Ellen M. Sukovich

Relationships between Barrier Jet Heights, Orographic Precipitation Gradients, and Streamflow in the Northern Sierra Nevada

Abstract The rate of precipitation increase with elevation, termed the orographic precipitation gradient (OPG), is critically important for hydrologic forecasting in mountain basins that receive both rain and snow. Here, the following are examined to see how well they are able to predict the OPG and how it changes between storms and years: 1) a linear model of orographic precipitation forced by upstream radiosonde data, 2) monthly Parameter-Elevation Regressions on Independent Slopes Model (PRISM) precipitation data, and 3) seven years of hourly wind profiler data used to identify characteristics of the Sierra barrier jet (SBJ). These are compared against 124 daily resolution (four of which also had quality controlled, hourly resolution) precipitation gauge records in the northern Sierra Nevada. All methods represent the OPG well in the mean and during a year when less than 30% of the precipitation occurred on days with SBJs. However, the linear model and PRISM do not adequately capture annual variations ...

A Seven-Year Wind Profiler–Based Climatology of the Windward Barrier Jet along California’s Northern Sierra Nevada

Abstract This wind profiler–based study highlights key characteristics of the barrier jet along the windward slope of California’s Sierra Nevada. Between 2000 and 2007 roughly 10% of 100 000 hourly wind profiles, recorded at two sites, satisfied the sierra barrier jet (SBJ) threshold criteria described in the text. The mean magnitude of the terrain-parallel flow in the SBJ core (i.e., Vmax) was similar at both sites (∼17.5 m s−1) and at a comparable altitude, 500–1000 m above the surface. The cross-mountain wind speed was weak at the altitude of Vmax, consistent with blocked conditions. The seasonal cycle of SBJ occurrences showed a maximum during the cooler months and a minimum in summer. Additionally, the SBJ was stronger in winter than in summer. Because the warm-season (May–September) SBJs were different than their cool-season (October–April) counterparts and occurred during California’s dry season, they were not discussed in detail. An inventory of ∼250 cool-season SBJ cases from the two sites was ge...

Assessment of extreme quantitative precipitation forecasts and development of regional extreme event thresholds using data from HMT-2006 and COOP observers

Extreme precipitation events, and the quantitative precipitation forecasts (QPFs) associated with them, are examined. The study uses data from the Hydrometeorology Testbed (HMT), which conducted its first field study in California during the 2005/06 cool season. National Weather Service River Forecast Center (NWS RFC)griddedQPFs for24-hperiods at24-h(day1), 48-h(day2), and72-h(day3)forecastleadtimesplus24-h quantitativeprecipitationestimates(QPEs)fromsitesinCalifornia(CA)andOregon‐Washington(OR‐WA) are used. During the 172-day period studied, some sites received more than 254 cm (100 in.) of precipitation. The winter season produced many extreme precipitation events, including 90 instances when a site received more than 7.6 cm (3.0 in.) of precipitation in 24 h (i.e., an ‘‘event’’) and 17 events that exceeded 12.7 cm (24 h) 21 [5.0 in. (24 h) 21 ]. For the 90 extreme events f.7.6 cm (24 h) 21 [3.0 in. (24 h) 21 ]g, almost 90% of all the 270 QPFs (days 1‐3) were biased low, increasingly so with greater lead time. Of the 17 observed events exceeding 12.7 cm (24 h) 21 [5.0 in. (24 h) 21 ], only 1 of those events was predicted to be that extreme. Almost all of the extreme events correlated with the presence of atmospheric river conditions. Total seasonal QPF biases for all events fi.e.,

Sierra Barrier Jets, Atmospheric Rivers, and Precipitation Characteristics in Northern California: A Composite Perspective Based on a Network of Wind Profilers

0.025 cm (24 h) 21 [0.01 in. (24 h) 21 ]g were sensitive to local geography and were generally biased low in the California‐Nevada River Forecast Center (CNRFC) region and high in the NorthwestRiverForecastCenter(NWRFC)domain.ThelowbiasinCAQPFsimprovedwithshorterforecast lead time andworsened forextreme events. Differences were also noted between the CNRFC andNWRFC in terms of QPF and the frequency of extreme events. A key finding from this study is that there were more precipitation events .7.6 cm (24 h) 21 [3.0 in. (24 h) 21 ] in CA than in OR‐WA. Examination of 422 Cooperative Observer Program (COOP) sites in the NWRFC domain and 400 in the CNRFC domain found that the thresholds for the top 1% and top 0.1% of precipitation events were 7.6 cm (24 h) 21 [3.0 in. (24 h) 21 ]a nd 14.2 cm (24 h) 21 [5.6 in. (24 h) 21 ] or greater for the CNRFC and only 5.1 cm (24 h) 21 [2.0 in. (24 h) 21 ] and 9.4 cm (24 h) 21 [3.7 in. (24 h) 21 ] for the NWRFC, respectively. Similar analyses for all NWS RFCs showed that the threshold for the top 1% of events varies from ;3.8 cm (24 h) 21 [1.5 in. (24 h) 21 ] in the Colorado BasinRiverForecastCenter(CBRFC)to ;5.1 cm (24 h) 21 [3.0in. (24 h) 21 ] in thenortherntierof RFCsand ;7.6 cm (24 h) 21 [3.0 in. (24 h) 21 ] in both the southern tier and the CNRFC. It is recommended that NWS QPF performance in the future be assessed for extreme events using these thresholds.

Climatology and Environmental Characteristics of Extreme Precipitation Events in the Southeastern United States

AbstractFive 915-MHz wind profilers and GPS receivers across Californias northern Central Valley (CV) and adjacent Sierra foothills and coastal zone, in tandem with a 6-km-resolution gridded reanalysis dataset generated from the Weather Research and Forecasting Model, document key spatiotemporal characteristics of Sierra barrier jets (SBJs), landfalling atmospheric rivers (ARs), and their interactions. Composite kinematic and thermodynamic analyses are based on the 13 strongest SBJ cases observed by the Sloughhouse profiler between 2009 and 2011. The analyses show shallow, cool, south-southeasterly (i.e., Sierra parallel) flow and associated water vapor transport strengthening with time early in the 24-h compositing period, culminating in an SBJ core at <1 km above ground over the eastern CV. The SBJ core increases in altitude up the Sierras windward slope and poleward toward the north end of the CV, but it does not reach the westernmost CV. Above the developing SBJ, strengthening southwesterly flow des...

Understanding the Role of Atmospheric Rivers in Heavy Precipitation in the Southeast United States

AbstractThis paper documents the characteristics of extreme precipitation events (EPEs) in the southeastern United States (SEUS) during 2002–11. The EPEs are identified by applying an object-based method to 24-h precipitation analyses from the NCEP stage-IV dataset. It is found that EPEs affected the SEUS in all months and occurred most frequently in the western portion of the SEUS during the cool season and in the eastern portion during the warm season. The EPEs associated with tropical cyclones, although less common, tended to be larger in size, more intense, and longer lived than “nontropical” EPEs. Nontropical EPEs in the warm season, relative to those in the cool season, tended to be smaller in size and typically involved more moist, conditionally unstable conditions but weaker dynamical influences. Synoptic-scale composites are constructed for nontropical EPEs stratified by the magnitude of vertically integrated water vapor transport (IVT) to examine distinct scenarios for the occurrence of EPEs. Th...

Extreme Quantitative Precipitation Forecast Performance at the Weather Prediction Center from 2001 to 2011

AbstractAn analysis of atmospheric rivers (ARs) as defined by an automated AR detection tool based on integrated water vapor transport (IVT) and the connection to heavy precipitation in the southeast United States (SEUS) is performed. Climatological water vapor and water vapor transport fields are compared between the U.S. West Coast (WCUS) and the SEUS, highlighting stronger seasonal variation in integrated water vapor in the SEUS and stronger seasonal variation in IVT in the WCUS. The climatological analysis suggests that IVT values above ~500 kg m−1 s−1 (as incorporated into an objective identification tool such as the AR detection tool used here) may serve as a sensible threshold for defining ARs in the SEUS.Atmospheric river impacts on heavy precipitation in the SEUS are shown to vary on an annual cycle, and a connection between ARs and heavy precipitation during the nonsummer months is demonstrated. When identified ARs are matched to heavy precipitation days (>100 mm day−1), an average match rate of...

Assimilation of GPS Radio Occultation Data for an Intense Atmospheric River with the NCEP Regional GSI System

AbstractExtreme quantitative precipitation forecast (QPF) performance is baselined and analyzed by NOAA’s Hydrometeorology Testbed (HMT) using 11 yr of 32-km gridded QPFs from NCEP’s Weather Prediction Center (WPC). The analysis uses regional extreme precipitation thresholds, quantitatively defined as the 99th and 99.9th percentile precipitation values of all wet-site days from 2001 to 2011 for each River Forecast Center (RFC) region, to evaluate QPF performance at multiple lead times. Five verification metrics are used: probability of detection (POD), false alarm ratio (FAR), critical success index (CSI), frequency bias, and conditional mean absolute error (MAEcond). Results indicate that extreme QPFs have incrementally improved in forecast accuracy over the 11-yr period. Seasonal extreme QPFs show the highest skill during winter and the lowest skill during summer, although an increase in QPF skill is observed during September, most likely due to landfalling tropical systems. Seasonal extreme QPF skill d...

Representation of the Sierra Barrier Jet in 11 years of a high-resolution dynamical reanalysis downscaling compared with long-term wind profiler observations

AbstractThis paper uses a case study to explore the potential of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and Challenging Minisatellite Payload (CHAMP) global positioning system (GPS) radio occultation (RO) satellite data over the eastern Pacific Ocean to improve analyses and mesoscale forecasts of landfalling atmospheric rivers (ARs) along the U.S. West Coast. The case study is from early November 2006 and was a very high-impact event in the Pacific Northwest where it created torrential rainfall and severe flooding. Recent studies have shown that the COSMIC data offshore have the ability to better define the vertical and horizontal structure of the strong AR. This paper extends the earlier work by assessing the impact of assimilating the COSMIC data into the Advanced Research Weather Research and Forecasting (ARW-WRF) mesoscale numerical model (using a nested mode with 36-, 12-, and 4-km grid sizes) on a key 24-h forecast.The data are assimilated using NCEP’s Gridp...