Impact of surface meteorological observations on RAMS forecast of monsoon weather systems over the Indian region

Summary An attempt has been made to study the impact of surface meteorological observations on the Regional Atmospheric Modelling System (RAMS) simulation of a monsoon depression and two low pressure systems. The surface observations are blended with the GEWEX Asian Monsoon Experiment (GAME) gridded analyses for these cases. In one set of experiments the model is run in 12 hour nudging mode initially and then in forecast mode using GAME gridded data without incorporating surface observations. In another set of experiments surface data are incorporated to enhance the signature of the systems in the large scale GAME analyses and nudging is applied initially for twelve hours. Subsequently the model is run in forecast mode to see the temporal and spatial evolution of different meteorological features associated with the systems. It is found that inclusion of the surface data has in general enhanced the signature of the systems in the analysis and subsequently shows improvement in the forecast of sea-level pressure, geopotential, wind field, etc. and the associated forecast of heavy rainfall, in particular. To make a quantitative comparison of the predicted rainfall with the observed one, equitable threat score and bias are calculated for different threshold values of rainfall. It is clearly noted that inclusion of surface data has improved the precipitation forecast over the Indian land mass as indicated by the equitable threat score and bias for all the threshold rainfall categories.     

Controls of dimethyl sulphide in the Bay of Bengal during BOBMEX-Pilot cruise 1998

The air-sea exchange is one of the main mechanisms maintaining the abundances of trace gases in the atmosphere. Some of these, such as carbon dioxide and dimethyl sulphide (DMS), will have a bearing on the atmospheric heat budget. While the former facilitates the trapping of radiation (greenhouse effect) the latter works in the opposite direction through reflectance of radiation back into space by sulphate aerosols that form from oxidation of DMS in atmosphere. Here we report on the first measurements made on DMS in the Bay of Bengal and the factors regulating its abundance in seawater. Phytoplankton alone does not seem to control the extent of DMS concentrations. We find that changes in salinity could effectively regulate the extent of DMSP production by marine phytoplankton. In addition, we provide the first ever evidence to the occurrence of DMS precursor, DMSP, in marine aerosols collected in the boundary layer. This suggests that the marine aerosol transport of DMSP will supplement DMS gaseous evasion in maintaining the atmospheric non-sea salt sulphur budget.     

A comparative study of Pleistocene phosphorites from the continental slope off western India

Two types of phosphorite recovered from the continental slope off western India are described. The first type, phosphorite 1, comprises a hard, grey nodule composed of carbonate fluorapatite (CFA) and calcite as major minerals. The phosphorite consists of light‐brown microcrystalline apatite containing a few skeletal fragments and planktonic foraminifera. Scanning electron microscope (SEM) studies show evidence of dissolution of skeletal calcite and filling of the resulting cavities by phosphate composed of ovoid to rod‐shaped apatite microparticles. Apatite also occurs as coatings on these particles. The P₂O₅ content of the phosphorite is 29%, and the CO₂ content of the CFA is about 4·5%. The rare‐earth element (REE) abundance (ΣREE=2·02 μg g^–1) is lower than in other modern phosphorites. The ⁸⁷Sr/⁸⁶Sr ratio and ?Nd value of this sample are 0·70921 and –9·9 respectively. The ¹⁴C age found through accelerator mass spectrometry (AMS) dating (18 720 ± 120 years BP) is much younger than that determined by the U‐series method (100 ka). The second type, phosphorite 2, comprises a friable, light‐brown nodule consisting of CFA as the only major mineral, with a CO₂ content of the CFA of 4·5%. In thin section, the phosphate is light brown and homogeneous, and a few bone fragments are present. The P₂O₅ content is 33%, and REE contents (ΣREE = 0·18 μg g^–1) are lower than in phosphorite 1. The age of phosphorite 2 is >300 ka. Phosphorite 1 appears to have formed during the late Pleistocene through replacement of carbonate by phosphate; phosphorite 2 is also of Pleistocene age but is much older than phosphorite 1. The initial substrate for phosphorite 2 was a fish coprolite, which was subsequently phosphatized during slow sedimentation under low‐energy conditions. Microbial mediation is evident in both phosphorites. The colour, density and P₂O₅ content of the phosphorites are found to be dependent on the nature of the initial substrates and physico‐chemical conditions during phosphatization. The CO₂ content of the CFA is not related to the precursor carbonate phase. The nature of sediments, rates of sedimentation and the time spent undergoing phosphogenesis at the sediment–water interface may control REE concentrations in phosphorites.     

Influence of fluvial‐tidal interactions on the nature of cross‐stratified packages in a deltaic setting: examples from the Barakar Coal Measure (Permian), Satpura Gondwana Basin,central India

Nine different types of cross‐stratified packages from the coal‐bearing, deltaic succession of the Barakar Formation (Permian) of the Satpura Gondwana Basin, central India, are described. The deposits are characterized by periodic mudstone drapes, reactivation surfaces including all other features suggestive of deposition from periodically unsteady, tidally‐influenced flows. The inferred flow patterns varied from purely bidirectional to pulsating unidirectional. The different types of cross‐stratified packages are interpreted to have resulted from superimposition of ebb‐oriented, steady, unidirectional fluvial currents of variable strength on the tidal flow in a deltaic setting. The study helps to distinguish cross‐strata that may develop in settings where fluvial and tidal currents interact. Copyright © 2004 John Wiley & Sons, Ltd.     

Identification of flood seasonality and drivers across Canada

Floods are the most frequently occurring natural hazard in Canada. An in-depth understanding of flood seasonality and its drivers at a national scale is essential. Here, a circular, statistics-based approach is implemented to understand the seasonality of annual-maximum floods (streamflow) and to identify their responsible drivers across Canada. Nearly 80% and 70% of flood events were found to occur during spring and summer in eastern and western watersheds across Canada, respectively. Flooding in the eastern and western watersheds was primarily driven by snowmelt and extreme precipitation, respectively. This observation suggests that increases in temperature have led to early spring snowmelt-induced floods throughout eastern Canada. Our results indicate that precipitation (snowmelt) variability can exert large controls on the magnitude of flood peaks in western (eastern) watersheds in Canada. Further, the nonstationarity of flood peaks is modelled to account for impact of the dynamic behaviour of the identified flood drivers on extreme-flood magnitude by using a cluster of 74 generalized additive models for location scale and shape models, which can capture both the linear and nonlinear characteristics of flood-peak changes and can model its dependence on external covariates. Using nonstationary frequency analysis, we find that increasing precipitation and snowmelt magnitudes directly resulted in a significant increase in 50-year streamflow. Our results highlight an east–west asymmetry in flood seasonality, indicating the existence of a climate signal in flood observations. The understating of flood seasonality and flood responses under the dynamic characteristics of precipitation and snowmelt extremes may facilitate the predictability of such events, which can aid in predicting and managing their impacts.     

Agricultural management impacts on groundwater: simulations of existing and alternative management options in Peninsular India

Understanding the principal causes and possible solutions for groundwater depletion in India is important for its water security, especially as it relates to agriculture. A study was conducted in an agricultural watershed in Andhra Pradesh, India to assess the impacts on groundwater of current and alternative agricultural management. Hydrological simulations were used as follows: (1) to evaluate the recharge benefits of water‐harvesting tillage through a modified Soil and Water Assessment Tool (SWAT) model and (2) to predict the groundwater response to changing extent and irrigation management of rice growing areas. The Green–Ampt infiltration routine was modified in SWAT was modified to represent water‐harvesting tillage using maximum depression storage parameter. Water‐harvesting tillage in rainfed croplands was shown to increase basin‐scale groundwater recharge by 3% and decrease run‐off by 43% compared with existing conventional tillage. The groundwater balance (recharge minus irrigation withdrawals), negative 11 mm/year under existing management changed to positive (18–45 mm/year) when rice growing areas or irrigation depths were reduced. Groundwater balance was sensitive to changes in rice cropland management, meaning even small changes in rice cropland management had large impacts on groundwater availability. The modified SWAT was capable of representing tillage management of varying maximum depression storage, and tillage for water‐harvesting was shown to be a potentially important strategy for producers to enhance infiltration and groundwater recharge, especially in semi‐arid regions where rainfall may be becoming increasingly variable. This enhanced SWAT could be used to evaluate the landscape‐scale impacts of alternative tillage management in other regions that are working to develop strategies for reducing groundwater depletion. Copyright © 2013 John Wiley & Sons, Ltd.