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Zhao, Chun

Publications and source records attributed to Zhao, Chun.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Southern Himalayas rainfall as a key driver of interannual variation of pre-monsoon aerosols over the Tibetan Plateau

The Tibetan Plateau (TP) is one of the most climate-sensitive regions around the world. Aerosols imported from adjacent regions reach their peak during the pre-monsoon season and play a vital role in the TP environment. However, the strong interannual variation in aerosols transported to the TP has not been fully understood. Here, we show that the interannual variability of pre-monsoon aerosols transported to the TP is influenced more by rainfall over the southern Himalayas than near-surface wind. Rainfall modulates fire events and biomass burning emissions and reduces aerosols over the TP by wet scavenging. Contrary to the role of wind in increasing aerosol transport, the positive correlation between wind and aerosols in the TP reported in previous studies is contributed by the negative interannual correlations between wind and rainfall and between rainfall and fire events over the southern Himalayas. This study highlights the co-variability of wind and rainfall and their confounding impacts on aerosols in the southern Himalayas and over the TP. With pre-monsoon rainfall projected to increase in adjacent regions of southern TP, aerosol transport to the TP may be mitigated in the future.

Liu, Weichien↗
DOE OSTI2023

Technical descriptions of the experimental dynamical downscaling simulations over North America by the CAM–MPAS variable-resolution model

Abstract. Comprehensive assessment of climate datasets is important for communicating model projections and associated uncertainties to stakeholders. Uncertainties can arise not only from assumptions and biases within the model but also from external factors such as computational constraint and data processing. To understand sources of uncertainties in global variable-resolution (VR) dynamical downscaling, we produced a regional climate dataset using the Model for Prediction Across Scales (MPAS; dynamical core version 4.0) coupled to the Community Atmosphere Model (CAM; version 5.4), which we refer to as CAM–MPAS hereafter. This document provides technical details of the model configuration, simulations, computational requirements, post-processing, and data archive of the experimental CAM–MPAS downscaling data. The CAM–MPAS model is configured with VR meshes featuring higher resolutions over North America as well as quasi-uniform-resolution meshes across the globe. The dataset includes multiple uniform- (240 and 120 km) and variable-resolution (50–200, 25–100, and 12–46 km) simulations for both the present-day (1990–2010) and future (2080–2100) periods, closely following the protocol of the North American Coordinated Regional Climate Downscaling Experiment. A deviation from the protocol is the pseudo-warming experiment for the future period, using the ocean boundary conditions produced by adding the sea surface temperature and sea-ice changes from the low-resolution version of the Max Planck Institute Earth System Model (MPI-ESM-LR) in the Coupled Model Intercomparison Project Phase 5 to the present-day ocean state from a reanalysis product. Some unique aspects of global VR models are evaluated to provide background knowledge to data users and to explore good practices for modelers who use VR models for regional downscaling. In the coarse-resolution domain, strong resolution sensitivity of the hydrological cycles exists over the tropics but does not appear to affect the midlatitude circulations in the Northern Hemisphere, including the downscaling target of North America. The pseudo-warming experiment leads to similar responses of large-scale circulations to the imposed radiative and boundary forcings in the CAM–MPAS and MPI-ESM-LR models, but their climatological states in the historical period differ over various regions, including North America. Such differences are carried to the future period, suggesting the importance of the base state climatology. Within the refined domain, precipitation statistics improve with higher resolutions, and such statistical inference is verified to be negligibly influenced by horizontal remapping during post-processing. Limited (≈50 % slower) throughput of the current code is found on a recent many-core/wide-vector high-performance computing system, which limits the lengths of the 12–46 km simulations and indirectly affects sampling uncertainty. Our experience shows that global and technical aspects of the VR downscaling framework require further investigations to reduce uncertainties for regional climate projection.

58 GEOSCIENCES↗
DOE OSTI2022

Simulations of aerosol pH in China using WRF-Chem (v4.0): sensitivities of aerosol pH and its temporal variations during haze episodes

Aerosol pH is a fundamental property of aerosols in terms of atmospheric chemistry and its impact on air quality, climate, and health. Precise estimation of aerosol pH in chemical transport models (CTMs) is critical for aerosol modeling and thus influences policy development that partially relies on results from model simulations. We report the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) simulated PM 2.5 pH over China during a period with heavy haze episodes in Beijing, and explore the sensitivity of the modeled aerosol pH to factors including emissions of nonvolatile cations (NVCs) and NH 3 , aerosol phase state assumption, and heterogeneous production of sulfate. We find that default WRF-Chem could predict spatial patterns of PM 2.5 pH over China similar to other CTMs, but with generally lower pH values, largely due to the underestimation of alkaline species (NVCs and NH 3 ) and the difference in thermodynamic treatments between different models. Increasing NH 3 emissions in the model would improve the modeled pH in comparison with offline thermodynamic model calculations of pH constrained by observations. In addition, we find that the aerosol phase state assumption and heterogeneous sulfate production are important in aerosol pH predictions for regions with low relative humidity (RH) and high anthropogenic SO 2 emissions, respectively. These factors should be better constrained in model simulations of aerosol pH in the future. Analysis of the modeled temporal trend of PM 2.5 pH in Beijing over a haze episode reveals a clear decrease in pH from 5.2 ± 0.9 in a clean period to 3.6 ± 0.5 in a heavily polluted period. The increased acidity under more polluted conditions is largely due to the formation and accumulation of secondary species including sulfuric acid and nitric acid, even though being modified by alkaline species (NVCs, NH 3 ). Our result suggests that NO 2 oxidation is unlikely to be important for heterogeneous sulfate production during the Beijing haze as the effective pH for NO 2 oxidation of S(IV) is at a higher pH of ~6.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Convection-permitting hindcasting of diurnal variation of Mei-yu rainfall over East China with a global variable-resolution model

Two sets of 48-hr hindcast experiments for Mei-yu season using non-hydrostatic global variable-resolution model (MPAS-Atmosphere) with regional refinement at 4km resolution (V4km) and uniform 60km resolution (U60km) are first-time conducted to attempt at simulating the Mei-yu rainfall in 2015 over East China. The analysis focuses on the seasonal average of diurnal variation of precipitation amount (PA), intensity (PI), and frequency (PF). Both simulations reasonably reproduce the spatial distribution of PA. V4km is more skillful in simulating the spatial distributions and magnitudes of PF and PI. The diurnal cycle of Mei-yu rainfall shows a major early morning peak and a minor afternoon peak, contributed by precipitation during two sub-periods with distinct synoptic circulations. With strong Mei-yu in the first sub-period, the diurnal variation of PA is controlled by nocturnal southwesterly jet. V4km overestimates the morning peak mainly due to its bias in simulating boundary layer inertial oscillation. Although differences in PA between the simulations are small, U60km overestimates PF and underestimates PI. With weak Mei-yu in the second sub-period, the diurnal variation of PA is controlled by both synoptic circulation and local convection. At both resolutions, deviations in the large-scale circulation modulated by a few typhoons lead to positive biases in the morning peak of PA. After removing the typhoon impacts, V4km captures the observed diurnal cycle of PA well, while U60km significantly underestimates PA and PI particularly in the afternoon. Future studies focusing on advancing modeling of southwesterly jet and typhoons may further improve convection permitting simulation of Mei-yu rainfall.

Xu, Mingyue↗
DOE OSTI2020

High concentration of ultrafine particles in the Amazon free troposphere produced by organic new particle formation

The large concentrations of ultrafine particles consistently observed at high altitudes over the tropics represent one of the world’s largest aerosol reservoirs, which may be providing a globally important source of cloud condensation nuclei. However, the sources and chemical processes contributing to the formation of these particles remain unclear. Here we investigate new particle formation (NPF) mechanisms in the Amazon free troposphere by integrating insights from laboratory measurements, chemical transport modeling, and field measurements. To account for organic NPF, we develop a comprehensive model representation of the temperature-dependent formation chemistry and thermodynamics of extremely low volatility organic compounds as well as their roles in NPF processes. We find that pure-organic NPF driven by natural biogenic emissions dominates in the uppermost troposphere above 13 km and accounts for 65 to 83% of the column total NPF rate under relatively pristine conditions, while ternary NPF involving organics and sulfuric acid dominates between 8 and 13 km. The large organic NPF rates at high altitudes mainly result from decreased volatility of organics and increased NPF efficiency at low temperatures, somewhat counterbalanced by a reduced chemical formation rate of extremely low volatility organic compounds. These findings imply a key role of naturally occurring organic NPF in high-altitude preindustrial environments and will help better quantify anthropogenic aerosol forcing from preindustrial times to the present day.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

Impact of Dust-Cloud-Radiation-Precipitation Dynamical Feedback on Subseasonal-to-Seasonal Variability of the Asian Summer Monsoon in Global Variable-Resolution Simulations With MPAS-CAM5

In this study, we investigate the effects of increased dust emission from the Middle East deserts on subseasonal-to-seasonal (S₂S) variability of the Asian summer monsoon (ASM). Numerical experiments are performed using the Model for Prediction Across Scales (MPAS) coupled with the Community Atmosphere Model (CAM5) physics, with regional refinement at 30 km grid resolution over South Asia and the surrounding regions. Result shows that increased dust emission and transport from the Middle East/West Asia region induces a strong dust-cloud-radiation-precipitation-circulation feedback, resulting in a colder surface over the desert regions and the western Tibetan Plateau, but warmer and moister troposphere with enhanced cloudiness and precipitation over the Pakistan/Northwest India (PNWI) region. The latter changes are amplified by the dust aerosol induced Elevated Heat Pump (EHP) mechanism along the West Himalayas/Iranian Plateau foothill regions, most pronounced during May-June. During July-August, cloud radiation feedback further enhances the warming of the upper troposphere, and cooling of the land surface over the PNWI and adjacent regions over West Asia. The upper tropospheric heating and increased precipitation over PNWI spur a large-scale anomalous Rossby wavetrain and a northward displacement of the subtropical jetstream, manifesting in a contraction of the South Asian High and westward shift of the Western Pacific Subtropical High. As a result, the entire ASM precipitation-cloud system is displaced westward. Precipitation and cloudiness are intensified over northwest and western India and west Asia, but suppressed over southern and central East Asia. Analyses of the S₂S variability of the upper level vorticity balance suggests that heating by Middle East dust plays an important role in exciting, and anchoring a teleconnection pattern through interactions among dust-cloud radiation, precipitation heating over the PNWI, the development of an upper level Rossby wavetrain, and the northward shift of the boreal summer jetstream over Eurasia.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Trans-Pacific transport and evolution of aerosols: spatiotemporal characteristics and source contributions

Aerosols in the middle and upper troposphere have a long enough lifetime for trans-Pacific transport from East Asia to North America to influence air quality on the west coast of the United States (US). Here, we conduct quasi-global simulations (180 °W–180 °E and 70 °S–75 °N) from 2010 to 2014 using an updated version of WRF-Chem (Weather Research and Forecasting model fully coupled with chemistry) to analyze the spatiotemporal characteristics and source contributions of trans-Pacific aerosol transport. We find that trans-Pacific total aerosols have a maximum mass concentration (about 15 µg m –3 ) in the boreal spring with a peak between 3 and 4 km above the surface around 40 °N. Sea salt and dust dominate the total aerosol mass concentration below 1 km and above 4 km, respectively. About 80.8 Tg of total aerosols (48.7 Tg of dust) are exported annually from East Asia, of which 26.7 Tg of aerosols (13.4 Tg of dust) reach the west coast of the US. Dust contributions from four desert regions in the Northern Hemisphere are analyzed using a tracer-tagging technique. About 4.9, 3.9, and 4.5 Tg year –1 of dust aerosol emitted from north Africa, the Middle East and central Asia, and East Asia, respectively, can be transported to the west coast of the US. The trans-Pacific aerosols dominate the column-integrated aerosol mass (~65.5 %) and number concentration (~80 %) over western North America. Radiation budget analysis shows that the inflow aerosols could contribute about 86.4 % (–2.91 W m –2 ) at the surface, 85.5 % (+1.36 W m –2 ) in the atmosphere, and 87.1 % (–1.55 W m –2 ) at the top of atmosphere to total aerosol radiative effect over western North America. However, near the surface in central and eastern North America, aerosols are mainly derived from local emissions, and the radiative effect of imported aerosols decreases rapidly. This study motivates further investigations of the potential impacts of trans-Pacific aerosols from East Asia on regional air quality and the hydrological cycle in North America.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Modeling the Impacts of Urbanization on Summer Thermal Comfort: The Role of Urban Land Use and Anthropogenic Heat

Urban land use and anthropogenic heat (AH) emission can considerably influence the human thermal comfort during extreme heat events. In this study, a spatially heterogeneous AH emission data and updated urban land use data are integrated into the Weather Research and Forecasting model to simulate the physical processes of urban warming during summer. Simulations conducted in the Yangtze River Delta (YRD) of east China suggest that the mean urban heat island intensity reaches 1.49 °C in urbanized areas during summer, with AH emission making a considerable contribution. The warming effect due to urban land use is intensified during extremely hot days, but in contrast, the AH effects are slightly reduced. Urban development increases the total thermal discomfort hours by 27% in the urban areas of YRD, with AH and urban land use contributing nearly equal amount. By limiting the daytime latent heat release, urban land use reduces the daily maximum heat stress particularly during extremely hot days; however, such alleviations can be offset by the AH emission. We conclude strategies for mitigation of urban heat island effect and heat stress in cities should therefore include measures to reduce AH emission.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Modeling the contributions of Northern Hemisphere dust sources to dust outflow from East Asia

Mineral dust aerosols can substantially influence the Earth's climate by altering the radiation budget and modifying cloud microphysical and radiative properties. Through long-range transport, dust aerosols could have a global impact. Here, using a fully coupled meteorology-chemistry model (WRF-Chem) and the tracer-tagging technique, we conduct quasi-global simulations to investigate the characteristics of dust intercontinental transport in the Northern Hemisphere and the source contributions to dust outflow from East Asia. Model results show that total dust emission from the main deserts (i.e., North Africa desert (NFD), Middle East desert (MED) and East Asia desert (EAD)) is about 4531 Tg yr-1, in which 66% is from NFD, 24% is from MED and 10% is from EAD. During long-range transport, the NFD and MED dust plumes are separated into two branches by the Tibetan Plateau, with higher dust concentration of 4 µgm-3 in the northern branch (37° N-60° N). The imported dust mass from NFD and MED to East Asia is 16.0 Tg yr-1 and 33.8 Tg yr-1, respectively; the local dust mass emitted from EAD can circumnavigate and import about 0.41 Tg yr-1 of dust into East Asia. As part of the “Asian outflow”, the MED and NFD dust is transported above 600 hPa with concentration of 2 µgm-3, while the EAD dust is mainly transported below 600 hPa with concentration of 3 µgm-3. The total outflow dust mass from East Asia is 12.0, 9.7 and 7.7 Tg yr-1 from EAD, NFD and MED, respectively. Dust particles dominate the aerosol mass concentration with a fraction of 56.5% in southern East Asia and 75.4% in northern East Asia. Moreover, dust number from EAD dominates the total aerosol number north of 37oN below 600 hPa, with a size range of 0.156–0.625 µm, but the NFD dust number is mainly above 600 hPa. MED dust number dominates the total column aerosol number south of 37 oN, with a size range of 0.312–0.625 µm. Quantifying dust source contributions and the associated dust number loading and size distribution over East Asia is important for understanding the role of East Asian dust outflow in climate and hydrological cycle.

dust aerosol, mass flux, source contribution, dust↗
DOE OSTI2019

Urban pollution greatly enhances formation of natural aerosols over the Amazon rainforest

One of the least understood aspects in atmospheric chemistry is how urban emissions influence the formation of natural organic aerosols, which affect Earth’s energy budget. The Amazon rainforest, during its wet season, is one of the few remaining places on Earth where atmospheric chemistry transitions between preindustrial and urban pollution-influenced conditions. Here we integrate insights from several laboratory measurements and simulate the formation of secondary organic aerosols (SOA) in the Amazon using a high-resolution chemical transport model. Simulations show that urban emissions of oxides of nitrogen from Manaus, a city of ~2 million people, greatly enhance production of biogenic SOA by 60-200% on average with peak enhancements of 400%, through the increased oxidation of gas-phase organic carbon emitted by the forests. Simulated enhancements agree with aircraft measurements, and are much larger than those reported over other locations. Furthermore, the implication is that increasing anthropogenic emissions in future years might substantially enhance biogenic SOA formation in pristine locations like the Amazon.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Urbanization effect on winter haze in the Yangtze River Delta Region of China

The impact of urbanization-induced land-cover change and increase in anthropogenic emissions on the air quality of the megacity cluster of the Yangtze River Delta (YRD) is investigated using the WRF-Chem model coupled with an Urban Canopy Model at cloud-resolving resolution. The urban land-cover effect results in considerable reduction of near-surface aerosol concentrations over urban regions and an increase in particle concentrations at higher altitudes over the surrounding rural areas. The UHI effect increases the lower atmospheric instability and vertical velocity, thus increasing the planetary boundary layer height and ventilation over the urban area, favoring the dispersion of pollutants from urbanized areas to their immediate vicinities. However, the urbanization-induced increases in aerosol emissions outweigh that of land-cover change, resulting in a net increase in surface particle concentrations by up to 50 µg m -3 and the occurrence of winter haze by more than 10 d season-1 in the YRD region.

urbanization-induced land-cover change, Urban Cano↗
DOE OSTI2018

New particle formation leads to cloud dimming

New particle formation (NPF), nucleation of condensable vapors to the solid or liquid phase, is a significant source of atmospheric aerosol particle number concentrations. With sufficient growth, these nucleated particles may be a significant source of cloud condensation nuclei (CCN), thus altering cloud albedo, structure, and lifetimes, and insolation reaching the Earth's surface. Herein we present one of the first numerical experiments to quantify the impact of NPF on cloud radiative properties that is conducted at a convection permitting resolution and that explicitly simulates cloud droplet number concentrations. Consistent with observations, these simulations suggest that in spring over the Midwestern U.S.A., NPF occurs frequently and on regional scales. However, the simulations suggest that NPF is not associated with enhancement of regional cloud albedos as would be expected from an increase of CCN. These simulations indicate that NPF reduces ambient sulfuric acid concentrations sufficiently to inhibit growth of preexisting particles to CCN sizes. This reduction in CCN-sized particles reduces cloud albedo, resulting in a domain average positive top of atmosphere cloud radiative forcing of 10 W m-2 and up to ~ 50 W m-2 in individual grid cells relative to a simulation in which NPF is excluded.

Sullivan, Ryan C. (ORCID:0000000235001842)↗
DOE OSTI2018

Impacts of aerosols on seasonal precipitation and snowpack in California based on convection-permitting WRF-Chem simulations

Here, a version of the WRF-Chem model with fully coupled aerosol–meteorology–snowpack is employed to investigate the impacts of various aerosol sources on precipitation and snowpack in California. In particular, the impacts of locally emitted anthropogenic and dust aerosols, and aerosols transported from outside California are studied. We differentiate three pathways of aerosol effects: aerosol–radiation interaction (ARI), aerosol–snow interaction (ASI), and aerosol–cloud interaction (ACI). The convection-permitting model simulations show that precipitation, snow water equivalent (SWE), and surface air temperature averaged over the whole domain (34–42° N, 117–124° W, not including ocean points) are reduced when aerosols are included, therefore reducing large biases in these variables due to the absence of aerosol effects in the model. Aerosols affect California water resources through the warming of mountaintops and the reduction of precipitation; however, different aerosol sources play different roles in changing surface temperature, precipitation, and snowpack in California by means of various weights of the three pathways. ARI by all aerosols mainly cools the surface, leading to slightly increased SWE over the mountains. Locally emitted dust aerosols warm the surface of mountaintops through ASI, in which the reduced snow albedo associated with dusty snow leads to more surface absorption of solar radiation and reduced SWE. Transported aerosols and local anthropogenic aerosols play a dominant role in increasing nonprecipitating clouds but reducing precipitation through ACI, leading to reduced SWE and runoff on the Sierra Nevada, as well as the warming of mountaintops associated with decreased SWE and hence lower surface albedo. The average changes in surface temperature from October 2012 to June 2013 are about –0.19 and 0.22 K for the whole domain and over mountaintops, respectively. Overall, the averaged reduction during October to June is about 7 % for precipitation, 3 % for SWE, and 7 % for surface runoff for the whole domain, while the corresponding numbers are 12, 10, and 10 % for the mountaintops. The reduction in SWE is more significant in a dry year, with 9 % for the whole domain and 16 % for the mountaintops. The maximum reduction of ~20 % in precipitation occurs in May and is associated with the maximum aerosol loading, leading to the largest decrease in SWE and surface runoff over that period. It is also found that dust aerosols can cause early snowmelt on the mountaintops and reduced surface runoff after April.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Biomass smoke from southern Africa can significantly enhance the brightness of stratocumulus over the southeastern Atlantic Ocean

Significance Each year biomass burning (BB) aerosols emitted in southern Africa during the fire season (July–October) transport westward over the southeast Atlantic Ocean and interact with underlying stratocumulus decks. Previous studies showed that BB aerosols greatly perturb the top of atmosphere radiation balance by scattering and absorbing solar radiation and by altering cloud properties via changing the lower tropospheric stability (direct and semidirect effects). Using a state-of-the-art model in combination with satellite observations, we found that BB aerosols that are entrained into the clouds function as cloud condensation nuclei, and increase the brightness of stratocumulus clouds (indirect effect). In these models, this indirect effect dominates the direct and semidirect effects of BB aerosols over this region.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

How Do Microphysical Processes Influence Large-Scale Precipitation Variability and Extremes?

Convection permitting simulations using the Model for Prediction Across Scales-Atmosphere (MPAS-A) are used to examine how microphysical processes affect large-scale precipitation variability and extremes. An episode of the Madden-Julian Oscillation is simulated using MPAS-A with a refined region at 4-km grid spacing over the Indian Ocean. It is shown that cloud microphysical processes regulate the precipitable water (PW) statistics. Because of the non-linear relationship between precipitation and PW, PW exceeding a certain critical value (PWcr) contributes disproportionately to precipitation variability. However, the frequency of PW exceeding PWcr decreases rapidly with PW, so changes in microphysical processes that shift the column PW statistics relative to PWcr even slightly have large impacts on precipitation variability. Furthermore, precipitation variance and extreme precipitation frequency are approximately linearly related to the difference between the mean and critical PW values. Thus observed precipitation statistics could be used to directly constrain model microphysical parameters as this study demonstrates using radar observations from DYNAMO field campaign.

58 GEOSCIENCES↗
DOE OSTI2017

The implications of dust ice nuclei effect on cloud top temperature in a complex mesoscale convective system

Mineral dust is the most important natural source of atmospheric ice nuclei (IN) which may significantly mediate the properties of ice cloud through heterogeneous nucleation and lead to crucial impacts on hydrological and energy cycle. The potential dust IN effect on cloud top temperature (CTT) in a well-developed mesoscale convective system (MCS) was studied using both satellite observations and cloud resolving model (CRM) simulations. We combined satellite observations from passive spectrometer, active cloud radar, lidar, and wind field simulations from CRM to identify the place where ice cloud mixed with dust particles. For given ice water path, the CTT of dust-mixed cloud is warmer than that in relatively pristine cloud. The probability distribution function (PDF) of CTT for dust-mixed clouds shifted to the warmer end and showed two peaks at about -45°C and -25°C. The PDF for relatively pristine cloud only show one peak at -55°C. Cloud simulations with different microphysical schemes agreed well with each other and showed better agreement with satellite observations in pristine clouds, but they showed large discrepancies in dust-mixed clouds. Some microphysical schemes failed to predict the warm peak of CTT related to heterogeneous ice formation.

54 ENVIRONMENTAL SCIENCES↗