Estimating trajectory uncertainties due to flow dependent errors in the atmospheric analysis Department of Meteorology, Stockholm University, Stockholm, Sweden
Received: 25 June 2009 – Published in Atmos. Chem. Phys. Discuss.: 24 July 2009 Abstract. The uncertainty of a calculated trajectory is dependent on the uncertainty in
the atmospheric analysis. Using the Ensemble Transform method (originally adapted
for ensemble forecasting) we sample the analysis uncertainty in order to create
an ensemble of analyses where a trajectory is started from each perturbed
analysis. This method, called the Ensemble analysis method (EA), is compared
to the Initial spread method (IS), where the trajectory receptor point is
perturbed in the horizontal and vertical direction to create a set of trajectories
used to estimate the trajectory uncertainty. The deviation growth is examined
for one summer and one winter month and for 15 different geographical locations.
We find up to a 40% increase in trajectory deviation in the mid-latitudes using
the EA method. A simple model for trajectory deviation growth speed is set up
and validated. It is shown that the EA method result in a faster error growth
compared to the IS method. In addition, two case studies are examined to
qualitatively illustrate how the flow dependent analysis uncertainty can impact
the trajectory calculations. We find a more irregular behavior for the EA
trajectories compared to the IS trajectories and a significantly increased
uncertainty in the trajectory origin. We conclude that by perturbing
the analysis in agreement with the analysis uncertainties the error
in backward trajectory calculations can be more consistently estimated.
Revised: 29 October 2009 – Accepted: 06 November 2009 – Published: 23 November 2009
Citation: Engström, A. and Magnusson, L.: Estimating trajectory uncertainties due to flow dependent errors in the atmospheric analysis, Atmos. Chem. Phys., 9, 8857-8867, doi:10.5194/acp-9-8857-2009, 2009.