New satellite study reveals a widespread transition zone in the sky, challenging climate models

A new study published in Advances in Atmospheric Sciences has shed light on a previously overlooked but common atmospheric phenomenon: the transition zone (TZ) where clouds and aerosols blend together, making it difficult to tell where one ends and the other begins. These conditions in the sky are more prevalent than previously thought and could be a key to reducing uncertainty in future climate projections.

Clouds and aerosols (tiny suspended particles in the air) are critical players in regulating Earth's temperature, but they remain one of the largest sources of uncertainty in climate models. Traditionally, scientists have classified atmospheric layers as either cloud or aerosol. However, this new research confirms that the real atmosphere is not so binary, often featuring a gradual transition filled with features like wispy cloud fragments or hydrated aerosols that defy simple classification.

The research, led by Jaume Ruiz de Morales from the Universitat de Girona, was honored with the Best Poster Prize at the 2024 International Radiation Symposium (IRS2024) and has been solicited for publication in the symposium's special issue. The work was conducted by an international team from the Universitat de Girona (Spain), the Karlsruhe Institute of Technology (Germany), and the Universitat de Barcelona (Spain).

"Our findings show that the atmosphere is far less black-and-white than climate models assume," said lead author Jaume Ruiz de Morales. "Nearly one in ten measurements reveals this ambiguous transition zone. Ignoring it means we might be missing a critical piece of the puzzle in understanding how the atmosphere manages the Earth's energy budget. Our work calls for questioning how we represent these suspended particles in climate models to achieve more accurate predictions, especially regarding their radiative effects."

To conduct their analysis, the team used a year of high-resolution data from the CALIOP lidar aboard the CALIPSO satellite. They identified these ambiguous TZ layers as those falling within the no-confidence range of a standard cloud-aerosol discrimination algorithm, plus the fuzzy edges of cirrus clouds.

The global assessment revealed that these transition zones are remarkably common, appearing in nearly 10% of all atmospheric profiles measured by the satellite. The study further identified three distinct types of TZ layers:

• Type 1: Layers classified as “cirrus fringes” by the algorithm.
• Type 2: Found at higher altitudes, with properties intermediate between thin ice clouds and aerosols, resembling high wispy clouds.
• Type 3: Found at lower altitudes, with properties between water clouds and aerosols, such as large, hydrated aerosol particles.

Geographically, TZ layers are found worldwide. Type 1 and 2 layers are most frequent in the Intertropical Convergence Zone and mid-latitudes. In contrast, type 3 layers are predominantly found over oceans off the coasts of West Africa and East Asia, regions known for elevated smoke and dusty marine aerosols.

This high frequency of transition zones poses a significant challenge for current climate models, which typically treat clouds and aerosols as separate, distinct entities, that do not fully represent the gradual change detected in observations of the real atmosphere. The research underscores the need to develop more sophisticated representations of atmospheric particles, moving beyond a simple two-category system to improve future climate projections. The team proposed two strategies to improve such parametrizations. To either include an intermediate phase between clouds and aerosols or to treat all suspended particles in the atmosphere as a continuum of states.