The accurate aerosol optical thickness is indispensable for estimating the radiative forcing of aerosols in the atmosphere. Sun photometry is one of the most popular methods, which is simple and easy to use, but it should be noted that some errors due to forward scattering effect can be introduced in the observation of the direct normal irradiance. Consequently, the estimated optical thickness of aerosols can be under-estimated even if the calibration constant is correct. This possibility depends on an optical geometry of the measuring instrument as well as aerosol characteristics. This report assesses these effects by assuming several aerosol types and instrumental parameters quantitatively.
Forward scattering ratio γλfwd, which is defined as a ratio of the forward scattering part to the true direct normal irradiance (Iλ), by Iλobs=Iλ(1+γλfwd), is approximately proportional to the product of the optical thickness (τλaer) and the single scattering albedo (ωλ) of aerosols and the relative air mass (m), γλfwd≈ελωλτλaerm. The coefficient ελ is a proportional constant which is dependent on the opening angle of the instrument as well as the optical characteristics of aerosols. The variation of ελ is tabulated for several aerosol types and opening angles. Then the error for the estimate of τλaer can be approximately expressed by
The Global Precipitation Measurement (GPM) core observatory satellite launched in 2014 features more extended latitudinal coverage (65°S-65°N) than its predecessor Tropical Rainfall Measuring Mission (TRMM, 35°S-35°N). The Ku-band radar onboard of the GPM is known to be capable of characterizing the 3D structure of deep convection globally. In this study, GPM’s capability for detecting mesoscale convective systems (MCSs) is evaluated. Extreme convective echoes seen by GPM are compared against an MCS database that tracks convective entities over the contiguous US. The tracking is based on geostationary satellite and ground-based Next Generation Radar (NEXRAD) network data obtained during the 2014-2016 warm seasons. Results show that more than 70 % of the GPM-detected Deep-Wide Convective Core (DWC) and Wide Convective Core (WCC) objects are part of NEXRAD identified MCSs, indicating that GPM-classified DWCs and WCCs correlate well with typical MCSs containing large convective features. By applying this method to the rest of the world, a global view of MCS distribution is obtained. This work reveals GPM’s potential in MCS detection at the global scale, particularly over remote regions without dense observation network.
In this study, we evaluated the impacts of 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 1.5 km convective-scale model. Out of 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 be improved 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. We found that 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, although the effects were small.
This paper examines southwesterly flows and their relationship 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, chance of rain greatly rose in Mei-yu and summer and mean rain intensity increased for all three seasons. In northern Taiwan, the percentage of southwesterly flow appearance was the highest in spring and decreased over warm seasons, while the trend reversed in southern Taiwan.
Southwesterly flows formed in spring primarily due to a deepening mid-latitude trough over eastern China. 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 formation of southwesterly flows was more equally contributed to by the mid-latitude trough and the southwestward extending Pacific high than in spring. The southwesterly flow axis was located roughly over Taiwan. This flow axis 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 island of Taiwan. As a result, moisture-laden air was transported to the Taiwan area by the strong southwesterly flow, providing favorable conditions for continuous rain in Taiwan. In summer, southwesterly flows formed 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.
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 2075–2099 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 700 hPa 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, tend 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 a 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.
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 is detected while there are 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, i.e., 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 enhance 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 variation 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, though this correlation also depends on the intensity of the El Niño forcing.
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 at-mosphere general circulation model (AGCM) is further evaluated. The evaluation present-ed here is based on six recently proposed dynamics-oriented diagnostic metrics. Lag-longitude correlation maps of surface precipitation in the East Indian and West Pacific Oceans confirm the previously found improved representation of the MJO in the modi-fied ECHAM6.3 model compared to the standard configuration. In fact, the modified ECHAM6.3 outperforms the default ECHAM6.3 in five of the six MJO-related diagnos-tics evaluated here. In detail, the modified ECHAM6.3 (1) successfully models the east-ward propagation of boundary layer moisture convergence (BLMC); (2) captures the rearward tilted structure of equivalent potential temperature (EPT) in the lower tropo-sphere and forward tilted structure of EPT in the upper troposphere; (3) exhibits the rear-ward tilted structure of equatorial diabatic heating in the lower troposphere; (4) adequate-ly simulates the MJO-related horizontal circulation at 850 and 200 hPa as well as 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.
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 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/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 and its 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 rainfall in the upper 85th, 95th, and 99th percentile were determined using daily rainfall averaged from eight synoptic stations in northwestern Philippines. More than 90 % of HPE days coincide with TC occurrence in the WNP and landfalling TCs only account for 12.8-15.1 % of HPE days. The present study looks at the non-landfalling TCs coincident with the HPEs. The result shows that these non-landfalling TCs are critical in remotely affecting almost all local HPEs in northwestern Philippines.
Analysis of the TC tracks and their influence on the southwesterly of the summer monsoon flow in Southeast Asia during HPE days show that most of the TCs moved 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 of the 1005-1007 hPa sea level isobar along 20°N that extends to at least 135°E longitude over the northern half of the Philippines, and a deepening of the monsoon trough in northern South China Sea. The 1005-1007 hPa trough induces an eastward shift of the southwesterly that increased the mean zonal wind along western Luzon by 1.94-4.69 times and water vapor flux by 2.67-6.92 times by way of the ‘moisture conveyor belt’. In addition, significant 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 for the upper 85th percentile daily rainfall. These are attributed to the recent changes in WNP TC tracks.