Journal of Geophysical Research: Atmospheres

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ISSN / EISSN : 2169-897X / 2169-8996
Published by: American Geophysical Union (AGU) (10.1029)
Total articles ≅ 7,307
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Journal of Geophysical Research: Atmospheres; https://doi.org/10.1029/2021jd034650

Abstract:
In the zonal direction, the downward branch of the Walker circulation above the Indian Ocean is only 20 degrees wide, whereas the Pacific counterpart is 90 degrees wide. This zonal sharpness is notable because atmospheric disturbances smaller than the planetary scale can interact with the planetary-scale Walker circulation through this branch. This zonal sharpness is also imprinted on a unique zonal discontinuity of the tropical rain belt above Northeast Africa. Therefore, we refer to this narrow downward branch as the “Wall”, investigate its climatology and interannual variability, and aim at determining its reason for existence. The strongest season of the lower tropospheric Wall in boreal summer is sustained by horizontal cold advection associated with the Asian Summer Monsoon, whereas another peak in boreal spring is explained by the seasonal migration of zonal winds. Two weak phases of the Wall correspond to two rainy seasons at the Eastern Horn of Africa, which are not reproduced well by the state-of-the-art global climate models. Experiments using a convection-permitting atmospheric model show that vertical mixing forced by mountain waves in East Africa is necessary for sustaining the Wall. After flattening the East African topography, zonal discontinuity of the tropical rain belt disappears. As for interannual variability, one standard deviation variability of vertical motions at the Wall is associated with one degree of sea surface temperature in the tropical Pacific, and the relationship are strongest in boreal autumn. Nevertheless, total variance is explained more by sea surface temperature in the tropical Indian Ocean.
Journal of Geophysical Research: Atmospheres; https://doi.org/10.1029/2021jd035126

Abstract:
During summer, the Southern Ocean is largely unaffected by anthropogenic emissions, which makes this region an ideal place to investigate marine natural aerosol sources and processes. A better understanding of natural aerosol is key to constrain the preindustrial aerosol state and reduce the aerosol radiative forcing uncertainty in global climate models. We report the concentrations of gaseous sulfuric acid, iodic acid, and methanesulfonic acid (MSA) together with a characterization of new particle formation (NPF) events over a large stretch of the Southern Ocean. Measurements were conducted on board the Russian icebreaker Akademik Tryoshnikov from January to March 2017. Iodic acid is characterized by a particular diurnal cycle with reduced concentration around noon, suggesting a lower formation yield when solar irradiance is higher. Gaseous MSA does not have a diurnal cycle and measured concentrations in gas and condensed phase are compatible with this species being primarily produced via heterogeneous oxidation of dimethyl sulfide and subsequent partitioning into the gas phase. We also found that NPF in the boundary layer is mainly driven by sulfuric acid but it occurred very rarely over the vast geographical area probed and did not contribute to the CCN budget in a directly observable manner. Despite the near absence of NPF events in the boundary layer, Aitken mode particles were frequently measured, supporting the hypothesis of a free tropospheric source. Iodic acid and MSA were not found to participate in nucleation, however, MSA may contribute to aerosol growth via heterogeneous formation in the aqueous phase.
, S. C. Van Den Heever
Journal of Geophysical Research: Atmospheres; https://doi.org/10.1029/2021jd034814

Abstract:
Convective dust storms, or haboobs, form when strong surface winds loft loose soils in convective storm outflow boundaries. Haboobs are a public safety hazard and can cause a near instantaneous loss of visibility, inimical air quality, and contribute significantly to regional dust and radiation budgets. Nevertheless, reliable predictions of convective dust events are inhibited by a lack of understanding regarding the complex and non-linear interactions between density currents, or convective cold pools, and dust radiative effects. In this paper, the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) is utilized to simulate the effect dust radiation interactions have on a long-lived haboob case study that spans three distinct radiative regimes: day (high shortwave), evening (low shortwave), and night (longwave only). A sophisticated algorithm is used to track and identify the numerous convective old pool boundaries in the simulations and assemble statistics that represent the impact of dust radiative effects. To first order, dust scattering of shortwave radiation in the day leads to a colder, dustier, and faster moving convective cold pool. In the transition period of early evening, the shortwave effects diminish while longwave dust absorption leads to warmer, slower density currents that loft less dust as they propagate onward. At night, the haboob is again warmer due to dust absorption, but gustier in the more stable nocturnal surface layer, leading to enhanced dust emissions.
, , E. Gutierrez, , , M. Barlage, D. Zardi,
Journal of Geophysical Research: Atmospheres; https://doi.org/10.1029/2021jd035002

Abstract:
This paper describes and evaluates physical parameterizations accounting for the effect of rooftop mitigation strategies (RMSs) on the urban environment, in the context of the mesoscale model Weather Research and Forecasting (WRF). Through the new implementation, the sensitivity of near-surface air temperature and building energy consumption to different RMSs is evaluated by means of numerical simulations in idealized urban areas, for typical summer and winter conditions. Rooftop mitigation strategies considered include cool roofs, green roofs, and rooftop photovoltaic panels. The reference case simulations are performed assuming buildings made by bricks, with roof composed of clay tiles. Results indicate that near-surface air temperature is reduced by cool and green roofs during summer: cool roofs are the most efficient in decreasing air temperature, followed by irrigated green roofs. Photovoltaic panels, instead, induce a temperature increase during daytime and a small decrease during nighttime. Cool roofs reveal to be the most efficient strategy in reducing the energy consumption by air conditioning systems. During wintertime, green roofs maintain a higher near-surface air temperature than clay-tile roofs and largely decrease energy consumption. Even PVPs increase air temperature, as in the summer case. On the other hand, cool roofs reduce near-surface air temperature during daytime, inducing an increase in energy consumption. The results presented here show that the parameterization schemes implemented in the WRF model can be a valuable tool to evaluate the effects of mitigation strategies in the urban environment.
Journal of Geophysical Research: Atmospheres; https://doi.org/10.1029/2021jd035199

Abstract:
The circulation of the stratosphere transports important trace gases, including ozone, and can be thought of as having a fast horizontal mixing component and a slow meridional overturning component. Measuring the strength of the circulation directly is not possible, and so it must be inferred from tracer measurements. Long-lived trace gases can be related to the idealized tracer age of air, which describes how long an air parcel has been in the stratosphere. In this paper, we derive a quantitative relationship between the vertical gradient of age and the horizontal mixing between the tropics and the extratropics using a “leaky pipe” framework in isentropic coordinates. Mixing rates of air into and out of the tropics are related to the vertical gradient of age in the tropics and in the extratropics, respectively. The derivation is repeated with the hemispheres separated so that the vertical structure of the mixing in the two hemispheres can be compared directly. These theories are applied to output from an idealized model of the stratosphere and from a realistic chemistry-climate model to test our assumptions and calculate the mixing rates in the models. We then perform a quantitative comparison of the mixing rates in the Northern and Southern hemisphere along with an examination of where such a separation is valid. Finally, we perform a very preliminary calculation of mixing efficiency with satellite data to demonstrate the use of the mixing metric to compare mixing models and data.
, , , , Qian Lu, Zhuoqi Liang
Journal of Geophysical Research: Atmospheres; https://doi.org/10.1029/2021jd035057

Abstract:
Using reanalysis, observations, and subseasonal to seasonal (S2S) forecasts, the most recent sudden stratospheric warming (SSW) in January 2021 and its predictability are explored. Previous work has shown that SSWs can be forecasted at relatively long lead time if favorable conditions are present. However, favorable conditions were not present for this most recent SSW, which occurred under the tropical westerly QBO and weak convection over tropical Pacific. In mid-December 2020 and early January 2021, the Ural ridge and East Asian trough were anomalously strong, corresponding to enhanced climatological wavenumber 2. In late December 2020 and mid-January 2021, negative (positive) height anomalies over the North Pacific (Atlantic) enhanced the climatological wavenumber 1. Alternate wave pulses by the wavenumber 1 and 2 finally led to the SSW onset and the long lifetime of the January 2021 SSW. As the composite for QBO westerlies displays a strengthened stratospheric polar vortex, the weak vortex in January 2020 was not attributed to this tropical forcing. Therefore, the predictability for the occurrence of the January 2021 SSW is not beyond two weeks with the required hit ratio > 50%. Splitting of the vortex adds further difficulty to prediction of this event. The cold anomalies over North Eurasia initiated one to two weeks before the SSW onset were likely due to the SSW precursors, which then persisted until mid-January 2021 as the SSW signal began to propagate downward. However, the persistent cold anomalies can only be forecasted in SSW-hit members and SSW-hit S2S models. The observed Arctic sea ice loss is unlikely to extend the predictability of this event in S2S models.
Jianfeng Sun, , Jingbo Mao, Junri Zhao, , , Xinke Wang, Huiling Ouyang, Xu Tang, , et al.
Journal of Geophysical Research: Atmospheres; https://doi.org/10.1029/2021jd035139

Abstract:
Secondary inorganic ions are important constituents of PM2.5 that play an important role in global climate change through direct and indirect radiative forcing. To investigate the influence of ship emissions on coastal cities, we launched a cruise ship campaign offshore of the East China Sea (ECS) and two field observation sites in the megacity of Shanghai using one-hour time-resolved water-soluble ion chromatography from June 3rd to June 27th, 2017. Three main secondary inorganic soluble ions in the atmosphere of the ECS, i.e., non-sea-salt sulfate (nss-SO42-), NO3- and NH4+, were 8.87, 4.94 and 4.65 μg·m-3 , respectively. While their values at the Pudong (PD) station in urban Shanghai were 6.33, 9.47 and 4.74 μg·m-3 and at the Dianshanhu (DSH) station in suburban Shanghai were 7.33, 10.01 and 6.78 μg·m-3, respectively. Nss-SO42- was dominant in the PM2.5 in the atmosphere of the ECS (nss-SO42-/NO3- =1.8), and mainly distributed near the Zhoushan Islands of the ECS. This indicated significant contributions from ship emissions. Based on the Community Multiscale Air Quality (CMAQ) model, two main sources of nss-SO42- were identified: emissions and horizontal transport. These comprised over 90% of the nss-SO42-. The influence of ship emissions reached up to 31.6% of the total nss-SO42- alongside the cruise ship and near coastal parking ports in the Yangtze River Delta (YRD) area, suggesting that more attention should be paid to ship emissions.
Poonam Bikkina, , , Hayato Yamaguchi
Journal of Geophysical Research: Atmospheres; https://doi.org/10.1029/2020jd033347

Abstract:
To understand the sources and transport pathways of organic compounds associated with soil microbes and higher plant waxes in the East Asian outflow, we assessed source-specific tracers such as α-, β- and ω-hydroxy fatty acids (FAs) in remote marine aerosols collected at Chichijima Island in the western North Pacific (WNP) during 2001-2003. Molecular distributions of hydroxy FAs are characterized by strong even-carbon numbered predominance, indicating biogenic sources. Hydroxy FAs showed a strong seasonality with higher loadings during winter/spring than summer/autumn. Cluster analysis of backward air mass trajectories, satellite-based fire counts and dust extinction data reveal an impact of the East Asian outflow over the WNP in winter/spring. In the spring, there are larger relative abundances of short-chain β-hydroxy C10-C18 FAs (a proxy for soil microbes), consistent with the higher loadings of non-sea-salt Ca2+ (dust tracer). The molecular distributions of β-hydroxy FAs in spring are in agreement with those of the reference materials of Chinese loess (CJ-1) and simulated Asian mineral dust (CJ-2), suggesting their probable sources in East Asia. A comparison of relative abundances of short-chain β-hydroxy C10-C18 FAs and long-chain ω-hydroxy C20-C32 FAs (a proxy for higher plant metabolites) in Chichijima aerosols between this (2001-2003) and previous (1990-1993) studies have unveiled an increment of 20% and 30%, respectively. Such an increase was likely caused by the changes in source strength on a decadal-scale and warrants further investigation. Furthermore, cluster analysis of trajectories and the overall distributions of hydroxy FAs between both datasets have shown their similar provenance in winter/spring.
Journal of Geophysical Research: Atmospheres, Volume 126; https://doi.org/10.1029/2021jd034904

Abstract:
Surface fluxes and atmospheric boundary layer budgets of enthalpy and momentum are quantified using bulk and integral methods based on measurements obtained during a research vessel transect across the Terra Nova Bay polynya during an intense late autumnal katabatic wind event. The surface sensible, latent and net radiation heat fluxes had maximum upward values of 2500 ± 600, 400 ± 100, and 100 ± 7 Wm-2, respectively, which occurred over open regions that had been cleared of sea ice by the wind stress. In these open areas, sea spray enhanced the sensible and latent heat fluxes by an estimated 106% and 18%, respectively. As sea ice formed on the surface and became thicker in the downwind direction, the sensible and latent heat fluxes decreased to 50% of their maximum values over pancake ice and to 5% at the downwind end of the transect, where snow-covered young ice flows were present. Snow growth removed over 50% of the water vapor that came from the surface.. The surface wind stress ranged from mean values of 2.9 Nm-2 in the most upwind regions to 0.9 Nm-2 at the end of the transect, with smaller scale variations of almost a factor of three due to different surface types (roughnesses) and gustiness. The downwind slowing and turning of the wind vector can be almost entirely explained by frictional and inertial forces, indicating the horizontal pressure gradient was weak. This case could serve as a case study for future modeling studies of coastal polynyas.
Journal of Geophysical Research: Atmospheres, Volume 126; https://doi.org/10.1029/2021jd035209

Abstract:
Passive satellite observations play an important role in monitoring global aerosol properties and helping quantify aerosol radiative forcing in the climate system. The quality of aerosol retrievals from the satellite platform relies on well-calibrated radiance measurements from multiple spectral bands, and the availability of appropriate particle optical models. Inaccurate scattering phase function assumptions can introduce large retrieval errors. High-spatial resolution, dual-view observations from the Advanced Baseline Imagers (ABI) on board the two most recent Geostationary Operational Environmental Satellites (GOES), East and West, provide a unique opportunity to better constrain the aerosol phase function. Using dual GOES reflectance measurements for a dust event in the Gulf of Mexico in 2019, we demonstrate how a first-guess phase function can be reconstructed by considering the variations in observed scattering angle throughout the day. Using the reconstructed phase function, aerosol optical depth retrievals from the two satellites are self-consistent and agree well with surface-based optical depth estimates. We evaluate our methodology and reconstructed phase function against independent retrievals made from low-Earth-orbit multi-angle observations for a different dust event in 2020. Our new aerosol optical depth retrievals have a root-mean-square-difference of 0.019– 0.047. Furthermore, the retrievals between the two geostationary satellites for this case agree within about 0.059±0.072, as compared to larger discrepancies between the operational GOES products at times, which do not employ the dual-view technique.
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