I was pleased to see the importance of atmospheric rivers being recognized through the convening of a dedicated international conference—see the meeting summary by Ralph et al. (2017) in a recent issue of this Bulletin. However, I was surprised to read in this report the assertion that the concept emerged as recently as the 1990s. Although the name “river” was not applied until the 1990s (Newell et al. 1992), narrow moist low level jets ahead of cold fronts were documented in the U.K. in the 1970s.
In a study of several fairly unexceptional cold fronts, Browning and Pardoe (1973) identified moist prefrontal LLJs, often thousands of kilometers long, with peak velocities near 900 hPa of 25 to 31 m s–1 and a rather narrow water-vapor flux half-width of from 650 km to as little as 170 km (see their Table 1). They remarked that the existence of these very narrow flows would lead to substantial underestimates of water vapour flux on the basis of the routine observations that were then available. These LLJs were attributed to the then recently identified phenomenon of the warm conveyor belt.
Recognition of the importance of pre-cold frontal LLJs for heavy orographic rainfall also dates back to the 1970s. Orographic rain over the modest hills of the U.K. is dominated by the seeder–feeder mechanism for which the strength of the LLJ is a key ingredient. In a study of 14 years of data over the hills of south Wales, Nash and Browning (1977) identified 20 cases for which the 24-hour rainfall amount exceeded 85 mm, and as many as 17 of these were characterised by LLJs with winds at 900 m between 25 and 43 m s–1. Although rain can continue after the passage of the surface cold front, the orographic enhancement ceases abruptly with the passage of the surface front because the LLJ is an entirely prefrontal phenomenon (Browning et al. 1975).
In my home county of Cumbria, where the hills face the prevailing on-shore winds, atmospheric rivers have caused two devastating flooding events within the most recent decade. In one of these events, a pre-cold frontal LLJ with a peak velocity of about 36 m s–1 lingered over the Cumbrian hills for a long period, producing rainfall totals of over 250 mm (Lean and Browning 2013). A similar, near-stationary, prefrontal LLJ gave large orographic falls over the hills of south Wales [see Figs. 1 and 8 in the review by Browning (1980)]. These cases highlight the increased risk of severe falls of orographic rain and associated flooding when an atmospheric river becomes slow moving or stationary over a vulnerable range of hills.
REFERENCES
Browning, K. A., 1980: Structure, mechanism and prediction of orographically enhanced rain in Britain. Orographic Effects in Planetary Flows, R. Hide and P. W. White, Eds., WMO GARP Publications Series No. 23, 85–114.
Browning, K. A., and C. W. Pardoe, 1973: Structure of low-level jet streams ahead of mid-latitude cold fronts. Quart. J. Roy. Meteor. Soc., 99, 619–638, https://doi.org/10.1002/qj.49709942204.
Browning, K. A., C. W. Pardoe, and F. F. Hill, 1975: The nature of orographic rain at wintertime cold fronts. Quart. J. Roy. Meteor. Soc., 101, 333–352, https://doi.org/10.1002/qj.49710142815.
Lean, H. W. and K. A. Browning, 2013: Quantification of the importance of wind drift to the surface distribution of orographic rain on the occasion of the extreme Cockermouth flood in Cumbria. Quart. J. Roy. Meteor. Soc., 139, 1342–1353, https://doi.org/10.1002/qj.2024.
Nash, J. and K. A. Browning, 1977: Structure of the lower atmosphere associated with heavy falls of orographic rain in south Wales. Meteorological Office Radar Research Laboratory Rep. 7.
Newell, R. E., N. E. Newell, Y. Zhu and C. Scott, 1992: Tropospheric rivers?—A pilot study. Geophys. Res. Lett., 19, 2401–2404, https://doi.org/10.1029/92GL02916.
Ralph, F. M., and Coauthors, 2017: Atmospheric rivers emerge as a global science and applications focus. Bull. Amer. Meteor. Soc., 98, 1969–1973, https://doi.org/10.1175/BAMS-D-16-0262.1.
