Ge growth on ion-irradiated Si self-affine fractal surfaces

We have carried out scanning tunneling microscopy experiments under ultrahigh vacuum condition to study the morphology of ultrathin Ge films eposited on pristine Si(100) and ion-irradiated Si(100) self-affine fractal surfaces. The pristine and the ion-irradiated Si(100) surface have roughness exponents of alpha=0.19+/-0.05 and alpha=0.82+/-0.04 respectively. These measurements were carried out on two halves of the same sample where only one half was ion-irradiated. Following deposition of a thin film of Ge (~6 A) the roughness exponents change to 0.11+/-0.04 and 0.99+/-0.06, respectively. Upon Ge deposition, while the roughness increases by more than an order of magnitude on the pristine surface, a smoothing is observed for the ion-irradiated surface. For the ion-irradiated surface the correlation length xi increases from 32 nm to 137 nm upon Ge deposition. Ge grows on Si surfaces in the Stranski-Krastanov or layer-plus-island mode where islands grow on a wetting layer of about three atomic layers. On the pristine surface the islands are predominantly of square or rectangular shape, while on the ion-irradiated surface the islands are nearly diamond shaped. Changes of adsorption behaviour of deposited atoms depending on the roughness exponent (or the fractal dimension) of the substrate surface are discussed.Comment: 5 pages, 2 figures and 1 tabl

The challenge of weather prediction: What makes it difficult? 3. Old and new ways of weather prediction

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The challenge of weather prediction 1. The basic driving

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Seminal role of clouds on solar dimming over the Indian monsoon region

In contrast to most of the world where solar dimming has changed over to solar brightening since late eighties, dimming continues unabated over the Indian region. This study investigates new insight into the origin of dimming over India. As the insolation at the surface is controlled by aerosols and clouds, we tried to separate out the two controlling factors by examining clear and cloudy sky days. From 1981-2006, the rate of dimming is found to be twice as large during cloudy conditions (~12 W/m2/decade) compared to that during clear sky conditions (~6 W/m2/decade). The clear sky dimming is attributed to increasing aerosols. While the rate of dimming by clouds is similar during summer and winter monsoon seasons, the increased contribution to dimming by clouds during summer seems to come from increasingly deeper clouds covering increasingly larger area. During winter, dimming in cloudy conditions appears to be due to indirect effect of aerosols

A new study of shower age distribution in near vertical showers by EAS air shower array

The air shower array has been developed since it started operation in 1931. The array covering an area of 900 sq m now incorporates 21 particle density sampling detectors around two muon magnetic spectrographs. The air showers are detected in the size range 10 to the 4th power to 10 to the 6th power particles. A total of 11000 showers has so far been detected. Average values of shower age have been obtained in various shower size ranges to study the dependence of shower age on shower size. The core distance dependence of shower age parameter has also been analyzed for presentation

Multiscale interaction with topography and extreme rainfall events in the northeast Indian region

Flash floods associated with extreme rain events are a major hydrological disaster in the northeast Indian (NEI) region because of the unique topographic features of the region as well as increased frequency of occurrence of such events. Knowledge of the spatiotemporal distribution of these events in the region and an understanding of the factors responsible for them, therefore, would be immensely useful for appropriate disaster preparedness. Using daily rainfall data from 15 stations over the region for 32 years (1975-2006), it is shown that the frequency of occurrence of these events is largest not during the premonsoon thunderstorm season but during the peak monsoon months (June-July-August). This fact together with the fact that most of these events occur during long rainy spells indicate that the extreme events in the NEI region largely occur in association with the monsoon synoptic events rather than isolated thunderstorms. We also find that the aggregate of extreme rain events over the region has a significant decreasing trend in contrast to a recent finding of an increasing trend of such events in central India (Goswami et al., 2006). This decreasing trend of extreme events is consistent with observed decreasing trend in convective available potential energy and increasing convective inhibition energy over the region for the mentioned period. Examination of the structure of convection associated with the extreme rain events in the region indicates that they occur through a multiscale interaction of circulation with the local topography. It is found that at all the stations, the events are associated with a mesoscale structure of convection that is embedded in a much larger scale convective organization. We identify that this large-scale organization is a manifestation of certain phases of the tropical convergence zone associated with the northward propagating active-break phases of the summer monsoon intraseasonal oscillation. Further, it is shown that the mesoscale circulation interacting with the local topography generates southward propagating gravity waves with diurnal period. The strong updrafts associated with the gravity waves within the mesoscale organization leads to very deep convective events and the extreme rainfall. The insights provided by our study would be useful when designing models to improve the prediction of extreme events

Climate Change and Potential Demise of the Indian Deserts

In contrast to the wet gets wetter and dry gets drier paradigm, here, using observations and climate model simulations, we show that the mean rainfall over the semi-arid northwest parts of India and Pakistan has increased by 10 to 50 percent during 1901 to 2015 and is expected to increase by 50 to 200 percent under moderate greenhouse gas (GHG) scenarios, e.g, SSP2 4.5. The GHG forcing primarily drives the westward expansion of the Indian summer monsoon (ISM) rainfall and is a result of a westward expansion of the inter-tropical convergence zone (ITCZ), facilitated by a westward expansion of the Indian Ocean warm pool. While an adaptation strategy to increased hydrological disasters is a must, harvesting the increased rainfall would lead to a significant increase in food productivity, bringing transformative changes in the socio-economic condition of people in the region

Nanoscale self-affine surface smoothing by ion bombardment

The topography of silicon surfaces irradiated by a 2-MeV Si+ ion beam at normal incidence and ion fluences in the range 1015-1016ions/cm2 has been investigated using scanning tunneling microscopy. At length scales below ~50 nm, surface smoothing is observed; the smoothing is more prominent at smaller length scales. The smoothed surface is self-affine with a scaling exponent α=0.53± 0.03

Extension of potential predictability of Indian summer monsoon dry and wet spells in recent decades

An understanding of the limit on potential predictability is crucial for developing appropriate tools for extended-range prediction of active/break spells of the Indian summer monsoon (ISM). The global low-frequency changes in climate modulate the annual cycle of the ISM and can influence the intrinsic predictability limit of the ISM intraseasonal oscillations (ISOs). Using 104-year (1901-2004) long daily rainfall data, the change in potential predictability of active and break spells are estimated by an empirical method. It is found that the potential predictability of both active and break spells have undergone a rapid increase during the recent three decades. The potential predictability of active spells has shown an increase from one week to two weeks while that for break spells increased from two weeks to three weeks. This result is interesting and intriguing in the backdrop of recent finding that the potential predictability of monsoon weather has decreased substantially over the same period compared to earlier decades due to increased potential instability of the atmosphere. The possible role of internal dynamics and external forcing in producing this change has been explored. The changes in energy exchange between the synoptic and ISO scale and the different ISO modes as evidenced by energetics computations in frequency domain also support the increased potential predictability of ISO. Our finding provides optimism for improved and useful extended-range prediction of monsoon active and break spells