A Climatology of the Vertical Structure of Water Vapor Transport to the Sierra Nevada in Cool Season Atmospheric River Precipitation Events

Abstract

This study presents the climatology of the vertical structure of water vapor flux above the Sierra Nevada during significant cool season (November-April) precipitation events. Atmospheric river (AR) and non-AR events are analyzed to better understand the effect of this structure on precipitation patterns. Daily measurements of cool season precipitation at seven weather stations around the Tahoe basin from 1974 to 2012 and NCEP/CPC gridded daily precipitation analysis along the Sierra crest for the period 1948-2012 are examined. NCEP-NCAR reanalysis and soundings from Oakland are used to look at upper atmospheric conditions, including the presence of vapor transport by low-and midlevel jets on storm days as well as upstream static stability in relation to significant precipitation events. Key findings are as follows: 1) ARs play a disproportionately large role in generating Tahoe basin precipitation during the cool season; 2) strong midlevel vapor transport needs to occur in tandem with low-level transport to achieve the most extreme 2-day precipitation in the Tahoe basin; 3) when low-to midlevel vapor transport is present on days with a defined AR, the local maximum in 2-day precipitation intensity decreases with distance from the Sierra crest, and on non-AR days, the relative increase in 2-day precipitation intensity due to low-and midlevel vapor transport does not vary based on distance from the Sierra crest; 4) AR and non-AR moisture fluxes are significantly modified by upstream static stability; and 5) understanding the impacts of ARs and their lower-and midlevel moisture flux structure are crucial components of the hydrometeorology in this region.

Detection of atmospheric rivers: Evaluation and application of an algorithm for global studies

“…The AR width is defined as the effective width of the object, calculated as the total Earth surface area of the object divided by the length. Other methods, with various degrees of complexity, have been used in previous studies to define the AR characteristic width [ Wick et al ., ; Jiang et al ., ; Backes et al ., ]. In section 3.7, we discuss the sensitivity of AR detection to the geometry threshold and show that length is a more important consideration than width, consistent with the notion in Rutz et al .…”

“…For example, the two gaps associated with the unfilled blue boxes around 105°W, 42°N in Figure b are not counted in the AR length. Previous studies have used the maximum distance between any two boundary grid cells to represent the AR length, which works well for ARs with a simple elongated shape [ Rutz et al ., ; Backes et al ., ]. A more sophisticated method uses the maximum combined distance between any three grid cells on the AR axis to account for some curvature of the AR [ Wick et al ., ].…”

“…Compared to IWV, the integrated water vapor transport (IVT) is a more desirable variable for AR detection due to its direct relationship with orographically induced precipitation [ Neiman et al ., ; Neiman et al ., ] and its usefulness in understanding AR's role in the global water cycle [ Zhu and Newell , ]. IVT‐based AR detection techniques using the third type of method above have been developed for studying ARs affecting the western U.S. and facilitated the investigation of AR's characteristics during landfall and inland penetration [ Rutz et al ., ; Backes et al ., ]. AR's global signatures [ Waliser et al ., ], including those in less explored areas such as Greenland [ Neff et al ., ] and Antarctica [ Gorodetskaya et al ., ], and the need for understanding the representation of ARs in global weather/climate models motivate the development and evaluation of an AR detection technique suitable for global studies.…”

Atmospheric River Impacts on Greenland Ice Sheet Surface Mass Balance

“…AR impacts on Greenland can occur at any time during the year but are most common during summer (JJA), with this summer peak in frequency especially pronounced in northern and western Greenland (Figure S4 in the supporting information). Moisture transport by ARs occurs predominantly in the lowest few kilometers of the troposphere (Backes et al, ; Neiman et al, ; Ralph et al, ), and thus, ARs are much more common along the low‐elevation coastal regions of Greenland compared with the high interior GrIS (see Figure S4 in the supporting information). Maps of AR frequency across SOM nodes (Figure ) demonstrate the close agreement between moisture transport patterns identified by the SOM classification and the object‐based AR detection algorithm (compare to Figure S2 in the supporting information).…”

Responses and impacts of atmospheric rivers to climate change

“…The vertical distribution of water vapour, wind speed and, consequently, water-vapour transport within ARs is another factor subject to change in future climate scenarios. Mid-level water-vapour transport, for example, is an important process producing heavy precipitation on the lee side of the northern Sierra Nevada 67,68 . These studies have a specific regional focus; however, the importance of mid-level transport in extending AR conditions inland of major topographic barriers can be generalized to many regions.…”