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Ahmad Hasan Nury,Khairul Hasan 대한환경공학회 2016 Environmental Engineering Research Vol.21 No.1
The study explored droughts using the Standardized Precipitation Index (SPI) in the northwestern region of Bangladesh, which is the drought prone area. In order to assess the trend and variability of monthly rainfall, as well as 3-month scale SPI, non-parametric Mann-Kendall (MK) tests and continuous wavelet transform were used respectively. The effect of climatic parameters on the drought in this region was also evaluated using SPI, with the Southern Oscilation Index (SOI) by means of the wavelet coherence technique, a relatively new and powerful tool for describing processes. The MK test showed no statistically significant monthly rainfall trends in the selected stations, whereas the seasonal MK test showed a declining rainfall trend in Bogra, Ishurdi, Rangpur and Sayedpur stations respectively. Sen`s slope of six stations also provided a decreasing rainfall trend. The trend of the SPI, as well as Sen`s slope indicated an increasing dryness trend in this area. Dominant periodicity of 3-month scale SPI at 8 to 16 months, 16 to 32 months, and 32 to 64 months were observed in the study area. The outcomes from this study contribute to hydrologists to establish strategies, priorities and proper use of water resources.
Formation mechanism of self-organized nano-ripples on quartz surface using femtosecond laser pulses
Ahsan, Md. Shamim,Lee, Man Seop,Hasan, Mohammad Khairul,Noh, Young-Chul,Sohn, Ik-Bu,Ahmed, Farid,Jun, Martin B.G. Elsevier 2015 OPTIK -STUTTGART- Vol.126 No.24
<P><B>Abstract</B></P> <P>This article demonstrates the formation of self-organized nano-ripples on a quartz surface by irradiation of femtosecond laser pulses. Self-oriented nano-ripples are printed on the quartz surface by controlling various laser parameters: average pulse energy, pulse width, and number of laser pulses on each irradiated spot. We validate that the direction of the self-organized nano-ripples are oriented perpendicularly to the laser polarization direction. We also investigate and analyze the dependency of nano-ripples period on various laser parameters including the pulse energy, number of laser pulses, and pulse width. The nano-ripples show increasing trend with the increase of laser energy and pulse width, whereas show decreasing trend with the increase of irradiated laser pulses. We also explain, qualitatively, the formation mechanism of the self-organized nano-ripples on quartz surface.</P>