These data are produced at the ECMWF as part of a special project run by Prof. Brian Hoskins and Dr. Paul Berrisford of Reading University. The project is part of the Joint Diagnostics Project (JDP) between Reading University, The UK Meteorological Office and ECMWF (European Centre for Medium Range Weather Forecasts).
The aim of the project is to provide a database of routine back trajectories. During 1995, 252 5-day back trajectories were computed on a regular basis from a time of 12z. The arrival points were chosen to be on three regular latitude / longitude grids centred over the U.K., the mid-Atlantic storm track region and the eastern U.S.A. and were all located at 900 hPa.
The computations were performed using the ECMWF trajectory package and the 6 hourly spectral T106 uninitialised operational analyses of the three components of the wind and ln(surface pressure). These data are interpolated to a 1.5 x 1.5 degree regular latitude / longitude grid. The package uses a simple mid-point type of algorithm with linear interpolations in both time and space and a timestep of 15 minutes. The output gzipped ASCII files contain information giving latitude, longitude and pressure at each timestep. The dataset is available both at ECMWF and the BADC.
For 1996 several changes were made to the basic system. Firstly the algorithm was changed : a 4th order Runge-Kutta scheme was introduced and the vertical interpolations were made to be cubic. When a timestep of 30 minutes is used this scheme consumes the same amount of computer resources as the mid-point type scheme mentioned above. The difference in horizontal position between calculations performed with the old and the new algorithm are usually quite small, though sometimes they can differ markedly. The pressure difference, however, is typically a few hPa, sometimes 20 - 30 hPa and occasionally as large as 200 hPa.
In addition to the arrival points used in 1995 112 extra points were added in order to provide data for the ACSOE community (Atmospheric Chemistry Studies in the Oceanic Environment - a NERC thematic programme, PI is Stuart Penkett of University of East Anglia). These points are located in the vicinity of the U.K. and the eastern Atlantic and their pressures range between 1000 and 700 hPa. The trajectories are computed for clusters of arrival points in order to estimate how sensitive the results are to small changes in the arrival position. This knowledge is useful for determining how accurate the results are likely to be. The package has been run four times daily (from 00z, 06z, 12z and 18z) starting from 1st March 1996.
On 30th January 1996 the ECMWF analysis system was changed to 3D variational analysis. Hence, there are no separate uninitialised / initialised analyses from this time. The data gap of January and February 1996 (caused when making the changes to the system) has been filled in using initialised analyses.