GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L09815, doi:10.1029/2008GL033706, 2008 Click Here for Full Article A 2-year climatology of polar low events over the Nordic Seas from satellite remote sensing A.-M. Blechschmidt1 Received 20 February 2008; revised 17 March 2008; accepted 31 March 2008; published 15 May 2008. [1] A new 2-year data set of polar low events over the Kolstad, 2005]. Another study about polar mesocyclone Nordic Seas is presented. The detection of polar lows is activity over the North-East Atlantic and Nordic Seas is based on the combined use of thermal infrared AVHRR available from Harold et al. [1999] for the period October imagery and SSM/I derived wind speeds from the satellite 1993 to September 1995. This study based completely on climatology HOAPS (Hamburg Ocean Atmosphere eye inspection of thermal infrared AVHRR (Advanced Very Parameters and Fluxes from Satellite Data). A total of High Resolution Radiometer) images. All cloud systems 90 polar lows are found in 2004 and 2005 with a maximum with a cyclonic curvature and a diameter of less than of polar low activity during the winter months. The main 1000 km appearing on satellite images were defined as polar low genesis regions lie between Iceland and Finnmark polar mesocyclones. It was mentioned that the term ‘‘polar in the Norwegian Sea, in the Barents Sea and in the lee low’’ should be used for a subgroup of these polar meso- region of Cape Farewell. Interannual variability in polar low cyclones with surface wind speeds exceeding 15 m/s. activity results mostly from more frequent cold air However, the study remained limited to polar mesocyclone outbreaks in 2004. Statistics for several atmospheric activity because no wind speed information was included in parameters (e.g., wind speed, precipitation, cloud top the detection scheme. An unexpectedly large number (4054) temperatures) which describe the intensity of the cyclones of polar mesocyclones were found which mainly developed are retrieved from satellite observations. The data set builds during the winter months. The highest density for cyclone a basis for studies about polar low forcing mechanisms and formation occurred over the north Norwegian Sea. Another for the validation of mesoscale numerical models. preferred region for cyclone formation was the Greenland Citation: Blechschmidt, A.-M. (2008), A 2-year climatology of Sea. Only few cyclones formed in the southern part of the polar low events over the Nordic Seas from satellite remote study area and over the Barents Sea. There was a significant sensing, Geophys. Res. Lett., 35, L09815, doi:10.1029/ difference in the amount of cyclone activity in the 2 years of 2008GL033706. the study with the first year being much more active. However, the seasonal and geographical distribution was 1. Introduction comparable for the 2 years. Noer and Ovhed [2003] studied polar lows near Norway in the period 1999 to 2003 between [2] Polar lows are intense maritime polar mesocyclones. October and May using more restrictive criteria for polar Knowledge about their phenomenology, origin and life low detection. This included a horizontal extent between cycle is limited. This is mostly due to a lack of metero- 100 km and 500 km and the development in cold air logical in situ data in polar low genesis regions. Moreover, outbreaks from the polar ice cap. Only 41 cases fulfilled the small scale and short lifetime of these systems compli- their selection criteria for polar lows. cates their representation in numerical models. A large [4] Overall, no clear conclusion about the number and number of studies about polar lows therefore benefit from characteristics of polar lows that form over the Nordic Seas satellite observations. It was only with the general avail- can be drawn from the above references. This is mainly due ability of satellite imagery in the 1960s that it was realized to different areas and different data sets on which the studies that these phenomena are quite common [Rasmussen and about polar low activity are based. Moreover, a broad range Turner, 2003]. of definitions for the term polar low can be found in [3] Only few climatologies of polar low events or at least literature [e.g., Dannevig, 1954; Reed, 1979; Noer and polar mesocyclones over the Nordic Seas exist. For exam- Ovhed, 2003] and confuses the reader. However, the study ple, Wilhelmsen [1985] mainly scrutinized weather maps to by Harold et al. [1999], though limited to polar mesocy- document gale-producing polar lows with horizontal scales clone activity, is probably one of the most comprehensive between 100 km and 500 km near the Norwegian coast for ones and often cited in literature regarding polar lows [e.g., the period 1972 to 1977. She found a total of 33 cases out of Rasmussen and Turner, 2003; Claud et al., 2007; Kolstad, which most polar lows originated in the regions around 2005]. Spitsbergen and the Barents Sea where large SST-gradients [5] In the present paper polar low activity in the Nordic occur. December and January were months of high polar Seas (i.e., the North Atlantic between 60° and 80° latitude low activity whereas only two polar lows occurred in and between Greenland and Novaya Zemlya) for the years February. A February minimum has also been identified 2004 and 2005 is investigated. A useful compromise in later studies [e.g., Lystad, 1986; Noer and Ovhed, 2003; between the different definitions of polar lows is given by Rasmussen and Turner [2003, p. 12] and applied in this 1Meteorological Institute, University of Hamburg, Hamburg, Germany. study:‘‘A polar low is a small, but fairly intense maritime cyclone that forms poleward of the main baroclinic zone. Copyright 2008 by the American Geophysical Union. The horizontal scale of the polar low is approximately 0094-8276/08/2008GL033706$05.00 L09815 1of5 L09815 BLECHSCHMIDT: CLIMATOLOGY OF POLAR LOW EVENTS L09815 between 200 and 1000 kilometers and surface winds near or the Norwegian Sea and 3 to 13 hours in the Barents Sea and above gale force.’’ The new detection scheme is an exten- the region between East Greenland and Iceland. Conse- sion of the detection method for polar mesocyclones by quently, the number of undetected polar lows resulting from Harold et al. [1999] to polar lows. It is based on the varying coverage of AVHRR images over the study area can combined use of thermal infrared AVHRR imagery and only be insignificant given that their lifetime is shorter than SSM/I derived wind speeds from the satellite climatology the time interval between two consecutive images. HOAPS (Hamburg Ocean Atmosphere Parameters and [10] HOAPS is a completely satellite based climatology Fluxes from Satellite data; Andersson et al., 2007). The of precipitation, evaporation and all basic state variables purpose is to build a data set of polar lows consistent with needed for the derivation of these fluxes over the global ice the definition given by Rasmussen and Turner [2003] for free oceans [Andersson et al., 2007]. Except for SST, all further studies about the dynamics that lead to polar low variables are derived from SSM/I passive microwave radio- formation and for the validation of mesoscale numerical meters. Pixels containing substantial atmospheric contami- models. nation due to heavy precipitation are filtered in HOAPS-3 to avoid misdetection of wind speeds. For this study a multi- 2. Methodology satellite composite data set with a spatial grid separation of 0.5° has been produced. Due to the remapping, each grid- [6] The detection scheme consists of two steps. In the point represents a mean value over about one to three first step mesocyclones with horizontal scales not bigger instantaneous SSM/I observations, which have a footprint than 1000 km are detected by eye inspection of thermal size of approximately 50 km. The data is available twice- infrared AVHRR quicklook images. In the second step it is daily with at least 50 km distance to the coast. Each grid- checked, whether HOAPS shows a wind speed of at least cell contains data from one of the three DMSP (Defence 15 m/s anywhere in the region of the mesocyclone on the Meterological Satellite Programm) satellites (F13, F14, AVHRR imagery. As described in section 3, HOAPS wind F15), there is no average from two or more satellites. Early speeds are derived from instantaneous satellite observations. passes are overwritten by later passes. The composited data This may result in minor underestimation of the number of is much better suited for polar low detection than a data set polar lows if the velocity criterion was not fulfilled at the derived from averaging data of several DMSP satellites time of the satellite overpass but at another point in time. because the latter can lead to artificial effects like repeated [7] In general, there is a good coverage of HOAPS data wind speed maxima or double fronts. over the study area. Nonetheless, it is not always possible to [11] MODIS is a key instrument aboard the Terra and check the wind speed criterion for polar lows, especially if Aqua satellites. In this study the operational Level-2 cloud their center is located close to land or ice. In very few cases, product is used, which has a spatial resolution of 5 km. no HOAPS wind speed is available because heavy precip- itation inhibits the calculation of wind speeds. If no HOAPS wind speed is available the mesocyclone is termed ‘‘polar 4. Results low-like mesocyclone’’ to indicate that it has the potential to [12] After the detection of the polar lows, their tracks, be a polar low. In contrast to the study by Harold et al. genesis regions, diameters and observed lifetimes have been [1999] however, all mesoscale cyclones that are recognized estimated from AVHRR imagery.