Das S

Microphysics of clouds and rain over the Western Ghat

In an attempt to unravel the interactions between cloud microphysics and dynamics that make shallow clouds precipitate heavily in this region, some unique observations of rain and cloud microphysical parameters are presented here from two stations, Pune and Mahabaleshwar, one each on the lee and windward sides, respectively, of the Western Ghat (WG) mountains in peninsular India. To elucidate rain microphysics, we used the raindrop size distribution (DSD) by fitting three parameter Gamma functions to the observed raindrop spectra. Over Pune, during stratiform rain with bright band (BB) at 0°C isotherm; concave upward DSD shapes are observed below the BB which becomes concave downward at lower altitudes. It is due to breakup process of large raindrops which increases drop concentration at midsizes suggesting coalescence, collision, and breakup processes. Both slope and intercept parameters of Gamma DSD decrease during no BB condition as altitudes decrease, signifying collision and coalescence processes. Over Mahabaleshwar, bimodal and monomodal DSD are observed during light and heavy rainfall, respectively. With shallow storm heights, small raindrops mainly contribute to both types of rainfall. The DSDs are parameterized, and their radar reflectivity factor-rainfall intensity relationships are evaluated suggesting the dominance of collision-coalescence processes. Aircraft measurements of orographic clouds over the WG suggest interaction of cloud mass with the ambient updraft speed. The orographically forced updrafts foster rapid condensational growth of cloud droplets triggering coalescence process within few hundred meters of cloud depth. Hence, these clouds are dynamically forced to produce precipitation over the WG.

A climatological perspective of water vapor at the UTLS region over different global monsoon regions: observations inferred from the Aura-MLS and reanalysis data

The Aura-MLS observations of eight years from 2004 to 2011 have been utilized to understand the hydration and the dehydration mechanism over the northern and the southern hemispheric monsoon (NH and SH) regions. The monsoon regions considered are the Asian Summer Monsoon, East Asian Summer Monsoon, Arizona Monsoon (AM), North African Monsoon, South American Monsoon and the Australian Monsoon. The annual cycle of water vapor as expected shows maxima over the NH during June–August and during December–February over the SH. The time taken by the air parcels over the NH monsoon regions is found to be different compared to that over the SH monsoon regions. The analysis shows the concentration of water vapor in the upper troposphere and the lower stratosphere (UTLS) has not changed over these eight years in both the hemispheres during their respective monsoon seasons. The present analysis show different processes viz., direct overshooting convection, horizontal advection, temperature and cirrus clouds in influencing the distribution of water vapor to the UTLS over these different monsoon regions. Analysis of the UTLS water vapor with temperature and ice water content shows that the AM is hydrating the stratosphere compared to all the other monsoon regions where the water vapor is getting dehydrated. Thus it is envisaged that the present results will have important implications in understanding the exchange processes across the tropopause over the different monsoon regions and its role in stratosphere chemistry.

Variability of mesopause temperature derived from two independent methods using meteor radar and its comparison with SABER and EOS MLS and a collocated multi-wavelength dayglow photometer over an equatorial station, Thumba (8.5° N, 76.5° E)

Two independent methods for deriving mesopause temperature using meteor radar installed at an equatorial station, Thumba (8.5° N, 76.5° E), are discussed in this article. This meteor radar-derived mesopause temperature is then compared with two different types of spaceborne measurement, namely (i) Sounding the Atmosphere using Broadband Emission Radiometry (SABER) and (ii) the Earth Observing System Microwave Limb Sounder (EOS MLS), and a collocated multi-wavelength dayglow photometer (DGPM). The meteor radar-derived temperature is in fairly good agreement with all the three measurement techniques, with an uncertainty of ±10°. This study focuses on a detailed evaluation and inter-comparison of mesopause temperature derived from different measurement techniques. An attempt is also made to compare the suitability of these observations to study planetary waves and other oscillation activities in the mesospheric region.

Statistical Characteristics of Convective Clouds over the Western Ghats Derived from Weather Radar Observations

X-band radar observations at Mandhardev (18.04°N, 73.85°E) are used to investigate statistics of convective clouds over the Western Ghats during monsoon season (June-September, 2014). Convective storms (cells) are identified using an objective-tracking method to examine their spatio-temporal variability, thus quantifying the time continuous aspects of convective cloud population over the region for the first time. An increased frequency of storm location and initiation along the windward mountains compared to coastal and lee side highlights orographic response to southwesterly flow, with superimposed diurnal cycle. An eastward progression of convective activity from upstream the barrier through windward slopes of mountains over to the lee side is observed. Storm area, height and duration follow lognormal distributions; wherein, small-sized storms contribute more to total population and unimodal distribution of 35-dBZ top heights (peaking at 5.5 km) depicts the dominance of shallow convection. Storms exhibit a pronounced diurnal cycle with a peak in afternoon hours, while the convective area maximum is delayed by several hours to that of precipitation flux. Cell lifetime and propagation show that cells move with slow speeds and have mean duration of 46 minutes. They align east-west nearly parallel to mountain ridges and their direction of movement is steered mostly by large-scale winds at lower levels. Based on top heights, convective cells are further classified into cumulus, congestus and deep clouds. In general, congestus (deep) cells are most abundant in the windward (leeward) side. A lead-lag relationship between congestus and deep cells indicates midtroposphere-moistening by congestus cells prior to deep convection.

The Assessment of Global Precipitation Measurement estimates over the Indian Subcontinent

Accurate and real-time precipitation estimation is a challenging task for current and future space-borne measurements, which is essential to understand the global hydrological cycle. Recently, the Global Precipitation Measurement (GPM) satellites were launched as a next-generation rainfall mission for observing the global precipitation characteristics. The purpose of the GPM is to enhance the spatio-temporal resolution of global precipitation. The main objective of the present study is to assess the rainfall products from the GPM, especially the Integrated Multi-satellitE Retrievals for the GPM (IMERG) data by comparing with the ground-based observations. The multi temporal scale evaluations of rainfall involving sub-daily, diurnal, monthly, and seasonal scales were performed over the Indian subcontinent. The comparison shows that the IMERG performed better than the TRMM-3B42, although both rainfall products underestimated the observed rainfall compared to the ground-based measurements. The analyses also reveal that the TRMM-3B42 and IMERG data sets are able to represent the large-scale monsoon rainfall spatial features, but are having region-specific biases. The IMERG shows significant improvement in low rainfall estimates compared to the TRMM-3B42 for selected regions. In the spatial distribution, the IMERG shows higher rain rates compared to the TRMM-3B42, due to its enhanced spatial and temporal resolution. Apart from this, the characteristics of raindrop size distribution (DSD) obtained from the GPM mission dual-frequency precipitation radar (DPR) is assessed over the complex mountain terrain site in the Western Ghats, India using the DSD measured by a Joss-Waldvogel disdrometer.

Upper tropospheric ozone transport from the sub-tropics to tropics over the Indian region during Asian summer monsoon

In this study, we investigate the role of the Asian summer monsoon (ASM) anticyclone in the distribution of ozone over the southern India and tropical Indian Ocean. We present the horizontal and vertical structure of ozone in the upper troposphere and lower stratosphere (UTLS) region. The analysis shows that the region within the ASM anticyclone has low ozone, and high tropopause altitude, as compared to the region outside the anticyclone during boreal summer. The southern edge of the ASM anticyclone, i.e. the southern India and tropical Indian Ocean show a remarkably high ozone concentration in the UTLS region during summer. Analysis of daily fields shows that ozone concentration in the upper troposphere over the southern India and tropical Indian Ocean increases with the strength of the tropical easterly jet, which is an outcome of ASM circulation. Different mechanisms responsible for the ozone enhancement in the UTLS region over the tropical Indian region have been discussed in this paper. The in situ ozonesonde observations from six Indian stations also support the space-based Aura-MLS observations, concluding that ASM anticyclone effectively transports ozone from the mid-latitude stratosphere to deep tropics. Shear generated turbulence and mixing in the vicinity of easterly jet also likely to play a minor role in the local ozone distribution.

TheRole of Alloy Matrix and Dispersoid on the CorrosionBehaviour of Cast AluminiumAlloy Composites

The potentiodynamic behaviour of annealed and cold worked 9Cr-1Mo, modified with V0.198 wt% and Nb 0.072 wt%, ferritic steel has been studied in H2SO4 in the concentration range 0.1–5 N at a scan rate of 45 mV min−1 at 25°C. Ecorr becomes more noble while the other electrochemical parameters, i.c. icorr, icrit, ip and Ecp for both states of the modified steel decrease with the increase in acid concentration. This trend was more marked in the annealed than in the cold worked alloy. This alloy shows a higher corrosion resistance in IN H2SO4 than pure 9Cr-1Mo steel. Slight passivation at higher potential in both cold worked and annealed modified 9Cr-1Mo was observed in seawater whereas no passivation was indicated in the case of annealed pure 9Cr-1Mo steel under the same experimental condition. The different corrosion behaviour has been explained as a result of microstructural differences.Sol–gel coatings were applied on AISI 304L stainless steel and their pitting resistance was evaluated in different concentrations of chloride ions in 0.01 M H2SO4 solution. AFM and XRD analyses of the coated surface confirmed the presence of nano structured α-alumina phase in the coating. Anodic polarisation indicated a considerable shift of pitting potential of the substrate in the noble direction after applying coatings. The SEM micrographs of the polarised specimens revealed the presence of relatively less number of pits on the coated surface. EIS analysis demonstrated the occurrence of diffusion process at the coating/metal interfaces in chloride ions presence.