Scientific Online Letters on the Atmosphere (SOLA) has been a fully Open Access journal under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) License (http://creativecommons.org/ license/by/4.0) since 2018, and has been added in the DOAJ (Directory of Open Access Journals) list. Last year, SOLA has updated the Guide for Authors and renewed its own web site (https://www.metsoc.jp/sola/).
In response to the extreme weather events in the summers of 2017 and 2018, SOLA welcomed submissions of paper on the related topics and has published the special edition “Extreme Rainfall Events in 2017 and 2018” as Volume 15A. The published papers cover the large-scale atmospheric and oceanic features of extreme weather in the summer of 2018 (Shimpo et al. 2019; Sekizawa et al. 2019; Takemura et al. 2019; Takaya 2019), the mesoscale environmental properties of the extreme rainfall in July 2018 (Takemi and Unuma 2019), the predictability of extreme weather in the summer of 2018 (Kotsuki et al. 2019; Matsunobu and Matsueda 2019; Kobayashi and Ishikawa 2019), the relationship between a typhoon and the rainfall in July 2018 (Moteki 2019; Enomoto 2019), the use of a gridded precipitation product in investigating extreme rainfalls (Yatagai et al. 2019), and the impacts of global warming on the extreme hot summer in 2018 (Imada et al. 2019).
We are growing. SOLA welcomes submission from the international community in meteorology, atmospheric sciences, and the related fields.
The Editorial Committee of Scientific Online Letters on the Atmosphere (SOLA) gives The SOLA Award to outstanding paper(s) published each year. I am pleased to announce that The SOLA Award in 2019 is going to be presented to the paper by Dr. Yukiko Imada et al., entitled with “The July 2018 high temperature event in Japan could not have happened without human-induced global warming”.
Attribution of individual extreme weather and climate events on human-induced climate change, called event attribution (EA), is often obscured by strong atmospheric fluctuations. By exploiting the large ensembles of d4PDF global and regional atmospheric model simulations, this study performs EA of the extreme heat event in Japan in July 2018. The authors first show, based on a comparison of two global ensembles, that the event would have had virtually no chance to occur without human-induced global warming. Second, with the regional ensemble dataset, they propose a new method to estimate changes in the occurrence of extreme heat events in Japan in accordance with global warming. These analyses answer public concerns on how global warming affected the devastating heat event and how it will under future greenhouse gas warming. The authors further proceed to examine atmospheric circulation features that induced the heat wave. With the model simulations and a reanalysis dataset, they conclude that the enhancement of the lower-tropospheric subtropical anticyclone in the North Pacific and the anomalous eastward expansion of the upper-level Tibetan high are part of naturally-driven teleconnection patterns and human influence on the circulation anomalies is undetectable.
The Editorial Committee of SOLA highly evaluates this process-based EA as well as the social impacts of the paper.
This study aims to investigate the tradeoff between the computational time and forecast accuracy with different data assimilation (DA) windows of four-dimensional local ensemble transform Kalman filter (4D-LETKF) for a single-case severe rainfall event. We perform a series of Observing System Simulation Experiments (OSSEs) with 1-, 3-, 5- and 15-minute DA window in a severe rainstorm event in Kobe, Japan, on July 28, 2008, following the prior OSSEs by Maejima et al. (2019). Running 1-minute DA cycles showed the best forecast accuracy but with the highest computational cost. The computational cost could be reduced by taking a long DA window, but the forecast became less accurate even though the same number of observations were used. A significant gap was found between the 3-minute window and 5-minute window. With the 1- and 3-minute windows, the forecasts captured the intense rainfall, while with the 5-minute window or longer, the rainfall intensity was drastically underestimated. This single-case study suggests that 3-minute or shorter DA window be a promising method for a severe rainfall forecast, although more case studies are necessary to draw general conclusion.
This study investigated the environmental factors responsible for the development of heavy rainfall in eastern Japan during the passage of Typhoon Hagibis (2019) by using mesoscale gridded analysis data as well as observed data. Environmental indices for diagnosing stability and moisture conditions were examined. It was found that the whole troposphere is almost saturated and the column total water vapor content is extremely large. In the lower troposphere we identified layers of moist absolutely unstable states with the thickness deeper than 2 km. Such deep moist absolutely unstable layers as well as abundant moisture content and almost saturated troposphere set a high potential for convective development. Under these favorable environmental conditions, the fact that the heights of the absolutely unstable layers' bottom are comparable to the mountain elevations is considered to be favorable for topographic lifting of unstable, moist air, which will trigger and activate strong convection and hence heavy rainfall. In spite of a moderate amount of convective available potential energy and a nearly moist-adiabatic lapse rate, moist absolute instability, abundant moisture, and high humidity jointly play a key role to increase the potential for generating the present heavy rainfalls.
This study investigated future changes in extreme precipitation associated with tropical cyclones (TCs) around Japan using large ensemble regional climate simulations for historical and +4 K climates. Under the warmer climate, extreme TC precipitation, defined as the 90th percentile value of the maximum daily precipitation derived from each TC (TCP90), is projected to increase throughout Japan from Kyushu to Kanto. We attributed most of the increase in TCP90 to increased atmospheric moisture due to global warming. Furthermore, it was found that TCP90 is projected to increase for all TC intensity categories. However, the projected increase in intense TCs affects TCP90 in only a limited area. Stronger TCs enhance TCP90 over east- and north-facing slopes of mountainous terrain, while TCP90 in most other areas is insensitive to TC intensity. These results suggest that even relatively weak TCs could have potential to produce extreme precipitation that might cause natural disasters.
Heavy precipitation frequently occurs over Kyushu, southwestern Japan, during the Baiu season, and abundant moisture transport is a key driving factor. To statistically understand the intensification of moisture transport to Kyushu during the Baiu season, synoptic-scale atmospheric conditions are examined using a composite analysis of reanalysis data. A heavy precipitation day is defined as a day with area-averaged daily precipitation over Kyushu that is larger than 1.0 mm and ranked in top 10% during May 31 to July 19 from 1981 to 2015. During such heavy precipitation days, the precipitation observed over Kyushu exceeds 100 mm day−1. For several days before the occurrence of heavy precipitation over Kyushu, a plateau-scale disturbance develops over the Tibetan Plateau associated with daytime surface heating, and is characterized by cloud convection formation and eastward extension. During the eastward extension, latent heating from the cloud and upper-level high potential vorticity maintains the disturbance. The disturbance reaches northwest Kyushu on the heavy precipitation day, and a pair of positive and negative anomalies of relative vorticity over northwestern and southeastern Kyushu intensify the anomalous moisture transport.
The 2015/16 El Niño is compared with the two previous strongest events, the 1982/83 and 1997/98 El Niño. The 2015/16 winter features a basin warming in the Indian Ocean, a negative sea surface temperature (SST) anomaly shifted to the north in the western Pacific Ocean in addition to a positive SST anomaly shifted to the west in the eastern Pacific Ocean. These SST distributions lead to suppressed convection in the Maritime Continent, and to a weakened Hadley circulation in the western Pacific Ocean. The eastern Asian monsoon in the 2015/16 winter was also weakened due to the dominance of the western Pacific (WP) pattern. On the other hand, the third and fourth centers of action of Pacific/North American (PNA) pattern in the 2015/16 case are obscure. This may be due to weak divergence in the eastern Pacific Ocean.
The observation operator for the Phased Array Weather Radar in the SCALE-LETKF data assimilation system is revisited, and the impact of its improvement on the analyses and forecasts is examined. The observation operator provides a functional relationship between equivalent radar reflectivity factor (Ze) and hydrometeor mass density (W) of each precipitation particle category. The W–Ze relationship is obtained by a radar simulator. This study performs a radiation code calculation with the parameters regarding particle size distribution of graupel consistent with the cloud microphysics scheme in the SCALE model. The newly obtained observation operator provides much stronger sensitivity of graupel mixing ratio to observed Ze compared to the operator originally used in the model. To examine the impact on the SCALE-LETKF analyses and forecasts, an experiment on a 13 July 2013 heavy rain case is performed with the new observation operator and is compared with the previous study. The forecast initiated by the analysis using the new operator shows much more realistic evolution of Ze in the middle troposphere, where a large amount of graupel is located. The overestimation of forecast Ze is significantly alleviated by the new observational operator. The 30-minute forecast of surface precipitation rate is also improved.
Japan Meteorological Agency (JMA) best track data indicate that the number of rapid intensification (RI) tropical cyclone events in the western North Pacific increased from 1987 to 2018. To clarify whether this increase is due to climatological changes or qualitative changes in the data, the long-term trend of RI events in JMA operational Dvorak data, which have been used as the first guess for best track analysis, was investigated. Because the JMA Dvorak analysis procedure has remained almost unchanged since 1987, the temporal homogeneity of the Dvorak data is expected to be much better than that of the best track data. The results showed no discernable trend in Dvorak-based RI events over the 32 years. Although the frequency distribution of 24-h intensity changes changed slightly in the Dvorak analysis, that of the best track data changed significantly; as a result, the frequency of best track-based RI events increased after 2006. JMA started using microwave satellite imagery for best track analysis in 2006. This change likely affected the temporal homogeneity of the best track data. These results suggest that the increase in best track-based RI events was due mainly to qualitative changes related to advances in observational techniques.