Journal of Geophysical Research: Atmospheres最新文献

Extreme El Niño events (e.g., 1982–1983, 1997–1998) are characterized by strong, eastward-shifted warm Sea Surface Temperature anomalies, and a southward migration of the eastern Pacific Inter-Tropical Convergence Zone (ITCZ) to the equator. Using an ensemble simulation with a single AMIP6 model, Beniche et al. (2024, https://doi.org/10.1038/s41598-024-52580-9) suggested that such events uniquely yield an eastward shift of the Pacific-North American (PNA) teleconnection pattern, with specific impacts over North America. Here, we first examine the robustness of these results in 135 ensemble members from 23 different AMIP6 models. The specific, eastward-shifted extreme El Niño teleconnection pattern is robust in all models but one. It is also highly reproducible across years and ensemble members, due to stronger teleconnection amplitude than that of internal atmospheric noise. This yields specific, predictable impacts (defined as $��>�$ 0.5 STD) such as warm conditions over Northeast America (69% chances), and wet anomalies over California (77%) and Florida (97%). We then show that 26 out of the 42 CMIP6 models we examined reproduce extreme El Niño events, defined as El Niño events associated with large eastern Pacific rainfall anomalies. These models tend to have a weaker cold tongue bias than the rest of CMIP6. Despite a degradation in performance from AMIP6 to CMIP6, 18 out of the 26 selected models capture the specific extreme El Niño teleconnections, albeit with some underestimation of wet anomalies over California and Florida. We end by discussing implications for future North American climate projections based on CMIP6.

The Victoria Mode (VM) plays a vital role in shaping Pacific Sea Surface Temperature (SST) variability. However, its impact on late-season tropical cyclone genesis (TCG) over the western North Pacific (WNP) remains underexplored. This study investigated the influence of VM on WNP TCG from 1965 to 2020. Significant relationships emerge in interannual (6 year) and decadal (11 year) timescales. On the interannual timescale, VM correlates positively with TCG west of 145°E and negatively to the east. On the decadal timescale, the negative relationship spans the entire WNP. Victoria Mode directly modulates vertical motion and induces low-latitude anticyclonic circulation across the low-latitude North Pacific by altering surface wind convergence and diabatic heating. This, in turn, shapes environmental conditions, affecting TCG. Dynamic factors are more important in WNP TCG changes than thermodynamic factors. Critical environmental conditions in VM's impact on TCG include four dynamic factors: relative vorticity, vertical velocity, meridional gradient of zonal wind, and vertical shear of zonal winds. Vertical shear of zonal winds is particularly influential. Distinct correlation patterns and mechanisms are diagnosed between different timescales, attributed to anomalous SST observed over the WNP on the interannual timescale but not on the decadal timescale. Furthermore, this study illustrates the complexity of region- and time-dependent relationships between environmental conditions and TCG. Such dependencies pose challenges for conventional genesis potential index models in accurately capturing the spatial characteristics of the VM-TCG relationship. Overall, our findings uncover VM's substantial influence on WNP TCG, enhancing understanding of key environmental conditions and mechanisms driving interannual and decadal TCG variations.

Based on the hourly rainfall gauge data and ERA5 reanalysis data of 1980–2022, this study reveals the characteristics of regional precipitation events (RPEs) over the middle and lower Yangtze River basin (MLYRB) in summer and associated mechanisms. Results show that the RPEs contribute more than 60% of the summer total rainfall over most of MLYRB and dominate the morning diurnal peak, interannual variation, and phased increasing trend for the summer total precipitation over MLYRB. The RPEs mainly occurring under two monsoonal types with shear line (Type1 and Type2; 91.4%) and landfalling tropical cyclone type (Type3; 8.6%) show higher precipitation occurrence probability in southern/northern and eastern MLYRB. The synoptic types of summer RPEs over MLYRB show a clear sub-seasonal variation associated with the development of East Asian summer monsoon, with Type1, Type2, and Type3 concentratively occurring in June to early July (78%), late June to July (62%) and late July to August (81%), respectively. Meanwhile, the diurnal variation of RPE precipitation amount under the synoptic Type1 and Type2 (Type3) exhibits a predominant morning (afternoon) peak. Meanwhile, the increasing trend of total RPE rainfall during 1980–1999 (2000–2022) is mainly contributed by the RPEs under Type1 (Type1 and Type2 together). The reduced coverage rate and the occurrence hours of RPEs under the monsoonal synoptic types dominate the weakened diurnal variation of the RPE precipitation amount during 2000–2022 compared with 1980–1999. This study provides a new scientific reference for the differences in the characteristics of summer RPEs in the MLYRB under different synoptic types.

The satellite microwave emissivity difference vegetation index (EDVI) has been used in previous studies to estimate FCs and FRP using traditional multivariate linear regression models. However, the nonlinear effects and contributions of numerous factors that affect forest fires cannot be disentangled by this model. Using the random forest (RF) model, this study utilized multiple EDVIs and the optical normalized difference vegetation index (NDVI) as key fuel properties to resolve the physical driving mechanisms of forest fires and to estimate the daily FCs and FRP over East Asia. The results showed that the estimated FCs and FRP were in good agreement with satellite observations, with a spatial R of 0.59 for FCs and 0.63 for FRP and a temporal R of 0.80 for FCs and 0.81 for FRP. The integration of EDVIs and NDVI into the RF model was found to improve model performance and generate overall lower systematic errors than the model without vegetation variables. Model performance was better than that in previous studies using multivariate linear regression models. In addition, EDVIs showed greater importance than NDVI. This was largely due to their daily temporal resolution that allowed EDVIs to capture forest fire dynamics in time. The combination of the RF model with satellite microwave and optical observations shows good performance and has great potential for FC and FRP estimations in global fire danger assessment.

We quantify CO₂ emissions from smaller anthropogenic point sources compared with earlier satellite studies, which have mostly focused on mid-sized (∼10 MtCO₂/year) and larger fossil fuel burning power plants. Two types of Orbiting Carbon Observatory (OCO) observation modes are used: OCO-2 Target mode and OCO-3 Snapshot Area Mapping (SAM) mode. Methods previously used with OCO-3 SAMs are adapted to quantify CO₂ emissions with OCO-2 Targets for the first time, demonstrating a similar capability to track emission changes at the Bełchatów Power Station. SAMs and Targets are then applied to quantify emissions from smaller sources in Canada: the Boundary Dam and Poplar River Power Stations in Saskatchewan, and the Suncor and Syncrude Mildred Lake mined oil sands processing facilities in northern Alberta. We verify our method on the nearby Colstrip Power Station in Montana by comparison with hourly reported values. For Canadian sources, only annual emissions are reported, to which our emission estimates cannot be directly compared. Emissions derived from a single satellite overpass correspond to daily or finer temporal scales and thus do not account for source intermittency or variability, which requires multiple revisits to reliably estimate annual emissions. Finally, we average OCO-3 SAMs on repeated revisits to improve weak enhancement signals above background noise. Averaging SAMs yields mixed results, with improvements achieved only under certain conditions. These studies help to clarify the capabilities and limitations of CO₂ point source emission quantification with current satellites in advance of plans for operational monitoring with future CO₂ satellite missions.

A heavy rainfall (HR) event caused by a bow echo struck South China on 11 April 2019. Two extremely HR periods were identified within this event, and the second rainfall period led to severe flooding in Shenzhen city, resulting in 11 fatalities. The first rainfall period was dominated by warm-rain processes, while the development of the second period was closely related to the intensification of ice-phase processes. The contribution of raindrops from the melting process played a crucial role in the formation of extreme rainfall, which achieved a high rain rate (RR) exceeding 120 mm hr⁻¹. The enhancement of the ice-phase processes during the second rainfall period was found to be closely associated with the development of a low-level mesoscale vortex (MV). Due to the complementary non-linear dynamical accelerations induced by the MV, the vertical velocity within the convective system rapidly intensified, leading to a more upright and deeper convective organization. As a result, more water vapor and supercooled water were lifted above the freezing level, which increased the presence of ice-phase particles with the potential to melt, subsequently contributing to the extreme high RR. This study investigates the microphysical characteristics of two periods of HR that occurred during and after the development of a MV within a bow echo event, and examines the key microphysical processes affected by the MV, which partially contributed to the second HR period.

The separate effects of the sensible heat flux (SHF) and latent heat flux (LHF) on the outer spiral rainbands of tropical cyclones (TCs) have not received sufficient attention. This study examines the separate contributions of the SHF and LHF to the outer spiral rainbands of TCs via a series of sensitivity experiments using the three-dimensional cloud-resolving Weather Research and Forecasting model. The results indicate that removing the outer SHF suppresses the activity of the outer spiral rainbands and results in a stronger and smaller-scale TC, whereas decreasing the outer LHF by the same amount has only a slight impact on the outer spiral rainbands. Further investigations indicate that the positive radial gradient of the potential temperature at the cold-pool outer edge is crucial for the strength of outer spiral rainbands. The strong positive radial gradient of the potential temperature gives rise to a negative radial gradient of the horizontal pressure at the cold-pool outer edge, and thus an outward radial pressure gradient force. As a result, strong outflow exists near the cold-pool outer edge, producing horizontal convergence and eventually lifting air to generate new convective cells. Distinct from the LHF, the SHF directly modulates the potential temperature immediately outside the cold pool and thus the radial gradient of the potential temperature at the cold-pool outer edge. Removing the outer SHF leads to a significantly lower potential temperature outside the cold pool, which is averse to the formation of a larger radial gradient of the potential temperature at the cold-pool outer edge.

Oceanic emission of gaseous elemental mercury (Hg⁰ or GEM) is an important source for atmospheric mercury (Hg), but existing estimates of global gross oceanic Hg⁰ emissions are highly variable (800–7,220 Mg yr⁻¹). This study measured atmospheric GEM concentrations and isotopic compositions at two coastal sites in Terengganu, Malaysia, a region that receives air masses from both hemispheres, during 2019–2021 to diagnose the amount of oceanic Hg⁰ emissions. Significantly lower mean (±1sd) concentration (1.28 ± 0.20 ng m⁻³), Δ¹⁹⁹Hg (−0.23 ± 0.03‰), and Δ²⁰⁰Hg (−0.066 ± 0.018‰) and significantly higher δ²⁰²Hg (0.43 ± 0.12‰) were observed during the wet season when air masses were predominantly from the Southern Hemisphere, compared with those (mean concentration, Δ¹⁹⁹Hg, Δ²⁰⁰Hg, and δ²⁰²Hg of 1.77 ± 0.09 ng m⁻³, −0.17 ± 0.03‰, −0.045 ± 0.023‰, and 0.25 ± 0.11‰, respectively) during the dry season when air masses were predominantly from the Northern Hemisphere, suggesting interhemispheric difference in GEM concentrations and its isotopic compositions. Using a Δ²⁰⁰Hg mass balance model, we estimated that the oceanic Hg⁰ emissions from Hg^II reduction should be below 2,250 ± 891 Mg yr⁻¹ (±1sd), which is at the low-end range of the literature reported values. We then used the constrained value as emission input to a three-dimensional atmospheric Hg isotope model and reproduced well the global distributions and interhemispheric gradient of atmospheric GEM Δ²⁰⁰Hg. The findings from the present study will help to better understand Hg⁰ emissions from global oceans and their roles in global atmospheric Hg cycling.

Circulation budgets can identify physical processes underpinning tropical cyclones, mesoscale convective vortices, and other weather systems where there are interactions across scales. It is unclear, however, how well these budgets close in practice. The present study uses the rapid intensification of Tropical Cyclone Nepartak (2016) as a case study to quantify the practical limitations of calculating circulation budgets using standard reanalyzes and numerical weather model data. First, we evaluate the circulation budget with ERA5. The budget residual can be reduced considerably by including contributions to circulation changes from subgrid-scale momentum transports, and reduced further with 24-hr smoothing, which dampens the discontinuous effects of data assimilation. Second, using a high-resolution Met Office Unified Model simulation, we examine how the choice of the path used (the domain boundary) affects the budget closure. Third, the truncation errors associated with numerical differentiation in time and space are investigated. The circulation budget improves as the model data are analyzed with more frequent time output intervals, and as the output grid spacing decreases. For the tropical convective examples evaluated here, the column mean budget residuals increase by up to 50% as the output intervals increase from 5 min to 3 hr. Errors also increase if the data are regridded to a coarser horizontal grid spacing and when convection straddles the domain boundary. A key result is that the circulation budget need not close for physical inferences made about the circulation and its evolution to be meaningful, thus validating the use of the technique in prior studies.

Acetone photolysis is a potentially important source of hydroperoxyl and hydroxyl radicals (HO_x) to the upper troposphere. The extent to which acetone photolysis is a significant source of HO_x in the upper troposphere is unclear in part because of scarce measurements of the acetone photolysis quantum yield (Φ_acetone) in the actinic region (i.e., λ > 300 nm). Past measurements of the Φ_acetone have derived temperature- and pressure-dependent parameterizations that lead to significantly different conclusions about the importance of acetone to HO_x formation in the upper troposphere. Here, we focus on previously published data to derive the recommended Φ_acetone for atmospheric chemical modeling. Using Stern-Volmer analyses, we determine temperature- and pressure-dependent Φ_acetone using updated measurements of fundamental acetone photolysis parameters. We also use simulations of the Φ_acetone produced from recent photophysical modeling to derive temperature- and pressure-dependent parameterizations of the Φ_acetone. In contrast to the current Φ_acetone parameterization used in atmospheric chemical modeling, our parameterization reflects a predicted nonzero Φ_acetone in the wavelength region above 320 nm. Despite the increased Φ_acetone values in the higher wavelength region of the acetone photolysis action spectrum, the modeled effect on HO_x production is not significantly different from HO_x produced using the current recommended Φ_acetone parameterization. Uncertainties in the acetone photolysis mechanism remain; thus, more direct temperature- and pressure-dependent measurements of Φ_acetone are warranted.