Emma Fiedler

1. Aircraft Observations of the Atmospheric Boundary Layer over the Ronne Polynya, Southern Weddell Sea, Antarctica Emma Fiedler Ian Renfrew (UEA) Tom Lachlan-Cope (BAS) John King (BAS)

2. sensible heat (Open University 1998)

3. Antarctic Peninsula Study area AVHRR

4. Aircraft Observations • 3 flights betweenFebruary 25th-28th 2007 • Cold air outbreak off Ronne Ice Shelf • High frequency measurements u, v, w, T

5. 3 flights over Ronne Polynya 4 ~120 km low altitude legs 2 at ~15 m, 2 at ~30 m (within surface layer) • Quality control: • - spectral analysis, cospectra, etc • (c.f. French et al. 2007; Drennan et al. 2007) • – averaging period (run) of 140 sec for turbulent fluxes appropriate • – 6 runs rejected out of 49 runs • – 1 run rejected due to instrument malfunction

6. Results Decrease in sensible heat flux with fetch due to reduction in air-surface temperature difference – CIBL warming – surface temperature decrease

7. Results Decrease in sensible heat flux with fetch due to reduction in air-surface temperature difference – CIBL warming – surface temperature decrease

8. Results

9. Transfer coefficients Only values from 15 m legs used due to increased scatter at 30 m Mean values same (flights 24hrs apart) CDN10 = 1.1 x 10-3 CHN10 = 0.7 x 10-3 No obvious trend with fetch, no difference between means for different regimes

10. Is there a variation with surface characteristics? Relationships between CD and CH with albedo and Tsurface examined as indicator of surface ice type

11. Modelling of sensible heat fluxes • Validation of CIBL model (Renfrew and King, 2000) – designed for use over polynyas – 1-D, zero-order jump model – Qs, h and θm vary with fetch Model changes: • Ice covered surface observed rather than open water – surface roughness length not dependent on wind speed – two-regime initial Tsurface – revised CH and CD

12. Sensible heat fluxes Ts = -7.8 oC Ts = -11.7 oC

13. Sensible heat fluxes Ts = -1.9 oC CHN10= 1.14 x 10-3 (DeCosmo et al., 1996) Ts = -7.8 oC CHN10= 0.70 x 10-3 (Ronne Polynya observations) Ts = -11.7 oC

14. Sensible heat fluxes Ts = -1.9 oC Ts = -1.9 oC Ts = -7.8 oC Ts = -11.7 oC

15. AMSR-E http://iup.physik.uni-bremen.de:8084/archive.html

16. Conclusions : Observations • Low-level, aircraft-based turbulence observations successfully collected over Ronne Polynya • Decrease in Qs with fetch • Polynya thin ice, rather than open water • Ice surface type can be split into two regimes • Both regimes considered part of polynya due to large heat fluxes • New values of CH and CD to add to limited data sets • CDN10 = 1.1 x 10-3 • CHN10 = 0.7 x 10-3 • No significant difference in CH and CD between regimes

17. Conclusions : Modelling • For these cases the Ronne Polynya was thin ice (not open water) • Modelling of QS requires appropriate – CH10N and CD10N – Tsurface – no Charnock parameter • Other coastal polynyas can be modelled in same way • unless large areas of open water • Implications for polynya satellite ice concentration algorithms & modelling • Should thin ice be considered in the same way as thicker ice or open water? • This study suggests thin ice similar to thicker ice • Similarity of CH and CD between regimes • Reduced QS compared to open water