• Atmosphere
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• v.30 no.4
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• pp.421-437
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• 2020

The effects of the terminal fall velocity-diameter relationship for raindrops, which is prescribed based on the measurement, on the simulated surface precipitation over Korea during summer season were investigated in our study. Two rainfall cases, 1-month summer precipitation and mesoscale rainfall, have been simulated using the Weather Research and Forecasting (WRF) model. The selected cloud microphysics parameterizations are WRF Single-Moment 5-class (WSM5) and WRF Single-Moment 6-class (WSM6) in the WRF model. The measured terminal fall-diameter relationship for raindrops by Gunn and Kinzer (1949) was applied in both WSM5 and WSM6. The sensitivity experiments with WSM5 and WSM6, applying the measured fall-diameter relationship, presents the different responses in simulated precipitation amount for the 1-month summer precipitation case. Precipitation increases with WSM5, thus enhancing the precipitation statistical skills. However, precipitation decreases with WSM6 leading to the deterioration of precipitation statistical skills. For the mesoscale rainfall case, precipitation increases with both WSM5 and WSM6, which further enhances the positive bias in precipitation amount.

• Atmosphere
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• v.26 no.3
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• pp.435-444
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• 2016

In spite of considerable progress in the recent decades, there still remain large uncertainties in numerical cloud models. In this study, effects of uncertainty in terminal velocity of graupel on cloud simulation are investigated. For this, a two-dimensional bin microphysics cloud model is employed, and deep convective clouds are simulated under idealized environmental conditions. In the sensitivity experiments, the terminal velocity of graupel is changed to twice and half the velocity in the control experiment. In the experiment with fast graupel terminal velocity, a large amount of graupel mass is present in the lower layer. On the other hand, in the experiment with slow graupel terminal velocity, almost all graupel mass remains in the upper layer. The graupel size distribution exhibits that as graupel terminal velocity increases, in the lower layer, the number of graupel particles increases and the peak radius in the graupel mass size distribution decreases. In the experiment with fast graupel terminal velocity, the vertical velocity is decreased mainly due to a decrease in riming that leads to a decrease in latent heat release and an increase in evaporative cooling via evaporation, sublimation, and melting that leads to more stable atmosphere. This decrease in vertical velocity causes graupel particles to fall toward the ground easier. By the changes in graupel terminal velocity, the accumulated surface precipitation amount differs up to about two times. This study reveals that the terminal velocity of graupel should be estimated more accurately than it is now.

• Journal of The Geomorphological Association of Korea
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• v.23 no.3
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• pp.105-122
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• 2016

To evaluate the changes in the physical characteristics of open rainfall related to canopy effects and rainfall intensity in Korea, the terminal velocity of raindrops and drop size distributions(DSD) were continuously measured by an optical-laser disdrometer in an open site(Op) and in two forest stands(Th1: Larix leptolepis, Th2: Pinus koraiensis) during five rainfall events in 2008. The terminal velocity, DSD and two forms of kinetic energy(KE, $Jm^{-2}$ $mm^{-1}$; KER, $Jm^{-2}$ $h^{-1}$) of open rainfall drops were determined and were compared with those of throughfall drops under two different canopy heights. The effects of the canopy and rainfall intensity, together with wind speed, on the changes in drop size and kinetic energy of throughfall were evaluated. Throughfall drops were larger than open rainfall drops. The distribution of terminal velocities for the drop sizes measured at Th2 was lower than that at Op; however, at Th1 the distribution was similar to that at Op. The total kinetic energy of throughfall at Th1 and Th2 was higher than the total kinetic energy of open rainfall, and the kinetic energy distribution for the drop sizes wassimilar to the drop size distribution. The observed throughfall-KER at Th1 was lower than an estimate previously produced using a model. The overestimation from the modeled value at Th1 was likely to be due to overestimated values of a square root transformation of fall height and its coefficient in the model because the distributions of terminal velocity for the drop size measured at Th1 were similar to those of open rainfall.

• Proceedings of the KAIS Fall Conference
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• 2003.06a
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• pp.98-101
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• 2003

폴리머 수용액인 점탄성 유체속에서 물체를 떨어뜨리면 시간 간격에 따라 종말속도가 다르게 측정된다. 이는 폴리머 수용액이 점성과 탄성을 포함하고 있는 것으로 수용액 내부에서 처음 통과한 물체를 기억하는 기억유체(memory fluid)라고 정의 할 수도 있다. 낙하에 사용된 물체는 속이 빈 알루미늄 구를 사용하였으며 구 내부에 일정량의 철분을 삽입하여 밀도를 변화시켰다. 이 결과 구를 떨어뜨리는 시간 간격이 작을수록 종말속도 값은 크게 나타나며 시간 간격이 충분히 크면 처음 구를 떨어뜨릴 때의 종말속도와 같게 된다. 또한, 농도가 증가할수록 처음 구를 떨어뜨려 측정된 종말속도는 서서히 감소하다가 측정시간간격의 85％를 통과하며 다시 처음의 속도에 도달하게 된다. 구의 종말속도는 점탄성 유체의 농도가 각각 500wppm, 1000wppm, 2000wppm의 경우 처음 구를 떨어뜨린 후에 구의 밀도와 온도변화에 따라 최고속도에 도달하는 시간도 증가하였으며, 시간이 경과할수록 감소하여 일정시간이 경과한 후에는 처음의 상태로 복귀되는 것을 알 수 있다.

• Tunnel and Underground Space
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• v.8 no.3
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• pp.194-199
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• 1998

This research was aimed to figure out the trend of dust diffusion at open pit limestone mine for assessing the environmental impacts on the high voltage power transmission line. It is rather easy to assess the dust generation and size distribution of limestone dust at the blasting site, but it is very hard to assess the expected area of dust diffusion and amount of dust fall by the distances from the dust source. In this research, a 3-dimensional fluid dynamic simulation software (3D-Flow) was used for analysing the above mentioned matters to assess the impacts to the insulators on the transmission tower by the blasting dust. It was verfied that the 3D-Flow is reliable tool for simulating dust movement, and the limestone dust is not much hazardous to the power transmission line.

• Atmosphere
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• v.29 no.1
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• pp.105-112
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• 2019

Measurements of vertical air motion and microphysics are essential for improving our understanding of convective clouds. In this paper, the author reviews the current research on the retrieval of vertical air motions using the cloud radar. At radar wavelengths of 3 mm (W-band radar; 94-GHz radar; cloud radar), the raindrop backscattering cross-section (${\sigma}b$) varies between successive maxima and minima as a function of the raindrop diameter (D) that are well described by Mie theory. The first Mie minimum in the backscattering cross-section occurs at D~1.68 mm, which translates to a raindrop terminal fall velocity of ${\sim}5.85m\;s^{-1}$ based on the Gunn and Kinzer relationship. Since raindrop diameters often exceed this size, the signal is captured in the radar Doppler spectrum, and thus, the location of the first Mie minimum can be used as a reference for retrieving the vertical air motion. The Mie technique is applied to radar Doppler spectra from the surface-based and airborne, upward pointing W-band radars. The contributions of aircraft motion to the vertical air motion are also described and further the first-order aircraft motion corrected equation is presented. The review also shows that the separate spectral peaks due to the cloud droplets can provide independent validation of the Mie technique retrieved vertical air motion using the cloud droplets as a tracer of vertical air motion.