The new Meteorological Research Institute Earth System Model version 2.0 (MRI-ESM2.0) has been developed based on previous models, MRI-CGCM3 and MRI-ESM1, which participated in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). These models underwent numerous improvements meant for highly accurate climate reproducibility. This paper describes model formulation updates and evaluates basic performance of its physical components. The new model has nominal horizontal resolutions of 100 km for atmosphere and ocean components, similar to the previous models. The atmospheric vertical resolution is 80 layers, which is enhanced from the 48 layers of its predecessor. Accumulation of various improvements concerning clouds, such as a new stratocumulus cloud scheme, led to remarkable reduction in errors in shortwave, longwave, and net radiation at the top of the atmosphere. The resulting errors are sufficiently small compared with those in the CMIP5 models. The improved radiation distribution brings the accurate meridional heat transport required for the ocean and contributes to a reduced surface air temperature (SAT) bias. MRI-ESM2.0 displays realistic reproduction of both mean climate and interannual variability. For instance, the stratospheric quasi-biennial oscillation can now be realistically expressed through the enhanced vertical resolution and introduction of non-orographic gravity wave drag parameterization. For the historical experiment, MRI-ESM2.0 reasonably reproduces global SAT change for recent decades; however, cooling in the 1950s through the 1960s and warming afterward are overestimated compared with observations. MRI-ESM2.0 has been improved in many aspects over the previous models, MRI-CGCM3 and MRI-ESM1, and is expected to demonstrate superior performance in many experiments planned for CMIP6.
The influence of tropical cyclones (TC) on the western North Pacific (WNP) summer monsoon flow—as well as the impact on rainfall in the Philippines during the months of June to September from 1958 to 2017—were investigated. High precipitation event (HPE) days with measured rainfall in the upper 85th, 95th, and 99th percentiles were determined using daily rainfall averages via data acquired from eight synoptic stations in northwestern Philippines. More than 90 % of HPE days coincide with TC occurrence in the WNP, whereas landfalling TCs only account for 12.8-15.1 % of HPE days. The present study looks at the non-landfalling TCs that are coincident with HPEs. The result shows that these non-landfalling TCs remotely play a key role that affects almost all local HPEs in northwestern Philippines.
Analysis of the TC tracks and their influence on southwesterly summer monsoon flow in Southeast Asia during HPE days shows that most of the TCs move along a line segment connecting northern Luzon and Okinawa, Japan. The composite low-level flow of all HPE days is characterized by a zonally-oriented eastward trough at the 1005-1007 hPa isobar along 20°N that extends to at least 135°E longitude over the northern half of the Philippines; a deepening of the monsoon trough in northern South China Sea also occurs. The 1005-1007 hPa trough induces an eastward shift on the southwesterly flow causing a 1.94-4.69 times increase in mean zonal wind velocity and 2.67-6.92 times increase in water vapor flux (via moisture conveyor belt) along western Luzon. In addition, increasing trends of 6.0 % per decade in the mean annual number of HPE days per decade and 12.7 % per decade in the annual total HPE precipitation are found to be significant in the upper 85th percentile of daily rainfall. These increases are attributed to the recent changes in WNP TC tracks.
Based on the preceding work, the influence of the stochastic multicloud model (SMCM) on the Madden–Julian oscillation (MJO) in the state-of-the-art ECHAM6.3 atmospheric general circulation model (AGCM) is further evaluated. The evaluation presented here is based on six recently proposed dynamics-oriented diagnostic metrics. Lag–longitude correlation maps of surface precipitation in the eastern Indian Ocean and West Pacific Ocean confirm the previously discovered improved representation of the MJO in the modified ECHAM6.3 model compared with the standard configuration. In fact, the modified ECHAM6.3 outperforms the default ECHAM6.3 in five of the six MJO-related diagnostics evaluated here. In detail, the modified ECHAM6.3 (1) successfully models the eastward propagation of boundary layer moisture convergence (BLMC); (2) captures the rearward-tilted structure of equivalent potential temperature (EPT) in the lower troposphere and forward-tilted structure of EPT in the upper troposphere; (3) exhibits the rearward-tilted structure of equatorial diabatic heating in the lower troposphere; (4) adequately simulates the MJO-related horizontal circulation at 850 and 200 hPa and the 300 hPa diabatic heating structure. These evaluations confirm the crucial role of convective-parameterization formulation on GCM-simulated MJO dynamics and support the further application and exploration of the SMCM concept in full-complexity GCMs.
Based on the monthly outgoing longwave radiation (OLR) data from 1979 to 2013, a significant correlation of convective activity over the western Pacific warm pool between June and August was detected, while there were no significant correlations between June and July and between July and August. The analysis results indicate that consistent anomalies in June and August usually occur during the years with strong warm pool convection. Moreover, two prerequisites are necessary for this consistent anomaly, that is, a higher sea surface temperature (SST) over the warm pool during the preceding spring and a relatively weak El Niño and Southern Oscillation (ENSO). An analysis based on the selected typical years indicates that convection in June tends to be enhanced when the warm pool SST is higher in the spring. The enhanced convection, in turn, reduces the solar insolation and local SST and consequently suppresses convection in July. In contrast to June, the local SST tends to increase due to the suppressed convection in July. Accordingly, the warm pool convection in August is subsequently enhanced again. In this process, the local air–sea interaction plays a major role in regulating SST anomalies from June to August and forming the consistent warm pool convection anomalies in June and August. There are additional complications in understanding intraseasonal variations in the warm pool convection from June to August as related to the ENSO forcing. During strong El Niño decaying years (e.g., 1998), the warm pool convection is suppressed with consistent positive OLR anomalies from June to August, implying that the El Niño forcing contributes to the significant positive correlation of convective activity between June and August. During moderate El Niño decaying years (e.g., 2007), however, the convection anomaly in June is opposite to that in August. In general, the local air–sea interaction effect plays an essential role in the significant correlation of convective activity between June and August, although this correlation also depends on the intensity of the El Niño forcing.
In this study, we evaluated the impacts of the revised observation error on ground-based global navigation satellite system (GNSS) zenith total delay (ZTD) data in the data-assimilation system of the Korea Meteorological Administration's 1.5 km convective-scale model. Out of the 100 total stations on the Korean Peninsula, 40 ground-based GNSS data stations were assimilated using three-dimensional variational (3D-Var) data assimilation. The ZTD observation errors were diagnosed for each station using a posteriori methods, giving errors with a variety of spatial and temporal characteristics. These station-specific error data were then implemented using the data-assimilation system, and their impacts were evaluated for a one-month period in July 2016.
The root mean square error (RMSE) of the relative humidity in the lower troposphere was found to improve for the period from T+0 to T+36 hours, when using GNSS data. Replacing the errors used in the previous model with the average diagnosed errors also provided better results, but they were not as good as the results obtained using station-specific errors. The observation error is closely related to precipitable water vapor (PWV); therefore, correction values reflecting seasonal characteristics should be applied. In addition, the quantitative precipitation forecasts were improved in all experiments using GNSS data, but the effects were minimal.
This study examines southwesterly flows and their association with rainfall in Taiwan during the warm seasons: spring, Mei-yu, and summer. We found that the percentage of southwesterly flow events in the lower troposphere was the highest in the Mei-yu season, followed by summer. When southwesterly flows occurred, the chance of rain greatly increased in Mei-yu and summer and the mean rain intensity increased for all three seasons. In northern Taiwan, the percentage of southwesterly flow events was the highest in spring and decreased over warm seasons, whereas the reverse scenario occurred in southern Taiwan.
Southwesterly flows occurred in spring primarily due to a deepening midlatitude trough over eastern China. The chance of rain in Taiwan increased during southwesterly flow events when the Pacific subtropical high retreated eastward and the trough moved closer to Taiwan. In the Mei-yu season, there was greater moisture, and the occurrence of southwesterly flows was more equally caused by the midlatitude trough and the southwestward extending Pacific high than in spring. The southwesterly flow axis was located roughly over Taiwan, which then shifted southeastward as the Pacific subtropical high weakened. At the same time, the high-moisture zone covered the northern South China Sea and the entire Taiwan. As a result, moisture-laden air was transported to the Taiwan area by the strong southwesterly flow, providing favorable conditions for continuous rain to occur in Taiwan. In summer, southwesterly flows occurred when the Pacific high extended southwest and a low/tropical cyclone moved over southeastern China. Rain tended to be more intense when the low was stronger and closer to Taiwan.
In order to investigate the dependence of future projections for summertime East Asian precipitation on their present-day model climatology, the models well reproducing the observed climatology over East Asia are focused on in the analysis of the fifth phase of the Coupled Model Intercomparison Project (CMIP5) future projections for the period from 2075 to 2099 under the Representative Concentration Pathway 8.5 global warming scenario. The future projection by these models indicates that summertime monthly climatological precipitation in future East Asia is more likely systematically decreased in some regions rather than evenly increased in every wet region.
The CMIP5 36-model ensemble mean monthly circulation change at 700hPa is characterized through the future summertime by a cyclonic circulation change to the south of Japan and the associated downward motion changes around Japan. The models showing the above features more clearly tend to simulate stronger westerlies over East Asia and more tropical precipitation in the present-day northern summer climatology. Therefore, an ensemble of the models reproducing the observed westerlies over East Asia, which are stronger than the 36-model ensemble mean, tends to simulate a strong downward motion change regionally in the future East Asian summer so that the possibility of a decrease in monthly precipitation is enhanced there against the “wet-getting-wetter” effect.
The future circulation change over East Asia was considered as part of the western North Pacific circulation change that responds to the future reduction of vertical motion in the vertically stabilized tropics. Large future reduction of the tropical vertical motion necessary for the strong downward motion change in East Asia can be attributed to the present-day climatology of much precipitation and large upward motion in the tropics.