Mechanism of high rainfall over the Indian west coast region during the monsoon season

The mechanism responsible for high rainfall over the Indian west coast region has been investigated by studying dynamical, thermodynamical and microphysical processes over the region for the monsoon season of 2009. The European Centre for Medium-Range Weather Forecasts wind and NCEP flux data have been used to study the large scale dynamical parameters. The moist adiabatic and multi-level inversion stratifications are found to exist during the high and low rainfall spells, respectively. In the moist adiabatic stratification regime, shallow and deep convective clouds are found coexisting. The Cloud Aerosol Interaction and Precipitation Enhancement EXperiment aircraft data showed cloud updraft spectrum ranging from 1 to 10 m s^?1 having modal speed 1–2.5 m s^?1. The low updrafts rates provide sufficient time required for warm rain processes to produce rainfall from shallow clouds. The low cloud liquid water is observed above the freezing level indicating efficient warm rain process. The updrafts at the high spectrum end go above freezing level to generate ice particles produced due to mixed-phase rainfall process from deep convective clouds. With aging, deep convection gets transformed into stratiform type, which has been inferred through the vertical distribution of the large scale omega and heating fields. The stratiform heating, high latent heat flux, strong wind shear in the lower and middle tropospheric levels and low level convergence support the sustenance of convection for longer time to produce high rainfall spell. The advection of warm dry air in the middle tropospheric regions inhibits the convection and produce low rainfall spell. The mechanisms producing these spells have been summarized with the block diagram.     

Decrease in evaporation over the Indian monsoon region: implication on regional hydrological cycle

Using surface observations from 58 widely distributed stations over India, a highly significant (99.9 %) decreasing trend of pan evaporation (E_pan) of 9.24 mm/a/a is calculated for 1971 to 2010. This constitutes a ~10 % reduction of E_pan over the last four decades. While E_pan is decreasing during the wet summer monsoon season (JJAS), as well as during the dry rest of the year, the rate of decrease during the dry season is much larger than that during the wet season. Apart from increasing solar dimming, surface winds are also persistently decreasing over the Indian sub-continent at the rate of ?0.02 m/s/a resulting in ~40 % reduction over the last four decades. Based on PenPan model, it is shown that both the above factors contribute significantly to the decreasing trend in E_pan. On a continental scale, annual mean potential evaporation (E_p) is larger than rainfall (P or E_p-P > 0, moisture divergence) indicating that India is water-limited. However, during wet monsoon P > E_p (or E_p-P < 0, moisture convergence) indicating that India is energy-limited during this season. Long term data shows that annually E_p-P follows a significant decreasing trend indicating that water limitation is decreasing with time. This is largely due to stronger decreasing trend of E_p-P during the dry season compared to weaker increasing trend of E_p-P during the wet monsoon season. The scatter plot of E_p-P versus E_p also conveys that the decrease in E_p leads to increase in moisture convergence in wet season and decrease in moisture divergence in dry season.     

Hydrogeochemistry and water quality assessment of shallow aquifers in the western flanks of Southern Western Ghats,SW India

The present study deals with the hydrogeochemistry and water quality of shallow aquifers in two important river basins—the Ithikkara and Kallada river basins—draining the south western flanks of Western Ghats in Kerala, South West India. Well water samples were collected from 20 dug wells with a depth range of 1 m below ground level (mbgl) to 18.2 mbgl during pre-monsoon, monsoon, and post-monsoon seasons of the year 2011–2012. These samples were analyzed for various physico-chemical parameters following standard methods and were evaluated for their interrelations and drinking water suitability. The pH of the water samples shows wide variation from highly acidic to highly alkaline water. About 80% of pre-monsoon samples recorded Fe²⁺ concentration above the permissible limit of drinking water standard. Water Quality Index (WQI) shows that majority of the well water samples fall in the category of excellent–good for drinking purpose. The results of the irrigation suitability assessment using the procedures like Percent Sodium, Sodium Absorption Ratio, Residual Sodium Carbonate, Kelly Index, Permeability Index, and Magnesium Hazard reveal that the well waters of the study area are fit for irrigation purpose. Na⁺/Cl^? ratio reflects the release of sodium to water due to silicate weathering. The samples have a Ca²⁺/Mg²⁺ ratio equal or greater than 2 indicating the effect of silicate minerals in contributing Ca²⁺ and Mg²⁺ ions to the well water. The saturation indices reveal that groundwater is supersaturated with SiO₂. Among the causative factors that determine the hydrochemical quality of well water samples, silicate weathering plays a pivotal role with significant input of ions from anthropogenic sources.     

Rainfall mechanism over the rain-shadow region of north peninsular India

Climate Dynamics - Rainfall mechanism over the rain-shadow region of north peninsular India during the summer monsoon season has been investigated using dynamic, thermodynamic, cloud microphysics...     

Intra-seasonal and Inter-annual variability of Bowen Ratio over rain-shadow region of North peninsular India

Intra-seasonal and inter-annual variability of Bowen Ratio (BR) have been studied over the rain-shadow region of north peninsular India during summer monsoon season. Daily grid point data of latent heat flux (LHF), sensible heat flux (SHF) from NCEP/NCAR Reanalysis for the period 1970–2014 have been used to compute daily area-mean BR. Daily grid point rainfall data at a resolution of 0.25° × 0.25° from APHRODITE’s Water Resources for the available period 1970–2007 have been used to study the association between rainfall and BR. The study revealed that BR rapidly decreases from 4.1 to 0.29 in the month of June and then remains nearly constant at the same value (≤0.1) in the rest of the season. High values of BR in the first half of June are indicative of intense thermals and convective clouds with higher bases. Low values of BR from July to September period are indicative of weak thermals and convective clouds with lower bases. Intra-seasonal and inter-annual variability of BR is found to be inversely related to precipitation over the region. BR analysis indicates that the land surface characteristics of the study region during July–September are similar to that over oceanic regions as far as intensity of thermals and associated cloud microphysical properties are concerned. Similar variation of BR is found in El Nino and La Nina years. During June, an increasing trend is observed in SHF and BR and decreasing trend in LHF from 1976 to 2014. Increasing trend in the SHF is statistically significant.