2008 report
Progress: 50%
Objectives (task 2.2)
(1) Realisation of meteorological observations during the Tara ice drift, cruises with R/V Oceania and R/V Polarstern.
(2) Studies with atmosphere models aiming at the test and improvement of parameterizations of processes over sea ice covered regions.
Work done and achievements made
In this second DAMOCLES period the observational program was intensified with continuous meteorological observations at the Tara drifting ice camp (UT), synoptic observations from R/V Polarstern during late summer 2007 and during an aircraft campaign in March 2007 near Svalbard. Furthermore, aerosol observations were carried out from R/V Oceania over the North Atlantic Ocean.
Process studies of the polar atmospheric boundary layer were made with mesoscale models run at AWI and at FMI.
At the Tara ice camp most of the meteorological equipment deployed in September 2006 had suffered due to a storm and hard weather/ice conditions in the previous winter. In April, the seriously damaged meteomast and some sensors were replaced by new ones from members of UT (Timo Palo and Erko Jakobson); the radiometer mast was moved to the new unharmed location. At the same time, the Vaisala DigiCORA Tethersonde System was installed and the first profiles were obtained. Erko Jakobson left the camp at the end of April; Timo Palo became a member of the Tara crew through the summer until the end of September.
Low-level measurements of meteorological parameters could be continued using the 10 m meteomast (Aanderaa Automatic Weather Station) equipped with wind and temperature sensors at four levels and a humidity and pressure sensor at one level. Data in one minute intervals are available now from the whole drift. Furthermore, two ultrasonic anemometers (Metek) were used measuring wind and temperature with high frequency, which allows to derive turbulent fluxes of momentum and heat.
Meteorological parameters in the atmospheric boundary layer (ABL) were measured using a tethered balloon rising up to 2000 m. Three sensors, suspended on a Kevlar line in 10-15 m intervals, measured air temperature, relative humidity, wind speed, and direction. Altitude was derived from the barometric pressure. One sensor was left on the surface in order to get surface data during the soundings. The measuring program included two half a day soundings per week and one 24 hour sounding per month. Some additional soundings were made studying e.g., the evolution of the profiles over 24 hours and processes in and close to the inversion layer. There were no soundings possible during bad weather conditions (e.g., wind speeds more than 10 m/s at the surface or hard rain/snowfall). Some problems occurred due to icing of the tetherline and of the balloon and due to a high humidity shortening the lifetime of sensors.
The tethersonde system could be used at 39 measuring days during 18 weeks in this summer (May to September). 95 vertical profiles were obtained, including three profiling experiments lasting 24 hours. The vertical profiles together with the low-level measurements give an invaluable reference material which helps to understand the meteorological situation over Arctic sea ice and to validate models. A first analysis showed already that the data contain interesting features like strong temperature inversions and a very high variability of the relative humidity in the ABL.
During the summer cruise of R/V Oceania (June 10 - August 23) to the Norwegian Sea and Svalbard region the following parameters were measured by IOPAN: meteorological data with an automatic station, vertical aerosol gradients with a particle counter, 3D wind speed with a sonic anemometer, and aerosol nanoparticles with a fast CPC counter.
From the collected data, vertical fluxes of aerosol as well as sensible and latent heat fluxes will be calculated as for the cruise of R/V Oceania performed in summer 2006. The sonic anemometer & CPC data will be used for eddy correlation estimations of aerosol fluxes. It is not clear until now, if a necessary correction of the ship movement can be carried out successfully. Alternatively, the inertial dissipation method will be used which does not need a ship movement subtraction.
A cruise was carried out by AWI with R/V Polarstern to the inner Arctic between end of July and end of September 2007. Routine meteorological observations were obtained from the ship including soundings twice a day. The data will be made available for DAMOCLES (see also task 2.1 and task 2.3).
AWI carried out an airborne campaign (ICESAR) over a sea ice covered region of the Fram Strait and over Storfjord. This campaign was conducted in close cooperation with ESA and DLR ( Germany). The DAMOCLES related part of the project focussed on the impact of leads and of sea ice topography on the near-surface fluxes of energy and momentum. Data will be used for mesoscale modelling.
Modelling
Modelling studies at AWI and FMI addressed boundary layer processes above regions with high sea ice cover and with leads. FMI has studied the evolution of the atmospheric boundary layer (ABL) during warm-air advection over sea ice in the Fram Strait. A hierarchy of numerical models has been applied, and the results have been validated against aircraft observations of the University of Hamburg. Operational HIRLAM (HIgh Resolution Limited Area Model) runs suffered from inaccurate information on the location of the sea ice edge. When this was improved, together with improving the horizontal resolution, the HIRLAM results became much better.
Two-dimensional model runs with inflow boundary conditions prescribed according to the aircraft observations demonstrated that the contribution of thin ice (refrozen leads) is important. The modelled turbulent fluxes of momentum, sensible heat and latent heat showed a large vertical divergence from the surface to the height of the aircraft observations. Above a 4-km-wide open lead, the modelled heat fluxes at the observation height of 24 m agreed reasonably well with the observations but were on average only 6% (sensible heat) and 13% (latent heat) of the surface values. Model experiments showed that in this case relatively high values for the momentum roughness length over sea ice yielded better agreement with the observed wind speed than values commonly used in numerical models.
Processes above leads were also addressed by AWI using the mesoscale model METRAS. A 3D modelling study over the northern Fram Strait with prescribed sea ice cover revealed that the lead impact on sea ice is comparable with the effect of a modified synoptic forcing. It was shown that a 5 % increase in the sea ice cover yielded approximately the same effect on the profiles of turbulent heat fluxes as a doubling of the geostrophic wind speed. This result is valid for regions with sea ice concentrations larger than 90 % during Arctic winter.
In a second study of AWI (and FMI) the interaction of the ABL with sea ice was studied for clear-sky conditions during polar night with a 1D model version. The focus was on the maximum occurring effect of leads on the ABL. It was found that the energy emanated from leads strongly influences both the ABL temperature and the temperature of the snow surface. It was shown that in the range of parameters studied a 1 % change in sea ice concentration can cause 1-4 K change in ABL temperature. This demonstrates the need for very accurate observations of sea ice concentration in the central Polar Regions, which generates a challenge for the development of new remote sensing methods for daily sea ice observations. Furthermore, errors in the sea ice concentration calculated in climate models could have a strong impact on their results. The results suggest that for a sea ice concentration of 95 %, the upward sensible heat flux from leads practically balances the downward flux over ice which is supported by observations.
Another important result of this joined AWI/FMI study was that for ABL processes over sea ice (with leads) two wind regimes can be distinguished which where modelled and also identified in SHEBA data. In the weak-wind regime a decoupling of ABL and snow surface temperatures is possible. This decoupling depends on the surface layer stability function used for stable stratification. The model results also suggest that the effect of leads in further stabilizing the lowest tens of meters of air over sea ice is important in maintaining the decoupling over periods of up to two days.
FMI has studied the stability dependence of the sensible heat flux from the atmosphere to sea ice. Empirical equations on the stability dependence of the heat transfer coefficient can be utilized to calculate the stability (as expressed by the Richardson number or Obukhov length) that corresponds to the maximum heat flux. The calculations show no dependence on the wind speed and surface roughness but a high sensitivity to the form of the empirical function. These results were compared against SHEBA data. The data showed some dependence on the wind speed, but were otherwise roughly in agreement with the predictions of the well-known stability functions of Dyer and Högström. The modelled air-surface temperature difference can be extremely sensitive to the functional form, because in very stable conditions the sensible heat flux decreases with increasing temperature difference, yielding to decoupling between the surface and the air (see above).
FIMR and FMI have studied the exchange of heat and moisture between the atmosphere and the Arctic sea ice on the basis of ECMWF operational analyses and NCEP/NCAR reanalysis. The focus has been on understanding the importance of accurate atmospheric forcing on sea ice thermodynamics. The study addressed the May-September 2003 period using the observations of the Chinese National Arctic Research Expedition 2003 (CHINARE-03) as a reference.
As in 2006, there was some overlap of activities in task 2.2 with those in task 2.1 concerning the studies of cyclones. Contributions of University of Bremen to a joint study (FMI/ Universities of Bremen and Hamburg) on polar low detection on the basis of AMSU-B data, which is part of both task 2.1 and 2.2, is described in the report of task 2.1.
Several manuscripts on task 2.2 work have been submitted by FMI, FIMR, AWI, and University of Hamburg to Tellus, Journal of Geophysical Research, and Geophysical Research Letters.
