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For 200 hPa zonal wind the largest uncertainties are in the tropical regions. | For 850 hPa temperature, the uncertainties are generally larger in the Southern Hemisphere (this corresponds well with the fact that we have fewer observations in the Southern Hemisphere). | For MSLP the Antarctic region has the largest spread/uncertainty. |
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For all variables it is clear that the uncertainties are decreasing with time, i.e. the spread values are smaller for recent periods than for older ones.
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MSLP spread | 850 hPa temperature spread | 200 hPa zonal wind spread |
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The main reason for this is the limited ensemble size of 10 members that introduces considerable sampling noise. On the other hand, if we consider the mean seasonal (JJA 1980 in this case) spread for the three variables the fields are much smoother and easier to interpret. For seasonal mean fields, this sampling noise is averaged out and as a result will provide smoother spread fields:
| MSLP (mean seasonal (JJA) | 850 hPa temperature (mean seasonal (JJA) | 200 hPa zonal wind (mean seasonal (JJA) |
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(14) When I look at active systems such as extra-tropical cyclones or tropical cyclones I expect a larger uncertainty, yet I do not see that clearly in the ensemble spread
The main problem with the extra-tropical and tropical cyclones in terms of uncertainty is the fact that due to the lower resolution of the EDA system, the EDA members systematically overestimate the central pressure of the cyclone (i.e. the pressure is not sufficiently low). This means that the spread among the members remains small and consequently the EDA shows lower uncertainties than in reality. On the other hand the spatial pattern of the uncertainties correspond rather well with the actual cyclones. This is demonstrated for some extra-tropical cyclones like cyclone Desmond: 2015120500, cyclone Xaver: 2013120500 or the Great Storm of 1987 in the UK. In all of these cases the maximum spread values don't exceed 1 hPa, which is quite small. The pattern of large spread values is scattered throughout the domain, though the primary cyclones are reasonably well-marked in the uncertainty field (particularly for the 1987 storm). For tropical cyclones the spread values can be larger, as it is for a cyclone near to Japan in 1987 or for the Haiyan typhoon near to the Philippines. It is very interesting to see the case of Hurricane Sandy, where the region with the largest uncertainties is not fully in agreement with the location of the hurricane's eye, but with some peaks to the east and west of it (the values are larger to the east). This indicates the uncertainties related to the position of the hurricane. So overall the EDA spread can give a qualitative idea of the uncertainties relating to active systems such as cyclones, but it is unable to provide the right uncertainty amplitude due to the lower resolution of the EDA system.
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