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Bhate J.,National Atmospheric Research Laboratory
Climate Dynamics

The summer monsoon rainfall over India exhibits strong intraseasonal variability. Earlier studies have identified Madden Julian Oscillation (MJO) as one of the most influencing factors of the intraseasonal variability of the monsoon rainfall. In this study, using India Meteorological Department (IMD) high resolution daily gridded rainfall data and Wheeler-Hendon MJO indices, the intra-seasonal variation of daily rainfall distribution over India associated with various Phases of eastward propagating MJO life cycle was examined to understand the mechanism linking the MJO to the intraseasonal variability. During MJO Phases of 1 and 2, formation of MJO associated positive convective anomaly over the equatorial Indian Ocean activated the oceanic tropical convergence zone (OTCZ) and the resultant changes in the monsoon circulation caused break monsoon type rainfall distribution. Associated with this, negative convective anomalies over monsoon trough zone region extended eastwards to date line indicating weaker than normal northern hemisphere inter tropical convergence zone (ITCZ). The positive convective anomalies over OTCZ and negative convective anomalies over ITCZ formed a dipole like pattern. Subsequently, as the MJO propagated eastwards to west equatorial Pacific through the maritime continent, a gradual northward shift of the OTCZ was observed and negative convective anomalies started appearing over equatorial Indian Ocean. During Phase 4, while the eastwards propagating MJO linked positive convective anomalies activated the eastern part of the ITCZ, the northward propagating OTCZ merged with monsoon trough (western part of the ITCZ) and induced positive convective anomalies over the region. During Phases 5 and 6, the dipole pattern in convective anomalies was reversed compared to that during Phases 1 and 2. This resulted active monsoon type rainfall distribution over India. During the subsequent Phases (7 and 8), the convective and lower tropospheric anomaly patterns were very similar to that during Phase 1 and 2 except for above normal convective anomalies over equatorial Indian Ocean. A general decrease in the rainfall was also observed over most parts of the country. The associated dry conditions extended up to northwest Pacific. Thus the impact of the MJO on the monsoon was not limited to the Indian region. The impact was rather felt over larger spatial scale extending up to Pacific. This study also revealed that the onset of break and active events over India and the duration of these events are strongly related to the Phase and strength of the MJO. The break events were relatively better associated with the strong MJO Phases than the active events. About 83% of the break events were found to be set in during the Phases 7, 8, 1 and 2 of MJO with maximum during Phase 1 (40%). On the other hand, about 70% of the active events were set in during the MJO Phases of 3 to 6 with maximum during Phase 4 (21%). The results of this study indicate an opportunity for using the real time information and skillful prediction of MJO Phases for the prediction of break and active conditions which are very crucial for agriculture decisions. © 2009 Springer-Verlag. Source

Patra A.K.,National Atmospheric Research Laboratory
Journal of Geophysical Research: Space Physics

Gadanki radar observations of the daytime 150 km echoes displaying descending features, resembling the descending ion layer behavior, are presented. The descending pattern is intriguing since it indicates the possible role of density gradient associated with the ion layer in generating the irregularities responsible for 150 km radar echoes, not envisioned before. Unusually strong SNR and narrow spectral features of these echoes clearly indicate the role of sharp density gradients in the echoing process. Given the fact that Gadanki is located at magnetically low latitude, it is argued that these descending echoing layers are produced by interchange instability on the gradient of daytime descending ion layer formed by meridional wind shear associated with tidal/gravity waves quite similar to that observed during nighttime. The observations reported here are first of its kind illustrating the role of density gradient layers and are important in furthering our understanding on the puzzling 150 km echoing phenomenon. Source

Das S.K.,National Taiwan University | Jayaraman A.,National Atmospheric Research Laboratory
Atmospheric Research

Estimation of the effect of long-range transportation of anthropogenic aerosols in India is a real challenge due to the strong influence of local sources. This study addresses this issue from the measurements of aerosol optical and physical properties during 16-31 March 2006 at Kalpakkam (12.56° N and 80.12° E), a remote eastern coastal station in India. Increased anthropogenic aerosols were observed due to long-range transport from Indo-Gangetic Basin (IGB) in comparison to two other sources: central Bay of Bengal (CBoB) and northern Indian Ocean (NIO) as grouped from back-trajectory analyses of air parcels. AOD is found to be maximum of about 0.32 during IGB wind regime followed by CBoB (0.27) and NIO (0.20) winds. MODIS observed AOD is found to be high all along the wind back-trajectories connected from IGB indicating IGB as a source. Black carbon (BC) during IGB wind (2.0μg·m -3) is 65% greater than that observed during NIO wind (1.2μg·m -3). As a result, single scattering albedo becomes as low as 0.89 during IGB wind while 0.92 during NIO wind. These long-range transported aerosols cause about 65% enhancement of atmospheric radiative forcing and consequently, aerosol heating rate is also increased by about 70% during IGB wind regime (0.36K/day) compared to NIO wind regime (0.21K/day). Being a coastal region, Kalpakkam experiences strong diurnal variation of aerosol properties due to land and sea breezes that introduce about 30% increase of atmospheric forcing during land breeze by short-range transport of BC from nearby urban region. The present study concludes that long-range transported anthropogenic aerosols over coastal region of India cause significant enhancement of regional aerosol radiative forcing and their heating effect can have significant consequences for regional climate change by altering hydrological cycle over the tropical continental area. © 2012 Elsevier B.V. Source

Sridharan S.,National Atmospheric Research Laboratory | Sathishkumar S.,Indian Institute of Geomagnetism | Gurubaran S.,Indian Institute of Geomagnetism
Journal of Atmospheric and Solar-Terrestrial Physics

The present study demonstrates how the relationship between the high latitude northern hemispheric major sudden stratospheric warming (SSW) events of 2006 and 2009 and low-latitude mesospheric tidal variability in zonal winds observed by the MF radar at Tirunelveli (8.7°0N, 77.8°E) exists. It is found that the ozone mixing ratio increases at low latitudes during the SSW and it could probably be due to the SSW induced reversal of meridional circulation towards southward, which may aid the transport of ozone from high to low latitudes, but prevent the same from low to high latitudes. As semi-diurnal tide is produced due to solar insolation absorption of ozone and the increase in the ozone mixing ratio could be a reason for the increase in the semi-diurnal tidal amplitude. The variabilities of diurnal tide appear to be governed mostly by variation of specific humidity at 300. hPa over equator and intraseasonal variability dominates the variabilities in both the parameters. © 2011 Elsevier Ltd. Source

Sathishkumar S.,Indian Institute of Geomagnetism | Sridharan S.,National Atmospheric Research Laboratory
Journal of Geophysical Research: Space Physics

Mesospheric wind observations by the medium frequency radar and geomagnetic field observations at Tirunelveli (8.7°N, 77.8°E, 1.75°N dip angle) are used to study the relative importance of solar and lunar influences in the variabilities of mesospheric tides and equatorial electrojet (EEJ) strength during the unprecedented major stratospheric sudden warming (SSW) of 2009. It is observed that the afternoon reversal in the EEJ, popularly known as counter electrojet, occurs consecutively for several days during the SSW event, when there is an enhancement of solar semidiurnal tide in both zonal wind at 90 km and EEJ strength over Tirunelveli. Although the amplitude of lunar tides also shows enhancement, it is much less than that of solar. The diurnal tidal amplitude in zonal wind and EEJ strength also shows large enhancement just before the onset of SSW. However, solar semidiurnal tide dominates diurnal tide during the SSW. The diurnal tidal phase in zonal wind shifts to a few hours earlier during the SSW. The lunar semidiurnal tidal phase shifts to later hours in both zonal wind and EEJ strength. The main observation of the present study is that the large semidiurnal tide observed during the SSW 2009 is mostly solar driven and only partly lunar driven, although tidal planetary wave interaction also may play a vital role. Although a similar behavior is noticed during the SSW 2006 also, the large lunar semidiurnal tide observed in the EEJ strength without having large lunar semidiurnal tide in the underlying mesospheric winds needs further investigation. © 2012. American Geophysical Union. All Rights Reserved. Source

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