19,371 research outputs found
Study of the Aerosol Indirect Effect by Large-Eddy Simulation of Marine Stratocumulus
A total of 98 three-dimensional large-eddy simulations (LESs) of marine stratocumulus clouds covering both nighttime and daytime conditions were performed to explore the response of cloud optical depth (Ο) to various aerosol number concentrations (Na = 50β2500 cmβ3) and the covarying meteorological conditions (large-scale divergence rate and SST). The idealized First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) and the Atlantic Stratocumulus Transition Experiment (ASTEX) Lagrangian 1 sounding profiles were used to represent the lightly and heavily drizzling cases, respectively. The first and second aerosol indirect effects are identified. Through statistical analysis, Ο is found be to both positively correlated with Na and cloud liquid water path (LWP) with a higher correlation associated with LWP, which is predominantly regulated by large-scale subsidence and SST. Clouds with high LWP occur under low SST or weak large-scale subsidence. Introduction of a small amount of giant sea salt aerosol into the simulation lowers the number of cloud droplets activated, results in larger cloud droplets, and initiates precipitation for nondrizzling polluted clouds or precedes the precipitation process for drizzling clouds. However, giant sea salt aerosol is found to have a negligible effect on Ο for lightly precipitating cases, while resulting in a relative reduction of Ο of 3%β77% (increasing with Na, for Na = 1000β2500 cmβ3) for heavily precipitating cases, suggesting that the impact of giant sea salt is only important for moist and potentially convective clouds. Finally, a regression analysis of the simulations shows that the second indirect effect is more evident in clear than polluted cases. The second indirect effect is found to enhance (reduce) the overall aerosol indirect effect for heavily (lightly) drizzling clouds; that is, Ο is larger (smaller) for the same relative change in Na than considering the Twomey (first indirect) effect alone. The aerosol indirect effect (on Ο) is lessened in the daytime afternoon conditions and is dominated by the Twomey effect; however, the effect in the early morning is close but slightly smaller than that in the nocturnal run. Diurnal variations of the aerosol indirect effect should be considered to accurately assess its magnitude
Binary matrices of optimal autocorrelations as alignment marks
We define a new class of binary matrices by maximizing the peak-sidelobe
distances in the aperiodic autocorrelations. These matrices can be used as
robust position marks for in-plane spatial alignment. The optimal square
matrices of dimensions up to 7 by 7 and optimal diagonally-symmetric matrices
of 8 by 8 and 9 by 9 were found by exhaustive searches.Comment: 8 pages, 6 figures and 1 tabl
Enhanced spin injection efficiency in a four-terminal double quantum dot system
Within the scheme of quantum rate equations, we investigate the spin-resolved
transport through a double quantum dot system with four ferromagnetic
terminals. It is found that the injection efficiency of spin-polarized
electrons can be significantly improved compared with single dot case. When the
magnetization in one of four ferromagnetic terminals is antiparallel with the
other three, the polarization rate of the current through one dot can be
greatly enhanced, accompanied by the drastic decrease of the current
polarization rate through the other one. The mechanism is the exchange
interaction between electrons in the two quantum dots, which can be a promising
candidate for the improvement of the spin injection efficiency.Comment: 10 pages and 5 figure
- β¦