The climatology of polar lows over the Nordic Seas has been investigated using infrared satellite images for the period between 2000 and 2009. The same region was studied in the 1980s using traditional weather charts for the period between 1972 and 1982. One motivation for the present study was to revisit this climatology, but using a different decade and taking advantage of the vastly improved coverage and dissemination of infrared satellite images since the 1980s. The fact that forecasters at the Norwegian Meteorological Institute had introduced a routine to register polar-low events systematically from 2000 and onward also provided a unique opportunity for extending the existing repository of subjectively identified polarlow observations. On average we found 12 polar-low events per year in the region of study. This is more than the earlier investigation, but we believe that this can be explained by the fact that the previous study relied almost uniquely on weather charts with very little information from ocean areas in the Nordic Seas. The largest numbers were found in January with an average of 2.8 polar-low events per year. The study reconfirms the February minimum found in previous studies, but on the basis of our data we could not show that this minimum is statistically significant. It is suggested that this may be explained as a manifestation of the coldest winter month, when a surface-pressure high over the Scandinavian mainland is common and the large-scale atmospheric flow is less favourable to polar-low formation. This hypothesis was tested by calculating the mean sea-level pressure (MSLP) anomaly for January, February and March from an atmospheric reanalysis. This revealed a positive anomaly over Scandinavia and northwest Russia not found in the pressure distributions for January and March.

Preface This techrical report describes research by Dr. Cardone that began in 1 966 and has been sponsored by three different contracts as its scope increased and as the many applications that it will have become apparent. The first application of this work is to use available data more intelligently in the development of numerical wave hindcasting and forecasting procedures. In 1964, a wave climatology for the North Atlantic was produced that used ship reports to generate the wind fields for the wave hindcasts. It took one half an hour on a CDC 1604 to generate the wind fields for every six hours for a year. Although the hindcasts gave quite good results, it was clear that higher resolution wind fields and a better theory for the winds in the planetary boundary layer would improve the quality of wave predictions. This report more than adequately makes up for the nalvet of previous wind field models in the planetary boundary layer. A second goal for this study was to aid in the development of the software to be used should radar scatterometry and passive microwave data become available from a spacecraft. The definition of the winds over the ocean depends on many factors. Work is actively in progress to combine the results of this paper with simulated data such as might be obtained from remote .sensing techniques so that an optimum analysis of the planetary boundary layer can be made. This report makes it possible to develop ways to use the widely scattered ship reports over the ocean obtained on a synoptic basis in an intelligent way for the extrapolation and interpolation of spacecraft data into areas not observed by ships. A numerical model of the North Atlantic Ocean is presently under development for the Office of Naval Research. For this model, the wind stress at the sea surface and the sensible and latent heat fluxes at the air sea boundary are needed. The procedures described in this report define the wind stress at the sea surface, the atmospheric stability and the air sea temperature differences on an oceanic scale.

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The objective of the EuroROSE (European Radar Ocean Sensing) project was to combine area covering ground-based remote-sensed wave and current data with high-resolution numerical forecast models to provide nowcasts and forecasts for coastal marine operators. Two experiments to test and to demonstrate the system took place: one on the coast of Norway, north of Bergen in March 2000 and the second on the north coast of Spain at Gijon in October -November 2000. Qualitative and quantitative intercomparisons of the wave measurements and wave model products from these experiments are presented. These include measurements using the Wellen Radar (WERA) high-frequency (HF) radar, the WaMoS (Wave Monitoring System) Xband radar, a directional Waverider and output from the WAM wave model. Comparisons are made of the full directional spectra and of various derived parameters. This is the first-ever intercomparison between HF and X-band radar wave measurements and between either of these and WAM. It has provided a data set covering a much wider range of storm and swell conditions than had been available previously for radar wave-measurement validation purposes and has clarified a number of limitations of the radars as well as providing a lot of very useful radar wave data for future model-validation applications. The intercomparison has led to improvements in the data quality control procedures of both WaMoS and WERA. The two radar sytems measured significant wave height with mean biases of 3% and 6%, respectively, and mean direction differences of less than 2j in both cases. Limitations in the WAM model implementation are also discussed. D

A real time assimilation and forecasting system for coastal currents is presented. The purpose of the system is to deliver current analyses and forecasts based on assimilation of high frequency radar surface current measurements. The local Vessel Traffic Service monitoring the ship traffic to two oil terminals on the coast of Norway received the analyses and forecasts in real time. A new assimilation method based on optimal interpolation is presented where spatial covariances derived from an ocean model are used instead of simplified mathematical formulations. An array of high frequency radar antennae provide the current measurements. A suite of nested ocean models comprise the model system. The observing system is found to yield good analyses and short range forecasts that are significantly improved compared to a model twin without assimilation. The system is fast; analysis and six hour forecasts are ready at the Vessel Traffic Service 45 minutes after acquisition of radar measurements.Comment: 15 figures, one tabl

A method for estimating return values from ensembles of forecasts at advanced lead times is presented. Return values of significant wave height in the North-East Atlantic, the Norwegian Sea and the North Sea are computed from archived +240-h forecasts of the ECMWF ensemble prediction system (EPS) from 1999 to 2009. We make three assumptions: First, each forecast is representative of a six-hour interval and collectively the data set is then comparable to a time period of 226 years. Second, the model climate matches the observed distribution, which we confirm by comparing with buoy data. Third, the ensemble members are sufficiently uncorrelated to be considered independent realizations of the model climate. We find anomaly correlations of 0.20, but peak events (> P 97 ) are entirely uncorrelated. By comparing return values from individual members with return values of subsamples of the data set we also find that the estimates follow the same distribution and appear unaffected by correlations in the ensemble. The annual mean and variance over the 11-year archived period exhibit no significant departures from stationarity compared with a recent reforecast, i.e., there is no spurious trend due to model upgrades.

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