Asia-Pacific Journal of Atmospheric Sciences最新文献

The stable isotopes of hydrogen and oxygen in precipitation provide a useful reference for the study of hydrological processes. However, the interpretation of stable isotopes in a monsoon climate zone remains uncertain. To investigate isotopic variations and the controlling factors in the midlatitude monsoon region, continuous observations of precipitation isotopes in Dongying were made. We investigate the controlling factors of precipitation δ¹⁸O by analyzing their relationship with temperature, precipitation amount, relative humidity, surface atmospheric pressure, and outgoing longwave radiation (OLR) data. Back trajectory analysis of the HYSPLIT model based on precipitation events was also used to trace moisture sources. The results show that there is a significant spatial correlation between stable isotopes of precipitation and precipitation amount in both monsoon and non-monsoon periods. The integration of large-scale convection over several days (0–10 days) preceding each event was determined as the main driver of precipitation isotopes in Dongying. The difference is that in the monsoon period, the isotope of precipitation records the convective activity of upstream water vapor in the past 10 days, while in the non-monsoon period, the precipitation isotope reflects the convective activity of upstream water vapor in the past 3 days. These findings improve regional-scale understanding of hydrological cycles in the East Asian mid-latitude monsoon region and have the potential to improve our understanding of isotopic variations in the proxy archives of the East Asian monsoon region.

This paper presents the results of the recent development of the all-sky radiance assimilation system in the Korean Integrated Model (KIM). In the cycled analysis and forecast experiments, the increased coverage of radiance data in cloudy regions improved the quality of initial fields for mass variables, temperature and humidity. The experimental period covered the record-breaking heavy rainfall event on August 9, 2022. We examined the simulation accuracy of the western North Pacific subtropical high (WNPSH) in both clear- and all-sky experiments. In the clear-sky experiment, northward propagation of the WNPSH was restricted. A humid bias exists with clear-sky radiance assimilation over the WNPSH region. Since humid air is lighter than dry air, in this situation, the geopotential height (GPH) should be lower to achieve the same pressure, and a low-pressure bias occurs. All-sky radiance assimilation dries the moisture field, which helps elevate the GPH over the WNPSH region. The expansion of the WNPSH yielded a steeper confrontation in the air between the land and ocean around the southeastern sea of the Korean Peninsula to predict the strength of rainfall events more accurately. A more accurate simulation of the jet stream outlet was also demonstrated in an all-sky experiment. This study shows that the all-sky radiance assimilation can help to more accurately predict extreme rainfall events via proper simulations of large-scale fields.

This study examines the characteristics of the urban heat island (UHI) in Dhaka, the densely populated capital city of Bangladesh under the influence of the South Asian monsoon, and its interaction with heat waves. For this, meteorological data at Dhaka (urban) and Madaripur (rural) stations and reanalysis data for the period of 1995–2019 are used for analysis. Here, the UHI intensity is defined as the urban-rural difference in 2-m temperature, and a heat wave is defined as the phenomenon which persists for two or more consecutive days with the daily maximum 2-m temperature exceeding its 90th percentile. The UHI intensity in Dhaka is in an increasing trend over the past 25 years (0.21 °C per decade). The average UHI intensity in Dhaka is 0.48 °C. The UHI is strongest in winter (0.95 °C) and weakest in the monsoon season (0.23 °C). In all seasons, the UHI is strongest at 2100 LST. The average daily maximum UHI intensity in Dhaka is 2.15 °C. Through the multiple linear regression analysis, the relative importance of previous-day daily maximum UHI intensity (PER), wind speed, relative humidity (RH), and cloud fraction which affect the daily maximum UHI intensity is examined. In the pre-monsoon season, RH is the most important variable followed by PER. In the monsoon season, RH is the predominantly important variable. In the post-monsoon season and winter, PER is the most important variable followed by RH. The occurrence frequency of heat waves in Dhaka shows a statistically significant increasing trend in the monsoon season (5.8 days per decade). It is found that heat waves in Bangladesh are associated with mid-to-upper tropospheric anticyclonic-flow and high-pressure anomalies in the pre-monsoon season and low-to-mid tropospheric anticyclonic-flow and high-pressure anomalies in the monsoon season. Under heat waves, the UHI intensity is synergistically intensified in both daytime and nighttime (nighttime only) in the pre-monsoon (monsoon) season. The decreases in relative humidity and cloud fraction are favorable for the synergistic UHI-heat wave interaction.

The moisture sources, transport paths and the quantitative moisture contribution of each source region and path of the South-West, West, North-East and South-East heavy rainfall types in the Three-River-Headwater region (TRHR) of Tibetan Plateau (TP) in summer are tracked, calculated and compared using the FLEXPART model. The results show that: the southern TP and the local target region of TRHR contribute the most moisture to the four types of precipitation. In addition, the northern TP is the third predominant moisture source region to the South-West and West rainfall types, which are distributed in the west of TRHR. Nevertheless, the third critical source region of the North-East and South-East rainfall types, which occur in the east of TRHR, is the eastern areas outside the TP. Four kinds of rainfall events have four identical moisture transport paths: Southern short-distance path, Southern long-distance path, Southwest path and Northwest path. The Southern short-distance path contributes the most moisture to the South-West (24.2%), West (19.8%) and South-East (15.9%) rainfall types, the second most moisture of which respectively comes from the Northwest path, Southwest path and Southeast path. In addition, the Southern short-distance path and Southwest path are the most active moisture transport channels of the three types of precipitation (more moisture trajectories are transported through these two paths). The moisture of North-East rainfall type is primarily contributed by the East path (26.0%) and the Northwest path (18.2%), and the most active moisture transport channels are the East path (21.9%) and the Southern long-distance path (19.9%).

Typhoons Morakot (2009) and Mindulle (2004) were two of the rainiest and most damaging typhoons to hit Taiwan on record, where both cases are associated with a strong low-level southwesterly monsoon flow. The moisture-rich southwesterly monsoon flow and the typhoon-induced northwesterly current usually converge on Taiwan’s Central Mountain Range to produce catastrophic rainfall. The two storms are simulated with a cloud-resolving model (CRM) using the pseudo-global-warming (PGW) methodology to assess the fraction of precipitation attributable to long-term climate change. For each storm, two scenarios are simulated and compared—the control run in present-day climate and the sensitivity test in a past environment four decades ago, where the climate-change signal (“deltas”) is computed using global reanalysis data as the difference between 1990–2009 and 1950–1969. Being realistically reproduced by the CRM at a 3-km grid size in the control run, both typhoons progress in the sensitivity test with highly similar evolution to their present-day counterpart, even though the background in the sensitivity run is slightly cooler and drier than the present. Under the current climate, Morakot and Mindulle produce more rainfall by about 5 mm per day within 300–400 km from the center during their lifespan (equal to an increase of ~4–8%) compared to their counterparts in past climates. Such results are in close agreement with previous studies, and the shift in mean daily rainfall is tested as statistically significant at a confidence level of 99.5%. The water budget analysis shows that the increased rainfall from past to present climate is accounted for mainly by the low-level convergence of moisture associated with a more vigorous secondary circulation and a higher precipitable water amount.

The precipitation prediction of the Korean Integrated Model (KIM) is evaluated over South Korea for the summer season of July–August 2022, and key factors for accurate predictions are examined using various approaches, including case studies under distinct synoptic patterns and physics sensitivity experiments. In this study, a five-day prediction experiment was conducted using the latest version of KIM in a near real-time full cycle configuration with 8-km grid spacing, while additional case simulations and prediction tests were conducted on low-resolution or cold-run testbeds. For verification, a newly designed synoptic pattern verification was introduced to assist to the conventional dichotomous verification for daily precipitation. It was found that heavy rainfall events over South Korea are determined by two dominant patterns: frontal and cyclonic. KIM can successfully discriminate between synoptic patterns with a detection rate of approximately 85% for these two types within a short-range prediction. However, it is evident that the precise prediction of precipitation requires an accurate location of the precipitation system within a specified timeframe, wherein KIM shows weakness in delaying the movement of extratropical cyclones with forecast lead times. The significance of moist physics is also highlighted by sensitivity experiments that control convective trigger conditions. This demonstrates that large-scale precipitation from a microphysics scheme must be enhanced to properly represent the strong development of inland rain systems over South Korea, which are highly sensitive to convective precipitation activity in the numerical model, especially in upwind ocean regions.

The microphysical properties of ice pellets (IP) are analyzed, and associated relevant thermodynamic conditions are investigated using rawinsonde soundings and model reanalysis data in the Yeongdong region of Korea. During the intensive observation campaign of snowfall, two distinctive IP events of 1 March 2021 (IP1) and 15 March 2018 (IP2) were observed when strong cold advection was prevalent below about 2 km as accompanied by distinctive inversion strength (4.7 ~ 9.3 ℃) above the cold layers. Cold air intrusion along the eastern side of Taebaek mountains appeared to abruptly decrease low level (850 hPa) temperature up to -4.7 ~ -3.4 ℃, but warmer than 8-year average (-9.5 ℃), respectively. Both episodes had smaller maximum size (1.8 mm in average) of ice pellets with greater fallspeed (4.2 m s⁻¹) in comparison to general snow crystals. Ice pellets occurred in the synoptic condition of the High in the north and the Low passing by the south, which resulted in cold northeasterly over the Yeongdong region. Rawinsonde soundings show a melting layer between 800 and 700 hPa just above the freezing layer of 900 ~ 800 hPa existed, such as a reversed S temperature profile, which is also consistent with the model reanalysis. The IPs’ life time was short within a couple of hours since it occurred along with low-level strong cold advection (IP1) or rapidly-moving squall line (IP2).

We note that the atmosphere has distinct tropical and extratropical regimes. The tropical regime is significantly dependent on the greenhouse effect and is characterized by temperatures that are largely horizontally homogenized. The extratropical regime is dominated by large scale unstable convective eddies that transport heat between the tropics and the poles (leaving the poles warmer than they otherwise would be) and serve to determine the temperature difference between the tropics and the poles. Changes in tropical temperature and in the tropics-to-pole temperature difference both contribute to changes in global mean temperature. It turns out that changes in global mean temperature associated with major climate change (i.e., the last glacial maximum and the warm period of the Eocene about 50 million years ago) were associated primarily with changes in the tropics-to-pole temperature differences. By contrast, changes in global mean temperature over the past 150 years or so are almost entirely associated with changes in tropical temperature. Thus, there is no intrinsic amplification associated with a change in the tropics-to-pole temperature difference. However, model simulations of climate behave differently from both observations and from each other. In particular, they all show more significant contributions for the tropics-to-pole temperature difference – sometimes much more significant. They also show excessive tropical warming.

The most frequent and concentrated rainfall in the pre-flood season in South China usually occurs around the Dragon Boat Festival every year, locally known as ‘Dragon-boat Rainfall (DBR)’. In 2022, a record-breaking DBR attacked South China, causing disastrous flooding. We suggest that this extreme DBR was jointly regulated by the tropical convective forcing and Tibetan Plateau (TP) heating. Distinctly strong low-level southwesterlies and ascending motions over South China were the key atmospheric conditions. And the abnormal low-level southwesterlies were contributed by both the anomalous anticyclone over the western North Pacific and the anomalous westerlies at the southern side of the TP. On the one hand, during the period of 2022 DBR, stronger-than-normal convective forcing over the Maritime Continent induced the low-level anomalous anticyclone over the western North Pacific through triggering the meridional vertical circulation and further promoted the upward motions over South China. On the other hand, positive diabatic heating over the TP forced abnormal warm anticyclone in the mid-upper troposphere, more warm air advected downstream by the background westerlies, intensifying the upward motions over South China. Meanwhile, the TP heating could induce the anomalous low-level westerlies at the southern side of the TP, which further merged into and intensified the southwesterlies over South China and greatly enhanced the moisture transport and convergence there. Therefore, we highlight the strong thermal forcing over the TP, exerting a combined and amplified effect with the convective forcing over the Maritime Continent, dominated the record-breaking DBR in 2022.