Interaction of katabatic winds and mesocyclones near the eastern coast of Greenland

Transcription

1 Meteorol. Appl. 9, (2002) DOI: /S Interaction of katabatic winds and mesocyclones near the eastern coast of Greenland Thomas Klein and Günther Heinemann, Meteorologisches Institut der Universität Bonn Satellite images occasionally show the existence of mesocyclones (MCs) close to the eastern coast of Greenland, especially in the region of Angmagssalik/Tasiilaq. The forcing mechanisms of such MCs are investigated by means of numerical simulations with a three-dimensional mesoscale model. The special characteristics of the East Greenland topography are shown to be a key factor for the development of the MCs. The channeling of the flow in large valleys along the East Greenland coast leads to a convergence, implying a vertical stretching of the flow through the valleys and the generation of cyclonic vorticity. This convergence can be strong during intense katabatic storms, so-called Piteraqs, which are a much-feared phenomenon in that area of Greenland. During these situations the synoptic environment enforces the katabatic flow. The results of the simulations suggest a close relationship between the occurrence of Piteraqs and the generation of mesoscale vortices close to the East Greenland coast. An improved understanding of these processes will help forecasters to advise aircraft and ship operations near the coast of East Greenland. 1. Introduction Mesocyclones (MCs) in the Arctic have been studied extensively during the last two decades. Although our knowledge of MCs and the scientific understanding of their forcing mechanisms have significantly increased during that period, MCs can still represent a major problem for operational weather forecasting. Baroclinic instability associated with upper level cold air advection and cold air masses over relatively warm water seems to be the dominating mechanism for most MCs developing over the Norwegian Sea (see Grønås et al., 1987; Nielsen, 1997; Nordeng & Rasmussen, 1992), but also certain characteristic topographical structures can play a role in mesocyclogenesis. In the present study, the term mesocyclones is used in agreement with the European Polar Low Working Group definitions (Heinemann & Claud 1997), that is the classical term polar low is included, which is restricted to maritime systems with near-surface winds exceeding 15 ms 1. The ice sheet of Greenland (Figure 1a) seems to be a favourable orographic structure in order to initiate or contribute to cyclogenesis on different scales. Kristjánsson & McInnes (1999) investigate the importance of Greenland for the position and strength of the Icelandic low by examining individual cases using simulations with the Norwegian Limited Area Model (NORLAM). In one of the cases, a secondary synoptic cyclone development between Iceland and Greenland occurs on the rearside of a northeastward propagatinglow pressure system. The secondary development is strongly related to the Greenland orography. The southern tip of the Greenland ice sheet extends about 500 km in north-south direction and represents a comparatively narrow barrier to the prevailing westerly large-scale flow. This barrier appears to be an ideal location for leecyclones to occur and can also cause flow patterns as the Greenland tip jet discussed in Doyle & Shapiro (1999). Furthermore, the orography of the East Greenland coast near Angmagssalik (Tasiilaq, see Figure 1b) seems to be important for the development of MCs. Satellite observations occasionally show the existence of mesoscale vortices close to the coast near Angmagssalik. In Figure 2 examples of MCs occurring in the vicinity of Angmagssalik are displayed. Figure 2a shows two MCs between Greenland and Iceland embedded in a westerly flow pattern. Figure 2b shows a mesoscale system on the rearside of a large synoptic low pressure system passing south of Iceland. In Figure 2c a small mesoscale cloud spiral close to Angmagssalik can be seen embedded in a convective flow. Finally, another MC can be observed south of Angmagssalik close to the Greenland coast in Figure 2d. In general, the village of Tasiilaq/Angmagssalik (TA in Figure 1b) is comparatively well sheltered from intense katabatic winds and has a relatively low annual mean wind speed compared to most other Greenland sites (Hedegaard, 1982; Rasmussen, 1989). Using analyses of 15 years of wind data obtained at Greenland synoptic 407

2 Thomas Klein and Günther Heinemann Lat in deg Pituffik Figure 1. a) Map of Greenland with orography (isolines every 500 m) from high-resolution (2 km) topography data (Ekholm, 1996). Triangles mark the radiosonde stations (abbreviations: Sco=Scoresbysund, Dan=Danmarkshavn, KG=Kangerdlugssuaq) b) Topography (isolines every 200 m) from high-resolution (2 km) topography data (Ekholm, 1996) for the area of Tasiilaq/Angmagssalik (TA). The fjord Sermilik (SK) and the valleys Ikertivaq (IK1) and Ikeq (IK2) are marked. 408 Davis Strait Aasiaat Narsaq IK2 KG Dan. Sco. Tasiilaq/Angmagssalik IK1 SK TA 100 km Lon in deg stations, Hedegaard (1982) finds the winds on 59% of that period to be calm (less than 0.5 ms 1 ) at Tasiilaq. Yet, the maximum wind speed to be encountered in a period of 50 years turns out to be 57 ms 1. The pure katabatic flow over the more homogeneous parts of the ice sheet generally does not reach Tasiilaq. But, during certain synoptic situations (e.g., a synoptic low located between the east coast of Greenland and Iceland), the pure katabatic flow can be significantly supported by a synoptic pressure gradient. Such conditions can lead to hazardous katabatic storms called Piteraq by the Inuits, during which Angmagssalik is no longer protected from the katabatic flow. These storms with extraordinary wind speeds of hurricane force are a much-feared phenomenon, and the correct prediction of such Piteraqs is one essential task for the operational forecast for that region. Rasmussen (1989) describes a disastrous Piteraq occurring in 1970, during which wind gusts of 72 ms 1 were estimated and which caused significant damage in the Angmagssalik area. But even minor Piteraqs with lower wind speeds than those encountered in the 1970-Piteraq can endanger life in that area. Kaas & Isaksen (1989) discuss two Piteraqs with measured wind speeds of up to 33 and 15 ms 1, respectively. In the first case investigated in Kaas & Isaksen (1989) and in the disastrous 1970-event discussed in Rasmussen (1989) the Piteraqs were mainly forced by the synoptic environment. A favourable synoptic situation for a Piteraq to occur in the area of Angmagssalik is the location of a strong synoptic low pressure system between about Kangerdlugssuaq (see Figure 1a) and Iceland. During such conditions, the winds at Kangerdlugssuaq close to the centre of the low can be relatively weak, but to the southwest the associated pressure gradient leads to a northwesterly nearsurface flow, which is channeled and thereby additionally intensified in the valleys and fjords close to Angmagssalik. A second mechanism, which Kaas & Isaksen (1989) consider to be the main forcing of the second Piteraq discussed in their study, is the presence of very cold and therefore dense air over the top of Greenland. Reaching the vicinity of the steeper slopes of the ice sheet, the cold dense air can be significantly accelerated under the impact of gravity and, taking into account additional channeling effects in the valleys close to the coast of Angmagssalik, lead to katabatic storms. However, in most Piteraq cases a combination of these two mechanisms rather than only one individual forcing seems to be present. In the present study the interaction of katabatic storms and MCs near Angmagssalik at the eastern coast of Greenland is investigated using the mesoscale model NORLAM. Four different cases are discussed: the first scenario is an idealized simulation, where the model is started with an atmosphere at rest with horizontally homogeneous temperature and humidity fields. The second and the third example are cases of real MC events near Angmagssalik. The fourth case is an idealized simulation with an artificial synoptic forcing, in

3 Katabatic wind/mesocyclone interaction, East Greenland 50 W 30 W 20 W 10 W 65 N TA 60 N AVHRR data, ch4 Brightness Temperatures, NOAA 7, :15 TB [K] W 30 W 20 W 10 W 65 N TA 60 N AVHRR data, ch4 Brightness Temperatures, NOAA 11, :18 TB [K] Figure 2. a) AVHRR channel 4 image from global area coverage (GAC) data (4 km resolution, polarstereographic projection, brightness temperature scale in K) at 1515 UTC on 1 January Tasiilaq/Angmagssalik (TA) is indicated by the filled circle. b) Same as Figure 2a, but for 1818 UTC 30 March which a typical synoptic situation for an interaction of katabatic storms and MCs is sketched. 2. The numerical model The limited area primitive equation model NORLAM used for the simulations of MCs and katabatic storms at the eastern coast of Greenland is the former operational model of the Norwegian Meteorological Institute (DNMI) at Oslo. NORLAM has been successfully applied for the simulation of MCs, katabatic winds and other mesoscale phenomena in the Arctic and Antarctic (e.g. Grønås et al., 1987; Grønås & Skeie, 1999; Skeie & Grønås, 2000; Klein & Heinemann, 2001; Klein et al., 2001). General information on the model can be found in Grønås & Hellevik (1982), while Nordeng (1986) provides a description of the basic model physics and parameterization schemes. For the simulations discussed in this work, resolutions of 50 km and 25 km are used in a nested mode. Global analyses of the European Centre for Medium-range Weather Forecasts (ECMWF) are used as initial and boundary fields for the NORLAM runs with a grid spacing of 50 km (LAM50). Runs with 25 km grid spacing (LAM25) are then performed with the results of the LAM50 run as boundary conditions, using either the 0 h initialized LAM50 analysis or the 6 h LAM50 forecast as initial state. The model is used with a domain size of grid points for the LAM50 and LAM25 simulations. The model domains are 6000 km 5000 km (LAM50, see 409

4 Thomas Klein and Günther Heinemann 50 W 30 W 20 W 10 W 65 N TA 60 N AVHRR data, ch4 Brightness Temperatures, NOAA 12, :19 TB [K] W 30 W 20 W 10 W 65 N TA 60 N AVHRR data, ch4 Brightness Temperatures, NOAA 14, :57 TB [K] Figure 2 (contd) c) Same as Figure 2a, but for 1919 UTC 22 January d) Same as Figure 2a, but for 1457 UTC 2 April 1997 from high resolution picture transmission (HRPT) data (reduced resolution of 3 km). Figure 13) and 2500 km 3000 km (LAM25, see Figure 4a). 30 σ-levels are used for the vertical discretization with about half the levels located below 850 hpa in order to resolve the boundary layer well. 3. Results of numerical simulations 3.1 The structure of the katabatic wind system near Angmagssalik 410 In order to separate the typical structure of the boundary layer over the Greenland ice sheet from any synoptic influences, the results of an idealized simulation without synoptic forcing are discussed first. In this model run, the atmosphere is initially at rest, while the temperature and relative humidity fields are horizontally homogeneous in the initial state. Constant boundary conditions are assumed for LAM50. The initial vertical profiles of potential temperature and relative humidity used for this simulation are displayed in Figure 3. Wintertime radiation conditions and sea ice coverage are assumed for this simulation (according to 1 January 1990). Figures 4a and 4b show the near-surface wind field (lowest NORLAM σ-level at which wind components are held, about 30 m above ground) after 24 h of simulation (model start at 0000 UTC) for Greenland and in detail for the East Greenland region of Angmagssalik and Kangerdlugssuaq. The divergence of the net radiation and sensible heat fluxes over the ice sheet leads to a cooling of the boundary layer. Over the ice slopes, the air is colder than the environmental air further away from the ice surface and moves downward under gravity, friction and the Coriolis force, i.e.

5 Katabatic wind/mesocyclone interaction, East Greenland katabatic winds develop over the slopes of the ice sheet. After 24 h of integration, winds with speeds of up to 16 ms 1 are present over the slopes. The intensity of these winds varies according to the strength of the orographical gradient. Wind speeds are moderate over the gentle slopes and increase significantly in the steeper areas. Furthermore, the intensity of the winds depends on the horizontal scale of the respective slope: over the relatively homogeneous large slopes, e.g. in West Greenland, North Greenland and Northeast Greenland, winds are stronger than in South Greenland. This appears to be a result of a larger cold air reservoir created over the Greenland plateau compared to that over the southern tip of Greenland. due to channeling effects, it is important to note the cyclonic curvature and shear of the flow in the valleys. 3.2 The MC event of 1-2 April 1997 The East Greenland MC event discussed in this section Additional intensifications can be seen in valleys, where the orographical structure leads to a channeling of the flow. This effect is especially obvious from the valleys southwest of Angmagssalik (valley IK2 in Figure 4b) and northeast of Angmagssalik in the region of Kangerdlugssuaq (KG in Figure 4b). Wind speeds of up to 13 ms 1 are present in these valleys. In general, the flow dissipates within only a short distance from the bottom of the slopes. However, this is not the case in the valley southwest of Angmagssalik (note that the valley IK1 in Figure 1b is not resolved in LAM25, so that this valley can be identified with the northeastern part of the valley/bay of IK2 in the LAM25 orography). In that area, the katabatic outflow from the valley region converges with a second boundary layer flow, which is associated with the temperature gradient at the sea ice edge and which is in approximate thermal wind balance. Besides the variation of the strength of the katabatic flow Figure 3. Initial vertical profiles of potential temperature (θ in K) and relative humidity (f in %) for the idealized simulation for the wintertime case (1 January 1990). Figure 4. a) Mean wind vectors (only every second vector plotted) at the lowest NORLAM σ-level after 24 h for an idealized LAM25-simulation for wintertime conditions (1 January 1990, atmosphere at rest used as initial conditions) and topography (isolines every 500 m). The thick line shows the coast line according to the model resolution, while the thick dashed line marks the ice edge. Greenland radiosonde stations are marked by triangles. The rectangle marks the part of the domain shown in detail in Figure 4b. b) Same as Figure 4a, but for the area framed by the rectangle in Figure 4a (all wind vectors). The locations of Sermilik and Kangerdlugssuaq are indicated by SK and KG, respectively, IK2 marks the Ikeq valley. 411

6 Thomas Klein and Günther Heinemann 45 W 35 W 30 W TA 65 00'N IK 'N M2 Figure 5. Same as Figure 2d, but for the channel 2 image with the region of the MC enlarged and for the full resolution HRPT image (1.1 km nadir, albedo greyscale in %) m/s 70 N 65 N M2 60 N 50 W 30 W Figure 6. NSCAT-derived 10 m wind vectors (50 km resolution, sampled at about 1220 UTC, 1400 UTC and 1540 UTC 1 April 1997). The position of the MC is indicated (M2) Figure 7. Mean sea level pressure (full isolines every 2 hpa), potential temperature at 850 hpa (dashed isolines every 2 K) and relative humidity at 850 hpa (shaded if larger than 90%) for 1 April 1997 at 0600 UTC (6 h LAM25 forecast, part of the model domain, isolines are not shown for higher orography). IK2 indicates the region of Ikeq valley southwest of Angmagssalik (triangle ANG), while SL and LT mark a synoptic low over Iceland and a mesoscale lee-trough at the southern tip of Greenland, respectively. The thick dashed line marks the ice edge took place during 1 to 2 April On satellite images for 1457 UTC 2 April 1997 (Figures 2d and 5) a MC can be observed south of Angmagssalik close to the Greenland coast (centre at about 63 N/37 W). For this case data of the NSCAT (NASA Scatterometer) are available with 50 km resolution. NSCAT-derived 10 m wind vectors (obtained from the Center for Ocean- Atmospheric Prediction Studies, COAPS) for the afternoon of 2 April are shown in Figure 6. The use of scatterometer data is restricted to the ice-free ocean, but a clear signal of the MC (marked M2) with wind speeds of ms 1 can be seen in the NSCAT wind field. Although NSCAT has a better capability to detect MCs compared to the ERS scatterometer (see Lieder & Heinemann 1999), MCs of a typical size of km are still difficult to detect because of the coarse scatterometer resolution and data gaps (see Figure 6). 412

7 Katabatic wind/mesocyclone interaction, East Greenland The MC is first visible on satellite images in the morning of 2 April Images on 1 April show only scattered thin clouds in the Angmagssalik area (not displayed). On 3 April the MC is no longer detectable. However, results of a NORLAM simulation for that case started at 0000 UTC 1 April (shown below) yield the development of a MC (named M hereafter) already at 1200 UTC 1 April, i.e. about 24 h prior to the time of the MC signal in the satellite images. This MC is not associated with a cloud structure and develops close to the East Greenland coast. According to the simulation results, this first system M weakens during the following 6 h and seems to dissipate, although a mesoscale trough remains close to the coast. In that area of cyclonic flow, a re-intensification or new development occurs in the morning of 2 April. The location of this second MC (named M2 hereafter) simulated by NOR- LAM is in very good agreement with the position of the MC observed on the satellite images and in the NSCAT-derived wind field. The synoptic situation at 0600 UTC 1 April 1997 is displayed in Figure 7. On the rear side of a synoptic low (SL) with its centre over Iceland, a moderate synoptic flow from the Greenland ice in the direction of the open water is present southwest of Angmagssalik. The Angmagssalik area is cloud-free at that time. At the southern tip of Greenland a weak lee-trough (LT) can be seen, which moves eastwards during the following 12 h and dissipates fast. A MC (M) is simulated by NORLAM in the region of Ikeq valley (IK2) slightly southwest of Angmagssalik 6 h later (Figure 8). The MC has a horizontal extent of about km and is associated with a closed low-level circulation. The wind speeds at the lowest NORLAM σ-level have values of up to 23 ms 1 over the Greenland ice sheet. The strong convergence of the winds at the lower part of the slope (especially in the valley region of Ikeq) is associated with an anomaly of the potential vorticity (PV), which is indicated in Figure 8, where values larger than 1.5 PVU (potential vorticity unit, 1 PVU=10 6 m 2 s 1 kg 1 K) at 925 hpa are shaded for 0600 and 1200 UTC. In order to provide insight into the vertical structure of the simulated MC on 1 April, a cross-section along the line F1 F2 indicated in Figure 8 is shown in Figure 9. The tangential wind vectors indicate the strong katabatic flow over the slope and upward motion near the coast. The circulation associated with M can be seen in the two jets at about 600 m height, which blow in southward and northward directions with wind speeds (normal to the plane of the cross-section) of about 10 ms 1 and 5 ms 1, respectively. The potential temperature field shows a cold air pool at the centre of M with a vertical extent of about 300 m, while a warm air anomaly is present above that height. A tongue of PV with values larger than 1.5 PVU extends from the slope into the centre of the vortex M. The model results at 0600 UTC 1 April show that this cyclonic PV anomaly is transported from the Ikeq valley to the coast and the sea ice (see Figure 8). In the NORLAM simulation, the MC M moves southeastwards over the open water and weakens during the Figure 8. Topography (isolines every 500 m) and wind vectors at the lowest NORLAM σ-level for 1200 UTC on 1 April 1997 (12 LAM25 forecast). M marks the MC southwest of Angmagssalik. Dark and light shadings indicate areas with 1.5 PVU (at 925 hpa) at 0600 UTC and 1200 UTC (1 April), respectively. F1-F2 indicates the location of a cross-section shown in Figure 9. The thick dashed line marks the ice edge. Figure 9. Cross-section F1-F2 (see Figure 8) after 12 h LAM25 simulation (valid at 1200 UTC 1 April 1997). Wind vectors (reference vectors in upper right corner of figure) tangential to cross-section and potential temperature (long-dashed isolines every 2 K). Full lines indicate winds directed into the plane; isolines dashed with short dashes indicate winds directed out of the plane (isolines every 1 ms -1 ). Areas with PV 1.5 PVU are shaded. 413

8 Thomas Klein and Günther Heinemann Figure 10. a) Topography (isolines every 500 m) and geopotential height (isolines every 10 m, not shown above topography) and potential temperature (dashed isolines every 2 K) at 950 hpa for 1200 UTC 1 April 1997 (12 h LAM25 simulation). M indicates the position of the MC southwest of Angmagssalik. The thick dashed line marks the ice edge. b) Same as a), but after 36 h (valid at 1200 UTC 2 April 97). M2 indicates the second MC south of Angmagssalik over the open water. The location of the first MC M (24 h before) is also shown. following 6 h (not shown). During this phase, M merges with a preexisting synoptic trough axis, which is a remainder of the dissipating synoptic system SL over Iceland. In the following time, a new development or re-intensification occurs in the dissipation area of M, leading to the MC M2. In Figure 10 the geopotential height and the potential temperature at 950 hpa are shown after 12 h and 36 h, respectively, displaying the different positions of the two MCs M and M2. Between 1200 UTC 2 April (36 h) and 0000 UTC 3 April (48 h) the second MC M2 moves only slightly (about 50 km) southwestwards compared to the position in Figure 10b, thereby approaching the Greenland coast again. The dissipation stage of M2 on 3 April is not covered by the 48 h NORLAM simulation discussed here. Considering the simulation results of this case of mesocyclogenesis near the East Greenland coast, the question arises about the forcing mechanisms and the potential relevance of the Greenland topography for the development. As could be seen from the PV distribution in Figures 8 and 9, the cold airflow from the Greenland ice sheet appears to be very important for the development of M. Assuming conservation of PV, the barotropic vertical stretching of this airflow leads to the generation of cyclonic relative vorticity. In contrast to the pure katabatic wind situation as described above in the idealized case, the vertical extent of the downflow is larger, which is a result of an additional synoptic support of the flow. The stretching effect is therefore more pronounced than in the pure katabatic wind case. For the second MC M2, also low-level baroclinicity seems to be important. The airflow southwest of Angmagssalik provides a transport of cold air over the open water and thereby enhances the baroclinicity in the genesis region of M2. This can be seen from Figure 11, where the wind vectors and the potential temperature at the lowest NORLAM 5-level at 0000 UTC 2 April are displayed. The cold air from the slopes south of Ikeq flows into the genesis region of the vortex M2. On the other hand, the MC M2 represents an additional forcing of the katabatic wind system by increasing the downslope pressure gradient force, leading to a positive feedback between the katabatic wind and the mesocyclogenesis at this stage. Figure 12 shows the fully developed system M2 at 1200 UTC 2 April. The comparison with the satellite images (Figures 5 and 2d) shows good agreement for the position of the MC. A cyclonic cloud pattern with a vertical extent of up to about 600 hpa is associated with M2, which is also in agreement with the cloud temperatures in that area on infrared satellite images (see Figure 2d). The simulated near-surface winds are strongest in the western part of M2, and the wind field is in agreement with the NSCAT observations (Figure 6). In order to investigate the importance of the Greenland ice sheet orography for the MC case of 2 April 1997, a sensitivity study was performed. In this study the surface elevation was set to zero at all grid points of the model domain, i.e. the Greenland ice has no orography in this run. In this sensitivity run neither the MC M nor M2 develop, which clearly indicates the importance of the orography of the Greenland ice sheet for both MC developments. While the non-development of the MC M, for which the role of the topography is obvious, could be expected, it is an interesting result of the sensitivity study that also the development of M2 is prevented. Thus, it has to be concluded that the cold airflow from the Greenland ice sheet and/or the preexistence of M are important prerequisites for the development of M2. The MC case of 1 2 April 1997 is therefore a complex example of how the Greenland ice sheet orography and the associated katabatic near-surface 414

9 Katabatic wind/mesocyclone interaction, East Greenland Figure 11. Topography (isolines every 500 m) and wind vectors (only every second vector plotted) and potential temperature (dashed isolines every 1 K) at the lowest σ-level after 24 h (valid at 0000 UTC 2 April 1997, LAM25). M2 indicates the genesis region of the second MC, C and W show areas with cold and warm air, respectively. The thick dashed line marks the ice edge. Figure 12. Topography (isolines every 500 m) and wind vectors at the lowest σ-level after 36 h (valid at 1200 UTC 2 April 1997, LAM25). The MC is marked M2. Areas where the relative humidity at 900 hpa exceeds 90% are shaded. The thick dashed line marks the ice edge flows in the Angmagssalik region can contribute to the genesis of MCs in that area. 3.3 The Angmagssalik MC of 24 March 1986 In this section, a second case study of an interaction of synoptically supported katabatic winds in the area of Angmagssalik and the development of a MC is presented. The synoptic situation at 1800 UTC 24 March 1986 (30 h LAM50 forecast) is displayed in Figure 13 for the whole model domain of LAM50. Two major low pressure systems can be seen: A large low pressure system (L1) extends from Iceland to the Norwegian coast, while a second low (L2) with a well defined frontal system is located over the southern part of the Davis Strait between Labrador and South Greenland. Figure 14a shows the wind vectors at the lowest NOR- LAM σ-level for a part of the model domain at 1800 UTC on 24 March (24 h LAM25 forecast). The convergence zone along the west coast of Greenland is associated with the coldfront of pressure system L2. Particularly interesting is the MC M near the East Greenland coast in the region southwest of Angmagssalik. The MC has a diameter of about 300 to 350 km and can already be seen in the simulation results at 1200 UTC on 24 March, developing in connection with a strong katabatic flow through the valley IK2 (note also the mesoscale high south of M). The MC dissipates during the following 12 h, while moving to the northeast. Strong near-surface winds of up to 20 ms 1 are present at the western side of M in the IK2 valley region near point A (Figure 14a). At 1800 UTC on 24 March, the pressure anomaly associated with the vortex M is most pronounced with about 5 hpa (not shown). Figure 14b shows the full-resolution wind field for the Angmagssalik area. The intense katabatic flow through the IK2 valley is obvious. A PV anomaly (exceeding 2 PVU) at low levels is associated with M. As in the case of 1 2 April 1997 the valley of Ikeq is found to be the source region of this PV anomaly, which is being transported over the sea ice and the open ocean. In order to provide a more detailed view of the MC M, a cross-section along the line M1 M2 (marked in Figure 14a) is displayed in Figure 15 for 1800 UTC 24 March. A strong wind component tangential to the plane of the cross-section can be seen for the katabatic wind layer above the slope southwest of Angmagssalik. Figure 13. Mean sea level pressure (isolines every 5 hpa) and potential temperature (isolines every 4 K, dashed) at 850 hpa for 1800 UTC 24 March 1986 (30 h LAM50 forecast). The solid thick line marks the coastline, while the dashed thick line represents the ice edge for March L1 and L2 indicate low pressure systems. 415

10 Thomas Klein and Günther Heinemann Figure 15. As Figure 9, but for the cross-section M1-M2 (location shown in Figure 14a) through MC M at 1800 UTC on 24 March 1986 (24 h LAM25 simulation). The wind component normal to cross-section is shown with isolines every 2 ms -1. Light shading indicates regions with relative humidity exceeding 90%. The dark shading marks the area where the PV exceeds 2 PVU. Figure 14. a) Wind vectors (only every second vector plotted) at the lowest 5-level for a part of the model domain of LAM25 (24 h LAM25 forecast started with 6 h LAM50 forecast, valid at 1800 UTC 24 March 1986). The topography is shown by full lines (isolines every 500 m). M marks a MC near Angmagssalik (triangle ANG), A indicates a location over the ice slope near the Ikeq valley. The line M1-M2 indicates the location of a cross-section shown in Figure 15. b): As part a), but for a smaller section of the model domain with every wind vector plotted. Additionally, regions with PV 2 PVU (at 925 hpa) at 1200 UTC 24 March 1986 and at 1800 UTC 24 March 1986 are indicated by dark and light shading, respectively. The Ikeq valley is marked IK2. The wind field normal to the plane reveals two pronounced jets, which are associated with the circulation of the MC M and the intense katabatic winds over the Greenland ice. The jet over the slope has a maximum of about 18 ms 1 and is mainly a result of the channeled northwesterly katabatic flow in the Ikeq valley region (see Figure 14b). The second jet is located over the ice-free water at the eastern side of the vortex M. Above this jet, the relative humidity exceeds 90%, which seems to be partly due to the convergence in the lower atmosphere (below 850 hpa) and subsequently ascent in that region. As in the case of 1 April 1997, a low-level cold air pool is present in the centre of the MC. A PV anomaly (exceeding 2 PVU) at low levels (below 1000 m) is associated with M (see also Figure 14b). In agreement with Figure 14b, the displacement of this PV anomaly from the slopes of the valley of Ikeq to the sea ice can also be seen from an earlier cross-section at 0600 UTC 24 March (not shown). At that time (0600 UTC) large parts of the Angmagssalik region are covered by clouds associated with the synoptic low pressure system L1. With L1 moving to the east, the Angmagssalik area becomes cloud-free, and the katabatic wind system develops over the ice sheet. The simulated propagation of L1 as well as the structure of the associated cloud cover are in agreement with satellite observations (not displayed). Figure 16 displays vertical profiles valid at the position of the filled square A in the valley region Ikeq southwest of Angmagssalik (see Figure 14), where the strongest winds are simulated, at different times on 24 March. At 0000 UTC the wind speed is nearly zero throughout the lowest 1000 m of the atmosphere, and the stratification is about neutral, since cloudy conditions prohibit the development of a surface inversion (see below). With the wind direction backing from northeast to about north-northwest, a dramatic intensification takes place during the following 6 12 h. The profiles of the potential temperature at 0600 UTC and at 1200 UTC show large stability between about 100 m and 400 m, while a neutral stratification is present in about the lowest 100 m as a result of strong mixing. A low-level jet with winds of gale force (maximum speed of 33 ms 1 at 200 m above the ice and 25 ms 1 at a height of 10 m) is simulated for 1200 UTC 24 March. Above 1200 m (not displayed) the wind speeds reach 10 ms 1 only. At 1200 UTC the wind direction in the lowest 416

11 Katabatic wind/mesocyclone interaction, East Greenland 1000 m of the atmosphere is close to the direction of the local fall-line. The structure of the potential temperature profile at 1800 UTC resembles those at 0600 UTC and at 1200 UTC, but the stability is significantly weaker. The winds at 1800 UTC are also weaker, but still strong with about 17 ms 1 at 10 m height. The simulated boundary layer structure between 0600 UTC and 1800 UTC resembles that observed by aircraft measurements in the katabatic wind layer during conditions of strong synoptic support (Heinemann, 1999). Figure 17 shows a time series of NORLAM-simulated near-surface variables and cloud cover at the site A (see Figure 14). Clouds are simulated at the 850 hpa and 500 hpa levels at 0000 UTC on 24 March, which are associated with the synoptic low pressure system L1 (Figure 13). As a consequence of the eastward propagation of L1, the area near Angmagssalik is cloud-free in the early morning of 24 March. As a result, the net radiation and the sensible heat flux at the surface decrease significantly, and a temperature fall of about 10 C is simulated between 0000 UTC and 0600 UTC. This cooling is partly a local effect due to the decrease of the cloud cover and the consequent decrease in net radiation down to values of about 80 Wm 2. However, a second part of the cooling seems to be associated with cold air being advected from the top of the Greenland ice sheet, i.e. a classical Piteraq situation as described in Rasmussen (1989) and Kaas & Isaksen (1989). Prior to the strong wind event of 24 March, cold air with temperatures down to 30 C is present northwest of Angmagssalik over the higher parts of the Greenland ice sheet (not shown). A moderate upper level (700 hpa) synoptic flow from the northwest transports the cold air towards the steeper slopes, thereby generating the initial conditions for the subsequent onset of the Piteraq in the Ikeq area southwest of Angmagssalik. Parallel to the decrease of the potential temperature an enormous increase in 10 m wind speed from about 1ms 1 to 20 ms 1 is present at the location A. After 1200 UTC 24 March the wind speed decreases again. Clouds form again in the evening of 24 March (associated with the eastward propagation of the synoptic low L2), leading to a net radiation of about 0 Wm 2. It is interesting to note that Piteraq events as the case of 24 March can also be important for the snow-atmosphere energy exchange, particularly for questions of the ablation of the ice sheet. While the latent heat flux over the ice is generally only small, a significant increase of up to 80 Wm 2 is present during the Piteraq, when cold dry air from the higher regions over the ice sheet flows through Ikeq valley. In contrast, the sensible heat flux reaches values down to 90 Wm 2 during the Piteraq. In order to examine the forcing mechanisms of the MC M, the vorticity budget equation on pressure surfaces was evaluated. The results agree with the PV analysis shown above. The generation of cyclonic vorticity associated with the vertical stretching of the flow channeled in the valley southwest of Angmagssalik seems to be very important for the development of M. Cyclonic vorticity is produced over the slopes southwest of Angmagssalik, which is then transported eastward by horizontal advection (not displayed). As for the case of 1 2 April 1997 a sensitivity run with the Greenland orography removed was performed for the case of 24 March In the no-orography run no Location A: 24 March 1986 θ (K) height (m) ff (m/s) 24 March 0000UTC 24 March 0600UTC 24 March 1200UTC 24 March 1800UTC dd ( ) Figure 16. Vertical profiles at the location A (position shown in Figure 14): wind speed, potential temperature and wind direction at 0000 UTC, 0600 UTC, 1200 UTC and 1800 UTC on 24 March 1986 (LAM25 results after 06 h, 12 h, 18 h and 24 h, respectively). Figure 17. Time series (LAM25 forecast, 1800 UTC 23 March UTC 26 March 1986) at the location A in the Ikeq valley southwest of Angmagssalik (see Figure 14). From top to bottom: cloud cover [clouds at 300 hpa (CL300), 500 hpa (CL500) and 850 hpa (CL850)]; net radiation (Q) and 2 m temperature (T); surface flux of sensible heat (H) and latent heat (E); 10 m wind speed (ff) and 10 m wind direction (dd). Q, H and E are 6 h average values. 417

12 Thomas Klein and Günther Heinemann MC develops in the vicinity of Angmagssalik, and a northwesterly flow without significant curvature is simulated instead. This result again underlines the importance of the structure of the East Greenland topography for MC developments near Angmagssalik. A shallow cloud band between about 850 hpa and 700 hpa is simulated by NORLAM in a convergence zone at the eastern side of M at 1200 UTC 24 March, where also fractions of the cloud cover associated with the synoptic low L1 are present (not shown). No significant signal of M can be detected on satellite images in the afternoon of 24 March (Figure 18). But, at about 35 W and 65 N shallow convective clouds are visible, which appear to be cyclonically organized. The convective clouds are in about that area, where the cloud band is simulated by NORLAM (see Figure 15), but the simulated clouds are of a much larger vertical extent. Additionally, the satellite image at 1618 UTC 24 March 1986 with the Angmagssalik region enlarged (Figure 18b) shows a significant gap in the sea ice coverage (coastal polynia) close to the East Greenland 60 W 50 W 30 W 65 N TA 60 N % W TA 65 N IK2 Figure 18. a) AVHRR channel 2 image from HRPT data (reduced resolution, albedo greyscale in %) for 1618 UTC 24 March 1986 for South Greenland. b) Same as a), but for the full HRPT resolution (1.1 km nadir) with the Angmagssalik/Tasiilaq (TA) region enlarged. IK2 marks the area of Ikeq

13 Katabatic wind/mesocyclone interaction, East Greenland coast at Ikeq, which appears to be a result of strong northwesterly winds in that area. Since this coastal polynia is also present on earlier satellite images, it cannot be concluded to what extent the specific Piteraq event of 24 March modifies the coastal sea ice coverage. Nevertheless, the existence of the coastal polynia is one indicator for a generally strong northwesterly flow associated with channeled katabatic winds prevailing in the Ikeq region. Unfortunately, no other observations are available for the area of the MC and the Piteraq at Ikeq. Only weak winds of about 5 ms 1 were recorded at Angmagssalik. This is in agreement with the simulation results (Figure 14b), since the MC and the katabatic storm are simulated southwest of Angmagssalik. In the NORLAM results, winds at Angmagssalik are also weak, since the simulated Piteraq does not directly affect Angmagssalik. In the NORLAM simulation the MC M dissipates in a region east of Angmagssalik (about km to the northeast of its position in Figure 14a) during the morning of 25 March 1986, i.e. its lifetime is about 1 day. 3.4 Idealized scenario of the interaction of downslope storms and mesocyclogenesis According to the results of the previous sections and to previous studies of Piteraqs (Rasmussen 1989, Kaas & Isaksen 1989) the synoptic conditions are of crucial importance. A precursor for a Piteraq to occur is the existence of very cold air on top of the Greenland ice, and under certain conditions the synoptic environment can support or trigger the katabatic flow from the ice sheet. The presence of a synoptic low located between about Greenland and Iceland appears to be particularly suitable to trigger intense Piteraqs. Figure 19. Initial mean sea level pressure (full isolines every 2 hpa) for an idealized LAM50 simulation (synoptic low L between East Greenland and Iceland). The thick line shows the coast line according to the model resolution, while the thick dashed line marks the ice edge. Greenland radiosonde stations are marked by triangles. A moderate Piteraq situation is sketched in this section by prescribing a certain synoptic situation in the initial fields for an idealized NORLAM run. As in the simulation without synoptic forcing (Section 3.1) wintertime conditions are assumed. A single vertical profile of temperature and relative humidity is used for the NORLAM initialization, i.e. the initial distribution of temperature and relative humidity is horizontally homogeneous as in the idealized study in Section N 0 70 N H L M 60 W 20 W 60 N Figure 20. Same as Figure 19, but after 6 h of LAM25 simulation and for a part of the LAM25 domain. H, L, and M indicate a high, a low and a MC, respectively. Figure 21. Wind vectors at the lowest s-level after 12 h for an idealized LAM25-simulation (synoptic low between East Greenland and Iceland) and topography (isolines every 500 m). The line XA-XB indicates the position of the cross-section shown in Figure 22. The regions of Ikeq (IK2), Sermilik (SK) and the location of Kangerdlugssuaq (KG, filled square) are marked for reference. M indicates the MC. 419

14 Thomas Klein and Günther Heinemann Figure 22. Cross-section of the terms of the vorticity budget equation after 6 h for the idealized LAM25 simulation (synoptic low near Iceland) along the line XA-XB shown in Figure 21. Left from top to bottom: stretching term (DIV), local tendency (DZ) and horizontal advection (HADV). Right from top to bottom: tilting term (TT), diffusion term (DF) and vertical advection (VADV). DZ is plotted with isolines every 0.5x10-8 s 2, all other terms with isolines every 1.0x10-8 s 2. Positive values are shown by solid lines, negative values are dotted. 420

15 Katabatic wind/mesocyclone interaction, East Greenland In addition, a negative geopotential height anomaly (i.e. a low pressure system) is introduced at the 1000 hpa level. The geopotential height at pressure levels above 1000 hpa is then obtained by integrating the hydrostatic equation. The procedure is described in detail in Klein & Heinemann (2001). The prescribed geopotential height anomaly corresponds to a pressure anomaly of about 12 hpa with a minimum pressure of 988 hpa at the centre of the low. The diameter of the low is 1500 km, i.e. the artificial synoptic cyclone represents a synoptic low pressure system of moderate intensity. The centre of the low is initially located between East Greenland and Iceland. Figure 19 shows the initial LAM50 mean sea level pressure field. After 6 h LAM25 simulation (started with the 0 h prognosis of LAM50), the synoptic low has started to fill up (central pressure of 991 hpa, Figure 20). A large trough has formed along the eastern coast of Greenland, while an anticyclone is present over the interior regions of the Greenland ice sheet. Close to Angmagssalik, a MC M is simulated. During the following 6 h, M weakens slightly. The wind vectors at the lowest NORLAM σ-level after 12 h are shown in Figure 21. Relatively strong winds are present along the east coast of Greenland with maximum speeds of up to 18 ms 1 in the Ikeq valley region. The centre of the simulated MC is located over the sea ice. In contrast to the western part of the MC, which is influenced by the strong katabatic flow, the winds at the eastern part of the MC are relatively moderate with speeds less than 10 ms 1. The results of this idealized run resemble the simulated MC M in the real case study for April However, the comparison of Figure 21 and Figure 8 shows much more downslope flow for the April 1997 case. The development of this MC is clearly a result of the particular topographic structure of the east coast of Greenland. An important forcing mechanism is the vertical stretching of the flow descending through the valley area of Ikeq southwest of Angmagssalik. This is demonstrated in Figure 22, where the vorticity budget equation in pressure coordinates is evaluated for a cross-section through that valley (after 6 h): ζ r r ζ ω ζ r r = vh h( + f) ( ζ + f) hvh + t p DZ HADV VADV DIV u ω v ω Fζ p y p x + TT DF The positive tendency of vorticity (DZ), i.e. the generation of cyclonic vorticity, is mainly a result of the convergence of the flow (stretching term DIV) in the Ikeq valley. The vertical advection (VADV) of existing vorticity (previously generated by the stretching mechanism DIV) contributes to a positive DZ to a small extent. The tilting term (TT), horizontal advection (HADV) and diffusion (DF) tend to counteract the local vorticity production. Six hours later, the mesocyclone is fully developed (Figure 21). At the time of Figure 22 (valid at +06 h) the flow over the slopes of Ikeq has a more pronounced downslope component compared to Figure 21 (not shown). This extreme forcing by the stretching mechanism can, apparently for continuity reasons, not continue for a longer period, resulting in a more contour-parallel flow at +12 h. In a sensitivity run without orography no MC develops. As in the no-orography simulations for the real cases discussed above, northwesterly winds are simulated in the Angmagssalik region. Despite the high degree of idealization, this experiment captures the development mechanism of Piteraqs and orographically forced MCs in the region of Angmagssalik. Similar idealized simulations (not shown) with slightly varied positions and diameters of the synoptic low (but still with the synoptic low between Iceland and Greenland) suggest the position of the MC development to be strongly coupled to the specific structure of the orography southwest of Angmagssalik (in particular, Ikeq valley) and to be less sensitive to the exact location of the synoptic low. The results of the idealized simulations support the thesis that the presence of a synoptic low pressure system between about Greenland and Iceland favours a strong katabatic flow in Ikeq valley near Angmagssalik and subsequent mesocyclogenesis associated with this flow pattern. 4. Discussion and conclusions The orography of the Greenland ice sheet in the Angmagssalik area can lead to severe katabatic storms called Piteraq by the Inuits. Such katabatic storms seem to be able to contribute to or even trigger the development of short-lived MCs close to the east coast of Greenland. The downslope flow from the Greenland ice can be important for mesocyclogenesis because of two mechanisms: Vertical stretching of the converging drainage flow leads to the generation of cyclonic vorticity. The katabatic flow provides a cold air transport, which can significantly enhance the low-level baroclinicity, especially in the vicinity of a maritime environment. For the generation of extreme katabatic winds, the cooling of the boundary layer over the sloped ice sheet does not seem to be sufficient. Idealized simulations without synoptic forcing have shown maximum nearsurface winds in the valley regions of East Greenland of 13 ms 1 only. The synoptic environment can enforce the katabatic flow in different ways. The temperature contrast between the cold boundary layer air over the 421

16 Thomas Klein and Günther Heinemann Greenland ice sheet and the free atmosphere can be increased by warm air advection associated with synoptic cyclones. A favourable synoptic scenario for an intensification of the katabatic flow at East Greenland consists of a synoptic low located between Greenland and Iceland. The synoptic pressure gradient at the western side of such a low enforces the drainage flow from the ice sheet, which in turn can lead to the development of MCs in the Angmagssalik area. Synoptic support for the mesocyclogenesis appears to be particularly necessary for the mechanism of vortex stretching, since the vertical extent of the pure katabatic wind layer is generally only small (rarely more than 300 m). When a MC has formed close to the coast, a positive feedback on the katabatic wind system can occur. The results of this study depict the basic mechanisms for the interaction of katabatic storms and mesocyclogenesis near the coast of Southeast Greenland, and are based on idealized and realistic case studies. Further investigations are necessary in order to determine the significance of such MC events and in order to validate numerical simulations for Piteraq cases. In addition, an extensive analysis of satellite imagery could be helpful to estimate the frequency of Angmagssalik MCs, although not all MCs seem to be associated with cloud patterns (e.g. the case of 24 March 1986). Similar mechanisms associated with large-scale katabatic flows seem to be present in some areas of Antarctica, where the importance of the specific orography of the Ross Sea or the Weddell Sea regions for mesocyclogenesis has been pointed out by several studies (e.g. Carrasco & Bromwich, 1995; Gallée, 1995; Engels & Heinemann, 1996). Acknowledgments The present study was funded by the Deutsche Forschungsgemeinschaft under grant He 2740/1. The ECMWF provided the analyses taken as initial and boundary conditions for the simulations. The Norwegian Meteorological Institute (DNMI) at Oslo made the NORLAM model available. SSM/I data used for the derivation of the sea ice coverage for April 1997 were provided by the Global Hydrology Resource Center (GHRC) at the Global Hydrology and Climate Center (Huntsville, Alabama, USA). SSM/I-derived sea ice coverage for January 1990 and March 1986 was provided by the National Snow and Ice Data Center (NSIDC) at Boulder, Colorado. AVHRR satellite data were provided by Dundee Satellite Receiving Station (HRPT data) and NOAA/NESDIS (GAC data). NSCAT data were provided by COAPS. References Carrasco, J. F. & Bromwich, D. H. (1995). A Midtropospheric Subsynoptic-scale Vortex that developed over the Ross Sea and Ross Ice Shelf of Antarctica. Antarctic Sci., 7: Doyle, J. D. & Shapiro, M. 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Wind vector and extreme wind statistics in Greenland. Weather Service Report No. 1., ISBN , Danish Meteorological Institute, Copenhagen, Denmark, 106pp. Heinemann, G. (1999). The KABEG 97 field experiment: An aircraft-based study of katabatic wind dynamics over the Greenland ice sheet. Boundary-Layer Meteor., 93: Heinemann, G. & Claud, C. (1997). Report of a workshop on Theoretical and observational studies of polar lows of the European Geophysical Society Polar Lows Working Group. Bull. Am. Meteorol. Soc., 78: Kaas E. & Isaksen, L. (1989). To Piteraq er forudsagt af DK-LAM. Vejret, 2, Danish Meteorological Society: Klein, T. & Heinemann, G. (2001). On the forcing mechanisms of mesocyclones in the eastern Weddell Sea region, Antarctica: Process studies using a mesoscale numerical model. Meteorologische Zeitung N.F., 10: Klein, T., Heinemann, G., Bromwich, D. H., Cassano, J. J. & Hines, K. M. (2001). Mesoscale Modeling of Katabatic Winds Over Greenland and Comparisons with AWS and Aircraft Data. Meteor. Atmosph. Phys., 78: Kristjánsson, J. E. & McInnes, H. (1999). The impact of Greenland on cyclone evolution in the North Atlantic. Q. J. R. Meteorol. Soc., 125: Lieder, M. & Heinemann, G. (1999). A summertime Antarctic mesocyclone event over the Southern Pacific during FROST SOP3: A meso-scale analysis using AVHRR, SSM/I, ERS and numerical model data. Weather and Forecasting, 14: Nielsen, N. W. (1997). An early Autumn polar low formation over the Norwegian Sea. J. Geophys. Res., 102: Nordeng, T. E. (1986). Parameterization of Physical Processes in a Three-Dimensional Numerical Weather Prediction Model. Technical Report No. 65, ISSN , The Norwegian Meteorological Institute, Oslo, Norway, 48pp. Nordeng, T. E. & Rasmussen, E. A. (1992). A most beautiful polar low: A case study of a polar low development in the Bear Island region. Tellus, 44A: Rasmussen, L. (1989). Den dag, Angmagssalik naesten blaeste i havet. Vejret, 2, Danish Meteorological Society: Skeie, P. & Grønås, S. (2000). Strongly stratified easterly flows across Spitsbergen. Tellus, 52A: