In the late 1990s, NOAA Physical Sciences Division (PSD) scientists took advantage of their observing system expertise to begin studying the winter storms that impact the Western U.S. on an annual basis. The first of these focused field campaigns was the award-winning (1998 DOC Silver Medal) California Land-falling Jets Experiment (CALJET). Additional field campaigns followed in the early 2000’s and were eventually organized under the umbrella of the NOAA Hydrometeorology Testbed (HMT; hmt.noaa.gov). HMT has continued research on the underlying physical processes associated with extreme precipitation and flooding.
The California Department of Water Resources (CA-DWR) understood the importance of this research, especially as demands for water increased due to population and agriculture growth. In an effort to provide a more permanent solution than episodic field campaigns could provide, CA-DWR asked the nominees to work with them to design, build, and implement a permanent statewide observing system to help stakeholders detect, monitor, and forecast the conditions that can lead to dangerous flooding and/or valuable water supply. The nominees successfully accomplished this enormous task over the past eight years, with the last observing system becoming operational in January of 2017. California now owns a network consisting of over 100 observing sites that provide terrestrial and atmospheric information to CA-DWR and their stakeholders and is part of CA-DWR’s Enhanced Flood Response and Emergency Preparedness program. There is no other observing network of this magnitude on the planet. The specific contributions of the nominees to the various components of the CA-DWR observing network are highlighted in the paragraphs that follow.
A major finding from HMT is the role that atmospheric rivers (ARs), narrow regions of enhanced water vapor transported in the warm sectors of mid-latitude cyclones, play in creating heavy precipitation that can lead to flooding. Water vapor is the fuel that generates precipitation, and Global Navigation Satellite Systems such as Global Positioning System (GPS) offer a reliable method of calculating vertically integrated water vapor (IWV) under all weather conditions. For this part of the project, the nominees formed a collaborative partnership with UNAVCO, a university-based consortium focused on geodetic science, to upgrade existing GPS receivers across California (part of UNAVCO’s Plate Boundary Observatory) with meteorological measurements and real-time communications to allow for continuous retrievals of IWV. There are now 54 sites across California providing cost-effective measurements of IWV in near real time. The success of this program has motivated UNAVCO to add additional IWV sensing sites in Oregon and Washington.
In mountainous watersheds, the altitude in the atmosphere where snow changes into rain (a.k.a., the snow level) can determine whether a storm augments the snowpack or creates a flood. Because of the importance of the snow level in mountain hydrology, the PSD began making snow-level measurements in 2002 using vertically pointing radars. This work resulted in a U.S. patent. Unfortunately, the radars used for this purpose were relatively expensive and PSD only possessed a handful of them. Therefore, specifically for the CA-DWR project, the nominees and their CI partners successfully designed, prototyped, and perfected a new, frequency-modulated, continuous-wave (FM-CW) radar to routinely provide snow-level measurements during precipitation events. This “snow-level” radar is about 1/5th the cost of the radars PSD used previously to make snow-level measurements. CA-DWR invested in ten of these snow-level radars and the nominees deployed them in major watersheds across the state. The nominees are currently working with Radiometrics, Inc., to form a new CRADA to create a market for and increase the visibility of this unique instrument.
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 storm event. The nominees used commercially available soil temperature and moisture sensors that they have used in previous HMT research to create a network of 39 soil moisture stations across the state, many of which are located near fire stations operated by the California Department of Forestry and Fire Protection. For little added cost, the nominees added precipitation and surface meteorological sensors to help portray the current fire weather conditions.
Finally, the importance of ARs to water supply and flooding motivated CA-DWR to invest in a coastal network of atmospheric river observatories (AROs), a unique collection of instruments designed by the nominees to detect and monitor the key features in ARs. Combined with collocated GPS water vapor, a 449-MHz Doppler wind profiler measures the transport of water vapor at the level in the atmosphere that largely controls the precipitation formed by lift over the coastal mountains and Sierra Nevada. A radio acoustic sounding system (RASS) provides temperature profiles that are important for determining atmospheric stability. A surface meteorological tower provides additional measurements to fill in the near surface conditions. CA-DWR invested in four AROs, which the nominees deployed at roughly 300 km spacing along the coast. The success of this project motivated the U.S. Department of Energy to invest in three additional AROs for Oregon and Washington to support wind energy studies in those states. The nominees constructed the wind profilers from components that were manufactured by PSD’s CRADA partners, Vaisala, Inc. and Scintec, AG. As with the ARO and other instrument installations listed above, the nominees were required to search for suitable sites, obtain leases with land owners, as well as install electric utilities and data communications. Already dozens of trips have been taken by the nominees to conduct site surveys, install and maintain the instruments, as well as to forge collaborative relationships with CA-DWR and their stakeholders.
All of the data from the observing network in California are telemetered to a data hub in Boulder using cellular internet technology. The data are organized and archived into a readily accessible data base and data visualization products are generated by the nominees and publicly displayed in near real time (https://www.esrl.noaa.gov/psd/data/obs/datadisplay/). In addition, the data are transmitted to CA-DWR’s data exchange center (CDEC), to NOAA’s data clearing house (MADIS), and to NWS Western Region in specialized formats used by the River Forecast Centers. In 2013, CA-DWR signed a second five-year MOU with PSD to finish the instrument installations, to operate and maintain their observing network, and to conduct further research to increase our physical understanding of West Coast winter weather. By taking advantage of lessons learned in HMT, including both new and commercially available observing technology designed to monitor the impacts of ARs, California has built out an unprecedented statewide observing network that will provide detailed climate quality observations well into the future, to aid in their decisions for managing their water resources.