A sensitivity of squall-line intensity to environmental static stability under various shear and moisture conditions

Squall lines develop in various climate regions having diverse environmental profiles of wind shear, moisture, and temperature. In order to explore the sensitivity of squall lines to these environmental profiles, we have performed an extensive set of numerical simulations under various shear and moisture conditions in midlatitude-continental and tropical–oceanic temperature environments. From the results of the sensitivity simulations and the analyses of the environmental parameters, it is found that the static stability in a convectively unstable layer is of primary importance in determining the strength of squall lines. Under temperature environments having the same static stability, convective available potential energy (CAPE) and precipitable water content (PWC) well describe the squall-line intensity. Vertical shear also plays an important role in determining the squall-line structure as well as the intensity through the interaction with surface cold pool. The combination of the static stability in a convectively unstable layer, CAPE, and PWC should be examined in diagnosing the intensity of squall lines that develop with an optimal shear for their environment.     

Variation of crustal thickness in the Philippine Sea deduced from three-dimensional gravity modeling

Abstract   Crustal thickness of the northern to central Philippine Sea was gravimetrically determined on the simple assumption of four layers: seawater, sediments, crust and lithospheric mantle, with densities of 1030, 2300, 2800 and 3300 kg/m³, respectively. As for the correction of the regional gravity variation, a 15 km difference of the lithospheric thickness with a density difference of 50 kg/m³ against the asthenosphere below between both sides of the Kyushu-Palau Ridge was taken into consideration. Mantle Bouguer anomalies were calculated on the assumption of constant crustal thickness of 6 km, and then the crustal thickness was obtained by three-dimensional gravity inversion method. The results show occurrence of thin crust areas with a thickness of approximately 5 km in the southern part and at the western margin of the Shikoku Basin and also of thick crust areas in the northwestern and northeastern parts of the Parece Vela Basin. We suggest that these are because of the variation of magma supply at the time of sea floor spreading in the Shikoku and Parece Vela Basins, which is possibly related to the variation of spreading rate and enhanced magmatism near the past arc volcanic fronts. The results further show the occurrence of crust thinner than 5 km in the northeastern part of the West Philippine Basin, of crust thicker than 15 km in the Amami Plateau, the Daito and Oki-Daito Ridges, and also in the northern part of Kyushu-Palau Ridge, whereas the southern part of the Kyushu-Palau Ridge the crust is thicker than 10 km. It was also inferred that small basins in the Daito Ridge province have the thinnest oceanic crust of less than 5 km in the Kita-Daito Basin.     

Large-Eddy-Simulation Study of the Effects of Building-Height Variability on Turbulent Flows over an Actual Urban Area

Large-eddy simulation (LES) is used to investigate the effects of building-height variability on turbulent flows over an actual urban area, the city of Kyoto, which is reproduced using a 2-m resolution digital surface dataset. Comparison of the morphological characteristics of Kyoto with those of European, North American, and other Japanese cities indicates a similarity to European cities but with more variable building heights. The performance of the LES model is validated and found to be consistent with turbulence observations obtained from a meteorological tower and from Doppler lidar. We conducted the following two numerical experiments: a control experiment using Kyoto buildings, and a sensitivity experiment in which all the building heights are set to the average height over the computational region \(h_{all}\). The difference of Reynolds stress at height \(z=2.5h_{all}\) between the control and sensitivity experiments is found to increase with the increase in the plan-area index (\(\lambda _p\)) for \(\lambda _p > 0.32\). Thus, values of \(\lambda _p\approx 0.3\) can be regarded as a threshold for distinguishing the effects of building-height variability. The quadrant analysis reveals that sweeps contribute to the increase in the Reynolds stress in the control experiment at a height \(z= 2.5h_{all}\). The exuberance in the control experiment at height \(z=0.5h_{all}\) is found to decrease with increase in the building-height variability. Although the extreme momentum flux at height \(z=2.5h_{all}\) in the control experiment appears around buildings, it contributes little to the total Reynolds stress and is not associated with coherent motions.     

Aeolian dust experiment on climate impact: An overview of Japan–China joint project ADEC

The Aeolian Dust Experiment on Climate Impact (ADEC) was initiated in April 2000 as a joint five-year Japan–China project. The goal was to understand the impact of aeolian dust on climate via radiative forcing (RF). Field experiments and numerical simulations were conducted from the source regions in northwestern China to the downwind region in Japan in order to understand wind erosion processes temporal and spatial distribution of dust during their long-range transportation chemical, physical, and optical properties of dust and the direct effect of radiative forcing due to dust. For this, three intensive observation periods (IOP) were conducted from April 2002 to April 2004.The in situ and network observation results are summarized as follows: (1) In situ observations of the wind erosion process revealed that the vertical profile of moving sand has a clear size dependency with height and saltation flux and that threshold wind velocity is dependent on soil moisture. Results also demonstrated that saltation flux is strongly dependent on the parent soil size distribution of the desert surface. (2) Both lidar observations and model simulations revealed a multiple dust layer in East Asia. A numerical simulation of a chemical transport model, CFORS, illustrated the elevated dust layer from the Taklimakan Desert and the lower dust layer from the Gobi Desert. The global-scale dust model, MASINGAR, also simulated the dust layer in the middle to upper free troposphere in East Asia, which originated from North Africa and the Middle East during a dust storm in March 2003. Raman lidar observations at Tsukuba, Japan, found the ice cloud associated with the dust layer at an altitude of 6 to 9 km. Analysis from lidar and the radio-sonde observation suggested that the Asian dust acted as ice nuclei at the ice-saturated region. These results suggest the importance of dust's climate impact via the indirect effect of radiative forcing due to the activation of dust into ice nuclei. (3) Studies on the aerosol concentration indicated that size distributions of aerosols in downwind regions have bimodal peaks. One peak was in the submicron range and the other in the supermicron range. The main soluble components of the supermicron peak were Na⁺, Ca²⁺, NO₃⁻, and Cl⁻. In the downwind region in Japan, the dust, sea salt, and a mixture of the two were found to be dominant in coarse particles in the mixed boundary layer. (4) Observation of the optical properties of dust by sky-radiometer, particle shoot absorption photometer (PSAP), and Nephelometer indicated that unpolluted dust at source region has a weaker absorption than originally believed.A sensitivity experiment of direct RF by dust indicated that single scattering albedo is the most important of the optical properties of dust and that the sensitivity of instantaneous RF in the shortwave region at the top of the atmosphere to the refractive index strongly depends on surface albedo. A global scale dust model, MASINGAR, was used for evaluation of direct RF due to dust. The results indicated the global mean RF at the top and the bottom of the atmosphere were − 0.46 and − 2.13 W m^− 2 with cloud and were almost half of the RF with cloud-free condition.     

Gravimetric determination of the density of the Zenisu Ridge

The subject of this paper concerns the analysis of gravity, Seabeam bathymetry and single channel seismic data over the Zenisu Ridge. Gravity effects due to the topographies of the sea bottom and the basement surface of Enshunadaoki Seamount are calculated by means of Fast Fourier Transform. The density of the basement of the seamount is determined by a linear least-squares fit between observed free-air anomalies and calculated gravity effects. The results show that the most probable density of the basement is 2.87 g/cm³ and that the sediment has a density in the range of 2.05 to 2.3 g/cm³.

This result confirms the hypothesis that the ridge consists of uplifted oceanic crust. The value of 2.87 g/cm³ is higher than densities of ordinary seamounts in the Bonin Arc and is reasonable as an average density of an oceanic crust.     

Diffusion of trapped particles due to the bounce-drift resonance interaction in the magnetosphere

Within the framework of the quasi-linear approximation, the hybrid diffusion process due to the bounce-drift resonance interaction between trapped particles and low-frequency field fluctuations is examined. The diffusion coefficients obtained, which are valid for particles with large pitch angles, cover the previous results in a few limiting cases. In general, the diffusion coefficients depend strongly on the spatial structure of the power spectrum along field lines, as well as the frequency dependence. The relative importance of the radial diffusion and field-aligned acceleration for ringcurrent particles is discussed. It is shown that the field-aligned acceleration exceeds the inward penetration of the particles near the plasmapause.     

An eddy-resolving simulation of the diurnal variation of fair-weather convection and tracer transport

Tracer transport in the atmosphere is controlled not only by synoptic-scale to mesoscale weather disturbances but also by microscale boundary-layer processes especially under fair-weather conditions. The present study investigates numerically the diurnal variation of boundary-layer convection and cumulus clouds and their role in transporting tracers by conducting high-resolution simulations that explicitly resolve turbulent eddies. The transport of dust aerosols in a desert area under two distinct stability conditions is specifically examined. Convection plays a significant role in transporting dust upward; in other words, the vertical depth of the dust transport is critically determined by the depth of convection. Deep convection is effective in transporting dust into the free atmosphere. The early morning stratification strongly regulates the temporal evolution and the vertical growth of convection and therefore the amount of tracer emission and transport. A sensitivity to model resolution of O (1 km) in a cloud-resolving simulation range is also examined. A proper parameterization for activating microscale convection is required for representing the diurnal variation of convection and tracer transport.