Radiometric observations of relative humidity profile at Kolkata show a significant fall at around 1 to 2 km height during convective rain events. An extensive investigation shows that the fall of relative humidity is not seen during calm conditions but is strongly related to the characteristics of temperature lapse rate profiles. Moreover, the phenomenon may have strong association with boundary layer structure. The reason for such anomalies in the planetary boundary layer humidity profile might be due to the release of latent heat at the mentioned altitude. The abundance of pollutant aerosols in urban regions has also been found to contribute to this relative humidity anomaly. It has also been reported that this boundary layer relative humidity is accompanied by high latent heat release and condensation of vapour to liquid which is not much prominent in other rain types as observed in stratiform rain. Hence, convective rain produces some unique boundary layer characteristics which have also been partially supported with allied satellite and multi-station observations. © 2017 Elsevier B.V.

R CHAKRABORTY

S TALUKDAR

S JANA

A MAITRA

University of Calcutta (informally known as Calcutta University or CU) is a collegiate public state university located in Kolkata, West Bengal, India.&nbsp;Within India it is recognized as a &quot;Five-Star University&quot; and accredited &quot;A&quot; Grade by National Assessment and Accreditation Council.

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The University of Calcutta has secured the Fifth position among the Universities of India in the prestigious &quot;Indian University Ranking 2019&quot; list, released by the NIRF of the Ministry of Human Resource Development, Government of India.

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It was established on 24 January 1857, and was one of the first institutions in Asia to be established as a multidisciplinary and Western-style university. Its alumni and faculty include several heads of state, heads of government, social reformers, prominent artists, only academy award winner and Dirac medal winner in India, many Fellows of the Royal Society and five Nobel laureates as of 2019 which is highest in South Asia.

University Of Calcutta

Atmospheric Research

The profiles of rain microstructures have been investigated for different types of precipitation using microrain radar observations at a tropical location Kolkata (22.57° N, 88.37° E). A prominent dip in radar reflectivity ( Z ) profile has been observed near the boundary layer at high rain rates. This phenomenon is due to the break-up of raindrops into smaller sizes at around 2 km height during intense convective events. The boundary layer dynamics decides the dominance of one of the two processes, namely, coalescence and break-up of rain drops, and determines the drop size distribution profile near the boundary layer. A sharp gradient of vertical velocity at 1-2 km heights is responsible for enhanced drop break-up resulting in an increase in the small drop number density. © 2004-2012 IEEE.

IEEE Geoscience and Remote Sensing Letters

Effect of Boundary Layer Dynamics on the Profiles of Rain Drop Size Distribution during Convective Rain

A nowcasting technique has been proposed to estimate the impending rain accumulation using ground-based radiometric measurements at Kolkata (22.65°N, 88.45°E), a tropical location. It has been observed that the normalized variation of brightness temperature (BT) at 31 GHz along with the standard deviation of BT at 22 GHz and instability indices, namely, lifting index, have shown definite changes before rain events. A combination of these three parameters can be effective in predicting rain events both qualitatively and quantitatively. Accordingly, a prediction model is developed and tested on several intense rain events during the period 2014-2015. The model is found to perform reasonably well in predicting intense rain about 70-75 min in advance with an efficiency of 80%. © 1980-2012 IEEE.

IEEE Transactions on Geoscience and Remote Sensing

Prediction of Rain Occurrence and Accumulation Using Multifrequency Radiometric Observations

The behaviors of various meteorological parameters during 1981–2010 are investigated to obtain any asymmetric variability of summertime near surface wind over Indian coastal boundaries. No significant changes were obtained in the trends of surface pressure, surface relative humidity, 2-metre temperature and surface precipitation; although, near surface wind speed is found to have significantly declined on the eastern coast with respect to the western coast during this period. Summertime surface wind speed on the eastern coast have decreased from 3.5 to 2.5 m s− 1 (7 to 5 knots) whereas 4.5 to 4 m s− 1 (9 to 8 knots) during the last three decades (statistical significance level ~ 95%). A decrease in the atmospheric instability may serve as the potential reason for the suppression of severe convective occurrences manifested by a parallel decrease in surface wind speeds over these regions. The local heating up of middle atmosphere (300–500 hPa pressure level) due to increased humidity and the difference in net heat flux over Arabian Sea and Bay of Bengal due to the variance of temperature gradient (1000–925 hPa) along the coastal boundaries might be responsible for this climatic disparity between the coastal regions of India since the last three decades. Summertime near surface wind speed projections for Indian sub-continent based on 7 best climate models, for RCP8.5 scenarios, has been calculated to show a mean increase by ~ 10–15% on the eastern coast (Eastern Ghats), ~ 1–2% on the western coasts (Western Ghats), ~ 1–5% decrease in the Indo-Gangetic Basin and ~ 3% decrease in the Gangetic West Bengal and adjoining Bangladesh. © 2017 Elsevier B.V.

Global and Planetary Change

East-west coastal asymmetry in the summertime near surface wind speed and its projected change in future climate over the Indian region

In this present investigation, a threefold analysis has been portrayed to comprehend the atmospheric dynamics at the Tropopause height on both short and long term basis over the Indian sub-continent. The Tropopause height as determined from the gridded temperature profiles from ERA Interim reanalysis is using a concept of “threshold lapse-rate”. It has also been noticed that an exchange between water vapour and ozone occurs along the Tropopause (100 hPa) region during occurrences of deep moist convection. The eastern Indian coastal region experiences enormous convective strength during summer (March–May), hence an increase in specific humidity (as proxy of water vapour) with subsequent decrease in ozone levels in the east-coastal stations are found comparable with the inland stations. Again, the vertical convective transport during deep convection has been found to be supported by stronger vertical wind in the coastal regions compared to inland locations. Finally, it has been comprehended that vertical transport of water vapour gradually decreases in the eastern Indian coastal regions while not much change has been noticed in inland regions. © 2017 Elsevier B.V.

Tropopause height characteristics associated with ozone and stratospheric moistening during intense convective activity over Indian sub-continent

The present analysis investigates the spatio-temporal variability of the convective parameters and associated lightning flash rates during the period 1997–2013 including the El Niño and La Niña episode. It reveals that north-western and north-eastern part along the foothills of Himalayas as well as Indo-China peninsular region and South China Sea are much convective prone zones over the South/South-East Asia. The terrain/orography of the Himalayan range, the influence of cross-equatorial low-level jet and large-scale circulation during pre-monsoon and active phase of monsoon, the western Pacific Warm Pool with increased sea surface temperature as well as the solar-heating-originated local instability instigate the convective anomaly to propagate over the north-western and north eastern Indian sub-continent along with the Indo-China peninsula and South China Sea respectively. The land surfaces of the Indian sub-continent and the sea surface of South China Sea possesses significant correlation with lightning flash rates and convective parameters whereas the sea surface surrounding Indian sub-continent do not show such good correlations among them. Although, the occurrence of convective activities during the El Niño (La Niña) gets reduced (increased), the occurrence of lightning flashes gets enhanced (diminished) during this period which may be the direct consequence of warming atmosphere in relation to changing patterns of regional climate. © 2017 Elsevier B.V.

Spatio-temporal variability of lightning and convective activity over South/South-East Asia with an emphasis during El NiÃ±o and La NiÃ±a

The association of lightning activity with the long-term as well as seasonal spatio-temporal distribution of convective available potential energy (CAPE), surface convective precipitation, vegetation cover and anthropogenic aerosol loading over the Indian sub-continent has been studied for the period 2000–2014. The north-east to north-west arc including the foothills of the Himalayas is the primary seats of lightning occurrences. The correlations of lightning activity with each of aerosol loading, vegetation cover, convective instability and convective precipitation helps us in understanding the definite entity that is responsible for changing the lightning activity in different parts of this tropical region. Lightning flash rate (LFR) has significant positive correlations (r ~ 0.5–0.7) with AOD, CAPE and surface convective precipitation but significant negative correlation (r ~− 0.4) with Normalized Difference Vegetation Index (NDVI). Using global circulation models from the Climate Model Intercomparison Project Phase 5 (CMIP5), time-series of observed and projected upper tropospheric water vapor, surface convective precipitation and aerosol optical depth (AOD) from the historical simulations (1996–2005) and RCP8.5 emission scenario (2036–2045) are analyzed over the Indian region that are vulnerable to climate change in terms of occurrence of convective events and associated hazardous lightning phenomena. This study indicates that upper tropospheric water vapor (300 hPa) has a significant linkage with the lightning occurrences associated with convective activities and strong updraft. During the mid- 21st century, AOD, surface convective precipitation and specific humidity are projected to increase by 1.42%, 2.01% and 1.40%, respectively which may result in regional changes in lightning activity over the Indian sub-continent. © 2016 Elsevier B.V.

On the association of lightning activity and projected change in climate over the Indian sub-continent

Collocated measurements of aerosol optical depth (AOD) and black carbon at different wavelengths over Kolkata, an urban region in eastern India, have been used to calculate aerosol singlescattering albedo (SSA). The wavelength dependence of SSA and AOD has been presented to discriminate the aerosol types over this highly populated metropolitan area. The spectral pattern shows that SSA decreases with wavelength for most of the time in a year and corresponding Ångström coefficient is greater than unity. These optical properties indicate the dominance of fine-mode pollutant particles over the city. The temperature lapse rate profile within the surface boundary layer has been found to be significantly influenced by the heating effect of fine-mode pollutants, and consequently, the growth of the convective processes in the lower troposphere is notably affected. In addition, a back trajectory analysis has also been presented to indicate that transported air masses can have significant impact on spectral pattern of SSA. © 2016 American Geophysical Union. All Rights Reserved.

Fulltext

Journal of Geophysical Research

Dominance of pollutant aerosols over an urban region and its impact on boundary layer temperature profile

Convection has a significant role in maintaining the atmospheric dynamics and thermodynamics, particularly in the tropical regions during pre-monsoon season, which may be due to the changing patterns in atmospheric instability and pollution. A critical analysis is done on the variability of instability indices and their significant signature to meteorological parameters and atmospheric pollution over Indian region in the warming atmosphere during 2005–2015. The present study represents that the solid as well as gaseous pollutants, in combination, produce a damping force in suppressing convective activities over the eastern coastal regions of India. A significant anti-correlation (r ~ − 0.6 to − 0.8) between instability parameters [Convective Available Potential Energy (CAPE) and Lifted Index (LI)] and atmospheric pollutants [gaseous (NO2 and SO2) and solid (BC and PM2.5)] has been obtained in the eastern coastal regions on a long-term basis. To improve the level of agreement between pollution and instability, a unitless and dimensionless index called KLURT index has been introduced, which provides correlation (r) value as high as ~ 0.6. On a real time basis, KLURT index is found to be useful as an effective precursor of thunderstorm events. The final part of this study indicates a prediction technique using KLURT index which gives a high prediction efficacy of 75%, low FAR value, extremely good BS 0.06, an excellent bias (~ 0.96) and a good lead time of 1 h for a threshold value of 12.5 in terms of predicting intense convections at the urban location, Kolkata. Thus, the present study provides an appropriate means to manifest convection as a function of changing anthropogenic factors both in long and short-term basis with high correlation values and provides significant efficacy in predicting severe weather, thereby demonstrating the usefulness of this hypothesis in various socio-economic aspects especially at the current tropical urban location. © 2017 Elsevier B.V.

Association of atmospheric pollution and instability indices: A detailed investigation over an Indian urban metropolis

In the present study, the effectiveness of nowcasting convective activities using a microwave radiometer has been examined for Kolkata (22.65° N, 88.45° E), a tropical location. It has been found that the standard deviation of brightness temperature (BT) at 22. GHz and instability indices like Lifting Index (LI), K Index (KI) and Humidity Index (HI) has shown definite changes before convective events. It is also seen that combination of standard deviation of BT at 22. GHz and LI can be most effective in predicting convection. A nowcasting algorithm is prepared using 18 isolated convective events of 2011 and in all cases, a marked variation of these parameters has been seen an hour before the event. Accordingly, a prediction model is developed and tested on convective events of 2012 and 2013. It is seen that the model gives reasonable success in predicting convective rain about 7075. min in advance with a prediction efficiency of 80%. © 2015 Elsevier B.V.

Journal	Data powered by TypesetAtmospheric Research
Publisher	Data powered by TypesetElsevier Ltd
ISSN	0169-8095