Lavers, D.A., G. Villarini, R.P. Allan, E.F. Wood, and A.J. Wade, The detection of atmospheric rivers in atmospheric reanalyses and their links to British winter floods and the large-scale climatic circulation, Journal of Geophysical Research, 117, D20106, doi:10.1029/2012JD018027, 2012.
Atmospheric rivers (ARs) are narrow bands of enhanced water vapor transport in the lower troposphere, and are the cause of extreme precipitation and floods over mid-latitude regions. This study introduces an algorithm (based on the vertically-integrated horizontal Water Vapor Transport, IVT) for the detection of persistent ARs (lasting 18 hours or longer) in five atmospheric reanalysis products. The reanalyses considered were: (1) NCEP Climate Forecast System (CFSR), (2) ECMWF ERA-Interim (ERAIN), (3) Twentieth Century Reanalysis (20CR), (4) NASA Modern Era Retrospective-Analysis for Research and Applications (MERRA), and (5) NCEP–NCAR.
Figure 1: Time series of the number of persistent ARs in each winter half-year (October to March) over 1980–2010 in the five reanalyses (left y-axis). The black dashed line represents the winter half-year Scandinavian Pattern index (anomaly values shown on the right y-axis). The total number of ARs for each reanalysis product is given in the legend.
Time series of the number of detected ARs in each winter half-year over 1980–2010 in the five reanalyses are shown in Figure 1 (taken from JGR paper). The number of ARs varies between about 2 and 14 events per winter. Each product identifies a different number of ARs ranging from 190 in CFSR to 264 in 20CR, which may be partly caused by the different IVT threshold values used for each reanalysis, as well as the different assimilating models and data used. As shown in Figure 1, a negative dependence was found between AR frequency and the winter half-year Scandinavian Pattern. In conclusion, the generally good agreement of AR occurrence between the reanalyses suggests that realistic sea surface temperatures and atmospheric circulation, used in the five products, are sufficient for simulating the AR structures.
Figure 2: The IVT (in kg m-1 s-1) for (a) 20CR, (c) CFSR, (d) ERAIN, (e) MERRA, (f) NCEP–NCAR and (b) 20CR MSLP field (in hPa) at 1200 UTC 10th December 1994 before the largest flood event on 11th December 1994 in the Ayr at Mainholm basin in Scotland. The “L” and “H” in panel (b) refer to the Low and High pressure centres respectively; the black dots in the panels mark the location of the Ayr at Mainholm basin.
An example of an AR captured in the five reanalyses is shown in Figure 2 (taken from JGR paper); this AR was behind the largest flood in one of the study river basins. The effect of the different reanalysis grid resolutions is shown, with the peak IVT and hence AR region (as shown by the red and orange colors) in the finer resolution CFSR, ERAIN and MERRA products occupying a smaller region than in the 20CR or NCEP-NCAR.
A strong link exists between the detected ARs and the biggest winter floods in the nine study basins. In one western British basin about 80% of the 31 largest floods followed a persistent AR. As the largest floods in these basins occur in the winter, these results provide evidence that ARs control a large part of the upper tail of the flood peak distribution.