4
Observing Systems and Technologies: Successes and Challenges

In this chapter we highlight current and emerging observing systems and technologies to address the observational needs discussed in Chapters 2 and 3. Systems and technologies are considered in two broad categories: those based on the surface and those based in space. In the spirit of considering the issue From the Ground Up, those based at the surface are given more emphasis and further categorized according to whether the technology provides in-situ or remotely sensed observations. Surface-based remote sensing systems are discussed according to whether the sensing technology is active or passive. We discuss systems that may be based at the surface but provide both in-situ and remotely sensed observations in the vertical dimension at heights well above near-surface. Some of these systems are mobile (e.g., aircraft). Others are designed to provide targeted observations.

Following the discussion of technologies and systems, we summarize several particular observational challenges, including those posed by the surface and the planetary boundary layer, and mountains, cities, and coasts. We conclude the chapter with a discussion of the global context within which U.S. mesoscale observations are embedded. The global context is important because, for many applications, the utility of limited-area mesoscale observations is highly dependent on larger domains of observations, for example, in the provision of initial and boundary conditions for mesoscale numerical weather prediction models.



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4 Observing Systems and Technologies: Successes and Challenges In this chapter we highlight current and emerging observing systems and technologies to address the observational needs discussed in Chapters 2 and 3. Systems and technologies are considered in two broad categories: those based on the surface and those based in space. In the spirit of consid- ering the issue From the Ground Up, those based at the surface are given more emphasis and further categorized according to whether the technol- ogy provides in-situ or remotely sensed observations. Surface-based remote sensing systems are discussed according to whether the sensing technology is active or passive. We discuss systems that may be based at the surface but provide both in-situ and remotely sensed observations in the vertical dimen- sion at heights well above near-surface. Some of these systems are mobile (e.g., aircraft). Others are designed to provide targeted observations. Following the discussion of technologies and systems, we summarize several particular observational challenges, including those posed by the surface and the planetary boundary layer, and mountains, cities, and coasts. We conclude the chapter with a discussion of the global context within which U.S. mesoscale observations are embedded. The global context is important because, for many applications, the utility of limited-area meso- scale observations is highly dependent on larger domains of observations, for example, in the provision of initial and boundary conditions for meso- scale numerical weather prediction models. 

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 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP SURFACE-BASED OBSERVING SYSTEMS Mesoscale meteorology is closely identified with surface observing sys- tems, perhaps because disruptive weather is intrinsically at the mesoscale, and impacts are most often experienced at or near the surface. The United States has enormous diversity and complexity within its inventory of sur- face-based observing assets, which are operated by federal, state, and local agencies, numerous segments of the private sector, universities, schools, and hobbyists and other enthusiasts. Surface-based observing systems employ both in-situ sensing as well as active and passive remote sensing technolo- gies. A number of efforts have summarized observational capabilities in the United States. For the last decade and with funding from the Global Energy and Water Cycle Experiment (GEWEX) America’s Prediction Project (GAPP), University Corporation for Atmospheric Research/National Center for Atmospheric Research (UCAR/NCAR) has developed a database that describes and maps what is available (http://www.eol.ucar.edu/projects/ hydrometnet). The National Science Foundation (NSF) recently sponsored development of another database to serve the dual purpose of providing users with information about available resources and to identify future observational needs in atmospheric research (see http://www.eol.ucar.edu/ fadb/). The National Oceanic and Atmospheric Administration (NOAA) is currently developing an Observing Systems Architecture website with a comprehensive list of NOAA networks at http://www.nosa.noaa.gov (check “Observing System Inventory” on the left side of the page). A summary table based on these websites appears in Appendix B. Other useful websites for such information include http://madis.noaa.gov and http://www.met. utah.edu/cgi-bin/databbase/mnet_no.cgi. Networks for Surface Observations: Land-Based Most commonly, “surface” measurements consist of temperature and relative humidity, wind, precipitation, and air pressure. World Meteoro- logical Organization (WMO) standards prescribe wind measurements at a height of 10 m in open areas, and pressure, temperature, and humidity at about eye level (1.5 m), but many surface measurements deviate from these standards, often for good reason. For example, routine observations are made for applications in transportation, agriculture, the power industry, air quality, and public safety, nearly all of which have specific criteria that differ from WMO standards. There are many thousands of surface sites gathering weather and related information. Based on the UCAR/NCAR and NSF surveys, approximately 500 surface networks operate in the United States and its coastal waters. Federal and state agencies as well as universities and the private sector take

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 OBSERVING SYSTEMS AND TECHNOLOGIES observations off the coasts of the lower 48 states, Alaska, and Hawaii. The federal government alone operates approximately 25,000 sites for numer- ous applications, including climate monitoring, weather forecasting, and monitoring conditions near fires; however, many sites do not report data in real time. Many state departments of transportation, working with private- sector transportation weather service providers, operate networks along highways, and at least one railroad collects observations along its tracks. States, cities, and universities maintain mesoscale networks for air quality monitoring, as part of their flash-flood warning procedures, for agriculture, research, and general weather information. A relatively recent develop- ment is urban networks for use in the case of deliberate or accidental toxic releases. Other groups that collect data are power companies, chemical pro- cessing plants, and television stations. Even private citizens have automated weather stations in their homes, some of which produce real-time data. Although surface sites are numerous, abundance doesn’t necessarily translate into utility. The sites are not evenly distributed: There are gaps in rural areas, areas with limited access, and in complex terrain. It is a major effort to keep information on multiple networks up to date, so some of net- works included in Appendix Table B.1 may have languished due to lack of funds: The numbers are always changing. On the other hand, some smaller networks may not be documented. Figure 4.1 maps the surface coverage for meteorological data over Washington State. The data are from NorthwestNet, which collects and integrates measurements made by multiple groups. On the map, one can see areas of dense and sparse data coverage. The latter areas typically have low population density or are difficult to reach due to terrain or other factors. The densely covered areas have data from multiple sources, including weather hobbyists, air pollution networks, and road networks, as well as more conventional sources. Not all of obser- vations are suitable for all applications. For example, roadside weather stations are installed along stretches of road with frequent hazardous weather, such as high winds or icing, so they are often not “representa- tive” of synoptic conditions. However, such non-representativeness on the synoptic scale is strong evidence of value at the mesoscale and is the primary driver for such extensive private and public investment in surface stations nationwide. Likewise, data from individual homes and schools may not meet the accuracy standards or exposure criteria that are required for numerical weather prediction or research. However, nearly all observations are suit- able for some purposes, such as identifying the passage of a strong front with a well-defined wind and temperature change. It is possible that some specialized networks have sites with higher quality data than one would expect, but the supporting metadata are absent.

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0 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP FIGURE 4.1 Sample map of NorthwestNet surface observations. SOURCE: Figure 4-1.eps provided courtesy of Cliff Mass, University of Washington. bitmap image While the technology for many of the “weather” variables is mature, measurement of rainfall and especially surface snowfall and precipitation type remains a challenge. Rainfall measurements from gauges are reason- ably accurate, but rainfall varies on scales smaller than the typical spacing between gauges; this problem has been alleviated to some degree by com- bining gauge and weather radar measurements. The Natural Resources Conservation Center operates the Snowpack Telemetry (Snotel) network of “snow pillows” that weigh the snow using pressure sensors to estimate the water supply. Data are routinely available daily but are accessed at higher rates for special needs. The type and amount of frozen precipitation is criti-

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 OBSERVING SYSTEMS AND TECHNOLOGIES cal to keeping the roadways passable (NRC, 2004a). Precipitation type and snowfall rate are critical information at airports. While networks that collect weather data can be dense, as illustrated in Figure 4.1, networks that collect soil moisture can seem sparse by com- parison, as illustrated in Figure 4.2. A notable exception is the Oklahoma Mesonet (see Box 4.1). Soil-moisture estimates are relevant for numerical prediction and agricultural applications, among others. Automated tech- niques exploit, for example, the variation of the dielectric constant for soils (time-domain reflectometry), neutron scatter by water in the soil (neutron probes), and measuring how a ceramic block embedded in the soil reacts to heat pulse. The dearth of soil-moisture data is currently being addressed by running land-surface models that integrate precipitation, solar radiation, etc., for a period of time. Satellites, to be discussed in the next section, have potential to supply near-surface soil-moisture data, but estimates are limited by clouds and thick vegetation. Larson et al. (2008) have suggested a new technique for tracking soil-moisture fluctuations that is independent of FIGURE 4.2 Soil-moisture networks in the United States documented at http:// 4-2.eps www.eol.ucar.edu/fadb/. NOTES: The black dots represent the Oklahoma Mesonet; bitmap image green, the Illinois State Water Survey network; yellow, ARM/CART; white, Ameri- Flux sites; red, United States Department of Agriculture/Natural Resources Conser- vation Service (USDA/NRCS) Soil Climate Analysis Network (SCAN). SOURCE: Courtesy of Scot Loehrer.

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 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP BOX 4.1 The Oklahoma Mesonet The most prominent state mesonet is the Oklahoma mesonet (Figure 4.1.1), which is used for emergency response, agriculture, severe storms forecasting, re­ search, and other applications (McPherson et al., 2007). The Oklahoma mesonet consists of 120 automated stations, with at least 1 station in each of Oklahoma’s 77 counties. At each site, the environmental variables are measured by a set of instruments located on or near a 10­m­tall tower. The Oklahoma Climatological Survey (OCS) at the University of Oklahoma receives the observations, verifies the quality of the data, and provides the data to mesonet customers. It takes only 5 minutes to make measurements available to the public. FIGURE 4.1.1 Map of the Oklahoma mesonet. 4-3.eps NOTE: Multiple agencies are involved in the individual sites. bitmap image The standard measurements include temperature and humidity (1.5 m), wind (10 m), air pressure, precipitation, incoming solar radiation, and soil temperature at 10 cm either below natural cover or bare ground. Most sites also sample air temperature at 9 m above ground, wind speed at 2 and 9 m above ground, soil moisture at 5, 25, and, 60 cm below ground, soil temperatures at 5 and 30 cm below ground under the natural sod cover, and soil temperature at 5 cm below bare ground. At 10 sites, turbulence fluxes of heat, moisture, and momentum are sampled at half­hour intervals in addition to the soil and weather variables.

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 OBSERVING SYSTEMS AND TECHNOLOGIES cloudiness; it exploits the effect of soil moisture on the reflection of Global Positioning System (GPS) radio waves. Currently, available remote sensing technologies cannot provide soil moisture below approximately 5 cm. In addition to numerical data, there is a growing network of web cameras monitoring the nation’s streets and highways. While not especially useful for numerical weather prediction, cameras are highly useful for road trans- portation, providing drivers and road managers a check on road conditions (weather, traffic flow, state of the road due to precipitation), and for moni- toring wind and weather changes in other applications, such as fighting forest fires or warning about the spread of noxious substances. Coastal Ocean Networks The Integrated Ocean Observing System (IOOS) provides real-time quality-controlled data for both the oceans and the Great Lakes, “from the global scale of ocean basins to local scales of coastal ecosystems.”1 IOOS is an end-to-end system that involves observations, data communications and management, and data-analysis and modeling, through its three inter- acting subsystems, Observation and Data Telemetry, Data Management and Communications, and Data Analysis and Modeling. These challenging tasks involve partnerships among federal and state agencies, the private sector, and universities. IOOS has a coastal component, which involves the U.S. Exclusive Economic Zone (EEZ, which extends 200 nautical miles or 370 km offshore) and the Great Lakes, and a global component. Coastal and interior waters in the United States are monitored by a diverse network of buoys operated by both the public and private sectors. These diverse measurements are being incorporated into 11 Regional Coastal Ocean Observing Systems (RCOOSs), parts of which also participate in a National Backbone of coastal observations. Most of the RCOOS buoys measure meteorological variables. NOAA’s National Data Buoy Center col- lects and quality-checks, and then distributes the data via the GTS in real time. The core variables measured by the National Backbone sites include ocean data on composition (salinity, dissolved nutrients, dissolved oxygen, chemical contaminants), life (fish species and abundance, zooplankton and phytoplankton species and abundance, waterborne pathogens), and other physical characteristics (temperature, sea level, surface waves and currents, heat flux, bathymetry and bottom character, sea ice, optical properties). The RCOOSs (Figure 4.3) are being coordinated by regional associa- tions that will in turn contribute to the evolving IOOS. There are of the order of 700 coastal observation sites in approxi- mately 50 networks. Since these sites must cover the Great Lakes and the 1 See http://www.ocean.us/what_is_ios.

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 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP 4-4.eps FIGURE 4.3 Regional Coastal Observing Systems. NOTE: LME = Large Marine Ecosystems. SOURCE: Nationalbitmap image http://www.ndbc.noaa.gov/. Data Buoy Center, U.S. coastline plus the EEZ, the coverage is sparse compared to the land surface. To illustrate, the continental United States has a surface area of 7,700,000 km2, while a conservative estimate of the EEZ is slightly less than one-third that value. As shown in Appendix Table B.1, the number of meteorological sites on land reporting in real time exceeds 10,000. Signifi- cant deficiencies exist over the coastal waters despite the fact that oceanic regions tend toward greater uniformity over larger regions. The 700 coastal ocean sites, which include Alaska and Hawaii, clearly do not resolve either the atmospheric or oceanic mesoscale (Figures 4.4 and 4.5). To counter this large difference in the density of surface observations between land and sea, satellite scatterometer winds and sea-surface tem- perature estimates provide high-quality information at high resolution over the oceans. However, some of these measurements become problematic very close to the coasts, owing to strong gradients and land-contaminated satel- lite footprints. The low density of measurements immediately offshore is a matter of considerable concern, given that 50 percent of the U.S. popula- tion lives within 50 miles of the coast and the increased complexity and importance associated with coastal airflow near large cities.

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 OBSERVING SYSTEMS AND TECHNOLOGIES FIGURE 4.4 Coastal networks along U.S. Pacific coastlines. 4-5.eps bitmap image 4-6.eps FIGURE 4.5 Sites in the Pacific Northwest. SOURCES: GAPP/NCAR Earth bitmap image Observing Laboratory, http://www.eol.ucar.edu/projects/hydrometnet. Figure from National Data Buoy Center, http://www.ndbc.noaa.gov/.

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 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP The Vertical Dimension: Surface-Based In-Situ Technologies The high cost of atmospheric measurements relative to surface mea- surements is the reason why the government bears the cost of many obser- vations taken well above ground. The one major system, the radiosonde network, launched from the ground at fixed times to collect observations at various altitudes, is described below. Radiosondes are balloon-borne instrument packages that measure temperature, relative humidity, and wind as a function of pressure, from near the surface to stratospheric altitudes (generally 10 hPa or higher). Radiosondes have been the standard for atmospheric measurements in the troposphere and lower stratosphere since World War II. The vertical resolu- tion of the measurements is good, better than 10 hPa. The roughly 80 U.S. raob sites are widely spaced, several hundred kilometers apart. Balloons are usually launched twice a day at 0000 and 1200 UTC. Thus, the sampling density of the radiosonde network is poorly matched to the large amplitudes and small scales of lower tropospheric variability. In 2000, the National Research Council (NRC) Panel on Geoscience, Environ- ment, and Resources discussed improved temperature monitoring capabili- ties from this network (NRC, 2000). It found the radiosonde network to be in decline and insufficient even for global monitoring. The trend has persisted and is not likely to reverse. In addition to the standard temperature, relative humidity, and wind radiosonde data, a subset of the U.S. network contributes to the WMO’s Global Atmosphere Watch Ozone monitoring network. With ozonosondes launched in tandem with a modified radiosonde, telemetered ozone profiles are available at approximately 100 sites globally. These data are important for stratospheric ozone, but the slow sensor response limits the applica- tion of such profiles in lower tropospheric applications unless balloons with a slower rise rate are used or inexpensive fast-response sensors are developed. Evolution of the technology associated with disposable sondes con- tinues; the dollar cost per sounding has declined, and the quality of data continues to improve. Small disposable nanosensors are currently being tested, which may make more parameters (trace gases, for example) pos- sible to measure from profiling sondes. Further development of sensor technology for carbon dioxide, ozone, and other priority pollutants is encouraged since the technology to profile these variables via remote sens- ing is not mature or sufficiently cost-effective. This need for obtaining pro- files for “chemical weather” variables described in the “Decadal Survey” (NRC, 2007a) would suggest that the current network be maintained as a source for profiling information for additional variables.

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 OBSERVING SYSTEMS AND TECHNOLOGIES The Vertical Dimension: Surface-Based Remote Sensing Technologies Passive and active remotesensing techniques have been employed in up- looking configurations at the surface. As examples, several types of sensors are described below that utilize microwave, infrared, and visible parts of the electromagnetic spectrum. Passive Sensors Microwave radiometers. The microwave spectrum from a few to 180 GHz frequency contains a wealth of information on water and hydrometeors in the atmosphere. Outside of a broad O2 absorption feature at 60 MHz, the spectrum is dominated by the pressure- and temperature-dependent spec- trum of water vapor, liquid water, and ice. Up-looking microwave spectro- radiometry has been used to retrieve profiles of temperature, water vapor, and cloud liquid water (Solheim et al., 1998). The temporal resolution of the profiles is excellent—5 minutes—but the vertical resolution decreases quickly with altitude and is coarser than that for radiosondes. Several Atmospheric Radiation Measurement/Clouds and Radia- tion Testbed (ARM/CART) sites operate microwave radiometer profilers (MWRP) that measure downwelling microwave radiation in two frequency ranges: 22-30 GHz and 51-59 GHz (Liljegren, 2007). The former range contains a weakly absorbing water vapor resonance band; measurements in five channels are used to infer water vapor profiles. The latter frequency range lies on one shoulder of the broad oxygen absorption band mentioned above. Measurements in seven channels are used to infer temperature pro- files. The profiles, along with cloud liquid water path are derived at roughly 5-minute intervals.2 GPS Integrated Precipitable Water. An analysis of GPS signal delays that result from the radio refractive index profile leads to estimates of (columnar) Integrated Precipitable Water (IPW; Bevis et al., 1992). IPW indicates the depth of liquid water that would result if all water vapor in a vertical col- umn were condensed. Except during maneuvers of GPS satellites, IPW esti- mates are stable, accurate except during intense rainfall, and do not need calibration. GPS/IPW measurements are thus used as a reference standard to calibrate rawinsondes There are 300 to 400 ground-based receivers in the United States that report hourly (Figure 4.6). Most of the GPS receiver sites in the United States were in place before it was recognized that water vapor, a nuisance for geodetic applications, was producing a useful sig- nal for atmospheric applications. Both IPW and slant-path water vapor 2 For more information, see http://www.arm.gov/instruments/instrument.php?id=mwrp.

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 OBSERVING SYSTEMS AND TECHNOLOGIES EScan Panels FIGURE 4.11 Conceptual design of microwave radar antenna panels mounted on 4-12.eps the corner of a building (D. McLaughlin, UMass-Amherst/CASA). bitmap image with vector arrows & type These measurement issues are being studied in testbeds such as the Helsinki study discussed earlier, the Pentagon Shield program (Warner et al., 2007), and in urban field experiments.13 Lessons learned here and elsewhere need to be factored into the urban component of the national mesoscale observing network. Urban networks provide unique challenges such as the need for three-dimensional measurements at dense scales and communications. In addition, the sensors cannot be deployed easily, and dealing with building architectural codes, real estate costs, and societal acceptance becomes very important. These challenges are already being addressed in other disciplines such as cell phone antenna deployments and in weather via a planned CASA project. Figure 4.11 shows a conceptual deployment scenario of low-cost microwave radar sensors in an urban environment where the radar antenna panels are attached to the edges of the taller buildings. The electronic-scanning sensors merge seamlessly with the background and have no moving parts (McLaughlin et al., 2007). Also shown in the figure are the communication antennas. 13 E.g.,the December 2007 special issue of Journal of Applied Meteorology and Climatology on Joint Urban 2003.

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0 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP Mountains Mountains affect the weather by initiating convection and deep snow- falls, focusing water or wind into narrow valleys, and generating turbulence aloft and severe (≥100 mph) windstorms at the surface on their lee side. They present the danger of slick roads, high winds, poor visibility, and avalanches, rockfalls, and mudslides. They also cause significant down- stream effects in weather and stream flow, the latter causing water resource management challenges up to 1000 km. Knowing the water content in the mountain snowpack is important to water managers and their customers. At the same time, mountains present special observational challenges. The weather conditions—temperature, winds, and precipitation—around moun- tains are so variable that the small number of measurement sites cannot capture the complexity. Radars and data transmission are both limited by blockage, and traditional flux measurements in complex terrain are difficult to interpret correctly. Measurement sites are difficult to install and main- tain. Forest fires present a particular challenge, because wind and moisture measurements in remote terrain are critical. Current observations in the mountains can be characterized as sam- pling sparse but sampling smart. Decades of experience have determined where Snowpack Telemetry measurements are sited, and methods are being developed to incorporate satellite information. Likewise, state DOTs know the weather-vulnerable portions of major roadways and the locations of site stations. The dangerous stretches of major roads are often equipped with web-cams to help travelers. Larger metropolitan areas have instrumented the watershed upstream to alert them of the possibility of flash floods. At the mesoscale and smaller scales, challenges remain—particularly with respect to convective precipitation and wildfires. Because mountains block radar beams, many areas are without coverage. We have the tools to begin to address this, for example, gap-filling radars and lidars that operate in adaptive-collaborative modes with rain gauges, stream gauges, and satellites. As cell-phone towers proliferate, these offer platforms not only for radars and lidars but also for communication of data from remote sites. High-resolution numerical models need to be part of the observational mix. Mountains provide strong forcing, making precipitation and wind pat- terns more predictable. The combination of good upstream conditions with some boundary-layer, surface, and radar data that the model can assimilate has the potential to provide the three-dimensional picture needed by fire meteorologists, snowpack and runoff analysts, and flash-flood or downslope windstorm forecasters. Thus the components needed for a “Mountain Net” are included in our architecture as proposed, the primary challenge being to address the severe under-sampling problem.

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 OBSERVING SYSTEMS AND TECHNOLOGIES The Coastal Zone Coastal regions have both natural and made-made features that create complex spatial and temporal variability in weather and sea-state condi- tions, much of which may go undetected. For example, prevailing offshore flow could be replaced by a sea breeze along one stretch of coast while adjacent stretches remain offshore, affecting forecasts of convective initia- tion and energy demand. Varying winds also can affect the destination of a hazardous chemical leak and the towing of ships and barges in a harbor. Coastal fronts can move onshore ahead of winter storms, affecting where different types of precipitation fall (solid, partly frozen, and unfrozen). Unmeasured air-sea interactions occur offshore, creating moisture and sta- bility conditions that are ripe for severe weather outbreaks in the return flow regions. The vulnerability of coastal zones is increasing annually, as more coastal regions become large population centers. Coastal counties are growing three times faster than other U.S. counties, and coastal and marine waters are an annual tourist destination for 90 million Americans. In addition, many coastal regions have significant topography, suggesting that their special observing and network needs are congruent with those of the Urban Net and Mountain Net described above. Additional requirements, though, exist for offshore weather and sea-state data, including profiling of winds, temperature and moisture above the surface, and temperature, current, and salinity at and below the surface. Thus the U.S. mesoscale network of networks should include a suite of additional buoys and land stations, and remote sensing capabilities extending 100-200 km from the coast. The Planetary Boundary Layer Challenge One of the most difficult to measure and yet one of the most important parameters is the height of the daytime and nighttime planetary boundary layer (PBL). Driven in the daytime by heating of the surface and convec- tion and driven at night by winds and infrared radiative cooling of the surface, the PBL height is critically important in forecasting constituent concentrations in numerical models (since this is the height of the box into which constituents mix and react). It is now believed that the imprecision with which the PBL height is known is a major source of uncertainty in the predictive capability of current numerical chemical forecast models. It really is astounding after nearly sixty years of remote sensing observations in meteorology that such an important meteorological variable is not mea- sured with regularity throughout its diurnal cycle. The only area of relative strength relates to winds from ultra-high fre- quency (UHF) and very high frequency (VHF) wind profiles when combined

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 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP with AMDAR and TAMDAR observations. This combination thoroughly captures the synoptic scale, and also some of the larger mesoscale circula- tions. However, the characteristic spacing of radar wind profilers is too large, often missing medium-sized mesoscale circulations that spawn dis- ruptive and severe weather. The number of commercial airline observations has a large-amplitude diurnal cycle, which deprives the composite observing system of needed data for ~8 hours per day and leaves the system vulnerable during large storms (or terrorist attacks) when there are wholesale flight cancellations. The national condition for thermodynamic, trace-gas, and aerosol profiling is one of significant inadequacy to address mesoscale predic- tion needs. A major improvement in thermodynamic profiling is needed. Radiosonde sites are several hundred kilometers apart and only address the synoptic scale. The vertically resolved water vapor field, especially in the BOX 4.2 An Example Core Observing Site to Address the Planetary Boundary Layer Challenge For the last 20 years, Howard University of Washington, D.C., has operated a research station at Beltsville, Maryland. Since 2001, when a NOAA Center for A tmospheric Sciences was founded at Howard as part of a NOAA Cooperative Agreement, the Beltsville facility has grown into a high­level core mesoscale observation site. A tall tower to measure CO2 fluxes was installed by the Univer­ sity of Virginia, and a Raman lidar was constructed in cooperation with NASA. Radiosonde observations and ozonesonde releases have been carried out for validation of NASA’s Tropospheric Emission Spectrometer and Ozone Monitoring Instrument. NOAA has contributed many of the radiosondes as part of its modern­ ization program and the Pennsylvania State University has contributed the ozone soundings as part of the NASA INTEX Ozonesonde Network Study (IONS). Baron Meteorological Services has contributed a weather radar to the site. The EPA and the State of Maryland have contributed a radar wind profiler and a ground­based chemical monitoring capability (PM, O3, NOx) to the site. The U.S. Department of Agriculture (USDA) has contributed a shadowband radiometer to the site to mea­ sure aerosol optical depth, and NASA has contributed an AERONET site. Surface energy fluxes and subsurface temperature and moisture are measured at the site. Sonic anemometers measure turbulence at the site. Surface solar radiation fluxes (as in the Baseline Surface Radiation Network, NOAA) are being routinely made at the site. Arguably, Beltsville is the type of station which can be expected to arise from the efforts recommended in this report. Multiple agencies, with disparate needs, can contribute to a single site and leverage resources. It is interesting that this site was founded and is operated by a Minority Serving Institution, which clearly

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 OBSERVING SYSTEMS AND TECHNOLOGIES lowest 1 km, is most critical, being essential for improved prediction of all high-impact weather. The needs of chemical weather predication are on a similar plane, requiring national-scale coverage of major pollutant species including aerosols, thereby enabling urban and regional pollutant forecasts. Some research stations have been established that include many of the core, ground-based remote sensing systems that supply these types of observa- tions (see Box 4.2). Yet, there is no national coverage of sufficient scope to address the planetary boundary-layer challenge. Recommendation: As a high infrastructure priority, federal agencies and their partners should deploy lidars and radio frequency profilers nationally at approximately 400 sites to continually monitor lower trospospheric conditions. could not have accomplished a project of this scope without contributions from the federal and private sector. FIGURE 4.2.1 The Howard University Beltsville site showing the instrumentation component and training. SOURCE: Whiteman et al. (2006). Figure 4-13 now Figure 4.2.1

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 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP Wind, diurnal boundary-layer structure, and water vapor profiles are the highest priority for a network, the sites for which should have a char- acteristic spacing of approximately 150 km but could vary between 50 and 200 km based on regional considerations such as those just discussed for urban areas, mountains, and coastal zones. Such observations, while not mesoscale resolving, are essential to improved performance by high- resolution numerical weather prediction models and chemical weather pre- diction at the mesoscale. Through advanced data assimilation techniques, data from these 400 sites, when used in combination with geostationary satellite measurements, GPS constellation “wet delay” measurements, and commercial aviation soundings, could effectively fill many of the critical gaps in the national observing system. Any sensors that measure air chemistry or aerosol properties above ground should be located with or near the meteorological profilers. Most chemical models use vertical grid spacing of 100-200 m in the lowest 3 km (finer near the surface), but chemical and aerosol measurements at even two or three levels within the lowest 3 km would be a marked improve- ment over present capability. Above 3 km, satellite measurements become increasingly effective at greater altitudes. Some of the measurements could be from towers, others from remote sensing by lidars or differential optical absorption spectroscopy.14 Challenges for Space-Based Observations The “Decadal Survey” (NRC, 2007a) has identified a path forward for the next generation Earth observational satellite system for the United States. Measurements that are relevant to mesoscale applications include soil moisture using the L-band, soil composition and vegetation character- ization from a hyperspectral spectrometer, columns of atmospheric trace gases to high horizontal resolution, aerosol and cloud profiles, land-surface topography, temperature and humidity soundings, tropospheric winds that don’t depend on feature tracking (from Doppler lidar), and subsurface water. The United States was expected to play a leading role in developing many of the space-based capabilities mentioned above, which in turn would have contributed substantially to mesoscale observations of the Earth, its ocean, and atmosphere. However, as pointed out in the “Decadal Survey’s” preliminary report (NRC, 2005), “The national system of environmental satellites is at risk of collapse.” Further deterioration in the U.S. plans led to an even more pessimistic assessment in the final report (NRC, 2007a): “Those concerns have greatly increased in the period since the interim 14 Described at http://www.atmos.ucla.edu/~jochen/research/doas/DOAS.html.

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 OBSERVING SYSTEMS AND TECHNOLOGIES report was issued, because NASA had cancelled additional missions, and NOAA’s polar and geostationary satellite programs have suffered major declines in planned capabilities.” The “Decadal Survey” links observations in solid earth, water, weather, climate, health, and ecosystem science areas to meeting societal challenges regarding water, food, and energy security, early warnings of hazardous weather, ecosystems services, and improvements in public health and envi- ronmental quality. Specific recommendations are made to both NOAA and NASA concerning GOES-R hyperspectral sounding capability, the elimina- tion of climate monitoring sensors in NPOESS, deletion of the Conical image: Rain rate Scanning Microwave Imager/Sounder, and removal of key meteorological sensors from the early-morning orbiting satellite (0530 LST Equator crossing), and a series of other missions relevant to this study. From a mesoscale perspective, the most disturbing finding was the elimination of hyperspectral infrared temperature and water vapor sounding capability from geostationary altitude. Support for the “Decadal Survey” conclusions is widespread. The National Weather Association, primarily representing operational forecasters, strongly advocates for “inclusion of a capable high spectral resolution atmospheric infrared sounder on the next generation of GOES-R series of spacecraft.” The American Meteorological Society’s Committee on Satellite Meteorology and Oceanography issued a consensus statement “On the Importance of Deploying a GEO Advanced Sounder without Delay.” Further, a recent NRC workshop on “Ensuring the Climate Measurements from NPOESS and GOES-R” found strong advocacy for geostationary hyperspectral sounding, as did the coincident WMO “Workshop on the Re-design and Optimization of the Space-based Global Observing System.” In conclusion, satellites will play an increasingly important role in mesoscale observation, but limitations of frequency, resolution, and preci- sion near the surface mean that satellite profiles will not replace ground- based observations in the near future. Finding: It is a national imperative to sustain and improve operational geostationary satellite observations as a critical adjunct to the surface- based mesoscale network. Observations from geostationary orbit are unique and inherently Mesoscale, owing to the high rate of time domain sampling and excellent horizontal resolution. Visible and infrared imagery are invaluable to severe weather forecasts and warnings. Estimates of assimilation of radiances, cloud-drift winds, and free troposphere water vapor enable the initializa- tion of global and mesoscale models. Over land, the vertical resolution of water vapor and temperature data normally obtained from geostationary

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 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP orbit is not independently sufficient but absolutely essential. Continuous cloud layers prevent infrared soundings below cloud top; however, micro- wave imaging array technology (Lambrigtsen et al., 2006) offers a useful lower-resolution alternative under cloud cover. On the other hand, it is impractical to establish a stand-alone surface-based network with adequate horizontal resolution throughout the depth of atmospheric boundary layer and lower troposphere. Soundings obtained from ground-based profilers (including aircraft of opportunity) and geostationary satellites complement each other optimally, each of their strengths compensating for the other’s relative weaknesses. Recommendation: As a high satellite instrument priority, NASA and NOAA, in cooperation with foreign space agencies, should seek to improve the quality of geostationary satellite water vapor and tempera- ture soundings within continental atmospheric boundary layers. Infrared hyperspectral soundings and soundings from microwave syn- thetic thinned aperture arrays, each in geostationary orbit, offer unique opportunities to improve mesoscale prediction. While potentially costly, the benefits from improved geostationary soundings would be large, likely enabling more skillful forecasts of convective rainfall and attendant severe weather and flooding. The geostationary platform is unique among satel- lites, offering the sampling frequency required in this application. GLOBAL CONTEXT AND INFRASTRUCTURE Much of the data collected are managed globally through the evolving Global Observing System (GOS),15 which is coordinated by the WMO’s World Weather Watch. Data from the the GOS are used for a variety of applications that span time scales from nowcasting to climate, and include land, ocean, atmosphere, and ecological applications. The GOS provides valuable examples of how a variety of user needs and requirements for various applications are addressed as well as how important areas such as data exchange are handled. GOS is composed of two major subsystems, space- and ground-based. Each may be thought of as a system of systems. The ground-based sub- system provides observations from surface observing stations on land, upper air observing stations, ships at sea, moored and drifting buoys, and aircraft. While some of these systems are owned and operated by WMO members, the aircraft system is operated by various airlines and coordinated 15 Detailedinformation on the observing system component of the GOS can be found at http://www.wmo.int/pages/prog/www/OSY/gos-components.html.

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 OBSERVING SYSTEMS AND TECHNOLOGIES within WMO through the AMDAR System Panel.16 Some of the observing systems are coordinated with other international organizations (mainly the Global Ocean Observing System [GOOS], Global Terrestrial Observing System [GTOS], and Global Climate Observing System [GCOS]). Data from the space-based subsystem of the GOS17 are provided by operational satellites in low-Earth and geostationary orbits and selected research satel- lites in low-Earth orbits. Those satellites are operated by various countries or consortia of countries with WMO activities through mechanisms such as Coordination Group for Meteorological Satellites (CGMS) and Committee of Earth Observing Satellites (CEOS). The GOS Ground-Based Sub-System Over land, a relatively sparse network of nearly 11,000 stations delivers observations of conventional meteorological parameters. About 4000 of those stations comprise the Regional Basic Synoptic Networks, whose data are exchanged globally in real time in compliance with WMO Regulation 40.18 Over the oceans, ships and moored and drifting buoys also pro- vide information for GOS. On any given day, about 2800 ships and 900 drifting buoys provide near-surface meteorological parameters as well as sea-surface temperature.19 Solar radiation observations, surface lightning network observations, and tide-gauge measurements are also provided via the GOS, but in limited numbers. Upper air observations are provided mainly by land-based radiosonde and aircraft data, with a limited number of observations from ground-based wind profilers and radiosonde releases from ships at sea. Close to 900 land-based upper air stations provide radiosonde sound- ings to the GOS twice a day: at 1200 and 0000 UTC. The AMDAR system provides observations of temperature and wind from commercial aircraft at flight level as well as soundings during ascent and descent. As noted by the 2007 WMO Expert Team on the Evolution of the GOS,20 the global AMDAR program exchanges between 220,000 and 250,000 observations 16 “The goal of the Panel shall be to enhance the upper-air component of the Observing System of the World Weather Watch through cooperation among Members in the acquisition, exchange and quality control of meteorological observations from aircraft using automated reporting systems.” The AMDR Panel’s goals are found at http://www.wmo.int/amdar/Goal_ TOR.html. 17 Detailed information about the space-based component of the GOS can be obtained from the WMO Space Program web site: http://www.wmo.int/pages/prog/sat/index_en.html. 18 Regulation 40 addresses the free exchange over the Global Telecommunications System of 6-hourly RBSN and all upper air, ocean, and satellite data (some Members provide surface observations on an hourly basis). 19 See http://www.wmo.int/pages/prog/www/OSY/gos-components.html. 20 See http://www.wmo.int/pages/prog/www/OSY/Reports/ET-EGOS-_Final-Report.pdf.

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 OBSERVING WEATHER AND CLIMATE FROM THE GROUND UP per day over the WMO Global Telecommunications System. Most AMDAR observations are in the Northern Hemisphere, and programs like EUCOS (EUMETNET Composite Observing System) are working to optimize AMDAR ascent and descent data for use by EUCOS member countries. For example in 2006, EUMETNET-AMDAR provided approximately 750 soundings per day.21 The RBSN observing stations and conventional upper air network do not all report on a routine basis, with the performance varying greatly by WMO region.22 Reports from stations over the United States are very reliable. In addition to the GOS, specialized observing networks such as the Global Atmospheric Watch for chemistry and the World Hydrological Cycle Observing System provide data that may or may not be in real time. Approximately one-fourth of the RBSN stations make up the Global Cli- mate Observing System (GCOS) Surface Network,23 and approximately 20 percent of the upper air sites make up the GCOS Upper Air Network. As with the GOS, performance of the GCOS sub-set of the GOS is not at 100 percent. The GOS Space-Based Subsystem The space-based subsystem of the GOS embraces the concept of a com- posite observing system with research and operational satellite data used in synergy.24 Data are provided by both operational satellites and low-Earth orbit research satellites. Examples of the research products are hyper- spectral sounding data from AIRS, altimetry measurements from JASON, precipitation measurements from TRMM, and sea-surface winds from ENVISAT. Much of the satellite data flowing into the GOS are used for routine analysis, nowcasting, and forecasting applications at the National Meteorological and Hydrological Services (NMHS) across the globe. Global NWP centers use the data for a variety of forecast guidance products. How the GOS is expected to evolve over the coming decades was recently discussed in WMO Technical Document No. 1267, “Implemen- tation Plan for Evolution of Space and Ground-based Subsystems of the 21 See http://www.wmo.ch/pages/prog/www/OSY/Meetings/ET-EGOS_Geneva00/Doc-. doc. 22 GOS performance is routinely monitored by major NWP centers (see for example http:// www.ecmwf.int/products/forecasts/d/charts/monitoring/coverage/), however, the WMO for- mally evaluates GOS performance during special observing periods each year and the per- formance for various regions can be accessed from the reports of the Commission on Basic Systems reports on the following web site: http://www.wmo.int/pages/prog/www/CBS-Reports/ CBSsession-index.html. 23 See http://www.wmo.int/pages/prog/gcos/documents/GSN_Stations_by_Region.pdf. 24 See http://www.wmo.int/pages/prog/www/OSY/gos-components.html.

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 OBSERVING SYSTEMS AND TECHNOLOGIES GOS,”25 which makes specific recommendations concerning the evolution of the space-based and surface-based subsystem of the GOS. Those recom- mendations were based on guidance from the Rolling Requirements Review process26 as well as observing system experiments, and observing system simulation experiments performed by various NWP centers. Results from these experiments are presented at WMO-sponsored workshops, such as the Fourth WMO Workshop on the Impact of Various Observing systems on NWP. Because of the long lead times for satellite systems, plans for the evolution of the space-based portion of the GOS have been based mainly on the long-term planning of both operational and research satellite operators. Future research missions will continue to contribute to the space-based component of the GOS while influencing its evolution. Those planned research missions include investigations of atmo- spheric chemistry and trace gases, the Earth’s gravity field, soil moisture and ocean salinity, atmospheric winds using lidar, disaster and environmental monitoring, integrated atmospheric column water vapor, cloud ice content, cloud droplet properties and distribution, aerosols, and polar ice and snow water equivalent. Instrumentation under development to accomplish these measurements include space-borne lidar, high-resolution and hyperspectral imaging and sounding instrumentation, active and passive microwave sen- sors, cloud resolving radars, and L-band radars. 25 Information on WMO activities with respect to redesign of the GOS can be found at http://www.wmo.int/pages/prog/www/OSY/GOS-redesign.html, with a link to WMO Technical Document 1267 at http://www.wmo.int/pages/prog/www/OSY/Documentation/ Impl-Plan-GOS_Sept00.pdf. 26 The Rolling Requirements Review (RRR) process is used to determine how well the GOS is meeting WMO user requirements in a variety of applications area. The RRR procedure consists of four steps: review of user requirements for observations; assessment of the capabili- ties of existing and planned observing systems; critical review (gap analysis), comparing the requirements with system capabilities, in terms of present and planned networks; and state- ment of guidance, which lists conclusions and identifies priorities for action. This information is made available to all users (WMO 2007).