Dr. K. Niranjan Kumar

Aridity is typically associated with deep and dry daytime boundary layers, stable nighttime stratification, divergent flows and limited large-scale moisture advection. All these factors are paramount in regulating the hydro-climatology of hyper-arid regions, resulting in extremely intermittent – and often intense – local precipitation patterns. However, the link between synoptic-scale forcing and intense precipitation over arid regions has been scarcely investigated in the literature, and still remains exceedingly unexplored. We present here a diagnostic study of intense precipitation in the Central Arabian Peninsula, based on the analysis of local extreme signatures embedded in synoptic patterns. Special emphasis is given to the genesis of winter extremes over the Peninsula, and to possible effects of synchronization between the atmospheric circulation over the Mediterranean and the Indian Ocean. Based on composites of the tropospheric circulation for a large ensemble of intense events, we show that moisture necessary to trigger winter extremes over the Peninsula starts to build up in average 8 days before heavy rainfall occurrence, mainly as a consequence of the interplay between the Mediterranean and the Monsoonal circulation. Moisture advection is in turn associated with an upper-troposphere cyclonic circulation and pronounced potential vorticity intrusions. Overall, our results demonstrate how large-scale precursors can be effectively used to improve the predictability of rainfall extremes in hyper-arid regions.

We present first report on the periodic wave-like signatures (WLS) in the D-region ionosphere during 22 July 2009 total solar eclipse using JJI, Japan, VLF navigational transmitter signal (22.2 kHz) observations at stations, Allahabad, Varanasi and Nainital in Indian Sector, Busan in Korea and Suva in Fiji. The signal amplitude increased on 22 July by about 6 and 7 dB at Allahabad and Varanasi and decreased by about 2.7, 3.5, and 0.5 dB at Nainital, Busan and Suva, respectively, as compared to 24 July 2009 (normal day). The increase/decrease in the amplitude can be understood in terms of modal interference at the sites of modes converted at the discontinuity created by the eclipse intercepting the different transmitter-receiver great circle paths. The wavelet analysis shows the presence of WLS of period ~16-40 minutes at stations under total eclipse and of period ~30-80 minutes at stations under partial eclipse (~85-54 % totality) with delay times between ~50-100 minutes at different stations. The intensity of WLS was maximum for paths in the partially eclipsed region and minimum in the fully eclipsed region. The features of WLS on eclipse day seem almost similar to WLS observed in the nighttime of normal days (e.g., 24 July 2009). The WLS could be generated by sudden cutoff of the photo-ionization creating nighttime like conditions in the D-region ionosphere and solar eclipse induced gravity waves coming to ionosphere from below and above. The present observations shed additional light on the current understanding of wave induced D-region ionospheric perturbations.

The present study investigates the role of equatorial Pacific sea surface temperatures (SSTs) on the precipitation variability over the United Arab Emirates (UAE) and adjoining Middle East regions. Monthly precipitation data (1981–2011) assembled from rain gauge stations located in the UAE along with other global reanalysis data sets are used to explore the teleconnections. It is observed that statistically significant correlations exist between precipitation over the UAE and the equatorial Pacific and North Atlantic SSTs. Canonical correlation analysis between the monthly winter precipitation and the global SSTs (60°S to 60°N) reveals that the major portion of the precipitation variability is influenced by equatorial Pacific SSTs associated with El Niño–Southern Oscillation (ENSO). The moisture budget analysis reveals the distinct change in the anomalous circulation (cyclonic and anticyclonic) associated with strong convergence and divergence of the moisture flux during the warm and cold phases of ENSO, respectively. Further, the composite analysis of upper troposheric zonal wind shows the equatorward shift (~2° latitude) of subtropical jet stream (STJ) over the Middle East during the warm phase of ENSO affecting the weather in the UAE. The findings suggest that the teleconnection linking ENSO and the precipitation over UAE and adjoining regions is mediated by the response of STJ to Rossby waves.

In this study, for the first time, planetary wave signatures in ionospheric Total Electron Content (TEC) retrieved from global positioning service (GPS), mesospheric wind and temperature at low latitude station has been identified during January-February 2009. Our investigations revealed that planetary waves with 3–5 days periodicity characterized as ultrafast Kelvin (UFK) waves caused by stratospheric warming event during January 2009. The UFK waves are observed to be propagated from lower atmosphere to ionosphere. The UFK perturbations during SSW event are discussed in the light of current understanding of role of non linear interaction of planetary waves to modulate low latitude ionosphere.

Arid and semiarid climates occupy more than 1/4 of the land surface of our planet, and are characterized by a strongly intermittent hydrologic regime, posing a major threat to the development of these regions. Despite this fact, a limited number of studies have focused on the climatic dynamics of precipitation in desert environments, assuming the rainfall input – and their temporal trends – as marginal compared with the evaporative component. Rainfall series at four meteorological stations in the United Arab Emirates (UAE) were analyzed for assessment of trends and detection of change points. The considered variables were total annual, seasonal and monthly rainfall; annual, seasonal and monthly maximum rainfall; and the number of rainy days per year, season and month. For the assessment of the significance of trends, the modified Mann-Kendall test and Theil-Sen’s test were applied. Results show that most annual series present decreasing trends, although not statistically significant at the 5% level. The analysis of monthly time series reveals strong decreasing trends mainly occurring in February and March. Many trends for these months are statistically significant at the 10% level and some trends are significant at the 5% level. These two months account for most of the total annual rainfall in the UAE. To investigate the presence of sudden changes in rainfall time-series, the cumulative sum method and a Bayesian multiple change point detection procedure were applied to annual rainfall series. Results indicate that a change point happened around 1999 at all stations. Analyses were performed to evaluate the evolution of characteristics before and after 1999. Student’s t-test and Levene’s test were applied to determine if a change in the mean and/or in the variance occurred at the change point. Results show that a decreasing shift in the mean has occurred in the total annual rainfall and the number of rainy days at all four stations, and that the variance has decreased for the total annual rainfall at two stations. Frequency analysis was also performed on data before and after the change point. Results show that rainfall quantile values are significantly lower after 1999. The change point around the year 1999 is linked to various global climate indices. It is observed that the change of phase of the Southern Oscillation Index (SOI) has strong impact over the UAE precipitation. A brief discussion is presented on dynamical basis, the teleconnections connecting the SOI and the change in precipitation regime in the UAE around the year 1999.

Long-period modulations are shown in aerosol optical depth measured by the Microtops II Sun photometer over a high-altitude site the central Himalayan region (Nainital, 29.4° N, 79.5° E, 1958 m a.m.s.l.) for the first time. Fourier analysis of aerosol optical depth showed dominant 25–45 day oscillations observed in MODerate-resolution Imaging Spectro radiometer data. Further, a Hovmiller diagram showed westward (northward) propagation at a different longitude (latitude), confirming that the modulations are associated with Rossby waves. It is also shown that the Rossby wave amplitude causes an additional warming of 4.16 ± 0.98 W m−2 over the observational site. Hence, the present study illustrates the importance of wave-induced aerosol dynamics and the corresponding radiative effects.

A Doppler Lidar was installed at Manora Peak, Nainital (29.4°N; 79.2°E; 1958 amsl) to estimate mixing layer height for the first time by using vertical velocity variance as basic measurement parameter for the period September – November 2011. Mixing layer height is found to be located ∼0.57±0.1 km and 0.45±0.05 km AGL during day and nighttime, respectively. The estimation of mixing layer height shows good correlation (R2>0.8) between different instruments and with different methods. Our results show that wavelet co-variance transform is a robust method for mixing layer height estimation.

This study reports characteristics of inertia-gravity waves (IGWs) in the atmospheric boundary layer during the passage of Tropical Cylone-03B, using the Doppler Sound Detection and Ranging (SODAR) observations at the Indian tropical station of Gadanki (13.45°N, 79.2°E; near the east coast of India). Wavelet analysis of horizontal winds indicates significant wave motion (˜60 h) near the characteristic inertial period. The hodograph analysis of the filtered winds shows an anti-cyclonic turning of horizontal wind with height and time, indicating the presence of IGW. This study finds important implications in boundary layer dynamics during the passage of tropical cyclones.

In the present study, the observed variability of monsoon droughts over India have been examined using a drought monitoring index, namely the Standardized Precipitation Evapo-transpiration Index (SPEI). For calculating the SPEI at different time periods, long term (1901–2010), high resolution, monthly gridded temperature and rainfall data sets have been used. The drought time series shows significant interannual, decadal and long term trends. The analysis suggests a general increase in the intensity and percent area affected by moderate droughts during the recent decades. In particular, the frequency of multi-year (24 months) droughts has shown a statistically significant increase, which is attributed to increase in surface air temperatures and thus drying of the atmosphere. The wavelet analysis of SPEI suggests significant spectral peaks at quasi-biennial (2–3 years), ENSO (5–7 years) and decadal (10–16 years) time scales, with significant multi-decadal variations. The variability of monsoon droughts over India is significantly influenced by the tropical sea surface temperature anomalies. The Canonical correlation analysis (CCA) suggests that the major portion of the drought variability is influenced by the El Nino/Southern Oscillation (ENSO). Global warming, especially the warming of the equatorial Indian Ocean represents the second coupled mode and is responsible for the observed increase in intensity of droughts during the recent decades.

During recent years, an increase in the intensity of pre-monsoon tropical cyclones (TC) is observed over the Arabian Sea. The present study suggests this increase is due to epochal variability in the intensity of TCs and is associated with epochal variability in the storm-ambient vertical wind shear and Tropical Cyclone Heat Potential (TCHP). There is a significant increase (0.53 kJcm-2yr-1) of TCHP during recent years. The warmer upper ocean helps TCs to sustain or increase their intensity by an uninterrupted supply of sensible and latent heat fluxes from the ocean surface to the atmosphere.

Daily rainfall datasets of 10 years (1998–2007) of Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) version 6 and India Meteorological Department (IMD) gridded rain gauge have been compared over the Indian landmass, both in large and small spatial scales. On the
larger spatial scale, the pattern correlation between the two datasets on daily scales during individual years of the study period is ranging from 0.4 to 0.7. The correlation improved significantly (∼0.9) when the study was confined to specific wet and dry spells each of about 5–8 days. Wavelet analysis of intraseasonal
oscillations (ISO) of the southwest monsoon rainfall show the percentage contribution of the major two modes (30–50 days and 10–20 days), to be ranging respectively between ∼30–40% and 5–10% for the various years. Analysis of inter-annual variability shows the satellite data to be underestimating seasonal rainfall by ∼110 mm during southwest monsoon and overestimating by ∼150 mm during northeast monsoon season.
At high spatio-temporal scales, viz., 1◦×1◦ grid, TMPA data do not correspond to ground truth. We have proposed here a new analysis procedure to assess the minimum spatial scale at which
the two datasets are compatible with each other. This has been done by studying the contribution to total seasonal rainfall from different rainfall rate windows (at 1 mm intervals) on different
spatial scales (at daily time scale). The compatibility spatial scale is seen to be beyond 5◦×5◦ average spatial scale over the Indian landmass. This will help to decide the usability of TMPA products,
if averaged at appropriate spatial scales, for specific process studies, e.g., cloud scale, meso scale or synoptic scale.

Mountain waves in the stratosphere have been observed over elevated topographies using both nadir-looking and limb-viewing satellites. However, the characteristics of mountain waves generated over the Himalayan Mountain range and the adjacent Tibetan Plateau are relatively less explored. The present study reports on three-dimensional (3d) properties of a mountain-wave event that occurred over the western Himalayan region on 9 December 2008. Observations made by the Atmospheric Infrared Sounder (AIRS) onboard the Aqua and Microwave Limb Sounder (MLS) onboard the Aura satellites are used to delineate the wave properties. The observed wave properties such as horizontal (λx, λy) and vertical (λz) wavelengths are 276 km (zonal), 289 km (meridional) and 25 km respectively. A good agreement is found between the observed and modeled/analyzed vertical wavelength for a stationary gravity wave determined using the Modern Era Retrospective-analysis for Research and Applications (MERRA) reanalysis winds. The analysis of both the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis and MERRA winds shows that the waves are primarily forced by strong flow across the topography. Using the 3d properties of waves and the corrected temperature amplitudes, we estimated wave momentum fluxes of the order of ~0.05 Pa, which is in agreement with large amplitude mountain wave events reported elsewhere. In this regard, the present study is considered to be very much informative to the Gravity Wave Drag (GWD) schemes employed in current general circulation models (GCM) for this region.

In this paper we study the planetary scale wave features using concurrent observations of mesospheric wind and temperature, ionospheric h’F, and tropospheric wind from Tirunelveli, Gadanki and Kolhapur, all located in the Indian low latitudes, made during February 2009. Our investigations reveal that 3 to 5 day periodicity, characterized as ultra fast Kelvin (UFK) wave, was persistent throughout the atmosphere during this period. These waves show clear signatures of upward wave propagation from troposphere to the upper mesosphere linking the ionosphere through a clear correlation between mesospheric winds and h’F variations. We also note the amplitude of this wave to decrease as we go away from the equator. These results are first of their kind from Indian sector which portray the vertical as well as latitudinal characteristics of the 3-5 day UFK waves simultaneously right from the troposphere to the ionosphere.

South peninsular India experiences a large portion of the annual rainfall during the northeast monsoon season (October to December). In this study, the facets of diurnal, intra-seasonal and inter-annual variability of the northeast monsoon rainfall (the NEMR) over India have been examined. The analysis of satellite derived hourly rainfall reveals that there are distinct features of diurnal variation over the land and oceans during the season. Over the land, rainfall peaks during the late afternoon/evening, while over the oceans an early morning peak is observed. The harmonic analysis of hourly data reveals that the amplitude and variance are the largest over south peninsular India. The NEMR also exhibits significant intra-seasonal variability on a 20–40 day time scale. Analysis also shows significant northward propagation of the maximum cloud zone from south of equator to the south peninsula during the season. The NEMR exhibits large inter-annual variability with the co-efficient of variation (CV) of 25%. The positive phases of ENSO and the Indian Ocean Dipole (IOD) are conducive for normal to above normal rainfall activity during the northeast monsoon. There are multi-decadal variations in the statistical relationship between ENSO and the NEMR. During the period 2001–2010 the statistical relationship between ENSO and the NEMR has significantly weakened. The analysis of seasonal rainfall hindcasts for the period 1960–2005 produced by the state-of-the-art coupled climate models, ENSEMBLES, reveals that the coupled models have very poor skill in predicting the inter-annual variability of the NEMR. This is mainly due to the inability of the ENSEMBLES models to simulate the positive relationship between ENSO and the NEMR correctly. Copyright © 2012 Royal Meteorological Society

Precise specification of the vertical distribution of cloud optical properties is important to reduce the uncertainty in quantifying the radiative impacts of clouds. The new global observations of vertical profiles of clouds from the CloudSat mission provide opportunities to describe cloud structures and to improve parameterization of clouds in the weather and climate prediction models. In this study, four years (2007–2010) of observations of vertical structure of clouds from the CloudSat cloud profiling radar have been used to document the mean vertical structure of clouds associated with the Indian summer monsoon (ISM) and its intra-seasonal variability. Active and break monsoon spells associated with the intra-seasonal variability of ISM have been identified by an objective criterion. For the present analysis, we considered CloudSat derived column integrated cloud liquid and ice water, and vertically profiles of cloud liquid and ice water content. Over the South Asian monsoon region, deep convective clouds with large vertical extent (up to 14 km) and large values of cloud water and ice content are observed over the north Bay of Bengal. Deep clouds with large ice water content are also observed over north Arabian Sea and adjoining northwest India, along the west coast of India and the south equatorial Indian Ocean. The active monsoon spells are characterized by enhanced deep convection over the Bay of Bengal, west coast of India and northeast Arabian Sea and suppressed convection over the equatorial Indian Ocean. Over the Bay of Bengal, cloud liquid water content and ice water content is enhanced by ~90 and ~200 % respectively during the active spells. An interesting feature associated with the active spell is the vertical tilting structure of positive CLWC and CIWC anomalies over the Arabian Sea and the Bay of Bengal, which suggests a pre-conditioning process for the northward propagation of the boreal summer intra-seasonal variability. It is also observed that during the break spells, clouds are not completely suppressed over central India. Instead, clouds with smaller vertical extent (3–5 km) are observed due to the presence of a heat low type of circulation. The present results will be useful for validating the vertical structure of clouds in weather and climate prediction models.

Using a Mesosphere Stratosphere Troposphere (MST) radar, operating at 53 MHz at Gadanki (13.5°N, 79.2°E), India, the present study reports on the temporal and spatial characteristics of inertia-gravity waves (IGWs) generated from four tropical cyclones that formed over the Bay of Bengal. IGWs are observed with intrinsic frequencies, vertical and horizontal wavelengths in the ranges of 1.2f–3.0f, 2.5–5, and 300–1600 km, respectively, where f is the Coriolis frequency at the Gadanki site. It is found that both the convective and geostrophic adjustment processes in the troposphere play a major role in the generation of IGWs. Also it is found that the horizontal propagation direction of IGWs is aligned along the motion of convective rain bands.► We examine the MST radar observation of cyclones generated inertia-gravity waves. ► These four Bay of Bengal cyclones: TC03B, FANOOS, KHAI_MUK and NISHA are considered. ► Lower atmospheric winds during these four cyclones show cyclonic structures. ► Hodograph analysis of the inertia-gravity waves tracks the movement of cyclones. ► The inertia-gravity waves propagate normal outward from the cyclones as expected.