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An assessment of wind forcing impact on a spectral wave model for the Indian Ocean
Get PDFThe focus of the present study is the assessment of the impact of wind forcing on the spectral wave model MIKE 21 SW in the Indian Ocean region. Three different wind fields, namely the ECMWF analyzed winds, the ECMWF blended winds, and the NCEP blended winds have been used to drive the model. The wave model results have been compared with in-situ observations and satellite altimeter data. This study also evaluated the performance of the wind products during local phenomenon like sea breeze, since it has a significant impact on the wave prediction in the Indian coastal region. Hence we explored the possibility of studying the impact of diurnal variation of winds on coastal waves using different wind fields. An analysis of the model performance has also been made during high wind conditions with the inference that blended winds generate more realistic wave fields in the high wind conditions and are able to produce the growth and decay of waves more realistically
Observations of black carbon induced semi direct effect over Northeast India
No full textThis article reports observational evidence of Black Carbon (BC) induced cloud burning effect (Semi direct effect) for the first time over a mountainous location in North east India. Simultaneous aircraft observations of Black Carbon (BC) mass concentrations and cloud microphysical parameters were carried out over Guwahati, in Northeast India during Cloud Aerosol Interactions and Precipitation Enhancement Experiment (CAIPEEX) Phase-I in 2009. Elevated pollution layers of BC (concentration exceeding 1μgm-3) were observed over the site up to 7km on different experimental days (August 30, September 4 and 6 in 2009) in the cloud regime. The vertical heating rate and radiative forcing induced by elevated BC layers in the cloud regime were estimated using an optical model along with a radiative transfer model. The instantaneous vertical heating rate induced by BC in cloud layers is found to be as high as 2.65K/day. The instantaneous vertical heating by BC is found to be inducing a significant reduction in the measured cloud liquid water content (LWC) over the site. Subsequently, the BC stimulated heating has been found to be reducing the cloud fraction (CFR) and thus inducing a "cloud burning effect (Semi direct effect)", over the region. The estimated instantaneous BC induced radiative forcing in the cloud regime is found to be+12.7-+45.1Wm-2 during the experimental periods. This large warming and reduction in cloudiness can decrease the precipitation over the region. However, more simultaneous BC-cloud observations and further research are necessary to establish a stable "semi-direct effect" over the region
Erratum to: Changes in western disturbances over the Western Himalayas in a warming environment
No full textPossible evidence of new particle formation and its impact on cloud microphysics from airborne measurements over Bay of Bengal
No full textAirborne measurements conducted under a special mission over Bay of Bengal (BoB) during the CAIPEEX (Cloud Aerosol Interaction and Precipitation Enhancement EXperiment) in 2011 were analyzed in the present study. Research flights were carried out on 19 and 20 October, 2011 (referred as RF1 and RF2), in the region over BoB, which was influenced by a depression to evaluate the aerosol–cloud interactions over marine environment. The increased concentration of aitken/accumulation mode particles was observed at 500 m above sea surface level over the ocean after the passage of the depression. The source of these particles and their subsequent growth during RF1 at about 200 km from coastline has been attributed to (i) increased production of aerosols due to oxidation of dimethyl sulfide (DMS) because of upwelling of the deep ocean water during the depression and (ii) anthropogenic aerosols transported from inland. Moreover, measurements of accumulation and coarse mode particles with diameter ranging from 0.1 to 3 μm and cloud droplets in the range 3 to 47 μm show systematic growth associated with cloud microphysical/rain formation process. On the other hand, no such evidence of increasing particle concentration and growth has been observed at about 60 km from coastline towards southeast during RF2. Evidently, the rain event observed during the night hours of 19 October caused the washout and scavenging of aerosols which contributed towards the decreased aerosol concentration observed near the coast
Prolonged monsoon droughts and links to Indo-Pacific warm pool: A Holocene record from Lonar Lake, central India
No full textConcerns about the regional impact of global climate change in a warming scenario have highlighted the gaps in our understanding of the Indian Summer Monsoon (ISM, also referred to as the Indian Ocean summer monsoon) and the absence of long term palaeoclimate data from the central Indian core monsoon zone (CMZ). Here we present the first high resolution, well-dated, multiproxy reconstruction of Holocene palaeoclimate from a 10 m long sediment core raised from the Lonar Lake in central India. We show that while the early Holocene onset of intensified monsoon in the CMZ is similar to that reported from other ISM records, the Lonar data shows two prolonged droughts (PD, multidecadal to centennial periods of weaker monsoon) between 4.6-3.9 and 2-0.6 cal ka. A comparison of our record with available data from other ISM influenced sites shows that the impact of these PD was observed in varying degrees throughout the ISM realm and coincides with intervals of higher solar irradiance. We demonstrate that (i) the regional warming in the Indo-Pacific Warm Pool (IPWP) plays an important role in causing ISM PD through changes in meridional overturning circulation and position of the anomalous Walker cell; (ii) the long term influence of conditions like El Niño-Southern Oscillation (ENSO) on the ISM began only ca. 2 cal ka BP and is coincident with the warming of the southern IPWP; (iii) the first settlements in central India coincided with the onset of the first PD and agricultural populations flourished between the two PD, highlighting the significance of natural climate variability and PD as major environmental factors affecting human settlements
Whistlers detected and analyzed by Automatic Whistler Detector (AWD) at low latitude Indian stations
No full textRecently, at three Indian low latitude stations: Varanasi (geomag. lat. 14°55′N, geomag. long. 153°54′E, L: 1.078), Allahabad (geomag. lat. 16.05°N; geomag. long. 155.34°E, L: 1.081) and Lucknow (geomag. lat. 17.6°N, geomag. long. 154.5°E, L: 1.104) an Automatic Whistler Detector (AWD) has been installed in December, 2010 for detection and analysis of whistlers. This instrument automatically detects and collects statistical whistlers data for the investigation of whistlers generation and propagation. Large numbers of whistlers have been recorded at Varanasi and Allahabad during the year 2011 which is analyzed in the present study. Different types of whistlers have been recorded at Varanasi and Allahabad. The correlation between recorded whistlers and causative lightning strikes were analyzed using data provided by World-Wide Lightning Location Network (WWLLN). We observed that for both the stations more than 50% of causative sferics of whistlers were observed to match closely with the times of WWLLN detected lightning strikes within the propagation times of causative tweeks. All of these lightning strikes originated from the region within 500–600 km radius circle from the conjugate point of Varanasi and Allahabad supports the ducted propagation at low latitude stations. The dispersion of the observed whistlers varies between 8 and 18 s1/2, which shows that the observed whistlers have propagated in ducted mode and whole propagation path of whistlers lies in the ionosphere. The ionospheric columnar electron contents of these observed whistlers vary between 13.21 TECU and 56.57 TECU. The ionospheric parameters derived from whistler data at Varanasi compare well with the other measurements made by other techniques
Relationship between size of cloud ice and lightning in the tropics
Get PDFThe association of lightning flashes with mean cloud ice size over continental and oceanic region in the tropical areas has been analyzed using the observations from various satellite platforms (MODIS, TRMM, and LIS) for the period 2000-2011. We found that frequency of lightning in general is higher over the continental region compared to oceanic region, whereas larger size of cloud ice is observed over the oceanic regions compared to the continental regions. Relationship between lighting and cloud ice size shows similar features over both continental and oceanic regions. For the first time, we show that total lighting increases with increase in the cloud ice size; attends maximum at certain cloud ice size and then decreases with increase in cloud ice size. Maximum lightning occurred for the mean cloud ice size of around 23-25 μm over the continental region and mean cloud ice size of around 24-28 μm over the oceanic region. Based on our observation we argue that the relation between lightning and mean cloud ice size follow the curve linear pattern, and not linear
Ozone trends in the vertical structure of Upper Troposphere and Lower stratosphere over the Indian monsoon region
No full textOzone trends in the Upper Troposphere and Lower Stratosphere over the Indian region are investigated using three satellite data sets namely Halogen Occultation Experiment (1993-2005), Stratospheric Aerosol and Gas Experiment (1993-2005) II, and Aura Microwave Limb Sounder (MLS, 2005-2011). Estimated ozone trends using multi-variate regression analysis are compared with trends at two Indian ozonesonde stations (Delhi, 28°N, 77°E and Pune, 18°N, 73°E), and a 3-D Chemical Transport Model (CTM, SLIMCAT) for the 1993-2005 time period. Overall, all the observational data sets and model simulations indicate significant increasing trend in the upper troposphere (0-2.5 /year). In the lower stratosphere, estimated trends are slightly positive up to 30 mb and are negative between 30 and 10 mb. Increasing trends in the upper troposphere is probably due to increasing trends in the tropospheric ozone precursor gases (e.g. CO, NOx, NMHCs). Here, we argue that these contrasting ozone-trend profiles might be partially responsible for insignificant long-term trends in the tropical total column ozone. On seasonal scale, positive trends are observed during all the seasons in the upper troposphere while structure of trend profile varies in lower stratosphere. Seasonal variations of ozone trends and its linkages with stratospheric intrusions and increasing trends in lightning flashes in the troposphere are also discussed
Mechanism of high rainfall over the Indian west coast region during the monsoon season
No full textThe 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
An assessment of the diversity in scenario-based tsunami forecasts for the Indian Ocean
No full textThis work examines the extent to which tsunami forecasts from different numerical forecast systems might be expected to differ under real-time conditions. This is done through comparing tsunami amplitudes from a number of existing tsunami scenario databases for eight different hypothetical tsunami events within the Indian Ocean. Forecasts of maximum tsunami amplitude are examined at 10 output points distributed throughout the Indian Ocean at a range of depths. The results show that there is considerable variability in the forecasts and on average, the standard deviation of the maximum amplitudes is approximately 62 of the mean value. It is also shown that a significant portion of this diversity can be attributed to the different lengths of the scenario time series. These results have implications for the interoperability of Regional Tsunami Service Providers in the Indian Ocean