Transcript Ship Based Turbulence Measurements Under Heavy Seas

Ice-Cloud Coupling in the Central Arctic Ocean – Measurements from the ASCOS Campaign
Ian M. Brooks1, Cathryn E. Birch1, Thorsten Mauritsen2, Joseph Sedlar2, Michael Tjernström2, P. Ola. G. Persson3, Matthew Shupe3,
Barbara J. Brooks1, Sean F. Milton4, Paul Earnshaw4
1University
of Leeds, UK; 2Stockholm University, Sweden; 3University of Colorado / NOAA; 4Met Office, UK
Background
Objectives
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.
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
NASA DC8
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; gasphase chemistry; ocean microstructure profiles & turbulent fluxes; marine biochemistry; bubble spectra…
tethersonde
sodar
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
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.
Above: probability distributions (contoured), mean (solid), and median (dashed)
profiles from radiosondes launched every 6 hours throughout the campaign.
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)
Tethersonde path
Near-neutral
Cloud top
unstable
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.
stable
unstable
stable
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.
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
Comparison with UM
Remote sensing retrievals
sodar

RH
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.
School of Earth and Environment
INSTITUTE FOR CLIMATE AND ATMOSPHERIC SCIENCE
UNIVERSITY OF LEEDS