or garnet‐bearing mafic source with the involvement of sediments from the subduction zone. The medium‐ to high‐K basic rocks, common in subduction‐related magmatic arcs, can also explain the generation of the high‐K granodiorites of the Chhiplakot klippe. The augen gneisses from the eastern Almora nappe and Chhiplakot klippe along with the Askot klippe further show geochemical similarity with the associated granodiorites, suggesting there is a genetic linkage with one another.     

Application of third generation shallow water wave models in a tidal environment

Wave modeling was performed in the German Bight of the North Sea during November 2002, using the spectral wave models, namely the K-model and Simulating WAves Nearshore (SWAN), both developed for applications in environments of shallow water depths. These models mainly differ with respect to their dissipation source term expressions and in exclusion or inclusion of nonlinear wave–wave interactions. The K-model uses nonlinear dissipation and bottom dissipation, and neglects quadruplet wave–wave interaction whereas, SWAN includes, besides bottom dissipation, dissipation by white-capping and depth induced wave breaking and triad wave–wave interaction. The boundary spectra were extracted from the WAM model results of a North Sea hindcast of the HIPOCAS project, wind fields, tidal current and water level variations from the results of models used in the Belawatt project. The purpose of this study was to test the performance of both wave models to see whether they were able to predict near-shore wave conditions accurately. The runs were performed with and without tidal current and level variations to determine their effect on the waves. Comparisons of model results with buoy measurements show that taking into account tides and currents improve the spectral shape especially in areas of high current speeds. Whereas SWAN performed better in terms of spectral shape, especially in case of two peaked spectra, the K-model showed better results in terms of integrated parameters.Responsible Editor: Hans Burchard     

A central difference method with low numerical dispersion for solving the scalar wave equation

In this paper, we propose a nearly‐analytic central difference method, which is an improved version of the central difference method. The new method is fourth‐order accurate with respect to both space and time but uses only three grid points in spatial directions. The stability criteria and numerical dispersion for the new scheme are analysed in detail. We also apply the nearly‐analytic central difference method to 1D and 2D cases to compute synthetic seismograms. For comparison, the fourth‐order Lax‐Wendroff correction scheme and the fourth‐order staggered‐grid finite‐difference method are used to model acoustic wavefields. Numerical results indicate that the nearly‐analytic central difference method can be used to solve large‐scale problems because it effectively suppresses numerical dispersion caused by discretizing the scalar wave equation when too coarse grids are used. Meanwhile, numerical results show that the minimum sampling rate of the nearly‐analytic central difference method is about 2.5 points per minimal wavelength for eliminating numerical dispersion, resulting that the nearly‐analytic central difference method can save greatly both computational costs and storage space as contrasted to other high‐order finite‐difference methods such as the fourth‐order Lax‐Wendroff correction scheme and the fourth‐order staggered‐grid finite‐difference method.     

On the dominance of pre-existing swells over wind seas along the west coast of India

Wave data collected off Goa along the west coast of India during February 1996-May 1997 has been subjected to spectral analysis, and swell and wind sea parameters have been estimated by separation frequency method. Dominance of swells and wind seas on monthly and seasonal basis has been estimated, and the analysis shows that swells dominate Goa coastal region not only during southwest monsoon (93%), but also during the post-monsoon (67%) season. Wind seas are dominant during the pre-monsoon season (51%). The mean wave periods (T_m) during southwest monsoon are generally above 5 s, whereas T_m is below 5 s during other seasons. Co-existence of multiple peaks (from NW and NE) was observed in the locally generated part of the wave spectrum, especially during the post-monsoon season. NCEP reanalysis winds have been used to analyse active fetch available in the Indian Ocean, from where the predominant swells propagate to the west coast of India. A numerical model was set up to simulate waves in the Indian Ocean using flexible mesh bathymetry. The correlation coefficients between measured and modelled significant wave heights and mean wave periods are 0.96 and 0.85, respectively. Numerical simulations reproduced the swell characteristics in the Indian Ocean, and from the model results potential swell generation areas are identified. The characteristics of swells associated with tropical storms that prevail off Goa during 1996 have also been analysed.     

基于多台站OH全天空气辉成像仪观测的中国中纬地区重力波传播特性

利用位于中国中纬地区6个OH气辉成像仪2012年1月至2013年12月两年的观测数据, 我们研究分析了重力波传播特征.结果表明重力波的水平波长、观测周期和水平相速度分别主要分布于10~35 km, 4~12 min和30~100 m·s-1范围. 夏季,重力波主要沿极向方向传播. 然而, 冬季, 他们有向赤道方向和平行于赤道方向的传播趋势. 同时, 我们结合TRMM卫星和ECMWF数据, 发现在夏季重力波的北向传播趋势可能主要由观测台站南方的对流活动导致. 然而, 对流层顶附近的急流可能在冬季重力波的主要传播方向方面做出较大贡献. 分析结果也表明, 低层-中层大气背景风的滤波效应仅在夏季与中国中纬地区重力波纬向传播方向各向异性吻合较好.     

Variability of Convective Activity over the North Indian Ocean and its Associations with Monsoon Rainfall over India

The present study is an attempt to examine the variability of convective activity over the north Indian Ocean (Bay of Bengal and Arabian Sea) on interannual and longer time scale and its association with the rainfall activity over the four different homogeneous regions of India (viz., northeast India, northwest India, central India and south peninsular India) during the monsoon season from June to September (JJAS) for the 26 year period (1979 to 2004). The monthly mean Outgoing Long-wave Radiation (OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this study and the 26-year period has been divided into two periods of 13 years each with period-i from 1979 to 1991 and period -ii from 1992 to 2004. It is ascertained that the convective activity increases over the Arabian Sea and the Bay of Bengal in the recent period (period -ii; 1992 to 2004) compared to that of the former period (period -i; 1979 to 1991) during JJAS and is associated with a significantly increasing trend (at 95% level) of convective activity over the north Bay of Bengal (NBAY). On a monthly scale, July and August also show increase in convective activity over the Arabian Sea and the Bay of Bengal during the recent period and this is associated with slight changes in the monsoon activity cycle over India. The increase in convective activity particularly over the Arabian Sea during the recent period of June is basically associated with about three days early onset of the monsoon over Delhi and relatively faster progress of the monsoon northward from the southern tip of India. Over the homogeneous regions of India the correlation coefficient (CC) of OLR anomalies over the south Arabian Sea (SARA) is highly significant with the rainfall over central India, south peninsular India and northwest India, and for the north Arabian Sea (NARA), it is significant with northwest India rainfall and south peninsular rainfall. Similarly, the OLR anomalies over the south Bay of Bengal (SBAY) have significant CC with northwest India and south peninsular rainfall, whereas the most active convective region of the NBAY is not significantly correlated with rainfall over India. It is also found that the region over northeastern parts of India and its surroundings has a negative correlation with the OLR anomalies over the NARA and is associated with an anomalous sinking (rising) motion over the northeastern parts of India during the years of increase (decrease) of convective activity over the NARA.     

Rainfall and temperature trends in India

An investigation was carried out to identify trends in the rainfall and temperature regimes of the Ganga basin in India and in India as a whole. Long-term data on the monsoon and annual rainfall and the average annual temperature for India as a whole, and on the monsoon rainfall, number of rainy days and annual maximum temperature of the Ganga basin were analysed. The trends in these data were detected using non-parametric methods. The results of this study showed that the rainfall variables had a decreasing trend and the temperature had an increasing trend. These trends were observed to have begun around the second half of the 1960s, and have implications for the Indian economy. As the Indian economy continues to be based on agriculture, water resource management for irrigation plays a vital part in its growth. Present methods of design and management for water resource systems assume the climatic regime to be stationary. If indeed the climatic regime has changed, it will be necessary to develop new management approaches which consider this change.     

Variability in the vertical distribution of ozone over a subtropical site in India during a winter month

Six sets of electrochemical ozonesondes along with radiosondes were launched during 11–29 December 2004 from Kanpur (26.03N, 80.04E). Large variabilities in the vertical distribution of ozone have been observed during the campaign period. Higher ozone levels as compared to the average of all the profiles during this period have been observed in the height ranges of 3–7 and 10–18 km on December 18 and 25, respectively. Ozone levels in the 11–14 km range were observed to be much lower on December 29. These events have been analyzed in detail using meteorological parameters, back trajectories and potential vorticity. Higher ozone on December 18 may be associated with lateral transport from Africa and Gulf countries, where higher CO had been observed along the trajectory path. However, on December 25, enhanced ozone layers could be associated with transport from the stratosphere. Potential vorticity data suggest that a jet stream from midlatitude was approaching this location along the isentropic surface (350 K) towards the southeast direction. The lower ozone observed on December 29 originated from the marine region near the equator. These sharp changes in this period reflecting changing meteorology have given evidence of transport of ozone from different regions including stratospheric intrusion.