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Journal of Geophysical Research
American Geophysical Union
Journal of Geophysical Research

American Geophysical Union

2169-897X

Journal of Geophysical Research/Journal Journal of Geophysical Research
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    The Influence of Dust-Smoke Mixtures on Boundary Layer Processes and Nocturnal Warming in the Sahel

    Christopher PhillipsUdaysankar NairSundar Christopher
    17页
    查看更多>>摘要：Using a single column model with ground-based, aircraft, and satellite data sets we assess the combined role of smoke and dust aerosols, land degradation/aridization (LDA), and their impact on the planetary boundary layer (PBL) in influencing near-surface air temperature over the Sahel. Our study is unique because it assesses the combined role of smoke and dust aerosols on PBL evolution and near-surface air temperatures during both day and nighttime. More importantly, using a theoretical framework, we provide a careful explanation of the geophysical processes responsible for the changes in PBL and near-surface air temperature. Our results indicate that during northern hemisphere winter months, dust, and smoke over Sahel radiatively combine to impact the PBL. We show that aerosol mixtures dominated by dust modify PBL height in a manner that minimizes/maximizes surface layer cooling/warming at times when daytime maximum/ nocturnal minimum temperatures occur. Furthermore, we find that increasing smoke contribution to total column aerosol optical extinction counteracts nighttime warming through daytime cooling. When smoke constitutes half or more of to the total column aerosol optical extinction, the ratio of longwave to shortwave radiative forcing is less than 10%, and nighttime cooling ensues. Minimum temperature is most sensitive to changes in mid-visible aerosol optical depth (AOD) values <1 and doubling of dust AOD within this range during the 1950–1980 Sahelian LDA event is estimated to have a nocturnal warming potential of 0.6°C.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Radar Diagnosis of the Thundercloud Electron Accelerator

    E. WilliamsH. MkrtchyanB. MailyanG. Karapetyan...
    32页
    查看更多>>摘要：Thunderstorm Ground Enhancements (TGEs) refer to correlated enhancements in surface electric field and gamma ray flux that are manifestations of electron runaway in the storm overhead. The electric field enhancements can be of positive or negative polarity. In this study, altitude-resolved S-band radar observations of graupel are used to demonstrate distinct differences in storm structure linked with these “positive” and “negative” TGEs. The physical interpretation rests on the well-established temperaturedependent tripole structure of thunderstorms, with the main negative charge of the tripole acting as an electron repeller. This interpretation is supported by case studies showing altitude-stable convection, with shallow (deep) development linked with “positive” (“negative”) TGEs, and by case studies of collapsing storms that show upper dipole dominance early and lower inverted dipole dominance later when graupel particles descend from a colder to warmer temperature domain. In the case of many TGEs on Mt Aragats (3.2 km MSL), the temperature-dependent altitude of downward electron acceleration and avalanching may be sufficiently distant (>500 m) from surface detectors that the energetic electrons (1–10 MeV) are not likely avalanche/runaway electrons. Instead, they are Compton-scattered and pair-produced electrons from bremsstrahlung gamma radiation emanating from the high-field avalanche region aloft. These inferences are consistent with GEANT4 calculations that identify the physical origins of energetic electrons at the surface.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Impacts of Lake Surface Temperature on the Summer Climate Over the Great Lakes Region

    Jiali WangPengfei XueWilliam PringleZhao Yang...
    20页
    查看更多>>摘要：The surface of the Great Lakes interacts with the atmosphere, influencing the weather and climate over the Great Lakes Region (GLR). However, most climate models were not designed with sufficient emphasis on lake–atmosphere interactions, which could potentially cause model biases over the GLR. To understand how lake surface temperature (LST) affects the regional summer climate over the GLR, we conducted twin experiments using the Weather Research and Forecasting model at a spatial resolution of 4 km using two different LST data sets as the bottom boundary condition over the Great Lakes. Our simulations include 10 ensemble members for the summer of 2018 and a single multiyear run for the summers of 2014–2020. Results show that variations in LST influence atmospheric temperature and moisture at a local scale-while affecting the convective environment and precipitation processes over a much larger spatial scale. In particular, an LST that is only 1°C–3°C warmer (depending on the lake) increases near-surface air temperature by 1.93°C and 0.97°C over Lake Superior and Lake Erie, respectively, and increases evaporation over the lakes by 0.23 and <f>1.1 mm day~(?1). The warmer LST reduces mesoscale convective precipitation upstream of the GLR; however, it increases isolated deep convective precipitation and nonconvective precipitation downstream of GLR due to increased local instability and enhancement of moisture transport. Our analyses confirm the robustness of these impacts, which is at least 2 times larger than the model internal variability and is seen across all simulated summer seasons.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Physical and Chemical Properties of Cloud Droplet Residuals and Aerosol Particles During the Arctic Ocean 2018 Expedition

    Linn KarlssonAndrea BaccariniPatrick DuplessisDarrel Baumgardner...
    20页
    查看更多>>摘要：Detailed knowledge of the physical and chemical properties and sources of particles that form clouds is especially important in pristine areas like the Arctic, where particle concentrations are often low and observations are sparse. Here, we present in situ cloud and aerosol measurements from the central Arctic Ocean in August–September 2018 combined with air parcel source analysis. We provide direct experimental evidence that Aitken mode particles (particles with diameters ?70 nm) significantly contribute to cloud condensation nuclei (CCN) or cloud droplet residuals, especially after the freeze-up of the sea ice in the transition toward fall. These Aitken mode particles were associated with air that spent more time over the pack ice, while size distributions dominated by accumulation mode particles (particles with diameters ?70 nm) showed a stronger contribution of oceanic air and slightly different source regions. This was accompanied by changes in the average chemical composition of the accumulation mode aerosol with an increased relative contribution of organic material toward fall. Addition of aerosol mass due to aqueous-phase chemistry during in-cloud processing was probably small over the pack ice given the fact that we observed very similar particle size distributions in both the whole-air and cloud droplet residual data. These aerosol– cloud interaction observations provide valuable insight into the origin and physical and chemical properties of CCN over the pristine central Arctic Ocean.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Mesosphere and Lower Thermosphere Changes Associated With the 2 July 2019 Total Eclipse in South America Over the Andes Lidar Observatory, Cerro Pachon, Chile

    H. BourneF. VargasA. LiuG. Swenson...
    18页
    查看更多>>摘要：This article presents the results of a week of observations around the 2 July 2019, total Chilean eclipse. The eclipse occurred between 19:22 and 21:46 UTC, with complete sun disc obscuration at 20:38– 20:40 UTC (16:38–16:40 LT) over the Andes Lidar Observatory (ALO) at (30.3°S, 70.7°W). Observations were carried out using ALO instrumentation with the goal to observe possible eclipse-induced effects on the mesosphere and lower thermosphere region (MLT; 75–105 km altitude). To complement our data set, we have also utilized TIMED/SABER temperatures and ionosonde electron density measurements taken at the University of La Serena's Juan Soldado Observatory. Observed events include an unusual fast, bow-shaped gravity wave structure in airglow images, mesosphere temperature mapper brightness as well as in lidar temperature with 150 km horizontal wavelength 24 min observed period, and vertical wavelength of 25 km. Also, a strong zonal wind shear above 100 km in meteor radar scans as well as the occurrence of a sporadic E layer around 100 km from ionosonde measurements. Finally, variations in temperature and density and the presence of a descending sporadic sodium layer near 98 km were seen in lidar data. We discuss the effects of the eclipse in the MLT, which can shed light on a sparse set of measurements during this type of event. Our results point out several effects of eclipse-associated changes in the atmosphere below and above but not directly within the MLT.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Upper-Troposphere Saddle-Like Response to Springtime Surface Sensible Heating Over the Tibetan Plateau: Combined Effect From Baroclinic and Barotropic Process

    Yu ZhaoRuizao SunZhiang XieAnmin Duan...
    18页
    查看更多>>摘要：The surface sensible heating over the Tibetan Plateau (TP) is a significant, elevated heating source in spring (March-April-May, MAM). It exerts considerable thermal effects on both regional and global climate. During MAM, the surface sensible heating over the TP triggers a giant saddle-like circulation anomaly in the upper troposphere at 200 hPa. Although this structure was observed a long time ago and it contributes to the stationary wave, its formation mechanism remains unclear. Using the Linear Baroclinic Model, we reveal the evolution of this structure in detail. In the initial state, a typical baroclinic anticyclone with a vertical phase tilt forms around the heating source. When the wave propagates downstream due to strong relative vorticity advection and favorable lower level circulation, an anticyclone with a barotropic structure forms downstream. Our analysis indicates that both the barotropic process and baroclinic process contribute to the formation of the saddle-like structure, and the latter is associated with the spring unique background circulation and vertical heating structure over the TP.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Atmospheric Triggers of the Brunt Ice Shelf Calving in February 2021

    Diana FrancisRicardo FonsecaKyle S. MattinglyOliver J. Marsh...
    15页
    查看更多>>摘要：The calving of Antarctic ice shelves remains unpredictable to date due to a lack of understanding of the role of the different climatic components in such events. In this study, the role of atmospheric forcing in the calving of the Brunt Ice Shelf (BIS) in February 2021 is investigated using a combination of observational and reanalysis data. The occurrence of a series of extreme cyclones around the time of the calving induced an oceanward sea-surface slope of >0.08° leading to the calving along a pre-existing rift. The severe storms were sustained by the development of a pressure dipole on both sides of the BIS associated with a La Ni?a event and the positive phase of the Southern Annular Mode. Poleward advection of warm and moist low-latitude air over the BIS area just before the calving was also observed in association with atmospheric rivers accompanying the cyclones. Immediately after the calving, strong offshore winds continued and promoted the drift of the iceberg A-74 in the Weddell Sea at a speed up to 700 m day~(?1). This study highlights the contribution of local atmospheric conditions to ice-shelf dynamics. The link to the larger scale circulation patterns indicates that both need to be accounted for in the projections of Antarctic ice shelf evolution.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Improving the Understanding of Atmospheric River Water Vapor Transport Using a Three-Dimensional Straightened Composite Analysis

    Guangzhi XuLin WangPing ChangXiaohui Ma...
    22页
    查看更多>>摘要：Irregular shapes of atmospheric rivers (ARs) hamper easy AR composite analyses and understandings of AR's moisture transport mechanisms. We develop a method to composite AR-related variables from a reanalysis data set. By averaging a large number of samples, the three-dimensional structure and some evolutionary features of a typical North Pacific AR are revealed. An AR is typically located along and in advance of the surface cold front of an extratropical cyclone. A mesoscale secondary circulation is observed in the cross sections of the AR corridor, where both geostrophic and ageostrophic winds make indispensable contributions to the moisture fluxes. Geostrophic moisture advection across the cold front within the Equatorward half of the AR is created by baroclinicity of the system and serves as the primary moisture source for the AR. Moisture fluxes from the warm sector of the cyclone are primarily due to boundary layer ageostrophic winds and are more important within the poleward half of the AR, particularly during the genesis stage. Faster AR movement compared with low-level winds enables the AR to collect downwind moisture. In an AR-relative view, this is represented as an easterly flow, consistent with the “feeder-airstream” concept, and emphasizes the role of local moisture recycling. Moisture transport within the Equatorward half is mostly due to geostrophic advection of the propagating AR-cyclone couple in an Earth-relative view. Driven by intensifying geostrophic winds, ARs tend to reach peak moisture transport intensity about 2 days after genesis. Then, reduced moisture level and influxes from lateral boundaries prevent further intensification.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Sublimation Origin of Negative Deuterium Excess Observed in Snow and Ice Samples From McMurdo Dry Valleys and Allan Hills Blue Ice Areas, East Antarctica

    Jun HuYuzhen YanLaurence Y. YeungSylvia G. Dee...
    20页
    查看更多>>摘要：The oxygen and hydrogen isotopic composition in snow and ice have long been utilized to reconstruct past temperatures of polar regions, under the assumption that post-depositional processes such as sublimation do not fractionate snow. In low-accumulation (<0.01 m yr ?1) areas near the McMurdo Dry Valleys in Antarctica, surface snow and ice samples have exceptionally low deuterium excess values (d-excess ≡ δD – 8*δ~(18)O)-sometimes as negative as ?5‰-an uncommon phenomenon that is not fully understood. Here we use both an isotope-enabled general circulation model and an ice physics model and establish that such exceptionally low d-excess values can only arise from precipitation if the majority of the moisture is sourced from the Southern Ocean (south of 55°S). However, the model results show that moisture sourced from oceans north of 55°S contributes significantly (>50%) to precipitation in Antarctica today. We thus propose that sublimation must have occurred to yield the low d-excess values in snow observed in and near the Dry Valleys, and that solid-phase-diffusion in ice grains is sufficiently fast to allow Rayleigh-like isotopic fractionation in similar environments. We calculate that under present-day conditions at the Allan Hills outside the Dry Valleys, 3%–24% of the surface snow is lost due to sublimation. Because the magnitude of sublimation may be nonstationary (i.e., it could vary in time) during past cold periods, we suggest that sublimation-induced fractionation can alter the relationship between the snow isotopic composition and polar temperatures.
      原文链接:
    • NSTL
    • Amer Geophysical Union

    Terrestrial Gamma-Ray Flashes With Accompanying Elves Detected by ASIM

    Ingrid Bj?rge-EngelandNikolai ?stgaardAndrey MezentsevChris Alexander Skeie...
    21页
    查看更多>>摘要：The Atmosphere-Space Interactions Monitor was designed to monitor Terrestrial Gammaray Flashes (TGFs) and Transient Luminous Events (TLEs) from space, enabling the study of how these phenomena are related. In this paper, we present observations of 17 TGFs with accompanying Elves. TGFs are short and highly energetic bursts of gamma photons associated with lightning discharges, whereas Elves are TLEs that are observed as concentric rings of ultraviolet (UV) and visible light at ionospheric altitudes, produced by the excitation of N_2 molecules when an electromagnetic pulse hits the base of the ionosphere. Elves were identified when optical detections in the UV band could be clearly distinguished from other optical signals from lightning strokes. The TGFs they accompanied had short durations and were associated with particularly high peak current lightning. Lightning sferics associated with these events were detected by the global lightning network GLD360 and the World Wide Lightning Location Network, and they were, with the exception of one event, observed over ocean or coastal regions. It is likely that these events were associated with Energetic In-cloud Pulses. We show that short duration TGFs tend to be associated with higher peak currents than long duration TGFs.
      原文链接:
    • NSTL
    • Amer Geophysical Union