A Twenty-First-Century California Observing Network for Monitoring Extreme Weather Events [Journal of Atmospheric and Oceanic Technology]

By Coleman, T

Proquest LLC

ABSTRACT

During Northern Hemisphere winters, the West Coast of North America is battered by extratropical storms. The impact of these storms is of paramount concern to California, where aging water supply and flood protection infrastructures are challenged by increased standards for urban flood protection, an unusually variable weather regime, and projections of climate change. Additionally, there are inherent conflicts between releasing water to provide flood protection and storing water to meet requirements for the water supply, water quality, hydropower generation, water temperature and flow for at-risk species, and recreation. To improve reservoir management and meet the increasing demands on water, improved forecasts of precipitation, especially during extreme events, are required. Here, the authors describe how California is addressing their most important and costliest environmental issue-water management-in part, by installing a state-of-the-art observing system to better track the area's most severe wintertime storms.

1. Introduction

Since the late 1990s, scientists from the National Oceanic and Atmospheric Administration (NOAA)'s Earth System Research Laboratory (ESRL) and their partners have been studying the winter storms that impact the U.S. West Coast each year. Beginning in 2004, this work was organized under the umbrella of NOAA's Hydrometeorology Testbed (HMT-West; hmt.noaa. gov; Ralph et al. 2005; Morss and Ralph 2007). This paper describes a California HMT-Legacy project that has three main goals: 1) to install a twenty-first-century observing system to help address California's water and emergency management needs, 2) to provide a state-of-the-art numerical weather forecast model ensemble with a high-resolution nest over California, and 3) to develop decision support tools for weather and river forecasters and water managers. This project is part of the California Department of Water Resources (CA-DWR) Enhanced Flood Response and Emergency Preparedness Program.

The HMT-Legacy project is intended to help address some of the most extreme challenges that California faces regarding water and flood management in the face of climate change. California's population and economies (including agriculture), and thus its demands for water, are expected to grow rapidly in coming decades, in a time when floods and storms are being projected to increase in magnitude and frequency (Das et al. 2011), when the state's snowpacks are expected to retreat and decline (Cayan et al. 2008, 2013), and when the state may face increasingly intense droughts.

The tension between increasing floods and decreasing snowpacks is tightly bound because California's reservoirs are used for both flood risk management and water supply purposes, with a volume of open space maintained for flood capture each winter that is nearly equal to the most optimistic projections of the volume of water that will no longer be stored in the state's snowpacks by midcentury under global warming (Knowles and Cayan 2004). The water that is not stored as snowpack most likely will run off in the winter months instead, often as flood flows. The projected earlier runoff thus is likely to become an important reason for keeping even more open space behind the state's dams (for even more flood control) but also corresponds to water that ideally could be saved until later in the year (behind those same dams) to meet growing warm-season urban, agricultural, and environmental demands (Cayan et al. 2010).

This dilemma facing reservoir managers is a paramount concern. If the future skill of week 1 and week 2 precipitation forecasts would be sufficient to be used in making water management decisions, this concern would be ameliorated. However, because there are no guarantees that sufficient forecast skill can be achieved, the HMT-Legacy project, in essence, is an insurance policy for California. The additional information about storms and floods and improvements in short-term (0-3 days) and perhaps even longer lead time forecasts that the new observations and numerical model ensemble may provide are of the utmost importance to the state's water and flood managers. Long-term operation of the observing network also will allow the state to track intraseasonal-to-decadal climate changes and better manage their consequences.

2. Selected scientific achievements from HMT-West

Following are some of the scientific achievements from HMT-West that motivated CA-DWR to invest in the HMT-Legacy project. A major finding from HMT-West is the role that atmospheric rivers (ARs), narrow regions of enhanced water vapor transported in the warm sectors of midlatitude cyclones, play in creating heavy precipitation that can lead to flooding (Ralph et al. 2004, 2006; Neiman et al. 2008; Guan et al. 2010; Lavers et al. 2011; Moore et al. 2012). As defined by Ralph et al. 2004, ARs are long (.2000 km), relatively narrow (,1000km), and concentrated (.2 cm of integrated water vapor) moisture plumes. Globally, ARs are a critical component of Earth's energy budget (Zhu and Newell 1998). In addition, climate projections suggest that the intensity and frequency of AR events in California may increase in response to global climate change (Dettinger 2011). An example of an AR impacting the U.S. West Coast as viewed from satellites (Wick et al. 2013) is shown in Fig. 1. The continents are black in Fig. 1 because the satellite microwave retrievals of water vapor that work over the oceans currently are not available over land, given the poorly known microwave emissivity of land surfaces (Prigent et al. 2000). In addition, satellites do not measure the winds in the low-level jet (Neiman et al. 2002) that focus the transport of moisture onshore and determine which watershed(s) will be impacted most by the AR (Ralph et al. 2003).

Water vapor is the fuel that generates precipitation, and Global Navigation Satellite Systems (GNSS) such as GPS offer a robust and reliable method of calculating vertically integrated water vapor (IWV; Bevis et al. 1992; Duan et al. 1996) with high temporal resolution under all weather conditions (Gutman et al. 2004). Also, unlike microwave satellite retrievals, GPS can provide accurate water vapor estimates over land. Peixoto and Oort (1992) showed that approximately 80% of the water vapor in the Northern Hemisphere atmosphere at mid-latitudes exists in the lowest 700 mb, so IWV serves as a good proxy for the low-level moisture that fuels precipitation. For example, using four winters of IWV measurements collected on the northern coast of California, Neiman et al. (2009) showed that in order to produce 12 mm h"1 of rainfall in the coastal mountains, there needed to be at least 2 cm of IWV. This work helped to define the threshold of IWV that is now used to detect an AR.

In mountain watersheds, the altitude in the atmosphere where snow changes to rain (hereafter referred to as the snow level) can determine whether a storm augments the snowpack or creates a flood. White et al. (2002) used the National Weather Service (NWS) River Forecast System to simulate how changing the snow level would impact runoff in four California watersheds. For some of the watersheds they examined, a rise in the snow level of 600 m could more than triple the peak runoff in the watershed for the precipitation associated with a modest storm. Because of the importance of the snow level in mountain hydrology, White et al. (2010) began to evaluate the accuracy of snow-level forecasts produced by the NWS using snow-level observations collected with vertically pointing precipitation profilers (White et al. 2000) and found that significant forecast errors (300-900 m) occurred for some of the wettest storms.

The timing of a storm within the winter wet season can also determine whether a flood will ensue. For early season storms the antecedent soil conditions are normally dry, such that much of the precipitation is absorbed by the ground, thereby minimizing runoff. Later in the wet season, the timing between subsequent storms determines whether the soils dry out sufficiently to absorb some or all of the rainfall from the next precipitation event (Zamora et al. 2011). An example indicating the streamflow response to soil moisture conditions in the Russian River watershed in Sonoma County, California, is shown in Fig. 2. The watershed was impacted by three separate precipitation events within a 5-day period from late November through early December 2012. Peaks in the Russian River streamflow were observed each time the observed precipitation rate and amount kept the 10-cm soil at field capacity for a period longer than 3 hours. The 424.8 m3 s21 (15 000 cfs) flow peak occurred early on 3 December after the soil at 15-cm depth exceeded the field capacity by 14% volumetric water content, as a resuit of the saturation-excess runoff (Dunne and Black 1970). The maximum flow stage corresponding to this peak streamflow was 5.98 m, which is 0.42 m below flood stage for this particular location on the Russian River.

3. A tiered approach to observing system enhancements

All of the aforementioned findings from HMT-West influenced the design of the observing network that ESRL proposed to CA-DWRin 2007. The basic strategy was to organize different observing projects in a series of successive tiers, forming a pyramid. Each tier incorporates and builds on the previous tier(s) by adding new projects with increased scope, complexity, and/or cost. For example, tier 1 involves networks of sensors that have a proven track record and are relatively inexpensive to acquire, deploy, operate, and maintain. This tier consists of precipitation gauges, soil moisture probes, integrated water vapor sensors using existing GPS/GNSS receivers, and a new snow-level radar (Johnston et al. 2012) that was designed specifically for the HMT-Legacy project. Tier 1 also takes advantage of existing observing infrastructure within California. For example, NOAA is partnering with the UniversityNAVSTAR Consortium(UNAVCO;www. unavco.org)1 to upgrade existing GPS receivers across California with meteorological measurements and realtime communications to allow for continuous retrievals of IWV.

Tier 2 consists of observing technology that is mature but that comes at a higher cost than observing technology in tier 1. Given the importance of ARs in generating heavy precipitation and floods and the gaps associated with satellite remote sensing, ESRL scientists had previously designed, deployed, and tested a combination of sensors, called an atmospheric river observatory (ARO; White et al. 2009, section 4d), that could detect and monitor the important physical parameters of ARs as they make landfall. A statewide network of AROs was proposed to CA-DWR under tier 2.

The upper tiers (3 and 4) have observing projects that may not have been fully evaluated in the research community and/or are significantly more expensive to implement than the observing projects in tiers 1 and 2. Examples include buoy-mounted wind profilers (Jordan et al.1998),gap-filling radars (Matrosovet al. 2005; Jorgensen et al. 2011), and a Pacific winter storms reconnaissance program akin to the hurricane reconnaissance program conducted each year over the Atlantic Ocean. These ideas are scientifically tractable, and NOAA has made progress in each of these areas over the past several years. For example, NOAA, the National Center for Atmospheric Research (NCAR), and the National Aeronautics and Space Administration (NASA) used a new automated dropsonde system on an unmanned aircraft to study atmospheric rivers over the Pacific Ocean in February 2011 for a project called Winter Storms and Pacific Atmospheric Rivers (WISPAR). ESRL is also collaborating with the NWS and NOAA's National Severe Storms Laboratory to evaluate the benefit of gap-filling radar to improve quantitativeprecipitationestimationinanareaof California that has particularly poor coverage from the NWS operational radar network Next Generation Weather Radar (NEXRAD). This project is in conjunction with the Sonoma County Water Agency and the San Francisco CBS television network affiliate, KPIX, who installed a Doppler weather radar on Mount Vaca in Napa and Solano Counties, based largely on HMT's prior demonstration of gap-filling radar on the Sonoma County coast (Matrosov et al. 2005). A similar method of tiers (not discussed) was used to propose projects involving numerical modeling, information display, and decision support.

The original agreement signed with CA-DWR in 2008 was to implement the observing, numerical modeling, display, and decision support projects from tier 1. The Scripps Institution of Oceanography (SIO) is a coin-vestigator on the observing implementation plan. In 2010, CA-DWR amended the agreement to include a coastal network of four AROs from tier 2. Figure 3 shows a map of where each of the observing networks is being deployed. A follow-on agreement will define how the observing networks will be operated and maintained after 2013.

4. Observing system and forecast model descriptions

a. Soil probes and surface meteorological sensors

The HMT-Legacy project calls for the installation of 43 integrated soil moisture, soil temperature, and surface meteorology stations. ESRL is responsible for installing 27 of the 43 stations. ESRL decided to partner with the California Department of Forestry and Fire Protection (CAL FIRE) for the bulk of these installations because the numerous CAL FIRE station locations offered a variety of soil and meteorological conditions, site access was easy, and site security is more than adequate. In addition, the CAL FIRE station staff appreciate having access to the local surface meteorological data that are being provided as part of this project to help portray fire weather conditions during the dry season. Table 1 lists the instruments comprising the ESRL installations. Initially, soil probes are being installed at two depths at each site: 10 and 15 cm. Some of the soil probe installations in key watersheds will receive or will be retrofitted with additional probe depths that are consistent with both the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Soil Climate Analysis Network (SCAN) and the U.S. Climate Reference Network (USCRN) probe depths (5,10, 20, 50, and 100 cm). The soil probe and meteorological sensor signals are acquired through a Campbell Scientific, Inc. CR800 datalogger. Soil and surface meteorology data (2-min averages) are transmitted once every hour to a data hub in Boulder, Colorado, through one of three communication methods: telephone, satellite, or cellular services. Figure 4 shows a typical ESRL soil probe and surface meteorology installation in the HMT-Legacy network.

CA-DWR and SIO are jointly responsible for installing the remaining 16 soil/surface meteorology stations. Because many of these stations were intended to be at higher elevations, it was both efficient and cost effective to take advantage of the existing infrastructure available at the Remote Automated Weather Station (RAWS) network run by the U.S. Forest Service and Bureau of Land Management and monitored by the National Interagency Fire Center. Table 2 lists the soil-probing and surface meteorology instruments comprising the CA-DWR/SIO installations. The installation depths are 5, 10, 20, and 50 cm. Because of the soil structure, not all depths will be populated at all installation sites. Table 3 lists the site locations where NOAA and CA-DWR/SIO are installing soil and surface meteorology equipment for the HMT-Legacy project.

b. GPS integrated water vapor

Hundreds of continuously operating GPS receivers have been installed in California primarily for surveying and geodetic science purposes. Outfitting a GPS receiver site with temperature and pressure measurements allows real-time retrieval of the IWV. We will refer to GPS receiver sites with this configuration as GPS-Met sites, which is also the name of the program in NOAA that provides IWV estimates retrieved from GNSS signal delays to NOAA weather forecasters, NOAA weather forecast models, and researchers around the world. Many of the existing GPS receiver sites in California are part of the National Science Foundation'sPlate Boundary Observatory (PBO) that is operated by UNAVCO.

The HMT-Legacy project calls for 36 GPS-Met sites to provide estimates of IWV throughout California. Some of these sites were only equipped with GPS receivers and are being retrofitted by UNAVCO with the necessary meteorological sensors. Others are existing GPS receiver sites in the PBO network that were already GPS-Met compatible but needed real-time communications to make them useful for operational weather forecasting applications. Because of the initial success of the project, UNAVCO has added six additional GPS-Met sites to the California network to support this application in areas devoid of atmospheric or geodetic observations. Finally, some of the GPS-Met sites are collocated with other new or existing HMT-West observing sites in this project, particularly where it made scientific sense to have IWV measurements available with another type of atmospheric measurement. Table 4 lists the new GPS-Met sites that were made available as part of the HMT-Legacy project.

The GPS receiver signals and surface meteorological data from the GPS-Met stations are transmitted to Boulder via the internet, where they are combined with continuously updated GPS satellite orbit information provided by the Scripps Orbit and Permanent Array Center at the University of California San Diego to calculate IWV in near-real time. Currently IWV is estimated every 30 minutes for numerical weather prediction and satellite calibration/validation purposes. However, an experiment underway in 2013 is examining whether shorter (5-15 min) averaging periods can provide accurate estimates of IWV that are useful to forecasters during rapidly changing extreme weather conditions associated with ARs, the North American monsoon, and Santa Anna conditions. Estimates of IWV from the HMT-Legacy GPS-Met network are available on NOAA's GPS-Met home page (http://gpsmet.noaa.gov/). Values of IWV are also combined with satellite observations to provide a blended IWV product that is available from the Cooperative Institute for Research in the Atmosphere (http://amsu.cira.colostate.edu/gpstpw/) and the National Centers of Environmental Prediction (NCEP; http:// www.osdpd.noaa.gov/bTPW/).

c. Snow-level radars

The pulsed Doppler radars that have been used in HMT-West to provide measurements of the snow level during precipitation are relatively expensive to acquire, transport, deploy, operate, and maintain. One of the goals of the HMT-Legacy project was to develop a less expensive instrument that would be easier to transport, deploy, operate, and maintain. Radar engineers at ESRL and the University of Colorado'sCooperative Institute for Research in the Environmental Sciences designed and prototyped a new frequency modulated-continuous wave (FM-CW) radar operating at 10-cm wavelength for this project (Johnston et al. 2012).

Instead of transmitting a pulsed signal, the FM-CW radar transmits continuously, which requires separate antennas to transmit and receive so the transmitter does not saturate the receiver. The range of the targets is determined by changing the transmitted frequency during the observations. When the echoes are received, the frequency is measured and converted into range. Constant transmission also allows the radars to be low powered, which simplifies the radar electronics and allows the design to take advantage of readily available components. In production mode, the parts to build one of these new FM-CM radars would be about an order of magnitude less expensive than the parts required to build a higher-powered pulsed radar designed for the same purpose.

These small ''snow-level radars'' (Fig. 5) use two vertically pointed 1.2-m-diameter parabolic reflectors for antennas. The antennas are enclosed in shrouds that have steep covered openings so that snow can slide off and not impact operation of the radar. These antennas have asymmetrical side lobes that allow the radars to be situated at sites that otherwise would produce ground clutter for other types of vertically pointing radars. The electronics for the snow-level radar are located in the narrow compartment between the antennas. The compartment is insulated and has a heater and air conditioner. This allows the radar to be all-weather capable, while using commercial-grade computers and electronics. The entire radar is mounted on a flat 4.5-m-long utility trailer so it can be easily transported, positioned, and leveled, although the installations for the HMT-Legacy project are intended to be permanent. Table 5 lists the engineering characteristics of the snow-level radar. Table 6 lists the locations of the 10 snow-level radars that are being installed near major reservoirs across California for the HMT-Legacy project.

During precipitation, an automated algorithm based on White et al. (2002) analyzes profiles of radar reflectivity and Doppler vertical velocity measured by the snow-level radar to determine if a radar brightband (Battan 1959) is present. If a brightband exists, the algorithm chooses the peak radar reflectivity in the brightband to represent the snow level. The algorithm is applied to 10-min blocks of radar data and the results are transmitted hourly to the data hub in Boulder, Colorado, via one of the three communication services described earlier. An example of the real-time snow-level product display derived from a snow-level radar in the HMT-Legacy project is shown in Fig. 6. If a brightband is not detected, the time-height cross section of Doppler vertical velocity is still displayed.

d. Atmospheric river observatories

The original ARO concept (White et al. 2009) consisted of an observation couplet: one site at the coast instrumented with a Doppler wind profiler (Carter et al. 1995) to measure the incoming airflow profile and a GPS-Met station to measure the IWV and surface meteorology and a second site downwind in the coastal mountains instrumented with an S-band precipitation profiling radar (White et al. 2000), disdrometer, and surface meteorology to characterize the bulk microphysics of the orographically enhanced rainfall (White et al. 2003; Neiman et al. 2005; Kingsmill et al. 2006; Martner et al. 2008), as well as the orographic precipitation enhancement ratio. Measuring the wind profile is critical because the winds in the low-level jet are most highly correlated with the orographically enhanced rainfall, while the winds near the surface can often be blocked by the terrain (Neiman et al. 2002). Combining the winds in the low-level jet with the measured IWV, used as a proxy for the low-level moisture, allows the calculation of the bulk flux of water vapor, which Neiman et al. (2009) showed to be more highly correlated with orographic rainfall than either the winds in the low-level jet core or the IWV, treated separately. Figure 7 illustrates the scientific concepts behind the ARO development.

Where possible, given noise considerations, the AROs will include a Radio Acoustic Sounding System (RASS) for temperature profiling (Moran and Strauch 1994). The RASS is particularly useful for characterizing the atmospheric stability in AR conditions and is also useful for measuring the depth and strength of the marine inversion, which is often prevalent along the coast during the dry season. Table 7 lists the engineering specifications for the 449-MHz wind profiler with RASS, the particular wind profiler technology chosen for the AROs in this project based largely on a yearlong wind profiler technology evaluation conducted by ESRL from September 2005 to August 2006 (see http://www.esrl. noaa.gov/psd/psd2/programs/ioos/). Figure 8 shows the ARO installed on San Nicolas Island off the coast of Southern California. This particular installation is supported by the U.S. Navy, but the same technology and setup will be used for the four coastal AROs supported by the HMT-Legacy project.

For the HMT-Legacy project, CA-DWR gave priority to installing a ''picket fence'' of single-site AROs along the coast rather than investing in fewer AROs and using saved resources to support the observing couplets, as in the original ARO concept. ESRL has operated an ARO couplet in Sonoma County, California, as part of the HMT-West since the winter of 1997/98. They plan to continue operating this particular ARO couplet during each upcoming winter wet season for as long as possible to gather more insight into the orographic processes working at relatively short distances (~10km) from the coast and to provide long-term observations of ARs making landfall in an important agricultural and ecological region. Table 8 lists the locations of the four AROs that are being installed for the HMT-Legacy project. These specific locations were chosen to form an ARO picket fence, as CA-DWR desired, but they were also places where ESRL had experience successfully operating an ARO in the past for a variety of projects related to HMT-West.

e. Data ingest and display

ASCII data files and display graphics from the observing networks are generated within minutes after being received at the data hub and are made publicly available online (http://www.esrl.noaa.gov/psd/data/ obs/). Data are also distributed through NOAA's Meteorological Assimilation Data Ingest System (MADIS; http://madis.noaa.gov/), the California Data Exchange Center (CDEC; http://cdec.water.ca.gov/), and are distributed in a specialized NWS data format to NWS Weather Forecast Offices (WFOs) and the California Nevada River Forecast Center (CNRFC) through NWS Western Region Headquarters.

Data from the HMT-Legacy project observing networks are also being displayed in Google Maps, as in Fig. 9. This display mimics the type of observational displays used by NWS field offices. Currently the following near-real-time surface meteorology measurements are available in this display: temperature, integrated water vapor, snow depth, wind speed, wind direction, and accumulated precipitation for the past 1-, 3-, 6-, 12-, or 24-h periods. In addition, the following remotely sensed data products are available: snow level, integrated water vapor flux, NEXRAD reflectivity mosaic, and NEXRAD 1-h precipitation mosaic. Time series displays of these and other HMT-West datasets, excluding the NEXRAD products, are available through the product availability table (http://www.esrl.noaa.gov/psd/data/obs/). A similar Google Maps display tool is available to view instrument inventories and to see where different types of ESRL instruments have been deployed for HMT-West and other field projects (http://www.esrl.noaa.gov/psd/data/ obs/sitemap/psdmapsite/data.php).

f. The HMT weather forecast model

To take full advantage of the observing networks being installed and to provide advanced lead time for high-impact weather events, the HMT-Legacy project includes a data assimilation and numerical weather prediction system. The weather forecast model is the most current release (v3.4.1) of the Weather Research and Forecasting (WRF) model (Skamarock and Klemp 2008). The configuration employed for the HMT-Legacy project uses the Advanced Research WRF (ARW) dynamic core. An eight-member ensemble covering North America (beginning in 2013) is run at 9-km grid spacing with 35 vertical levels. A variety of initial and boundary conditions, as well as physical parameterizations, are used to differentiate the ensemble members. Initial conditions are provided by blending the Global Forecast System (GFS) with local observations using the Local Analysis and Prediction System (LAPS; Albers et al. 1996; Toth et al. 2012). Lateral boundary conditions are updated every 3 h using the GFS ensemble. A subset of forecast fields produced by the model is also publicly available (http://laps.noaa.gov/hmt/hmt.html).

To provide hourly model forecasts for the water vapor flux tool (see section 5), a separate WRF 3-km grid spacing (10-km grid spacing prior to 2013) model run is initialized every hour using LAPS. LAPS analyses are produced over the same West Coast domain and with the same horizontal grid spacing as the model. By reproducing the analysis every hour, the latest observations, both operational and experimental, are included for the next forecast cycle (Jian et al. 2003). The physics packages used in the model include the Thompson microphysics scheme (Thompson et al. 2004) and the nonlocal mixing Yonsei University (YSU) planetary boundary layer scheme (Noh et al. 2003). These schemes were chosen based on 5 years of experience gained in running the WRF model over the western United States for HMT (Jankov et al. 2007,2009,2011; Yuan et al. 2008, 2009). The analysis production starts 20 min after the hour in order to allow the latest data collected during the previous hour to arrive. The updated analysis grid is available approximately 45 min after the top of the hour. This new LAPS analysis is used to initialize the model, which then produces a 12-h forecast. The forecast, along with hourly output, is available approximately 2 h after the observations are collected. Gridpoint data extraction necessary for the water vapor flux tool is done almost instantaneously. The model output displayed on the right side of the flux tool (see section 5) is the 3-h forecast available from each successive hourly model run.

5. Examples of integrated observational and model forecast display products

Once the raw observations and model forecasts associated with the HMT-Legacy project are acquired and ingested, value-added data displays are produced in near-realtime (www.esrl.noaa.gov/psd/data/obs/). Figure 10 illustrates a multipanel display of the snow-level product derived from 6 of the 10 snow-level radars stretching from Northern California to the south-central Sierra (see Fig. 3). This information is of primary importance to river forecasters to verify the snow levels predicted by numerical weather prediction models. In addition to providing information on the snow level, the snow-level radar network provides detail on the timing of precipitation and the depth (up to the radar's maximum range and subject to the radar's minimum sensitivity) of the precipitating cloud layer. For example, in the left-hand side of Fig. 10 the network depicts the time lag required for the onset of a storm's precipitation to proceed from north to south as the storm progresses down the coast of California.

Figure 11 shows an example of the water vapor flux tool display derived from a prototype ARO deployed in Sonoma County, California, as part of the HMT 2008/09 field season. This display, developed jointly by operational weather forecasters and HMT research scientists, combines observations with numerical weather prediction output to help monitor and forecast the forcings associated with landfalling ARs. Weather forecasters and other end users can use this tool to verify how well the HMT weather forecast model is portraying the AR conditions and the resulting precipitation. In the near future, the tool will include operational Rapid Refresh model output. HMT research is also being conducted to determine how far inland atmospheric rivers impact precipitation, runoff, and the potential for flooding.

The snow-level radar and water vapor flux tool displays have received positive feedback from NWS and the U.S. Army Corps of Engineers (ACE). Both agencies have noted that these products have increased their situational awareness of storm impacts. For example, Arthur Henkel, the development and operations hydrologist at the CNRFC, has said that the snow-level measurements generated by HMT have ''changed the way we do business with respect to snow-level forecasting.'' In another case, Larry Schick, meteorologist with the ACE office in Seattle, used the water flux display from an ARO deployed on the Washington coast, as part of the Howard A. Hanson Dam (HHD) flood risk mitigation project (White et al. 2012), to help make significant water management decisions during a series of storms that impacted western Washington in January 2012. He stated, ''Yesterday, I used the new coastal radar and ARO in tandem to refine the forecast and give our dam regulator engineers critical forecast information...Of course, I was monitoring local WFO Seattle NWS forecasts and Northwest River Forecast Center as well and they were right on, but the ARO does allow a strong confirmation for making these rapidly changing but important dam operational decisions.'' White et al. (2012) also includes specific examples of and statistics on how the ARO observations were used in daily forecast operations during the HHD flood risk mitigation project.

6. Decision support tools

An important step in impacting forecast operations and end-user decisions is to develop decision support tools (DSTs) tailored to their needs, based on state-of-the-art knowledge and near-real-time data provided by this new observing and modeling system. An important component of this project's DST development is the role of atmospheric rivers in creating the heavy precipitation that can lead to flooding or to beneficial water supply (Dettinger et al. 2011). Based on HMT research, clearly defined criteria have now been established that identify when an AR is about to strike (e.g., Fig. 11). The location at landfall and intensity of ARs are also critical, and both of these parameters can now be monitored with the newly installed observing network. Numerical model forecast-based tools have been developed to better predict these events out to several days. For example, there is now an automated AR detection tool (Wick et al. 2013; http://www.esrl.noaa.gov/psd/psd2/coastal/ satres/data/html/ar_detect_gfs_new.php) applied to the NWS operational Global Forecast System produced by NCEP.

An example of this advanced warning capability occurred in December 2010 when a major AR struck Southern California. Because of previous wild fire scars on the mountains of Southern California, it was recognized that any heavy rainfall event could lead to large debris flows. A training session provided by one of the authors to all western region offices of the NWS just a few weeks earlier highlighted the importance and recognition of ARs. The positive impact of this training was demonstrated by forecasters having the confidence to alert state and local emergency management to the potential threat several days in advance. In fact, up to 48 h before 15-20 in. of rain fell in the San Bernardino Mountains of Southern California, forecasters were predicting up to 20 in. of rain to fall and warning of major debris flows, which allowed for earlier warnings and preparations. Over $60 million (U.S. dollars) in damages were reported in San Bernardino County from flash flooding and landslides. Some major cities in Southern California received over 50% of their average annual rainfall in just 7 days. Major flooding occurred along the Santa Margarita, San Diego, and Mojave Rivers.

A new direction in the DST realm is the development of performance measures for predictions that relate more effectively to the key conditions associated with ARs and extreme precipitation. Both Ralph et al. (2010) and White et al. (2010) describe new performance measures for forecast variables related to flooding, and these measures are now available for testing and implementation. Another tool that is now available is a scaling for extreme rainfall (Ralph and Dettinger 2012) that is more intuitive to nonspecialists and that is not sensitive to changes in climate. This scaling is simply four ''rainfall categories'' (R-Cats) based on 3-day-total rainfall, and these R-Cats can be applied to observations or predictions. R-Cat 4 (.500 mm) is the most extreme rainfall category, and California is the only state outside of the southeastern United States, where the impacts of tropical storms and hurricanes are most prevalent, that has experienced R-Cat 4 events during the period 1950-2008.

7. Summary and future work

a. Summary

Some of the winter storms that are responsible for the bulk of California's water supply throughout the year are also responsible for generating destructive floods that result in the loss of lives and property. In California, as for the nation as a whole, floods produce more annual property damage, on average, than any other type of natural disaster. Recently, a U.S. Geological Survey Multi Hazards Demonstration Project called ARkStorm (for atmospheric river 1000; Porter et al. 2011) studied the impacts of a scientifically plausible epic storm hitting California and found that such an event could result in $725 billion in losses. Furthermore, this project estimated that improved forecasting and warnings could reduce losses by tens of billions of dollars.

To provide forecasters, water managers, and the general public with the atmospheric and surface conditions that lead to heavy precipitation and flooding, CA-DWR is working with HMT-West and partners to install an unprecedented observing system across the state. The system consists of four synergistic observing networks that monitor the atmospheric and terrestrial conditions that can lead to dangerous floods and debris flows: 43 soil moisture, soil temperature, and surface meteorology stations; 36 GPS-Met integrated water vapor-observing sites; 10 snow-level radar and surface meteorology stations; and four coastal atmospheric river observatories. Through data assimilation, observations from these networks will provide improved initialization fields to drive weather forecast models. Long-term operation of the observing system will provide data to interpret how California's climate is changing and whether adaptation through new water management strategies will be required.

b. Future work

To maximize the impact of the HMT-Legacy project, future work will include developing training modules to increase the usage of the observations, models, and decision support tools within the NWS and also with water managers in the U.S. Army Corps of Engineers, U.S. Bureau of Reclamation, and local water agencies throughout California. ESRL is currently working on a new agreement with CA-DWR to 1) implement a plan to sustain the HMT-Legacy observations, 2) develop new decision support tools, and 3) optimize observing network expansion to provide watershed-scale information on extreme events. The last element will employ data denial experiments to help quantify the benefit of the additional observations on numerical weather forecasts.

In the spring of 2013, the HMT will begin a pilot project in North Carolina. This HMT-Southeast pilot study (HMT-SEPS) will have a warm-season precipitation focus, but ESRL's observing assets will be available year-round. In 2014, after the launch of the Global Precipitation Measurement mission's core satellite, NASA will bring a number of observing assets to bear on HMT-Southeast, including scanning radars, disdrometers, and rain gauges.

The HMT-Legacy project already has generated action on at least two fronts. First, UNAVCO and NOAA have expanded the GPS water vapor monitoring network by 25 stations in the western United States, including 13 in Oregon and Washington combined. This expansion will help with tracking the inland penetration of ARs throughout the Pacific Northwest. Second, the Western States Water Council (http://www.westernstateswater. org/) adopted Position 322 (Western States Water Council 2011) in July 2011, which includes the following statement: ''Be it further resolved, that the Western States Water Council supports development of an improved observing system for Western extreme precipitation events, to aid in monitoring, prediction, and climate trend analysis associated with extreme weather events.'' At the time of this publication, an implementation plan for this western states-wide observing system was being developed at the request of the Western Governors' Association (http://www.westgov.org/).

Acknowledgments. The authors acknowledge the highly skilled engineering and technical staff at DWR, NOAA, SCRIPPS, and UNAVCO who have designed, implemented, operated, and maintained the observing networks described in this paper. We also thank CA-DWR, California Department of Forestry and Fire Protection, U.S. Forest Service, U.S. Army Corps of Engineers, U.S. Bureau of Reclamation, U.S. Navy, city of Santa Barbara, Humboldt County, Merced Irrigation District, Potter Valley Irrigation District, and University of California'sBodega Marine Laboratory, all of whom have provided facilities for the observing system deployments.

*Denotes Open Access content.

1 Operators of the PBO, the geodetic component of EarthScope funded by the National Science Foundation.

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A. B. WHITE,* M. L. ANDERSON,+ M. D. DETTINGER,# F. M. RALPH,* A. HINOJOSA,+ D. R. CAYAN,#

R. K. HARTMAN,@ D. W. REYNOLDS,& L. E. JOHNSON,** T. L. SCHNEIDER,++ R. CIFELLI,* Z. TOTH,*

S. I. GUTMAN,* C. W. KING,* F. GEHRKE,+ P. E. JOHNSTON,& C. WALLS,## D. MANN,## D. J. GOTTAS,*

AND T. COLEMAN&

* NOAA/Earth System Research Laboratory, Boulder, Colorado

+ California Department of Water Resources, Sacramento, California

# U.S. Geological Survey, and Scripps Institution of Oceanography, La Jolla, California

@ NOAA/California Nevada River Forecast Center, Sacramento, California

& Cooperative Institute for Research in the Environmental Sciences, NOAA/University of Colorado Boulder,

and NOAA/Earth System Research Laboratory, Boulder, Colorado

** Cooperative Institute for Research in the Atmosphere, NOAA/Colorado State University, Fort Collins,

and NOAA/Earth System Research Laboratory, Boulder, Colorado

++ NOAA/Office of Hydrologic Development, Boulder, Colorado

## UNAVCO, Inc., Boulder, Colorado

(Manuscript received 14 September 2012, in final form 10 May 2013)

Corresponding author address: Dr. Allen B. White, NOAA/ Earth System Research Laboratory, R/PSD2, 325 Broadway, Boulder, CO 80305.

E-mail: [email protected]

Copyright:	(c) 2013 American Meteorological Society
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