Journal of Geophysical Research: Atmospheres最新文献

Due to limited water vapor measurements, vapor isotopes have been traditionally estimated under the assumption of isotopic equilibrium between rain and vapor below cloud base. However, recent advancements in analytical instruments allow more vapor isotopic measurements that have challenged this assumption. To enhance our understanding of rain-vapor interactions below cloud base in tropical regions, we established an automated system to measure rain and vapor isotopes simultaneously and continuously in real time at minute intervals in Singapore. Among 324 rain events monitored from 2016 to 2019, 81% exhibited a substantial departure of rain and vapor isotopes from the expected equilibrium. This departure suggests that raindrop evaporation plays a larger role in determining their isotopes. The conclusion is supported by the generally lower slopes of the local meteoric water line. Seasonal variations in rain event characteristics indicate changing influences of rain-vapor interactions: during monsoons, more frequent heavy rainfall maintains relatively high humidity below cloud base, favoring rain-vapor isotopic equilibrium, whereas during inter-monsoons, more light rain events lead to pronounced rain evaporation and larger isotopic differences. Furthermore, rain-vapor interactions below cloud base significantly modulated their isotope evolution during individual events. As events progressed, reduced humidity favored evaporation, increasing rain isotope values and decreasing its d-excess, whereas vapor isotope values decreased and its d-excess increased. Our study introduces a new approach to capturing real-time high-resolution rain and vapor isotopes at minute intervals to understand the dynamics of rain-vapor interactions below cloud base. Findings underscore the crucial role of these interactions in influencing rain and vapor isotopes during tropical rain events.

It is important to understand the mechanisms of heavy rainfall events, as such information could improve forecasting of these events and help mitigate their adverse impacts on life and property. In this study, we analyzed hourly stable isotopic compositions in water vapor (δ¹⁸O_v and d-excess_v) during heavy rainfall events in the summer monsoon season (June to September) from 2013 to 2023 in Nanjing, eastern China. These data were extracted from the longest data set of high-resolution and continuous in situ observations of water vapor isotopes globally. Based on these data, we identified four evolution patterns of δ¹⁸O_v during heavy rainfall events, corresponding to different weather systems: slow-declining (tropical cyclone interacting with mid- and high-latitude system), W-shaped (tropical cyclone), U-shaped (cold vortex system), and inclined L-shaped (upper-level trough system). The isotopic variations suggest that heavy rainfall events in eastern China were mainly sustained by moisture from adjacent oceans (including the South China Sea and the East China Sea) and terrestrial environment rather than from the distant Indian Ocean as previously suggested. In addition, for some heavy rainfall events with an intermittent period, the nearby oceanic moisture transport alters before and after the intermittent period due to an intensity change or overall transition of low-level weather systems. This study serves as a benchmark for tracing heavy rainfall processes in East Asia using high-resolution water vapor isotopes.

The Tibetan Plateau (TP) has experienced “south drying-north wetting” extreme precipitation changes over the past half century. The effects of water vapor transport at different timescales on TP extreme precipitation changes remain unexplored. Here, we utilize the reanalysis data sets to quantify the contributions of stationary and transient processes of water vapor transport to the long-term changes in the extreme precipitation (R95p) during wet season (Jun-Jul-Aug-Sep) over the TP and surrounding regions. We find that the daily scale transient processes dominate the dipole trend of extreme precipitation with a contribution of 55.1% in the northern and 79.5% in the southern TP, respectively, whereas the contribution of monthly scale stationary processes is of 19.0% and 20.5%. The long-term changes in extreme precipitation are dominated by the transient dynamic component. We identified the synoptic circulation patterns affecting the changes of R95p over the northern and southern TP by using k-means clustering. The patterns featured with a 500 hPa trough, 200 hPa wind divergence and low transient geopotential height are identified. The frequency of the dominant circulation patterns increases in the northern TP and decreases in the southern TP, which leads to the dipolar changes of extreme precipitation over the TP and surrounding regions.

Transport of exogenous anthropogenic mercury (Hg) is an important source of Hg pollution in the Tibetan Plateau (TP) and its downstream water ecosystems, but the origins and contributions of Hg sources remain uncertain. Here, we investigate the concentrations and isotopic compositions of gaseous elemental mercury (GEM) at four rural sites in the TP and three urban sites surrounding the TP to quantify the sources of GEM in the TP. GEM concentrations in the surrounding cities (site-specific means: 2.36–9.12 ng m⁻³) were highly elevated mainly due to strong local anthropogenic emissions as indicated by their negative δ²⁰²Hg and near zero Δ¹⁹⁹Hg and Δ²⁰⁰Hg signatures. GEM isotopes indicate that GEM pollution in the TP, typically observed during the summer monsoon and the pre-monsoon, were mainly caused by trans-boundary transport of anthropogenic Hg from surroundings. Using an Hg isotope mixing model, we estimate that exogenous anthropogenic emissions on average contributed 26 ± 5% (1sd) to the GEM in the TP. Further analysis of the transport of anthropogenic Hg emissions based on the backward trajectory and gridded anthropogenic Hg emissions suggests that 16 ± 9% and 6 ± 13% of the GEM in the TP were derived from anthropogenic sources in South Asia and China, respectively. Our study suggests that anthropogenic Hg emissions in South Asia could be effectively transported to the TP across the Himalayan range. Future studies are needed to better assess the role of rapidly increasing anthropogenic Hg emissions in South Asia on the regional to global scale atmospheric Hg cycling.

Regarded as the Asian Water Tower, the Tibetan Plateau (TP) collects atmospheric precipitation from a vast area of land and feeds into major rivers that sustain the livelihood of billions of people in East, South and Central Asia. It is critical to reasonably simulate the hydrological cycle over the TP in order to assess future climate risks to agriculture, water resources and ecosystem services. To address the chronic wet biases over the TP in state-of-the-art climate models, we have compared 12 high-resolution (HR) climate models (25–50 km) to their corresponding low-resolution versions (100–200 km) with respect to the 1979–2014 climatology. It is found that the HR models consistently reduce about half of the wet biases over the TP, mainly from better resolved orography. The wet biases are reduced by 41% over the northern and western TP, mainly contributed by decreased frequency of light precipitation (0.1–10 mm day⁻¹), which is attributed to reduced evaporation because of weakened surface wind by raised orography. The most significant reduction of biases (53%) rising from decreased frequency of mid-heavy precipitation (10–50 mm day⁻¹), appears over the southern and eastern TP, on the leeside of elevated orography where steeper orography enhances rain shadow effect by stronger downward motion along the sharper slope, while partly compensated by air column convergence due to vertical stretching of the downward flow for potential vorticity conservation. This study highlights the importance of surface processes and resolving complex orography in simulating precipitation and large-scale hydrology around the TP which potentially benefits the future hydrological projection.

Arctic amplification (AA), characterized by a more rapid surface air temperature (SAT) warming in the Arctic than the global average, is a major feature of global climate warming. Various metrics have been used to quantify AA based on SAT anomalies, trends, or variability, and they can yield quite different conclusions regarding the magnitude and temporal patterns of AA. This study examines and compares various AA metrics for their temporal consistency in the region north of 70°N from the early twentieth to the early 21st century using observational data and reanalysis products. We also quantify contributions of different radiative feedback mechanisms to AA based on short-term climate variability in reanalysis and model data using the Kernel-Gregory approach. Albedo and lapse rate feedbacks are positive and comparable, with albedo feedback being the leading contributor for all AA metrics. The net cloud feedback, which has large uncertainties, depends strongly on the data sets and AA metrics used. By quantifying the influence of internal variability on AA and related feedbacks based on global climate model ensemble simulations, we find that water vapor and cloud feedbacks are most heavily affected by internal variability.

High resolution extended-range cloud condensation nuclei (CCN) spectral comparisons with cloud microphysics and drizzle of the Physics of Stratocumulus Tops (POST) field experiment confirmed results in the Marine Stratus/Stratocumulus Experiment (MASE). Both of these stratus cloud projects demonstrated that bimodal CCN spectra typically caused by cloud processing were associated with clouds that exhibited higher concentrations of smaller droplets with narrower distributions and less drizzle than clouds associated with unimodal CCN spectra. Resulting brighter clouds and increased cloudiness could enhance both indirect aerosol effects (IAE). These stratus findings are opposite of analogous measurements in two cumulus cloud projects, which showed bimodal CCN associated with fewer larger droplets more broadly distributed and with more drizzle than clouds associated with unimodal CCN. Resulting reduced cumulus brightness and cloudiness could reduce both IAE. Physics of Stratocumulus Tops (POST) flights in air masses with higher CCN concentrations, N_CCN, showed more extremes of the stratus characteristics. However, POST flights with lower N_CCN showed opposite droplet characteristics similar to the cumulus clouds, yet still showed less drizzle in clouds associated with bimodal CCN, but not as much less as the flights with higher N_CCN. Since all MASE clouds were in polluted air masses, while the two cumulus projects were in clean air masses we deduce from these four projects that both the dynamic stratus/cumulus differences (vertical wind) and N_CCN are responsible for the microphysics and drizzle differences among these projects. This is because the clean POST characteristics are a hybrid between MASE/POST high N_CCN and the two cumulus projects.

The rapid development of the wind industry is accompanied by increasing environmental impacts. Currently, there is a lack of research on the impacts of offshore wind farm (OWF) on tropical cyclone (TC) intensity, including the mechanisms involved. This research is carried out by using a coupled and an uncoupled numerical model to investigate the impact of OWF on an autumn TC in the northeastern South China Sea. The results show that the wind speed deficit caused by OWF leads to an increase in surface pressure on the inflow side. This causes the surface pressure in the TC periphery to increase by advection, even if the TC is some distance away from the OWF. The increase in pressure gradient from the periphery to the TC center enhances the TC secondary circulation, thereby intensifying the TC. When the TC enters the OWF, the above mechanisms weaken and the ocean dominates the TC intensification. This is because the reduction in wind speed caused by the OWF results in a weaker sea surface current velocity, which weakens the flow of upstream cold water into the OWF, warming the sea surface temperature (SST) within the OWF. This implies that the horizontal gradient of the local SST is an important factor to be considered in the development of OWF. Sensitivity experiments indicate that OWF can also intensify other types of TC, and that higher cut-out wind speeds lead to stronger intensification effects. These results also provide a new perspective on TC intensity forecasts.

The Hadley circulation (HC) has been expanding poleward in recent decades. The Coupled Model Intercomparison Project Phase 6 (CMIP6) models predict that the expansion will accelerate in the future, more so in the Southern Hemisphere (SH). However, the extent of the expansion varies widely among the models. We investigate the mechanisms driving the intermodel spread in SH HC expansion predictions. The intermodel spread is obtained by an empirical orthogonal function analysis on the SH HC trend patterns of 16 CMIP6 model simulations using the historical and shared socioeconomic pathway 5–8.5 scenarios. The leading mode, showing a mean meridional stream function anomaly at the poleward SH HC extent, explains 49.73% of the variance and significantly correlates (r = 0.94) with the SH HC expansion. By analyzing the extended Kuo-Eliassen equation, we find that the intermodel difference in the representation of diabatic heating is responsible for about 14% of the intermodel spread. The meridional eddy momentum and heat fluxes contribute to about 21% and 18% of the intermodel spread, respectively. The models simulating a relatively large SH HC expansion tend to show increased precipitation in the Southern Pacific Convergence Zone, reduced baroclinic instability in the subtropics, and an enhanced poleward shift of jet stream in the midlatitudes. This suggests that the uncertainty in the HC projection may be constrained by reducing the bias in the trend of the mean fields.

The 12-year continuous observation of gravity wave momentum fluxes (GWMFs) estimated by the Mohe meteor radar (53.5°N, 122.3°E) revealed prominent intraseasonal variability around the extratropical mesopause (82–94 km) during boreal winters. Composite analysis of the December‒January‒February (DJF) season according to the Madden‒Julian Oscillation (MJO) phases revealed that the zonal GWMFs notably increased in MJO Phase 4 (P4) by ∼2–4 m²/s², and a Monte Carlo test was designed to examine the statistical significance. The response in zonal winds lags behind the GWMF response by two MJO phases (i.e., 1/2π), indicating a “force‒response” interaction between them. Additionally, time-lagged composites revealed that strengthened westward GWMFs occurred ∼25–35 days after MJO P4, coincident with the MJO impact on the zonal winds in the stratosphere. The analysis results also suggested that the mechanism of MJO by which the MJO influences the stratospheric circulation might involve poleward propagating effects of stationary planetary waves with zonal wavenumber one. This work emphasizes the importance of GW intraseasonal variability, which impacts tropical sources from the troposphere to the extratropical mesopause.