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Rajeevan M.,National Atmospheric Research Laboratory NARL
Meteorological Applications | Year: 2010

The spatial and temporal variability of extreme rainfall events over India and their long-term trend have been analysed during the southwest monsoon season from June to September (JJAS) by taking daily rainfall data for 55 years (1951-2005). The contribution of extreme rainfall events (Category iii with rainfall >124.4 mm day-1) to the total seasonal rainfall is also analysed for the same period. The analysis shows that the occurrence of extreme rainfall events over India during JJAS shows spatial variations with preferred regions of occurrence over the west coast, central parts of India and northeast India. The average frequency of extreme rainfall events along with the contribution of extreme rainfall events to the seasonal rainfall shows a significant increasing trend (above the 98% confidence level) over the Indian region during JJAS, and also during June and July. It is also found that the increasing trend of contribution from extreme rainfall events during JJAS is balanced by a decreasing trend in Category i (rainfall ≤ 64.4 mm day-1) rainfall events. Using the filtered data it is found that the increasing trend of extreme rainfall over central India is mainly contributed by synoptic scale systems (periodicity between 3 and 7 days). The seasonal mean moist convective instability (CI) during JJAS averaged over central India indicates a significant increasing trend (99.9% confidence level) and is basically due to the increasing trend of the number of days with greater degree of moist CI during JJAS, which may be one possible cause for the increasing trend in the frequency of extreme rainfall events over the region. © 2009 Royal Meteorological Society.


Dwivedi S.,SRM University | Narayanan M.S.,SRM University | Venkat Ratnam M.,National Atmospheric Research Laboratory NARL | Narayana Rao D.,SRM University
Atmospheric Chemistry and Physics | Year: 2016

Monsoon inversion (MI) over the Arabian Sea (AS) is one of the important characteristics associated with the monsoon activity over Indian region during summer monsoon season. In the present study, we have used 5 years (2009-2013) of temperature and water vapour measurement data obtained from satellite sounder instrument, an Infrared Atmospheric Sounding Interferometer (IASI) onboard MetOp satellite, in addition to ERA-Interim data, to study their characteristics. The lower atmospheric data over the AS have been examined first to identify the areas where MIs are predominant and occur with higher strength. Based on this information, a detailed study has been made to investigate their characteristics separately in the eastern AS (EAS) and western AS (WAS) to examine their contrasting features. The initiation and dissipation times of MIs, their percentage occurrence, strength, etc., has been examined using the huge database. The relation with monsoon activity (rainfall) over Indian region during normal and poor monsoon years is also studied. WAS δT values are ∼2 K less than those over the EAS, δT being the temperature difference between 950 and 850 hPa. A much larger contrast between the WAS and EAS in δT is noticed in ERA-Interim data set vis-à-vis those observed by satellites. The possibility of detecting MI from another parameter, refractivity N, obtained directly from another satellite constellation of GPS Radio Occultation (RO) (COSMIC), has also been examined. MI detected from IASI and Atmospheric Infrared Sounder (AIRS) onboard the NOAA satellite have been compared to see how far the two data sets can be combined to study the MI characteristics. We suggest MI could also be included as one of the semipermanent features of southwest monsoon along with the presently accepted six parameters. © Author(s) 2016.


Venkat Ratnam M.,National Atmospheric Research Laboratory NARL | Shravan Kumar M.,National Atmospheric Research Laboratory NARL | Basha G.,National Atmospheric Research Laboratory NARL | Anandan V.K.,ISTRAC | Jayaraman A.,National Atmospheric Research Laboratory NARL
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2010

This paper documents the effect of the annular solar eclipse of 15 January 2010 on the lower atmospheric boundary layer dynamics over a complex terrain environment at Gadanki (13.5°N, 79.2°E,) using a suite of instruments namely automatic weather station, mini boundary layer mast (15. m), Doppler SODAR, GPS radiosonde and ozonesonde observations. The net heating rates are estimated using radiative transfer algorithm before, during and after the eclipse. Effect on soil temperature is seen clearly up to 20. cm depth and at all the levels up to 15. m. Decrease in the thermal plume level, a dip in the surface layer and a strong vertical downdrafts (subsidence) are noticed during the peak eclipse. Upper layer winds did not show any variation during the eclipse. It is also found to have pronounced effect on all the surface meteorological parameters for a two-day period. © 2010 Elsevier Ltd.


Venkat Ratnam M.,National Atmospheric Research Laboratory NARL | Krishna Murthy B.V.,B1 | Jayaraman A.,National Atmospheric Research Laboratory NARL
Geophysical Research Letters | Year: 2013

Indian Summer Monsoon (ISM) is mainly characterized by seasonal wind reversal in low level jet stream and tropical easterly jet (TEJ) among several other elements of monsoon systems. TEJ is observed in general between 100 and 150 hPa during June-September over the Indian region and its strength is directly related to the monsoon rainfall. In the context of changing climate, large reduction in its extent and weakening of its strength were reported. Using high resolution measurements, we report here the observation of a sharp strengthening of the TEJ during the recent warmest decade (2001-2010), reaching its 1970s value. We also show that this change is reflected in the tropical cyclone systems and finally on the precipitation patterns over the Indian region as they are interlinked. We attribute this unusual change partly to the change in the circulation due to the tropospheric warming and lower stratospheric ozone recovery. Key Points Showed that Indian Summer Monsoon circulation has changed in recent past Its effect on number of cyclones over Bay of Bengal is also going to change ISM modelers/forecasters need to consider these issues in future modeling ©2013. American Geophysical Union. All Rights Reserved.


Gayathri B.,Sri Venkateswara University | Bhavani Kumar Y.,National Atmospheric Research Laboratory NARL
International Journal of Engineering and Technology | Year: 2013

A portable laser radar system was installed at Manora Peak, Nainital (29° 22' N, 79°27' E, 1960 m MSL) under a scientific collaborative programme between National Atmospheric Research Laboratory (NARL), Gadanki and Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital. The laser radar system provides the extinction of vertical distribution of airborne particles in the atmosphere. In this paper, we show the laser radar observations obtained on two consecutive days. A drastic reduction in the height distribution of airborne particles due to heavy rain scavenging processes in the atmosphere was seen in the laser radar observations obtained on 17 May 2006. The airborne particle optical depth (AOD) obtained on 16 May 2006 was about 0.21 at λ=0.532 μm, which is typical for the month of May at this high altitude site. However, on 17 May 2006 this value reduced to 0.08 due to the rain washout effect in the height range of 0.2 to 3.0 km AGL.


Pramitha M.,National Atmospheric Research Laboratory NARL | Venkat Ratnam M.,National Atmospheric Research Laboratory NARL | Taori A.,National Atmospheric Research Laboratory NARL | Krishna Murthy B.V.,B1 | And 2 more authors.
Atmospheric Chemistry and Physics | Year: 2015

Sources and propagation characteristics of high-frequency gravity waves observed in the mesosphere using airglow emissions from Gadanki (13.5° N, 79.2° E) and Hyderabad (17.5° N, 78.5° E) are investigated using reverse ray tracing. Wave amplitudes are also traced back, including both radiative and diffusive damping. The ray tracing is performed using background temperature and wind data obtained from the MSISE-90 and HWM-07 models, respectively. For the Gadanki region, the suitability of these models is tested. Further, a climatological model of the background atmosphere for the Gadanki region has been developed using nearly 30 years of observations available from a variety of ground-based (MST radar, radiosondes, MF radar) and rocket- and satellite-borne measurements. ERA-Interim products are utilized for constructing background parameters corresponding to the meteorological conditions of the observations. With the reverse ray-tracing method, the source locations for nine wave events could be identified to be in the upper troposphere, whereas for five other events the waves terminated in the mesosphere itself. Uncertainty in locating the terminal points of wave events in the horizontal direction is estimated to be within 50-100 km and 150-300 km for Gadanki and Hyderabad wave events, respectively. This uncertainty arises mainly due to non-consideration of the day-to-day variability in the tidal amplitudes. Prevailing conditions at the terminal points for each of the 14 events are provided. As no convection in and around the terminal points is noticed, convection is unlikely to be the source. Interestingly, large (~9 ms-1 km-1) vertical shears in the horizontal wind are noticed near the ray terminal points (at 10-12 km altitude) and are thus identified to be the source for generating the observed high-phase-speed, high-frequency gravity waves. © Author(s) 2015.


Leena P.P.,National Atmospheric Research Laboratory NARL | Venkat Ratnam M.,National Atmospheric Research Laboratory NARL | Krishna Murthy B.V.,B1
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2012

In the present study, characteristics of inertia gravity waves (IGWs), associated momentum and heat fluxes, and their source mechanisms have been studied using five years (2006-2011) of high resolution radiosonde observations collected from the tropical station, Gadanki (13.5°N, 79.2°E), India. The deduced horizontal wavelengths are of the order of a few 1000s km in the troposphere and stratosphere in contrast to those low horizontal wavelengths reported earlier from this location. The estimated horizontal wavelengths match well with those reported very recently using satellite (HIRDLS and SABER) measurements. Clear link between the fluxes and background wind are noticed with enhanced values during the westward phase of zonal wind. Although two sources for the generation of IGWs namely strong convection and wind shears coexist during monsoon season, wind shear is found to be mainly responsible. © 2012 Elsevier Ltd.


Leena P.P.,National Atmospheric Research Laboratory NARL | Venkat Ratnam M.,National Atmospheric Research Laboratory NARL | Krishna Murthy B.V.,B1 | Vijaya Bhaskara Rao S.,Sri Venkateswara University
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2012

Until now most of the gravity waves (GWs) characteristics reported using radiosonde observations are of low frequency waves. In the present study, a method to detect high frequency GWs using radiosonde observations has been presented. Making use of this method, long-term high resolution radiosonde data at Gadanki (13.5°N, 79.2°E), a tropical station in India, has been analyzed. The vertical (horizontal) wavelengths of the GWs lie in the range of 6-12 km (100-300 km) and 3-7 km (100-500 km) in the troposphere and lower stratosphere, respectively. From the simultaneous MST radar observations the periods of these GWs are found to be in the range of 2-6. h. The propagation direction is towards south-east/north-west and south-east in the lower troposphere and lower stratosphere, respectively. These characteristics are quite different from those reported for the inertial period GWs. This analysis, if extended to the global network of radiosonde observations, will help to parameterize the high frequency GWs in the global models. © 2012 Elsevier Ltd.


Satheesh Kumar S.,National Atmospheric Research Laboratory NARL | Narayana Rao T.,National Atmospheric Research Laboratory NARL | Taori A.,National Atmospheric Research Laboratory NARL
Atmospheric Measurement Techniques | Year: 2015

The paper explores the possibility of implementing an advanced photogrammetric technique, generally employed for satellite measurements, on airglow imager, a ground-based remote sensing instrument primarily used for upper atmospheric studies, measurements of clouds for the extraction of cloud motion vectors (CMVs). The major steps involved in the algorithm remain the same, including image processing for better visualization of target elements and noise removal, identification of target cloud, setting a proper search window for target cloud tracking, estimation of cloud height, and employing 2-D cross-correlation to estimate the CMVs. Nevertheless, the implementation strategy at each step differs from that of satellite, mainly to suit airglow imager measurements. For instance, climatology of horizontal winds at the measured site has been used to fix the search window for target cloud tracking. The cloud height is estimated very accurately, as required by the algorithm, using simultaneous collocated lidar measurements. High-resolution, both in space and time (4 min), cloud imageries are employed to minimize the errors in retrieved CMVs. The derived winds are evaluated against MST radar-derived winds by considering it as a reference. A very good correspondence is seen between these two wind measurements, both showing similar wind variation. The agreement is also found to be good in both the zonal and meridional wind velocities with RMSEs < 2.4 m s-1. Finally, the strengths and limitations of the algorithm are discussed, with possible solutions, wherever required.


Ratnam M.V.,National Atmospheric Research Laboratory NARL | Basha S.G.,National Atmospheric Research Laboratory NARL
Atmospheric Science Letters | Year: 2010

In this study, we introduce a robust method for precise determination of atmospheric boundary layer (ABL) top from COSMIC global positioning system radio occultation measurements. We apply a wavelet covariance transform to compute the convolution of COSMIC-observed bending angle/refractivity profile with a Haar function and use the maximum covariance to identify the ABL top, making detection of even small transitions possible. Results obtained were compared with radiosonde N profiles for verification of the ABL top. This procedure developed was used to study the global distribution of ABL top with special reference to the inter-tropical convergence zone. Copyright © 2010 Royal Meteorological Society.

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