Community Atmosphere Model Version Research Articles

Connection between Barents Sea Ice in May and Early Summer Monsoon Rainfall in the South China Sea and Its Possible Mechanism

The impacts of Arctic sea ice on climate in middle and high latitudes have been extensively studied. However, its effects on climate in low latitudes, particularly on summer monsoon rainfall in the South China Sea (SCS), have received limited attention. Thus, this study investigates the connection between the Arctic sea ice concentration (SIC) anomaly and the early summer monsoon rainfall (ESMR) in the SCS and its underlying physical mechanism. The results reveal a significant positive correlation between the Barents Sea (BS) SIC in May and the ESMR in the SCS. When there is more (less) SIC in the Barents Sea (BS) during May, this results in a positive (negative) anomaly of the local turbulent heat flux, which lasts until June. This, in turn, excites an upward (downward) air motion anomaly in the vicinity of the BS, causing a corresponding downward (upward) motion anomaly over the Black Sea. Consequently, this triggers a wave train similar to the Eurasian (SEU) teleconnection, propagating eastward towards East Asia. The SEU further leads to an (a) upward (downward) motion anomaly and weakens (strengthens) the western Pacific subtropical high (WPSH) over the SCS, which is accompanied by a southwest adequate (scarce) water vapor anomaly transporting from the Indian Ocean, resulting in more (less) precipitation in the SCS. Furthermore, the response of ESMR in the SCS to the SIC in the BS is further verified by using the Community Atmosphere Model version 5.3 (CAM5.3). This study introduces novel precursor factors that influence the South China Sea summer monsoon (SCSSM), presenting a new insight for climate prediction in this region, which holds significant implications.

Exploring covariabilities of the high-summer subtropical upper-level pressure systems in the Northern Hemisphere

The Northern Hemisphere subtropical upper-tropospheric pressure systems, i.e., South Asian high (SAH), mid-Pacific Trough (MPT), and mid-Atlantic Trough (MAT), play key roles in regulating the Eurasian–Pacific–North American climate, but their covariabilities and drivers remain unclear. Here, we reveal two seesaw-like teleconnections between MPT and western SAH (WSAH) and between MPT and MAT during the boreal high summer, respectively, and explore their associations with the tropical Pacific and Atlantic sea surface temperature (SST) anomalies. The MPT-WSAH teleconnection, featured by coherent variations between WSAH and MPT, is coupled with enhanced western Pacific subtropical high and Asian mid-latitude low in the low-level troposphere. In contrast, the MPT-MAT teleconnection features an out-of-phase relationship between MPT and MAT, which is associated with westward expanded western Pacific subtropical high but weakened North Atlantic subtropical high. Both two teleconnections are closely linked to Pacific SST anomalies. However, the former is mainly driven by the hybrid La Niña in the tropical Pacific, where cold SST anomalies spreading eastward since preceding winter. Whereas the latter corresponds to the central Pacific La Niña since late-spring mainly driven by preceding successive SST warming in the tropical North Atlantic. Above conclusions are further confirmed by the simulated results from the numerical experiments, which are performed using the US National Center for Atmospheric Research global Community Atmosphere Model version 4.

Responses of Westerly Jets over Asia versus North America in the Last Glacial Maximum and Their Attributions

Abstract By using the Community Atmosphere Model version 4 (CAM4) coupled to a slab ocean model, the total responses of westerly jets (WJ) over Asia and North America during the Last Glacial Maximum (LGM), as well as the relative influences of different individual LGM forcing on the changes in WJ, are evaluated in this study. The results show that the seasonal variation of Northern Hemisphere WJ in LGM is characterized by distinct regional discrepancies. The WJ is intensified and shifts southward in North America, while it generally weakens and slightly displaces northward in central Asia. Over Japan, the WJ is enhanced and moves southward in summer but is attenuated and shifts northward in winter. The change of WJ has a close relationship with the midtroposphere temperature anomalies which result from the different external forcings. In summer, the ice sheet albedo plays a leading role in wind fields at mid-to-high latitudes of the Northern Hemisphere by causing a zonal wave–like response of temperature. In winter, the ice sheet topography dominates the wind fields across North America to North Atlantic by inducing a northwest–southeast-oriented tripole temperature response. The effects of orbital parameters and greenhouse gases largely contribute to the alteration of upper wind fields over Asia and North Pacific by causing a dipole response of temperature.

Improving BC Mixing State and CCN Activity Representation With Machine Learning in the Community Atmosphere Model Version 6 (CAM6)

AbstractRepresenting mixing state of black carbon (BC) is challenging for global climate models (GCMs). The Community Atmosphere Model version 6 (CAM6) with the four‐mode version of the Modal Aerosol Module (MAM4) represents aerosols as fully internal mixtures with uniform composition within each aerosol mode, resulting in high degree of internal mixing of BC with non‐BC species and large mass ratio of coating to BC (RBC, the mass ratio of non‐BC species to BC in BC‐containing particles). To improve BC mixing state representation, we coupled a machine learning (ML) model of BC mixing state index trained on particle‐resolved simulations to the CAM6 with MAM4 (MAM4‐ML). In MAM4‐ML, we use RBC to partition accumulation mode particles into two new modes, BC‐free particles and BC‐containing particles. We adjust RBC to make the modeled BC mixing state index (χmode) match the one predicted by the ML model (χML). On a global average, the mass fraction of BC‐containing particles in accumulation mode decreases from 100% (MAM4‐default) to 48% (MAM4‐ML). The globally averaged χmode decreases from 78% (MAM4‐default) to 63% (MAM4‐ML, 19% reduction) and agrees well with χML (66%). The RBC decreases by 52% for accumulation mode and better agrees with observations. The hygroscopicity drops by 9% for BC‐containing particles in accumulation mode, leading to a 20% reduction in the BC activation fraction. The surface BC concentration increases most (6.9%) in the Arctic, and the BC burden increases by 4%, globally. Our study highlights the application of the ML model for improving key aerosol processes in GCMs.

Sharp decline of dust events induces regional wetting over arid and semi-arid Northwest China in the NCAR Community atmosphere model

Multiple lines of observational evidence have indicated a significant wetting over the arid and semi-arid Northwest China (NWC) during recent decades, coinciding with a simultaneous sharp decline of dust events. Although recent studies have attributed NWC wetting to different anthropogenic and natural forcings, the mechanisms are not definitive and the regional wetting has been greatly underestimated in the Coupled Model Intercomparison Project historical simulations. Based on sensitivity experiments with different dust emission amounts using the NCAR Community Atmospheric Model version 5 (CAM5), here we find that decreasing dusts exert significant impacts on mixed-phase clouds through reducing the concentration of ice nucleating particles, increase the NWC precipitation and thus induce regional wetting through enhancing convection precipitation. A possible convection invigoration mechanism whereby the atmospheric vertical temperature gradient and convective instability are strengthened by reduced dusts, leading to convection invigoration and increased precipitation. These results are reinforced by simulations over the dust region in North Africa where mixed-phase and ice clouds are rare and reduced dusts do not increase precipitation. This study highlights the possible mechanism of dust-ice cloud interactions in recent NWC wetting and future regional climate change.

Characteristics and mechanisms of the severe compound cold-wet event in southern China during February 2022

A severe compound cold-wet event occurred in southern China (hereafter referred to as CWESC ) in February 2022, leading to enormous socioeconomic losses. In this study, we proposed a new index to denote the severity of the compound cold-wet event. Based on the multivariate survival method, the CWESC in February 2022 is identified as the severest event during the past six decades. Our results indicate that the CWESC in 2022 is jointly regulated by the La Niña-like SST condition in the tropical Pacific and the warm SST anomalies in the North Atlantic, and a teleconnection in the Northern Hemisphere during winter (hereafter referred to as TNHW) plays the key role. The TNHW pattern originates from the tropical Pacific, and it splits into two routes over the North Atlantic. The northern branch of TNHW propagates via the Arctic and Siberia, causing intensified near-surface northerly wind and partially inducing an anomalous anticyclone over the western North Pacific (WNP). The southern branch of TNHW propagates via the Mediterranean and western Asia, inducing a deepened India–Burma trough and partially inducing the anomalous anticyclone over WNP. The intensified near-surface northerly wind causes enhanced cold advection over southern China, while the deepened India–Burma trough and the anomalous anticyclone over WNP cause increased southerly warm and moist air flow towards southern China, resulting in the CWESC in 2022. Moreover, four groups of numerical experiments forced by tropical Pacific, North Pacific, and North Atlantic SST anomalies are conducted based on the Community Atmosphere Model version 5. The results confirm the important roles of the La Niña-like condition and the warm SST anomalies in the North Atlantic in causing the CWESC in 2022.

Extreme heat waves in June 2021 over Europe regulated by shrinking Eurasian snow cover in mid–high latitudes

Europe has experienced many extreme heat waves over the past few decades. In this study, the physical processes underlying these long-lasting and wide-ranging heat wave events are investigated based on a case study in Europe in June 2021. Heat waves are associated with barotropic anticyclonic anomalies accompanied by positive geopotential height anomalies locally. These anomalies persist under the conditions of increased meridional air temperature gradients of the mid–upper troposphere in the high latitudes of Eurasia and the formation of the Arctic front jet. The shrinking high-latitude snow cover in April–May favors higher surface temperatures and larger meridional temperature gradients in June in the mid–upper troposphere due to the soil moisture–evaporation–temperature positive feedback process. The summer Arctic front jet is then strengthened, and the mid-latitude westerly winds are weakened. This atmospheric circulation background favors waveguide formation and wave resonance that produces high-amplitude atmospheric waves and the stagnation of ridges in the mid-latitudes. Numerical experiments using the Community Atmosphere Model version 5 verify the proposed physical mechanisms, with the climatic responses in sensitivity experiments to anomalous snowfall rates closely resembling the observational results. Therefore, in June 2021, under the identified atmospheric circulation background and the perturbation of the upstream positive phase of the North Atlantic Oscillation, the large-scale barotropic high pressure and barotropic anticyclonic circulation in the study region tended to be stable and persistent, which is favorable for the production of long-lasting and wide-ranging heat wave events.

An Aerosol Optical Module With Observation‐Constrained Black Carbon Properties for Global Climate Models

AbstractAtmospheric black carbon (BC) aerosols have been long‐lasting uncertain components in environmental and climate studies. Global climate models (GCMs) potentially overestimate BC absorption efficiency due to a lack of consideration of complex BC microphysical and mixing properties. We extract multiple BC properties from observations and develop an aerosol optical module known as Advanced Black Carbon (ABC) in the framework of the Modal Aerosol Model version 4 (MAM4). The ABC module is implemented in the Community Atmosphere Model version 6 (CAM6) and evaluated by in situ and remote sensing observations. CAM6‐ABC addresses the shortcomings of CAM6‐MAM4 in terms of BC microphysical and mixing properties, particularly their size, mixing state and optical simulations. Sensitivity simulations show that the global BC absorption aerosol optical depth at 550 nm simulated by CAM6‐ABC is reduced by ∼29% compared with that in CAM6‐MAM4. The BC absorption enhancement simulated by CAM6‐ABC is reduced from ∼2.6 of the default MAM4 to ∼1.4, which is closer to the observed values (mostly less than 1.5). With improved BC absorption estimation, the biases of aerosol single‐scattering coalbedo simulations are reduced by 18%–69% compared with global Aerosol Robotic Network observations. Moreover, the globally averaged BC direct radiative effect is reduced from 0.37 to 0.28 W/m2 at the top of the atmosphere. Our new scheme alleviates the overestimation of BC absorption in GCMs by constraining BC microphysical and mixing properties when assessing aerosol radiative and climate effects, and it can be easily implemented in most modal‐based aerosol modules of climate models.

The Role of In‐Cloud Wet Removal in Simulating Aerosol Vertical Profiles and Cloud Radiative Forcing

AbstractAmong the physical processes controlling aerosol vertical profiles, in‐cloud wet removal is of utmost importance while its representation in global climate models (GCMs) is crude. In this study, we implement into the Community Atmosphere Model version 6 (CAM6) a physically‐based aerosol wet removal parameterization scheme that explicitly treats aerosol activation, removal and resuspension. Evaluation against in‐situ observations shows that the default scheme substantially overestimates the upper tropospheric black carbon (BC) and sea salt mass. Our physically‐based scheme reduces BC and sea salt mass by a factor of 10 and 1,000, respectively, in better agreement with observations. Also, the new scheme slightly increases number of aerosol particles between 12 nm and 4.8 μm in diameter, thereby mitigating the aerosol number underestimation in the default scheme. Our new scheme reduces the overestimation of coarse‐mode aerosol (0.5–4.8 μm) number. Overall, the aerosol property changes (mass decrease and number increase) reduce the cloud condensation nuclei (CCN) concentration at low supersaturation (i.e., 0.02% and 0.1%), and increase CCN at high supersaturations (i.e., 0.5% and 1%). Consequently, the global annual mean cloud liquid water path increases by 1.89 g m−2 and the ice water path increases by 0.51 g m−2. The global annual mean shortwave, longwave, and net cloud radiative forcing change by −1.06, 0.57, and −0.48 W m−2, respectively. Further improvement is needed to reflect the real physics that the removal efficiencies for aerosol mass and number are disproportionate and to advect cloud‐borne (activated) aerosols for a complete aerosol lifecycle.

Influence of ENSO on Tropospheric Ozone Variability in East Asia

AbstractAbundance of tropospheric ozone is determined by the interplay among emissions, chemistry, and climate dependent transport. It is important to identify the essential factors that modulate ozone in China and Korea because of increasing ozone trends in this area. In this study, we investigated the correlations of tropospheric ozone in East Asia with El Niño Southern Oscillations (ENSO). The model simulated ozone from 2000 to 2020 with Community Atmosphere Model version 6 with chemistry (CAM6‐chem), ECMWF Atmospheric Composition Reanalysis data, and ozone profiles measured in South Korea, Japan, and Hong Kong. These data were utilized to understand the ozone–ENSO relationship in this region. We found that tropospheric ozone in East Asia is significantly correlated with the wintertime Niño 3.4 index in El Niño/La Niña developing summer and decaying spring, with correlation coefficient up to 0.9. Particularly tropospheric ozone originated from stratosphere is strongly correlated with the Niño 3.4 index. Ozone concentrations in China and the Korean Peninsula are ∼5 ppb higher during El Niño developing summer compared to La Niña in a broad area, because of anomalous cyclonic/anticyclonic flows in the extratropics. In El Niño/La Niña decaying spring, the opposite occurs in the same region. In the decaying spring, stronger variability was observed in the subtropical regions of the lower to mid troposphere, indicating higher ozone concentrations during El Niño compared to La Niña. This variability is associated with the fluctuations in the Walker circulation, large‐scale sinking/rising motions, and the accompanying biomass burning events. The model reproduced the ozone variability associated with ENSO in the ozonesonde observations.

Insights of warm-cloud biases in Community Atmospheric Model 5 and 6 from the single-column modeling framework and Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) observations

Abstract. There has been a growing concern that most climate models predict precipitation that is too frequent, likely due to lack of reliable subgrid variability and vertical variations in microphysical processes in low-level warm clouds. In this study, the warm-cloud physics parameterizations in the singe-column configurations of NCAR Community Atmospheric Model version 6 and 5 (SCAM6 and SCAM5, respectively) are evaluated using ground-based and airborne observations from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign near the Azores islands during 2017–2018. The 8-month single-column model (SCM) simulations show that both SCAM6 and SCAM5 can generally reproduce marine boundary layer cloud structure, major macrophysical properties, and their transition. The improvement in warm-cloud properties from the Community Atmospheric Model 5 and 6 (CAM5 to CAM6) physics can be found through comparison with the observations. Meanwhile, both physical schemes underestimate cloud liquid water content, cloud droplet size, and rain liquid water content but overestimate surface rainfall. Modeled cloud condensation nuclei (CCN) concentrations are comparable with aircraft-observed ones in the summer but are overestimated by a factor of 2 in winter, largely due to the biases in the long-range transport of anthropogenic aerosols like sulfate. We also test the newly recalibrated autoconversion and accretion parameterizations that account for vertical variations in droplet size. Compared to the observations, more significant improvement is found in SCAM5 than in SCAM6. This result is likely explained by the introduction of subgrid variations in cloud properties in CAM6 cloud microphysics, which further suppresses the scheme's sensitivity to individual warm-rain microphysical parameters. The predicted cloud susceptibilities to CCN perturbations in CAM6 are within a reasonable range, indicating significant progress since CAM5 which produces an aerosol indirect effect that is too strong. The present study emphasizes the importance of understanding biases in cloud physics parameterizations by combining SCM with in situ observations.

Spatial distribution of aerosols burden and evaluation of changes in aerosol optical depth using multi-approach observations in tropical region

Understanding of Aerosol optical depth (AOD) parameter is important for air quality assessment. This study aims to evaluate and validate AOD measurements from combine datasets to improve air quality for a period 2005–2020 using Aerosol Robotic Network (AERONET) at Ilorin site (8.320° N, 4.340° E) in Nigeria. AOD outputs from Community Atmosphere Model Version 6 with chemistry (CAM6-chem) at 1° horizontal resolution and Modern-Era Retrospective analysis for Research and Applications (MERRA-2) are investigated in addition to validation of two satellites AOD retrievals: Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR). Result of spatial distribution of AOD shows high values > 1 in the North and Western Sahara compared to Central Africa. Desert dust shows largest contribution in the North and Western Africa that is up to 2 magnitude larger than other aerosol types. Primary organic matter (POM) and secondary organic aerosols (SOAs) both presents high burdens with later been dominant at around 10° band, and black carbon (BC) largest burden (2.6 × 10 − 5 kgm − 2) is seen in the model from oil and gas exploration site in Nigeria. Inter-comparison of MERRA/MISR/MODIS and AERONET AOD using linear correlation of the seasonal dependence demonstrated high correlation (r = 0.864 − 0.973) subjected to Root Mean Square Error (RMSE = 0.069 − 0.211), suggesting good agreement between the datasets. When compared to seasonal mean maximum AERONET AOD value of 0.978 MERRA is ∼5%, MISR ∼28% and MODIS ∼29% lower with stronger correlations observed in the wet and pre-harmattan seasons. Similarly, MODEL AOD at 550 nm and dust burden were found to be ∼34% and ∼67% lower in context to AERONET AOD annual mean value of 0.627. Positive relationships that indicate an upward slope exist between all the computed datasets with moderate value of AERONET/CAM-chem spearman partial correlation, and MERRA/MODIS and MODIS/MISR showing strong and significant relationship with p-value less than 0.05. Low variance is observed with all measurements except in MERRA.

A new simplified parameterization of secondary organic aerosol in the Community Earth System Model Version 2 (CESM2; CAM6.3)

Abstract. The Community Earth System Model (CESM) community has been providing versatile modeling options, with simple to complex chemistry and aerosol schemes in a single model, in order to support the broad scientific community with various research interests. While different model configurations are available in CESM and these can be used for different fields of Earth system science, simulation results that are consistent across configurations are still desirable. Here we develop a new simple secondary organic aerosol (SOA) scheme in the Community Atmosphere Model (CAM) version 6.3, the atmospheric component of the CESM. The main purpose of this simplified SOA scheme is to reduce the differences in aerosol concentrations and radiative fluxes between CAM and CAM with detailed chemistry (CAM-chem) while maintaining the computational efficiency of CAM. CAM simulation results using the default CAM6 and the new SOA schemes are compared to CAM-chem results as a reference. More consistent SOA concentrations are obtained globally when using the new SOA scheme for both temporal and spatial variabilities. The new SOA scheme shows that 62 % of grid cells globally are within a factor of 2 compared to the CAM-chem SOA concentrations, which is improved from 24 % when using the default CAM6 SOA scheme. Furthermore, other carbonaceous aerosols (black carbon and primary organic aerosol) in CAM6 become closer to CAM-chem results due to more similar microphysical aging timescales influenced by SOA coating, which in turn leads to comparable wet deposition fluxes. This results in an improved global atmospheric burden and concentrations at the high latitudes of the Northern Hemisphere compared to the full chemistry version (CAM-chem). As a consequence, the radiative flux differences between CAM-chem and CAM in the Arctic region (up to 6 W m−2) are significantly reduced for both nudged and free-running simulations. We find that the CAM6 SOA scheme can still be used for radiative forcing calculation as the high biases exist both in pre-industrial and present conditions, but studies focusing on the instantaneous radiative effects would benefit from using the SOA scheme developed in this study. The new SOA scheme also has technical advantages including the use of identical SOA precursor emissions as CAM-chem from the online biogenic emissions instead of pre-calculated emissions that may introduce differences. Future parameter updates to the CAM-chem SOA scheme can be easily translated to the new CAM SOA scheme as it is derived from the CAM-chem SOA scheme.

The Most Effective Remote Forcing in Causing U.S.‐Wide Heat Extremes as Revealed by CESM Green's Function Experiments

AbstractWe make use of the Community Atmosphere Model version 5 Green's function q‐flux perturbation experiments to explore the most effective remote forcing in driving U.S.‐wide summer heat extremes. We find that positive q‐flux forcing over the western North Pacific Ocean is the most effective in causing an increased heat extreme frequency. This works by driving increased sea surface temperature and precipitation over western North Pacific and an eastward propagating Rossby wave train with an anomalous ridge over the contiguous U.S. In comparison, negative q‐flux forcing over the eastern tropical Pacific and its resulting surface cooling also leads to an increased heat extreme frequency but is less effective. Furthermore, guided by the Green's function results, we separate the role of western North Pacific warming and eastern tropical Pacific cooling in U.S. heat extremes in prescribed sea surface temperature experiments and ERA5 reanalysis data and find overall consistent conclusions.

Why a large‐scale monsoon does not exist in North America: Orographic effects

AbstractTo understand why a large‐scale monsoon does not exist in North America, we performed a series of sensitivity simulations to investigate orographic effects of the Rocky Mountains (RMs) using the Community Atmosphere Model version 5.1. Results show that the height of the RMs plays a fundamental role in shaping the monsoon over North America that is confined to a small area from northwestern Mexico to the southwestern United States. When the RMs' height is increased by five times their actual height, a larger part of the non‐monsoon region over North America becomes a monsoon region. The mechanical effects of the RMs uplift dominate in winter, while thermal effects dominate in summer. During winter the mechanical effects induce an equivalent barotropic atmospheric response in the troposphere. With the uplift of the RMs, the ridge and trough located on the western and eastern sides of Canada are strengthened. Most areas of North America are influenced by the northerlies during winter such that enhanced descending motion over the eastern RMs favours a dry winter climate. However, thermal effects dominate during summer through enhanced baroclinic atmospheric responses. The Mexico high and lower‐level cyclonic circulation are strengthened with the RMs uplift, triggering large‐scale ascending motion. Eastern North America is mainly controlled by the enhanced southerly wind along the western flank of the North Atlantic subtropical high. Thus, the enhanced water‐vapour transport and upward motion on the eastern side of the RMs increase summer precipitation. As a result, an obvious seasonal variation with the feature of ‘dry winter and wet summer’ finally develops, indicating that the height of the RMs plays a crucial role in shaping the monsoon over the central United States. Additional experiments show that the base area of the RMs has little effect on the large‐scale monsoon formation over North America.

Connection between the Tropical Pacific and Indian Ocean and Temperature Anomaly across West Antarctic

West Antarctic and the Antarctic Peninsula have experienced dramatic warming in austral spring since the 1970s. Using observations and the Community Atmosphere Model version 4 (CAM4), this study explores the physical mechanism by which the tropical Pacific and Indian Ocean temperature anomaly mode (PIM) affects the dipolar surface air temperature (SAT) anomalies across the West Antarctic in austral spring. The positive phase of the PIM, characterized by positive sea surface temperature anomalies (SSTAs) in the tropical central-eastern Pacific and western Indian Ocean and negative SSTAs in the Maritime Continent, can generate two branches of stationary Rossby wave trains propagating from the tropical central Pacific and southeastern Indian Ocean to the West Antarctic, with an anticyclonic anomaly appearing over the Amundsen Sea. The northerlies advect warmer air to the Ross–Amundsen Seas, but southerlies advect colder air to the Antarctic Peninsula–Weddell Sea, resulting in the dipole of SAT anomalies over the West Antarctic. In this process, the role of tropical central-eastern Pacific SSTAs dominate, and it is amplified by the SSTAs around the Maritime Continent. The SSTAs in the western Indian Ocean combined with the SSTAs over the Maritime Continent further contribute to the western pole of the SAT. Only simulation that includes a prescribed PIM forcing can exactly reproduce the observations of the dipolar SAT response across the West Antarctic, indicating the need to treat the tropical Pacific and Indian Oceans as a unified whole.

Decadal variability of extreme high temperature in mid- and high-latitude Asia and its associated North Atlantic air–sea interaction

The decadal variability of extreme high temperature (EHT) in the mid and highlatitudes of continental Asia (MHLCA) and associated mechanisms were investigated. Observational analysis indicated that, after removing global warming, the first leading mode of the EHT events showed a meridional dipole pattern and had significant decadal variability. During the periods 1980–1994 and 2012–2019 (1995–2011), EHT events were more frequent and stronger (less frequent and weaker) in western-central Siberia than normal, whereas they were less frequent and weaker (more frequent and stronger) in the wide area to the south of Lake Baikal. Further Observational and CESM (Community Earth System Model) results based analysis suggested that decadal change in air–sea interaction in the North Atlantic play an important role in shaping the decadal variability of EHT events in MHLCA. On decadal timescales, meridional negative–positive–negative sea surface temperature (SST) anomalies in the North Atlantic and their coupled positive North Atlantic Oscillation can trigger stronger wave activity flux and cause a significant anticyclonic–cyclonic teleconnection wave train in the troposphere over the mid and high latitudes of the Eurasian continent. As a result, an anomalous high-pressure center is evident in western-central Siberia, and thereby reducing total cloud cover and causing stronger solar heating. Thus, EHT events become more frequent and stronger. On the contrary, for the areas south of Lake Baikal, EHT events are weakened by corresponding cyclonic circulation anomalies. Additionally, the CAM5 (Community Atmosphere Model version 5) experiments suggested more important driving role of the decadal North Atlantic tripole SST anomalies in the abovementioned processes.

Climate Impacts of Convective Cloud Microphysics in NCAR CAM5

Abstract We improved the treatments of convective cloud microphysics in the NCAR Community Atmosphere Model version 5.3 (CAM5.3) by 1) implementing new terminal velocity parameterizations for convective ice and snow particles, 2) adding graupel microphysics, 3) considering convective snow detrainment, and 4) enhancing rain initiation and generation rate in warm clouds. We evaluated the impacts of improved microphysics on simulated global climate, focusing on simulated cloud radiative forcing, graupel microphysics, convective cloud ice amount, and tropical precipitation. Compared to CAM5.3 with the default convective microphysics, the too-strong cloud shortwave radiative forcing due primarily to excessive convective cloud liquid is largely alleviated over the tropics and midlatitudes after rain initiation and generation rate is enhanced, in better agreement with the CERES-EBAF estimates. Geographic distributions of graupel occurrence are reasonably simulated over continents; whereas the graupel occurrence remains highly uncertain over the oceanic storm-track regions. When evaluated against the CloudSat–CALIPSO estimates, the overestimation of convective ice mass is alleviated with the improved convective ice microphysics, among which adding graupel microphysics and the accompanying increase in hydrometeor fall speed play the most important role. The probability distribution function (PDF) of rainfall intensity is sensitive to warm rain processes in convective clouds, and enhancement in warm rain production shifts the PDF toward heavier precipitation, which agrees better with the TRMM observations. Common biases of overestimating the light rain frequency and underestimating the heavy rain frequency in GCMs are mitigated.

Orographic Cirrus and Its Radiative Forcing in NCAR CAM6

AbstractCirrus clouds play an important role in the Earth's radiative energy budget, thereby affecting the climate state and climate change. Orographic gravity wave (OGW)‐induced sub‐grid scale vertical velocity (i.e., cooling rate) is not resolved by large‐scale models and its impact on ice formation in cirrus clouds is not well quantified. In this study, one sub‐grid scale OGW scheme (e.g., McFarlane) is used in the Community Atmosphere Model version 6 (CAM6) to generate vertical velocity variance (σw) for cirrus formation. Results from the default model and simulations with the OGW‐induced σw are evaluated against the DOE ARM Small Particles in Cirrus (SPARTICUS) campaign observations. The OGW based on the McFarlane scheme increases the sub‐grid scale σw over mountains compared to the default model and improves the model agreement with the SPARTICUS observations. Larger σw due to OGWs can trigger more frequent homogeneous nucleation in orographic cirrus and generates a higher number concentration of ice crystals observed during the SPARTICUS campaign. Moreover, our evaluation of the model simulations against satellite observations indicates that the McFarlane scheme generates high in‐cloud ice number concentrations (>200 L−1) in the upper troposphere over mountains and high plateaus at mid‐ and high‐latitudes of the winter hemisphere as shown in the observations. More ice crystals with smaller sizes absorb more infrared radiation (+0.523 ± 0.125 W m−2). The net radiative cloud forcing change at the top of the atmosphere is +0.330 W m−2 due to the orographic cirrus.

Extent of the Impact of Arctic Atmospheric Uncertainty on Extended‐Range Forecasting of Cold Events in East Asia

AbstractThe extended‐range predictability of three simulated extreme cold events in East Asia in the Community Atmosphere Model (version 4) control experiment, whose atmospheric circulation backgrounds are similar to two recent observational cases, is investigated. The results show that they have a predictability of four pentads. Then, we evaluate the extent of the forecast uncertainty in the 4th pentad caused by the initial atmospheric uncertainties in the Arctic, which are large due to sparse instrumental observations. The initial uncertainty leading to the largest forecast uncertainty is obtained by the conditional nonlinear optimal perturbation method, and referred to as the CNOP‐type initial uncertainty. The forecast of the cold surge in the 4th pentad fails after adding the CNOP‐type initial uncertainty at day 0. In comparison, the forecast uncertainties are much weaker when the initial conditions are perturbed by noises, and the weaker influence may be due to the noises' lack of spatial structure. In terms of how the CNOP‐type initial uncertainty develops, a baroclinic structure is seen in the uncertainties on the first 2 days, followed by a propagating Rossby wave feature from the 2nd pentad to the 4th pentad. Meanwhile, synoptic transient eddy feedback also plays an essential role. The results suggest that the CNOP‐type initial uncertainty has potential to identify sensitive areas for targeted observations; plus, it could serve as a member of ensemble initial perturbations, since it indicates the largest uncertainty growth.