Evaluation of Low-Level Clouds, Temperature, and Surface Radiation

  Southern Ocean clouds are one of climate science's greatest challenges       

The Southern Ocean plays a pivotal role in regulating Earth's climate, yet the clouds that blanket this remote region remain among the least understood features in atmospheric science. By controlling both incoming sunlight and outgoing heat, these clouds strongly influence Earth's energy balance. Low-level clouds over the Southern Ocean (SO) exert a strong influence on surface radiation, yet their representation in reanalyses and climate models remains uncertain. Study evaluated cloud properties and surface radiative fluxes from ERA5, MERRA-2, and the CAM-ATRAS model using observations obtained over the SO by Japan's R/V Shirase. ERA5 and MERRA-2 overestimate the frequency of low-level clouds with a base below 1 km. Although CAM-ATRAS also overestimates very low clouds, it shows the best overall agreement with observations in cloud occurrence and phase. Despite the high frequency of low-level clouds, all data sets underestimate downward longwave (DLW) radiation. Cold bias and cloud phase bias likely reduce cloud-base emissivity and contribute to underestimated DLW radiation. Aerosol sensitivity experiments using CAM-ATRAS indicate that enhanced cloud condensation nuclei concentrations increase low-level cloud frequency but have a limited impact on surface radiative fluxes.

Even small errors in representing them can introduce significant uncertainties into weather forecasts, climate models and projections of future global warming, making them a long-standing challenge for climate scientists. To better understand why these clouds remain so difficult to simulate, researchers from the National Institute of Polar Research (Japan) and Nagoya University analyzed cloud observations collected during the 64th Japanese Antarctic Research Expedition (JARE64) aboard the research icebreaker R/V Shirase. Professor Jun Inoue explains, "Numerical models have been reported to exhibit limited skill in reproducing clouds. In particular, over the Southern Ocean and Antarctica, where cloud representation remains especially challenging, cloud-related biases have been shown to increase errors in the surface energy budget through biases in the radiative budget." Clouds over the Southern Ocean (SO) exert disproportionate influence on Earth's radiation budget by strongly modulating both shortwave and longwave radiative fluxes. Cloud macrophysical properties, such as horizontal and vertical cloud fractions, play critical roles in controlling downward shortwave (DSW) and downward longwave (DLW) radiation at the surface. Therefore, numerous studies have investigated the cloud fraction over the SO. Specifically, ice and liquid clouds produce distinct radiative effects because ice clouds have lower emissivity than liquid clouds, generally reflect less shortwave radiation, and emit weaker longwave radiation.

Many observational studies have been conducted focusing on the cloud phase over the SO. Although pure supercooled liquid water (SLW) freezes homogeneously below −38°C, ice nucleating particles (INPs), such as bioaerosols, mineral dust, and organic aerosols originating from both local and remote sources, can trigger freezing at much higher temperatures. Satellite and ship-based observations suggest that bioaerosols and other INPs contribute to mixed-phase and ice cloud formation at relatively high temperatures above −15°C over the SO and Antarctic coastal regions during spring and summer, which are seasons with high biological activity. Despite these findings, SLW clouds remain prevalent over the SO in summer, with occurrence frequencies of approximately 40% reported in active satellite products. Shipborne observations further indicate that SLW clouds are prevalent in the middle troposphere at temperatures higher than −25°C. Additionally, sulfate and organic aerosols, which comprise the greatest proportion of cloud condensation nuclei (CCN), play a central role in regulating low-level liquid cloud microphysics over the SO. Accordingly, many previous studies focused on aerosols and their role in low-level cloud formation over the SO. Numerical weather prediction and climate models are used widely to investigate cloud processes on the global scale. However, many models underestimate low-level cloud cover and the liquid water path, leading to excessive absorption of shortwave radiation at the surface over the SO. Such biases are commonly attributed to deficiencies in the representation of cloud phase and aerosol–cloud interactions, as demonstrated by comparisons with satellite products. The radiative effects of SO clouds remain a major source of uncertainty in climate projections. Indeed, substantial intermodel spread exists in the representation of cloud phase, and improved cloud phase representation with realistic INP concentrations can lead to reduced biases in cloud top radiative effects. Consequently, the SO remains one of the most challenging regions for cloud modeling, where persistent surface radiation biases have been identified in numerical models.

From December 2022 to March 2023, ship-based instruments continuously measured cloud properties, atmospheric temperature and humidity, surface radiation and aerosol concentrations, providing a comprehensive benchmark for evaluating model performance. The team evaluated two widely used atmospheric reanalysis data sets, ERA5 and MERRA-2, alongside the CAM-ATRAS climate model using observations throughout the expedition. Although all three data sets broadly captured cloud patterns over the Southern Ocean, important differences emerged. ERA5 and MERRA-2 consistently overestimated the occurrence of low-level clouds, whereas CAM-ATRAS most closely matched the observations, particularly in reproducing cloud occurrence and cloud phase. Surprisingly, despite simulating abundant low-level clouds, all three data sets underestimated the amount of downward longwave radiation reaching the surface. Comparison with observations showed that the reanalysis data sets contain higher aerosol concentrations than observed. Therefore, the researchers also conducted sensitivity experiments with CAM-ATRAS by increasing aerosol emissions over the Southern Hemisphere to examine how aerosols influence cloud formation and surface radiation. However, the aerosol sensitivity experiments further showed that increasing aerosol concentrations produced more low-level clouds but had only a limited effect on surface radiation.

The researchers traced this discrepancy to the physical properties of the simulated clouds rather than to cloud amount alone. In the models, clouds contained excessive ice, reducing the heat emitted toward the surface. However, these results demonstrate that biases in cloud representation alone cannot explain the underestimated DLW. Instead, the numerical models exhibit an inherent cold temperature bias, which also plays a role in the underestimation of DLW. These findings indicate that accurately representing both cloud phase and temperature is more important than simply reproducing cloud frequency when simulating the Southern Ocean's surface energy budget. By identifying the processes responsible for persistent cloud biases, the study provides valuable guidance for improving weather and climate models. Better representation of cloud microphysics, aerosol–cloud interactions and the background environment will help reduce uncertainties in simulations of Earth's energy balance, leading to more reliable predictions of future warming, sea ice change and climate variability. CAM-ATRAS explicitly simulates aerosol processes, including new particle formation, condensation, coagulation, activation to cloud droplets, aqueous-phase chemistry, dry and wet deposition, aerosol–radiation interactions, and aerosol–cloud interactions. Aerosols are represented using 12 size bins spanning diameters from 1 nm to 10 μm. Major aerosol species, including sulfate, black carbon, organic matter, SS, dust, marine organic aerosols, and bioaerosols are explicitly simulated, allowing detailed investigation of aerosol number concentrations and mixing states. INP number concentrations in clouds are calculated based on the simulated concentrations of dust, marine organic aerosols, and bioaerosols, the temperature dependence of ice-nucleation active site density per unit mass for each species, ambient temperature, and cloud fraction. These INP concentrations are then used to calculate ice nucleation within the cloud microphysical scheme.

In this study, the simulation period corresponding to the JARE64 cruise was used for analysis. Monthly sea surface temperature and sea ice distributions were prescribed as boundary conditions. The model was nudged toward MERRA-2 reanalysis fields for temperature and horizontal wind components in the free troposphere (pressure levels <800 hPa). The horizontal resolution was 0.9° × 1.25°, with 30 vertical layers extending from the surface to 40 km. In addition to the base simulation, a sensitivity simulation in which aerosol emissions over the SH were enhanced by two orders of magnitude (hereafter, referred to as the CAM-ATRAS SH × 100 experiment). The researchers emphasize that continued progress will require not only expanded observations of clouds across the Southern Ocean and Antarctica but also increased observations of fundamental atmospheric variables, particularly temperature, to reduce the cold bias in numerical models. As Assistant Professor Kazutoshi Sato notes, "Because observations over Antarctica remain sparse, numerical models still contain substantial uncertainties in their representation of the Antarctic atmosphere. Therefore, incorporating existing but currently underutilized observations into numerical models may provide an effective solution. For example, assimilating observations from the PANSY radar at Japan's Syowa Station, which are not yet routinely used in numerical weather prediction systems, could help reduce model biases and improve forecast accuracy." This study represents one of the most comprehensive observational evaluations of cloud and radiation simulations over the Southern Ocean using data collected during the JARE64 expedition.

During JARE64, clouds with a wide range of cloud base heights were observed over the SO and the Antarctic coastal regions. We calculated the frequency distribution of cloud base height over the entire observation period using 500-m vertical bins. Observations showed that clouds with a base height of below 1 km exhibit both primary and secondary peaks in occurrence frequency. Clouds with a base height in the middle troposphere were also frequently observed over the SO and Antarctic coastal regions. To compare cloud base height between observations and models, the modeled cloud base height was defined as the lowest model level at which the cloud liquid or ice mixing ratio exceeds 0.001 g kg−1. ERA5 overestimates the frequency of clouds with a base height of below 1 km. For CAM-ATRAS, although the frequency of clouds with a base height below 0.5 km is still overestimated, the occurrence frequency of clouds with a base height of below 1 km is closer to that of the observations than that of the two reanalysis data sets (ERA5 and MERRA-2). By revealing why current models struggle to reproduce these clouds and identifying the processes responsible for long-standing biases, the findings provide an important step toward more accurate weather forecasts, improved climate models and more confident projections of Earth's changing climate.

The National Institute of Polar Research (NIPR) was founded in 1973, is an inter-university research institute dedicated to advancing scientific research and observations in the Arctic and Antarctic regions. As one of the four institutes under the [Research Organization of Information and Systems (ROIS)], NIPR conducts comprehensive polar research through observation stations and international collaborations. The institute also promotes polar science by supporting collaborative research projects and providing access to scientific data, samples, and materials. NIPR remains Japan’s only institution devoted to comprehensive research activities in both polar regions. ROIS is a parent organization of four national institutes (National Institute of Polar Research, National Institute of Informatics, the Institute of Statistical Mathematics and National Institute of Genetics) and the Joint Support-Center for Data Science Research. It is ROIS's mission to promote integrated, cutting-edge research which goes beyond the barriers of these institutions, in addition to facilitating their research activities, as members of inter-university research institutes.

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China's Y-20 Transport Aircraft

Chinese PLA Air Force's Y-20 Family 

The Y-20 is China's first domestically developed new-generation large military transport aircraft. Characterized by its long range, heavy payload capacity and high speed, it is capable of conducting long-distance airlift operations for both personnel and cargo under complex meteorological conditions.  When six Chinese heavy transport planes entered European airspace in April 2022, it was not just their cargo that drew international attention. Bound for Serbia, the Y-20 aircraft had flown thousands of km's from China carrying components of the Chinese-made FK-3 surface-to-air missile system. A second wave of six aircraft followed, bringing the operation to 12 sorties in what was then China’s biggest overseas airlift involving the strategic cargo plane. Coming just weeks after Russia invaded Ukraine, the delivery raised concerns among European capitals about Belgrade’s growing military ties with Beijing. But the operation also revealed something significant about Chinese capabilities: the People’s Liberation Army could now use its own heavy transport aircraft to move military equipment from East Asia deep into Europe. At the centre of that capability was the Y-20 Kunpeng, China’s first domestically developed heavy military transport. The Xi'an Y-20 is China's largest domestically developed, heavy-lift strategic military transport aircraft. The Y-20 significantly enhances the People's Liberation Army Air Force (PLAAF) mobility by enabling rapid deployment of heavy combat gear, armored vehicles and troops over long distances. Officially named "Kunpeng" after a mythical giant Chinese bird, and affectionately called "Chubby Girl" due to its wide fuselage. It has supported international humanitarian missions, disaster relief and long-range logistics across dozens of countries. Some  of the important specifications are as follows:-

Payload Capacity : Up to 66 metric tons.

Maximum Takeoff Weight : 220 metric tons.

Range : Approximately 4,400 to 4,500 km's with a heavy payload (and up to 10,000+ km under specific ferry configurations).

Variants

Y-20A : The initial baseline variant powered by Russian-designed D-30 engines.

Y-20B : An upgraded variant featuring fully domestic WS-20 turbofan engines, marking complete independence in Chinese engine and electronic manufacturing.

YY-20 / YY-20A : An aerial refueling tanker variant expanding the fleet's force projection and cross-platform capabilities (such as recent high-profile refuel drills).

KJ-3000 : An emerging airborne early-warning and control (AEW&C) derivative built on the Y-20 airframe currently undergoing advanced flight testing.

Four years on, the aircraft is evolving into something considerably more versatile. China has introduced an improved version powered by domestically produced engines, confirmed that the airframe can switch between transport and aerial-refuelling roles, and is using it as the basis for a new airborne early-warning aircraft. What began as China’s answer to a shortage of strategic airlift is becoming a family of support aircraft that could help the PLA move forces further from home and sustain and coordinate combat aircraft deeper into the western Pacific. China formally launched its large transport aircraft programme in 2007, with the Xian Aircraft Corporation leading development. The first Y-20 flew in January 2013 and entered PLA Air Force service in July 2016, giving Beijing an indigenous heavy airlifter after years of mostly relying on imported Russian Il-76 aircraft. The platform’s development was driven in part by Beijing’s difficulty in acquiring additional Russian Il-76 transports and Il-78 tankers. On 06 July, 2016, the Y-20 was officially commissioned into service with the Chinese People’s Liberation Army (PLA) Air Force, marking a pivotal step forward in the Chinese PLA Air Force's strategic projection capability.

Designed to fill that strategic void, the Y-20 can carry a maximum payload of roughly 66 tonnes, with an estimated range of about 3,700km (2,300 miles) at that load. But the Serbia mission highlighted how refuelling stops along the route could extend the airlifter’s reach far beyond its range on a single flight. One of the programme’s most important changes has come under the wings, however. Since its commissioning, the Y-20 has undertaken a broad spectrum of missions, ranging from military transport operations and participation in military parades to international humanitarian assistance. It has become an integral part of the Chinese PLA Air Force's realistic combat-oriented training system, while also playing an increasingly prominent role in military operations other than war (MOOTW), including emergency rescue and disaster relief, humanitarian assistance and transnational airlift missions. The Chinese PLA Air Force's main combat aircraft has entered a new era represented by the Y-20, the J-20 and other advanced aircraft at an unprecedented pace. The original Y-20A uses Russian D-30KP-II turbofan engines. The newer Y-20B instead uses four domestically produced WS-20 high-bypass turbofans.

The WS-20 offers greater thrust and better fuel efficiency than the D-30, although reliable public figures quantifying the resulting gains in range or payload are not available. More broadly, replacing Russian engines gives China greater control over the production and future development of an aircraft increasingly central to its long-range aviation capabilities. It is estimated that around 90 Y-20A-family transports and tankers were operational by August 2025, while close to two dozen Y-20Bs were in service or awaiting delivery. The Y-20 has also helped China address another constraint on its ability to project air power like aerial refuelling. China has developed a dedicated tanker derivative, the YY-20A, which allows combat and other aircraft to fly further and remain airborne longer, an increasingly important capability as PLA operations extend deeper into the western Pacific and across the South China Sea. Its growing utility was on show during joint exercises with Egypt in August, when a YY-20A was photographed refuelling an Egyptian Air Force Rafale, a rare example of a Chinese tanker supporting a foreign-made combat aircraft. But rather than relying solely on dedicated tankers, China is now adding greater flexibility to the Y-20 fleet itself. It was confirmed earlier this month that the Y-20B could switch between transport and aerial-refuelling missions by installing additional fuel tanks and removable refuelling equipment.

The evidence points towards the Y-20B becoming a common multi-role tanker-transport platform rather than China maintaining entirely separate production lines for B-model transports and tankers. Such an arrangement could allow commanders to shift aircraft between two critical tasks, moving troops and equipment or refuelling combat aircraft, as operational requirements changed. It would also allow the PLA Air Force to generate additional tanker capacity from its transport fleet when required, rather than relying entirely on dedicated tanker aircraft. This flexibility could be particularly valuable during sustained operations further into the western Pacific or over the South China Sea. However, no operational Y-20B has yet been publicly observed fitted with the complete tanker configuration. The necessary fuel-transfer systems and wiring may already be incorporated into production aircraft even when the external refuelling equipment is absent. Over the past decade, the Y-20 has evolved from a single transport platform into a broad family of aircraft comprising multiple variants, including the Y-20A, Y-20B and YY-20A aerial tanker. Its operational footprint now extends across Asia, Africa, Europe and Oceania, making it a defining symbol of the Chinese PLA Air Force's endeavor to build itself into a world-class armed force. Yet the platform’s evolution extends beyond transport and aerial refuelling. The Y-20B is also providing the basis for the KJ-3000 airborne early-warning and control aircraft, which has been undergoing flight testing since late 2024.

As PLA combat aircraft expand their operational reach, the KJ-3000 is intended to solve another problem: greater range matters little without the ability to detect threats, track targets and coordinate operations across an expanding battle space. The KJ-3000’s large dorsal radar is designed to provide airborne surveillance and command-and-control capabilities that could complement or eventually replace China’s ageing KJ-2000 fleet, which is based on the Russian Il-76. This capability is becoming more important as Beijing asks the PLA to operate further from home. A Taiwan conflict could require sustained high-intensity operations while potentially confronting US and allied forces across the western Pacific, while the South China Sea presents another vast operating area. Beijing sees Taiwan as part of China and has never ruled out the use of force to achieve reunification. Most countries, including the US, Taiwan’s main international backer, do not recognise the self-ruled island as an independent state. However, Washington is opposed to any attempt to take Taiwan by force and is committed to supplying it with weapons. Its development highlights a broader consequence of the Y-20 programme: China now has a domestically produced large-aircraft platform which can be adapted for specialised roles, further reducing its reliance on imported airframes as happened in the past.

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