CW3E Publication Notice: Global Analysis of Climate Change Projection Effects on Atmospheric Rivers

CW3E Publication Notice

Global Analysis of Climate Change Projection Effects on Atmospheric Rivers

May 24, 2018

Vicky Espinoza (UC Merced) and CW3E collaborators Bin Guan (UCLA), Duane Waliser (NASA/JPL), along with CW3E director Marty Ralph and David Lavers European Centre for Medium‐Range Weather Forecast, recently published a paper in Geophysical Research Letters, titled Global Analysis of Climate Change Projection Effects on Atmospheric Rivers.

Atmospheric rivers (ARs) are elongated strands of horizontal water vapor transport, accounting for over 90% of the poleward water vapor transport across midlatitudes. ARs have important implications for extreme precipitation when they make landfall, particularly along the west coasts of many midlatitude continents (e.g., North America, South America, and West Europe) due to orographic lifting. ARs are important contributors to extreme weather and precipitation events, and while their presence can contribute to beneficial rainfall and snowfall, which can mitigate droughts, they can also lead to flooding and extreme winds. This study takes a uniform, global approach that is used to quantify how ARs change between Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations and future projections under the Representative Concentration Pathway (RCP) 4.5 and RCP8.5 warming scenarios globally. The projections indicate that while there will be ~10% fewer ARs in the future, the ARs will be ~25% longer, ~25% wider, and exhibit stronger integrated water vapor transports under RCP8.5 (Figure 1). These changes result in pronounced increases in the frequency (integrated water vapor transport strength) of AR conditions under RCP8.5: ~50% (25%) globally, ~50% (20%) in the northern midlatitudes, and ~60% (20%) in the southern midlatitudes (Figure 2).

Figure 2 from Espinoza et al., 2018. AR frequency (shading; percent of time steps) and IVT (vectors; kg · m−1 · s−1) for (a) ERA‐Interim reanalysis for the historical period (1979–2002) with six green boxes depicting regions analyzed in Figures S2 and S3, (b) the MMM for the 21 CMIP5 models analyzed in this study for the historical period (1979–2002), (c) RCP4.5 warming scenario (2073–2096), and (d) RCP8.5 warming scenario (2073–2096).

This research was supported by the NASA Energy and Water cycle Study (NEWS) program. Vicky Espinoza’s contribution to this study was made possible by NASA Jet Propulsion Laboratory’s Year-Round Internship Program during her graduate studies at the University of Southern California. Please contact Duane Waliser at duane.waliser@jpl.nasa.gov with inquiries. More information can be found from the NASA website https://www.jpl.nasa.gov/news/news.php?feature=7141.

Espinoza, V., Waliser, D. E., Guan, B., Lavers, D. A., & Ralph, F. M. 2018: Global Analysis of Climate Change Projection Effects on Atmospheric Rivers. Geophysical Research Letters. 45. https://doi.org/10.1029/2017GL076968

CW3E Publication Notice: Evaluation of Atmospheric River Predictions by the WRF Model Using Aircraft and Regional Mesonet Observations of Orographic Precipitation and Its Forcing

CW3E Publication Notice

Evaluation of Atmospheric River Predictions by the WRF Model Using Aircraft and Regional Mesonet Observations of Orographic Precipitation and Its Forcing

April 16, 2018

CW3E project scientist Andrew Martin and co-authors have published a study characterizing predictability limits in Atmospheric River (AR) forecasts and apportioning Russian River precipitation forecast errors among vapor transport and orographic precipitation components. The article, titled Evaluation of Atmospheric River Predictions by the WRF Model Using Aircraft and Regional Mesonet Observations of Orographic Precipitation and its Forcing, is now in early online release at the Journal of Hydrometeorology.

This study leveraged airborne dropsonde observations of offshore Atmospheric Rivers completed during the CalWater experiment and the Atmospheric River Observatory at Bodega Bay and Cazadero, CA to verify forecasts of AR properties and their resulting precipitation. Forecasts were created by CW3E’s numerical weather prediction model, West-WRF, and compared to Global Forecast System reforecasts (GFSRe) valid for the same events. Forecast skill in AR properties and precipitation was evaluated at lead times up to 7 days ahead. Notably, the study found that deterministic skill in integrated vapor transport and other related fields degrades (meaning that forecasts created from climatology perform just as well or better) more than 4 days ahead for both models. However, West-WRF improves upon GFSRe skill in IVT at days 1, 2 and 3 ahead (see Fig. 1c).

Figure 1. a) Value added by GFSRe over GFSRe climatology validated against 145 CalWater dropsondes for the variables z500 (blue), IVT (black), IWV (green) and e925 (red). b) as in a, except for West-WRF value added over GFSRe climatology. c) as in b, except reference forecast is GFSRe.

The study also employed a novel forecast error separation technique to apportion precipitation forecast errors among the component caused by vapor transport simulation and orographic precipitation process simulation. Data from the Atmospheric River Observatory was used to demonstrate that West-WRF forecasts of orographic precipitation during landfalling AR are more accurate in simulating both components; but also that West-WRF forecasts of precipitation can be improved by improving the vapor transport component because its orographic precipitation process is accurate. This lends confidence that CW3E’s effort to improve west coast precipitation forecasts by assimilating offshore observations into West-WRF analyses can yield successful results.

Co-authors include Dr. F Martin Ralph, Reuben Demirdjian, Laurel DeHaan, and Dr. Rachel Weihs of CW3E with Dr. David Reynolds of the Cooperative Institute for Research in Environmental Sciences and Dr. Sam Iacobellis of Scripps Institution of Oceanography. The study was funded by the US Army Corps of Engineers, the California Department of Water Resources, and the National Science Foundation XSEDE program.

CW3E Publication Notice: High-Elevation Evapotranspiration Estimates During Drought: Using Streamflow and NASA Airborne Snow Observatory SWE Observations to Close the Upper Tuolumne River Basin Water Balance

CW3E Publication Notice

High-Elevation Evapotranspiration Estimates During Drought: Using Streamflow and NASA Airborne Snow Observatory SWE Observations to Close the Upper Tuolumne River Basin Water Balance

March 5, 2018

CW3E postdoc Brian Henn has published a study on estimating evapotranspiration (ET) in California’s Sierra Nevada in Water Resources Research titled High-Elevation Evapotranspiration Estimates During Drought: Using Streamflow and NASA Airborne Snow Observatory SWE Observations to Close the Upper Tuolumne River Basin Water Balance. The study leveraged NASA Airborne Snow Observatory (ASO) and distributed streamflow observations and a basin-scale mass balance approach to estimate ET across the upper Tuolumne River watershed region over three warm seasons (2013-2015), showing spatially coherent totals of about 200 mm per year of ET for these high-elevation areas during California’s recent drought. This represents a novel application of ASO and mass balance approaches to estimate ET at the watershed scale, which is difficult to observe directly. The Tuolumne watershed and others like it the Sierra Nevada are critical water supply areas for California, and changes in ET in the future could impact the reliability of major reservoirs.

The paper was written in collaboration with Tom Painter and Kat Bormann of the NASA ASO team, Bruce McGurk of McGurk Hydrologic, Lorraine and Alan Flint of the USGS, Vince White of Southern California Edison, and Jessica Lundquist of the University of Washington. Please contact Brian at bhenn@ucsd.edu with inquiries.

Figure 1. Figure (1) from Henn et al. (2018): (a) ASO lidar-derived 50 m SWE map for 3 April 2013, over the basin of the Tuolumne River at Highway 120. (b) Example plot for this ASO flight, showing how the basin’s water balance is quantified. All SWE from the 3 April flight is assumed to melt by 30 September ( math formula); cumulative streamflow ( math formula) and precipitation ( math formula) between the flight date and 30 September are then calculated. Uncertainty bounds at 95% confidence are shown for each variable.

Henn, B., Painter, T. H., Bormann, K. J., McGurk, B., Flint, A. L., Flint, L. E., White, V., Lundquist, J. D. (2018). High-elevation evapotranspiration estimates during drought: Using streamflow and NASA airborne snow observatory SWE observations to close the upper tuolumne river basin water balance. Water Resources Research, 54. https://doi.org/10.1002/2017WR020473

CW3E Publication Notice: Global Assessment of Atmospheric River Prediction Skill

CW3E Publication Notice

Global Assessment of Atmospheric River Prediction Skill

February 27, 2018

CW3E collaborators Michael DeFlorio (NASA/JPL), Duane Waliser (NASA/JPL), and Bin Guan (UCLA), along with CW3E director Marty Ralph and colleagues David Lavers and Frederic Vitart of the European Centre for Medium-Range Weather Forecasts (ECMWF), recently published a paper in the Journal of Hydrometeorology titled Global Assessment of Atmospheric River Prediction Skill (early online release; doi:10.1175/JHM-D-17-0135.1). The study introduces the Atmospheric River Skill (ATRISK) algorithm, which is an object-based approach used to quantify atmospheric river (AR) prediction skill using Subseasonal to Seasonal (S2S) Project global hindcast data from ECMWF. Two decades of data from this ensemble hindcast system were used in this work. The ATRISK algorithm determines the distance between the centroids of observed and forecasted ARs (an adjustable parameter; see Fig. 1), which can be used to compute relative operating characteristic (ROC) curves. DeFlorio et. al (2018) shows that climate variability conditions modulate regional AR forecast skill. In particular, over the US West Coast, AR forecast utility (defined as the ratio of hits to false alarms) decreases at 10-day lead during negative Pacific-North America (PNA) conditions, and increases at 10-day lead during positive El Nino and Southern Oscillation (ENSO) conditions, with an even larger increase in AR forecast skill during phase-locked El Niño and positive PNA conditions (Fig. 2).

Figure 1: Figure (2) from DeFlorio et al. (2018): Method of determining if a predicted atmospheric river (AR) is a “hit” or a “miss” relative to an observed AR. Predicted and observed ARs are shown as shaded light and dark shaded ovals, respectively. Their IVT-weighted centroids are shown as black dots, and the distances D1 and D2 between each predicted AR and the observed AR are shown as black arrows. The distance threshold DT, which indicates the acceptable horizontal distance between an observed and predicted AR for a prediction to be considered skillful, is shown as a black arrow. In this example, the prediction of AR1 is considered skillful (a “hit”) since its centroid falls within the distance threshold of the observed AR, while the prediction of AR2 is not considered skillful (a “miss”) since its centroid falls outside the distance threshold of the observed AR.

Figure 2. Figure (10a) from DeFlorio et al. (2018): Relative operating characteristic (ROC) curves composited on positive (red) and negative (blue) phases of the combined El Niño-Southern Oscillation (ENSO) & Pacific-North America teleconnection (PNA) modes in December-January-February (DJF) over the North Pacific/Western U.S. region. The 1000 km distance threshold is used, and positive and negative phases are defined using +/- 0.5 standardized values of the climate index for each mode. 3-day (solid), 7-day (dashed), and 10-day (dotted) lead times are shown. The number of positive and negative phase days for each combined mode phase are listed above the legend. Area under ROC curve distributions for both region/mode/lead times of relevance, calculated from a bootstrap process that was repeated 1000 times by using resampling of the composite positive and negative mode days (red and blue, respectively) and all days (white) distributions with replacement, are included beside the ROC curves.

Deflorio, M., D. Waliser, B. Guan, D. Lavers, F.M. Ralph, and F. Vitart, 2018: Global assessment of atmospheric river prediction skill. Journal of Hydrometeorology, early online release, doi:10.1175/JHM-D-17-0135.1

CW3E Publication Notice: Genesis, Pathways, and Terminations of Intense Global Water Vapor Transport in Association with Large-Scale Climate Patterns

CW3E Publication Notice

Genesis, Pathways, and Terminations of Intense Global Water Vapor Transport in Association with Large-Scale Climate Patterns

February 13, 2018

CW3E researchers Scott Sellars and Brian Kawzenuk and director Marty Ralph in collaboration with Phu Nguyen (UC Irvine) and Soroosh Sorooshian recently published a paper in Geophysical Research Letters titled Genesis, Pathways, and Terminations of Intense Global Water Vapor Transport in Association with Large-Scale Climate Patterns (http://onlinelibrary.wiley.com/doi/10.1002/2017GL075495/full). The study uses the CONNected objECT (CONNECT) algorithm applied to integrated water vapor transport (IVT) data for the period of 1980 to 2016 calculated from Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) to identify objects associated with extreme moisture transport (Sellars et al., 2013, 2015).

The algorithm generated a global dataset of life-cycle records in time and space of evolving strong water vapor transport events. Each object was associated with distinct physical and climatological features such as object size, location, and intensity, various climatological teleconnection patterns, and many other characteristics. This algorithm identified various weather phenomena associated with strong moisture transport such as atmospheric rivers, hurricanes and tropical cyclones, monsoon transport, and various other systems that produced extreme moisture transport. It was illustrated that these events typically occurred in five distinct regions located in the midlatitudes (off the coast of the southeast United States, eastern China, eastern South America, off the southern tip of South Africa, and in the southeastern Pacific Ocean) (Figure 1a). Additional analysis showed distinct genesis and termination regions and global seasonal peak frequency during Northern Hemisphere late fall/winter and Southern Hemisphere winter (Figure 1c and d). In addition, the frequency and location of these events were shown to be strongly modulated by the Arctic Oscillation, Pacific North American Pattern, and the Quasi-Biennial Oscillation. Moreover, a positive linear trend in the annual number of objects was reported, increasing by 3.58 objects year-over-year. The vast dataset produced in this study will be used for various future research opportunities focused on extreme moisture transport and its connection to large-scale climate dynamics.

Figure 1:(a) Total number of IVT objects from January 1980 to August 2016. (b) Average duration in hours of object at each grid cell. (c) The number of objects at genesis (starting) locations for all IVT objects. (d) The number of objects at termination (ending) locations for all IVT objects. The gray areas represent landmass.

CW3E Publication Notice: An Inter-comparison Between Reanalysis and Dropsonde Observations of the Total Water Vapor Transport in Individual Atmospheric Rivers

CW3E Publication Notice

An Inter-comparison Between Reanalysis and Dropsonde Observations of the Total Water Vapor Transport in Individual Atmospheric Rivers

February 2, 2018

CW3E collaborators Bin Guan (UCLA), Duane Waliser (NASA/JPL), along with CW3E director Marty Ralph, recently published a paper in the Journal of Hydrometeorology, titled An Inter-comparison Between Reanalysis and Dropsonde Observations of the Total Water Vapor Transport in Individual Atmospheric Rivers (https://doi.org/10.1175/JHM-D-17-0114.1). The paper is included in the journal’s special collection on MERRA-2, Modern-Era Retrospective analysis for Research and Applications version 2.

Using airborne observations from various field campaigns over the northeastern Pacific along with two atmospheric reanalysis products (ERA-Interim and MERRA-2), the study validated key characteristics of atmospheric rivers (ARs) depicted by reanalyses against observations, as well as evaluating how well the 21 observed ARs represent the total of about 6000 ARs that occurred during the winters of 1979-2016 over the northeastern Pacific.

Results showed that the reanalysis products accurately depict the strength of the observed ARs in terms of the total water vapor flowing along an individual AR across its entire width, with a mean error of only +3% or -1% depending on the reanalysis product being evaluated. Additionally the 21 observed ARs well represent the mean strength of the total of about 6000 ARs identified in reanalysis products, with a mean difference of 5% or 14% depending on the reanalysis product being compared. Similar comparisons were also done for AR width, and for ARs in other regions and seasons. The study highlights the values of both dedicated observations of specific cases and spatiotemporally more complete global reanalysis products in understanding the characteristics and impacts of ARs.

Figure Caption: (left) Histogram of AR widths based on all ARs detected in ERA-Interim over the northeastern Pacific (AR centroids within 163.4–124.6°W, 23–46.4°N) during 15 January to 25 March of 1979–2016 (gray bars). Also shown are the mean AR width (km) based on all reanalysis ARs that contributed to the histogram (red solid), the subset of the reanalysis ARs that correspond to the 21 dropsonde transects (red dashed), and the observed value based on the 21 dropsonde transects as reported in Ralph et al. (2017b) (blue dashed for the mean, and blue circles for individual transects). The mean AR width value is also indicated in the figure legend for each sample. Red shading indicates the 95% confidence interval of the mean reanalysis AR width for a random 21-member sample drawn from the pool of reanalysis ARs based on 10,000 iterations. The error bar centered on the blue dashed line indicates the 95% confidence interval of the difference between the blue and red dashed lines based on a two-tailed, paired t-test. (right) As in the left but for total integrated water vapor transport (108 kg s−1) across AR widths.

CW3E Publication Notice: Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States

CW3E Publication Notice

Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States

December 1, 2017

CW3E long-time collaborator, Mike Dettinger, and USGS colleague, recently published a paper in Geophysical Research Letter titled: Flood runoff in relation to water vapor transport by atmospheric rivers over the western United States.

In the study they analyzed historical flood flows at over 5000 streamgages across the western US in relation to landfalling atmospheric-river storms. Specifically, they focused on the probabilities of floods flows occurring as conditioned by the presence of an atmospheric river and by the water vapor-transport rates in the atmospheric river. Through this analysis they were able to show that stronger the atmospheric river, the more likely are flood flows to develop.

Along the west coast, these peak flows coincide with atmospheric rivers about 80+% of the time, falling off to about 40-50% of the time in southern California, and falling off the farther inland the river basin (with notable regional anomalies, e.g., around Phoenix and in northern Idaho).

CW3E Publication Notice: Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers

CW3E Publication Notice

Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers

Spetember 14, 2017

F. Martin Ralph, director of CW3E, along with collaborators, recently published a paper in the American Meteorological Society’s Journal of Hydrometeorology: Ralph, F. M., S. Iacobellis, P. Neiman, J. Cordeira, J. Spackman, D. Waliser, G. Wick, A. White, and C. Fairall, 2017: Dropsonde Observations of Total Integrated Water Vapor Transport within North Pacific Atmospheric Rivers. J. Hydrometeor., 18, 2577-2596, https://doi.org/10.1175/JHM-D-17-0036.1

This study uses vertical profiles of water vapor, wind, and pressure obtained from 304 aircraft dropsondes across 21 ARs, in the midlatitudes as well as the subtropics, which were deployed during various experiments since the winter of 1998, including CALJET (Ralph et al. 2004), Ghostnets (Ralph et al. 2011), WISPAR (Neiman et al. 2014), CalWater-2014, CalWater-2015 (Ralph et al. 2016), and AR Recon-2016. Dropsondes provide the best measurements to date of horizontal water vapor transport in atmospheric rivers (ARs) and can document AR structure. Different methods of defining AR edges, using either integrated vapor transport (IVT) or integrated water vapor (IWV), were compared.

The study found that total water vapor transport (TIVT) in an AR averaged nearly 5×108 kg s-1, which is 2.6 times larger than the average discharge of liquid water from the Amazon River. The mean AR width was 890 ± 270 km. Subtropical ARs contained larger IWV but weaker winds than midlatitude ARs, although average TIVTs were nearly the same. Mean TIVTs calculated with an IVT-threshold versus an IWV- threshold produced results that differed by only 4% on average, although they did vary more between midlatitudes and subtropical regions. In general, important AR characteristics such as width and TIVT are less dependent on latitude when the IVT-threshold is used, and the IWV threshold often was not crossed on the warm side of subtropical ARs, so IVT represents a more robust threshold across a wider range of conditions than IWV.

Results were summarized in a schematic to illustrate the AR structure in 3 dimensions (see below). This schematic was used in the AR definition that was recently published in the American Meteorological Society’s Glossary of Meteorology.

Figure 1: Schematic summary of the structure and strength of an atmospheric river based on dropsonde measurements analyzed in this study, and on corresponding reanalyses that provide the plan-view context. (a) Plan view including parent low pressure system, and associated cold, warm, stationary 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 panel (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. Magnitudes of variables represent an average mid-latitude atmospheric river with lateral boundaries defined using the IVT threshold of 250 kg m-1 s-1. Depth corresponds to the altitude below which 75% of IVT occurs. Adapted primarily from Ralph et al. 2004 and Cordeira et al. 2013.

CW3E Publication Notice: The Chiricahua Gap and the Role of Easterly Water Vapor Transport in Southeastern Arizona Monsoon Precipitation

CW3E Publication Notice

The Chiricahua Gap and the Role of Easterly Water Vapor Transport in Southeastern Arizona Monsoon Precipitation

Spetember 13, 2017

Click here for personal use pdf file

This study is a collaborative effort between CW3E and University of Arizona that identifies a terrain feature along the Arizona-New Mexico border just north of Mexico that is potentially important to the weather and climate of the southeast Arizona summer monsoon. The terrain feature is a “gap” that is approximately 250 km across and 1 km deep and represents the lowest terrain elevation along the 3000-km length the Continental Divide from 16-45°N. The name “Chiricahua Gap” is introduced to identify this key terrain feature, which reflects the name of a nearby mountain range in southeast Arizona and the region’s Native American history. The importance of the Chiricahua Gap is that it represents the primary pathway in which low altitude atmospheric water vapor is transported across the Continental Divide.

Motivated by identification of the Chiricahua Gap, upper-air observations from a wind profiling radar in Tucson, model reanalyses (Climate Forecast System Reanalysis), and gridded daily precipitation data (NCEP Stage-IV) are used to construct a case study and 15-year climatology to link summer monsoon rainfall events in southeast Arizona to low-altitude water vapor transport within the Chiricahua Gap. The results show that 76% of the wettest summer monsoon days in southeast Arizona during 2002-2016 occurred in conditions of low-altitude easterly water vapor transport in the Chiricahua Gap on the previous day. This result highlights how low-altitude water vapor associated with the wettest summer monsoon days in southeast Arizona originates from the east side of the Continental Divide, which differs from previous studies published since the 1970s. Much of the recent scientific literature points to southwesterly surges of low-altitude water vapor from over the Gulf of California as the primary driver of rainfall over southern Arizona during the summer monsoon. The current study by F. M. Ralph and T. J. Galarneau shows that the source region of low-altitude water vapor in southeast Arizona during the summer monsoon is potentially more complex, and is significantly influenced by source regions east of the Divide.

The paper is an example of CW3E expanding its research to examine the dynamics of the North American monsoon. Because monsoon is an important source or water for the US southwest and can cause flooding events, particularly flash floods, better understanding and improving forecasts of the North American monsoon is and important component of CW3E achieving its goal of revolutionizing the physical understanding, observations, weather predictions, of extreme events in Western North America and their impacts on floods, droughts, hydropower, ecosystems and the economy.

Figure 1: Terrain height (shaded in m) over Arizona, New Mexico, western Texas, and northern Mexico. Key terrain features are labeled in black. The location of Tucson, Arizona, is labeled by the black-filled circle. Low-altitude easterly water vapor transport through the Chiricahua Gap is shown by the blue arrows. This figure is modified from Fig. 1b in Ralph and Galarneau (2017).

CW3E Publication Notice: Characterizing the Influence of Atmospheric River Orientation and Intensity on Precipitation Distribution over North-Coastal California

CW3E Publication Notice

Characterizing the Influence of Atmospheric River Orientation and Intensity on Precipitation Distributions over North-Coastal California

Spetember 12, 2017

Chad Hecht, a CW3E staff researcher, and Jason Cordeira, a CW3E affiliate and professor at Plymouth State University, recently published an article in AGU Geophysical Research Letters: Hecht, C. W., and J. M. Cordeira, 2017: Characterizing the Influence of Atmospheric River Orientation and Intensity on Precipitation Distribution over North-Coastal California. Geophys. Res. Lett., 44, doi:10.1002/2017GL074179. click here for personal use pdf file.

The key result of the study found that south-southwesterly oriented atmospheric rivers (ARs) produce significantly more Russian River watershed areal-average precipitation compared to westerly ARs (median areal-precipitation of 13 mm vs. .5 mm). This difference in precipitation accumulations is attributed to both the orientation of water vapor flux relative to the watershed topography and large-scale forcing that results in ascent.

The study uses clustering to objectively identify different orientations and intensities of ARs that make landfall over the California Russian River watershed (Fig. 1). Daily averaged IVT was calculated using 11-years of National Centers for Environmental Prediction (NCEP)–Climate Forecast System Reanalysis (CFSR) data spanning from 1 January 2004 to 31 December 2014. The paper analyzed the synoptic-scale flow configurations and resulting precipitation accumulations and distributions of westerly and south-southwesterly oriented ARs (Orange and Blue clusters in Fig. 1b).

Figure 1. (a) Domain averaged daily IVT direction (angular coordinate) and magnitude (kg m/s ; radial coordinate) for all days from 1 January 2004 to 31 December 2014 that data were available. Markers are color-coded based on 24-h accumulated precipitation (mm). The colored lines illustrate the average IVT for days with precipitation >10 (black), >25 (blue) and >50 mm (red). The 200 kg/m/s threshold that was applied in this study is shown by the black circle. (b) As in (a) except for days with daily average IVT ≥200 kg/m/s and color-coded based on K-means cluster.

Composite analyses illustrate the vastly different synoptic-scale characteristics associated with westerly and south/southwesterly ARs (Cluster 2 and Cluster 3). These different synoptic-scale flow configurations result in differences in synoptic scale forcing co-located over the composite AR and the Russian River watershed (Fig. 2).

Figure 2. (a, b) Composite mean IVT (kg/m/s ; plotted according to the reference vector in the upper right), SLP (hPa; contoured), and IWV (mm; color-coded according to scale), (c, d) composite mean 250-hPa geopotential height (dam; contoured), wind speed (m s–1 ; colorcoded according to scale), and IWV (mm; dashed blue contour), and (e, f) composite mean 700-hPa geopotential height (dam; solid contours), Q-vectors (1011 K/m/s ; plotted according to the reference vector in the bottom right), Q-vector divergence (1016 K/m/s ; color-coded according to scale) and potential temperature (K; dashed red contours) at t–12 h during (a,c,e) westerly and (b,d,f) south–southwesterly ARs.

The large difference in Russian River watershed area-averaged precipitation between westerly and south-southwesterly ARs (Fig. 3a) is not likely explained by statistically similar cluster IVT magnitudes (i.e., AR intensity; Fig. 3b) and IWV values (Fig. 3e) but likely a combination of a more favorable southwesterly IVT direction (i.e., AR orientation) relative to the orientation of the local topography and favorable synoptic-scale forcing for ascent (Fig. 2) illustrated by Q-vector convergence (Fig. 3d). While both AR types exhibit significantly statistically similar mean IVT, south-southwesterly ARs are associated with statistically significantly higher mean low-level IVT (1000–850 hPa; Fig. 3c).

Results from this study suggest that extreme precipitation produced by ARs is the result of both upslope moisture flux and quasi-geostrophic forcing for ascent.

Figure 3. Box and whisker plots of Russian River Watershed (a) area-average 24-h precipitation (mm), (b) domain average IVT (kg/m/s ), (c) domain average lower tropospheric (1000–850 hPa) IVT (kg/m/s ), (d) domain average Q-vector divergence (1016 K/m/s ), and (e) domain average IWV (mm) for westerly (orange) and south–southwesterly (blue) ARs. The boxes represent the interquartile range of the data and the whiskers represent upper and lower quartile of the data. The horizontal line within the boxes is the median value. The colored dots represent outliers and the asterisks represent extreme outliers. The box in the upper-left corner of each panel indicates the result of the independent samples t-test with 95% confidence (white indicates significantly statistically similar means and black indicates significantly statistically different).

Support for this project was provided by the State of California-Department of Water Resources and the U.S. Army Corps of Engineers, both as part of broader projects led by CW3E. A majority of this work was conducted while Chad was a graduate student at Plymouth State University. Dr. Cordeira and his graduate students at Plymouth State University actively collaborate with CW3E on topics related to atmospheric rivers, such as analyzing, understanding, and forecasting their impacts along the U.S. West Coast.