ASCOS ice drift on Icebreaker Oden August 12 – Sept 1 2008, 87-87.6  N, 1-11  W.

1. Near-neutral stable stable unstable unstable Ice-Cloud Coupling in the Central Arctic Ocean – Measurements from the ASCOS CampaignIan M. Brooks1, Cathryn E. Birch1, Thorsten Mauritsen2, Joseph Sedlar2, Michael Tjernström2, P. Ola. G. Persson3, Matthew Shupe3, Barbara J. Brooks1, Sean F. Milton4, Paul Earnshaw41University of Leeds, UK; 2Stockholm University, Sweden; 3University of Colorado / NOAA; 4Met Office, UK Background Numerical models do a poor job of representing Arctic clouds. This is a serious problem in climate models because the clouds play a dominant role in controlling the surface energy budget. The underlying problem is that Arctic clouds have properties very different from those elsewhere in the world (and on which model parameterizations are based). This is a result of the unique, very clean, environment – with very little aerosol on which to form droplets, cloud microphysics, and hence radiative & dynamic properties differ from that elsewhere. The underlying ice surface has a similar albedo to the cloud, so that long-wave radiative processes dominate the surface energy budget rather than solar radiation. Arctic stratus – unlike that at mid-latitudes – almost always acts to warm the surface. • Objectives • The ASCOS meteorological subprogram was focussed on understanding the physical processes controlling Arctic summer stratus: • Turbulent mixing between ice surface and cloud • Radiatively driven turbulence in cloud • Entrainment at cloud top • Cloud microphysical properties • and in turn, the effect of the cloud properties on the surface energy budget ASCOS ice drift on Icebreaker Oden August 12 – Sept 1 2008, 87-87.6N, 1-11W. Science team of 32 + 10 logistical support staff Measurements of: mean meteorology; turbulent fluxes; surface radiation budget; ice temperature & near-surface heat flux; aerosol physics & chemistry; remote sensing of winds, boundary-layer structure, & cloud properties; gas-phase chemistry; ocean microstructure profiles & turbulent fluxes; marine bio-chemistry; bubble spectra… NASA DC8 sodar tethersonde Boundary Layer Structure meteorological masts Meteorological conditions radiosondes Above: Sodar backscatter signal for the entire operational period. The presence of low cloud/fog stongly attenuates the signal so that the effective range is typically less than 500m (backscatter < ~5dB is considered to be background noise) Surface Turbulence Statistics smooth rough Above: Time-height record for all the tethersonde flights. The break in the record from 19th-21st is due to instrument repairs, other breaks are due to fog preventing work on the ice (due to polar bear risk) Excluded Above: The surface roughness length is an important parameter in bulk flux parameterizations schemes. Calculated values vary significantly with wind direction, reflecting flow over the relatively smooth & uniform local ice floe, or over the much rougher mixture of small open leads & broken ice. Flow through the mast lattice or from over the ship is excluded from the analysis. Tethersonde path Above: probability distributions (contoured), mean (solid), and median (dashed) profiles from radiosondes launched every 6 hours throughout the campaign. Cloud top Above: Surface layer stability parameters for the entire campaign, calculated from all turbulence measurement sites. Left: Monin-Obukhov stability parameter shows conditions to be near-neutral > 80% of the time (-0.1 < z/L < 0.1), right: A bulk Richardson number measure indicates near-neutral conditions (-0.05 < Rib > 0.05) all the time. 13/08 31/08 Above: (left) Surface temperature and relative humidities with respect to water and ice for the whole ice drift. Note periods of supersaturation with respect to ice. (right) temperature time-height section from radiosondes. Windspeed contours t-sonde Timeseries of surface fluxes. Gaps in series result from exclusion of flow distorted periods and instrument icing. Radar reflectivity showing cloud cover for the duration of the ice-drift. First week shows passage of several frontal systems associated with climatologically unusually strong and frequent storms sodar Comparison with UM  RH Remote sensing retrievals Top: backscatter for the 3.5 hour duration of a tethersonde flight, contours of sodar wind speed, and cloud top from remote sensing (cloud base is at the surface) – peaks in backscatter indicate high variability in air density, often a temperature inversion. Bottom: tethersonde profiles of potential temperature, RH (solid:up, dashed:down), wind speed, & turbulent dissipation rate, along with sodar backscatter and winds. Dashed line indicates top of well-mixed surface layer. Acknowledgments: Funded by NERC (NE/E010008/1), the Knut and Alice Wallenberg Foundation, DAMOCLES European Union 6th Framework Program Integrated Research Project, and NSF. CEB is partially funded by the Met Office. Thanks to the Swedish Polar Research Secretariat, Captain Mattias Peterson & the crew of Oden. ASCOS is an IPY & SOLAS project. Comparison of total cloud water content (liquid + ice) retrieved from the cloud radar (top) and predicted by the Met Office Unified Model (bottom). The model does a poor job of reproducing the observed cloud field.