Greenland Flow Distortion Experiment Research Articles

Arctic System Reanalysis improvements in topographically forced winds near Greenland

Southern Greenland is home to a number of weather systems characterized by high speed low‐level winds that are the result of topographic flow distortion. These systems include tip jets, barrier winds and katabatic flows. Global atmospheric reanalyses have proven to be important tools in furthering our understanding of these systems and their role in the climate system. However, there is evidence that their mesoscale structure may be poorly resolved in these global products. Here output from the regional Arctic System Reanalysis (ASRv1–30 km and ASRv2–15 km grid resolutions) are compared to the global ERA‐Interim Reanalysis (ERA‐I–80 km grid resolution), focusing on their ability to represent winds in the vicinity of southern Greenland. Comparisons are made to observations from surface and upper‐air stations, as well as from research aircraft flights during the Greenland Flow Distortion Experiment (GFDex). The ERA‐I reanalysis has a tendency to underestimate high wind speeds and overestimate low wind speeds, which is reduced in ASRv1 and nearly eliminated in ASRv2. In addition, there is generally a systematic reduction in the root‐mean‐square error between the observed and the reanalysis wind speeds from ERA‐I to ASRv1 to ASRv2, the exception being low‐level marine winds where the correspondence is similar in all reanalyses. Case‐studies reveal that mesoscale spatial features of the wind field are better captured in ASRv2 as compared to the ERA‐I or ASRv1. These results confirm that a horizontal grid size on the order of 15 km is needed to characterize the impact that Greenland's topography has on the regional wind field and climate. However even at this resolution, there are still features of the wind field that are under‐resolved.

Evaluation of WRF Model Resolution on Simulated Mesoscale Winds and Surface Fluxes near Greenland

Abstract Southern Greenland has short-lived but frequently occurring strong mesoscale barrier winds and tip jets that form when synoptic-scale atmospheric features interact with the topography of Greenland. The influence of these mesoscale atmospheric events on the ocean, particularly deep ocean convection, is not yet well understood. Because obtaining observations is difficult in this region, model simulations are essential for understanding the interaction between the atmosphere and ocean during these wind events. This paper presents results from the Weather Research and Forecasting (WRF) Model simulations run at four different resolutions (100, 50, 25, and 10 km) and forced with the ECMWF Re-Analysis Interim (ERA-Interim) product. Case study comparisons between WRF output at different resolutions, observations from the Greenland Flow Distortion Experiment (GFDex), which provides valuable in situ observations of mesoscale winds, and Quick Scatterometer (QuikSCAT) satellite data highlight the importance of high-resolution simulations for properly capturing the structure and high wind speeds associated with mesoscale wind events and surface fluxes of latent and sensible heat. In addition, the longer-term impact of mesoscale winds on the ocean is investigated by comparison of surface fluxes and winds between model resolutions over a two-month period.

Complexities in the climate of the subpolar North Atlantic: a case study from the winter of 2007

AbstractAs a result of its high topography, Greenland significantly distorts the atmospheric flow affecting local as well as remote weather systems and, via air–sea interaction processes, the coupled climate system. During February and March 2007, a field campaign was carried out to investigate this as part of the international Greenland Flow Distortion Experiment (GFDex). The observational programme occurred during anomalous conditions, relative to the climatological mean, that favoured the formation of barrier flow along the southeastern coast of Greenland and easterly tip jets at Cape Farewell, Greenland's southernmost point, while inhibiting the formation of westerly tip jets in the same region. The North Atlantic Oscillation (NAO) index was positive, indicating a deep Icelandic Low. However, there was a southeastward shift in the low's centre and a more zonally oriented storm track that we propose to be the result of the concurrent positive phase of the East Atlantic (EA) teleconnection. Unlike the usual situation for an NAO positive state, there was also a warming in the lower troposphere over the Labrador Sea that we argue was the result of a relatively weak Canadian Polar Trough over the eastern Canadian Arctic. It is shown that consideration of the strength of the Canadian Polar Trough in conjunction with information on the phases of the NAO and EA provides for a qualitatively better representation of the state of the atmosphere over the subpolar North Atlantic during the winter of 2007, and by inference in general, than is possible via the NAO alone. Copyright © 2011 Royal Meteorological Society

An assessment of a Greenland lee cyclone during the Greenland Flow Distortion experiment: An observational approach

AbstractOn 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

The impact of targeted observations made during the Greenland Flow Distortion Experiment

AbstractThe 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

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

AbstractAn 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

The Greenland Flow Distortion experiment

Quarterly Journal of the Royal Meteorological SocietyVolume 135, Issue 645 p. 1917-1918 EditorialFree Access The Greenland Flow Distortion experiment Ian A. Renfrew, Corresponding Author Ian A. Renfrew School of Environmental Sciences, University of East Anglia, UKSchool of Environmental Sciences, University of East Anglia, UKSearch for more papers by this authorJón Egill Kristjánsson, Jón Egill Kristjánsson Department of Geosciences, University of Oslo, NorwaySearch for more papers by this author Ian A. Renfrew, Corresponding Author Ian A. Renfrew School of Environmental Sciences, University of East Anglia, UKSchool of Environmental Sciences, University of East Anglia, UKSearch for more papers by this authorJón Egill Kristjánsson, Jón Egill Kristjánsson Department of Geosciences, University of Oslo, NorwaySearch for more papers by this author First published: 18 December 2009 https://doi.org/10.1002/qj.534Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Dobson GMB. 1966. Annual variation of ozone in Antarctica. Q. J. R. Meteorol. Soc. 92: 549– 552. Loewe F. 1967. On Antarctic pressure variations. Q. J. R. Meteorol. Soc. 93: 373– 380. Renfrew IA, Moore GWK, Kristjánsson JE, Ólafsson H, Gray SL, Petersen GN, Bovis K, Brown PRA, Føre I, Haine T, Hay C, Irvine EA, Lawrence A, Ohigashi T, Outten S, Pickart RS, Shapiro M, Sproson D, Swinbank R, Woolley A, Zhang S. 2008. The Greenland Flow Distortion experiment. Bull. Am. Meteorol. Soc. 89: 1307– 1324. Sampe T, Xie S-P. 2007. Mapping high sea winds from space: A global climatology. Bull. Am. Meteorol. Soc. 88: 1965– 1978. Stewart RHA. 1958. Summary of observations at Shackleton, Antarctica 1956–57. Q. J. R. Meteorol. Soc. 84: 176– 179. Wexler H. 1959. Seasonal and other temperature changes in the Antarctic atmosphere. Q. J. R. Meteorol. Soc. 85: 196– 208. Wilson CV, Godson WL. 1963. The structure of the arctic winter stratosphere over a 10-yr period. Q. J. R. Meteorol. Soc. 89: 205– 224. Citing Literature Volume135, Issue645Special Issue: The Greenland Flow Distortion ExperimentOctober 2009 Part BPages 1917-1918 ReferencesRelatedInformation

On the impact of high‐resolution, high‐frequency meteorological forcing on Denmark Strait ocean circulation

AbstractThis paper quantifies and discusses the impact of high‐resolution, high‐frequency atmospheric forcing on the ocean circulation in the vicinity of the Denmark Strait. The approach is to force a 2 km resolution regional ocean circulation model with atmospheric states from reanalysis products that have different spatial and temporal resolutions. We use the National Center for Environmental Prediction global reanalysis data (2.5° resolution, 6‐hourly output) and a specially configured regional atmospheric model (12 km resolution, hourly output). The focus is on the month‐long period in winter 2007 during the Greenland Flow Distortion Experiment. Diagnostics of upper‐ocean currents and mixing are sensitive to the small‐scale variability in the high‐resolution regional atmospheric model. The hydrographic state of the ocean model is insensitive over the month‐long experiments, however. Both sea ice and the fluxes of volume, heat, and freshwater across the east Greenland shelf break and through the Denmark Strait show a moderate response to the high‐resolution atmospheric forcing. The synoptic‐scale atmospheric state has a large role in controlling sea ice too, while internal ocean dynamics is the dominant factor controlling the flux diagnostics. It is the high spatial resolution, not the temporal resolution, that causes these effects, with O(10 km)‐scale features being most important. The sea‐level wind field is responsible, with the other atmospheric fields playing relatively minor roles. Copyright © 2009 Royal Meteorological Society

An overview of barrier winds off southeastern Greenland during the Greenland Flow Distortion experiment

AbstractDuring 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.Numerical simulations emphasize the non‐stationarity of the Greenland barrier flow and its dependence on the synoptic situation in the Greenland‐‐Iceland region. They show that the barrier jet originated north of the Denmark Strait and was drawn southward by a synoptic‐scale cyclone, with the strength and location of the maximum winds highly dependent on the location of the cyclone relative to the orography of Greenland. Experiments without Greenland's orography suggest a ∼20 m s−1 enhancement of the low‐level peak wind speeds due to the presence of the barrier. Thus, the Greenland barrier flows are not classic geostrophically balanced barrier flows but have a significant ageostrophic component and are precisely controlled by synoptic‐scale systems. Copyright © 2009 Royal Meteorological Society

A comparison of aircraft‐based surface‐layer observations over Denmark Strait and the Irminger Sea with meteorological analyses and QuikSCAT winds

AbstractA compilation of aircraft observations of the atmospheric surface layer is compared with several meteorological analyses and QuikSCAT wind products. The observations are taken during the Greenland Flow Distortion Experiment, in February and March 2007, during cold‐air outbreak conditions and moderate to high wind speeds. About 150 data points spread over six days are used, with each data point derived from a 2‐min run (equivalent to a 12 km spatial average). The observations were taken 30–50 m above the sea surface and are adjusted to standard heights. Surface‐layer temperature, humidity and wind, as well as sea‐surface temperature (SST) and surface turbulent fluxes are compared against co‐located data from the ECMWF operational analyses, NCEP Global Reanalyses, NCEP North American Regional Reanalyses (NARR), Met Office North Atlantic European (NAE) operational analyses, two MM5 hindcasts, and two QuikSCAT products.In general, the limited‐area models are better at capturing the mesoscale high wind speed features and their associated structure; often the models underestimate the highest wind speeds and gradients. The most significant discrepancies are: a poor simulation of relative humidity by the NCEP global and MM5 models, a cold bias in 2 m air temperature near the sea‐ice edge in the NAE model, and an overestimation of wind speed above 20 m s−1 in the QuikSCAT wind products. In addition, the NCEP global, NARR and MM5 models all have significant discrepancies associated with the parametrisation of surface turbulent heat fluxes. A high‐resolution prescription of the SST field is crucial in this region, although these were not generally used at this time. Copyright © 2009 Royal Meteorological Society

Aircraft‐based observations of air–sea fluxes over Denmark Strait and the Irminger Sea during high wind speed conditions

AbstractDuring 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.Over open water, the wind stress varied from 0.2 to 1.9 N m−2 and the surface sensible and latent heat fluxes from 50 to 300 W m−2, resulting in total surface heat fluxes of up to 600 W m−2. The exchange coefficients are at the upper end of those previously observed. Mean values for the 15–19 m s−1 range are CDN = 2.04 × 10−3, CHN = 1.63 × 10−3 and CEN = 1.57 × 10−3 for momentum, heat and moisture, respectively. The value of the momentum exchange coefficient is in line with previous studies; however, both the heat and moisture exchange coefficients are higher than in previous studies. Values of CDN over sea ice and the marginal ice zone were in the range 1.67–6.29 × 10−3 and were, for these conditions, generally higher than CDN over adjacent open water areas. No significant spatial patterns in the exchange coefficients over open water have been detected, although there is some suggestion of higher exchange coefficients immediately downwind of the sea ice. Copyright © 2009 Royal Meteorological Society

THE GREENLAND FLOW DISTORTION EXPERIMENT

Greenland has a major influence on the atmospheric circulation of the North Atlantic-western European region, dictating the location and strength of mesoscale weather systems around the coastal seas of Greenland and directly influencing synoptic-scale weather systems both locally and downstream over Europe. High winds associated with the local weather systems can induce large air-sea fluxes of heat, moisture, and momentum in a region that is critical to the overturning of the thermohaline circulation, and thus play a key role in controlling the coupled atmosphere-ocean climate system. The Greenland Flow Distortion Experiment (GFDex) is investigating the role of Greenland in defining the structure and predictability of both local and downstream weather systems through a program of aircraft-based observation and numerical modeling. The GFDex observational program is centered upon an aircraft-based field campaign in February and March 2007, at the dawn of the International Polar Year. Twelve missions were fl...

A reverse tip jet during the Greenland Flow Distortion experiment

Greenland is the third-largest mountain range in the world and, due to its inhospita-ble nature, it is one of the least-studied plac-es on Earth. Greenland’s vast size, extreme topography and its location in the Northern Hemisphere, however, means it has a signifi-cant impact on the weather and climate of Northern Europe. It influences the air flow in the local region where it plays a major part in the production of many of the weather phenomena observed in the area, includ-ing gravity wave generation, cyclogenesis, tip jets, katabatic flows and barrier winds. The Greenland Flow Distortion experiment (GFDex) is investigating the role of Greenland in defining both local weather systems and those further downstream.