A blocking analysis was performed for the ERA40 re-analysis (Uppala, 2003)
from the 500hPa geopotential in 6-hourly resolution with two different methods.
1. Blocking index employing the method of Tibaldi and Molteni (Tibaldi, 1990)
Every longitude (for ERA40: resolution T106, 320 values) is analysed separately.
Three zonal bands of the following latitude ranges are relevant: 78.75+-3.75,
60+-3.75 and 41.25+-3.75 degrees. All data within these bands are considered,
separately for the northern and the southern hemisphere.
A longitude is considered blocked if the geopotential in a mid-latitude region
(around 60 deg) is higher than in a corresponding region nearer to the equator
(around 41.25 deg) as well as much higher than in a corresponding poleward region
(around 78.75 deg, i.e. the gradient is above 10m/deglat).
The single necessary modification to the original method is to the delta interval
(Tibaldi, 1997) due to different data resolution.
The analysis is done from 1.1.1958 00UTC to 31.12.2001 18UTC. This is not the
whole ERA40 period, but only the completely available years.
2. Tibaldi-Molteni method with modifications (Schalge et al. 2011)
The Tibaldi-Molteni method was modified to reduce the results to synoptically
relevant blocking events.
The following modifications are realised:
(a) Instead of fixed distances every combination of latitudes within the delta
intervals is permitted.
(b) Only regions with a geopotential of above the 70% percentile (for the whole
latitude) in the mid latitude band at this time are taken into account.
(c) Blocked regions separated by a zonal distance less than 10 deg are
merged (the gap is closed by setting the originally unblocked regions to
blocked and the GHG to 0.000001, GHG=geopotential height gradient)
(d) Blocked regions with a zonal width of less than 20 deg are removed.
(e) Events with a duration of less than 3 days are removed.
The columns of the provided ASCII data correspond to the longitudes (320 values, start: 0.0, increment 1.125), the rows to the time steps (64284 values,
start: 1.1.1958 00UTC, increment 6 hours).