We reviewed the long-term trends and inter-annual variations in the surface shortwave irradiance in China and Japan. Pyranometer observations revealed decreases followed by increases in the shortwave irradiance in China and Japan between the 1960s and 2000s, while obvious long-term trends were not evident in the satellite observations after 1983. In China, surface shortwave irradiance decreased from 1961 until around 1990, but then began to increase. In Japan, on the contrary, the decreasing trend stopped in the 1960s, with little inter-annual variation during the 1970s and 1980s, and an increase began around 1990. The causes of the differences between the shortwave irradiance trends in China and Japan were ascribed to an increase in light-absorbing aerosols in China that began in the 1960s and a decrease in absorbing aerosols in Japan that began in the late 1970s. Absorbing aerosols decrease both direct and diffuse radiation, while non-absorbing aerosols decrease direct radiation but increase diffuse radiation. Although these aerosol influences are generally found under clear sky conditions, absorbing aerosols could have direct effects even under cloudy sky conditions. The trends of surface shortwave irradiance in China and Japan are in line with the so-called global dimming and brightening dimming processes, although the phases of the minimum periods in the two regions slightly differed. An increase in anthropogenic aerosol was responsible for the variation in the shortwave irradiance through the direct radiative effect of aerosol in the polluted area, while indirect radiative effects, i.e., changes in cloud cover due to an increase in cloud condensation nuclei, dominated in pristine areas. The effects of other factors, such as variations in water vapor and natural aerosol levels, appear to be small compared to the effects of cloud and anthropogenic aerosols.
Inverse analysis estimates the regional flux of greenhouse gases between the earth’s surface and atmosphere using observed atmospheric concentration data that include satellite data. In particular, this method is effective in estimating the flux in regions where observational flux data are limited. However, inverse analysis is basically a mathematical optimization method. Therefore, confirmation of the causal validity of the spatial and temporal changes in the estimated flux is necessary. One confirmation method is validation of the relationship with physical and biological observation data (analysis data) of confirmed accuracy. In this study, the features and validity of changes in the carbon dioxide (CO2) flux estimated by inverse analysis were verified via interrelation analysis, with changes in precipitation, short-wave radiation, surface temperature, and Normalized Difference Vegetation Index (NDVI) in regions of South America and Africa where CO2 flux observation data are limited. Sufficient accuracy of the land surface elements is required for the analysis results to confirm the CO2 flux estimated by inverse analysis. An examination of the correlation of anomalies showed consistent relationships among the precipitation, short-wave radiation, surface temperature, and NDVI data used in this study, which were independently created. The relationships between change in the estimated CO2 flux and characteristic changes in the land surface elements in South America and Africa were consistent for each region. This study confirmed the physical and biological validity of the changes in the CO2 flux estimated by inverse analysis. During the period of this study, the NDVI anomaly was influential in South America and the precipitation (soil wetness) anomaly was an essential factor in Africa for the CO2 flux anomaly. The short-wave radiation anomaly was also influential in both South America and Africa. The distinctive relationships are more clearly detected in the results of inverse analysis using both ground-based CO2 concentration data and the Greenhouse gases Observing SATellite (GOSAT) data than in the results using only ground-based CO2 concentration data. This demonstrates the usefulness of GOSAT data in regions with limited atmospheric CO2 concentration data.
Lightning features over the Tibetan Plateau were studied in relation to topography using the World Wide Lightning Location Network (WWLLN) dataset obtained from April 2009 until December 2014. To describe the strength of lightning strokes, lightning strokes with energies above the 90th percentile (7666 J) were defined as strong lightning (S-lightning) strokes, and the ratio of S-lightning strokes to the overall number of lightning (O-lightning) strokes was defined as the strong ratio (S-ratio). O-lightning density over the Tibetan Plateau was found to be high in general, except over the western part of analysis region. Minimum-density zones were observed along the Himalayas approximately 6 km above sea level and in deep valleys within the Tibetan Plateau. The maximum- and minimum-density zones also exhibited maximum and minimum annual rainfall amounts, respectively. S-lightning strokes were also found to frequently occur over the Tibetan Plateau, and most S-ratios in the analysis units exceeded 30 %, which corresponds to three times the global mean. In particular, the S-ratios over the southern part of the Tibetan Plateau, including the Himalayas, were found to be high (50 %) and to correspond with the zone that had the minimum O-lightning density. The maximum O-lightning density was observed to occur during the summer at an elevation approximately 0.2-1.0 km higher than the plateau level. The O-lightning and S-lightning densities around the Nagqu sonde station were negatively correlated with the Showalter stability index (SSI) and the vertical wind shear. The S-ratio and average stroke energy were found to be negatively correlated with the vertical shear but not with the SSI.
Regionally enhanced meshes that have quasi-uniformly fine circular region is proposed using a new transformation method with icosahedral grids to obtain a cost-effective simulation for waves, transports, and mixing processes, the behaviors of which depend strongly on the horizontal resolution. The target region, which is designed to be composed of a finer mesh, is connected to a coarser mesh region, which is generated with the Schmidt transformation to maintain an isotropy of grid shapes. To realize these requirements, the spring dynamics method can be used and the characteristic length of the spring connecting grid nodes should be determined through three parameters: (i) the number of grid points placed in the target region, (ii) the area of the target region, and (iii) a parameter of the Schmidt transformation. By introducing a set of mathematical formulae, the minimum grid interval in the target region can be uniquely determined as a function of the area of the target region only. It is confirmed that fine and quasi-homogeneous meshes in the target region are generated using the grid transformation proposed in this study. Numerical simulations under realistic atmospheric conditions are performed using a non-hydrostatic model with the grid system proposed in this study and in a previous study. As the new grid system has a more homogenous resolution in the target region compared with that of the previous study, the estimation of the momentum fluxes of gravity waves are less affected by their dependence of the grid resolution.
In this study, energy and water vapor exchange between the lake and atmosphere over the largest lake in Tibet, Lake Serling Co, was measured by an eddy covariance system from April 26 until September 26, 2014. The results demonstrated that the diurnal variations of the sensible heat flux (H) and latent heat flux (LE) were different from that of the net radiation (Rn). Rn reached its peak value at the local noon, whereas H peaked in the morning and LE peaked in the afternoon. On a seasonal scale, H and LE were also different from Rn. The maximum value of Rn occurred in June, while the maxima of H and LE were observed in September. Lake evaporation was quantified with a daily mean value of 2.7 mm d-1 and a total amount of 417.0 mm during the study period. In addition, evaporation from Lake Serling Co was compared with two types of pan evaporation (D20 pan and E601B pan) and potential evaporation on the land surface. The variability of conversion coefficients between lake evaporation, D20/E601B pan and potential evaporation indicate that coefficients varied depending on the month and could not be defined as a single experimental value.