JGR–Atmospheres

Syndicate content Journal of Geophysical Research: Atmospheres
Table of Contents for Journal of Geophysical Research: Atmospheres. List of articles from both the latest and EarlyView issues.
Updated: 1 day 1 hour ago

Updraft and Downdraft Core Size and Intensity as Revealed by Radar Wind Profilers: MCS Observations and Idealized Model Comparisons

Fri, 06/05/2020 - 19:00
Abstract

This study explores the updraft and downdraft properties of mature stage mesoscale convective systems (MCSs) in terms of draft core width, shape, intensity, and mass flux characteristics. The observations use extended radar wind profiler (RWP) and surveillance radar data sets from the U.S. Department of Energy Atmospheric Radiation Measurement program for midlatitude (Oklahoma, USA) and tropical (Amazon, Brazil) sites. MCS drafts behave qualitatively similar to previous aircraft and RWP cloud summaries. The Oklahoma MCSs indicate larger and more intense convective updraft and downdraft cores, and greater mass flux than Amazon MCS counterparts. However, similar size‐intensity relationships and draft vertical profile behaviors are observed for both regions. Additional similarities include weak positive correlations between core intensity and core width (correlation coefficient r ∼ 0.5) and increases in draft intensity with altitude. A model‐observational intercomparison for draft properties (core width, intensity, and mass flux) is also performed to illustrate the potential usefulness of statistical observed draft characterizations. Idealized simulations with the Weather Research and Forecasting model aligned with midlatitude MCS conditions are performed at model grid spacings (△x ) that range from 4 km to 250 m. It is shown that the simulations performed at △x = 250 m at similar mature MCS lifecycle stages are those that exhibit draft intensity, width, mass flux, and shape parameter performances best matching with observed properties.

Evaluation of an Improved Convective Triggering Function: Observational Evidence and SCM Tests

Fri, 06/05/2020 - 19:00
Abstract

This study provides a strong observational support for a recently developed convective triggering function that uses the large‐scale dynamic convective available potential energy (dCAPE) as a constraint combined with an unrestricted air parcel launch level (ULL) to relax the unrealistic strong coupling of convection to surface heating and capture nocturnal elevated convection. Both case study and statistical analysis are conducted using the observations collected from the Department of Energy's Atmospheric Radiation Measurement program at its Southern Great Plains and Manaus (MAO) sites. They show that dCAPE has a much stronger correlation with precipitation than convective available potential energy, and ULL is essential to detect elevated convection above boundary layer under both midlatitude and tropical conditions and for both afternoon and nighttime deep convection regimes. Sensitivity tests with the single‐column model (SCM) of the Department of Energy's Energy Exascale Earth System Model indicate that the role of dCAPE in suppressing daytime convection is more effective for tropical convection than midlatitude convection. Even though the dCAPE can suppress the overestimated convection, ULL plays a much bigger role in improving the diurnal cycle of precipitation than dCAPE. It not only helps capture nocturnal elevated convection but also significantly removes the spurious morning precipitation seen in the default model, due to the release of unstable energy at night. However, the use of ULL has led to an overestimation of light‐to‐moderate precipitation (1–10 mm/day) due to more convection being triggered above boundary layer.

Effects of Gentle Topography on Forest‐Atmosphere Gas Exchanges and Implications for Eddy‐Covariance Measurements

Fri, 06/05/2020 - 13:13
Abstract

The interpretation of tower‐based eddy‐covariance (EC) turbulent flux measurements above forests hinges on three key assumptions: (1) steadiness in the flow statistics, (2) planar homogeneity of scalar sources or sinks, and (3) planar homogeneity in the flow statistics. Large eddy simulations (LESs) were used to control the first two so as to explore the break‐down of the third for idealized and real gentle topography such as those encountered in Amazonia. The LES runs were conducted using uniformly distributed sources inside homogeneous forests covering complex terrain to link the spatial patterns of scalar turbulent fluxes to topographic features. Results showed strong modulation of the fluxes by flow features induced by topography, including large area with negative fluxes compensating “chimney” regions with fluxes almost an order of magnitude larger than the landscape flux. Significant spatial heterogeneity persisted up to at least two canopy heights, where most eddy‐covariance measurements are performed above tall forests. A heterogeneity index was introduced to characterize and contrast different scenarios, and a topography categorization was shown to have predictive capabilities in identifying regions of negative and enhanced fluxes.

The Impact of SST on the Zonal Variability of the Western Pacific Subtropical High in Boreal Summer

Fri, 06/05/2020 - 10:35
Abstract

Summertime western Pacific subtropical high (WPSH) is closely related to remote sea surface temperature anomalies (SSTA) in the tropical oceans (TO) and local SSTA in the western North Pacific (WNP). Based on atmospheric reanalysis and multimodel ensemble products, the warming (cooling) of the tropical Indian, tropical central‐eastern Pacific, and tropical Atlantic (tropical western Pacific and WNP) can lead to westward extension of the WPSH, and vice versa. A series of numerical experiments are conducted to quantify the relative importance of remote and local SSTA, including the separate impact of different TO basins. In response to warming (cooling) of the tropical Indian, tropical central‐eastern (western) Pacific, and tropical Atlantic, ascending (descending) motions develop locally and subsequently excite remote baroclinic and barotropic responses. The baroclinic anticyclone west of 140°E contributes to the westward extension of the WPSH. The barotropic anticyclone and descending motion over the WNP can further increase the sea level pressure and strengthen the WPSH. Based on the quasi‐geostrophic omega equation, we find that the anomalous vorticity advection (thermal advection) primarily accounts for the anomalous descending motion in response to remote (local) SSTA. Compared to local SSTA, the remote SSTA can induce larger perturbations, subsequently making the zonal variability of the WPSH more significant. Furthermore, it is found that the westward extension of the WPSH is primarily induced by atmospheric baroclinic (barotropic) response to the SSTA in tropical Indian and tropical Atlantic (tropical Pacific). Overall, the tropical Indian plays the most important role on the zonal movement of the WPSH.

Coherent Interannual Variations of Springtime Surface Temperature and Temperature Extremes Between Central‐Northern Europe and Northeast Asia

Fri, 06/05/2020 - 10:22
Abstract

This study reveals a strong coherent interannual variation in spring surface air temperature (SAT) and temperature extremes between central‐northern Europe (45–70°N, 0–30°E) and Northeast Asia (40–70°N, 110–140°E). The regional mean spring SAT over the two regions is highly correlated, with a correlation of 0.6 during 1950–2017. Arctic Oscillation (AO) and an atmospheric teleconnection, termed the North Atlantic‐Eurasian (NAE) pattern, both play important roles in connecting SAT anomalies over the two regions. Anticyclonic anomalies are seen over central‐northern Europe and Northeast Asia during positive phases of the spring AO and NAE teleconnections. The associated southerly and westerly wind anomalies to the northwestern side of the anomalous anticyclones lead to surface warming over the two regions via anomalous wind‐induced heat advection from lower latitudes. In addition, a decrease in cloud cover and an accompanying increase in surface downward shortwave radiation also contribute to the coherent SAT warming over central‐northern Europe and Northeast Asia. We suggest that synoptic‐scale eddies forcings are crucial to the maintenance of the spring NAE teleconnection. Atmospheric model experiments indicate that the North Atlantic sea surface temperature anomalies also partly contribute to the maintenance of the NAE‐related atmospheric anomalies over the North Atlantic and European regions. The observed coherent spring SAT variations over central‐northern Europe and Northeast Asia and their associations with the AO and midlatitude atmospheric teleconnections can be reproduced in the long historical simulation of a coupled model. This study has implication for the understanding of concurrent occurrence of extreme temperature events over different regions of Eurasia.

Significance of 4DVAR Radar Data Assimilation in Weather Research and Forecast Model‐Based Nowcasting System

Fri, 06/05/2020 - 09:42
Abstract

Accurate nowcasting of short‐lived extreme weather events is essential for saving millions of lives and property. Traditional methods of nowcasting are majorly focused on extrapolation of precipitation derived from radar reflectivity data, which often fail to capture the initiation and decay of weather systems. Earlier studies have shown the ability of high‐resolution Numerical Weather Prediction (NWP) models to better capture the structure and lifecycle of storms compared to data‐driven methods. However, the initial value problem of NWP makes it more challenging to be implemented for nowcasting applications. To handle such uncertainty from initial conditions, we have designed an NWP nowcasting system based on variational approach using WRF model. One of the major challenges of the variational methods in the nowcasting system is the choice of control variables used for generating background error statistics. Thus, we have investigated the impact of control variable options on improving the skill of variational‐based NWP nowcasting system. The proposed nowcasting system was tested for a heavy rainfall event that occurred over the Chennai city, India, on 1 December 2015, by assimilating Doppler Weather Radar data using different control variable options in Weather Research and Forecast—three‐dimensional (3DVAR)‐ and four‐dimensional variational data assimilation (4DVAR)‐based nowcasting system. Results show that control variables choices have a positive impact on 4DVAR analysis, particularly on radial velocity. Our results also indicate that assimilation of Doppler Weather Radar data with zonal and meridional momentum control variable in a 4DVAR system shows more than 30% improvement in precipitation forecast skill compared to the 3DVAR system.

Long‐Term Trends and Solar Responses of the Mesopause Temperatures Observed by SABER During the 2002–2019 Period

Thu, 06/04/2020 - 19:30
Abstract

The global distribution and variations of the monthly mesopause temperature are presented during 2002–2019 covering the latitudes of 83°S to 83°N based on Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations. To investigate the long‐term trend and solar response of the mesopause temperature, a three‐component harmonic fit is first applied to remove the seasonal variation from the monthly temperature data series. Then a multiple linear regression model is performed to residual temperatures versus constant, linear trend, solar activity, and geomagnetic activity terms. In this study, the mesopause temperature shows a cooling trend through all latitudes ranging from ~0 to −0.14 K/year with a mean of −0.075 ± 0.043 K/year. The cooling trends in the Southern Hemisphere are stronger than those in the Northern Hemisphere. For high latitudes (60–80°), significant negative trends can be observed during nonsummertime, while no significant trends are found for summertime. The mesopause temperature shows apparent positive responses to solar activity through all latitudes ranging from 3.03 to 4.80 K per 100 solar flux units (sfu) with a mean of 3.99 ± 0.49 K per 100 sfu, which is more significant and stable in the Northern Hemisphere. There is a pronounced drop for mesopause height at polar latitudes, which reflects the shrinking effect at lower altitudes mainly caused by greenhouse gas cooling. We show that the length of the time interval analyzed strongly influences the results. Our results, obtained from 18‐year SABER observations, are expected to be a robust measure of the mesopause temperature variability.

Intensification of the Atlantic Multidecadal Variability Since 1870: Implications and Possible Causes

Thu, 06/04/2020 - 19:22
Abstract

The Atlantic multidecadal variability (AMV) acts as a key source of multidecadal variability and plays an important role in climate change in many regions of the Northern Hemisphere, resulting in great social and economic impacts. This paper shows that, since the late 19th century, the AMV has experienced a significant increasing trend. The AMV intensified at a rate of 0.0040°C per decade from 1870 to 2014, which is statistically significant at the 95% confidence level. The AMV teleconnects the amplified signal from the North Atlantic across the Eurasian continent to North America through a circumglobal teleconnection pattern, which further intensifies the regional climate variability and climate extremes in North Africa, Europe, East Asia, and North America. Results from the single‐forcing and all‐but‐one‐forcing experiments of the Community Earth System Model, version 1, indicate that anthropogenic aerosol emissions have played a more important role in the AMV intensification compared to other external forcings, especially during the period from the 1900s to 1990s.

Mobile Laboratory Measurements of High Surface Ozone Levels and Spatial Heterogeneity During LISTOS 2018: Evidence for Sea Breeze Influence

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

The Long Island Sound Tropospheric Ozone Study (LISTOS) was organized to investigate ozone formation and transport in the New York City metropolitan area and locations downwind. During LISTOS, the University at Albany Atmospheric Sciences Research Center (ASRC) mobile laboratory was used for measuring surface O3, NO2, and aerosol number and mass concentration. Sharp O3 concentration gradients, with ΔO3 Δy−1 over 15 ppb km−1, were measured both at and near the land‐water interface and on the highway on days characterized by high regional O3 concentrations. These large O3 gradients at or near the land‐water interface, and in air masses relatively low in NO2, are shown to be influenced in part by the transport of highly oxidized air masses via sea breeze circulation and convergence with gradient flow. On the highway under regionally high O3 concentrations, strong anticorrelation (R 2 = 0.78, p  < 0.05) between O3 and NO2 and an absolute slope less than 1 suggested that Ox concentrations (O3 + NO2) increased with increasing NO2. Overall, the on‐road measurements made during LISTOS help to better characterize the interaction between the emitted pollution and the meteorological conditions on Long Island, thereby having potential policy implications.

Ozone Production in the Soberanes Smoke Haze: Implications for Air Quality in the San Joaquin Valley During the California Baseline Ozone Transport Study

Thu, 06/04/2020 - 09:55
Abstract

The Soberanes Fire burned 53,470 ha (132,127 acres) along the central California coast between 22 July and 12 October 2016, generating dense smoke and a variety of gaseous compounds that drifted eastward into the San Joaquin Valley Air Basin (SJVAB), an “extreme” nonattainment area for ozone (O3). These gases included nitrogen oxides (NO x ) and volatile organic compounds, the photochemical precursors of O3. The fire started during the California Baseline Ozone Transport Study, a field campaign that brought aircraft, surface, and remote sensing measurements of O3 and related species to central California. In this paper, we use the California Baseline Ozone Transport Study measurements to assess the impact of the Soberanes Fire on ozone and particulate air quality in the SJVAB. We focus our analysis on 27 July to 2 August when the smoke haze was heaviest and the highest O3 concentrations in the SJVAB during 2016 were recorded. Our analyses suggest that while 40 to 60 ppbv of fire‐generated O3 was transported to the eastern SJVAB in the 1‐ to 3‐km‐altitude range, relatively little smoke or fire‐generated O3 reached the surface in the Visalia area.

Increased dust aerosols in the high troposphere over the Tibetan Plateau from 1990s to 2000s

Wed, 06/03/2020 - 19:45
ABSTRACT

The dust aerosols are a major type of aerosol over the Tibetan Plateau (TP) and influence climate at local to regional scales through their effects on thermal radiation and snow‐albedo feedback. Based on the Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2) aerosol dataset, we report an increase of 34% in the atmospheric dust in the high troposphere over the TP during the spring season in the 2000s in comparison to the 1990s. This result is supported by an increase of 157% (46%) in the dust deposition flux in the Mugagangqiong (Tanggula) ice cores and an increase of 69% in the Aerosol Index (AI) from Earth Probe (EP) Total Ozone Mapping Spectrometer (TOMS), as well as by increases of simulated dust aerosols over the TP derived from the Community Earth System Model (CESM) and models from the Coupled Model Intercomparison Project Phase 6 (CMIP6). The increased atmospheric dust over the TP is caused in two aspects: (1) there was a higher dust emission over the Middle East during the 2000s than during the 1990s, which is explained by less precipitation and 25.8% higher in cyclone frequency over the Middle East. The increased cyclones uplift more dust from the surface over the Middle East to the central Asia in the middle troposphere. (2) Enhanced mid‐latitude zonal winds help transport more dust in the middle troposphere from the central Asia to the Northwest China and thereafter an increase in northerly winds over Northwest China propels dust southward to the TP.

Black Carbon and Precipitation: an Energetics Perspective

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

Black carbon aerosols (BC) influence precipitation through a range of processes. The climate response to the presence of BC is however highly dependent on its vertical distribution. Here, we analyze the changes in the energy budget and precipitation impacts of adding a layer of BC at a range of altitudes in two independent global climate models. The models are run with atmosphere‐only and slab ocean model setup to analyze both fast and slow responses, respectively. Globally, precipitation changes are tightly coupled to the energy budget. We decompose the precipitation change into contributions from absorption of solar radiation, atmospheric longwave radiative cooling, and sensible heat flux at the surface. We find that for atmosphere‐only simulations, BC rapidly suppress precipitation, independent of altitude, mainly because of strong atmospheric absorption. This reduction is offset by increased atmospheric radiative longwave cooling and reduced sensible heat flux at the surface, but not of sufficient magnitude to prevent reduced precipitation. On longer timescales, when the surface temperature is allowed to respond, we find that the precipitation increase associated with surface warming can compensate for the initial reduction, particularly for BC in the lower atmosphere. Even though the underlying processes are strikingly similar in the two models, the resulting change in precipitation and temperature by BC differ quite substantially.

Particle‐Size Distributions and Solubility of Aerosol Iron Over the Antarctic Peninsula During Austral Summer

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

This study provides new data on the properties of aerosol iron (Fe) over the Antarctic Peninsula, one of the fastest warming regions on Earth in recent decades. Atmospheric deposition delivers Fe, a limiting micronutrient, to the Southern Ocean, and aerosol particle size influences the air‐to‐sea deposition rate and fractional solubility of aerosol Fe. Size‐segregated aerosols were collected at Palmer Station on the West Antarctic Peninsula during austral summer 2016–2017. Results show single‐mode size distribution of aerosol Fe, peaking at 4.4 μm diameter. The average concentration of total aerosol Fe was 1.3 (±0.40) ng m−3 (range 0.74–1.8 ng m−3). High concentrations of total aerosol Fe occurred in January, implying increased Fe source strength then. Total labile Fe varied between 0.019 and 0.095 ng m−3, and labile Fe (II) accounted for ~90% of the total labile Fe. The average fractional solubility for total Fe was 3.8% (±1.5%) (range 2.5–7.3%). Estimated dry deposition fluxes for the study period were 3.2 μg m−2 year−1 for total labile Fe and 83 μg m−2 year−1 for total Fe in aerosols. We speculate that local and regional dust sources in Antarctica contributed to the observed aerosol Fe in austral summer and that warming on the Antarctic Peninsula during the past half century may have increased the formation of dust sources in this region. The potential biogeochemical impact of atmospheric Fe input to the West Antarctic Peninsula shelf waters and adjacent pelagic surface waters of the Southern Ocean may need to be re‐evaluated.

Understanding the Interfacial Behavior of Typical Perfluorocarboxylic Acids at Surfactant‐Coated Aqueous Interfaces

Wed, 06/03/2020 - 15:14
Abstract

Sea surface microlayers (SMLs) play an important role in the transport and fate of surfactants from both anthropogenic and biogenic sources. The formation of sea spray aerosols (SSAs) at the SML has been recognized as a critical pathway for the entry of surfactants into the atmosphere. Perfluorocarboxylic acids (PFCAs) have become increasingly concerning as a class of emerging organic pollutants, having high enrichment in surface seawaters due to their remarkable surface activity. Using artificial seawater coated by extensively existed surfactants, including stearyl alcohol (C18OH) and stearic acid (SA) as simplified models of the SML and fresh SSAs, the interfacial behaviors of soluble and insoluble PFCAs were examined. Information about lateral packing and chain conformation of films was obtained by means of Langmuir trough and infrared reflection‐absorption spectroscopy (IRRAS). Perfluorooctanoic acid (PFOA) dissolved in aqueous subphases was found to be incorporated into lipid monolayers via alterations in surface pressure‐area (π−A) isotherms. With negligible solubility, perfluorotetradecanoic acid (PFTA) formed condensed mixed monolayers with other lipids at the air‐water interface. The mixed monolayers of C18OH/PFTA and SA/PFTA were expanded by the addition of sea salts into the pure water subphase. Compared with the π−A isotherms of SA/PFTA, decreased molecular areas were observed for those of C18OH/PFTA. This research suggests that PFCAs can be incorporated into air‐water interfaces by acting as film‐forming materials. The introduction of PFCAs will alter the surface properties and thus the atmospheric fate and behavior of SSAs and SMLs.

Characterizing weekly cycles of particulate matter in a coastal megacity: The importance of a seasonal, size‐resolved, and chemically‐speciated analysis

Wed, 06/03/2020 - 10:15
Abstract

We present the first study of the weekly cycles (WC) of chemically‐speciated and size‐resolved particulate matter (PM) in Metro Manila, Philippines, a coastal megacity located within a highly complex meteorological environment that is subject to both anthropogenic and natural sources. To measure PM, Micro‐Orifice Uniform Deposit Impactors (MOUDIs) were deployed in Metro Manila from August 2018 to October 2019 and samples were analyzed for ionic and elemental species, including black carbon (BC). The WC in Metro Manila varied remarkably across seasons, linked to shifts in meteorology, transport, and aerosol source. Identified aerosol sources were traffic, local and regional burning, dust, sea salt, and secondary aerosol formation. Direct emissions induced a late workweek peak while secondary aerosol formation led to a weekend peak in response to precursor buildup mainly from traffic. Seasonal analysis revealed that local burning from solid waste management and agricultural fires induced a strong WC peak while regional burning emissions from the Maritime Continent (MC) and possibly the Asian continent elevated seasonal baseline concentrations of the WC. BC showed a seasonally persistent WC, consistent in magnitude, weekly peak timing, and particle size. The dominant submicrometer WC and the contribution of BC across seasons have important ramifications on public health and policymaking, which are also discussed. As many of the observed WC patterns are undetectable when using only bulk PM, this study demonstrates that a seasonal, size‐resolved, and chemically‐speciated characterization is required to more fully understand the driving mechanisms governing WCs.

Asymmetrical Linkages between Multi‐frequency Atmospheric Waves and Variations in Winter PM2.5 Concentrations in Northern China during 2013–2019

Wed, 06/03/2020 - 09:23
Abstract

Atmospheric waves have a broad spectrum of momentum at various frequencies and link to the PM2.5 concentrations in northern China. We show that the meaningful circulations depended on the wave frequency and the rank of the PM2.5 concentrations. The winter 2013–2019 PM2.5 had different correlations with the subseasonal 850 hPa height anomalies at low (>30 days), intermediate (10–30 days), and high (<10 days) frequencies. The low‐frequency waves increased PM2.5 concentrations by lowering the height over Eurasia and raising the height over the North Atlantic. The first two singular value decomposition (SVD) principal patterns at the low‐frequency were alike wavenumber 3 and 4 circulations, respectively. The two patterns accounted for the majority of the background variations of winter PM2.5 in China. The intermediate‐frequency waves increased PM2.5 concentrations via an anomalous dipole pattern over East Asia with a positive polarity over Japan and a negative polarity over Mongolia. The positive polarity is indicative of persistent haze episodes. The high‐frequency waves showed a small dipole correlation pattern with PM2.5, but they barely accounted for high PM2.5 concentrations. The high‐frequency waves traveled quickly. They raised or lowered PM2.5 concentrations in a short time, preventing particulate accumulations. The anomalous heights at the low‐ and intermediate‐frequency for the PM2.5 concentrations >90th showed a few characteristics consistent with the circulation anomalies proposed in previous climate studies with monthly mean data. Synergic effects of the waves at different frequencies result in the observed PM2.5 variations.

Zonal Similarity of Long‐term Changes and Seasonal Cycles of Baseline Ozone at Northern Mid‐latitudes

Wed, 06/03/2020 - 08:48
Abstract

The lifetime of ozone in the troposphere is approximately three weeks. Prevailing westerly winds at northern mid‐latitudes can transport air around the globe in that time. Hence, within these latitudes zonal similarity is expected in long‐term changes and seasonal cycles of concentrations of baseline ozone. We quantify the degree of zonal similarity by examining eight in situ baseline ozone data sets near the west coasts of North America and Europe, i.e. upwind of those continents and downwind of the Pacific and Atlantic Oceans, where the impacts of local and regional ozone sources have been largely mixed into the troposphere, giving the best‐defined baseline ozone signature. Zonal similarity is found in both long‐term changes and seasonal cycles. The decades‐long increase in northern hemisphere, mid‐latitude baseline mixing ratios (average ~ 0.60 ppb yr‐1 from 1980‐2000) has ended, with a maximum reached in the mid‐2000s, followed by slow decrease (average = ‐0.09 ± 0.08 ppb yr‐1 from 2000 to the present). The year of the ozone maximum exhibits little if any statistically significant difference with location, altitude or season. The ozone seasonal cycle differs markedly between sea level coastal stations representative of the marine boundary layer and the free troposphere sampled at elevated sites and by sondes and aircraft. However, within each of these broad tropospheric layers, the seasonal cycles are similar at all locations. Vertical profiles of the parameters that define the long‐term trends and the seasonal cycle are also similar between North America and Europe.

Issue Information

Tue, 06/02/2020 - 19:00

No abstract is available for this article.

An evaluation of the large scale atmospheric circulation and its variability in the Community Earth System Model 2 (CESM2) and other CMIP models

Tue, 06/02/2020 - 10:35
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 provided through 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 strength, stationary waves and blocking, although the SH jet stream is placed at approximately the correct location. This analysis also highlights systematic deficiencies in these features across the new CMIP6 archive, such as the continued tendency for the SH jet stream to be placed too far equatorward, the North Atlantic westerlies to be too strong over Europe, the storm tracks as measured by low level meridional wind variance to be too weak and a lack of blocking in the North Atlantic sector.

Characterization of aerosol type over East Asia by 4.4 km Multi‐angle Imaging SpectroRadiometer (MISR) product: first insight and general performance

Mon, 06/01/2020 - 13:29
Abstract

The lack of aerosol type information has largely hindered satellite products from further applications such as constraining model simulations and quantifying aerosol climate effects. The recent Version (V) 23 MISR aerosol products with an enhanced spatial resolution at 4.4 km enable an unprecedented chance to explore aerosol types and associated processes in the regional scale. Here we provide a comprehensive insight into the characterization of MISR aerosol optical and microphysical properties, as well as their performance, over East Asia. Ground validation shows a remarkable improvement in the accuracy of V23 MISR AOD with ~80% of its retrieval bias within ±(0.05+20%AODAERONET). However, an underestimation of MISR AOD is still prevalent in the high‐AOD (>0.6) conditions, due to the surface‐atmosphere separation problem and insufficient absorbing aerosol mixtures being selected. MISR AOD of different size bins agrees well with AERONET results, demonstrating an evident advantage in discriminating natural dust from anthropogenic particles. Although MISR nonspherical and absorbing retrievals display a consistent variation with the AERONET inversions, their component AODs have a poor reliability over East Asia due to the inappropriate aerosol component models or their mixtures as in V22. In particular, the most striking problem is the sparse and discrete MISR absorbing retrievals with spatial discontinuity. Generally, the high‐resolution V23 MISR products exhibit a great potential in characterizing the regional variations of aerosol type, which can be further refined by considering the prior aerosol knowledge over East Asia.

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