Tip Jets and Barrier Winds: A QuikSCAT Climatology of High Wind Speed Events around Greenland

Abstract

The high topography of Greenland results in a number of orographically induced high wind speed flows along its coast that are of interest from both a severe weather and climate perspective. Here the surface wind field dataset from the NASA-JPL SeaWinds scatterometer on board the Quick Scatterometer (Quik-SCAT) satellite is used to develop a wintertime climatology of these flows. The high spatial resolution and the twice-daily sampling of the SeaWinds instrument allows for a much more detailed view of the surface winds around Greenland than has been previously possible. Three phenomena stand out as the most distinctive features of the surface wind field during the winter months: the previously identified tip jets and reverse tip jets, as well as the hitherto unrecognized barrier flows along its southeast coast in the vicinity of the Denmark Strait. Peak surface wind speeds associated with these phenomena can be as large as 50 m s Ϫ1with winds over 25 m s Ϫ1 occurring approximately 10%-15% of the time at each location. A compositing technique is used to show that each type of flow is the result of an interaction between a synoptic-scale parent cyclone and the high topography of Greenland. In keeping with previous work, it is argued that tip jets are caused by a combination of conservation of the Bernoulli function during orographic descent and acceleration due to flow splitting as stable air passes around Cape Farewell, while barrier winds are a geostrophic response to stable air being forced against high topography. It is proposed that reverse tip jets occur when barrier winds reach the end of the topographic barrier and move from a geostrophic to a gradient wind balance, becoming supergeostrophic as a result of their anticyclonic curvature.

An easterly tip jet off Cape Farewell, Greenland. I: Aircraft observations

“…Although based on a synthesis of the observations presented here -i.e. for the 21 February 2007 case -this schematic is generally representative of other ETJ cases described in the literature via numerical simulations and satellite data, for example those of Moore and Renfrew (2005), Martin and Moore (2007) and Ohigashi and Moore (2009) are rather similar even for the cloud patterns. Figure 13 illustrates the high winds associated with the ETJ core as open arrows.…”

“…These scatterometer-based U 10N winds have been used extensively in the study of maritime weather and climate: for example, in climatological studies (e.g. Moore and Renfrew, 2005;Sampe and Xie, 2007); to identify small-scale wind features associated with, for example, SST fronts (e.g. Chelton et al, 2004); and in investigations of particular weather system events (e.g.…”

“…The reason for this climatological accolade is the frequent low-level 'tip jets' that occur there as a result of interactions between the synoptic-scale flow and the high topography of Greenland. Westerly tip jets were first investigated, through numerical model simulations, by Doyle and Shapiro (1999), while easterly or reverse tip jets were first investigated by Moore (2003) and Moore and Renfrew (2005). Doyle and Shapiro suggested tip jets were driven by a Bernoulli acceleration as flow descended down the lee side of Greenland, as well as acceleration in the lee due to flow distortion around the tip of Greenland.…”

“…It has been hypothesised that the forcing mechanism of easterly tip jets is quite different; that these are in fact barrier flows which, upon reaching the end of the barrier at Cape Farewell, turn anticyclonically and accelerate into a curved gradient wind balance (Moore and Renfrew, 2005). In other words, they are barrier flows undergoing a rapid adjustment due to the removal of the barrier.…”

“…Almost all previous studies have relied exclusively on either satellite-based observations, such as scatterometer winds (e.g. QuikSCAT winds: Moore and Renfrew, 2005;Sampe and Xie, 2007), meteorological analyses or reanalyses (Moore, 2003;Egger, 2006), or numerical weather prediction (NWP) simulations (Doyle and Shapiro, 1999;Petersen et al, 2003Petersen et al, , 2005Orr et al, 2005;Martin and Moore, 2007;Ohigashi and Moore, 2009). The only exceptions are two recent studies that jets'; here we use the adjective 'easterly' in preference to 'reverse' tip jet.…”