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Table of Contents for Geophysical Research Letters. List of articles from both the latest and EarlyView issues.
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Evaluation of the Consistency of ECMWF Ensemble Forecasts

Thu, 06/04/2020 - 19:53
Abstract

An expected benefit of ensemble forecasts is that a sequence of consecutive forecasts valid for the same time will be more consistent than an equivalent sequence of individual forecasts. Inconsistent (jumpy) forecasts can cause users to lose confidence in the forecasting system. We present a first systematic, objective evaluation of the consistency of the European Centre for Medium‐Range Weather Forecasts (ECMWF) ensemble using a measure of forecast divergence that takes account of the full ensemble distribution. Focusing on forecasts of the North Atlantic Oscillation and European Blocking regimes up to 2 weeks ahead, we identify occasional large inconsistency between successive runs, with the largest jumps tending to occur at 7–9 days lead. However, care is needed in the interpretation of ensemble jumpiness. An apparent clear flip‐flop in a single index may hide a more complex predictability issue which may be better understood by examining the ensemble evolution in phase space.

Effects of Nonlinear Resonance Broadening on Interactions Between Electrons and Whistler Mode Waves

Thu, 06/04/2020 - 19:48
Abstract

Effects of nonlinear resonance broadening on diffusion coefficients for interactions between parallel propagating whistler mode waves and relativistic electrons in a uniform magnetic field are investigated in this work using test particle simulations. In quasi‐linear theory, variations of pitch angle and energy are calculated along the unperturbed trajectory, resulting in difficulties when the pitch angle is near 90° or when wave amplitudes are large. Using a test particle simulation code, we investigate effects of nonlinear resonance broadening on scattering of electrons and compare the results with a previous nonlinear theory by Karimabadi et al. We demonstrate that Karimabadi et al.'s theory gives better agreement with test particle simulations compared with standard quasi‐linear theory, provided that the resonance broadening width is allowed to vary. Our result suggests further development of the nonlinear theory and should be useful to improve radiation belt modeling.

Evidence of Nighttime Production of Organic Nitrates During SEAC4RS, FRAPPÉ, and KORUS‐AQ

Thu, 06/04/2020 - 19:00
Abstract

Organic nitrates (RONO2) are an important NO x sink. In warm, rural environments dominated by biogenic emissions, nocturnal NO3‐initiated production of RONO2 is competitive with daytime OH‐initiated RONO2 production. However, in urban areas, OH‐initiated production of RONO2 has been assumed dominant and NO3‐initiated production considered negligible. We show evidence for nighttime RONO2 production similar in magnitude to daytime production during three aircraft campaigns in chemically distinct summertime environments: Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) in the rural Southeastern United States, Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) in the Colorado Front Range, and Korea‐United States Air Quality Study (KORUS‐AQ) around the megacity of Seoul. During each campaign, morning observations show RONO2 enhancements at constant, near‐background O x (≡O 3+N O 2) concentrations, indicating that the RONO2 are from a non‐photochemical source, whereas afternoon observations show a strong correlation between RONO2 and O x resulting from photochemical production. We show that there are sufficient precursors for nighttime RONO2 formation during all three campaigns. This evidence impacts our understanding of nighttime NO x chemistry.

Empirical Models for Predicting Water and Heat Flow Properties of Permafrost Soils

Thu, 06/04/2020 - 19:00
Abstract

Warming and thawing in the Arctic are promoting biogeochemical processing and hydrologic transport in carbon‐rich permafrost and soils that transfer carbon to surface waters or the atmosphere. Hydrologic and biogeochemical impacts of thawing are challenging to predict with sparse information on arctic soil hydraulic and thermal properties. We developed empirical and statistical models of soil properties for three main strata in the shallow, seasonally thawed soils above permafrost in a study area of ~7,500 km2 in Alaska. The models show that soil vertical stratification and hydraulic properties are predictable based on vegetation cover and slope. We also show that the distinct hydraulic and thermal properties of each soil stratum can be predicted solely from bulk density. These findings fill the gap for a sparsely mapped region of the Arctic and enable regional interpolation of soil properties critical for determining future hydrologic responses and the fate of carbon in thawing permafrost.

Regional MJO Modulation of Northwest Pacific Tropical Cyclones Driven by Multiple Transient Controls

Thu, 06/04/2020 - 19:00
Abstract

The Madden–Julian Oscillation (MJO) is widely acknowledged for its ability to modulate Northwest Pacific tropical cyclones (TCs), but a complete understanding of the underlying mechanisms remains uncertain. Beyond established effects of the MJO's relative humidity envelope, other dynamical factors have recently been invoked via new genesis potential indices and high‐resolution modeling studies. Here we revisit the ability of the MJO to modulate West Pacific TCs through a quasi‐explicit cyclone downscaling strategy driven by composited observations, paired later with a genesis index to investigate regional drivers of modulation. We reveal two distinct spatial modes of TC modulation in which the MJO's dynamic and thermodynamic effects act in tandem to increase TCs. In the South China Sea, for instance, shear reductions associated with the MJO's circulation lead to increasing potential intensity ahead of the arrival of a positive humidity anomaly, all of which combine for an extended period of cyclogenesis favorability.

Detecting Slow Slip Events From Seafloor Pressure Data Using Machine Learning

Thu, 06/04/2020 - 19:00
Abstract

Detecting slow slip events (SSEs) at offshore subduction zones is important to understand the slip behavior on offshore subduction megathrusts, where tsunamis can be generated. The most widely used method to detect SSEs is to measure the vertical seafloor deformation caused by SSEs using seafloor pressure data. However, due to the small signal‐to‐noise ratio and instrumental drift, such detection is very difficult. In this study, we trained a machine learning model using synthetic data to detect SSEs and applied it to real pressure data in New Zealand between 2014 and 2015. Our method detected five events, two of which are confirmed by the onshore GPS records. Besides, our model performs better than the traditional matched filter method. We conclude that machine learning could be used to detect SSEs in real seafloor pressure data. The method can be applied to other regions, especially where near trench GPS is not available.

Satellite observed positive impacts of fog on vegetation

Thu, 06/04/2020 - 15:18
Abstract

Fog is an important water source for many ecosystems, especially in drylands. Most fog‐vegetation studies focus on individual plant scale, the relationship between fog and vegetation function at larger spatial scales remains unclear. This hinders an accurate prediction of climate change impacts on dryland ecosystems. To this end, we examined the effect of fog on vegetation utilizing both optical and microwave remote sensing derived vegetation proxies and fog observations from two locations at Gobabeb and Marble Koppie within the fog‐dominated zone of the Namib Desert. Significantly positive relationships were found between fog and vegetation attributes from optical data at both locations. The positive relationship was also observed for microwave data at Gobabeb. Fog can explain about 10%–30% of variability in vegetation proxies. These findings suggested that fog impacts on vegetation can be quantitatively evaluated from space using remote sensing data, opening a new window for research on fog‐vegetation interactions.

Increased Likelihood of Appreciable Afternoon Rainfall Over Wetter or Drier Soils Dependent Upon Atmospheric Dynamic Influence

Thu, 06/04/2020 - 09:22
Abstract

The relationship between morning soil moisture and afternoon rainfall persists as an important yet unresolved challenge in land‐atmosphere interaction study, complicated in part by atmospheric influence. Here, we address this relationship by utilizing NASA's satellite soil moisture and precipitation data for the warm season (June–September) of 2015–2019 over Northern Hemisphere land (0–60°N). Raining days are partitioned into low, medium, and high regimes of atmospheric water vapor convergence. Under the low convergence regime, afternoon rainfall is more likely to occur over wetter soils or higher relative humidity; for days with high moisture convergence, occurrence favors drier soils or lower relative humidity. For each regime, afternoon rainfall occurrence favors warmer morning soil or air temperature. These conclusions are not affected by the threshold magnitude utilized to identify afternoon rainfall events by accumulation, but the threshold value does affect the soil moisture (or relative humidity)‐precipitation relationship when convergence regimes are not considered.

Remote sensing retrieval of isoprene concentrations in the Southern Ocean

Thu, 06/04/2020 - 08:33
abstract

Isoprene produced by marine phytoplankton acts as a precursor of secondary organic aerosol and thereby affects cloud formation and brightness over the remote oceans. Yet, the marine isoprene emission is poorly constrained, with discrepancies among estimates that reach 2 orders of magnitude. Here we present ISOREMS, the first satellite‐only based algorithm for the retrieval of isoprene concentration in the Southern Ocean. Sea surface concentrations from six cruises were matched with remotely‐sensed variables from MODIS Aqua, and isoprene was best predicted by multiple linear regression with chlorophyll‐a and sea surface temperature. Climatological (2002‐2018) isoprene distributions computed with ISOREMS revealed high concentrations in coastal and near‐island waters, and within the 40°‐50°S latitudinal band. Isoprene seasonality paralleled phytoplankton productivity, with annual maxima in summer. The annual Southern Ocean emission of isoprene was estimated at 63 Gg C yr‐1. The algorithm can provide spatially and temporally realistic inputs to atmospheric and climate models.

Global Modeling of Equatorial Spread F with SAMI3/WACCM‐X

Thu, 06/04/2020 - 08:30
Abstract

We report the first results of a global ionosphere/thermosphere simulation study that self‐consistently generates large‐scale equatorial spread F (ESF) plasma bubbles in the post‐sunset ionosphere. The coupled model comprises the ionospheric code SAMI3 and the atmosphere/thermosphere code WACCM‐X. Two cases are modeled for different seasons and geophysical conditions: the March case (low solar activity: F10.7 = 70) and the July case (high solar activity: F10.7 = 170). We find that equatorial plasma bubbles formed and penetrated into the topside F layer for the March case but not the July case. For the March case a series of bubbles formed in the Atlantic sector with irregularity spacings in the range 400‐1200 km, rose to over 800 km, and persisted until after midnight. These results are consistent with recent GOLD observations. Calculation of the generalized Rayleigh‐Taylor instability (GRTI) growth rate shows that the e‐folding time was much shorter for the March case than the July case.

The spatiotemporal variability of cloud radiative effects on the Greenland Ice Sheet surface mass balance

Thu, 06/04/2020 - 08:30
Abstract

To better understand and quantify the impact of clouds on the Greenland Ice Sheet surface mass balance (SMB), we study the spatiotemporal variability of the cloud radiative effect (CRE). The total CRE is separated in short‐term and long‐term impacts by performing multiple simulations with the SNOWPACK model for 2001‐2010. The annual total CRE is 16.8 ± 4.5 W m‐2, reducing the SMB with ‐157 ±3.8 Gt yr‐1. Summer cloud radiative cooling is ‐6.4 ± 5.7 W m‐2 in the ablation area, increasing the SMB with 121 ±$2.2 Gt yr‐1. The annual integrated impact is cloud‐reduced SMB of ‐36 Gt yr‐1. The short‐term effect dominates the opposing long‐term effects through the albedo‐melt feedback. A long‐term warming effect decreases the albedo and so preconditions the surface for enhanced (summer) melt. The impact of the CRE, determined by spatial, temporal and initial conditions, explains existing conflicted views on the role of cloud radiation and emphasises the need for accurate cloud and albedo representations in future studies.

Role of atmospheric variability in driving the “Warm‐Arctic, Cold‐continent” pattern over the North America sector and sea ice variability over the Chukchi‐Bering Sea

Thu, 06/04/2020 - 08:29
Abstract

While the observed decline of sea ice over the Chukchi‐Bering Sea (CBS) has coincided with the “warm‐Arctic, cold‐continent” (WACC) pattern over the North America (NA) sector, there is a debate on the causes of the WACC pattern. Here we present a very similar WACC pattern over the NA sector on both interannual and subseasonal time scales. Lead‐lag regression analyses on the shorter time scale indicate that an anomalous anticyclonic circulation over Alaska/Yukon in conjunction with the downward surface turbulent heat flux and long‐wave radiation anomalies over CBS leads the formation of the WACC pattern by about 1‐2 days, while the latter further leads CBS sea ice reduction by about 3 days. These results indicate that atmospheric variability may play an active role in driving both the WACC pattern over NA and CBS sea ice variability.

Understanding Re‐intensified Multi‐year El Niño Events

Thu, 06/04/2020 - 08:29
Abstract

To understand the unique temporal evolution of the 2014‐16 multi‐year El Niño event, which can be characterized as a successive and re‐intensified event preceded by a weak El Niño, this study examines similar events in a 2200‐yr simulation of Community Earth System Model, version 1. By contrasting multi‐year El Niño events with single‐year ones, we find that the succession characteristics of multi‐year El Niño events are primarily caused by a negative North Pacific Oscillation (NPO) in the 1st‐year winter via subtropical Pacific coupling processes. By contrasting re‐intensified multi‐year El Niño events with lingering ones, we find that the re‐intensification characteristics are caused by a negative Indian Ocean Dipole (IOD) in the 1st‐year fall via tropical inter‐basin coupling processes. The phase information of the preceding winter NPO and fall IOD together can be used to project the evolution characteristics of El Niño events, particularly the re‐intensified multi‐year and transitional single‐year events.

Crustal deformation in southern California constrained by radial anisotropy from ambient noise adjoint tomography

Thu, 06/04/2020 - 08:29
Abstract

We build a new radially anisotropic shear‐wave velocity model of southern California based on ambient noise adjoint tomography to investigate crustal deformation associated with Cenozoic evolution of the Pacific‐North American plate boundary. Pervasive positive radial anisotropy (4%) is observed in the crust east of the San Andreas Fault (SAF), attributed to subhorizontal alignment of mica/amphibole foliation planes resulting from significant crustal extension. Substantial negative anisotropy (6%) is revealed in the mid/lower crust west of the SAF, where high shear‐wave speeds are also observed. The negative anisotropy could result from steeply dipping amphibole schists in a shear zone developed during Laramide flat slab subduction. Alternatively, it could be caused by the crystal preferred orientation (CPO) of plagioclase, whose fast axis aligns orthogonally to a presumed subhorizontal foliation. The latter new mechanism highlights potentially complex CPO patterns resulting from different lithospheric mineralogy, as suggested by laboratory experiments on xenoliths from the region.

Changes in Soil Cohesion Due to Water Vapor Exchange: A Proposed Dry‐Flow Trigger Mechanism for Recurring Slope Lineae on Mars

Thu, 06/04/2020 - 08:19
Abstract

Recurring slope lineae (RSL) are seasonal flows on steep slopes on Mars. Their formation mechanism is unknown, but dry granular flows are a likely explanation. Any proposed trigger for these flows must be consistent with the observed temperature dependence of RSL: more active in warmer months or when sun‐facing. Here, we use atmospheric modeling and laboratory experiments to explore a potential mechanism that involves both wet and dry processes at Hale Crater, a known RSL location. We propose that dry flows are triggered by changes in soil cohesion due to the loss of water. When surface temperature and humidity were experimentally simulated, salts likely found in the soil only completely dehydrated during the active season for RSL. We propose that the loss of water from soil in warmer months (or when illuminated) lowers soil cohesion and maximum stability angle. Slope failure may occur, exposing darker underlying material and creating RSL.

Global heat uptake by inland waters

Thu, 06/04/2020 - 08:19
Abstract

Heat uptake is a key variable for understanding the Earth system response to greenhouse gas forcing. Despite the importance of this heat budget, heat uptake by inland waters has so far not been quantified. Here we use a unique combination of global‐scale lake models, global hydrological models and Earth system models to quantify global heat uptake by natural lakes, reservoirs and rivers. The total net heat uptake by inland waters amounts to 2.6 ± 3.2x1020 J over the period 1900‐2020, corresponding to 3.6% of the energy stored on land. The overall uptake is dominated by natural lakes (111.7%), followed by reservoir warming (2.3%). Rivers contribute negatively (‐14%) due to a decreasing water volume. The thermal energy of water stored in artificial reservoirs exceeds inland water heat uptake by a factor ~10.4. This first quantification underlines that the heat uptake by inland waters is relatively small, but non‐negligible.

Natural sources of ionization and their impact on atmospheric electricity

Thu, 06/04/2020 - 08:15
abstract

We present a study of atmospheric electricity using the chemistry‐climate model SOCOL considering ionization by solar energetic particles during an extreme solar proton event (SPE), galactic cosmic rays (GCR), and terrestrial radon (Rn‐222). We calculate the global distribution of the atmospheric conductivity and fair‐weather downward current density (J z ) using atmospheric ionization rates from all sources. We found that J z is enhanced (by more than 3.5 pA/m2) in radon source and polar regions. Contribution of Rn‐222 is essential at middle‐ and low latitudes/altitudes where GCR‐induced air conductivity is reduced. The model results are in good agreement with the available observations. We also studied the effects of an extreme SPE, corresponding to the 774 AD event, on the atmospheric electricity and found that it would lead to a large increase of J z on a global scale. The magnitude of the effects depends on location and can exceed background value more than 30 times over the high latitudes (a conservative upper bound). SSuch an assessment has been performed for the first time.

Distributed Acoustic Sensing (DAS) of Seismic Properties in a Borehole drilled on a Fast‐Flowing Greenlandic Outlet Glacier

Thu, 06/04/2020 - 08:15
Abstract

Distributed Acoustic Sensing (DAS) is a new technology in which seismic energy is detected, at high spatial and temporal resolution, using the propagation of laser pulses in a fibre‐optic cable. We show analyses from the first glaciological borehole DAS deployment to measure the englacial and subglacial seismic properties of Store Glacier, a fast‐flowing outlet of the Greenland Ice Sheet. We record compressional and shear waves in 1043 m‐deep vertical seismic profiles, sampled at 10 m vertical resolution, and detect a transition from isotropic to anisotropic ice at 84% of ice thickness, consistent with the Holocene‐Wisconsin transition. We identify subglacial reflections originating from the base of a 20 m‐thick layer of consolidated sediment and, from attenuation measurements, interpret temperate ice in the lowermost 100 m of the glacier. Our findings highlight the promising potential of DAS technology to constrain the seismic properties of glaciers and ice sheets.

A Post‐2013 Dropoff in Total Ozone at a Third of Global Ozonesonde Stations: Electrochemical Concentration Cell Instrument Artifacts?

Thu, 06/04/2020 - 08:09
Abstract

An international effort to improve ozonesonde data quality and to reevaluate historical records has made significant improvements in the accuracy of global network data. However, between 2014 and 2016, ozonesonde total column ozone (TCO; O3) at 14 of 37 regularly reporting stations exhibited a sudden dropoff relative to satellite measurements. The ozonesonde TCO drop is 3–7% compared to satellite and ground‐based TCO, and 5–10% or more compared to satellite stratospheric O3 profiles, compromising the use of recent data for trends, although they remain reliable for other uses. Hardware changes in the ozonesonde instrument are likely a major factor in the O3 dropoff, but no single property of the ozonesonde explains the findings. The bias remains in recent data. Research to understand the dropoff is in progress; this letter is intended as a caution to users of the data. Our findings underscore the importance of regular ozonesonde data evaluation.

The Lack of QBO‐MJO Connection in CMIP6 Models

Wed, 06/03/2020 - 19:00
Abstract

Observational analysis has indicated a strong connection between the stratospheric quasi‐biennial oscillation (QBO) and tropospheric Madden‐Julian oscillation (MJO), with MJO activity being stronger during the easterly phase than the westerly phase of the QBO. We assess the representation of this QBO‐MJO connection in 30 models participating in the Coupled Model Intercomparison Project 6. While some models reasonably simulate the QBO during boreal winter, none of them capture a difference in MJO activity between easterly and westerly QBO that is larger than that which would be expected from the random sampling of internal variability. The weak signal of the simulated QBO‐MJO connection may be due to the weaker amplitude of the QBO than observed, especially between 100 to 50 hPa. This weaker amplitude in the models is seen both in the QBO‐related zonal wind and temperature, the latter of which is thought to be critical for destabilizing tropical convection.

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