The Quarterly Journal of the Royal Meteorological Society

doi: 10.1002/qj.576pmid: N/A

Renfrew, Ian A.; Kristjánsson, Jón Egill

doi: 10.1002/qj.534pmid: N/A

Renfrew, I. A.; Outten, S. D.; Moore, G. W. K.

doi: 10.1002/qj.513pmid: N/A

An easterly tip jet event off Cape Farewell, Greenland, is described and analysed in considerable detail. In Part I of this study (this paper) comprehensive aircraft‐based observations are described, while in Part II of this study numerical simulations and a dynamical analysis are presented. The easterly tip jet of 21 February 2007 took place during the Greenland Flow Distortion experiment. It resulted through the interaction of a barotropic synoptic‐scale low pressure system in the central North Atlantic and the high topography of southern Greenland. In situ observations reveal a jet core at the coast with peak winds of almost 50 m s−1, about 600–800 m above the sea surface, and of 30 m s−1 at 10 m. The depth of the jet increased with wind speed from ∼1500 m to ∼2500 m as the peak winds increased from 30 to 50 m s−1. The jet accelerated and curved anticyclonically as it reached Cape Farewell and the end of the barrier. The easterly tip jet was associated with a tongue of cold and dry air along the coast of southeast Greenland, general cloud cover to the east, and cloud streets to the south of Cape Farewell. Precipitation was observed during the low‐level components of the flight. The very high wind speeds generated a highly turbulent atmospheric boundary layer and resulted in some of the highest surface wind stresses ever observed over the ocean. Copyright © 2009 Royal Meteorological Society

Outten, S. D.; Renfrew, I. A.; Petersen, G. N.

doi: 10.1002/qj.531pmid: N/A

An easterly tip jet that occurred on 21 February 2007 off Cape Farewell, Greenland, is examined. In Part I of this article aircraft observations were described. Now, in Part II, numerical simulations and an analysis of the dynamical forcing mechanisms are presented. The simulations make use of a limited‐area 12 km resolution configuration of the Met Office's Unified Model. Sea‐surface temperatures and sea‐ice concentrations have been replaced using the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) product, addressing a boundary‐layer temperature bias, while roughness lengths over sea ice have been updated, addressing a wind‐speed bias. These modifications ensured a reasonably accurate simulation: generally within 1–2 K and 2–3 m s−1 when compared with dropsonde observations. A momentum‐budget analysis along a curved locus through the core of the jet has been derived. Off southeast Greenland, the easterly tip jet was in cross‐jet geostrophic balance, but was being accelerated downstream by an along‐jet pressure gradient. Over the curved part of the locus, as the jet rounded Cape Farewell, a cross‐jet residual suggests that the jet was unbalanced at the height of the jet core. This residual decreases with height so that an approximate gradient wind balance applies in the upper part of the jet. The anticyclonic curvature, characteristic of easterly tip jets, was caused by a dramatic decrease in the cross‐jet pressure‐gradient force at the end of the barrier, after which the jet aligned with the synoptic‐scale isobars and returned to approximate geostrophic balance. The momentum budget is shown to be robust and applicable to other cases. Copyright © 2009 Royal Meteorological Society

Petersen, G. N.; Renfrew, I. A.; Moore, G. W. K.

doi: 10.1002/qj.455pmid: N/A

During the Greenland Flow Distortion experiment, barrier flow was observed by an instrumented aircraft on 1, 2, 5 and 6 March 2007 off southeastern Greenland. During this time period the barrier flow increased from a narrow jet, ∼15 m s−1, to a jet filling almost the whole of the Denmark Strait with maximum wind speed exceeding 40 m s−1. Dropsonde observations show that the barrier flow was capped by a sharp temperature inversion below mountain height. Below the inversion was a cold and dry jet, with a larger northerly wind component than that of the flow above, which was also warmer and more moist. Thus, the observations indicate two air masses below mountain height: a cold and dry barrier jet of northern origin and, above this, a warmer and moister air mass that was of cyclonic origin.

Mc Innes, Harold; Kristjánsson, Jón Egill; Schyberg, Harald; Røsting, Bjørn

doi: 10.1002/qj.524pmid: N/A

On 3 March 2007 between 0920 and 1545 UTC, the mesoscale structure of a mature lee cyclone southeast of Greenland was successfully captured during a flight with a research aircraft during the Greenland Flow Distortion experiment (GFDex). The cyclone was accompanied by hazardous weather conditions such as extensive icing and low‐level wind speeds exceeding 34 m s−1. Calculations based on atmospheric soundings from Egedesminde on the west coast of Greenland indicate that conditions were favourable for flow splitting and hence cyclogenesis in the lee of Greenland during the formation of the cyclone. PV inversion carried out on an upper‐level southward‐moving potential vorticity (PV) anomaly indicates that upper‐level forcing as well as orographic forcing had an important role in the cyclone development. The frontal seclusion of the cyclone's warm core was consistent with both the Norwegian cyclone model and Shapiro and Keyser's model. However, it was not possible to match this cyclone precisely with any of these models, which is probably a consequence of the strong influence from Greenland on the cyclone development. In situ measurements showing a combination of warm dry air with relatively high ozone concentrations in the centre of the cyclone together with trajectory calculations indicate that this air had experienced considerable descent, probably induced by Greenland's orography. The observations documented the cyclone's mesoscale structure, and clearly revealed a deep tropopause fold. This tropopause fold is a manifestation of the forcing on the cyclone from the upper‐level PV anomaly. Copyright © 2009 Royal Meteorological Society

Kristjánsson, Jón Egill; Thorsteinsson, Sigurdur; Røsting, Bjørn

doi: 10.1002/qj.497pmid: N/A

We present HIRLAM simulations of a deep extratropical cyclone that developed off the southeast coast of Greenland on 2–3 March 2007. The purpose of the simulations is to understand the role of orographic forcing for the cyclone evolution, relating the results to previous model studies. The cyclone evolution was preceded by a powerful cold air outbreak over Greenland, starting on 27 February, manifested by a southward movement of an upper‐level potential vorticity (PV) anomaly from 80°N to 60°N. In addition to a CONTROL run, starting at 0000 UTC 2 March, which captures the main features of the cyclone evolution quite well, we have carried out simulations in which Greenland's orography was removed (NOGREEN), as well as simulations with different starting times. In the NOGREEN simulation starting at 0000 UTC 2 March, the cyclone deepens more rapidly than in CONTROL, due to a stronger cold advection on the rear side, leading to a more rapid baroclinic energy conversion. Furthermore, the cyclone position is shifted northward by 500 km, compared to the CONTROL run. A very different result is found in the NOGREEN simulations that were started 24–36 hours earlier, as the cyclone off Greenland's southeast coast is now displaced eastwards by hundreds of km, and more so as the run starts earlier. The results indicate a phase‐locking by Greenland of a transient PV anomaly, indicating a mechanism for understanding cyclogenesis in this area. Without Greenland's orography, the PV anomaly is unconstrained, and the curvature of its southward trajectory is larger. Copyright © 2009 Royal Meteorological Society

Våge, Kjetil; Spengler, Thomas; Davies, Huw C.; Pickart, Robert S.

doi: 10.1002/qj.488pmid: N/A

The westerly Greenland tip jet is an intense, narrow and intermittent wind phenomenon located southeast of Cape Farewell that occurs frequently during the winter season. Using the ERA‐40 reanalysis dataset, a catalogue of 586 objectively detected westerly tip jet events is compiled for the winters 1957‐2002, and an analysis is undertaken of the character of the jet and its accompanying atmospheric features. It is shown that the tip jet frequency exhibits a significant positive correlation with both the NAO index and the latitude of the Icelandic Low. The peak wind speed and accompanying heat fluxes of the jet have values up to 30 m s−1 and 600 W m−2, respectively, and are sustained for less than one day. The air parcels constituting the tip jet are shown, based upon a trajectory model and the ERA‐40 dataset, to have a continental origin, and to exhibit a characteristic deflection and acceleration around southern Greenland. The events are almost invariably accompanied both by a notable coherence of the lower‐level tip jet with an overlying upper‐level jet stream, and by a surface cyclone located to the lee of Greenland. It is also shown that the cyclone originates upstream of and is advected to the lee of Greenland, and thereby it both precedes in time and contributes dynamically to the formation of the tip jet. On this basis, it is suggested that the tip jet arises from the interplay of the synoptic‐scale flow evolution and the perturbing effects of Greenland's topography upon the flow. Copyright © 2009 Royal Meteorological Society

Irvine, E. A.; Gray, S. L.; Methven, J.; Renfrew, I. A.; Bovis, K.; Swinbank, R.

doi: 10.1002/qj.499pmid: N/A

The impact of targeted sonde observations on the 1‐3 day forecasts for northern Europe is evaluated using the Met Office four‐dimensional variational data assimilation scheme and a 24 km gridlength limited‐area version of the Unified Model (MetUM). The targeted observations were carried out during February and March 2007 as part of the Greenland Flow Distortion Experiment, using a research aircraft based in Iceland. Sensitive area predictions using either total energy singular vectors or an ensemble transform Kalman filter were used to predict where additional observations should be made to reduce errors in the initial conditions of forecasts for northern Europe. Targeted sonde data was assimilated operationally into the MetUM. Hindcasts show that the impact of the sondes was mixed. Only two out of the five cases showed clear forecast improvement; the maximum forecast improvement seen over the verifying region was approximately 5% of the forecast error 24 hours into the forecast. These two cases are presented in more detail: in the first the improvement propagates into the verification region with a developing polar low; and in the second the improvement is associated with an upper‐level trough. The impact of cycling targeted data in the background of the forecast (including the memory of previous targeted observations) is investigated. This is shown to cause a greater forecast impact, but does not necessarily lead to a greater forecast improvement. Finally, the robustness of the results is assessed using a small ensemble of forecasts. Copyright © 2009 Royal Meteorological Society and Crown Copyright

Petersen, G. N.; Renfrew, I. A.

doi: 10.1002/qj.355pmid: N/A

During the Greenland Flow Distortion experiment (GFDex), aircraft‐based observations of air–sea fluxes were obtained over Denmark Strait and the Irminger Sea. High‐frequency observations of velocity, temperature and water vapour have been used to calculate turbulent fluxes of momentum, heat and moisture using the eddy covariance method. These are the first direct air–sea flux observations in this region, and add to the relatively small collection of direct air–sea flux observations made in high wind speed conditions. The aircraft‐based turbulence legs were flown at remarkably low levels, only 30–50 m above the sea‐surface and so within the atmospheric surface layer. Results are presented for 145 flux runs, each of 2 min (approximately 12 km), 131 over open water and 14 over sea ice and the marginal ice zone. The flux data were obtained in 10 m neutral wind speeds of up to 25 m s−1, with 80% of the flux data in the range 15–19 m s−1.