Impact of Moisture Vertical Distribution on Radiative Subsidence: Implications for Cloud Holes and Doldrums

Garg, Tanisha (2025) Impact of Moisture Vertical Distribution on Radiative Subsidence: Implications for Cloud Holes and Doldrums. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

This thesis examines the impact of the vertical distribution of moisture on radiatively driven subsidence in lower free troposphere, and the importance of radiative subsidence in maintaining cloud free regions in the mature stages of cold pools ("Cloud Holes") in the trades and low wind speed events in the ITCZ. Radiative subsidence has previously been proposed as an important mechanism for shallow mesoscale circulations but limited information on its controls has hindered a deeper understanding. The first part of the thesis aims to understand the control of moisture on radiative subsidence from idealized moisture profiles. Our findings reveal that the vertical gradient of water vapor mixing ratio is most crucial for determining radiative subsidence. This points to the importance of local moisture distribution in determining radiative subsidence. We use a one-column conceptual model for examining the evolution of the atmospheric column in clear-sky subsidence regimes. We expand on this model by adding interactive radiative cooling, using a radiative transfer model ARTS, and by adding a dynamic free troposphere. Using this model, we characterize the change in mixed layer height, moisture profiles and radiative subsidence velocity. We observe that the radiative subsidence weakens over time even as the free troposphere becomes drier. The second part of this study employs two observational case studies using EUREC4A and ARC campaigns’ data. The first examines a cloud hole from trades where we find that radiative subsidence maintains mixed layer height below the lifting condensation level, a key condition for suppressing cloudiness and forming cloud holes. The second is from a low wind speed event in doldrums where radiative subsidence initially supports calm conditions, but its diurnal variability limits its role in sustaining the event over time.

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