Abstract
Abstract. A new 3-D tracer tool is coupled to the WRF model to analyze the origin of the moisture in two extreme atmospheric river (AR) events: the so-called Great Coastal Gale of 2007 in the Pacific Ocean and the Great Storm of 1987 in the North Atlantic. Results show that between 80 and 90 % of moisture advected by the ARs, and a high percentage of the total precipitation produced by the systems have a tropical origin. The tropical contribution to precipitation is in general above 50 % and largely exceeds this value in the most affected areas. Local convergence transport is responsible for the remaining moisture and precipitation. The ratio of tropical moisture to total moisture is maximized as the cold front arrives on land. Vertical cross sections of the moisture content suggest that the maximum in tropical humidity does not necessarily coincide with the low-level jet (LLJ) of the extratropical cyclone. Instead, the amount of tropical humidity is maximized in the lowest atmospheric level in southern latitudes and can be located above, below or ahead of the LLJ in northern latitudes in both analyzed cases.
Highlights
Atmospheric rivers are long and narrow structures in the lower troposphere that carry large amounts of water vapor (Zhu and Newell, 1998). Guan and Waliser (2015) have estimated that atmospheric river (AR) have a median length of about 3600 km, a median length / width ratio of about 7 and a mean integrated vapor transport (IVT) of 370 kg m−1 s−1
These objective detection criteria have shown that AR structures of integrated water vapor (IWV) and IVT can extend from the tropics into midlatitudes; they do not provide information about the source and sink regions of AR water vapor
Whereas the simulated IVT field is realistic when compared to observations, Weather Research and Forecast (WRF) tends to overestimate precipitation
Summary
Atmospheric rivers (hereafter, ARs) are long and narrow structures in the lower troposphere that carry large amounts of water vapor (Zhu and Newell, 1998). Guan and Waliser (2015) have estimated that ARs have a median length of about 3600 km, a median length / width ratio of about 7 and a mean integrated vapor transport (IVT) of 370 kg m−1 s−1. Between three and five ARs can be found per hemisphere at any given time (Zhu and Newell, 1998), accounting for approximately 84 % of the meridional IVT for the Northern Hemisphere and about 88 % in the Southern Hemisphere (Guan and Waliser, 2015) Since these structures can transport an amount of precipitable water equivalent to several times the discharge of the Mississippi River (Ralph and Dettinger, 2011), ARs have been identified as a primary feature of the global water cycle. Eiras-Barca et al (2016) proposed a combined IVT and IWV variablethreshold detection algorithm, which operates both in summer and winter months These objective detection criteria have shown that AR structures of IWV and IVT can extend from the tropics into midlatitudes; they do not provide information about the source and sink regions of AR water vapor. The WRF tracer tool provides information about the vertical distribution of tropical moisture, as well as the position of the maximum of moisture with regard to the LLJ
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