The campaign was led by Dr. Husi Letu of the Aerospace Information Research Institute at the Chinese Academy of Sciences, working with collaborators from a broad consortium of Chinese institutions, including the Institute of Tibetan Plateau Research, the Institute of Atmospheric Physics, Xi’an University of Technology, the National University of Defense Technology, the National Satellite Meteorological Center, Peking University, and Tsinghua University. The team conducted its field work at Lulang, in the southeastern portion of the plateau, a region often described as part of the Third Pole because its high-elevation atmosphere behaves in ways that differ markedly from the lower, more aerosol-laden air over most of the inhabited world. The balloon payload carried instruments capable of measuring cloud droplet spectra, aerosol properties, standard meteorological variables such as temperature and humidity, and the vertical component of the atmospheric electric field, all sampled while the platform remained inside the cloud rather than passing through it.

The distinction between sampling from a tethered balloon and sampling from an aircraft is more than logistical. Research aircraft typically traverse a cloud at cruise speeds of tens to hundreds of meters per second, which means their instruments register a spatially compressed snapshot along a mostly horizontal track. Vertical structure, the dimension that governs how droplets nucleate, grow by condensation, and are redistributed by updrafts and entrainment, is often inferred indirectly. A tethered balloon inverts that geometry. Because it can ascend and descend slowly on its tether, or hold station at a fixed altitude, it resolves vertical gradients over very small height intervals and can revisit the same layer repeatedly as conditions evolve. “With the tethered balloon, we can stay in the cloud and observe how its microphysical and electrical properties change with height,” said Dr. Huazhe Shang, who helped design and coordinate the campaign. That capability is particularly valuable over complex terrain such as the Tibetan Plateau, where orographic forcing shapes cloud development in ways that are difficult to reproduce in models.

The campaign’s clearest finding concerns the relationship between aerosols near cloud base and the droplets that form above them. Aerosol particles serve as the seeds on which water vapor condenses; without such cloud condensation nuclei, droplets cannot form at the humidities found in most of the troposphere. The observations showed that clouds embedded in air with higher aerosol loading contained greater numbers of droplets, but that those droplets were smaller on average. This inverse relationship between droplet number and size is a classic expectation of aerosol-cloud interaction theory: when many particles compete for a finite supply of available water vapor, the condensed water is partitioned into more, smaller droplets. The measurements also indicated that cloud water content increased with aerosol abundance, a link that was especially evident under the comparatively clean conditions of the plateau.

That last point carries weight beyond the immediate measurements. In heavily polluted atmospheres, the signal of aerosols on cloud properties can be difficult to isolate because so many competing processes are at work. In the near-pristine air over the Tibetan Plateau, where the pool of particles capable of acting as cloud condensation nuclei is relatively limited, the observations raise the possibility that those particles exert a disproportionately strong control on how clouds develop. “These observations raise the possibility that aerosols may play a more pronounced role in shaping cloud microphysics under clean conditions,” said Dr. Hengqi Wang, who participated in the campaign from its planning stages through execution. If borne out by further work, the finding would suggest that pristine regions are not, as sometimes assumed, places where aerosol effects on clouds can be safely neglected, but rather places where a modest change in particle number could meaningfully alter droplet populations, cloud reflectivity, and precipitation formation.

The vertical profiles collected by the balloon added a second layer of insight into droplet behavior. As expected from basic cloud physics, droplets generally grew larger and cloud water content increased with height inside the cloud, a pattern consistent with continued condensation of water vapor as air rises and cools. Rising air expands and cools adiabatically, and as the temperature falls the air’s capacity to hold vapor diminishes, so excess vapor condenses onto existing droplets and enlarges them. Not every profile followed this simple trajectory, however. In some cases the number of droplets decreased with height, a signature most likely attributable to entrainment, the process by which drier, droplet-free air from outside the cloud is mixed into its interior. Entrainment dilutes the cloud, evaporates some droplets, and introduces spatial inhomogeneity that single-pass aircraft sampling often misses. Resolving these features directly, at fine vertical resolution, is precisely the kind of observation the tethered platform was designed to deliver.

The electrical measurements may prove to be the campaign’s most distinctive contribution. Historically, studies of cloud electrification have concentrated on thunderstorms, where charge separation is vigorous enough to generate lightning, and where electric fields can reach magnitudes of tens of kilovolts per meter. Warm, non-lightning clouds have received far less attention, largely because their electrical activity was assumed to be negligible. The Tibetan Plateau observations confirmed that the fields in such clouds are indeed weak, but they also showed that weak does not mean featureless. The measured field strengths showed no clear linear relationship with droplet number or droplet size, and the difference between readings inside and outside the cloud was small. Taken together, these results suggest that in weakly electrified warm clouds, the local electric field may be governed primarily by the background atmospheric electrical environment, the global circuit and fair-weather field in which all clouds are embedded, rather than by processes internal to the cloud itself.

At the same time, the researchers emphasize that the absence of lightning does not imply the absence of electricity. Some variability in the electric field remained within the observed clouds, and although these perturbations are far weaker than the fields associated with thunderstorms, they are not necessarily irrelevant. The electromagnetic environment inside even a weakly electrified cloud can matter for aircraft and other airborne platforms, particularly those operating sensitive instruments or relying on electrical systems whose performance could be influenced by ambient field variations. Documenting the magnitude and structure of these weak fields inside non-lightning clouds therefore has practical implications for aviation and for the design of future airborne measurement campaigns, in addition to its value for understanding the physics of charge distribution in shallow clouds.

The campaign also demonstrates a methodological template that its leaders intend to expand. Because the balloon measurements were combined with ground-based observations and satellite data over Lulang, the study illustrates how a relatively inexpensive and flexible platform can anchor a multi-instrument observing strategy over terrain that is difficult for conventional facilities to reach. “This campaign is only a beginning,” said Dr. Husi Letu, the corresponding author of the paper. “In the next step, we will coordinate multi-platform observations from aircraft, tethered balloons, satellites, and ground-based radars to further investigate and compare the microphysical and electrical characteristics of warm, mixed-phase, and ice clouds under different environmental conditions.” Such coordination would allow researchers to connect the fine-scale vertical structure captured by the balloon with the broader spatial context provided by radar and satellites, and to extend the analysis from liquid warm clouds to the mixed-phase and ice clouds that dominate precipitation over high mountains.

For a field in which cloud processes remain among the largest sources of uncertainty in climate projections, the ability to hover inside a cloud and watch droplets, aerosols, and electric fields evolve together is a genuine advance. The Tibetan Plateau, with its clean air, intense solar radiation, and complex orography, offers an unusually sensitive natural laboratory for these questions, and the first results from the tethered-balloon campaign suggest that even its gentlest clouds hold information that has been hidden from view. What looks tranquil from the ground is, on close inspection, a finely balanced system in which the smallest particles help set the size of the smallest droplets, and faint electric fields trace the architecture of the atmosphere itself.

Subject of Research: Cloud microphysics and electric field measurements in non-lightning clouds over the Tibetan Plateau using a tethered balloon

Article Title: Tethered balloon peers inside clouds to reveal hidden aerosol and electrical effects

Article References: Tethered balloon peers inside clouds to reveal hidden aerosol and electrical effects. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: tethered balloon, cloud microphysics, aerosols, electric fields, Tibetan Plateau, warm clouds, cloud droplets, atmospheric electricity, field campaign, Third Pole, non-lightning clouds, Advances in Atmospheric Sciences