New Study on observing the vertical humidity structure of the atmosphere with satellite-based instruments

18 November 2021

In a new study published in Atmospheric Measurement Techniques Marc Prange, Dr. Manfred Brath and Prof. Dr. Stefan Bühler show that sophisticated satellite-based instruments are capable of detecting so called elevated moist layers, rejecting the previously suggested notion that these layers may constitute an inherent blindspot for the satellite observing system.

The vertical humidity structure is a key characteristic of the atmosphere due to the strong greenhouse effect of water vapor. The ability to reliably observe the vertical humidity structure on a global basis is essential for both numerical weather prediction and understanding the atmosphere’s response to a warming climate.

Elevated moist layers are thought to be remnants of thunderstorms that pump up moist air from the surface high up into the atmosphere, manifesting as formidable deep anvil clouds that can extend all the way up to 15 km altitude in the tropics. The effectiveness at which moist air can be pumped upwards depends on the vertical temperature structure of the atmosphere. In layers around 5 km altitude this effectiveness is often particularly low, forcing the ascending air to diverge horizontally outwards into the thunderstorm’s rather dry environment, creating an extended shelf of unusually high humidity. Such moisture shelfs are thought to decay more slowly than the thunderstorms themselves and significantly impact the local radiation budget, which is important for the local weather.

In their new study, Prange et al. (2021) show that, contrary to previous assumptions, elevated moist layers can be captured by satellite observations. To understand the technical key to achieve this, one needs to understand that satellite observations only contain indirect information about the atmosphere, the interpretation of which requires physical radiation models and is also constrained computationally. The paper demonstrates that for a proper resolution of the humidity structure in the presence of an elevated moist layer, a good resolution of the vertical temperature structure is a crucial requirement. This is shown in a case study where the resolution of the moist layer shows a clear dependence on different assumptions about the temperature structure. The study also introduces a new framework to identify and characterise elevated moist layers, which can easily be applied to any large dataset of atmospheric humidity. This methodology may be used in the future to directly assess and compare different dataset’s capabilities to resolve elevated moist layers.

Reference to new study:

Prange, M., Brath, M., and Buehler, S. A.: Are elevated moist layers a blind spot for hyperspectral infrared sounders? A model study, Atmos. Meas. Tech., 14, 7025–7044, 10.5194/amt-14-7025-2021, 2021.

Contact:

Marc Prange
Universität Hamburg
Department of Earth Sciences, Meteorological Institute
International Max Planck Research School on Earth System Modelling