13B.3 Cloud and inversion characteristics over the Arctic pack ice

Thursday, 12 June 2008: 11:00 AM
Aula Magna Höger (Aula Magna)
J. Sedlar, Stockholm University, Stockholm, Sweden; and M. Tjernström

The interaction between the main inversion layer and cloud cover influencing the cloud topped atmospheric boundary layer over the Arctic pack ice is analyzed. A suite of observations were made from a 5mm scanning radiometer, an SBand cloud radar and radiosondes along with ground-based meteorological measurements taken during the Arctic Ocean Experiment 2001 (AOE-2001) over a 1.5 month expedition during the melt season, including a three week ice/drift period near 89°N.

Two regimes were observed (only cloudy cases were examined): cloud tops were capped by the main inversion layer or cloud tops were found to lie within the inversion layer; the later was most often observed, occurring nearly 5:1 times more often than the former. Inversion depths are thicker and tend to be stronger for cases when clouds were found within the inversion (CII – clouds inside inversion) although the deepest inversions were not always the strongest.

The surface radiative budget determines the onset and subsequent end of the melt season and is dependent upon cloud phase. Observed incoming radiation suggest that cloud phase may differ between cases of CII and cloud tops capped by the inversion (CCI – clouds capped by inversion). First, second and third radar moments are analyzed to provide hints at cloud phase. First moment radar returns were slightly stronger during CII cases although the difference in PDF statistics is generally small. A PDF of radar-determined fall speeds show a steep drop in occurrence of larger fall speeds as the icebreaker moved further from the ice edge towards its drift position in the pack ice; transport time across the pack ice seems to impact on cloud phase. Vertical PDFs of cloud radar moments suggest differences in cloud phase between CII and CCI cases; radar returns for CCI decrease towards inversion base where cloud top cooling is expected to be strongest, supportive of more ice-phase cloud particles. Radar spectrum width PDFs show a larger spread in fall speeds near inversion base for CCI rather than CII cases which may suggest that turbulence near cloud top – inversion base is dominating vertical motions.

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