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DeFlorio, MJ, Waliser DE, Guan B, Lavers DA, Ralph FM, Vitart F.  2018.  Global assessment of atmospheric river prediction skill. Journal of Hydrometeorology. 19:409-426.   10.1175/jhm-d-17-0135.1   AbstractWebsite

Atmospheric rivers (ARs) are global phenomena that transport water vapor horizontally and are associated with hydrological extremes. In this study, the Atmospheric River Skill (ATRISK) algorithm is introduced, which quantifies AR prediction skill in an object-based framework using Subseasonal to Seasonal (S2S) Project global hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The dependence of AR forecast skill is globally characterized by season, lead time, and distance between observed and forecasted ARs. Mean values of daily AR prediction skill saturate around 7-10 days, and seasonal variations are highest over the Northern Hemispheric ocean basins, where AR prediction skill increases by 15%-20% at a 7-day lead during boreal winter relative to boreal summer. AR hit and false alarm rates are explicitly considered using relative operating characteristic (ROC) curves. This analysis reveals that AR forecast utility increases at 10-day lead over the North Pacific/western U.S. region during positive El Nino-Southern Oscillation (ENSO) conditions and at 7-and 10-day leads over the North Atlantic/U.K. region during negative Arctic Oscillation (AO) conditions and decreases at a 10-day lead over the North Pacific/western U.S. region during negative Pacific-North America (PNA) teleconnection conditions. Exceptionally large increases in AR forecast utility are found over the North Pacific/western United States at a 10-day lead during El Nino + positive PNA conditions and over the North Atlantic/United Kingdom at a 7-day lead during La Nina + negative PNA conditions. These results represent the first global assessment of AR prediction skill and highlight climate variability conditions that modulate regional AR forecast skill.

Ralph, FM, Neiman PJ, Kiladis GN, Weickmann K, Reynolds DW.  2011.  A multiscale observational case study of a Pacific atmospheric river exhibiting tropical-extratropical connections and a mesoscale frontal wave. Monthly Weather Review. 139:1169-1189.   10.1175/2010mwr3596.1   AbstractWebsite

A case study is presented of an atmospheric river (AR) that produced heavy precipitation in the U.S. Pacific Northwest during March 2005. The study documents several key ingredients from the planetary scale to the mesoscale that contributed to the extreme nature of this event. The multiscale analysis uses unique experimental data collected by the National Oceanic and Atmospheric Administration (NOAA) P-3 aircraft operated from Hawaii, coastal wind profiler and global positioning system (GPS) meteorological stations in Oregon, and satellite and global reanalysis data. Moving from larger scales to smaller scales, the primary findings of this study are as follow: 1) phasing of several major planetary-scale phenomena influenced by tropical-extratropical interactions led to the direct entrainment of tropical water vapor into the AR near Hawaii, 2) dropsonde observations documented the northward advection of tropical water vapor into the subtropical extension of the midlatitude AR, and 3) a mesoscale frontal wave increased the duration of AR conditions at landfall in the Pacific Northwest.

Ralph, FM, Neiman PJ, Rotunno R.  2005.  Dropsonde observations in low-level jets over the northeastern Pacific Ocean from CALJET-1998 and PACJET-2001: Mean vertical-profile and atmospheric-river characteristics. Monthly Weather Review. 133:889-910.   10.1175/Mwr2896.1   AbstractWebsite

Dropsonde observations are used to document the mean vertical profiles of kinematic and thermodynamic conditions in the pre-cold-frontal low-level-jet (LLJ) region of extratropical cyclones over the eastern Pacific Ocean. This is the region within storms that is responsible not only for the majority of heavy rainfall induced by orography when such storms strike the coast, but also for almost all meridional water vapor transport at midlatitudes. The data were collected from NOAA's P-3 aircraft in 10 storms during the California Land-falling Jets Experiment (CALJET) of 1998 and in 7 storms during the Pacific Land-falling Jets Experiment (PACJET) of 2001. The mean position of the dropsondes was 500 km offshore, well upstream of orographic influences. The availability of data from two winters that were characterized by very different synoptic regimes and by differing phases of ENSO-that is, El Nino in 1998 and La Nina in 2001-allowed examination of interannual variability.The composite pre-cold-frontal profiles reveal a well-defined LLJ at 1.0-km altitude with a wind speed of 23.4 m s(-1) and a wind direction of 216.7 degrees, as well as vertical shear characteristic of warm advection. The composite thermodynamic conditions were also documented, with special attention given to moist static stability due to the nearly saturated conditions that were prevalent. Although the dry static stability indicated very stable conditions (4.5 K km(-1)), the moist static stability was approximately zero up to 2.8-km altitude. Although the composite winds, temperatures, and water vapor mixing ratios in 2001 differed markedly from 1998, the moist static stability remained near zero from the surface up to 2.8-3.0-km altitude for both seasons. Hence, orographic precipitation enhancement is favored in this sector of the storm, regardless of the phase of ENSO.The dropsonde data were also used to characterize the depth and strength of atmospheric rivers, which are responsible for most of the meridional water vapor transport at midlatitudes. The vertically integrated along-river water vapor fluxes averaged 525 X 10(5) kg s(-1) (assuming a 100-km-wide swath), while the meridional and zonal components were 387 X 10(5) kg s(-1) and 302 X 10(5) kg s(-1), respectively. Although the composite meridional transport in 2001 was less than half that in 1998 (230 X 10(5) kg s(-1) versus 497 X 10(5) kg s(-1)), the characteristic scale height of the meridional water vapor transport remained constant; that is, 75% of the transport occurred below 2.25-km altitude.

Andrews, ED, Antweiler RC, Neiman PJ, Ralph FM.  2004.  Influence of ENSO on flood frequency along the California coast. Journal of Climate. 17:337-348.   10.1175/1520-0442(2004)017<0337:Ioeoff>2.0.Co;2   AbstractWebsite

The influence of the El Nino-Southern Oscillation (ENSO) phenomenon on flooding in California coastal streams is investigated by analyzing the annual peak floods recorded at 38 gauging stations. The state of ENSO prior to and during flooding is characterized by the multivariate ENSO index (MEI), where MEI, 20.5 is defined as the La Nina phase and MEI>0.5 as the El Nino phase. Flood magnitude in all 20 streams located south of 35degreesN has a significant positive correlation (r=0.3 to 0.6), whereas in 3 of the 4 streams located north of 41degreesN flood magnitude has a significant negative correlation (r=-0.3 to 20.4), with MEI from -2.2 to +3.2. Correlations with MEI are uniformly weak and insignificant, however, when the floods are subdivided into El Nino and non-El Nino phases. A comparison of the geometric mean El Nino flood to the geometric mean non-El Nino flood determined that the means were statistically different at gauging stations south of 35degreesN and north of 41degreesN. For 20 streams located south of 35degreesN, the geometric mean of annual peak floods recorded at a stream gauge during El Nino phases is 2-14 times the geometric mean of annual peak floods recorded during non-El Nino phases. Thus, south of 35degreesN along the California coast, floods are significantly larger during an El Nino phase than a non-El Nino phase. For the three streams located north of 41degreesN, the geometric mean of annual peak floods during an El Nino phase was less than 70% of the geometric mean of annual peak floods during a non-El Nino phase. The relative strength of the El Nino phase, however, has, at most, a weak influence on flood magnitude. Flood exceedance probabilities for the El Nino and non-El Nino periods were calculated for all gauging stations using a three-parameter log gamma distribution. For exceedance probabilities from 0.50 to 0.02, the ratio of the El Nino to non-El Nino floods varies from greater than 10 near 32degreesN to less than 0.7 near 42degreesN. Latitude explains 76%-90% of the observed variation in the relative magnitude of El Nino versus non-El Nino floods over the range of exceedance probabilities.

Ralph, FM, Neiman PJ, Kingsmill DE, Persson POG, White AB, Strem ET, Andrews ED, Antweiler RC.  2003.  The impact of a prominent rain shadow on flooding in California's Santa Cruz Mountains: A CALJET case study and sensitivity to the ENSO cycle. Journal of Hydrometeorology. 4:1243-1264.   10.1175/1525-7541(2003)004<1243:Tioapr>2.0.Co;2   AbstractWebsite

Data from the California Land-Falling Jets Experiment (CALJET) are used to explore the causes of variations in flood severity in adjacent coastal watersheds within the Santa Cruz Mountains on 2-3 February 1998. While Pescadero Creek (rural) experienced its flood of record, the adjacent San Lorenzo Creek (heavily populated), attained only its fourth-highest flow. This difference resulted from conditions present while the warm sector of the storm, with its associated low-level jet, high moisture content, and weak static stability, was overhead. Rainfall in the warm sector was dominated by orographic forcing. While the wind speed strongly modulated rain rates on windward slopes, the wind direction positioned the edge of a rain shadow cast by the Santa Lucia Mountains partially over the San Lorenzo basin, thus protecting the city of Santa Cruz from a more severe flood. Roughly 26%+/-9% of the streamflow at flood peak on Pescadero Creek resulted from the warm-sector rainfall. Without this rainfall, the peak flow on Pescadero Creek would likely not have attained record status.These results are complemented by a climatological analysis based on similar to50-yr-duration streamflow records for these and two other nearby windward watersheds situated; 20 to 40 km farther to the east, and a comparison of this climatological analysis with composites of NCEP-NCAR reanalysis fields. The westernmost watersheds were found to have their greatest floods during El Nino winters, while the easternmost watersheds peaked during non-El Nino episodes. These results are consistent with the case study, that showed that the composite 925-mb, meridionally oriented wind direction during El Ninos favors a rain shadow over the eastern watersheds. During non-El Nino periods, the composite, zonally oriented wind direction indicates that the sheltering effect of the rain shadow on the eastern watersheds is reduced, while weaker winds, less water vapor, and stronger stratification reduce the peak runoff in the western watersheds relative to El Nino periods.These case study and climatological results illustrate the importance of conditions in the moisture-rich warm sector of landfalling Pacific winter storms. Although many other variables can influence flooding, this study shows that variations of +/-10degrees in wind direction can modulate the location of orographically enhanced floods. While terrain can increase predictability (e.g., rainfall typically increases with altitude), the predictability is reduced when conditions are near a threshold separating different regimes (e.g., in or out of a rain shadow).