# Blocking Detection

Calculation of the blocking index with two different methods:

The Tibaldi-Molteni-Method (Tibaldi et al., 1990 and 1997) and a modified Tibaldi-Molteni-Method (Schalge et al., 2011), which only detects synoptical relevant blocking events are available. The modified one was developed by Bernd Schalge as part of his doctoral thesis. Filters regarding the spatial and temporal extension of the blocking events are applied and can be adjusted.

1. Blocking index employing the method of Tibaldi and Molteni (Tibaldi, 1990)

Every longitude is analyzed 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).One necessary modification to the original method is to the delta interval (Tibaldi, 1997) to adjust it to the given data resolution.

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 realized:(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

Example:

To show the differences of both methods, the blocking index of ERAinterim data is calculated and a selection is shown ...

The blocking index of ERA40 data has been calculated . Information and data are available via the CliSAP Integrated Climate Data Center (ICDC).

Tool:

A program package for calculation and plotting is available together with a detailed description. Please contact Silke Schubert.

Requirements:

input data in SERVICE format (can easily be obtained from netCDF using CDOs) gfortran, CDO, GrADS

References:

• Schalge, B., R. Blender, and K. Fraedrich, 2011: Blocking detection based on synoptic filters. Advances in Meteorology, ID 717812, 11 pages. doi:10.1155/2011/717812.
• Schneidereit, Andrea, S. Schubert, P. Vargin, F. Lunkeit, X. Zhu, D. Peters, K. Fraedrich, 2012: Large scale flow and the long-lasting blocking high over Russia: Summer 2010. Mon. Wea. Rev., 140, 2967-2981, doi: 10.1175/MWR-D-11-00249.1
• Tibaldi S, Molteni F (1990) On the operational predictability of blocking. Tellus 42A : 343-365, doi:10.1034/j.1600-0870.1990.t01-2-00003.x
• Tibaldi S., F. D'Andrea, E. Tosi, E. Roeckner (1997) Climatology of Northern Hemisphere blocking in the ECHAM model. Climate Dynamics 13 : 649-666. PDF