Moisture-Driven Turbulence during AntarcticMoisture-Intrusion Regimes: Evidence fromRadiosondes over the Antarctic Peninsula

Technical Summary: Moisture-Driven Turbulence during Antarctic Moisture-Intrusion Regimes

Problem Statement
While moisture intrusions, including Antarctic atmospheric rivers (AARs), are recognized as primary drivers of cloud development and thermodynamic variability over the Antarctic Peninsula, their specific influence on atmospheric turbulence and vertical mixing remains poorly constrained. Existing models and diagnostics often rely on dry stability assumptions, which may fail to capture the complex thermodynamic interactions in polar environments where temperatures are well below freezing. Specifically, the role of phase changes—particularly deposition onto ice crystals in ice-supersaturated regions (ISSRs)—in modifying static stability and enhancing turbulent overturning is not fully understood. This study addresses the gap in quantifying how moist thermodynamics, distinct from dry dynamics, modulates turbulence during moisture-intrusion regimes in the Antarctic troposphere.

Methodology
The study utilizes high-resolution data from the Turbulence and Supercooled Clouds in Antarctica (T-SCAN) field campaign, conducted at Marambio Base (64.23°S) on the northern Antarctic Peninsula between January 12 and February 18, 2025.

• Observational Data: The core dataset comprises 32 high-resolution (1 Hz, ~3–5 m vertical resolution) radiosonde soundings using Vaisala RS41-SGE sondes, measuring temperature, relative humidity (RH), pressure, and wind. Of these, 19 flights included Anasphere vibrating-wire probes for in situ supercooled liquid water content (SLWC) measurements.
• Synoptic Context: ERA5 reanalysis data were used to characterize large-scale circulation, specifically identifying AAR periods via vertically integrated water vapor transport (IVT) and distinguishing direct AAR landfall from cyclonically driven moisture advection.
• Thermodynamic Processing:
  • RH profiles were corrected to reference ice saturation ($RH_{ice}$) for temperatures below 0°C using the Murphy and Koop formulation.
  • Saturated layers were identified using altitude-dependent thresholds adapted from Zhang et al. and Wilson et al.
  • A composite potential temperature ($\theta^*$) was constructed to account for both dry stability (in subsaturated layers) and moist stability (in saturated layers). In saturated layers, the moist Brunt-Väisälä frequency ($N^2_m$) was calculated, incorporating latent heat release from deposition (for ice) or condensation (for liquid).
• Turbulence Diagnostics: The Thorpe sorting method was applied to both the standard dry potential temperature ($\theta$) and the composite moist potential temperature ($\theta^*$). This allowed for the calculation of Thorpe displacements ($d_i$) and Thorpe scales ($L_T$), quantifying the vertical extent of overturning structures. Turbulent fractions were computed as the ratio of the total depth of accepted overturns to the tropopause height.

Key Results

• Prevalence of Ice Supersaturation: The campaign captured deep saturated and ice-supersaturated layers extending up to 6–8 km. On average, ice supersaturation accounted for approximately 60% of all saturated layers, reaching up to 90% in individual cases. In contrast, liquid-water saturation was largely confined to the lowest ~1 km of the troposphere, with supercooled liquid water (SLWC) signatures detected only in shallow, intermittent layers in three of the 19 SLWC-equipped flights.
• Moisture-Driven Instability: Accounting for moist thermodynamics ($\theta^*$) systematically reduced estimates of static stability compared to dry diagnostics ($\theta$). Consequently, the diagnosed turbulent overturning scales were larger when moist processes were included.
• Turbulent Fraction Enhancement: In 31 of the 32 soundings, the turbulent fraction derived from moist diagnostics exceeded that of dry diagnostics. In the strongest cases, moist turbulent fractions reached 60–70%, and exceeded 40% in 72% of all soundings. Dry diagnostics consistently underestimated the vertical mixing, with underestimations exceeding 50% in 3% of cases.
• Synoptic vs. Local Control: While moisture transport (IVT) provided the synoptic context for moisture availability, the magnitude of the turbulent correction was not monotonically related to IVT. Instead, the enhancement depended on the local vertical distribution of saturation, cloud phase, and static stability sampled by each radiosonde.
• SLWC Observations: SLWC signatures were observed in shallow layers (often <1 km) embedded within broader ice-dominated clouds. These observations support the presence of localized liquid pockets but do not indicate the vertically extensive liquid-topped mixed-phase structures common in the Arctic.

Significance and Claims
The paper claims that neglecting moist processes in polar environments leads to a systematic underestimation of vertical mixing and turbulence. The study demonstrates that during Antarctic moisture-intrusion regimes, including AAR conditions, the dominant mechanism for reducing static stability and enhancing turbulence is the latent heat release associated with ice-phase processes (deposition), rather than liquid-water condensation.

The authors assert that their approach, which applies moist Thorpe diagnostics to standard radiosonde data, offers a physically consistent and observationally accessible framework to distinguish generic moisture enhancement from the specific role of ice-phase thermodynamics. This method allows for the detection of turbulent overturning structures that remain undetected by dry stability estimates. The findings suggest that current numerical weather prediction and climate models may need to better represent these phase-dependent stability corrections to accurately simulate turbulence and vertical transport in the Antarctic troposphere. The study concludes that the Antarctic Peninsula troposphere is predominantly ice-phase above ~1 km, with turbulence significantly amplified by the specific thermodynamic structure of ice-supersaturated layers.