The 50 most-cited articles in Journal of Geophysical Research Atmospheres, 2020

Abstract We present a new version of the Met Office Hadley Centre/Climatic Research Unit global surface temperature data set, HadCRUT5. HadCRUT5 presents monthly average near‐surface temperature anomalies, relative to the 1961–1990 period, on a regular 5° latitude by 5° longitude grid from 1850 to 2018. HadCRUT5 is a combination of sea‐surface temperature (SST) measurements over the ocean from ships and buoys and near‐surface air temperature measurements from weather stations over the land surface. These data have been sourced from updated compilations and the adjustments applied to mitigate the impact of changes in SST measurement methods have been revised. Two variants of HadCRUT5 have been produced for use in different applications. The first represents temperature anomaly data on a grid for locations where measurement data are available. The second, more spatially complete, variant uses a Gaussian process based statistical method to make better use of the available observations, extending temperature anomaly estimates into regions for which the underlying measurements are…

Abstract We present the second update to a data set of gridded land‐based temperature and precipitation extremes indices: HadEX3. This consists of 17 temperature and 12 precipitation indices derived from daily, in situ observations and recommended by the World Meteorological Organization (WMO) Expert Team on Climate Change Detection and Indices (ETCCDI). These indices have been calculated at around 7,000 locations for temperature and 17,000 for precipitation. The annual (and monthly) indices have been interpolated on a 1.875°×1.25° longitude‐latitude grid, covering 1901–2018. We show changes in these indices by examining ”global”‐average time series in comparison with previous observational data sets and also estimating the uncertainty resulting from the nonuniform distribution of meteorological stations. Both the short and long time scale behavior of HadEX3 agrees well with existing products. Changes in the temperature indices are widespread and consistent with global‐scale warming. The extremes related to daily minimum temperatures are changing faster than the maximum. Spatial…

Abstract The Northern Hemisphere (NH) polar winter stratosphere of 2019/2020 featured an exceptionally strong and cold stratospheric polar vortex. Wave activity from the troposphere during December–February was unusually low, which allowed the polar vortex to remain relatively undisturbed. Several transient wave pulses nonetheless served to help create a reflective configuration of the stratospheric circulation by disturbing the vortex in the upper stratosphere. Subsequently, multiple downward wave coupling events took place, which aided in dynamically cooling and strengthening the polar vortex. The persistent strength of the stratospheric polar vortex was accompanied by an unprecedentedly positive phase of the Arctic Oscillation in the troposphere during January–March, which was consistent with large portions of observed surface temperature and precipitation anomalies during the season. Similarly, conditions within the strong polar vortex were ripe for allowing substantial ozone loss: The undisturbed vortex was a strong transport barrier, and temperatures were low enough to form…

Abstract The use of regional climate model (RCM)‐based projections for providing regional climate information in a research and climate service contexts is currently expanding very fast. This has been possible thanks to a considerable effort in developing comprehensive ensembles of RCM projections, especially for Europe, in the EURO‐CORDEX community (Jacob et al., 2014, 2020). As of end of 2019, EURO‐CORDEX has developed a set of 55 historical and scenario projections (RCP8.5) using 8 driving global climate models (GCMs) and 11 RCMs. This article presents the ensemble including its design. We target the analysis to better characterize the quality of the RCMs by providing an evaluation of these RCM simulations over a number of classical climate variables and extreme and impact‐oriented indices for the period 1981–2010. For the main variables, the model simulations generally agree with observations and reanalyses. However, several systematic biases are found as well, with shared responsibilities among RCMs and GCMs: Simulations are overall too cold, too wet, and too windy compared to…

Abstract This paper analyzes the ensemble of regional climate model (RCM) projections for Europe completed within the EURO‐CORDEX project. Projections are available for the two greenhouse gas concentration scenarios RCP2.6 (22 members) and RCP8.5 (55 members) at 0.11° resolution from 11 RCMs driven by eight global climate models (GCMs). The RCM ensemble results are compared with the driving CMIP5 global models but also with a subset of available last generation CMIP6 projections. Maximum warming is projected by all ensembles in Northern Europe in winter, along with a maximum precipitation increase there; in summer, maximum warming occurs in the Mediterranean and Southern European regions associated with a maximum precipitation decrease. The CMIP6 ensemble shows the largest signals, both for temperature and precipitation, along with the largest inter‐model spread. There is a high model consensus across the ensembles on an increase of extreme precipitation and drought frequency in the Mediterranean region. Extreme temperature indices show an increase of heat extremes and a decrease of…

Abstract The representation of the Northern Hemisphere (NH) storm tracks and jet streams and their response to climate change have been evaluated in climate model simulations from Phases 3, 5, and 6 of the Coupled Model Intercomparison Project (CMIP3, CMIP5, and CMIP6, respectively). The spatial patterns of the multimodel biases in CMIP3, CMIP5, and CMIP6 are similar; however, the magnitudes of the biases in the CMIP6 models are substantially lower. For instance, the multimodel mean RMSE of the North Atlantic storm track for the CMIP6 models (as measured by time‐filtered sea‐level pressure variance) is over 50% smaller than that of the CMIP3 models in both winter and summer, and over 40% smaller for the North Pacific. The magnitude of the jet stream biases is also reduced in CMIP6, but by a lesser extent. Despite this improved representation of the current climate, the spatial patterns of the climate change response of the NH storm tracks and jet streams remain similar in the CMIP3, CMIP5, and CMIP6 models. The SSP2‐4.5 scenario responses in the CMIP6 models are substantially larger…

Abstract Since its discovery in the early 1970s, the crucial role of the Madden‐Julian Oscillation (MJO) in the global hydrological cycle and its tremendous influence on high‐impact climate and weather extremes have been well recognized. The MJO also serves as a primary source of predictability for global Earth system variability on subseasonal time scales. The MJO remains poorly represented in our state‐of‐the‐art climate and weather forecasting models, however. Moreover, despite the advances made in recent decades, theories for the MJO still disagree at a fundamental level. The problems of understanding and modeling the MJO have attracted significant interest from the research community. As a part of the AGU's Centennial collection, this article provides a review of recent progress, particularly over the last decade, in observational, modeling, and theoretical study of the MJO. A brief outlook for near‐future MJO research directions is also provided.

Abstract More than 40 model groups worldwide are participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), providing a new and rich source of information to better understand past, present, and future climate change. Here, we use the Earth System Model Evaluation Tool (ESMValTool) to assess the performance of the CMIP6 ensemble compared to the previous generations CMIP3 and CMIP5. While CMIP5 models did not capture the observed pause in the increase in global mean surface temperature between 1998 and 2013, the historical CMIP6 simulations agree well with the observed recent temperature increase, but some models have difficulties in reproducing the observed global mean surface temperature record of the second half of the twentieth century. While systematic biases in annual mean surface temperature and precipitation remain in the CMIP6 multimodel mean, individual models and high‐resolution versions of the models show significant reductions in many long‐standing biases. Some improvements are also found in the vertical temperature, water vapor, and zonal wind speed…

Abstract Following the description of the basic climatic characteristics of Meiyu in East Asia, we present a comprehensive review of multi–time scale variabilities of the Meiyu together with the long‐term change of extreme precipitation during the Meiyu in this paper. The dynamic prediction skills of the Meiyu on subseasonal to seasonal time scales are further described and illustrated by using present numerical prediction models. For the multi–time scale variabilities of the East Asian Meiyu, they are closely related with the impacts of both quasi‐biweekly and 30–60 day oscillation from both the tropics and the middle and high latitudes on intraseasonal time scale, the influences of El Niño–Southern Oscillation and the Tropospheric Biennial Oscillation on interannual time scale, the variations of East Asian summer monsoon on decadal time scale, and the continued impacts of global warming, Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation on interdecadal scale and long‐term trend. Under the background of the global warming and the influences of urbanization and…

Abstract Climatic Research Unit temperature version 5 (CRUTEM5) is an extensive revision of our land surface air temperature data set. We have expanded the underlying compilation of monthly temperature records from 5,583 to 10,639 stations, of which those with sufficient data to be used in the gridded data set has grown from 4,842 to 7,983. Many station records have also been extended or replaced by series that have been homogenized by national meteorological and hydrological services. We have improved the identification of potential outliers in these data to better capture outliers during the reference period; to avoid classifying some real regional temperature extremes as outliers; and to reduce trends in outlier counts arising from climatic warming. Due to these updates, the gridded data set shows some regional increases in station density and regional changes in temperature anomalies. Nonetheless, the global‐mean timeseries of land air temperature is only slightly modified compared with previous versions and previous conclusions are not altered. The standard gridding algorithm…

Abstract Drought is among the costliest natural disasters that affect the economy, food and water security, and socioeconomic well‐being of about 1.4 billion people in India. Despite the profound implications of droughts, the propagation of meteorological to hydrological droughts in India is not examined. Here, we use observations and simulations from a well‐calibrated and evaluated Variable Infiltration Capacity (VIC) model to estimate drought propagation in India. Standardized Precipitation Index (SPI), Standardized Soil Moisture Index (SSMI), and Standardized Streamflow Index (SSI) were estimated for 223 catchments in India to represent meteorological, agricultural, and hydrological droughts, respectively. We estimated drought propagation time for these catchments located in 18 major Indian subcontinental river basins. Internal propagation of hydrological drought was estimated using optimal hydrological Instantaneous Development Speed (IDS) and Instantaneous Recovery Speed (IRS) from onset to the termination. Indus, Sabarmati, and Godavari river basins have higher propagation…

Abstract The Community Earth System Model 2 (CESM2) is the latest Earth System Model developed by the National Center for Atmospheric Research in collaboration with the university community and is significantly advanced in most components compared to its predecessor (CESM1). Here, CESM2's representation of the large‐scale atmospheric circulation and its variability is assessed. Further context is providedthrough comparison to the CESM1 large ensemble and other models from the Coupled Model Intercomparison Project (CMIP5 and CMIP6). This includes an assessment of the representation of jet streams and storm tracks, stationary waves, the global divergent circulation, the annular modes, the North Atlantic Oscillation, and blocking. Compared to CESM1, CESM2 is substantially improved in the representation of the storm tracks, Northern Hemisphere (NH) stationary waves, NH winter blocking and the global divergent circulation. It ranks within the top 10% of CMIP class models in many of these features. Some features of the Southern Hemisphere (SH) circulation have degraded, such as the SH jet…

Abstract The quasi‐biennial oscillation (QBO) of the zonal mean zonal wind is the primary mode of variability in the tropical lower stratosphere. The QBO is characterized by alternating easterly westerly shear layers that descend down from ∼10 to 100 hPa. The QBO is also seen in lower stratospheric tropical temperature, water vapor, and ozone and affects tropospheric variability through various teleconnections. We examine here the progress in simulating the QBO in the Coupled Model Intercomparison Project (CMIP) models, more specifically in CMIP3, CMIP5, and CMIP6 models. We show that the number of models that are able to simulate the QBO has increased from 0 in CMIP3, to 5 in CMIP5, to 15 in CMIP6. While the number of models with an internally generated QBO has tripled from CMIP5 to CMIP6, the fidelity of the simulation averaged over the CMIP models has not improved. We show that CMIP5 and CMIP6 models represent the QBO period and latitudinal extent quite well; however the QBO amplitude is shifted upwards relative to observations resulting in large underestimation of QBO amplitude…

Abstract Simulations from the models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6), which represent the most recent generation of climate models, are now available. Understanding the performance of these models in simulating historical climate extremes can provide a basis for producing reliable future climate projections. Here, we assess the simulation of 16 indices of temperature extremes defined by the Expert Team on Climate Change Detection and Indices using results from 24 CMIP6 models as compared with results from CMIP5. Comparisons with observations and reanalyses indicate that the CMIP6 models could capture the spatial patterns and temporal variations of the observed temperature extremes well for some indices, although less well for others. Based on spatial and temporal skill scores, CMIP6 ensemble means were more skillful in simulating absolute and threshold indices of extreme temperature than CMIP5 ensemble means were, but the performances of both the CMIP5 and CMIP6 ensemble means in simulating the spatial patterns for duration and…

Abstract Finer grids in global climate models could lead to an improvement in the simulation of precipitation extremes. We assess the influence on model performance of increasing spatial resolution by evaluating pairs of high‐ and low‐resolution forced atmospheric simulations from six global climate models (generally the latest CMIP6 version) on a common 1° × 1° grid. The differences in tuning between the lower and higher resolution versions are as limited as possible, which allows the influence of higher resolution to be assessed exclusively. We focus on the 1985–2014 climatology of annual extremes of daily precipitation over global land, and models are compared to observations from different sources (i.e., in situ‐based and satellite‐based) to enable consideration of observational uncertainty. Finally, we address regional features of model performance based on four indices characterizing different aspects of precipitation extremes. Our analysis highlights good agreement between models that precipitation extremes are more intense at higher resolution. We find that the spread among…

Abstract The comprehensive understanding of the occurred changes of permafrost, including the changes of mean annual ground temperature (MAGT) and active layer thickness (ALT), on the Qinghai‐Tibet Plateau (QTP) is critical to project permafrost changes due to climate change. Here, we use statistical and machine learning (ML) modeling approaches to simulate the present and future changes of MAGT and ALT in the permafrost regions of the QTP. The results show that the combination of statistical and ML method is reliable to simulate the MAGT and ALT, with the root‐mean‐square error of 0.53°C and 0.69 m for the MAGT and ALT, respectively. The results show that the present (2000–2015) permafrost area on the QTP is 1.04 × 10 6 km 2 (0.80–1.28 × 10 6 km 2 ), and the average MAGT and ALT are −1.35 ± 0.42°C and 2.3 ± 0.60 m, respectively. According to the classification system of permafrost stability, 37.3% of the QTP permafrost is suffering from the risk of disappearance. In the future (2061–2080), the near‐surface permafrost area will shrink significantly under different Representative…

Abstract Recent increases in computing power mean that atmospheric models for numerical weather prediction are now able to operate at grid spacings of the order of a few hundred meters, comparable to the dominant turbulence length scales in the atmospheric boundary layer. As a result, models are starting to partially resolve the coherent overturning structures in the boundary layer. In this resolution regime, the so‐called boundary layer “gray zone,” neither the techniques of high‐resolution atmospheric modeling (a few tens of meters resolution) nor those of traditional meteorological models (a few kilometers resolution) are appropriate because fundamental assumptions behind the parameterizations are violated. Nonetheless, model simulations in this regime may remain highly useful. In this paper, a newly formed gray zone boundary layer community lays the basis for parameterizing gray zone turbulence, identifies the challenges in high‐resolution atmospheric modeling and presents different gray zone boundary layer models. We discuss both the successful applications and the limitations…

Abstract Compound drought and heatwave (CDHW) events have garnered much attention in recent studies. However, thus far, the identification of such events is oversimplified, and their association with natural climate variability is not fully explored. Here, we derive anomalies in the weekly self‐calibrated Palmer Drought Severity Index (sc_PDSI) and daily maximum temperatures to identify CDHW events from 1982 to 2016 over 26 climate regions across the globe. Using a Poisson Generalized Linear Model (GLM), we analyze yearly occurrences of seasonal CDHW events and their association with the warm and cold phases of El Nino Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Atlantic Oscillation (NAO). ENSO exhibits robust association with CDHW events over the Southern Hemisphere during the austral summer and fall, while PDO influences their occurrences over the Western North America in the Northern Hemisphere during the boreal summer, which is supported by the composites of anomalies in the atmospheric circulations and surface energy budget. However, NAO…

Abstract All hydrological models need to be calibrated to obtain satisfactory streamflow simulations. Here we present a novel parameter regionalization approach that involves the optimization of transfer equations linking model parameters to climate and landscape characteristics. The optimization was performed in a fully spatially distributed fashion at high resolution (0.05°), instead of at lumped catchment scale, using an unprecedented database of daily observed streamflow from 4,229 headwater catchments (<5,000 km2) worldwide. The optimized equations were subsequently applied globally to produce parameter maps for the entire land surface including ungauged regions. The approach was evaluated using the Kling‐Gupta efficiency (KGE) and a gridded version of the hydrological model HBV. Tenfold cross validation was used to evaluate the generalizability of the approach and to obtain an ensemble of parameter maps. For the 4,229 independent validation catchments, the regionalized parameters yielded a median KGE of 0.46. The median KGE improvement (relative to uncalibrated parameters) was…

Abstract The Lightning Imaging Sensor (LIS) was launched to the International Space Station (ISS) in February 2017, detecting optical signatures of lightning with storm‐scale horizontal resolution during both day and night. ISS LIS data are available beginning 1 March 2017. Millisecond timing allows detailed intercalibration and validation with other spaceborne and ground‐based lightning sensors. Initial comparisons with those other sensors suggest flash detection efficiency around 60% (diurnal variability of 51–75%), false alarm rate under 5%, timing accuracy better than 2 ms, and horizontal location accuracy around 3 km. The spatially uniform flash detection capability of ISS LIS from low‐Earth orbit allows assessment of spatially varying flash detection efficiency for other sensors and networks, particularly the Geostationary Lightning Mappers. ISS LIS provides research data suitable for investigations of lightning physics, climatology, thunderstorm processes, and atmospheric composition, as well as real‐time lightning data for operational forecasting and aviation weather…

Abstract In 2018 and 2019, heatwaves set all‐time temperature records around the world and caused adverse effects on human health, agriculture, natural ecosystems, and infrastructure. Often, severe impacts relate to the joint spatial and temporal extent of the heatwaves, but most research so far focuses either on spatial or temporal attributes of heatwaves. Furthermore, sensitivity of heatwaves characteristics to the choice of the heatwave thresholds in a warming climate are rarely discussed. Here, we analyze the largest spatiotemporal moderate heatwaves—that is, three‐dimensional (space‐time) clusters of hot days—in simulations of global climate models. We use three different hazard thresholds to define a hot day: fixed thresholds (time‐invariant climatological thresholds), seasonally moving thresholds based on changes in the summer means, and fully moving thresholds (hot days defined relative to the future climatology). We find a substantial increase of spatiotemporally contiguous moderate heatwaves with global warming using fixed thresholds, whereas changes for the other two…

Abstract Surface wind speed has great impacts on the economy, environment, and society around the world. Based on 24 global climate models (GCMs) from Coupled Model Intercomparison Project Phase 6 (CMIP6), this paper assesses the historical surface wind speed over China, and quantifies the advancements of CMIP6 over CMIP5. In addition, future changes of surface wind speed under the Shared Socioeconomic Pathway scenarios of 1‐2.6, 2‐4.5, and 5‐8.5 (SSP1‐2.6, SSP2‐4.5, and SSP5‐8.5) are also provided, by using 18 out of the 24 models with all three scenarios. Results show that multimodel ensemble mean of CMIP6 can well capture the spatial distributions of the annual, summer, and winter surface wind speed and generally performs better than both the median of the individual CMIP6 GCMs and multimodel ensemble mean of CMIP5 in monthly, seasonal, and annual wind climatology. However, CMIP6 GCMs fail to reproduce the observed decreasing trends, in terms of magnitude and/or sign. Meanwhile, the annual cycle of the surface wind speed simulated by CMIP6 GCMs falsely demonstrates the maximum in…

Abstract Recent studies based on the ensemble mean of Coupled Model Intercomparison Project 6 (CMIP6) General Circulation Models (GCMs, CMIP6‐GCMs hereafter) reported a decline in drought frequency over South Asia in the projected future climate. Here using the simulations from 16 CMIP6‐GCMs, we examine the potential causes of declining droughts in South Asia. We show that the projections based on the multimodel ensemble mean CMIP6‐GCMs are not reliable over South Asia. The multimodel ensemble mean is influenced mainly by the low‐skill GCMs, which show high bias in simulating the monsoon (June–September) season precipitation during the observed period (1951–2014). The low‐skill GCMs show a higher (20–30%) increase in the convective precipitation with a rise in the global mean temperature under the warming (1.5°C, 2.0°C, and 2.5°C worlds) climate. The GCMs with less bias (BEST‐GCMs) in the monsoon season precipitation and better seasonal cycle representation show lower sensitivity of convective precipitation to rise in global mean temperature. BEST‐GCMs exhibit significantly…

Abstract The Indian Monsoon Data Assimilation and Analysis (IMDAA) is a regional high‐resolution atmospheric reanalysis over the Indian subcontinent. This regional reanalysis over India is the first of its kind and is produced by the National Centre for Medium Range Weather Forecasting and Met Office, UK, in collaboration with the India Meteorological Department under the National Monsoon Mission project of the Ministry of Earth Sciences, Government of India. The reanalysis runs from 1979 to 2018, to span the era of modern meteorological satellites. This article briefly describes the IMDAA system and discusses the performance of the IMDAA during summer monsoon (June–September). This study provides evidence for substantial improvements seen in IMDAA compared to the ERA‐Interim reanalysis fields over India. The evaluation is carried out for the period of 1979–1993 for all major features associated with the Indian Monsoon to highlight improvements compared to ERA‐Interim and to document the biases. The study also demonstrates the potential use of the IMDAA data for applications such as…

A sudden stratospheric warming (SSW) happened in September 2019 in the Southern Hemisphere (SH) with winds at 10hPa, 60°S reaching their minimum value on September 18. The evolution, favorable conditions, and predictability for this SSW event are explored. The favorable conditions include easterly equatorial quasi biennial oscillation (QBO) winds at 10hPa, solar minimum, positive Indian Ocean Dipole (IOD) sea surface temperatures (SST), warm SST anomalies in the central Pacific, and a blocking high near the Antarctic Peninsula. The predictive limit to this SSW is around 18 days in some S2S models, and more than 50% of the ensemble members forecast the zonal wind deceleration in reforecasts initialized around 29 August. A vortex slowdown in evident in some initializations from around 22 August, while initializations later than 29 August capture the wave-like pattern in the troposphere. The ensemble spread in the magnitude of the vortex deceleration during the SSW is mainly explained by the ensemble spread in the magnitude of upward propagation of waves, with an underestimated…

Abstract Satellite observations of the total column dry‐air CO 2 (X CO2 ) are expected to support the quantification and monitoring of fossil fuel CO 2 (ffCO 2 ) emissions from urban areas. We evaluate the utility of the Orbiting Carbon Observatory 2 (OCO‐2) X CO2 retrievals to optimize whole‐city emissions, using a Bayesian inversion system and high‐resolution transport modeling. The uncertainties of constrained emissions related to transport model, satellite measurements, and local biospheric fluxes are quantified. For the first two uncertainty sources, we examine cities of different landscapes: “plume city” located in relatively flat terrain, represented by Riyadh and Cairo; and “basin city” located in basin terrain, represented by Los Angeles (LA). The retrieved scaling factors of emissions and their uncertainties show prominent variabilities from track to track, due to the varying meteorological conditions and relative locations of the tracks transecting plumes. To explore the performance of multiple tracks in retrieving emissions, pseudo data experiments are carried out. The…

The Global Modeling and Assimilation Office (GMAO) has recently released a new version of the Goddard Earth Observing System (GEOS) Sub-seasonal to Seasonal prediction (S2S) system, GEOS-S2S-2, that represents a substantial improvement in performance and infrastructure over the previous system. The system is described here in detail, and results are presented from forecasts, climate equillibrium simulations and data assimilation experiments. The climate or equillibrium state of the atmosphere and ocean showed a substantial reduction in bias relative to GEOS-S2S-1. The GEOS-S2S-2 coupled reanalysis also showed substantial improvements, attributed to the assimilation of along-track Absolute Dynamic Topography. The forecast skill on subseasonal scales showed a much-improved prediction of the Madden-Julian Oscillation in GEOS-S2S-2, and on a seasonal scale the tropical Pacific forecasts show substantial improvement in the east and comparable skill to GEOS-S2S-1 in the central Pacific. GEOS-S2S-2 anomaly correlations of both land surface temperature and precipitation were comparable to…

Using the real-time predictions from 11 models, this study analyzes the prediction of the downward propagation and surface impact of the 2018 and 2019 sudden stratospheric warmings (SSWs). These two SSWs differed both in their morphology types (2018: split; 2019: displacement followed by split) and magnitudes (the former being stronger). With a large sample size (>2,200) of multimodel ensemble forecasts, it is revealed that the strength of the SSW is more important than the vortex morphology in determining the magnitude of its downward impact, with strong SSWs more likely to propagate downward than weak SSWs. Therefore, based on the probabilistic forecasts, the observed strong SSW in February 2018 was more likely to have a downward and surface impact than the January 2019 SSW. The relationship between the 10-hPa dominant wave number and the 100-hPa polar cap height (or the Northern Annular Mode) is weak, implying that the dominant wave number might not be the primary factor determining the downward propagation of SSWs in the two analyzed cases. Hence, the high polar cap height (or…

Abstract A carbonaceous aerosol plume associated with wildfires in British Columbia in August 2017 reached the stratosphere a few days following the fires onset. The Earth Polychromatic Imaging Camera (EPIC) sensor onboard the Deep Space Climate Observatory (DSCOVR) satellite observed the spatial and temporal evolution of the plume for about 6 weeks. EPIC's near‐hourly observations of the Earth's sunlit disk allowed for monitoring the smoke plume several times per day. High ultraviolet aerosol index values pointed to the presence of an aerosol layer in the upper troposphere and lower stratosphere, confirmed with Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar observations. EPIC aerosol optical depth retrievals and CALIOP information on layer height were used to estimate the stratospheric portion of the aerosol mass, reaching a maximum of 268 ± 86 Kt 3 days after the onset of the wildfires. About 10% of this aerosol mass reached the stratosphere on 13 August, whereas the remaining 90% did so over the following 2 days. Observed daytime and inferred nighttime rates of…

Abstract The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) deployed the NSF/NCAR C‐130 aircraft in summer 2018 across the western U.S. to sample wildfire smoke during its first days of atmospheric evolution. We present a summary of a subset of reactive oxidized nitrogen species (NO y ) in plumes sampled in a pseudo‐Lagrangian fashion. Emissions of nitrogen oxides (NO x = NO + NO 2 ) and nitrous acid (HONO) are rapidly converted to more oxidized forms. Within 4 h, ∼86% of the ΣNO y is in the form of peroxy acyl nitrates (PANs) (∼37%), particulate nitrate ( p NO 3 ) (∼27%), and gas‐phase organic nitrates (Org N (g) ) (∼23%). The average e ‐folding time and distance for NO x are ∼90 min and ∼40 km, respectively. Nearly no enhancements in nitric acid (HNO 3 ) were observed in plumes sampled in a pseudo‐Lagrangian fashion, implying HNO 3 ‐limited ammonium nitrate (NH 4 NO 3 ) formation, with one notable exception that we highlight as a case study. We also summarize the observed partitioning of NO y in all the smoke samples intercepted during…

Abstract Reactive nitrogen (Nr) within smoke plumes plays important roles in the production of ozone, the formation of secondary aerosols, and deposition of fixed N to ecosystems. The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) field campaign sampled smoke from 23 wildfires throughout the western U.S. during summer 2018 using the NSF/NCAR C‐130 research aircraft. We empirically estimate Nr normalized excess mixing ratios and emission factors from fires sampled within 80 min of estimated emission and explore variability in the dominant forms of Nr between these fires. We find that reduced N compounds comprise a majority (39%–80%; median = 66%) of total measured reactive nitrogen (ΣNr) emissions. The smoke plumes sampled during WE‐CAN feature rapid chemical transformations after emission. As a result, within minutes after emission total measured oxidized nitrogen (ΣNOy) and measured total ΣNHx (NH3 + pNH4) are more robustly correlated with modified combustion efficiency (MCE) than NOx and NH3 by themselves. The ratio of ΣNHx/ΣNOy displays…

Abstract Estimates of high‐resolution greenhouse gas (GHG) emissions have become a critical component of climate change research and an aid to decision makers considering GHG mitigation opportunities. The “Vulcan Project” is an effort to estimate bottom‐up carbon dioxide emissions from fossil fuel combustion and cement production (FFCO 2 ) for the U.S. landscape at space and time scales that satisfy both scientific and policy needs. Here, we report on the Vulcan version 3.0 which quantifies emissions at a resolution of 1 km 2 /hr for the 2010–2015 time period. We estimate 2011 FFCO 2 emissions of 1,589.9 TgC with a 95% confidence interval of 1,367/1,853 TgC (−14.0%/+16.6%), implying a one‐sigma uncertainty of ~ ±8%. Per capita emissions are larger in states dominated by electricity production and industrial activity and smaller where onroad and building emissions dominate. The U.S. FFCO 2 emissions center of mass (CoM) is located in the state of Missouri with mean seasonality that moves on a near‐elliptical NE/SW path. Comparison to ODIAC, a global gridded FFCO 2 emissions estimate,…

Abstract This study compares various drought indices, the Palmer Drought Severity Index (PDSI), the Standardized Precipitation Evapotranspiration Index (SPEI), and the Standardized Precipitation Index (SPI), and the uncertainties in each to the input data from which they are derived. The abilities of the PDSI, SPEI, and SPI to capture drought periods are assessed through a comparison with soil moisture estimates from two generations of the Global Land Data Assimilation System (GLDAS). This comparison shows that the skill with which a drought index represents variations in soil moisture does not necessarily improve when evapotranspiration is included (i.e., PDSI/SPEI rather than SPI), though this depends on location and the time scale of the drought. The differences in the abilities of the drought indices to represent soil moisture are also compared to the magnitude of the uncertainty in each index arising from the choice of input data. In many cases, the uncertainties in the variations of the PDSI, SPEI, and SPI to the choice of input data are larger in magnitude than the…

Abstract Evapotranspiration (ET) is a crucial quantity through which land surface conditions can impact near‐surface weather and vice versa. ET can be limited by energy or water availability. The transition between water‐ and energy‐limited regimes is marked by the critical soil moisture (CSM), which is traditionally derived from small‐sample laboratory analyses. Here, we aim to determine the CSM at a larger spatial scale relevant for climate modeling, using state‐of‐the‐art gridded data sets. For this purpose, we introduce a new correlation‐difference metric with which the CSM can be accurately inferred using multiple data streams. We perform such an analysis at the continental scale and determine a large‐scale CSM as an emergent property. In addition, we determine small‐scale CSMs at the grid cell scale and find substantial spatial variability. Consistently from both analyses we find that soil texture, climate conditions, and vegetation characteristics are influencing the CSM, with similar respective importance. In contrast, comparable CSMs are found when applying alternative…

Abstract Southern Ocean (S. Ocean) clouds are important for climate prediction. Yet previous global climate models failed to accurately represent cloud phase distributions in this observation‐sparse region. In this study, data from the Southern Ocean Clouds, Radiation, Aerosol, Transport Experimental Study (SOCRATES) experiment is compared to constrained simulations from a global climate model (the Community Atmosphere Model, CAM). Nudged versions of CAM are found to reproduce many of the features of detailed in situ observations, such as cloud location, cloud phase, and boundary layer structure. The simulation in CAM6 has improved its representation of S. Ocean clouds with adjustments to the ice nucleation and cloud microphysics schemes that permit more supercooled liquid. Comparisons between modeled and observed hydrometeor size distributions suggest that the modeled hydrometeor size distributions represent the dual peaked shape and form of observed distributions, which is remarkable given the scale difference between model and observations. Comparison to satellite observations of…

Abstract The dynamical downscaling method with a regional climate model (RCM) is widely used to assess the spatially detailed information about regional climate. However, the RCM result is considerably influenced by the systematic errors inherent to a general circulation model (GCM), which provides the initial and boundary conditions to the RCM. Such systematic errors sometimes lead to meaningless downscaled results. Many modified boundary dynamical downscaling (MBDDS) methods have been proposed to reduce the influences of the systematic errors of a GCM and extract meaningful signals for regional climate change. This study comprehensively reviews the MBDDS methods. The MBDDS methods partially modify the climate information projected by a GCM and use them as the boundary conditions of an RCM. The objectives of the methods are organized into two main objectives, that is, to obtain more reliable projections by correcting the biases in boundary conditions and to better understand the regional climate change mechanisms. To ensure comprehensive understanding of the MBDDS methods, this…

Decades of atmospheric research have focused on the Western North Atlantic Ocean (WNAO) region because of its unique location that offers accessibility for airborne and ship measurements, gradients in important atmospheric parameters, and a range of meteorological regimes leading to diverse conditions that are poorly understood. This work reviews these scientific investigations for the WNAO region, including the East Coast of North America and the island of Bermuda. Over 50 field campaigns and long-term monitoring programs, in addition to 715 peer-reviewed publications between 1946 and 2019 have provided a firm foundation of knowledge for these areas. Of particular importance in this region has been extensive work at the island of Bermuda that is host to important time series records of oceanic and atmospheric variables. Our review categorizes WNAO atmospheric research into eight major categories, with some studies fitting into multiple categories (relative %): Aerosols (25%), Gases (24%), Development/Validation of Techniques, Models, and Retrievals (18%), Meteorology and Transport…

Abstract Major sudden stratospheric warmings (SSWs), vortex formation, and final breakdown dates are key highlight points of the stratospheric polar vortex. These phenomena are relevant for stratosphere‐troposphere coupling, which explains the interest in understanding their future changes. However, up to now, there is not a clear consensus on which projected changes to the polar vortex are robust, particularly in the Northern Hemisphere, possibly due to short data record or relatively moderate CO2 forcing. The new simulations performed under the Coupled Model Intercomparison Project, Phase 6, together with the long daily data requirements of the DynVarMIP project in preindustrial and quadrupled CO2 (4xCO2) forcing simulations provide a new opportunity to revisit this topic by overcoming the limitations mentioned above. In this study, we analyze this new model output to document the change, if any, in the frequency of SSWs under 4xCO2 forcing. Our analysis reveals a large disagreement across the models as to the sign of this change, even though most models show a statistically…

Abstract A rare disturbance of the stratospheric Antarctic polar vortex in September 2019 led to a significantly higher than usual polar total ozone column. We use assimilation of ozone, HCl, and N2O data from the Microwave Limb Sounder with the Global Earth Observing System (GEOS) Constituent Data Assimilation System driven by reanalysis meteorology to study the evolution of the 2019 Antarctic polar ozone. We find that the maximum 2019 ozone hole area was near 10 × 106 km2, and as little as 20% of that in 2018 in mid‐September. However, the magnitude of vortex‐averaged chemical ozone depletion was not significantly different between the 2 years despite earlier chlorine deactivation in 2019. The assimilation results show that most of the differences between 2018 and 2019 Antarctic ozone resulted from two factors: (1) the geometry of the 2019 vortex, with ozone‐rich middle‐stratospheric air masses overlying the lower portion of the vortex and leading to a significant reduction of the total column, and (2) significantly reduced vortex volume. The anomalously small ozone hole of 2019…

Abstract We introduce a new climate model (GISS E2.2) that has been specially optimized for the middle atmosphere and whose output is being contributed to the CMIP6 archive. The top of the model is at a geopotential altitude of 89 km, and parameterizations of moist convection and various forms of gravity wave drag based on tropospheric processes are chosen specifically for this optimization. We first evaluate the model in its configuration as a coupled atmosphere‐chemistry model with respect to its simulation of the mean state of the middle atmosphere, from the mesosphere down through the upper troposphere/lower stratosphere. Then we assess its use as a coupled atmosphere‐ocean climate model by exploring its mean ocean climatology. To evaluate its variability, we report on its simulation of the primary modes in the troposphere, stratosphere, and ocean. Two climate change simulations are presented, the responses to instantaneous increases of 2xCO 2 and 4xCO 2 , run with two different ocean models. Sensitivity studies are performed to illustrate the effect of parameterizations on the…

Abstract Urbanization and global warming are the two major drivers of the warming environment in cities. The contributions of urbanization and global warming to the past occurrence of human‐perceived heatwaves (HWs) over China are evaluated in this study. Both observations and model simulations show HWs have become more intensive, longer‐lasting, and more frequent in recent decades. Urbanization and greenhouse gases contribute to 21.9% and 72.9% of the intensification of HWs, respectively. The occurrence probability of observed human‐perceived HWs has doubled over 1961–2012 and is projected to be about 4.36/5.92 times under the 1.5/2°C warming relative to the preindustrial level. At the 3.5°C warming, the average duration of HWs is projected to increase to 43.63 days/year, and the occurrence probability is expected to increase by 11.95 times, 91% of which is attributable to human‐induced warming. The highest sensitivity of the increases in HWs due to human‐induced warming is in Southern China.

Abstract A strong mountain wave, observed over Central Europe on 12 January 2016, is simulated in 2D under two fixed background wind conditions representing opposite tidal phases. The aim of the simulation is to investigate the breaking of the mountain wave and subsequent generation of nonprimary waves in the upper atmosphere. The model results show that the mountain wave first breaks as it approaches a mesospheric critical level creating turbulence on horizontal scales of 8–30 km. These turbulence scales couple directly to horizontal secondary waves scales, but those scales are prevented from reaching the thermosphere by the tidal winds, which act like a filter. Initial secondary waves that can reach the thermosphere range from 60 to 120 km in horizontal scale and are influenced by the scales of the horizontal and vertical forcing associated with wave breaking at mountain wave zonal phase width, and horizontal wavelength scales. Large‐scale nonprimary waves dominate over the whole duration of the simulation with horizontal scales of 107–300 km and periods of 11–22 minutes. The…

Abstract Nitrate (NO3−) is a major contributing species to haze formation in Northern China. So far, formation processes and source apportionments of nitrate aerosols during haze pollution have not yet been well understood. In this study, the PM2.5 samples were collected in Beijing from 13 November to 24 December 2018. In addition to water‐soluble ions, oxygen (δ18O‐NO3−) and nitrogen (δ15N‐NO3−) isotopes in particulate NO3− were also determined, in order to investigate the formation pathways and potential sources of NO3− aerosols. The results showed that NO3− was a dominant species (43%) of secondary inorganic aerosols during the sampling period. The δ18O‐NO3− and δ15N‐NO3− values averaged at 83.8 ± 13.4 and 11.5 ± 5.0‰, respectively. Combining isotope compositions and Bayesian isotope mixing model, we found that heterogeneous reaction and gas phase oxidation contributed equally to nitrate formation during the sampling period. However, the contribution of heterogeneous processes to nitrate increased from 39% during the clean period to 64% during the haze period. On average, coal…

Abstract Trends and variability in tropospheric hydroxyl (OH) radicals influence budgets of many greenhouse gases, air pollutant species, and ozone depleting substances. Estimations of tropospheric OH trends and variability based on budget analysis of methyl chloroform (CH 3 CCl 3 ) and process‐based chemistry transport models often produce conflicting results. Here we use a previously tested transport model to simulate atmospheric CH 3 CCl 3 for the period 1985–2018. Based on mismatches between model output and observations, we derive consistent anomalies in the inverse lifetime of CH 3 CCl 3 ( K G ) using measurements from two independent observational networks (National Oceanic and Atmospheric Administration and Advanced Global Atmospheric Gases Experiment). Our method allows a separation between “physical” (transport, temperature) and “chemical” (i.e., abundance) influences on OH + CH 3 CCl 3 reaction rate in the atmosphere. Small increases in K G due to “physical” influences are mostly driven by increases in the temperature‐dependent reaction between OH and CH 3 CCl 3 and…

Abstract The Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS) was launched on 19 February 2017 and began routine operation in June 2017. The first 2 years of SAGE III/ISS (v5.1) solar occultation ozone data were evaluated by using correlative satellite and ground‐based measurements. Among the three (MES, AO3, and MLR) SAGE III/ISS retrieved solar ozone products, AO3 ozone shows the smallest bias and best precision, with mean biases less than 5% for altitudes ~15–55 km in the midlatitudes and ~20–55 km in the tropics. In the lower stratosphere and upper troposphere, AO3 ozone shows high biases that increase with decreasing altitudes and reach ~10% near the tropopause. Preliminary studies indicate that those high biases primarily result from the contributions of the oxygen dimer (O4) not being appropriately removed within the ozone channel. The precision of AO3 ozone is estimated to be ~3% for altitudes between 20 and 40 km. It degrades to ~10–15% in the lower mesosphere (~55 km) and ~20–30% near the tropopause. There could be an altitude…

Abstract Demands are growing rapidly in the operational prediction and applications communities for forecasts that fill the gap between daily weather forecasts and seasonal climate outlooks. Recent scientific advances have identified sources of predictability on this time range, and modeling advances are leading to better forecasts. However, much remains to be done to further improve their skill and to develop new climate service forecast products to help countries and sectorial decision makers better manage weather risks and extremes and to adapt to climate change. This paper reviews the history and describes the main challenges and opportunities for the modeling and forecast‐applications communities to improve subseasonal to seasonal (S2S) forecasts and products, along with current developments catalyzed by the World Weather Research Programme and World Climate Research Programme's joint Sub‐Seasonal to Seasonal Prediction Project. The case of tropical cyclones is highlighted as an illustrative example of the points discussed.

Abstract As one of the major agricultural regions in the world, water scarcity problems in Northeast China have drawn much attention recently. Because of cold and long winter period, snow is an important component in the hydrological system. Yet few studies have been conducted to systematically assess its role. This study quantified the effects of snow on runoff and soil moisture in the entire region in a 30‐year time period (1982–2011) for the first time. A water and energy budget‐based distributed biosphere hydrological model with improved snow physics after calibration and validation is employed. A Standardized Snow depth Index (SSdI) is also proposed to quantify snow variations. Result shows that snow contributes 11% to runoff annually on average and 66% and 33% in April and May (main months for crop planting). Soil moisture could decrease by at least 20% in March–May if there would be no snow, and the major agriculture area suffers more than other regions. We also found that SSdI is indicative of standardized soil moisture index and standardized runoff index in spring,…

Abstract Atmospheric chemistry models—components in models that simulate air pollution and climate change—are computationally expensive. Previous studies have shown that machine‐learned atmospheric chemical solvers can be orders of magnitude faster than traditional integration methods but tend to suffer from numerical instability. Here, we present a modeling framework that reduces error accumulation compared to previous work while maintaining computational efficiency. Our approach is novel in that it (1) uses a recurrent training regime that results in extended (>1 week) simulations without exponential error accumulation and (2) can reversibly compress the number of modeled chemical species by >80% without further decreasing accuracy. We observe an ~260× speedup (~1,900× with specialized hardware) compared to the traditional solver. We use random initial conditions in training to promote general applicability across a wide range of atmospheric conditions. For ozone (concentrations ranging from 0–70 ppb), our model predictions over a 24‐hr simulation period match those of the…

Abstract Trichlorofluoromethane (CFC‐11, CFCl3) is a major anthropogenic ozone‐depleting substance and greenhouse gas, and its production and consumption are controlled under the Montreal Protocol. However, recent studies show that CFC‐11 emissions have been near constant or increasing since 2002. In this study, we use a two‐dimensional chemistry‐climate model to investigate the stratospheric ozone response to a range of future CFC‐11 emissions scenarios. A scenario with future emissions sustained at 10 gigagrams per year (Gg/year) above the baseline WMO (2018) A1 scenario results in minor additional global (90°S–90°N) ozone depletion of 0.13% by 2100, and a 1.5‐year delay in the global ozone recovery to 1980 levels, relative to the baseline. A scenario with 72.5 Gg/year (the 2013–2016 average) sustained to 2100 results in a substantial 15% increase in effective equivalent stratospheric chlorine and nearly 1% additional global ozone depletion by 2100, with a 7.5‐year delay in the recovery to 1980 global ozone levels, relative to the baseline. The ozone response averaged over time…

Abstract The dry‐wet abrupt alternation (DWAA) event, which is defined as the phenomenon of dry (or wet) spells abruptly following wet (or dry) spells, magnifies the influence of individual wet and dry events. The dynamic evolution of DWAA events has not been studied for different climate zones of China that is particularly susceptible to dry and wet extremes. This study explores the future changes in the abrupt alternations between dry and wet extremes across 10 climate divisions of China, with a thorough assessment of dry and wet conditions using the Standardized Precipitation Evapotranspiration Index (SPEI). We take advantage of an ensemble of regional climate model simulations including the Providing Regional Climate Impacts for Studies (PRECIS) experiment and five CORDEX East Asia experiments to produce high‐resolution climate information for a baseline period of 1975–2004 and a future period of 2069–2098. Our findings disclose that a total of 70% of China's land area suffered from the DWAA events at least once during 1975–2004. The wet‐dry alternation event is projected to…