Climatologically, an abrupt enhancement of convection occurs in mid-July over the western North Pacific (WNP) around 20°N, 150°E known as a convection jump (CJ), which is associated with a rapid diminishment of the Baiu rainband over central Japan through excitation of stationary Rossby wave. In view of the subtropical-origin teleconnection, we examined a projected future delay of the Baiu withdrawal under global warming conditions with relevance to future changes in seasonal evolution over the WNP. Based on reproducibility evaluation of the Coupled Model Intercomparison Project phase 3 (CMIP3) multi-model outputs with respect to seasonal evolution of summertime precipitation over central Japan, we computed weighted multi-model ensembles, which are used for diagnosis of the projected future CJ changes. The result shows that CJ under global warming will occur more southeastward. In the future projection, increases in sea surface temperature (SST) over the equatorial Pacific and subtropical North Pacific are larger than those in the adjacent regions. Sensitivity experiments with a general circulation model (MJ98 AGCM) utilizing these salient warm SST anomalies also show the southeastward shift of CJ in comparison with a control simulation. These results indicate that the change in the location of subtropical convections associated with the spatial distribution of warm SST anomalies could be an important factor for the late termination of the Baiu season over central Japan under global warming conditions.
A new method for identifying and tracking convective cells is proposed for the statistical analysis of convective cells embedded within mesoscale convective systems using a two dimensional radar reflectivity dataset. The algorithm for the identification and tracking of convective cells determines convective regions with radar reflectivity exceeding a given constant threshold. The threshold value is gradually increased to detect a cell region with a single peak of reflectivity. The algorithm includes a new cell-merging and -splitting scheme that assumes the conservation of total area of convective cells and the maintenance of their relative locations when merging or splitting occurs, and is termed AITCC (Algorithm for the Identification and Tracking Convective Cells). The AITCC performance was evaluated in an analysis of 2004 non-severe convective cells (30-40 dBZ) and in 1268 linkages of convective cells (i.e., two successive observations of the same convective cell) observed within meso-β convective systems in the Meiyu frontal region. We demonstrated that the AITCC decreased the number of incorrect cell assignments, especially in situations where convective cells are located close together. AITCC showed promising performance (false-alarm-rate < 10%) in the tracking of weak convective cells (30-40 dBZ) that seemed to be difficult for the previous tracking algorithms. AITCC is expected to enable us to calculate the statistical features of convective cells from their development to dissipation.
In order to understand the characteristics of low-level jets (LLJs) over coastal areas of China Continent and its linkages with rainfall during the warm seasons, half-hourly data from a wind profiler radar (WPR) at the Qingpu site during the Meiyu periods of 2008 and 2009 in addition to data from one month prior to and after (non-Meiyu periods) were used to develop a climatology of the LLJs over Shanghai, China. Two peaks in LLJ incidence were revealed at 500-800 m and 2100-2200 m altitudes. Thus, we classified the observed LLJs into two types: (1) boundary-layer jets (BLJs, below 1 km) and (2) synoptic-system-related LLJs (SLLJs, within 1-3 km). BLJs and SLJs showed different behavior in their temporal variation of the occurrence frequency, wind direction, and relation to rainfall. The BLJs displayed a more evident diurnal cycle than the SLLJs, with maximum incidence occurring in nighttime and early morning. The SLLJs occurred more frequently during Meiyu periods than non-Meiyu periods, whereas the occurrence frequency of BLJs increased throughout the warm seasons with no peak during Meiyu periods. BLJs are mostly southerly winds that might be induced by inertial oscillation with a strong background southerly geostrophic wind due to the west-east land-ocean thermal difference. SLLJs most frequently appeared as southwesterly and westerly winds embedded in the East Asian monsoon circulation. The relation between LLJs and precipitation was examined by comparing the frequency of LLJs occurrence on rainy days (cases), with daily (4-hourly) accumulated rainfall within 1 to 10 mm, and non-rainy days (cases) because the missing rate of WPR data is high during heavy rainfall events. Both BLJs and SLLJs occur more often on rainy days than on non-rainy days. In shorter time scales (4 hours), both BLJs and SLLJs tend to occur frequently during, before and after rainy cases, except SLLJs before rainy cases.
On 11 November 2007, record heavy rainfall was observed over Aomori Prefecture in northern Japan. Numerical simulations were performed using a non-hydrostatic model with a horizontal resolution of 5 km (5km-NHM) to clarify the formation mechanisms of this heavy rainfall. The results revealed that a meso-β scale vortical disturbance (MβV) originated over the offshore area with strong horizontal wind shear southeast of Aomori Prefecture at 0600 UTC 11 November. The MβV traveled north-northwestward over the sea and landed in Aomori Prefecture at 1200 UTC. The heavy rainfall was caused by convective systems associated with the MβV. A kinetic energy budget analysis was conducted using the results of 5km-NHM to account for the development of the MβV. Eddy kinetic energy (EKE) increased with time, while mean kinetic energy decreased. This relation implies that the MβV developed in association with kinetic energy conversion from the mean part to the eddy part. Horizontal shear below a height of 1 km played a key role in EKE production, and vertical advection by mean and perturbation flows transported EKE produced in the lower layers to the upper layers. The vertical shear and buoyancy did not significantly contribute to the EKE increase. The pressure gradient force normal to the shear line also contributed to EKE production by accelerating horizontal wind speed. A sensitivity experiment without diabatic heating indicates that convective activities affected the development and maintenance of horizontal shear, which significantly contributed to the MβV development.
Observation instruments are commonly upgraded because of technological advances and the convenience of the observation agency. However, great care is necessary when changing instruments to ensure data continuity for climatic data analysis. The Tateno upper-air observation station of the Japan Meteorological Agency replaced the Meisei RS2-91 type rawinsondes with Vaisala RS92-SGP type GPSsondes in December 2009. We carried out a total of 115 simultaneous dual launches for four seasons to investigate any differences in performance. The simultaneous sensor comparison results showed that Vaisala RS92-SGP temperature was 0.1-0.4K higher than Meisei RS2-91 temperature above the 100 hPa layer in night time observations; and that Meisei RS2-91 temperature was ∼0.1K higher above the 30 hPa layer in day time observations. Vaisala RS92-SGP relative humidity was ∼5% lower, particularly under humid conditions and in autumn. Vaisala RS92-SGP pressure was ∼0.5 hPa higher in the stratosphere. We also made pressure-level comparisons for temperature and relative humidity. Furthermore, comparison results are shown for precipitable water vapor measurements taken with a collocated GPS receiver for a sensitivity analysis on the number of dual soundings and for a reanalysis of upper-air temperature trends for 1956-2010, taking the three instrumental change events into consideration.
The relation between the synoptic variability of low cloud amounts and the wave disturbances over the eastern tropical Pacific is studied using data from an atmospheric reanalysis (ERA-Interim) and the International Satellite Cloud Climatology Project (ISCCP) over 12 years (from 1990 to 2001). Here, low clouds are defined as those having the tops between 800 and 680 hPa. A significant correlation is found between the low cloud amount and the meridional wind over 3-7 day periods. Composite analyses show that mixed Rossby-gravity waves in the tropics and Rossby waves in mid-latitudes contribute to the synoptic variability of the low cloud amount. These waves induce the fluctuations of the low cloud amount independently, and the low cloud variability is explained as their superposition. Quantitative analyses suggest that these waves affect the low cloud amount through horizontal advection of clouds and the modulation of static stability.
The Weather Research and Forecasting (WRF) model is used for regional transport simulations of atmospheric carbon dioxide (referred to as WRF-CO2) for the East Asia region at the horizontal resolution of 27 × 27 km. The domain extends from 18°N to 51°N in latitude and 101°E to 165°E in longitude, including the islands of Japan, South Korea, North Korea and a part of China. The simulation period is limited to the year 2002. To understand the role of surface fluxes and transport, we have simulated atmospheric CO2 using 5 different CO2 fluxes from ocean, fossil fuel and terrestrial biospheres at various horizontal resolutions, and at hourly to monthly time intervals. The model simulations are compared with observed time series at 9 stations, which are located under different ecological and climate conditions. The model simulations are evaluated at different seasonal, synoptic and diurnal time scales for CO2 and meteorological parameters, such as air temperature, relative humidity, wind speed and direction. The performance of WRF-CO2 model is found to be satisfactory in all aspects when compared to global model simulated results obtained under the TransCom (Transport Model Intercomparison Project) continuous experiment. The WRF-CO2 model is shown to have the ability to resolve distinct concentration variations, both diurnal and synoptic, at two closely spaced stations within 25 km. The diurnal cycles of terrestrial biospheric fluxes and the planetary boundary layer are found to be most dominant controls for CO2 diurnal variations, whereas the surface fluxes horizontal distributions and wind directions are identified as the dominant controls for CO2 synoptic variations. Further increase in horizontal resolution of biospheric fluxes and meteorology in WRF-CO2 simulation is required for improving the model-observation agreement.