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How Does Cloud Overlap Affect the Radiative Heating in the Tropical Upper Troposphere/Lower Stratosphere?
Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute (SMHI), Sweden.ORCID iD: 0000-0002-2551-1697
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0002-5940-2114
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0002-6908-7410
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Number of Authors: 52019 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 46, no 10, p. 5623-5631Article in journal (Refereed) Published
Abstract [en]

Characterizing two-layer cloud systems has historically been difficult. These systems have a strong radiative impact on the composition of and the processes in the upper troposphere-lower stratosphere (UTLS). Using 4 years of combined spaceborne lidar and radar observations, the radiative impact of two-layer cloud systems in the tropical UTLS is characterized, and its sensitivity to the properties of top- and bottom-layer clouds is further quantified. Under these overlapping cloud conditions, the bottom-layer clouds can fully suppress the radiative heating caused by high clouds in the UTLS, by inducing strong longwave cooling. If the vertical separation between the layers is <4 km, the radiative heating of the high cloud changes sign from positive to negative. Furthermore, the radiative effect at the top of the atmosphere is investigated, and it is found that the characteristic net warming by cirrus with ice water path <50 g/m(2) is suppressed in the two-layered system.

Place, publisher, year, edition, pages
2019. Vol. 46, no 10, p. 5623-5631
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-171159DOI: 10.1029/2019GL082602ISI: 000471237500068OAI: oai:DiVA.org:su-171159DiVA, id: diva2:1342161
Available from: 2019-08-13 Created: 2019-08-13 Last updated: 2019-12-17Bibliographically approved
In thesis
1. Improving the understanding of cloud radiative heating
Open this publication in new window or tab >>Improving the understanding of cloud radiative heating
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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, respectively.

Radiative heating from cirrus, located inside the TTL, is considered to play an important role in the mass transport from the troposphere to the stratosphere. This heating generates enough buoyancy so that the air can pass the barrier of zero net radiative heating. We find that high thin single-layer clouds can heat the upper troposphere by 0.07 K/day. If a thick cloud layer is present underneath, they will radiatively suppress the high cloud, causing it to cool the adjacent air instead. The optical depth and cloud top height of the underlying cloud are two crucial factors that radiatively impact the high cloud above.

Warm moist air is regularly transported from the mid-latitudes into the Arctic by low- and high-pressure systems. As the moist air enters the Arctic, it increases the cloudiness and warms the surface. This surface heating has the potential to affect the ice cover months after the intrusion. We find that during extreme moist intrusions, the surface temperature in the Arctic can rise by more than 5 K during the winter months with an increase in cloudiness by up to 30% downstream from the intrusion. These extra clouds radiatively heat the lower part of the atmosphere and cool the middle part, affecting the stability of the Arctic atmosphere.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2019. p. 36
Keywords
clouds, radiative effects, upper troposphere-lower stratosphere, atmospheric circulation, remote sensing
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-175365 (URN)978-91-7797-891-6 (ISBN)978-91-7797-892-3 (ISBN)
Public defence
2019-12-06, Vivi Täckholmsalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 84/11:1 och 84/11:2
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

In the printed version of the dissertation, the supporting information to paper 1 is missing. It can be found at the publisher's website. 

Available from: 2019-11-13 Created: 2019-10-22 Last updated: 2020-05-22Bibliographically approved

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