Using a regional climate model that includes a terrestrial biosphere model, numerical simulations were performed to clarify the mechanism of the carbon cycle between the terrestrial ecosystem and the atmosphere and to investigate the climate factors impact on the carbon cycle in the East Asian terrestrial ecosystem. Model verifications were performed with regard to the principal elements: precipitation and vegetation phenology. The variations of the atmospheric carbon dioxide concentration simulated by the model were validated using the data at six in situ observatories. After the confirmations of the model performance, regional features of the impact of climate factors on the gross primary production (GPP) were analyzed. The downward short-wave radiation (DSW), the soil wetness (SW), and the surface temperature (TA) were chosen as the effective climate factors. In the NE-Asia region, a positive anomaly of DSW in warm season induces a positive anomaly of GPP. High anomalous TA in the cold season also induces a large value of GPP. In the China region, similarly to that in NE-Asia, DSW is important for GPP. The effect of SW is also important in the spring. While the correlation between the variation of TA and that of GPP is positive and high in winter, the positive anomaly of TA in the warm season induces negative anomaly of GPP. In the Indochina region, DSW is important throughout the year. In contrast, the correlations between the variations of SW and those of GPP are negative in all seasons, indicating that GPP becomes a negative anomaly when the precipitation is a positive anomaly. In the India region, DSW in the warm season and SW in the cold season are important for increasing GPP. In the Mongolia and Inland regions, SW is a more important climate factor than DSW for GPP. High temperature in the cold season is also important. In summer, unusually high temperature and dry climate conditions reduce GPP. In the Philippines region, main factor affecting GPP is DSW.
In order to understand the structure and mechanism of the mesoscale convective system (MCS) under a vertical shear flow, numerical experiments are performed by use of a cumulus-convection-resolving model. The horizontal grid size is taken to be 1 km. This study was motivated to understand the orientation of a rainband which caused Fukui heavy rainfall in the Baiu season in 2004. Although the flow having strong low-level vertical shear was westerly, the rainband was not oriented in the west-east, but in the westnorthwest.eastsoutheast. One of the bases for understanding this problem is that rainbands tend to take a carrot shape when the environmental flow has unidirectional shear without any jet. This study discusses the importance of the north-south asymmetry such that the atmosphere is more latently unstable in the southern portion than in the northern portion, as observed in the Baiu frontal area. In this situation, convection is more enhanced in the southern portion of the rainband. As a result, such orientation of the rainband (MCS) as mentioned above is realized under the westerly flow. In the MCS treated in this study, the effect of rainwater evaporation is important, and the major convection in the western portion of the MCS tilts on the downshear side. Under such a situation, the so-called back-building takes place and plays an important role in the maintenance and slow movement of the MCS. The environmental wind near the surface is easterly relative to the moving MCS. The reason why the back-building takes place in spite of this situation is that convective activity induces strong vertical circulation and resulting low-level westerly wind which is stronger than the environmetal easterly wind. In addition, the low-level flow with pronounced southerly component of the wind is caused by convective activity. This flow as well as the northerly flow associated with downdrafts plays an important role to enhance mesoscale convection which constitutes the MCS. When the low-level air having the southerly component of the wind asecnds, most of the air maintain the southerly component. That is, the southerly component is dominant not only in the lower layer (below 2-km height) to the south of the MCS, but also in the middle and upper layers (above a height of 2-3 km) in the MCS area and to its north.
The atmospheric water balance over different domains within the South Asian monsoon region has been studied using moisture convergence (C) computed from JRA-25, ERA-40 and NCEP/NCAR reanalysis datasets, GPCP precipitation data (P) and evaporation data (E) as a residual of these two parameters. The seasonal climatology of P, C, and E for the selected regions shows generally large contribution of E to P. The inter-annual characteristics of P, C and E over selected key domains within the South Asian monsoon region have also been examined for both the early (June and July: JJ) and late summer (August and September: AS) monsoon periods from 1979 to 2000. The spatial and temporal characteristics of the hydrological cycle and the contribution of E and C to P are discussed in detail. One important aspect on the seasonal timescale is that from the dry regions in the northwest to the central and the wettest northeast regions, the monthly variations of E or C are large during the monsoon months specific to those regions. However, the interannual variability of P over each domain is not necessarily influenced by the same criteria like C or E, which influences the mean seasonal precipitation. It is also evident that the structure of variability for early (JJ) and late (AS) summer precipitation is different over the South Asian monsoon region. Over northwest India E is dominant on the seasonal timescale, but C contributes higher to interannual variability of P. On the other hand, over central India C is dominant during early summer (JJ) on the seasonal timescale, but E contributes higher to P variability on the interannual timescale, and during late summer (AS) E is dominant on the seasonal timescale, but C contributes higher to P variability on the interannual timescale. Over northeast India, C is dominant on the seasonal timescale, but E contributes higher to interannual variability of P. The importance of land-atmosphere interaction over each domain is discussed. The regionality in the mechanism of precipitation generation and its contribution to the India summer monsoon precipitation variability are also discussed in detail. The role of evaporation variability of precipitation is stronger over the Bay of Bengal sector and the role of convergence on the interannual variability of precipitation is stronger over the Arabian Sea sector.
During cold-air outbreaks in winter, a thick cloud band frequently appears over the northern Sea of Japan and produces localized heavy snowfall in the western coastal region of Hokkaido Island, northern part of Japan. The formation mechanism of this thick cloud band is investigated through a series of nonhydrostatic numerical simulations with a horizontal grid spacing of 5 km. The control simulation well reproduces the characteristics of an observed cloud band. The cloud band forms between relatively warm north-northwesterly winds on the northeast side and relatively cold northwesterly winds on the southwest side. Sensitivity experiments in which upstream topography is modified indicate that the formation and intensification of the cloud band depend on the following two effects; one is the effect of a specific mountain located near the coastline in the middle part of Russia’s Sikhote-Alin mountain range (SAMR), and the other is the effect of large-scale topography along the SAMR on synoptic-scale low-level cold northwesterlies. The specific mountain deflects the cold airflow and immediately a convergence zone forms downstream of the specific mountain, where the cloud band is initiated. On the northeastern side of this mountain, the Froude number is estimated to be about 0.4 from relatively high topography (∼1.2 km), stable stratification (∼0.02 s-1), and synoptic-scale wind speed of 10 m s-1. Thus, the relatively high topography strongly blocks a low-level cold air, whereas an upper air with high potential temperature flows downward over the sea. In contrast, on the southwestern side of the mountain, a low-level cold air can pass over the topography, because the Froude number is estimated to be about 1.6 from relatively low topography (∼0.8 km) and weak stable stratification (∼0.008 s-1). These two airs with different potential temperature create a mesoscale frontal zone over the sea, which causes the further development of the thick cloud band initiated by the coastal specific mountain in the SAMR.
This study uses a time-mode extended singular value decomposition (TESVD) analysis to identify the propagating coupled modes between the upper ocean heat content (UOHC) and sea surface temperature (SST) anomalies in the tropical Indo-Pacific. Four dominant TESVD modes stand out in the interannual timescale. The TESVD1 and TESVD2 depict the thermocline-SST feedback process during ENSO’ persistence period and its turnabout, respectively. With an intrinsic 42-month periodicity, both modes together form an UOHCA pattern propagating counterclockwise around the circuit of equatorial-to-northern tropical Pacific followed by the SSTA propagating eastward in the equatorial waveguide. The leading role of UOHCA is mainly associated with El Niño occurrence. In contrast, the TESVD3 depicts the onset stage of ENSO that has the westward propagating SSTA lead the UOHCA in the cold tongue region. Suppressed equatorial upwelling directly through the positive feedback between the warmer SSTs and anomalous wind convergence from the northwestern Pacific and Caribbean Sea is another factor to trigger El Niño over the NINO3 region, in addition to the thermocline-SST feedback. The TESVD4 depicts the low-frequency evolution of SSTA when ENSO signal is absent. It has the westward migrating SSTA lead the UOHCA 4-month in the equatorial East Pacific. However, the pace of coupled UOHCA is nearly stagnant elsewhere due to the lack of thermocline adjustment mechanism when the zonal wind anomalies largely disappear over the equatorial Indian and West Pacific Oceans. In the Indian Ocean, while the ENSO-related SSTA evolution mainly displays an eastward basin-scale change in the first two TESVD modes, SSTA pattern depicted by the TESVD3 (TESVD4) resembles the double (single) polarity of the Indian Ocean dipole (IOD) event. It is mainly in its east pole region of southeastern Indian Ocean that the SSTA evolution tends to decouple with the underlying UOHCA.
Multi-scale convective organization in a Madden-Julian Oscillation (MJO) event that occurred during December 2006 and January 2007 was studied by global numerical experiments using explicit moist physics. The simulations successfully reproduced the eastward-propagating (∼5 m s-1) convective envelope of the MJO with a zonal scale of 5000-10,000 km, which included eastward-propagating (10-15 m s-1) disturbances (EPDs) and westward-propagating cloud clusters (CCs) with zonal scales of 1000-2000 km and O (100 km), respectively. The simulated EPDs were composed of CCs, with new clusters growing to the east of older ones. When the large-scale circulation associated with the MJO intensified, the EPDs formed well-organized squall-type clusters (rainbands). The dynamical structure of the simulated EPDs was reminiscent of moist Kelvin waves. Relevance of westward-propagating wave disturbances including cross-equatorial flow to convective organization in the EPDs was also suggested.
In this study, we compare assimilation techniques of the full-rank extended Kalman filter (EKF) and the ensemble Kalman filter (EnKF), using a barotropic general circulation model, called barotropic S-model, under the perfect model configuration. We investigate the accuracy of the EnKF in reference to the direct computation of the EKF and examine the influence of the localization for EnKF. The barotropic S-model is based on the primitive equations and predicts the vertical mean state of the atmosphere. Although it has the predictability comparable to the operational prediction models, the direct computation of the EKF is possible. Therefore, we can assess the accuracy of the EnKF as a function of the ensemble members. In this study, the convergence of the EnKF to the EKF is examined using various ensemble members of 20, 50, 100, 410, and 1000. The EKF and EnKF directly assimilate the observation in the spectral space, and the observational elements are model variables. According to the result of the root mean square error (RMSE), the EnKF converges to the full-rank EKF filter when the ensemble member is increased to more than 50. It is demonstrated that the 20 ensemble members are insufficient with respect to the convergence. An empirical orthogonal function (EOF) analysis is conducted using the covariance matrices of analysis error for both filters. The structure of the first EOF (EOF-1) indicates the characteristics of the baroclinic instability waves in mid-latitudes in both filters, showing the same geographical distributions when it has converged. Interestingly, another large analysis error is detected in the Arctic region. Furthermore, the influence of the localization is examined by introducing the local ensemble transform Kalman filter (LETKF), which assimilates the observations in the physical space. The observations which are assimilated by the LETKF are retrieved from the spectral space to the physical space. It is found that the analysis error of the non-localized EnKF in the spectral space is smaller than that of the LETKF in the physical space. It is concluded from the comparison of the RMSE that more than 50 ensemble members are required for the non-localized EnKF to converge to the full-rank EKF for the practical assimilation in the spectral space under the perfect model configuration of the barotropic general circulation model of the atmosphere.