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This article focuses on the formation of complex trajectories of radiological background fields for atypical accidental discharges of radioactivity into the atmosphere during very low wind speed episodes (calms). Within several hours of a calm meteorological situation, a relatively significant level of radioactivity can be accumulated around the source. In the next stage, the calm situation is assumed to terminate and convective movement of the air immediately starts. A packet of accumulated radioactivity, which has the form of multiple Gaussian puffs, is drifted by wind. Consequently, the pollution is disseminated over the terrain. Significant transport of radioactivity even behind the protective zone of a nuclear facility (up to between 15 and 20 km) has been observed. Original optional statistical pre-processing of calm's results is inserted between the calm and convective stages of the trajectory generation which may improve performance of following computationally expensive methods of Bayesian filtering. Determination of complex trajectory passing through both calm and convective stages of the release scenario represents inevitable prior information for prospective assimilation techniques. Draft twin experiment is outlined for a simple assimilation scenario for re-estimation of the main model parameters based on a notional monitoring network in the outer convective region. [ABSTRACT FROM AUTHOR]