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A new mobile and portable scanning lidar for profiling the lower troposphere
No full textAn in-house developed mobile and portable three-dimensional scanning lidar system is discussed in this work. The system uses a stimulated Raman-scattering technique for the continuous observation of atmospheric aerosols, clouds and trace gases. This system has a fast scanning technique with a high-speed data acquisition, and permits the real-time measurement of atmospheric pollutants with the temporal resolution of 1 min. This scanning lidar system provides typical horizontal coverage of about 8–10 km while scanning; however, in zenith mode, good quality backscattered signals can be from 20 km, depending upon the laser power and sky conditions. This versatile lidar system has also overcome the drawbacks which are popular in the traditional scanning lidar systems such as complicated operation, overlap height between laser beam and telescope field of view In this system, the optical damage is reduced by using an integral coaxial transmitter and receiver. Some of the initial results obtained from the scanning lidar system are also presented. This study shows that boundary-layer structure and land–sea breeze circulation can be resolved from the developed scanning lidar system. The application of this lidar system to measure the pollutants over an industrial area is also discussed
Temporal and structural evolution of a tropical monsoon cloud system: A case study using X-band radar observations
No full textA mobile X-band (~9.535 GHz) dual-polarization Doppler weather radar system was operated at a tropical site Pune (18.5386°N, 73.8089°E, 582 m AMSL) by the Indian Institute of Tropical Meteorology, Pune, India for observing monsoon clouds. The measurement site was on the leeward (eastern) side of the Western Ghats (WG). This study focuses on the horizontal and vertical structure of monsoon precipitating clouds and its temporal evolution as observed by the X-band radar on August 27, 2011. The radar reflectivity factor (Z, dBZ) is used as a proxy for measure of intensity of cloud system. Result shows that the radar reflectivity has a strong temporal variation in the vertical, with a local peak occurring in the afternoon hours. Relatively shallow structure during the late night and early morning hours is noticed. The observed cloud tops were reached up to 8 km heights with reflectivity maxima of about 35 dBZ at ∼5 km. The spatial and vertical evolution of radar reflectivity is consistent with the large-scale monsoon circulation. The variations in the outgoing longwave radiation (OLR) from the Kalpana-1 satellite and vertical velocity and cloud-mixing ratio from the Modern Era-Retrospective Analysis for Research and Applications (MERRA) reanalysis data are also analyzed. As direct observations of clouds using radars are sparse over the Indian region, the results presented here would be useful to understand the processes related to cloud and precipitation formation in the tropical environment
Biological implications of cyclone Hudhud in the coastal waters of northwestern Bay of Bengal
Get PDFA very severe cyclonic storm, Hudhud, equivalent to a category-4 hurricane on the Saffir–Simpson hurricane wind scale (SSHWS), originated in the Andaman Sea on 6 October 2014. The cyclone propagated west-northwestward and made landfall near Visakhapatnam, northern coast of Andhra Pradesh on 12 October 2014. The study area, Gopalpur
(southern coast of Odisha) was in the active influence zone of Hudhud and in close proximity (~260 km north) to the
landfall point (Figure 1). This region is an important mass nesting rookery for vulnerable olive ridley sea turtles, which aggregate for breeding in the coastal waters off Odisha from November to May1. This region is also identified as a time-series station under the SATellite
Coastal and Oceanographic REsearch (SATCORE) programme coordinated by the Indian National Centre for Ocean
Information Services (INCOIS); it is being monitored since 2009
Inhibition of mixed-layer deepening during winter in the northeastern Arabian Sea by the West India Coastal Current
No full textThough the deep mixed layers (MLs) that form in the northeastern Arabian Sea (NEAS) during the winter monsoon (November–February) have been attributed to convective mixing driven by dry, cool northeasterly winds from the Indian subcontinent, data show that the deepest MLs occur in the northern NEAS and the maxima of latent-heat and net heat fluxes in the southern NEAS. We use an oceanic general circulation model to show that the deep MLs in the NEAS extend up to ~20°N till the end of December, but are restricted poleward of ~22°N (~23°N) in January (February). This progressive restriction of the deep mixed layers within the NEAS is due to poleward advection of water of lower salinity by the West India Coastal Current (WICC). The deep MLs are sustained till February in the northern NEAS because convective mixing deepens the ML before the waters of lower salinity reach this region and the wind stirring and convective overturning generate sufficient turbulent energy for the ML to maintain the depth attained in January. Though the atmospheric fluxes tend to cool the ML in the southern NEAS, this cooling is countered by the warming due to horizontal advection. Likewise, the cooling due to entrainment, which continues in the southern NEAS even as the ML shallows during January–February, is almost cancelled by the warming caused by a downwelling vertical velocity field. Therefore, the SST changes very little during December–February even as the ML shallows dramatically in the southern NEAS. These deep MLs of the NEAS also preclude a strong intraseasonal response to the intraseasonal variability in the fluxes. This role of horizontal advection implies that the ML depth in the NEAS is determined by an interplay of physical processes that are forced differently. The convective mixing depends on processes that are local to the region, but the advection is due to the WICC, whose seasonal cycle is primarily forced by remote winds. By inhibiting the formation of deep MLs in the southern NEAS, the WICC limits the region of formation of the high-salinity water masses of this region. Since the deep MLs in the NEAS have been linked to the high chlorophyll concentration there, our results imply that the conventional approach of averaging over boxes for studying the impact of physics on biogeochemistry can mask important details that are due to advection because it is the advective component of any budget that is most affected by the averaging process
Atmospheric transport of ozone between Southern and Eastern Asia
No full textThis study describes the effect of pollution transport between East Asia and South Asia on tropospheric ozone (O3) using model results from the Task Force on Hemispheric Transport of Air Pollution (TF HTAP). Ensemble mean O3 concentrations are evaluated against satellite-data and ground observations of surface O3 at four stations in India. Although modeled surface O3 concentrations are 1020 ppb higher than those observed, the relative magnitude of the seasonal cycle of O3 is reproduced well. Using 20% reductions in regional anthropogenic emissions, we quantify the seasonal variations in pollution transport between East Asia and South Asia. While there is only a difference of 0.05 to 0.1 ppb in the magnitudes of the regional contributions from one region to the other, O3 from East Asian sources affects the most densely populated parts of South Asia while Southern Asian sources only partly affect the populated parts of East Asia. We show that emission changes over East Asia between 2000 and 2010 had a larger impact on populated parts of South Asia than vice versa. This study will help inform future decisions on emission control policy over these regions
Has modulation of Indian summer monsoon rainfall by sea surface temperature of the equatorial Pacific Ocean, weakened in recent years?
No full textIn this paper, the east–west tropical ‘Walker circulation’ and its linear association with sea surface temperature (SST) of the Nino 3 region and Indian summer monsoon rainfall (ISMR) have been investigated. ‘Walker circulation’/‘reverse Walker circulation’ is primarily forced by SSTs of the equatorial Pacific Ocean. In this study, velocity potential field of 0.21 sigma level over the tropics was considered as proxy of the zonal tropical circulation (‘Walker circulation’/‘reverse Walker circulation’). Principal component analysis of the monsoon season tropical velocity potential data of 0.21 sigma level for the two periods 1951–1980 and 1981–2010, was done separately. We find that earlier, two different patterns of the velocity potential field, forced by probably two distinct modes of El Nino episodes, were associated with the ISMR. These two El Nino episodes, respectively, correspond to the strong El Nino events, where in warming was extended up to the date line (primarily zonal) and the moderate El Nino events in which, warming having north south extension, was limited to the eastern Pacific Ocean only. However, in recent years, only the first pattern of the velocity potential field, induced by the strong El Nino events (warming extending up to the date line), was correlated with the ISMR. Further, in the later period (1981–2010), velocity potential field at 0.21 sigma level over the tropical Pacific and Indian Oceans, which appeared to be primarily driven by SST anomalies of the equatorial Pacific Ocean in the first period, was found to be significantly correlated with the extra tropical circulation anomalies also. Therefore, modulation of the ISMR through velocity potential field over the tropical Indian and Pacific Oceans, in the later period, may have additional significant impact of the extra-tropical circulation anomalies. This might have led weak correlation between the ISMR and SSTs of the Nino 3 region, which is actually being observed in recent years
Monsoon onset signal in the stable oxygen and hydrogen isotope ratios of monsoon vapor
No full textWhereas data on deuterium (D) and oxygen (18O), effective tracers of hydrological processes, are available for global precipitation, such data on atmospheric water vapor are limited. With the advent of satellites capable of measuring D in atmospheric water vapor, mesoscale moisture transport processes (e.g. monsoons) could be detected early provided the signal is well above the noise. Our results of daily measurements of stable oxygen and hydrogen isotopic compositions of atmospheric water vapor (δ18Ovapor, δDvapor) and rain (δ18Orain, δDrain) during 2007–2008 CE over Ahmedabad, India, show that this is indeed so: (i) the onset of monsoon is marked by a dramatic decrease of ∼2.9‰ and ∼60‰, in δ18Ovapor, δDvapor, respectively, and 46‰ in deuterium excess (δD – 8. δ18O) of local atmospheric water vapor (ii) δ18Ovapor, δDvapor, δ18Orain and δDrain exhibit correlated variations during the monsoon season (mid-June to mid-September): the daily variations of δ18Ovapor and δDvapor are higher during large rain events, with sharp negative excursions from mean values. Deuterium excess of vapor and rain are indistinguishable from each other, within their natural variability, consistent with the achievement of isotopic equilibrium between them
Droughts of Indian summer monsoon associated with El Niño and Non-El Niño years
No full textUnderstanding the conditions of droughts are imperative for many purposes especially in planning and agricultural fields. In this paper an attempt is made to analyse rainfall distribution during droughts associated with El Niño and non-El Niño events using India Meteorological Department (IMD) daily rainfall data set having a spatial resolution of 0.25° latitude × 0.25° longitude grid. Patterns of rainfall during drought years which are not associated with El Niño have below normal rainfall over most places of Indian subcontinent, except peninsular India and eastern region. Most of the drought conditions of the Indian summer monsoon rainfall are associated with El Niño (13 of the 18 years) indicating that about 72% of the drought years are associated with the influence of Pacific Ocean. North India and most of the central Indian regions are under below normal rainfall especially over west coastal stations there the severity of drought is strong. The drought that are associated with El Nino are much intense in most parts of the subcontinent, it severely affected the entire west coastal belts, monsoon zone and eastern regions than the droughts associated with non-El Nino years. The spatial patterns of rainfall during flood years associated with La Niña events and drought years associated with El Niño reveal that the spatial structure of rainfall is highly non-linear. These results are also verified using APHRODITE rainfall data. Over central India and Western Ghats (WGs), the drought associated with El Niño gives clear indication of droughts from the early June onward, however, in the case of non-El Niño-related droughts, the indication of drought can be seen only after the first week of July. The study suggests that droughts associated with El Niño events bring severe drought conditions over WG region. However, over central India, there is no considerable difference in cumulative rainfall associated with the two types of droughts. The intraseasonal properties of rainfall are prominently different from non-El Niño to El Niño droughts. During El Niño droughts, the variance of rainfall in both the central Indian and WG regions is weaker than the droughts that are not associated with El Niño
Studying the effects of severe Aerosol Pollution of the atmosphere on the dynamics of cumulonimbus cloud charge structure by Numerical Modeling
No full textStudied are the effects of severe aerosol pollution of the atmosphere on the parameters of a cumulonimbus cloud (including its charge structure) and on precipitation. Considered is the example of the cloud that developed on May 11, 2009 near the town of Kharagpur (India) under conditions of severe aerosol pollution of the atmosphere due to dust transport from the desert. The in situ observations of the evolution of the cumulonimbus cloud of large vertical and horizontal extent and of its electric conditions were carried out on that day. It is found that the distribution of electric charges in the cloud was characterized by inverted polarity (i.e., the main positive charge is in the bottom of the cloud and the negative one is in the upper part of the cloud that contradicts usually observed cases). Using the small-dimension numerical model conducted are numerical experiments on the simulation of aerosol effects produced on the evolution of dynamic, microphysical, and electric structure of the cloud under study, namely, the cloud development under background conditions and in case of high aerosol concentration. It is assumed that aerosol particles possess ice-forming properties. It is obtained that the dynamic, microphysical, and electric structures of the cloud are significantly transformed under the influence of high aerosol concentrations; precipitation generation also significantly intensifies; polarity in the distribution of electric charge varies that agrees with the data of in situ observations
Acquisition of high resolution upper ocean spatial thermo-haline structure by Underway Conductivity Temperature and Depth (UCTD) system in the Bay of Bengal
Get PDFFirst time in the Indian ocean, Underway Conductivity-Temperature-Depth (UCTD) instrument was used measure the upper ocean thermo-haline strucuture with 500 m spatial resolution. The UCTD system was installed onboard ORV Sagar Nidhi and operated in the Bay of Bengal region during November-December 2013. While Argo float may provide the vertical structure at one place and drift according to the current, UCTD provide vertical structure with a spatial resolution of 100 m, if we operate in the top 150 m water column. Around 190 profiles with a spatial resolution of 500m and vertical resolution of 0.25 m temperature and salinity upto 150 m were acquired during this pilot cruise which was dedicated to characterize and study the upper ocean sub-mesoscale (1-10km) variability in the Bay of Bengal. The acquired data processed using standard Matlab tools and compared with onboard Thermosalinogrp