Head in a Cloud

troposphere and stratosphere meet blogosphere

Head in a Cloud random header image

The Influence of Changes in Cloud Cover on Recent Surface Temperature Trends in the Arctic

April 4th, 2008 by cgdenver · 1 Comment

Atmospheric circulation and the global energy budget are largely influenced by surface temperature. Thompson and Wallace (1998) showed that the Arctic Oscillation (AO) is the principal component of the Northern Hemisphere sea level pressure poleward of 20°N and Thompson et al (2000) showed that the AO accounts for as much as 50% of the winter warming over Eurasia due to warm air advection. Rigor et al (2000) showed that the AO accounts for 74% of the warming over the eastern Arctic Ocean and 14% of the cooling over the western Arctic during the winter.

Surface temperature changes are also related to sea ice cover. Cavalieri et al (2003) showed a decrease of total Arctic sea ice extent of about 36000 km/yr from 1979-2002. Studies shown by Parkinson et al (1999) and Parkinson and Cavalieri (2002) show similar results. Sea ice becomes important in a discussion of surface temperature because of a high albedo and associated feedback effects. Decreasing sea ice cover creates larger areas of open water and the varying atmospheric circulation changes moisture advection into the Arctic. Wang and Key (2005) show trends that the Arctic has become cloudier in spring and summer but less cloudy in winter. Trends in cloud amount have an impact on trends of surface temperature. Yinghui Liu, Jeffrey R. Key, and Xuanji Wang quantify the influence of changes in Arctic cloud cover on the surface temperature trend in their Journal of Climate article “The Influence of Changes in Cloud Cover on Recent Surface Temperature Trends in the Arctic” Volume 21 Issue 4 (February 2008).

Liu, Key and Wang used the APP-x dataset which extends the APP products(see (http://nsidc.org/data/docs/daac/nsidc0066_avhrr_5km.gd.html) to include cloud optical depth, cloud particle phase and size, cloud top temperature and pressure, all-sky surface temperature and broadband albedo, and radiative fluxes. Retrievals were done with the Cloud and Surface Parameter Retrieval System (CASPR) which was specifically designed for polar AVHRR (Advanced Very High Resolution Radiometer) daytime and nighttime data. The APP-x dataset includes daily composites at both 0400 and 1400 local solar standard time(LST) from January 1982 to December 2004. Monthly means of cloud cover and surface temperature under cloud free and cloudy and all-sky conditions were calculated from twice daily data.

In winter, the surface temperature increases over northern Canada with a maximum over Hudson Bay. The surface temperature decreases over the Arctic Ocean and eastern Arctic. Over the central Arctic Ocean, the decreasing trend is approximately -2.5K/decade. In spring, warming is seen over most of the Arctic with the trend about 2.0K/decade. In summer, surface temperature is generally increasing though with a smaller magnitude than in spring and trends are near 0 over the central Arctic Ocean. In autumn, there are strong increasing trends over the Beaufort Sea, the Chukchi Sea, Alaska and northern Canada with a maximum trend around 2.0K/decade.

The cloud free and cloudy surface temperature trends exhibit similar pattern as the all-sky trends. The differences in surface temperature trends under clear and cloudy conditions are due not only to changes in cloud cover but also to changes in cloud properties. A decreasing trend in APP-x cloud top height may contribute to a stronger cooling trend over the Chukchi Sea and central Arctic in winter under cloudy conditions than under clear conditions. Warmer clouds emit more longwave radiation upward. For regions where both clear and cloudy trends are positive, the magnitude of the warming trends under cloud free conditions is larger than under cloudy conditions. For regions where both trends are negative the magnitude of both trends are comparable. Under cloudy conditions, the trends are not as dramatic as under cloud free conditions. This tends to imply that clouds have a negative feedback on the surface temperature trends.

The article is very straightforward and the authors offer no speculation on what their conclusions may imply. They present some lengthy mathematics to quantify the influence of cloud cover on surface trends. This confirms that the atmosphere is very complex and that there are many things that interact with each other.

Tags: global warming · MTR3440 · troposphere

1 response so far ↓

  • 1 Cassie.Wheeler // May 6, 2008 at 2:49 pm

    More so than comments, I have some questions on this article. I have just recently started researching Arctic clouds or any type of clouds for that matter, so please forgive me if my questions seem naïve and the answers to them obvious. I am also open to references to other work that might be pertinent.

    The beginning of the article says that cloud properties can affect surface temperature change. I assume by properties it is referring to thickness, content, particle size, etc. How, specifically, do these affect temperature?

    One part of the article seemed to suggest that decreasing cloud top height may lead to stronger cooling in cloudy versus clear regions. I don’t understand how lowering the top (decreasing the thickness) of the cloud could lead to lower temperatures. It seems like more shortwave radiation would make it to the surface and the temperature would increase.

    Why do warmer clouds emit more longwave radiation upwards? On that note, what makes a cloud warmer or colder? Is it just the composition (colder clouds would have ice particles versus warmer clouds having liquid water)?

    Just for clarification, did the article imply clouds can act to stabilize the surface temperature more so than clear skies?

    I may have miss read this, but it seemed the overall conclusion of the article was that clouds act to reduce the surface temperature. In some of the reading I have done, there have been articles that show during the spring, summer and fall, the cloud cover increases and so to does the temperature. As a result, I am a little confused by the authors’ conclusion. Maybe this varies by location or region?

    Thanks for any input!

    Cheers,
    Cassie

Leave a Comment

*