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Observational data and simulations of the regional climate system Baltic integrated model system (BALTIMOS) were used to study precipitation in the Baltic Sea and its drainage basin with a special focus on the diurnal cycle. The study includes a general evaluation of BALTIMOS precipitation, showing that BALTIMOS has too many light rain events causing an overestimation of the total annual precipitation amount. The diurnal cycle as well as its spatial distribution was analysed. BALTIMOS captures the broad characteristics: a significant diurnal variability with an afternoon peak above land and weak variability with a nocturnal peak above sea. An algorithm to distinguish between frontal and convective precipitation was applied to examine the diurnal cycle more thoroughly. The local solar time of maximum rain in summer is about 1 to 2 h earlier in BALTIMOS than in radar observations of precipitation. [ABSTRACT FROM AUTHOR]

The BALTEX Integrated Model System (BALTIMOS) coupled atmosphere ocean model was compared to passive microwave observations of the Advanced Microwave Scanning Radiometer (AMSR-E). Emphasis was put on quantifying the uncertainties associated with the different variables based on data screening both in the model and observations. Monthly means of three atmospheric parameters, as well as sea surface temperature, were compared for a period of 1 year. Sea ice extent was also derived from AMSR-E and compared to the model data on a daily basis. It is shown that the accuracy of the comparisons on a monthly mean basis is limited by precipitation screening. Out of the three atmospheric parameters, surface wind speed and water vapor column amount agree with the model data to within the accuracy of the comparison. The vertically integrated cloud liquid water content diagnosed from BALTIMOS is systematically higher than the liquid water content derived from satellite, even if potential systematic errors are accounted for. In terms of coupling, the two most relevant variables discussed are sea surface temperature and sea ice extent. The temporal extent of sea ice in the investigation area is well represented, as are the periods of the main growing and decay periods. The total sea ice cover appears to be underestimated by BALTIMOS, especially in the peak season between January and the beginning of March. The amplitude of the annual cycle of sea surface temperature in BALTIMOS appears to be too weak compared to the observations, leading to too cold sea surface temperatures in summer and too warm sea surface temperatures in winter. This might also partially explain the underestimation of sea ice cover by BALTIMOS. [ABSTRACT FROM AUTHOR]

A method to classify precipitation events based on their spatial extent and texture has been developed and applied to 3 yr of BALTEX Radar Data Center weather radar composites over the Baltic region. The method is capable of distinguishing large-scale precipitation features typically associated with frontal systems from more small-scale features, which are usually found in convective systems. Data used for this study are 2-D radar images. The classification is performed in three steps. First, contiguous precipitation areas are identified in the radar data. In the second step, each of these areas is subjected to an analysis where different texture parameters are calculated. In a third step, these texture parameters are evaluated by means of a neural network, and a type of precipitation is assigned to each class. The neural network has been trained using a large set of visually classified radar scenes. A validation of the results has been performed by (1) comparing regions where U.K. Met Office analysis shows large contiguous frontal areas and (2) by using surface observations where the surface observer reported precipitation events that could clearly be associated either with intermittent convective precipitation or with frontal systems. The results of the different comparisons are generally in good agreement with each other, and the false classification rate ranges from 10% to 20%. The application to 3 yr of radar data has resulted in estimation of the frontal fraction of precipitation in the Baltic Sea area. About two-thirds of overall precipitation events are determined by frontal passages with high seasonal and diurnal variations. [ABSTRACT FROM AUTHOR]