613 Seminar: Seth Seidel/UC-Davis

The lightness of water vapor, clouds, and tropical climate

In this talk I tell two stories about tropical climate. In the first story we investigate a negative climate feedback due to the lightness of water vapor. The molecular weight of water vapor is less than that of dry air, making humid air less dense than dry air at a given temperature and pressure. Using observations and models, we demonstrate that this vapor buoyancy in the humid regions of Earth’s tropics must be balanced by warmer temperatures in the dry regions. This horizontal temperature difference gives rise to a negative lapse-rate-type climate feedback of about -0.2 W/m2/K, which we call the vapor buoyancy feedback. We further show that the feedback is absent in some GCMs.

In the second story we investigate the temperature of high clouds in the tropics. Tropical anvil clouds tend to warm modestly in models, though the exact reason for this is debated. I perform idealized experiments in a convection-permitting model. We note that the radiative tropopause – the location in the upper troposphere where radiative heating equals zero – warms at approximately the same rate as the cloud anvils. This behavior persists across a variety of modeling choices, suggesting a close relationship between the two temperatures.

Bio: Seth Seidel is a PhD candidate beginning his 5th year at the University of California Davis. He is advised by Da Yang, and his research focuses on radiation, clouds, and climate feedbacks in Earth’s tropics. He graduated from Texas A&M University in 2011 with a B.A. in Mathematics. In 2017 he completed an M.S. in Civil Engineering at UC Davis, where he studied turbulence in environmental flows.

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