The seasonal and wintertime interannual variability of the split jet and the storm-track activity minimum near New Zealand (NZ) have been examined based upon the European Centre for Medium Range Weather Forecast Reanalysis (ERA-40) 1979-2001 daily data. Using the split jet index defined in this paper, the climatological variation in the split jet is closely related to the storm-track activity minimum. In austral winter, the split jet is located near NZ along with the storm-track activity minimum, which is established by the climatological Rossby wave forced by the cross-equatorial flow in the Indian Ocean. In austral spring and autumn, in contrast, both the NZ split jet and the storm-track activity minimum are less clear in the absence of the Rossby wave, because of the near-zero Indian Ocean cross-equatorial flow. However, interannual variation in the wintertime Indian Ocean cross-equatorial flow is only weakly associated with interannual variability in the split jet, the correlation being 0.14. The interannual variation in the cross-equatorial flow further east, in the Indonesian region, shows a higher correlation of 0.29. The middle and higher latitude components of the split jet structure are more strongly related to the zonally asymmetric part of the Antarctic Oscillation. An index for this has a 0.43 correlation on interannual time scales with the split jet index. Other processes must also be important for the interannual variability of the split jet and storm-track activity minimum in the NZ region. It is hypothesized that blocking in that region may be of particular importance.
The interannual variation of summer climate in the western North Pacific and East Asia (WNP/EA) has been investigated in this study using an experiment forced by global observed sea surface temperatures (SSTs). The ensemble integrations enable us to separate externally forced variability from internal variability. It is found that the lower-tropospheric circulation anomaly over the WNP is dominated by the external variability forced by SSTs, while the anomaly of the East Asian jet (EAJ), is dominated by the atmospheric internal variability. The external variability in the WNP/EA sector is mainly reflected by the first leading empirical orthogonal function (EOF) mode of lower-tropospheric zonal wind, and partially by the second mode. The first mode is characterized by a cyclonic/anticyclonic circulation anomaly over the tropical WNP, reflecting changes in the WNP subtropical high. This mode is associated with precipitation and SST anomalies in the tropics. The second mode is characterized by a wave-like pattern of zonal wind in the meridional direction, with zonally-oriented cells over the WNP. This second mode is associated with very weak SST anomalies, and more like a mode of the internal variability. The internal mode is found to be well organized in the WNP/EA sector. The features associated with this mode include the meridional displacement, and intensity variation of EAJ, and precipitation anomaly over the WNP.
In Mongolia and northern China, most of the dust storms in spring occur in association with a passage of cold front formed at the leading edge of cold air outbreak. In this study, we propose an index to evaluate a cold frontal activity by identifying “cooling days”. These are days when there is a strong decrease in daily mean surface air temperature of greater than 5 K, using the European Centre for Medium-Range Weather Forecasts 45-year reanalysis data for 1967 through 2002. The geographical distribution of seasonal mean cooling day frequencies showed that Mongolia is the most frequent area over Eurasia with the highest frequency of cooling days in spring. Interannual variations in the area-averaged frequency of cooling days in Mongolia showed that most of the years with less-than-average frequencies in spring occurred in the last 20 years. To determine the main cause(s) for the differences in cooling day frequencies, we conducted a composite study for the 8 years with the highest area-averaged spring cooling day frequencies (active years), and the 8 years with the lowest (inactive years). This study revealed that there were no apparent differences between the active and inactive years with regard to geographical route of cold air, or total number of cooling events. In contrast, the cooling intensity in the inactive years was approximately 20% below that in the active years. These results suggest that recent weakening of cooling intensity over eastern Mongolia has resulted from warming over Siberia.
A simulation study was designed to investigate the error characteristics of retrieved local refractivity arising only from the assumption of spherical symmetry. The convergence of error estimation with respect to different horizontal resolutions was examined, and a resolution higher than 30 km was found to be reasonable for this simulation study. Two cases, one in summer 1997 and the other in winter 1998, were used to study errors of retrieved local refractivity. The error can reach 10 units in the lower troposphere under the assumption of spherical symmetry. Error decreases with height linearly, until about 3 km and then dramatically above 3 km. A local maximum error occurs at approximately 6 km in the 1997 case because there were large refractivity disturbances on the lee side of the Rocky Mountains around that level. The level of the maximum error over land (∼ 2-3 km) was higher than over ocean (∼ 1 km) due to complex topography, as well as a deeper boundary layer over land. As a result, the average root mean square error below 3 km presents a meandering feature over the entire domain of interest. Moreover, the error close to the surface over ocean was slightly higher than that over land. However, in the 2-4 km range the error was slightly lower over ocean than over land. Note that the altitude of maximum error over ocean can be shifted upward when strong disturbances, such as severe weather, exist over the region. The error during summer was larger than that during winter, and it was greater at lower latitudes than at higher latitudes, as reported in previous studies. When modeling retrieved local refractivity, the observational error is a function of the model horizontal resolution. The error was reduced as model resolution was decreased but there was a lower limit for an optimal resolution. This optimal resolution in the lower atmosphere was higher (∼ 190 km) than that in the middle and upper atmosphere (∼ 250 km) in this study, due to a stronger vertical gradient of refractivity and higher vertical resolution in the lower troposphere.
A meso-α-scale cloud cluster (CC) was observed in the Baiu frontal zone, located southwest of Kyushu, Japan, on July 7, 1996. It was characterized by a lifespan of approximately 20 hours, stationary motion, and heavy precipitation of over 200 mm in a period of 12 hours. This paper contains a report of the characteristic features of internal multiscale precipitation systems observed within the CC, and their periodic evolutions as newly found facts. The CC consisted of a convective rainfall region, characterized by a meso-βL-scale (100-200 km) line-shaped convective system (convective line, MβLCL), and a weak stratiform rainfall region on the lee side of the MβLCL. The MβLCL consisted of several band-shaped meso-βS-scale (20-100 km) convective systems (MβSCSs), and each MβSCS consisted of meso-γ-scale cumulonimbus clouds. The environmental atmosphere was characterized by a warm and moist inflow in the shallow layer (below 500 m in height) associated with a cyclonic circulation of slow moving and shallow depression. Additionally, the CC was located within a large temperature gradient zone in the lower atmosphere (below 4000 m in height) along the Baiu front. These were favorable environments for the generation and maintenance of theMβLCL. The MβLCLs were periodically generated with an interval of 5-6 hours in almost the same region, and they showed a common evolution of structure and processes. The behavior of cold pools formed by the MβLCLs on the northern side of the MβLCL played a key role in the periodic evolution. The developed cold pool intensified the MβSCSs in the MβLCL and modified one of the MβSCSs into an arc-shaped MβSCS, that had similar characteristics to those ofsquall lines. Simultaneously, the arc-shaped MβSCS became the sole MβSCS that constituted MβLCL. Finally, the MβLCL decayed with the expansion of the cold pool. Another MβLCL was generated when the favorable environment was restored after the cold pool dissipated, and the evolution process of the MβLCL was repeated.
Temperatures retrieved from NOAA-15 AMSU-A during the period from July to September in 2001 and 2002 are used to develop an algorithm for estimating the intensity of tropical cyclones (TCs) over the western North Pacific. The variance R2 explained by the algorithm is 76.9%, and the mean absolute error MAE (root-mean-square error, RMSE) is 5.6 (7.5) m s−1, all of which are comparable to the published results for Atlantic and Eastern Pacific TCs. In addition to the maximum temperature anomaly over the TC center, the height of the warm core, represented by the uppermost position of a certain temperature anomaly contour (1.0 and 0.5 K), is found to be another important predictor in the final equation. Both the jackknife method, and an independent test, are applied to verify the algorithm. Results show that R2 deteriorates slightly to 72.3% using the jackknife method, while the other three statistic, i.e., MAE, RMSE and standard deviation of residuals, increase by ∼0.5-1 m s−1 in both the jackknife and independent datasets. Most of large underestimations occur for small but strong TCs, while significant overestimations are either due to the lag of an upper-level warm core following the weakening of the surface circulation, or due to upper-level warming ahead of the surface circulation development.
Features of the polar-air outbreak and the energy balance in the transformed air-mass over the Japan Sea, during a period of strong polar-air outbreak in 1977, are studied by using upper observation data on a research vessel (Keifu-maru) and two coastal stations (Akita and Wajima). The analysis is made over an area of 3.8 × (100 km)2 using vertically dense (25-hPa interval) data interpolated from data at standard pressure levels and significant levels. The mixed layer capped by the stable layer is formed in the transformed air-mass. The large apparent heat and moisture sources are evaluated within the mixed layer. The apparent heat and moisture sources decrease abruptly at the top of the mixed layer. The large kinetic energy dissipation due to the sharing stress in the lower layer almost offsets the large generation of the kinetic energy due to the work down by the pressure gradient force. The present analysis indicates that the use of vertically dense data is important to resolve the multilayer structure of the transformed air-mass, and to obtain accurate venical distribution of apparent energy sources, in the transformed air-mass. The results of the present analysis are consistent, in general, with numerical simulations on the air-mass transformation over the Japan Sea. However, the apparent heat and moisture sources in the lower portion of the boundary layer in this study are considerably small as compared with results of the simulation studies. This discrepancy will be due to the use of data at the land stations on the outflow side of the budget analysis area.
Changes in rainfall characteristics over Mongolia and adjacent regions during summer were examined for the period 1960 to 1998, using daily and monthly precipitation data for Mongolia, China and the former USSR. Climatologically, mean summer (June to August) total precipitation was greater in northern Mongolia, and tended to decrease toward the south and southwest. Summer total precipitation contributed more than 60% of annual precipitation in Mongolia. ‘Wet days’ is defined as a day exceeding precipitation more than 0.1 mm. The number of wet days was approximately 40 days in northern Mongolia, while the number of wet days was less than 20 days in the south. Between 1960 and 1998, trends in summer total precipitation, the number of wet days in summer, and summer mean precipitation intensity were examined. The daily precipitation data were sorted into ascending order, and grouped into 10 classes which have an interval width equal to 10% of total number of wet days. Summer total precipitation increased in eastern and western Mongolia. Trends in heaviest precipitation class were also evaluated. The amounts of precipitation in the heaviest rainfall class, also increased in eastern and southern Mongolia and the Altai Mountains. The number of wet days increased over almost all of Mongolia. The frequency of relatively heavy rainfall events increased in eastern and southern areas, whereas during the same period weaker rainfall events became dominant in the northern part of central Mongolia.
This paper proposes the empirical correction of the correlated k-distribution method, to calculate the radiative heating rate in the 15 μm CO2 band. The transmittances produced by the correlated k-distribution method in CO2 15 μm band are systematically displaced from the exact transmittances obtained by the line-by-line method. When the empirical corrections are applied to the correlated k-distribution transmittances, the resultant heating rates of clear-sky are significantly improved in accuracy. In addition, this empirical correction is computationally efficient. Thus the method is promising for practical purposes, such as a radiation scheme in a dynamical model.