Atmospheric Science Letters最新文献

Climate models agree that a reduction of day-to-day temperature variability at mid-to-high latitudes during the cold season is a robust forced response to anthropogenic global warming. Although recent observations show a similar reduction, how much the observed change is forced and how much is internal variability is uncertain. Here, using large-ensemble simulations and a Ridge Regression detection tool, we decompose the observed day-to-day temperature variability changes since 1950 into contributions of forced and internal components. Our findings show that the observed reduction since the mid-1970s is dominated by a forced response (about 90%). Observations and models show consistent mechanisms responsible for this reduction in a warming world: the reduction is manifested as cold days warming faster than hot days, driven by Arctic sea-ice loss and associated reduction in the latitudinal temperature gradient, but not by large-scale atmospheric circulation changes. Overall, our study detects a robust influence of the Arctic changes on lower latitude day-to-day temperature variability, and suggests that this impact will continue in the coming decades.

Accurate precipitation observations are crucial for hydrological and climate monitoring, forecasting and research. However, sparse networks in regions with complex topography, like Ecuador, limit data availability. To address this gap, products such as ERA5 reanalysis, produced by ECMWF within the Copernicus Climate Change Service, and satellite-based datasets, including IMERG (Integrated Multi-satellite Retrievals for GPM) and MSWEP (Multi-Source Weighted-Ensemble Precipitation), offer near-real-time monitoring and spatial coverage in data-scarce areas. This study evaluates these three precipitation products over Ecuador using quality-controlled station data (1980–2024) to assess biases, extreme event detection, and the impact of topography. The results show that: (1) IMERG is the most skillful overall in estimating precipitation, particularly in lowland areas, though it declines in mountainous regions; (2) ERA5 and MSWEP underestimate precipitation in the Amazon, while ERA5 overestimates in high-altitude regions; (3) during the 1998 El Niño event, all products had challenges in capturing localized heavy precipitation, although ERA5 consistently captured but overestimated coastal heavy precipitation; (4) For 99th percentile precipitation extremes, ERA5 overestimated precipitation by 5.0 mm/day, while IMERG overestimated by 2.5 mm/day. This study highlights the need to analyse high-altitude precipitation estimates carefully and adapt bias-adjustment methods, providing insights for climate monitoring in tropical mountain regions.

Recent advances in data assimilation (DA) have focused on developing more flexible approaches that can better accommodate nonlinearities in models and observations. However, it remains unclear how the performance of these advanced methods depends on the observation network characteristics. In this study, we present initial experiments with the surface quasi-geostrophic model, in which we compare a recently developed ensemble filter using score-based diffusion models with the standard Local Ensemble Transform Kalman Filter (LETKF). Our results show that the analysis solutions respond differently to the number, spatial distribution, and nonlinear fraction of assimilated observations. We also find notable changes in the multiscale characteristics of the analysis errors. Given that standard DA techniques will eventually be replaced by more advanced methods, we hope this study sets the ground for future efforts to reassess the value of Earth observing systems in the context of newly emerging algorithms.

This study examines the formation, evolution, and merger mechanisms of two meso-β-scale vortices (V1 and V2) into a meso-α-scale vortex (V3) that triggered a heavy snowfall event over China's Loess Plateau on December 10–11, 2023. The merger of V1 and V2, occurring within the central region of an inverted trough northeast of the Tibetan Plateau, contributed ~71% of V3's initial cyclonic vorticity and intensified the snowfall event. Vorticity budget indicates distinct genesis pathways, with V1 forming primarily through horizontal vorticity advection and convergence-associated stretching, while V2 genesis was dominated by vertical stretching, with horizontal transport acting suppressively. After formation, V2 remained stretching-driven, whereas V1 shifted from stretching to horizontal-transport dominance. Kinetic energy budgets reveal that the strong southerly and southeasterly winds in the eastern inverted trough—critical for moisture transport—were sustained mainly by the pressure gradient force work, particularly its zonal component. The Fujiwhara-like merger of the two meso-β-scale vortices, guided by their respective northwesterly (V1) and southeasterly (V2) mean flows, exhibited a dominant role of the pressure gradient force work in the kinetic energy budget. Key differences in wind acceleration mechanisms emerged, with the zonal component of the pressure gradient force work primarily enhancing westerlies in V1's southern half, while the easterlies within V2's eastern half shifted from zonal to meridional dominance in the pressure gradient force work during the merger process.

South China experienced substantial rainfall from April 19 to April 22, 2024, with accumulated precipitation ranging from 200 mm to 350 mm. This significant precipitation was primarily driven by the combined synoptic influences of the southern branch trough, low-level jet stream, and the Jiang–Huai cyclone, affecting northern and central-eastern Guangxi as well as most parts of Guangdong. These synoptic conditions facilitated the initiation, development, and propagation of mesoscale convective systems (MCSs). Specifically, an MCS initiated in the evening over the eastern edge of the Yunnan–Guizhou Plateau (YGP), leading to the formation of a mesoscale vortex with closed cyclonic circulation at 850 hPa, which further promoted the merging of convection cells into an organized MCS. The intensified southerly winds appeared over the eastern parts of the mesoscale vortex, preceding the peak meridional component of moisture convergence by approximately 4–5 h. Moisture budget analysis revealed that the meridional component of horizontal moisture convergence was the primary contributor to the moisture increase over Guangxi and Guangdong provinces. Consequently, the enhanced mesoscale systems (vortices and MCSs) significantly increased horizontal convergence at lower levels, contributing substantially to the moisture accumulation over South China. The intensified low-level southerly wind associated with mesoscale vortices can be considered a potential forewarning parameter for short-range precipitation forecasting in such heavy rainfall events.

The Dabie vortex (DBV), a mesoscale system frequently generating severe weather in the Yangtze River Basin, exhibits spatial overlap with tornado-prone regions, yet its tornadic potential remains unexplored. This study identifies 23 tornadoes associated with DBV occurring between 2006 and 2023, primarily concentrated in the provinces of Jiangsu, Anhui, Hubei, and Henan. In northern Anhui, DBVs contribute up to 40% of tornadoes—surpassing Jiangsu (~29%) and rivaling half the frequency of tropical cyclone (TC)-induced events. Approximately 70% of tornadoes occur in the DBV's southeastern quadrant, favored by enhanced CAPE, low-level moisture, vertical wind shear, and storm-relative helicity. Most (87%) form during DBV development/maintenance phases, when dynamical forcing peaks. Compared to TC tornadoes, DBV tornadoes exhibit stronger instability and vertical wind shear. Relative to other-type tornadoes, they develop in environments with weaker instability but greater moisture, stronger shear, and greater storm-relative helicity. These findings underscore DBVs as a previously overlooked but critical driver of tornado activity in eastern China.

In this study, we applied the Community Earth System Model version 1 (CESM1) to investigate the impacts and physical and dynamic mechanisms of Anthropogenic Heat Release (AHR) due to global energy consumption on the Arctic climate in boreal summer from 1992 to 2013. AHR increases the air temperature in eastern Siberia and the Eastern European Plain obviously. AHR increases the air temperature significantly in eastern Siberia (60° N–70° N, 130° E–140° E) by 0.49 K on average, while it decreases the air temperature in the western Siberian plain regions in the Arctic. The results of our study demonstrate that AHR can affect lower-troposphere stability in the Arctic, which further affects the low cloud fraction and the surface energy balance. AHR can affect the atmospheric circulation in the Arctic, bringing more water vapor and amplifying the greenhouse effect, which leads to further warming in the Arctic in the boreal summer. The Relative Humidity (RH) in the Arctic is increased by an average of 0.12% due to AHR in the boreal summer. These thermal and dynamic effects of AHR lead to uneven warming in the Arctic in summer, indicating AHR acts as a non-negligible factor for the climate in the Arctic.

By using hourly rain gauge records from 1961 to 2020, we find that the annual precipitation differences between highland stations and lowland stations in eastern China are reducing. Specifically, precipitation frequency has decreased significantly in the highlands while precipitation intensity has increased considerably in the lowlands, both contributing to a reduced elevation–dependency of precipitation. The decreasing precipitation frequency correlates with reduced convective available potential energy (CAPE), whereas the increasing precipitation intensity is linked to enhanced low-level convergence.

The study of tropical cyclones re-entering the ocean or making ‘seafall’ has been limited. Here, idealised simulations are used to study the re-intensification of seafalling tropical cyclones. They follow a two-stage fast-slow process driven predominately by a change in surface friction initially and then by heating. The previous land decay causes seafalling tropical cyclones to be larger and intensify more slowly with milder inner-core contraction than in ocean-only cases. Nonetheless, they reach the same intensity but with almost twice the integrated kinetic energy, so that the second landfall made by seafalling tropical cyclones can cause more damage due to their larger footprint of destructive wind.

Drop size distributions (DSDs) obtained over ocean in the tropical warm pool using a shipboard optical disdrometer are investigated. Quality control procedures, including the suppression of effects primarily related to the ship-relative wind, are applied to extract reliable data. The procedures suppressed the number of data points to 2% of the total rain duration. The characteristics of the obtained DSD parameters such as averaged size and intercept parameter are reasonably consistent with those of previous studies, with the larger size for a specific rain rate. The categorization of the data according to precipitation characteristics contrasts the DSD parameters, especially for the stratiform precipitation over the coastal ocean prior to the MJO active period. This precipitation is also an outlier when the obtained DSDs are applied to the scattering simulation for the radar-based quantitative precipitation estimation, while the contrast between the open ocean and the coastal ocean is not clearly separated.