Unique Properties of the Arctic Stratiform Cloud-Top Region

Shupe, M., University of Colorado

Cloud Processes

Cloud Life Cycle

Sedlar J, MD Shupe, and M Tjernström. 2012. "On the relationship between thermodynamic structure and cloud top, and its climate significance in the Arctic." Journal of Climate, 25(7), 10.1175/jcli-d-11-00186.1.


Occurrence frequency of low-level, stratiform cloud cases used in the analysis (black), percentage of these cases where the cloud top was identified to occur within the inversion (CII, gray), and percentage where the cloud top was observed to be capped by the inversion (CCI, white) for the ASCOS, SHEBA, and Barrow locations. The total number of cases analyzed is given in brackets.


Occurrence frequency of low-level, stratiform cloud cases used in the analysis (black), percentage of these cases where the cloud top was identified to occur within the inversion (CII, gray), and percentage where the cloud top was observed to be capped by the inversion (CCI, white) for the ASCOS, SHEBA, and Barrow locations. The total number of cases analyzed is given in brackets.

Stratiform clouds are often formed and maintained by buoyancy-driven, cloud-scale turbulent motions. As a result of atmospheric radiation, mixing, and advective processes, temperature inversions typically occur near the top of these clouds. The strong static stability associated with these inversions limits the vertical extent of cloud-scale motions and thus usually constrains the cloud-top height. However, observations at multiple Arctic locations have shown a frequent occurrence of cloud top extending above the base of the temperature inversion suggesting that there may be unique processes in action near the top of Arctic stratiform clouds.

To examine this phenomenon, a combination of observations was used from Arctic land- and ocean-based facilities, including the DOE North Slope of Alaska site (2003–2008), the Surface Heat Budget of the Arctic Ocean (SHEBA) project in the Arctic sea-ice (1997–1998), and the Arctic Summer Cloud Ocean Study (ASCOS) near the North Pole (2008). At all of these stations periods with low-level stratiform clouds were examined using coincident atmospheric thermodynamic profiles from periodic radiosondings and cloud-top heights measured by vertically pointing cloud radar. Stratiform clouds appropriate for this analysis occurred during 20–45% of the radiosonding times at these facilities. All cases were classified as either the cloud extending into the temperature inversion (by at least 90 meters) or the cloud being capped by the inversion (< 90 meters above the inversion base).

In 58–73% of cases over the Arctic sea ice, the cloud top was found to extend into the temperature inversion. However, at Barrow this phenomenon was observed in only 31% of cases. At all locations, this cloud-top feature was most frequent in the spring and summer months and less frequent in fall and winter. During these cases, the clouds were generally lower in the atmosphere and associated with stronger temperature inversions. Cloud-top humidity inversions were also identified to occur 85% of the time that the cloud top extended into the temperature inversion.

Cloud water occurring within the statically stable temperature inversion is thought to be related to these layers being particularly moist relative to lower latitudes where the free troposphere above the inversion is typically quite dry. As a result of the strong static stability, radiative cooling within the moist Arctic inversion layer does not lead to buoyancy-driven motions, as typically occurs below the inversion base. Rather, this cooling directly forces condensation of cloud water within the inversion. Differences in the occurrence fraction of this phenomenon between the Alaska coast and Arctic sea-ice environments may be due to differences in the large-scale advection of moisture, the role that surface moisture sources play, and the dynamical interactions between the cloud and underlying surface.

This unique Arctic cloud-top structure can have important implications for cloud maintenance processes in a region where supercooled liquid water clouds are remarkably persistent in spite of the moisture sink from near-continuous ice formation. Cloud residing in the temperature inversion acts to redistribute the radiative cooling rate profile near cloud top such that less cooling occurs below the temperature inversion, potentially diminishing the buoyancy-driven turbulence in that region. Additionally, gravitational settling of condensed water droplets from within the inversion can serve as an efficient means of transporting water across the strongly stratified Arctic atmosphere and serve as a moisture source for the rest of the cloud layer below the inversion. The specific details of these processes and the impacts they have on Arctic cloud life cycles are the subject of ongoing observational and modeling studies.