Report of the workshop on "Theoretical and observational studies of polar lows" of the EUROPEAN GEOPHYSICAL SOCIETY POLAR LOWS WORKING GROUP
Günther Heinemann, Meteorologisches Institut, Universität Bonn, Auf dem Hügel 20, D-53121 Bonn, Germany
The sixth meeting of the EGS Polar Lows Working Group (EPLWG) was held at the State Scientific Center of the Russian Federation Arctic and Antarctic Research Institute (AARI), St.Petersburg, from 23 to 26 September 1996. For the first time, this workshop brought together scientists from western countries with scientists of AARI on the research field of polar mesocyclones. During the first two days of the meeting, 17 presentations of polar low studies were given, the next two days were devoted to more detailed discussions of two selected case studies in the frame of smaller working groups. The workshop was sponsored by the Office of Naval Research European Office (ONR). In the following, the presentations of the participants and the main points of the discussions of the working groups will be summarized. The meeting proceedings containing the extended abstracts of the presentations can be obtained from G. Heinemann end of November 1996.
2. Presentations of the participants and working groups
The meeting was structured into three scientific sessions, a session with current and future programs related to polar low research, working group meetings and a general concluding session. The meeting was opened by talks of A. Danilov, Deputy Scientific Director of AARI, and by G. Heinemann (GH), chairman of the EPLWG. GH pointed out that this was the first time for the EPLWG to hear about AARI's activities in polar low research. This opening of the PL group to eastern Europe, where the Russian coast belongs to the Arctic over several thousands of kilometers, and the fact that much of the Russian science is performed also in Antarctica, would probably widen the view of polar mesocyclones. As at previous meetings, an international group with participants from Canada, Denmark, Germany, Norway, Russia, United Kingdom and USA participated in the workshop.
2.1 Session 1: Climatological studies
Erik Rasmussen, Niels Bohr Institute, University of Copenhagen, Denmark, used three years of Advanced Very High-Resolution Radiometer (AVHRR) imagery for the investigation of polar lows over the Barents Sea. For the period 1991-1993, polar lows were found to occur relatively frequently in the Barents Sea region. The evaluation of synoptic maps indicate that most of the polar lows developed along the sea ice front and were associated with upper- level potential vorticity (PV) anomalies.
V. Lagun and E. Lutsenko from AARI presented preliminary classification results on polar mesocyclones over the Norwegian and Barents Seas for the period 1981-1995. Their polar low climatology was based on NOAA and METEOR satellite imagery and about 500 mesocyclones were detected during these 15 years. Twice-daily surface pressure and geopotential height fields from NCEP data (1981-1991) were used for a quantitative climatology.
J. Harold, School of Environmental Sciences, University of East Anglia (UEA), Norwich, UK, showed results for a climatology of polar mesocyclones over the North Atlantik based on AVHRR imagery. During the period October 1993-September 1995 more than 4000 individual vortices were detected and tracked. All cyclones were identified from cloud signatures using channel 4 (IR) of the AVHRR instrument. The two year data set was then used to examine the month by month variability in cyclone activity. The seasonal characteristics of cyclones, and their spatial variability was also studied. It was found that there is considerable variability in mesocyclone activity between months, but the seasonality was broadly consistent between the two years. The spatial variability revealed significant spatial and seasonal variability in cyclone activity, suggesting a similar variability in the coupling of the high latitude ocean and atmosphere.
Kari Wilhelmsen, Norwegian Meteorological Institute, Division for Northern Norway, reported on the importance of inversions upon the development of polar lows and weather forcasts in the Arctic. It was pointed out that the static stability of the air in a vertical column extending from the surface up to 500 hPa is of great importance for polar low developments. During cold air outbreaks, strong cold air advection is present in the boundary layer resulting from the diabatic heating of the cold air flowing from the sea ice over the open water. Upper level cold cores with 500 hPa temperatures below -38 deg Centigrades were found to be essential for the onset of deep convection and polar low formation, if they move over the open sea, where the near-surface air is typically 0 to +2 degrees above freezing level. The role of the inversion capping the boundary layer in preventing deep-layer convective instabilities was shown.
2.2 Session 2: Theoretical and numerical studies
I. Lavrenov and V. Bokov (AARI) have investigated problems of wind wave modelling and forecast for Arctic seas. An operational spectral model of AARI was used taking ECMWF analyses of the surface pressure field as initial data. While the results were quite satisfactorily for large-scale atmospheric conditions, significant underestimations of the wave height were found for the case of a MC over the Barents Sea 24-26 December 1995. More realistic initial fields from reanalyzed synoptic maps led to an increase from 3 to 7 m wave height in the vicinity of the MC.
T. Klein from the Meteorologisches Institut der Universität Bonn (MIUB), Germany, gave a presentation on idealized numerical simulations with a three-dimensional model for the investigation of the synoptic-scale forcing on mesocyclone development in the Weddell Sea region. Simulations with the NORLAM model with 50 and 25 km horizontal resolution and 30 vertical layers were performed using complex physical boundary conditions and different idealized synoptic-scale forcings as initial states. Sensitivity studies demonstrated that the topography and the diabatic heating of the surface fluxes of latent and sensible heat played an essential role in the formation of mesocyclones.
C. Köpken and G. Heinemann from MIUB presented a case study of an Arctic mesocyclone by means of aircraft data assimilated into a mesoscale numerical model. The mesocyclone occurred on 18 March 1989 near the sea-ice edge east of Jan Mayen and had already been investigated during a NOAA research flight campaign. Köpken and Heinemann studied the same case using the NORLAM meso-scale limited area model and analysis scheme in a modified version. The numerical simulation without data assimilation yielded no signs of a meso-scale development, which was in contrast to the cyclonic rotation with wind speeds up to 20 m/s observed by the aircraft and SSM/I wind speed retrievals. In order to obtain a dynamically consistent 3-dimensional analysis of the system, the aircraft flight level data were used in the objective analysis. The resulting analysis was then used for the further model integration. This allowed studies of the dynamical structure of the vortex and its further development which represents a more detailed analysis compared to the analysis relying solely on the aircraft data.
J. Harold (UEA) gave a presentation about the modelling the effects of polar mesocyclones on the underlying oceans. Harold explored the effect of polar mesocyclones on the high latitude North Atlantic. A Mellor and Yamada level 2.5 one dimensional mixed layer model was used to examine the individual effects of wind stress, change in wind direction of the wind stress, and heat transfer on the ocean. The combined effect of theses processes was quantified for a range of idealised cyclones.
The first talk about one of the working group's case studies was given by I. Johannessen, Geophysical Institute University of Bergen (GIUB), Norway. He presented numerical simulations of the "Le Cygne" polar low occurring from 13 to 16 October 1993 over the Norwegian Sea using the NORLAMS model (NORLAM model with the cloud water scheme of Sundquist et al., 1989) with 50 km and 25 km horizontal resolution and 18 and 30 sigma layers vertical resolution, respectively. For the genesis phase of the polar low, baroclinic instability coupled with upper level PV forcing was suggested as the triggering mechanism, with convection contributing at a later stage in the form of conditional instability of the second kind (CISK) or air-sea interaction instability (ASII). The NORLAMS predicted the evolution, maintenance and track of the depression very well, but a large sensitiviy on the starting time of the simulation was found (taking ECMWF analyses as initial fields and boundary values). The positive PV anomaly being present at upper levels during the initial stage of the polar low was found to be also present during the mature phase of the depression.
Simulation results including the second (SH) case study of the EPLWG were presented by G. Heinemann (MIUB). The two cases shown occured during 20-22 May 1986 (Case 1) and 30 July-1 August 1986 (Case 2) in the Weddell Sea region. During this period, the Winter Weddell Sea Program field phase 1986 was performed and additional observational data were available, in particular 3 hourly synoptic observations and 6 hourly radiosonde ascents from the German research vessel "Polarstern", which was situated close to the MC on 31 July 1986. Both cases developed close to the sea ice front of the Weddell Sea. Simulations over 54 h using the NORLAM model with 50 km horizontal resolution and 30 sigma layers showed a triggering by upper-level PV anomalies for both cases. In contrast to summertime mesocyclones, the MC events during winter were associated with strong convection and large surface energy fluxes. Sensitivity studies show that the diabatic heating by cloud water condensation was of great importance for the intensification during development of Case 2 during the first 24 h. Comparisons of the model results with radiosondes and synoptic observations showed the good quality of the model simulations.
The talk of C. Köpken (MIUB) about the potential use and difficulties of the assimilation of SSM/I data into a meso-scale numerical model for polar low research represented the link between model and satellite studies. SSM/I-derived parameters like near-surface windspeed (WS) and integrated water vapour (IWV) with a horizontal resolution of about 50 km have been used for the detection and study of meso- scale polar vortices in many cases. Köpken showed assimilation experiments using WS and IVW retrievals in the NORLAM model. WS was analysed with a 3-dimensional multivariate successive corrections scheme which converges towards the solution of an optimal interpolation (SC/OI). For the assimilation of IWV a two step method was chosen: First the SC/OI scheme was used in a 2-dimensional univariate version to analyse the field of integrated water vapour; in a second step the first guess profiles of specific humidity were adjusted to these analyzed total contents. The IWV assimilation improved the analysis and simulations significantly and had also large impact on the dynamical fields in the vicinity of fronts. But, in the cases considered, the improvement for the polar lows was limited, since they showed only a week signature in the integrated water vapour content due to their shallow vertical extent. The assimilation of WS retrievals presented more difficulties to the data analysis scheme, since the information about wind direction had to be taken from the model forecast.
2.3 Session 3: Satellite studies
The use of satellite-derived parameters for polar low research has increased considerably during recent years. The most important instruments used are the SSM/I on board the DMSP satellites since 1987, the European Remote Sensing Satellite scatterometer (ERS-SCAT) launched in 1991 and the AVHRR and the TIROS-N Operational Vertical Sounder (TOVS) on board the NOAA satellites since 1978. The application of these new types of satellite data for polar low research was the central point of the EPLWG meeting in 1992. Since polar MCs usually occur in data sparse regions, satellite images have been the only tool for the detection of these phenomena in many cases. With the advent of more advanced satellite instruments and the associated retrieval algorithms, the use of the satellite data has become more quantitative. High-quality sounding data from the TOVS have proven to be of significant value for retrievals of atmospheric temperatures and humidities in polar regions. AVHRR data are a valuable tool for the detection of meso-scale cloud structures and also for the determination of cloud parameters and sea surface temperatures for polar mesocyclone studies. The high precision passive microwave imaging sensor SSM/I allows the determination of near-surface WS and integrated atmospheric water parameters over ocean areas, but low-level structures are often smoothed out by averaging over the whole atmospheric column. ERS-SCAT data allow the determination of near-surface wind vectors over the ocean, but the main restriction is the narrowness of the scatterometer swath (about 500 km only), which leads to large gaps between subsequent satellite overpasses and therefore an insufficient areal coverage. The most efficient way to perform remote studies of MCs is to combine different satellite measurements in the form of a multi-sensor study using different data types being coinicident in time. This multi-sensor approach was followed in most of the papers presented in this section.
The paper of L.A. McMurdie, Dep. of Atmospheric Sciences, University of Washington (UW), Seattle, USA, and C. Claud, Laboratoire de Meteorologie Dynamique (LMD), Palaiseau, France, investigated MCs in cold airstreams poleward of major frontal zones over the southern oceans detected by geostationary GMS imagery. Quantitative information regarding the atmospheric temperature, humidity, water and wind fields of these systems was retrieved from SSM/I, ERS-SCAT and TOVS data. Fields of IWV, integrated cloud liquid water, liquid and solid precipitation occurrence, WS and geopotential thickness, but also ERS-SCAT surface wind vectors have been investigated for about 50 individual MCs. In few cases, the surface cyclonic circulation of the MC was confirmed by the ERS-1 winds. It was found that solid precipitation was more prevalent than liquid precipitation in most MCs and that MCs with spiral-shaped cloud systems have lower IWV content than comma-shaped MCs, reflecting the fact that the latter ones were generally detected more equatorwards. For a case occurring near the Antarctic ice edge on 2 June 1993, evidence was found that surface fluxes of heat and moisture were likely to have contributed significantly to its development.
M. Lieder and G. Heinemann (MIUB) presented two MC cases over the northern Amundsen Sea and Bellingshausen Sea, that occured during the special observing periods (SOP) July 1994 and January 1995 of the First Regional Observing Study of the Troposphere (FROST) in the Antarctic. The development of the two vortices with a lifetime of 48 hours and size of approximately 800-1000 km was studied using data from SSM/I, ERS-SCAT, AVHRR data and IR mosaics of the Antarctic region sampled from geostationary and polar orbiting sensors (Antarctic Meteorology Research Center, Wisconsin, USA). The MCs show a distinct signal in the remotely sensed parameters, in particular in the SSMI-derived WS and IWV. In the case of 5 to 6 July 1994, the ERS-1 winds show clearly the surface cyclonic circulation of the MC.
John Turner and Gareth Marshall, British Antarctic Survey, Cambridge, UK, showed results of a comprehensive study of Antarctic MCs in the Weddell Sea/Bellingshausen Sea area using AVHRR data collected at Rothera Station during the period March 1993 to February 1994 and ERS-SCAT data. All the vortices were first observed in the AVHRR imagery and a total of 452 mesocyclones were identified. But, because of the narrow 500 km swath of the scatterometer and the fact that winds can only be derived over the ice-free ocean, only 43 cases (9.4% of the total) were coincident with scatterometer observations. Most systems had a surface circulation and these were found to vary from a slight deviation from the broadscale geostrophic flow to a closed circulation. However, 25% of the MCs did not have any surface circulation. Fields of relative vorticity were also determined from the scatterometer data, with maximum values of up to 4x10-4 s-1. The problems of the ambiguity removal for the ERS-SCAT winds were addressed, which can lead to errors in the wind directions of 180 degrees especially near fronts and in the centre of vortices.
Satellite-derived aspects of the NH case study 13 to 16 October 1993 were eludicated by the paper of C. Claud (LMD), N.M. Mognard, CNES/CESBIO, Toulouse, France, and L.A. McMurdie (UW). Spatio-temporal fields were analyzed using TOVS and SSM/I retrievals, but also data from the Topex/Poseidon altimeter, which yields wind speed and sea state along a narrow swath. Using the different satellite measurements in synergy, the respective role of baroclinicity, surface fluxes, and upper-level support in the initiation and developement of the polar low was evaluated.
L. Mitnik, Pacific Oceanological Institute, Russia, gave an overview about the potential of the Real Aperture Radar (RAR) on board the Russian Okean satellites, which can be used in synergy with a passive microwave imager on the same satellite as well with SSM/I data.
Ian Renfrew, Aashish A. Clerk and Kent Moore, Dept. of Physics, University of Toronto, Canada, addressed the question of polar low movement from the viewpoint of cyclonic co-rotation of two or more MCs. Instead of being advected by the surrounding larger scale systems, two or more polar lows in the same vicinity were found to have binary interactions between pairs of polar lows. In geographic co-ordinates these interactions can appear as unusual loops, twists and turns in the low tracks. Satellite imagery from several case studies had been examined to accurately determine the polar low tracks. In addition, ground based and satellite derived wind data were utilised to provide estimates of the low magnitudes and a theoretical rotation rate for the binary systems, based on barotropic vortex dynamics, was calculated. A good correspondance between these calculated rotation rates and the observed rotation rates was found.
3. Programs related to polar low research and concluding remarks
During this session current and future projects related to polar MCs were presented. P. Twitchell, International GEWEX Project Office, Silver Spring, USA, reported on experimental activities in the frame of the GEWEX, FASTEX, and Labrador Sea Projects. The latter two have close links to polar low research, since field experiments will be performed in the Labador Sea area including intensive ship measurements in February 1997, increased frequency of radiosonde ascents of synoptic stations as well as possible instrumented aircraft missions between Greenland and Labrador.
J. Turner gave an overview about the FROST project, which included three SOPs (each lasting one month) for the SH and is now in the stage of reanalysing the atmosperic fields using numerical forecasts, satellite data and all available observational data.
G. Heinemann reported on the future activities of MIUB in the frame of the German contribution to ACSYS. An aircraft-based experiment is planned in April/May 1997 at the western side of Greenland aiming at the investigation of boundary layer fronts at the sea ice edge over the Davis Street and a the study of the three-dimesnsional structure of the katabatic wind system over Greenland. These studies will be complemented by numerical model studies and satellite studies, which will also be carried out in the Antarctic.
The plans of the University of Bergen were outlined by E. Raustein and include the atmospheric forcing on ocean circulations in Arctic oceans, simulations with a non-hydrostatic model and theoretical work on polar MCs.
I. Renfrew gave a viewback on the Canadian Beaufort and Arctic Storms Experiment (BASE) performed in September and October 1994 over the Canadian Arctic, where 11 SOPs with aircraft measurements and two ground-based radars were carried out.
The future polar low projects of AARI were presented by V. Lagun and A. Danilov. It is planned to set up and complete the data banks at AARI containing satellite data from 1981 to 1996 and observational data from Arctic and Antarctic stations. Apart from case studies, a synoptic climatology for Arctic MCs is planned using ECMWF and NCEP data. The next step will be energy budget studies for polar low events, wind wave modelling and boundary layer studies. Projects in the frame of ACSYS comprise the modelling of climate processes and ocean atmosphere interaction.
The workshop was sponsored by the Office of Naval Research European Office. The content of this workshop does not necessarily reflect the position or policy of the United States Government. I would like to thank the responsible persons from AARI, that we could hold our meeting at AARI. Thanks go also to the local organizers, in particular V. Lagun, S. Priamikov and the secretary Lena, for their assistance in preparing the workshop. I hope that the new contacts between people at this successful scientific meeting will be of advantage also for the future.