Improving the understanding of cloud radiative heating

• Main Author: Johansson, Erik
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Stockholms universitet, Meteorologiska institutionen (MISU) 2019
• Subjects: clouds; radiative effects; upper troposphere-lower stratosphere; atmospheric circulation; remote sensing; Meteorology and Atmospheric Sciences; Meteorologi och atmosfärforskning; Arctic; Indian; Sea ice
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-175365

Clouds play an essential role in regulating Earth’s radiation budget by reflecting and absorbing energy at different spectra. As they interact with radiation, they can radiatively heat or cool the adjacent atmosphere and the surface. This heating effect can have a strong implication for the circulation and can change the surface properties by, for example, melting sea ice. The lack of high-resolution global observations has previously been a limitation for our understanding of the vertical structure of cloud radiative heating, and for evaluating the cloud radiative effect in climate models. In this thesis, we will investigate and document cloud radiative heating derived from space-based observations. We will focus on two regions, the Arctic and the Tropics, where cloud radiative heating plays an important, but fundamentally different role. In the Tropics, radiative heating at high altitudes influences the large scale circulation. Stratiform, deep convective, and cirrus clouds have a strong radiative impact in the upper troposphere. We found while investigating the Indian monsoon, that thick stratiform clouds will radiatively heat the upper troposphere by more than 0.2 K/day when the monsoon is most intense during June, July and August. Deep convective clouds cause considerable heating in the middle troposphere and at the same time, cool the tropical tropopause layer (TTL). These two thick cloud types will also cool the surface during the monsoon, weakening the temperature gradient between land and ocean. During these months, cirrus clouds are frequently located inside the TTL. We further find that in the Tropics, the climate model, EC-Earth, can capture the seasonal variations in cloud radiative heating seen in the satellite observations. However, the model overestimates the radiative heating in the upper region and underestimates them in the middle region of the troposphere. This dissimilarity is caused by unrealistic longwave heating and low cloud fraction in the upper and middle of the troposphere, ...