A new algorithm is applied to study water vapor fluxes in the troposphere using wind and moisture data from the European Centre for Medium-Range Weather Forecasts. The fluxes are divided into filamentary structures known as tropospheric rivers and what are termed here broad fields. The results show that the tropospheric rivers may carry essentially the total meridional transport observed in the extratropical atmosphere but may occupy only about 10% of the total longitudinal length at a given latitude. The transient fluxes in traditional studies do not catch the filamentary structures completely and may therefore underestimate the fraction of transport assigned to moving systems, as well as omitting the geographical concentration. The mean flow and eddy fluxes evaluated by the new algorithm are considered to be more physically realistic.

/pdf/a-proposed-algorithm-for-moisture-fluxes-from-atmospheric-2j8yck2dwq.pdf

A Proposed Algorithm for Moisture Fluxes from Atmospheric Rivers

A Lagrangian-based method of analysis is introduced to examine the space-time structure and dynamics of extratropical cyclogenesis. the three-step method involves the calculation of extensive ensembles of trajectories, the evaluation of the Lagrangian time-trace of physical variables along these paths, and the application of objective selection criteria to identify significant Lagrangian structures. the approach can provide a qualitative depiction of, quantitative information about, and dynamical insight into, key features of mid-latitude cyclones. In this, the first of a two-part study, examples are given of the application of the method. A case-study approach has been adopted and used to identify coherent ensembles of trajectories (CETs) whose dynamics are seminal to an event of Atlantic cyclogenesis, to trace the origin of subsynoptic potential vorticity anomalies, and to analyse stratosphere-troposphere exchange accompanying extratropical cyclogenesis. In addition, a statistical investigation has been made of the climatology of CETs in the northern hemisphere for one particular winter month. Some consideration has been given to the shortcomings and potential of the method, and its complementarity with other approaches.

A lagrangian‐based analysis of extratropical cyclones. I: The method and some applications

This study uses a unique combination of airborne and satellite observations to characterize narrow regions of strong horizontal water vapor flux associated with polar cold fronts that occurred over the eastern North Pacific Ocean during the winter of 1997/98. Observations of these “atmospheric rivers” are compared with past numerical modeling studies to confirm that such narrow features account for most of the instantaneous meridional water vapor transport at midlatitudes. Wind and water vapor profiles observed by dropsondes deployed on 25–26 January 1998 during the California Land-falling Jets Experiment (CALJET) were used to document the structure of a modest frontal system. The horizontal water vapor flux was focused at low altitudes in a narrow region ahead of the cold front where the combination of strong winds and large water vapor content were found as part of a low-level jet. A close correlation was found between these fluxes and the integrated water vapor (IWV) content. In this case, 75%...

https://journals.ametsoc.org/doi/pdf/10.1175/1520-0493%282004%29132%3C1721%3ASACAOO%3E2.0.CO%3B2

Satellite and CALJET Aircraft Observations of Atmospheric Rivers over the Eastern North Pacific Ocean during the Winter of 1997/98

[1] Precipitation over and near mountains is not caused by topography but, rather, occurs when storms of a type that can occur anywhere (deep convection, fronts, tropical cyclones) form near or move over complex terrain. Deep convective systems occurring near mountains are affected by channeling of airflow near mountains, capping of moist boundary layers by flow subsiding from higher terrain, and triggering to break the cap when low-level flow encounters hills near the bases of major mountain ranges. Mesoscale convective systems are triggered by nocturnal downslope flows and by diurnally triggered disturbances propagating away from mountain ranges. The stratiform regions of mesoscale convective systems are enhanced by upslope flow when they move over mountains. In frontal cloud systems, the poleward flow of warm-sector air ahead of the system may rise easily over terrain, and a maximum of precipitating cloud occurs over the first rise of terrain, and rainfall is maximum on ridges and minimum in valleys. If the low-level air ahead of the system is stable, blocking or damming occurs. Shear between a blocked layer and unblocked moist air above favors turbulent overturning, which can accelerate precipitation fallout. In tropical cyclones, the tangential winds encountering a mountain range produce a gravity wave response and greatly enhanced upslope flow. Depending on the height of the mountain, the maximum rain may occur on either the windward or leeward side. When the capped boundary layer of the eye of a tropical cyclone passes over a mountain, the cap may be broken with intense convection resulting.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011RG000365

Orographic effects on precipitating clouds

[1] The most important sources of atmospheric moisture at the global scale are herein identified, both oceanic and terrestrial, and a characterization is made of how continental regions are influenced by water from different moisture source regions. The methods used to establish source-sink relationships of atmospheric water vapor are reviewed, and the advantages and caveats associated with each technique are discussed. The methods described include analytical and box models, numerical water vapor tracers, and physical water vapor tracers (isotopes). In particular, consideration is given to the wide range of recently developed Lagrangian techniques suitable both for evaluating the origin of water that falls during extreme precipitation events and for establishing climatologies of moisture source-sink relationships. As far as oceanic sources are concerned, the important role of the subtropical northern Atlantic Ocean provides moisture for precipitation to the largest continental area, extending from Mexico to parts of Eurasia, and even to the South American continent during the Northern Hemisphere winter. In contrast, the influence of the southern Indian Ocean and North Pacific Ocean sources extends only over smaller continental areas. The South Pacific and the Indian Ocean represent the principal source of moisture for both Australia and Indonesia. Some landmasses only receive moisture from the evaporation that occurs in the same hemisphere (e.g., northern Europe and eastern North America), while others receive moisture from both hemispheres with large seasonal variations (e.g., northern South America). The monsoonal regimes in India, tropical Africa, and North America are provided with moisture from a large number of regions, highlighting the complexities of the global patterns of precipitation. Some very important contributions are also seen from relatively small areas of ocean, such as the Mediterranean Basin (important for Europe and North Africa) and the Red Sea, which provides water for a large area between the Gulf of Guinea and Indochina (summer) and between the African Great Lakes and Asia (winter). The geographical regions of Eurasia, North and South America, and Africa, and also the internationally important basins of the Mississippi, Amazon, Congo, and Yangtze Rivers, are also considered, as is the importance of terrestrial sources in monsoonal regimes. The role of atmospheric rivers, and particularly their relationship with extreme events, is discussed. Droughts can be caused by the reduced supply of water vapor from oceanic moisture source regions. Some of the implications of climate change for the hydrological cycle are also reviewed, including changes in water vapor concentrations, precipitation, soil moisture, and aridity. It is important to achieve a combined diagnosis of moisture sources using all available information, including stable water isotope measurements. A summary is given of the major research questions that remain unanswered, including (1) the lack of a full understanding of how moisture sources influence precipitation isotopes; (2) the stationarity of moisture sources over long periods; (3) the way in which possible changes in intensity (where evaporation exceeds precipitation to a greater of lesser degree), and the locations of the sources, (could) affect the distribution of continental precipitation in a changing climate; and (4) the role played by the main modes of climate variability, such as the North Atlantic Oscillation or the El Nino–Southern Oscillation, in the variability of the moisture source regions, as well as a full evaluation of the moisture transported by low-level jets and atmospheric rivers.

Oceanic and terrestrial sources of continental precipitation

Computations of daily global tropospheric water vapor flux values show the presence of a filamentary structure The filaments, here called rivers, have lengths many times their widths and persist for many days while being translated through the atmosphere They are present in data analyzed for both 1981 and 1991 The water vapor flux maxima coincide quite closely to reflectivity features (averaged from wavelengths of 380 and 360 nm) as revealed by the Total Ozone Mapping Spectrometer (TOMS) It is suggested that the filamentary structure may also be present in other trace constituents

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/92GL02916%4010.1002/%28ISSN%291944-8007.ATMOS_RIVERS1

Courtney Scott

Papers

Tropospheric rivers? – A pilot study

Tropospheric rivers? A pilot study. [of filamentary structures of atmospheric water vapor