Journal of Geophysical Research: Atmospheres最新文献

Extreme integrated vapor transport (IVT) is a crucial driving factor of extreme precipitation events (EPEs). This paper presents a complex network-based characterization of propagations from extreme IVT to EPEs. Specifically, the propagations are tracked from extreme IVT to EPEs by event synchronization; and then the source zones of extreme IVT contributing to EPEs are identified by two-layer complex network. A case study is devised for North America based on the daily NCEP/NCAR Reanalysis 1 from 1948 to 2021. Overall, eight communities of EPEs are identified: the west coast of United States (US) tend to receive substantial EPEs from the Pacific Ocean; the Gulf of Alaska tends to receive oceanic EPEs propagating inland; western Canada typically experiences large amount of out tendencies and the EPEs tend to accumulate in the Baffin Island and Labrador Peninsula; the southeastern US and the northern Great Plains tend to experience northward propagations from Mexico. Along the west coast of North America, the propagations from extreme IVT to EPEs typically originate from the eastern North Pacific between 160°W and 110°W, and make landfalls in 4 days. These propagations are influenced by anomalous cyclonic circulations developing over the Gulf of Alaska forced by eastward Rossby waves. The coincidence rate of these propagations with atmospheric rivers is, respectively, 85.31% in autumn, 91.35% in winter, 73.94% in spring, and 64.52% in summer. Overall, the observed propagations from extreme IVT to EPEs yield insights into the mechanism of atmospheric moisture transport and the predictability of precipitation.

The local formation characteristics of thin supercooled liquid stratiform clouds on mid-level moisture advection layers have been revealed by the ∼2-year observations from routinely operating ground-based water vapor Raman Lidar, polarization Lidar, cloud radar, and conventional radiosondes at a subtropical site. These moisture advection layers were commonly observed during all seasons over our subtropical site, whereas thin supercooled liquid stratiform clouds occurred mainly in winter when the 0°C isotherm level occurred at lower altitudes. It was determined that these clouds mostly formed on the top of moisture advection layers capped by strong temperature inversions. The temperature-moisture structure collocation (the layer's upper edge with modest/low water vapor mixing ratio of ∼2–5 g kg⁻¹ occurred at the base of the temperature inversion) therein produced a narrow cool-moist water saturation layer. This collocation with a temperature inversion but no moisture inversion was peculiar to the upper boundary of the moisture advection layers. A relative minority (∼33%) of thin supercooled liquid stratiform clouds formed on the bottom or middle part of the moisture advection layers, where a moisture inversion coincided with a temperature inversion. Within the corresponding water saturation layer, the relative humidity peaked at the top of the coincided moisture inversion and temperature inversion, indicating warm-moist air mass advection. Two detailed case studies illustrated how shifts in the mesoscale meteorological environment pertinent to moisture advection layers determined the presence and absence of thin supercooled liquid cloud layers.

We examined large-amplitude inertia gravity waves (GWs) over Syowa Station, Antarctica using the PANSY (Program of the Antarctic Syowa MST/IS) radar data and the latest reanalysis (ECMWF reanalysis v5; ERA5) from October 2015 to September 2016. Focusing on large-amplitude events with large absolute momentum flux (AMF), hodograph analysis was applied to both data to estimate the wave parameters. It showed that the inertia GWs with a downward phase velocity becomes dominant in the stratosphere. Although their vertical wavelengths got shorter with altitude, their intrinsic periods and horizontal wavelengths got longer with altitude. In addition, their southward propagation was predominant in the stratosphere. Although height dependence of the estimated wave parameters is consistent with previous studies investigating inertia GWs over Syowa Station, some features specific to large-amplitude inertia GWs were also observed. The GW features obtained from PANSY were mostly consistent with those from ERA5 except for their amplitudes. Comparison of AMF between PANSY and ERA5 indicated that ERA5 significantly underestimated the AMF by a factor of 5 between 5 and 12.5 km altitudes and more above 12.5 km. Difference of horizontal and vertical wind power spectra between PANSY and ERA5 is quantitatively consistent with the difference of AMF and its height dependence. It was found that underestimation of vertical wind spectra primarily contributed to the underestimation of AMF in ERA5. The greater underestimation of AMF in the stratosphere might be due to larger vertical grid spacing in ERA5 and the shorter vertical wavelengths of the dominant GWs in the stratosphere.

Based on the 6-hourly tropical cyclone best track and global reanalysis data, statistical analyses and a machine learning approach are used to identify/quantify factors that affect the relative weakening rate (RWR) of landfalling TCs (LTCs) over China mainland during 1980–2020. Results show that the enhanced RWR of LTC events usually occurs when LTCs move into regions with large environmental vertical wind shear (VWS), large surface roughness (SURR), high land surface soil temperature (SOILT), low surface latent heat flux (SLHF), and with relatively faster translational speeds (SPD). The SPD and SURR are dominant factors determining the RWR of LTCs over China mainland, contributing about 20% and 18.5% to the RWR of LTCs. VWS is also a key factor affecting RWR of LTCs with mid-level VWS contributing 17.8% to RWR of LTCs and low- and deep-level VWS contributing about 12.9% and 11.2%, respectively. Furthermore, factors affecting the LTC weakening rate in south and north China are different. In north China, the VWS at different levels are all highly correlated with LTC RWR after landfall, whereas the influence of mid-layer VWS shows significant correlation with LTC RWR in south China. In addition, surface characteristics, including SURR, SLHF, and SOILT, have significant correlation with LTC RWR in south China. But the relationships between surface characteristics and LTC RWR in north China are not statistically significant. It is worth noting that although the correlation between DIV200 and LTC RWR is insignificant for the whole China mainland, it presents highly negative and positive correlations in south and north China, respectively.

Roll vortices can have a significant influence on the exchanges of momentum, sensible heat and moisture throughout the boundary layer. They have been shown to reinforce air-sea interactions under strong wind conditions. This raises the question of how air-sea exchanges can, in turn, affect the roll vortices. However, representing surface turbulent fluxes during extreme wind conditions is a major challenge in numerical weather prediction and can lead to large uncertainties in surface wind speed. The sensitivity of rolls to different representations of surface fluxes is investigated using large eddy simulations. The study focuses on Mediterranean windstorm Adrian, where rolls are responsible for the transport of strong winds to the surface. Considering the effects of sea spray increases surface heat fluxes and significantly influences the roll morphology. The rolls become narrower and extend over a greater height, while the downward transport of momentum is intensified and results in higher wind speed near the surface. Roll vortices vanish within a few minutes in the absence of momentum fluxes, which maintain the wind shear necessary for their organization. They also quickly weaken without sensible heat fluxes, which sustain the near-neutral stratification required for their development. In contrast, latent heat fluxes play a minor role. These findings emphasize the necessity of precisely representing the processes occurring at the air-sea interface, which do not only affect the thermodynamic surface conditions but also the vertical transport of momentum within the windstorm.

Australia has had several severe droughts in its recent history. Most studies have linked these droughts to large-scale modes of variability, whereas few studies have investigated droughts from the perspective of weather systems. The current study examines a wide range of weather systems focusing especially on heavy rainfall events, which are important to meteorological drought. Two distinct phases (development and recovery) are identified for the Millennium Drought based on the cumulative standardized precipitation index. Differences in precipitation from heavy rainfall events between the two drought phases are most pronounced in autumn and summer. The pronounced reduction in precipitation from autumn heavy rainfall events during the development phase is due to fewer, less intense, faster-moving warm conveyor belts. In contrast, increased precipitation from autumn heavy rainfall events in the recovery phase is explained by an interaction between warm conveyor belts and upper-level anticyclonic potential vorticity with a persistent anticyclonic circulation over the Tasman Sea acting to slow the eastward propagation of rainfall-producing weather systems. In summer, however, the difference in precipitation from heavy rainfall events in the two drought phases is due to changed moisture content within warm conveyor belts. In the recovery phase, the higher moisture content is associated with greater moisture transport over the Tasman Sea between Australia and New Zealand.

Volcanic and pyrocumulonimbus (pyroCB) injections into the stratosphere perturb the aerosol layer and can have important radiative and chemical impacts on timescales spanning from months to several years. Repeated in situ balloon-borne measurements of aerosol size and number concentration (>140 nm in diameter), ozone, water vapor, and atmospheric state variables made at midlatitudes in the southern hemisphere (SH) since 2019 enable us to better characterize such events. We use this record and coincident lidar extinction profiles to study several moderate to large stratospheric perturbations in the SH between 2019 and 2022 in detail, including the Australian New Year Super Outbreak (ANYSO) pyroCB in 2020. Median vertical profiles of aerosol number concentration, effective radius, and surface area in SH midlatitudes are also compared with those recorded in Northern Hemisphere midlatitudes under baseline conditions using an identical payload. These data depict the variability in stratospheric aerosol properties in the SH midlatitudes during this period and provide a benchmark for global sectional aerosol models. They reveal that sulfate particle size distributions under baseline conditions and in volcanic plumes are relatively well represented in the Community Earth System Model—Community Aerosol Radiation Model for Atmospheres (CESM-CARMA), but more observations of biomass burning plumes are needed to improve model skill in simulating pyroCB. Comparisons between in situ and lidar observations also highlight a need for more observations of aerosol composition and refractive index in both fresh and aging biomass burning plumes.

Major explosive volcanic eruptions were important triggers of abrupt climate changes during the Holocene and crucial sources of Hg to the atmosphere, yet there remains limited understanding regarding the long-range transportation of this volcanic Hg and its imprint in natural archives. Here, we present a reconstruction of Holocene global volcanism based on the anomalies in Hg concentrations, accumulation fluxes, and Hg/C ratios in three high-resolution peat profiles spanning Eurasia. Our reconstruction reveals that the two Tibetan peat profiles recorded 33 major explosive volcanic eruptions (with 11 eruptions being synchronously detected), which correspond with a French Pyrenees peat record and sulfate anomalies in polar ice cores. Additionally, the major explosive volcanic eruptions recorded in the TP peat profiles coincided with abrupt decreases in solar irradiance during the Holocene, suggesting these eruptions might have had a greater global climate impact. Our results suggest the atmospheric transport of volcanic Hg within the Northern Hemisphere and underscore the significant role played by major explosive volcanic eruptions in precipitating abrupt global climate and environmental changes during the Holocene. This study has implications for deciphering the configuration of volcanic eruption seasons, locations, and magnitudes during the Holocene and aligning the chronology of peat deposits with ice cores.

The summer precipitation in the middle and lower reaches of the Yangtze River (MLYR) is characterized by obvious intra-seasonal oscillations. This study investigates the propagation diversity of the 20–40-day summer precipitation oscillation in MLYR and its underlying mechanism. The 20–40-day oscillation events manifest as three types: northward propagation, southward propagation, and local oscillation. For northward and southward events, the propagation of precipitation is accompanied by the movement of 20–40-day low-frequency cyclonic vortices in the lower troposphere. Further investigation into the mechanism of anomalous vorticity propagation accompanying low-frequency vortices reveals that positive relative vorticity advection in the north of the vortex facilitates its northward propagation. Moreover, the positive advection is primarily reliant on the background southerly winds to transport the low-frequency vorticity. Diabatic heating dominates the southward propagation of low-frequency precipitation. The center of the latent heating is located in the south of the low-frequency vortex, which steers its southward migration. Furthermore, the uneven spatial distribution of latent heating across the vortex may be attributed to the non-uniform distribution of mean humidity. Based on the above results, sensitivity experiments are conducted using a regional climate model (RegCM4.6). The results demonstrate that when the southerly winds are increased (decreased) or the specific humidity gradient is decreased (increased), the southward (northward) events weaken and disappear, or transitioned into local oscillations or northward (southward) events. This further validates the physical processes through which the basic flow and humidity inhomogeneities affect the meridional propagation of the 20–40-day oscillations of precipitation.

The amplification effect of anthropogenic-biogenic interactions on secondary organic aerosol (SOA) formation remains debated, particularly regarding the impact of anthropogenic emissions on biogenic SOA (BSOA) formation in forests near megacities. This study concurrently measured typical biogenic and anthropogenic SOA tracers during day and night at the summit (1,690 m a.s.l.) and foot (200 m a.s.l.) of the Nanling mountains, a large subtropical forest adjacent to the Pearl River Delta (PRD) in southern China. Results revealed unexpectedly high concentrations of BSOA tracers (149.9 ± 70.5 ng m⁻³ at the summit and 109.7 ± 51.2 ng m⁻³ at the foot), surpassing those at most other background sites worldwide. Daytime BSOA tracer levels at the foot were consistent with nighttime levels, whereas the summit showed significantly higher concentrations at night. Nighttime correlations between O₃ and BSOA tracers at the summit suggest that high O₃ levels stimulate BSOA formation. Conversely, a negative correlation between O₃ and isoprene derived SOA (SOA_I) tracers at the foot indicates that other oxidants may also influence SOA_I formation. BSOA tracer concentrations rose significantly with the arrival of anthropogenic pollutants (e.g., SO₂ and NO₂), indicating that anthropogenic pollution amplifies BSOA formation by enhancing aerosol acidity (pH < 3). This amplification effect could be mitigated by the reduction of aerosol acidity due to increased NH₃ and relative humidity (RH). Our findings provide valuable insights into the interactions between anthropogenic and biogenic emissions on SOA formation and vertical distribution in forests surrounding megacities.