Under the influence of global climate change, wildfires are becoming increasingly significant within ecosystems. Satellite technology offers a critical perspective for studying fire. The synergy of high-spatial resolution narrowband imagers and hyperspectral infrared (IR) sounders allow for comprehensive observation and long-term global monitoring of fire characteristics and associated atmospheric changes on the pixel scale. This study demonstrates methods to utilize the pixel-scale collocated fire observations from the Visible Infrared Imaging Radiometer Suite (VIIRS) and the IR radiance spectra from the Cross Track Infrared Sounder (CrIS), onboard NASA's Suomi National Polar-orbiting Partnership (SNPP) satellite, to analyze fire and the atmospheric conditions before, during, and after fires. Two months of satellite observations over the Southwest United States and the Amazonia regions when large fires occurred in the regions of interest (October 2017 and August 2020) are used. Our findings reveal that CrIS has high sensitivity to fire that affects as little as 1% of its field of view (FOV) with FOV total fire radiative power (FRP) larger than 200 MW (MW) at night and 1000 MW during the day. By employing spectral principal component analysis (PCA), the CrIS spectral signature to atmospheric temperature, humidity, and trace gases corresponding to fire characteristics are quantified. This approach highlights that collocated imager and IR sounder data, when paired with PCA, provide a powerful method to effectively identify and monitor wildfires. This technique also allows for the observation of subsequent atmospheric alterations while managing data volume efficiently, ensuring that crucial spectral information is preserved. This methodology advances our ability to understand and respond to the multifaceted impacts of wildfires on both local and global scales, reinforcing the value of integrated satellite observations in environmental science.

This study explores the relationship between water vapor and rainfall intensities over three tropical lands (Amazon Basin, Sahel, southern South America) and three tropical ocean regions (Atlantic Ocean, Indian Ocean, Niño 4). We utilized daily total column water vapor (TCWV) data from the Atmospheric Infrared Sounder (AIRS) and daily precipitation records from the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation. Over tropical land, precipitation shows higher sensitivity to changes in water vapor, with a well-sorted pattern of an increased occurrence of higher daily precipitation as TCWV increases. Precipitation intensity over the Sahel, in particular, is extremely responsive to TCWV change. Over tropical oceans, the precipitation intensity is less sensitive to water vapor, particularly in the Indian Ocean and Niño 4 where precipitation intensities above the 40th percentile are no longer responding to the increasing TCWV. Quantifying water vapor and precipitation intensity aids in forecasting the occurrence of precipitation between tropical land and oceans.

In this study, the Atmospheric Infrared Sounder (AIRS) Observations for Model Intercomparison Projects (Obs4MIPs) V2.1 tropospheric air temperature, specific humidity, and relative humidity data are utilized to evaluate the global tropospheric temperature and humidity simulations in the fully coupled global climate models from the Coupled Model Intercomparison Project phases 3, 5, and 6 (CMIP3, CMIP5, and CMIP6), and possible simulation improvement in CMIP6 models in comparison to CMIP3 and CMIP5 models. Our analyses indicate that all three phases of CMIP models share similar tropospheric air temperature, specific humidity, and relative humidity biases in their multi‐model ensemble means relative to AIRS. Cold biases up to 4 K and positive relative humidity biases up to 20% are found in the free troposphere almost globally with maxima over the mid‐latitude storm tracks. Warm biases up to 2 K are seen over the Southern Ocean in the lower troposphere. Positive specific and relative humidity biases exist over the off‐equatorial oceans while negative specific and relative humidity biases are seen near the equator in the tropical free troposphere, which are related to the double‐intertropical convergence zone bias in the models. Both the air temperature and specific humidity biases are important to the relative humidity biases except in the tropical free troposphere where the specific humidity biases dominate. The tropospheric air temperature, specific humidity, and relative humidity biases are reduced from CMIP3 to CMIP5 and to CMIP6 at almost all pressure levels except at 300 hPa for specific humidity and in the boundary layer for relative humidity.

It is challenging to accurately characterize the three-dimensional distribution of horizontal wind vectors (known as 3D winds). Feature-matching satellite cloud top or water vapor fields have been used for decades to retrieve atmospheric motion vectors, but this approach is mostly limited to a single and uncertain pressure level at a given time. Satellite wind lidar measurements are expected to provide more accurate data and capture the line-of-sight wind for clear skies, within cirrus clouds, and above thick clouds, but only along a curtain parallel to the satellite track. Here we propose Vientos—a new satellite mission concept that combines two or more passive water vapor sounders with Doppler wind lidar to measure 3D winds. The need for 3D wind observations is highlighted by inconsistencies in reanalysis estimates, particularly under precipitating conditions. Recent studies have shown that 3D winds can be retrieved using water vapor observations from two polar-orbiting satellites separated by 50 min, with the help of advanced optical flow algorithms. These winds can be improved through the incorporation of a small number of collocated higher-accuracy measurements via machine learning. The Vientos concept would enable simultaneous measurements of 3D winds, temperature, and humidity, and is expected to have a significant impact on scientific research, weather prediction, and other applications. For example, it can help better understand and predict the preconditions for organized convection. This article summarizes recent results, presents the Vientos mission architecture, and discusses implementation scenarios for a 3D wind mission under current budget constraints.

This study investigates the possible linkage of sea surface height anomaly (SSHA), sea surface temperature (SST) and surface wind stress (TAU) with the falling ice radiative effects (FIREs) over the Pacific Ocean under the CMIP5 scenario of 1% CO2 increase per year (1pctCO2) for 140 years. Two sensitivity experiments using the fully-coupled ocean-atmosphere CESM1-CAM5 model are compared: one with FIREs (SON) and the other without (NOS). The warming period of the last 20 year is compared to the control period of the first 20 years. The mean state of the first 20 years exhibits a stronger west-east SSHA gradient in SON than in NOS over the Pacific Ocean, attributed to the stronger prevailing easterly trade winds and the SST pattern of being warmer in the western Pacific but colder in the eastern Pacific. In the last 20 years, SON continues to display a stronger SSHA west-east gradient with higher sea level and SST over the western Pacific Ocean compared to NOS. In the southwest Pacific, stronger trade winds are linked with the wind-evaporation-SST feedback, leading to a more significant decline in SSHA there in SON than in NOS. The CMIP5 ensemble mean shows similar relationships of SSHA with TAU and SST to those of NOS when both are compared against SON, highlighting the importance of FIREs in the SSH projection over the Pacific Ocean and the impact of FIREs on the changing relationships between SSHA, SST and TAU under global warming.

This study derives radiatively-active hydrometeors frequencies (HFs) from CloudSat-CALIPSO satellite data to evaluate cloud fraction in present-day simulations by CMIP5 models. Most CMIP5 models do not consider precipitating and/or convective hydrometeors but CESM1-CAM5 in CMIP5 has diagnostic snow and CESM2-CAM6 in CMIP6 has prognostic precipitating ice (snow) included. However, the models do not have snow fraction available for evaluation. Since the satellite-retrieved hydrometeors include the mixtures of floating, precipitating and convective ice and liquid particles, a filtering method is applied to produce estimates of cloud-only HF (or NPCHF) from the total radiatively-active HF (THF), which is the sum of NPCHF, precipitating ice HF and convective HF. The reference HF data for model evaluation include estimates of liquid-phase NPCHF from CloudSat radar-only data (2B-CWC) and ice-phase THF from CloudSat-CALIPSO 2C-ICE combined radar/lidar data. The model evaluation results show that cloud fraction from CMIP5 multi-model mean (MMM) is significantly underestimated (up to 30 %) against the total HF estimates, mainly below the mid-troposphere over the extratropics and in the upper-troposphere over the midlatitude lands and a few tropical convective regions. The CMIP5 cloud fraction biases are reduced dramatically when compared to the cloud-only HF estimates, but the area of overestimates expands from the tropical convective regions to mid-latitudes in the lower and upper troposphere. There is no CMIP5 standard output snow fraction available for comparison against CloudSat-CALIPSO estimate. The implications of these results show that hydrometeors frequency estimates from CloudSat-CALIPSO provide a reference for GCM’s cloud fraction from stratiform and convective form.

A two‐decade climatology of height‐resolved horizontal variance scaling exponents ( 𝛼 ) for temperature ( T ) and specific humidity ( q ) is described using Aqua Atmospheric Infrared Sounder (AIRS) sounding profiles. The AIRS Team Version 6 (V6), Version 7 (V7), and Community Long‐term Infrared Microwave Combined Atmospheric Product System (CLIMCAPS) retrieval algorithms are compared to European Centre for Medium‐Range Weather Forecasts Reanalysis v5 (ERA5) and Modern‐Era Retrospective analysis for Research and Applications, Version 2 (MERRA‐2) reanalyses. Large‐scale T exponents derived between 600 and 1,200 km ( 𝛼 L ) show close agreement between V6, V7, and CLIMCAPS algorithms. However, small‐scale q exponents derived between 150 and 400 km ( 𝛼 S ) are in poor agreement, including an unrealistically steep 𝛼 S in the planetary boundary layer (PBL) in the V7 and CLIMCAPS algorithms that is caused by a combination of algorithm damping and overconstraint by the first guess fields. ERA5 and MERRA‐2 reanalyses have large values of 𝛼 L and 𝛼 S for both T and q that indicate reduced small‐scale variability in the reanalysis fields. Differences in 𝛼 S between free‐running MERRA‐2 AMIP and MERRA‐2 are negligible, implying that suppressed small‐scale variability in reanalyses is imposed by the background model and not caused by the data assimilation process. AIRS has positively skewed T distributions in the tropical‐free troposphere that is consistent with positively buoyant air parcels in convection, and negative skewness in the PBL that is related to the existence of cold pools, behavior that is mostly absent in ERA5 and MERRA‐2. AIRS provides a global view of scale‐dependent variance and skewness that is useful for subgrid parameterization development and validation of weather and climate prediction models.

We examine the spatial distributions of CMIP6‐simulated cloud liquid water path (CLWP) and content (CLWC) against MODIS and CloudSat synthesized data over the tropical and subtropical Pacific. Three subsets of models are categorized based on their treatments of frozen ice‐radiative interactions. CLWP/CLWC are generally well simulated in subset with separately‐calculated radiative effects of cloud ice and falling ice (SON2). Too much warm clouds above 750 hPa are produced in either subset with total frozen ice radiative effects (SON1) or subset without radiative effects of falling ice (NOS) and thus CLWP/CLWC are overestimated over the open ocean including the trade‐wind regions. Stratocumulus clouds off the coasts of North and South America are severely underestimated in NOS models. We attribute the overestimates of clouds above the trade‐wind boundary layers to anomalous ascending motion associated with warmer sea surface temperature and weaker surface wind stress linked to indirect effects of falling ice‐radiation interactions.

This study uses derived 2C‐ICE estimates of frozen hydrometeors path (IWP) and vertical profile of ice water content (IWC) from CloudSat‐CALIPSO satellite measurements to evaluate stratiform floating ice (CIWP/CIWC), falling ice (snow) (SWP/SWC) and total ice (TIWP/TIWC) simulated by three subsets of CMIP6 models. They include those neglecting the falling ice (snow) radiative effects (NOS) or those considering these effects (SON) but with separate (SON2) or combined (SON1) frozen hydrometeors (cloud ice and falling ice) interacting with radiation. CIWP from NOS subset agrees with 2C‐ICE estimates better than SON2 except for overestimating over the trade‐wind regions. This is also the case for vertical profiles of regionally‐averaged CIWC, with the exception of overestimates in the lower troposphere of high latitudes by NOS. Falling ice (SWP/SWC) is simulated reasonably well in SON2 models against 2C‐ICE estimates, but with significant underestimates over mid‐ and high‐latitudes. Vertical shapes of regionally‐averaged SWC profiles are well reproduced but magnitudes are underestimated for all regions except for the middle troposphere of the tropical region. The inclusion of snow improves the agreement between the simulated TIWP/TIWC of SON1 and SON2 subsets and 2C‐ICE estimates, compared to the NOS subset. However, TIWP/TIWC are underestimated over the storm track and high latitude for all three subsets except for the tropical region of SON2. The aforementioned discrepancies are likely due to model physics, but observational estimates of frozen hydrometeors mass and content are also highly uncertain. Further studies are needed to improve both models and observational estimates of frozen hydrometeors mass.

This study attempts to infer the linkage of sea surface height anomaly (SSHA), surface wind stress and sea surface temperature with the falling ice (snow) radiative effects (FIREs) over the tropical and subtropical Pacific Ocean using CESM1-CAM5 sensitivity experiments with FIREs-off (NOS) and on (SON) under CMIP5 historical run. The obs4MIPs monthly SSH data based upon satellite measurements are used as a reference. The seasonal and annual mean spatial patterns of SSHA difference between NOS and SON are tightly linked to those of SST and TAU over the study domain, in particular, over the south Pacific. Compared with NOS, SON simulates improved seasonal and annual mean SSHA associated with improved sea surface temperature (SST), surface wind stress (TAU) over the trade-wind areas. In SON, the simulated mean absolute bias of SSHA over the study domain is reduced (up to 30%) against NOS relative to observations. The SSHA biases are then compared with CMIP5 models. Despite the biases of SST and SSHA over the south and north flanks of the equator in SON, the seasonal variations of improved SSHA are closely related to those of TAU and SST resulting from the FIREs; that is, higher SSHA is associated with weaker TAU and warmer SST changes and vice versa. The CMIP5 ensemble mean absolute biases of SSHA show similarities to NOS mainly over the south Pacific.

This study explores the linkage of frozen hydrometeors (cloud ice and falling ice/snow) with sea ice and adjacent lands through surface energy budget using model-observation comparisons to quantify the roles of the falling ice radiative effects (FIREs) in determining the extent and thickness of Arctic sea ice and adjacent land surface radiation budget and land surface skin (Ts) and surface (SAT) temperatures. The Coupled Model Intercomparison Project Phase 5 (CMIP5) models without FIREs tend to produce underestimated downward longwave radiation, and overestimated shortwave downward radiation and surface albedo, resulting in too-cold skin temperature (TS) and surface air temperature (SAT) and overestimated sea ice concentration (SIC) and thickness (SIT). By comparing two simulations of late 20th Century climate from CESM1-CAM5 model with inclusion and exclusion of FIREs, it is found that TS, SAT, radiation, SIC, and SIT and their seasonal cycles are improved with the inclusion of FIREs. Exclusion of FIREs results in underestimated net downward longwave radiative flux, which is highly correlated with overestimated surface albedo, colder TS, and SAT with a confidence level at 99% (p < 1%). These biases in CESM1-CAM5 resemble those in CMIP5 models without FIREs. With the inclusion of FIREs, the SIC bias is reduced by 2%–15% in summer, while the SIT is improved up to 90% in winter despite little improvement in SIC. These findings suggest a potential link among the increased downward longwave flux, decreased downward shortwave flux, and decreased surface air and land surface temperatures locally, which then drives SIC melting and SIT thinning when FIREs are included relative to when FIREs are excluded. It is suggested that the roles played by FIREs over the Arctic regions are of the same order of importance as those over the Southern Oceans despite the differences in geography and impact of human activity.

We evaluate the simulations of surface wind stress (TAU) and sea surface temperature (SST) over subtropical and tropical Pacific and Atlantic oceans in subsets of CMIP6 models that are categorized by frozen hydrometeors-radiation interactions. The CMIP6 models are divided into two subsets with combined (SON1) and separated (SON2) radiative properties of cloud ice and falling ice (snow) and compared to the set with cloud ice radiative effects only (NOS). There is evidence that these hydrometeors-radiation interaction treatments induce different atmospheric dynamic responses that influence the surface properties. Excessive westerly TAU and meridional TAU divergence away from convective zones are reduced significantly in SON1 and SON2 relative to NOS against QuikSCAT observations; while the differences between SON2 and SON1 are small. SON2 reduces cold SST biases over north oceans and equatorial zones drastically (1 to 2 K), and warm biases (up to 1 K) off the coasts of America and zonal TAU biases are reduced relative to NOS. Unlike SON2, SON1 improves SSTs mainly over south of Pacific Ocean and limited areas over the tropical belts relative to NOS although TAU is reduced drastically as in SON2, implying that other factors play a role in degrading the SST simulations in SON1 relative to SON2. SON2 outperforms NOS and SON1 in the seasonal cycles of SST mean biases and mean absolute biases averaged over the equatorial area, north ocean, and South Pacific against ERSST observations. Despite the significant improvements in TAU and SST simulations, SON2 models still exhibit non-trivial biases over south and north flanks of equatorial zones. These results suggest that there are direct linkages of TAU with SST changes resulting from the hydrometeors-radiation interactions in SON2, but not in SON1, relative to NOS, implying that a separated treatment of cloud ice and falling ice radiative properties in climate models is preferred.

To explore the impacts of hydrometeor radiative effects over subtropical and tropical Pacific and Atlantic Oceans, we quantify the mean radiation biases in historical climate simulations based on how frozen‐hydrometeors radiative properties are calculated in CMIP6 models. CMIP6 models are divided with cloud ice only (NOS), with combined (SON1), and with separate treatments (SON2) of cloud ice and falling ice (snow) radiative properties. Over the deep convective regions, NOS models overestimate outgoing longwave radiation (RLUT) and surface shortwave irradiance (RSDS), while underestimate top‐of‐atmosphere reflected shortwave radiation (RSUT). SON2 models reduce these biases by 4–14 W m −2 . However, this improvement is not seen in SON1 against NOS. Spatially averaged absolute biases in radiative fluxes for SON1 models are larger than those of NOS, suggesting that the SON1 approach of falling ice radiative effects may not produce the expected hydrometeor–radiation interactions. Over the south Pacific trade‐wind regions, both SON2 and SON1 show similar improvements in RLUT, RSUT, and RSDS with positive absolute bias differences up to 20 W m −2 against NOS, leading to improvement of CMIP6 over CMIP5 ensembles. The seasonal cycles are consistent with the annual means over these two regions except with larger differences between subsets of models during January–May than during June–December. In general, improvement from CMIP5 to CMIP6 due to more participating SON2 models is limited because of offset by SON1. These results suggest that a separate treatment of frozen‐hydrometeor radiative properties may be critical for reducing the spread of CMIP models.

The Aqua, SNPP (Suomi National Polar-orbiting Partnership), and JPSS (Joint Polar Satellite System) satellites carry a combination of hyperspectral infrared sounders (AIRS, Atmospheric Infrared Sounder, and CrIS, Cross-track Infrared Sounder) and high-spatial-resolution narrowband imagers (MODIS, Moderate Resolution Imaging Spectroradiometer, and VIIRS, Visible Infrared Imaging Radiometer Suite). They provide an opportunity to acquire high-quality, long-term cloud data records and are a key component of the existing Program of Record of cloud observations. By matching observations from sounders and imagers across different platforms at the pixel scale, this study evaluates the self-consistency and continuity of cloud retrievals from Aqua and SNPP by multiple algorithms, including the AIRS version 7 retrieval algorithm and the Community Long-term Infrared Microwave Combined Atmospheric Product System (CLIMCAPS) version 2 for sounders and the standard Aqua MODIS collection 6.1 and the NASA MODIS–VIIRS continuity cloud products for imagers. Metrics describing detailed statistical distributions at the sounder field of view (FOV) and the joint histograms of cloud properties are evaluated. These products are found to be highly consistent despite their retrieval from different sensors using different algorithms. Differences between the two sounder cloud products are mainly due to cloud clearing and the treatment of clouds in scenes with unsuccessful atmospheric profile retrievals. The sounder subpixel cloud heterogeneity evaluated using the standard deviation of imager retrievals at the sounder FOV shows good agreement between the standard and continuity products from different satellites. However, the impact of algorithm and instrument differences between MODIS and VIIRS is revealed in cloud top pressure retrievals and in the imager cloud distribution skewness. Our study presents a unique aspect to examine NASA's progress toward building a continuous cloud data record with sufficient quality to investigate clouds' role in global environmental change.

This study evaluates changes in simulated Pacific climate between two ECMWF re-analyses; the ERA Interim (ERAI) and the newest ERA5. Changes in the Integrated Forecasting System (IFS) and possibly sea surface temperature result in greatly reduced discrepancies in ERA5’s ice water path (IWP), radiative fluxes and precipitation relative to satellite-based observational products. IWP shows the largest percentage change, increasing by over 300% from ERAI to ERA5, due to inclusion of falling ice (snow) that impacts radiative calculation. ERAI to ERA5 changes in high-cloud fraction are generally anticorrelated as expected with outgoing longwave radiation, with ERA5 having smaller longwave discrepancies versus CERES observations compared with ERAI. Reflected shortwave discrepancies are similarly reduced from ERAI to ERA5, which appears to be due to changes in both cloud fraction and optical depth. Finally, ERA5 also reduces a longstanding precipitation excess relative to the GPCP observational product in the southern trade winds region between the Southern Pacific and intertropical convergence zones. This appears to be related to cooler prescribed sea surface temperatures, thereby reducing local moisture supply via suppressing net latent heat flux and stronger surface trade-winds. Compared with GPCP and CERES, ERA5 shows similar geographic patterns of discrepancies to ERAI in terms of precipitation and top-of-atmosphere radiation, but their magnitudes are greatly reduced in ERA5.

The impacts of falling ice (snow) radiative effects (FIREs) on simulated surface wind stress and sea surface temperature (SST) in Central Pacific El Niño (CP-El Niño) under a progressive warming climate are examined. Using controlled simulations with the CESM1 model, it is shown that the exclusion of FIREs (no snow: NOS) generates persistent westerly anomalies in surface wind stress relative to that with FIREs (snow on: SON). These anomalies subsequently lead to a weakening of the easterly trade winds associated with warmer SST anomalies in modeled life cycle. Results over three separated 40 year intervals (P1: 21–60 years; P2: 61–100 years; P3: 101–140 years) are compared with Coupled Model Intercomparison Project phase 5 (CMIP5) models without FIREs. Both NOS configuration and CMIP5 models simulate longer life cycles of CP-El Niño events with weakening easterlies and warmer SST anomalies on the equator, persistently propagating eastward from the mature to dissipating phases. Compared to NOS, SON, on the other hand, produces a shorter CP-El Niño life cycle together with stronger easterlies and colder SSTs over the eastern to central equatorial Pacific. The magnitudes of the simulated westerlies and warm SST anomalies tend to diminish without eastward shifting following the peak of the CP-El Niño activity. There are substantial differences in CP-El Niño characteristics from P1 to P3 between NOS and SON. During P1, both SON and NOS show patterns which are consistent with their present-day counterparts. In P2 and P3, SON exhibits a prolonged CP-El Niño life cycle, while NOS develops a double-peak El Niño evolution at the mature and decaying phases. Regarding El Niño diversity and the projections, the CMIP5 models have not reached a consensus. The inclusion of the FIREs would increase the confidence in simulating El Niño future behavior.

The falling ice (snow) radiative effects (FIREs) have previously been shown to contribute substantially to reduced discrepancies in simulations of present-day climatology of radiation, skin temperatures and sea ice concentration and thickness over the Southern Ocean.This study extends to examine the impacts of FIREs on simulation of sea ice changes under a scenario of gradual increase of atmospheric CO 2 concentration.We perform a pair of sensitivity experiments including (CESM1-SoN) and excluding (CESM1-NoS) FIREs using Community Earth System Model version 1.The differences in the annual and seasonal means between the initial and warmer periods are examined.Relative to CESM1-SoN, CESM1-NoS simulates more surface reflected shortwave and less downward longwave radiative warming, as well as colder surface temperature, resulting in larger annual-mean sea ice extent and thickness and slower seasonal and long-term sea ice melting and thinning.Over the Southern Ocean of CESM1-SoN, reduced downwelling longwave radiation in austral winter (June-July-August: JJA) leads to sea-ice growth with colder skin temperature while reduced net radiation resulting from increased shortwave reflection in austral summer reduces the melting of sea ice with little change in skin temperature.CESM1-NoS shows seasonal and long-term trends similar to those in CMIP5 models that exclude FIREs, hinting slower future warming-driven changes and larger amplitude of the annual cycle in sea ice concentration and thickness.The ice-free Southern Ocean in peak melting season is simulated at approximately year 130 for CESM1-NoS but year 100 for CESM1-SoN, about 30 years later than that of the Arctic.

This study compares radiation-circulation coupling over tropical and subtropical oceans by examining ice water path, radiation, low-level wind, and precipitation fields from the uncoupled prescribed sea surface temperature AMIP6 and the fullycoupled CMIP6 historical runs. Ice water path of the CMIP6 ensemble shows a closer agreement than the AMP6 ensemble, particularly in the subtropics. The inclusion of falling ice (snow) radiative effects (FIREs), in general, improves simulation of radiation, low-level wind, and precipitation fields over the northwest Pacific, Southeast Pacific Convergence Zone (SPCZ), equatorial eastern Pacific and Atlantic Intertropical Convergence Zone (ITCZ). When both AMIP6 and CMIP6 models are divided into two groups with inclusion and exclusion of FIREs, the impact of FIREs is most pronounced in ITCZ and the subtropical trade-wind regions in CMIP6 but over the tropical Pacific and SPCZ in AMIP6. This suggests that active ocean plays a significant role in radiation-circulation coupling. The CMIP6 models with FIREs have less over-estimated biases in upward longwave radiation over the convective zones in Pacific and Atlantic and less low-level divergence of anomalous flows over convective zones, i.e., stronger trade winds. The circulation changes stronger in CMIP6 over the trade wind regions than in AMIP6, which suggests that the role of active ocean is important in testing an improved physical process in models. This conclusion is also supported by more systematic improvements in groups of models with inclusion of FIREs in CMIP6 than in AMIP6, hinting the important roles of FIREs in radiationcirculation coupling.

A common deficiency in coupled atmosphere-ocean models is the lack of stratocumulus clouds near the west coasts of continents and shallow cumulus clouds over the trade wind regions. In this study, we examine changes of large-scale trade wind environment associated with hydrometeors-radiation-circulation interactions, focusing over the south-central Pacific, by carrying out experiments with falling ice (snow) radiative effects (FIREs) on or off using the CESM1-CAM5 coupled model. Compared to observations/reanalysis data and an experiment with FIREs on (SON), an experiment with FIREs off (NOS) tends to have too-high (0.3-0.9 K) sea surface temperature (SST) over the eastern half of the Pacific, and weaker surface wind stress over the northeast side but stronger stress over the southwest side of the study domain. The difference between NOS and SON shows lower-tropospheric wind fields with a cyclonic-like wind pattern, which is similar to surface wind stress, accompanied with weaker subsidence over the northeast side. The cyclonic-like wind pattern causes stronger moisture convergence, accompanied with horizontal warm and moist advections, and stronger effective ascending motion over the northeast side of the study domain, building large-scale environmental conditions to facilitate middle- and high-clouds instead of shallow cumulus clouds in nature. This large-scale environment in conjunction with the higher SSTs may be also responsible for the lack of the stratocumulus clouds near the coast of America. Remaining biases in SON such as the double intertropical convergence zone and parameterization deficiencies may also prevent a more realistic simulation of trade wind clouds and their associated large-scale environments.

Land skin temperature (Ts) is directly influenced by surface energy balance, in particular, radiative energy, which can be linked to model’s representation of radiative effects of hydrometeors in the atmosphere. This link is inferred by examining the changes of geographical distribution and seasonal cycle of surface radiation, surface turbulent fluxes and Ts between a pair of 140 years sensitivity experiments under 1% per year increase of atmospheric CO2. One is with radiative effects of falling ice (snow) hydrometeors on (SON) and the other off (NOS) using CESM1-CAM5 and the results are compared with CMIP5 models without these effects. For boreal winter, NOS relative to SON simulates less surface downward longwave and net flux (∼10–15 W m−2), resulting in colder Ts (∼2–3 K colder), over mid- and high latitudes, but more solar radiative flux, resulting in warmer Ts (∼1–3 K), over subtropical and tropical land. These differences between NOS and SON are amplified as the surface and the atmosphere become warmer. The results from CMIP5 ensemble generally match with those of NOS. Temporal correlation analysis indicates that the linkage between Ts and falling ice hydrometeor changes is through one between Ts and downward longwave and net fluxes at high latitudes, but strongly weakened by shortwave changes at low latitudes (and boreal summer). Relative to SON, land skin temperatures in NOS and CMIP5 are underestimated throughout the seasonal cycle but only slightly in summer.