Low Level Jets at the Ice Edge

Figure 1:  Sea ice extend on 29.June 2013. The figure is taken from the National Snow & Ice Data Center

Figure 1: Sea ice extend in the Arctic on 1. July 2013 from the National Snow & Ice Data Center

What are ice edge jets and why should we care?

The ice edge is the border between the sea ice and the open ocean (Figure 1), and jets are bands of high wind speeds in the atmosphere. Earlier studies showed that wind systems like the Greenland tip jet (Våge et al., 2009) or barrier winds at the east coast of Greenland (Harden et al., 2011) act on the ocean circulation and hence influence the climate. We expect that the ice edge also favors certain wind regimes, which have an impact on the climate. The effect of the ice edge on the weather is another point to consider. Low level jets are associated with high wind speeds. Over the ocean, these fast winds  are mixed down to the surface where they cause rough seas. Ships staying in this area can get into dangerous situations. We also suppose that strong shear zones at the ice edge are an aid for the development of polar lows. The challenge is to resolve the mesoscale ice edge jets in coarse resolution climate and numerical weather prediction models. However, first we need to understand under which conditions ice edge jets develop.

Figure 3: Schematic of the thermal wind relation (black) and the ageostrophic circulation (turquoise) inherent in the semi-geostrophic theory. Crossed circles indicate an acceleration into the paper. Circles with a centered dot illustrate an acceleration out of the paper. C,H represents Cold air and High surface pressure. W,L stands for Warm air and Low surface pressure. At the lower branch of the ageostrophic circulation both processes oppose each other, at the upper branch both processes enforce each other.

Figure 2: Schematic of the thermal wind relation (black) and the ageostrophic circulation (turquoise) inherent in the semi-geostrophic theory. Crossed circles indicate an acceleration into the paper. Circles with a centered dot illustrate an acceleration out of the paper. C,H represents Cold air and High surface pressure. W,L stands for Warm air and Low surface pressure. At the lower branch of the ageostrophic circulation both processes oppose each other, at the upper branch both processes enforce each other.

Why do you expect  low level jets at the ice edge?

The ice edge is characterized by a large surface temperature difference across the edge. Whereas the ocean surface temperature is close to the freezing point at around -1.9°C, the sea ice surface temperature is much lower, for example at -20°C. There are two theoretical concepts associated with horizontal temperature gradients, the semi-geostrophic theory and the thermal wind relation. The thermal wind relation states that the wind changes with height, whenever there is a horizontal temperature gradient. The semi-geostrophic theory connects a horizontal temperature gradient to a temporal change of the wind speed along the ice edge. Both mechanisms are illustrated in Figure 2.

Figure 2: Shown is a low level jet at the ice edge simulated with the WRF model.

Figure 3: WRF simulation of the 1993 case. The ice edge is to the north of the strong surface potential temperature gradient. A low level jet is visible at 400m above the sea ice.

What did you do to understand ice edge jets?

We used a numerical model for the simulation and analysis of two ice edge jet cases. Shapiro et al. (1989) reported about the observation of an ice edge jet during the aircraft-based Arctic Cyclone Experiment in 1984. The second case was based on the results of a modeled ice edge jet in 1993 by Grønås et al. (1999). The simulation of the 1993 case was successful (Figure 3), the ice edge jet in the 1984 case, however, could not be reproduced. For the purpose of validation of the 1993 case, we simulated a third case, which was based on measurements at the ice edge in 1997 (Drüe et al., 2001). The cases in 1993 and 1997 were comparable in terms of the relative alignment of the wind to the ice edge. Following the wind, the ice edge was in both cases on the right hand side. In fact, the simulation of the 1997 case showed a similar low level jet like in the 1993 case. However, no low level jet was found in the measurements by Drüe et al. (2001), suggesting that the ice edge jet in the 1993 case was only an artifact of the numerical models.

So, you found nothing?

Our simulations could not prove or disprove the existence of low level jets forced by the horizontal temperature gradient at the ice edge. However, we certainly gained understanding. The analysis of the three cases suggested following. The development of an ice edge jet occurs when the wind is close to parallel along the ice edge and whenever the ice edge is to the left hand side of the flow. The wind is then aligned with the accelerating effect of the thermal wind and the ageostrophic circulation, and hence, the wind speed increases at low levels. If the ice edge is to the right hand side of the flow, the wind is opposing the effect of the thermal wind and the ageostrophic circulation, and therefore decelerates. Thus, in this case we do not expect an ice edge jet.

What are you going to do then?

Because our analysis revealed that numerical models have issues simulating the flow at the ice edge, we will focus on measurements. Therefore, we are looking for more papers reporting about measurements at the ice edge, regardless whether they include low level jets or not. Ideally, we would like to conduct a measuring campaign ourselves, but this requires long term planning and solid funding.  If we find proof for our hypothesis, we can work on implementing ice edge jets in climate or numerical weather prediction models.

References

Drüe, C. and G. Heinemann, 2001: Airborne investigation of arctic boundary-layer fronts over the marginal ice zone of the Davis Strait. Boundary-layer meteorology, 101 (2), 261–292.

Grønås, S. and P. Skeie, 1999: A case study of strong winds at an Arctic front. Tellus A, 51 (5), 865–879.

Harden, B., I. Renfrew, and G. Petersen, 2011: A climatology of wintertime barrier winds off south-east Greenland. Journal of Climate, 24 (17), 4701–4717.

Shapiro, M. A. and L. S. Fedor, 1989: A case study of an ice-edge boundary layer front and polar low development over the Norwegian and Barents sea. Polar and Arctic Lows, Twitchell, P.F. Rasmussen, E.A. and Davidson,K.L.(eds.). A.Deepak Publishing, 257–277.

Våge, K., T. Spengler, H. C. Davies, and R. S. Pickart, 2009: Multi-event analysis of the westerly Greenland tip jet based upon 45 winters in era-40. Quarterly Journal of the Royal Meteorological Society, 135 (645), 1999–2011.

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About Stefan Keiderling

I have just finished my master degree in meteorology at the University of Bergen. My thesis was about low level jet streams at the ice edge, which was a nice combination of my research interests, atmospheric dynamics and polar regions.
This entry was posted in Air, Arctic, Climate, Cyclone, Forecasting, Ice, Jet, Modelling, Ocean, Sea, Wind and tagged , . Bookmark the permalink.

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