A new method to derive water vapor supersaturation at cumulus cloud base from in situ measurements
- 1Southern Cross University (Lismore, Australia)
Abstract
Current in situ measurements of water vapor supersaturation ( S v ) with respect to liquid water at cloud base are highly uncertain due to instrumental limitations. Consequently, S v is commonly estimated using cloud microphysical models and parameterizations. Here, we derive S v at warm cumulus cloud base from in situ measurements through calculations of the energy and water vapor budgets associated with cloud formation. The derived S v provides a thermodynamic analogue to the supersaturation established in a cloud condensation nuclei (CCN) chamber, enabling direct comparison with independently measured CCN activation spectra below cloud base. To this end, we formulate the energy budget of a saturated rising air parcel at cloud base and treat the transition between saturated thermodynamic states locally as quasi-isobaric. Under these conditions, the latent heat ( Q ) released during condensational droplet growth is determined by the corresponding sensible enthalpy difference between the saturated-vapor and water vapor–liquid states. According to the first law of thermodynamics, Q is partitioned between the change in internal energy (Δ U ) and water vapor expansion work ( W s ). Within this diagnostic framework, part of the condensational energy is, therefore, spent on W s , reducing the amount of liquid water formed relative to the limiting case in which the entire condensational energy contributes to droplet growth. Accordingly, the cloud supersaturation ( S c ) is diagnostically partitioned into liquid-phase ( S l ) and vapor-phase ( S v ) components based on the fractions of Q associated with Δ U and W s , respectively. This framework enables the derivation of S v from airborne cloud-base measurements through thermodynamic constraints on condensational energy partitioning. Here we use airborne measurements collected in growing convective cumuli over the Amazon Basin during the ACRIDICON-CHUVA campaign. Our results demonstrate that the derived cloud droplet number concentration as a function of S v agrees, within measurement uncertainties, with S v activation spectra of CCN measured below cloud base.
Versions
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BibTeX
@article{braga2026method,
title = {{A new method to derive water vapor supersaturation at cumulus cloud base from in situ measurements}},
author = {Ramon C. Braga and Manfred Wendisch and Rachel Albrecht and Daniel Harrison},
journal = {Atmospheric Chemistry and Physics},
year = {2026},
month = oct,
doi = {10.5194/acp-26-14769-2026},
url = {https://acp.copernicus.org/articles/26/14769/2026/},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - A new method to derive water vapor supersaturation at cumulus cloud base from in situ measurements
AU - Ramon C. Braga
AU - Manfred Wendisch
AU - Rachel Albrecht
AU - Daniel Harrison
PY - 2026
DA - 2026/10/20
JO - Atmospheric Chemistry and Physics
DO - 10.5194/acp-26-14769-2026
UR - https://acp.copernicus.org/articles/26/14769/2026/
AB - Current in situ measurements of water vapor supersaturation ( S v ) with respect to liquid water at cloud base are highly uncertain due to instrumental limitations. Consequently, S v is commonly estimated using cloud microphysical models and parameterizations. Here, we derive S v at warm cumulus cloud base from in situ measurements through calculations of the energy and water vapor budgets associated with cloud formation. The derived S v provides a thermodynamic analogue to the supersaturation established in a cloud condensation nuclei (CCN) chamber, enabling direct comparison with independently measured CCN activation spectra below cloud base. To this end, we formulate the energy budget of a saturated rising air parcel at cloud base and treat the transition between saturated thermodynamic states locally as quasi-isobaric. Under these conditions, the latent heat ( Q ) released during condensational droplet growth is determined by the corresponding sensible enthalpy difference between the saturated-vapor and water vapor–liquid states. According to the first law of thermodynamics, Q is partitioned between the change in internal energy (Δ U ) and water vapor expansion work ( W s ). Within this diagnostic framework, part of the condensational energy is, therefore, spent on W s , reducing the amount of liquid water formed relative to the limiting case in which the entire condensational energy contributes to droplet growth. Accordingly, the cloud supersaturation ( S c ) is diagnostically partitioned into liquid-phase ( S l ) and vapor-phase ( S v ) components based on the fractions of Q associated with Δ U and W s , respectively. This framework enables the derivation of S v from airborne cloud-base measurements through thermodynamic constraints on condensational energy partitioning. Here we use airborne measurements collected in growing convective cumuli over the Amazon Basin during the ACRIDICON-CHUVA campaign. Our results demonstrate that the derived cloud droplet number concentration as a function of S v agrees, within measurement uncertainties, with S v activation spectra of CCN measured below cloud base.
SN - 1680-7324
ER -