Observation of intermittent turbulence over Arctic sea-ice

The stable boundary layer (SBL) is less understood compared to the convective boundary layer. This is partly due to less available measured data in the SBL. This study focuses on small scale turbulence in the polar SBL (PSBL).

The PSBL is developed by radiational cooling from the surface or by warm air advection from the warmer ocean over the ice. The polar region is less influenced from the diurnal variation of the radiative forcing compared to mid-latitude areas. Also in the polar regions, the ground inversions can be very strong. The surface is relatively flat (less slopes) which results in low influence from katabatic flows and strong advective flows.

Within the polar SBL, turbulence is weak or sometimes intermittent. This requires a sensitive and fast measurement system, which is provided by the helicopter borne turbulence probe Helipod. The Helipod was developed for in-situ measurements of small-scale turbulent fluctuations of wind, temperature, humidity and the associated turbulent fluxes. This autonomously operating sensor package can be carried by a helicopter on a rope of 15 meter length. The operation speed is 40 m/s and the data is stored at a rate of 100 Hz. An advantage of the Helipod is that the flights can be performed at low altitudes above the surface (10 m or less) and that the Helipod can start from a ship. This is especially useful in the shallow SBL.

The database used for this (PSBL) study consist of three campaigns (Sep-Oct 1995, Jul-Sep 1996, Mar-Apr 2003) with the German research vessel Polarstern. Within these Artic campaigns, Helipod flights were performed in the East Greenland sea, the Fram Strait, Kara Sea, Laptew Sea, East-Siberian Sea and the central Artic sea. The surface consisted of sea which mostly was covered with sea-ice. In total 26 flight days were analysed. The meteorological cases varied between high SBL with weak stratification and shallow SBL with strong stratification as found from vertical profile flights.

Most of the Helipod flights were performed horizontally at different heights. The sensible heat flux is estimated for several flight levels within the PSBL.For some flight days it was possible to identify intermittent turbulence within the horizontal legs. Using stability indices, the flights were classified by stratification strength. The results and the influence of the stratification on intermittent turbulence will be showed.

Using spectral analysis, the spectral gap, which divide 2D and 3D turbulence, is identified for the different flights. The influence of different stratifications and wind speed is showed in the results.