¿What is an Atmospheric River?
Atmospheric Rivers (AR) are a long and narrow corridor with strong water vapor transport in the atmosphere. Although these corridors are observed over the continents and the oceans, they are commonly well-defined over the oceans, where they uptake moisture. AR typically form in conjunction with cold front systems of midlatitudes, and align parallel to and just ahead of a cold front in the warm air mass. Recently, a formal definition was formulated and published by the glossary of meteorology of the American Meteorological Society.
The definition is transcribed here from the AMS glossary as follows:
“Atmospheric River: A long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone. The water vapor in atmospheric rivers is supplied by tropical and/or extratropical moisture sources. Atmospheric rivers frequently lead to heavy precipitation where they are forced upward—for example, by mountains or by ascent in the warm conveyor belt. Horizontal water vapor transport in the midlatitudes occurs primarily in atmospheric rivers and is focused in the lower troposphere. Atmospheric rivers are the largest "rivers" of fresh water on Earth, transporting on average more than double the flow of the Amazon River”. Published by the AMS Glossary of Meteorology
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Fig. 1. Schematic summary of the structure and strength of an atmospheric river based on dropsonde measurements deployed from research aircraft across many atmospheric rivers and on corresponding reanalyses that provide the plan-view context. Magnitudes of variables represent an average midlatitude atmospheric river. Average width is based on atmospheric river boundaries defined by vertically integrated water vapor transport (IVT; from surface to 300 hPa) lateral boundary threshold of 250 kg m−1 s−1. Depth corresponds to the altitude below which 75% of IVT occurs. The total water vapor transport (a.k.a. flux) corresponds to the transport along an atmospheric river, bounded laterally by the positions of IVT = 250 kg m−1 s−1 and vertically by the surface and 300 hPa. (a) Plan view including parent low pressure system and associated cold, warm, and warm-occluded surface fronts. IVT is shown by color fill (magnitude; kg m−1 s−1) and direction in the core (white arrow). Vertically integrated water vapor (IWV; cm) is contoured. A representative length scale is shown. The position of the cross section shown in (b) is denoted by the dashed line A–A′. (b) Vertical cross-section perspective, including the core of the water vapor transport in the atmospheric river (orange contours and color fill) and the pre-cold-frontal low-level jet (LLJ), in the context of the jet-front system and tropopause. Water vapor mixing ratio (green dotted lines; g kg−1) and cross-section-normal isotachs (blue contours; m s−1) are shown. [Schematic is from Ralph et al. (2017). It was prepared by F. M. Ralph, J. M. Cordeira, and P. J. Neiman based on the composite of research-aircraft-based observations of 21 atmospheric rivers presented in Ralph et al. (2017), including adaptations of earlier results from Ralph et al. (2004), Cordeira et al. (2013), and others. The 21 observed cases were compared with thousands of ARs found in ERA and MERRA-2 over 37–38 years in the same region and season (Guan et al. 2018). The composite of the 21 aircraft-observed cases was shown to be representative of the broad spectrum of ARs in the reanalyses.
Atmospheric Rivers in southern South America
Similar to the findings in western North America, ARs have a high impact in precipitation and the hydrological cycle in western South America. When these strong corridor of water vapor impact against the coastal and the main Andes mountains, abundant snow and rain precipitate on the elevated terrain; and depending on the magnitude and the direction of ARs, with respect to the watershed orientation where they impact, ARs can be mostly beneficial or mostly hazardous, because they only provide water or they may also produce floods and landslides. The SSM/I sensor onboard of a polar-orbiting satellite retrieve the water vapor content in the whole column of the atmosphere, and the image of one retrieval illustrate a long and powerful AR making landfall on the west coast of South America. This AR produced heavy precipitation, floods and landslides on western side of the Andes, and a severe downslope windstorm on the eastern side of the Andes, causing fatalities in Chile and Argentina. More characteristics of this AR are highlighted in the Figure 2b by the output from the AR identification algorithm described in Viale et al. 2018. Further readings of ARs in South America are listed below
Figura 2: a) Chart of the IWV (mm) as seen by the SMM/I satellite imagery during the morning passes on 11 July 2006 showing an AR example. b) Chart of the IVT (kg m-1 s-1) magnitude and direction (color-code and vectors, respectively) showing the AR example detected at 0000UTC 11 July 2006 by the algorithm. The algorithm outputs as the shape boundary (green), axis (yellow), landfall location (red), and other key metrics are showed in the lower left box. This case was one of the most devastating AR over the period of 2001-2016. [Figure is from Viale et al. (2018).]
