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Simi K.G.,Kerala University | Thampi S.V.,Physical Research Laboratory | Chakrabarty D.,Physical Research Laboratory | Pathan B.M.,Indian Institute of Geomagnetism | And 2 more authors.
Journal of Geophysical Research: Space Physics | Year: 2012

A case of the drastic effects of an eastward prompt penetration and a westward overshielding electric field successively affecting the daytime equatorial ionosphere during the space weather event that occurred on 24 November 2001 is presented. Under the influence of the strong eastward prompt penetration electric field starting from 11:25 Indian standard time (IST), the equatorial electrojet (EEJ) strength reached the maximum value of 225 nT at 12:42 IST, almost 7 times greater than the monthly quiet time mean at the same time. This peak EEJ value exceeds the maximum observed values during the month of November for the entire solar cycle by more than 100 nT, irrespective of quiet or disturbed conditions. Further, owing to an ensuing overshielding event that occurred during the main phase of the storm rather than the end of the main phase, this unusually large EEJ showed an equally strong polarity reversal along with a weakening of the sporadic E layer over the equator. The EEJ strength was reduced from +225 to-120 nT at ∼13:45 IST, resulting in a strong counter electrojet condition. The latitudinal variation of the F region electron density data from the CHAMP satellite reveal an ill-developed equatorial ionization anomaly at 17:00 IST (11:24 UT) over the Indian sector due to this significant weakening of the zonal electric field. These observations showcase the significant degree to which the low-latitude ionosphere can be affected by the interplanetary electric field. Copyright 2012 by the American Geophysical Union.

Das S.S.,Vikaram Sarabhai Space Center | Uma K.N.,Vikaram Sarabhai Space Center | Das S.K.,Indian Institute of Tropical Meteorology
Radio Science | Year: 2012

Short-period gravity waves associated with the passage of tropical cyclone using mesosphere-stratosphere-troposphere (MST) radar located at Gadanki (13.5°N, 79.2°E) has been discussed. The observed stratospheric gravity wave is found to have a periodicity of∼42min, vertical and horizontal wavelength of∼3.5km and 14km, respectively. Maximum amplitude of gravity wave is observed in the upper troposphere and lower stratosphere (UTLS) region due to which periodic updrafts and downdrafts are observed. This weakens the stability of tropopause, which is observed in radar signal-to-noise ratio (SNR). The enhancement of vertical momentum flux of order∼-0.6m 2/s 2 observed in the lower stratosphere is attributed to the cyclone generated gravity waves. The obstacle effect is found to be the generative mechanism for the observed gravity waves associated with the tropical cyclone. Copyright 2012 by the American Geophysical Union.

Sinha P.R.,Tata Institute of Fundamental Research | Manchanda R.K.,Tata Institute of Fundamental Research | Subbarao J.V.,Tata Institute of Fundamental Research | Dumka U.C.,Tata Institute of Fundamental Research | And 3 more authors.
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2011

The first ever in-situ measurements of size-segregated vertical profiles of aerosol in the marine atmospheric boundary layer (MABL) over the Bay of Bengal (BoB), made at five different locations during the winter Integrated Campaign of Aerosols, gases and Radiation Budget (W_ICARB) experiment is presented which showed large spatial variability in aerosol properties over BoB with high aerosol concentration over northern BoB (>500cm-3) and low aerosol concentration (≤100cm-3) over southern BoB with a moderate aerosol concentration (250-450cm-3) over far east BoB. The altitude variation of aerosol number density is found to be steady in the convective boundary layer (up to ~400m) at all locations over BoB and above that the aerosol concentration is found to decrease, except at far eastern BoB. Over far eastern BoB, the altitude distribution of aerosol number concentration showed an increase at ~600m. Examination of the simultaneous air mass back trajectories along with the observations aerosol size distribution indicates that while the aerosols advected from IGP have a strong natural (coarse mode) component where as those from the east-Asia region are in general accumulation mode (anthropogenic) dominant. © 2010 Elsevier Ltd.

Dumka U.C.,Tata Institute of Fundamental Research | Dumka U.C.,Aryabhatta Research Institute of Observational science ARIES | Manchanda R.K.,Tata Institute of Fundamental Research | Sinha P.R.,Tata Institute of Fundamental Research | And 3 more authors.
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2013

Time variability of black carbon (BC) aerosols over different timescales (daily, weekly and annual) is studied over a tropical urban location Hyderabad in India using seven channel portable Aethalometer. The results for the 2-year period (January 2009-December 2010) show a daily-mean BC variability from ~1.00±0.12μgm-3 to 12.50±3.06μgm-3, with a remarkable annual pattern of winter high and monsoon low. The BC values maximize during winter (December-January), ~6.67±0.22μgm-3, and drop during summer (June-August), ~2.36±0.09μgm-3, which establishes a large seasonal variation. Furthermore, the BC mass concentration exhibits a well-defined diurnal variation, with a morning peak and early afternoon minimum. The magnitude of the diurnal variations is seasonal dependent, which maximizes during the winter months. Air mass back trajectories indicated several different transport pathways, while the concentration weighted trajectory (CWT) analysis reveals that the most important potential sources for BC aerosols are the Indo-Gangetic plain (IGP), central India and some hot spots in Pakistan, Arabian Peninsula and Persian Gulf. The absorbing Ångström exponent (αabs) estimated from the spectral values of absorption coefficient (σabs) ranges from 0.9 to 1.1 indicating high BC/OC ratio typical of fossil fuel origin. The annual average BC mass fraction to composite aerosols is found to be (10±3) % contributing to the atmospheric forcing by (55±10) %. The BC radiative forcing at the atmosphere shows strong seasonal dependency with higher values in winter (33.49±7.01) and spring (31.78±12.89) and moderate in autumn (18.94±6.71) and summer (13.15±1.66). The BC radiative forcing at the top of the atmosphere (TOA) is positive in all months, suggesting an overall heating of the regional climate over Hyderabad. © 2013 Elsevier Ltd.

Sinha P.R.,Tata Institute of Fundamental Research | Manchanda R.K.,Tata Institute of Fundamental Research | Kaskaoutis D.G.,Sharda University | Sreenivasan S.,Tata Institute of Fundamental Research | And 2 more authors.
Atmospheric Environment | Year: 2011

This work examines the aerosol physical properties and size distribution measured in the Marine Atmospheric Boundary Layer (MABL) over entire Bay of Bengal (BoB) and Northern Indian Ocean (NIO) during the Winter Integrated Campaign on Aerosols, Gases and Radiation Budget (W-ICARB). The measurements were taken using the GRIMM optical particle counter from 27th December 2008 to 30th January 2009. The results show large spatial heterogeneities regarding both the total aerosol number concentrations (NT) and the size distributions over BoB, which in turn indicates the variations in the source strength or advection from different regions. The aerosol number size distribution seems to be bi-modal in the 72% of the cases and can also be parameterized by uni-modal or by a combination of power-law and uni-modal distributions for the rest of the cases. The mode radius for accumulation and coarse-mode particles ranges from ∼0.1-0.2 μm and ∼0.6-0.8 μm, respectively. In the northern BoB and along the Indian coast, the aerosols are mainly of sub-micron size with effective radius (Reff) ranging between 0.25 and 0.3 μm highlighting the strong anthropogenic influence, while in the open oceanic areas they are much higher (0.4-0.6 μm). It was also found that the sea-surface wind plays a considerable role in the super-micron number concentration, Reff and mode radius for coarse-mode aerosols. Using the relation between NT and columnar AOD from Terra and Aqua-MODIS we found that the majority of the aerosols are within the lower MABL, while in some areas vertical heterogeneities also exist. © 2011 Elsevier Ltd.

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