Summary

According to the United Nations, three out of five people will be living in cities worldwide by the year 2030. The United States continues to experience urbanization with its vast urban corridors on the east and west coasts. Although urban weather is driven by large synoptic and meso-scale features, weather events unique to the urban environment arise from the characteristics of the typical urban setting, such as large areas covered by buildings of a variety of heights; paved streets and parking areas; means to supply electricity, natural gas, water, and raw materials; and generation of waste heat and materials. These all combine in various ways to create a very distinct local weather environment characterized by meso- and microscale urban heat island effects, urban flooding, changes in precipitation patterns, elevated concentration levels for gaseous pollutants and aerosols, and street canyon winds.

Given the high density of people and their dependence on infrastructure, urban areas are especially vulnerable to weather-related events like severe thunderstorms, heat and cold waves, winter storms with heavy ice and snow, air pollution, and the rapid spread of airborne disease. To better prepare and respond to these events, the field of urban meteorology has grown from simple observations and forecasts of the general weather for cities and surrounding metropolitan areas to scientific and technological advances that allow us to predict a wide set of environmental parameters at relatively fine temporal and spatial scales. As these capabilities have improved, the uses for urban weather information and its value to decision makers have increased.

Continued advances in these capabilities are needed, but most assessments of research and development priorities have come from discussions within the scientific research community. There is a need for more two-way interactions between urban meteorologists and end user communities, such as emergency managers, public utilities, and urban planners, to better



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Summary According to the United Nations, three out of five people will be liv- ing in cities worldwide by the year 2030. The United States continues to experience urbanization with its vast urban corridors on the east and west coasts. Although urban weather is driven by large synoptic and meso-scale features, weather events unique to the urban environment arise from the characteristics of the typical urban setting, such as large areas covered by buildings of a variety of heights; paved streets and parking areas; means to supply electricity, natural gas, water, and raw materials; and generation of waste heat and materials. These all combine in various ways to create a very distinct local weather environment characterized by meso- and microscale urban heat island effects, urban flooding, changes in precipitation patterns, elevated concentration levels for gaseous pollutants and aerosols, and street canyon winds. Given the high density of people and their dependence on infrastructure, urban areas are especially vulnerable to weather-related events like severe thunderstorms, heat and cold waves, winter storms with heavy ice and snow, air pollution, and the rapid spread of airborne disease. To better prepare and respond to these events, the field of urban meteorology has grown from simple observations and forecasts of the general weather for cities and sur- rounding metropolitan areas to scientific and technological advances that allow us to predict a wide set of environmental parameters at relatively fine temporal and spatial scales. As these capabilities have improved, the uses for urban weather information and its value to decision makers have increased. Continued advances in these capabilities are needed, but most assess- ments of research and development priorities have come from discussions within the scientific research community. There is a need for more two- way interactions between urban meteorologists and end user communities, such as emergency managers, public utilities, and urban planners, to better 1

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2 2 URBAN METEOROLOGY understand user information needs and the products and services available from urban meteorologists. This report, based largely on the information provided at a Board on Atmospheric Sciences and Climate community workshop, describes the needs for end user communities, focusing in particular on needs that are not being met by current urban-level forecasting and monitoring. This report also describes current and emerging meteorological forecasting and monitoring capabilities that have had and will likely have the most impact on urban areas, some of which are not being utilized by the relevant end user com- munities (see full Statement of Task in Appendix D). The Committee concludes that users of urban meteorological informa- tion need high-quality information available in a wide variety of formats that foster its use, within time constraints set by users’ decision processes. By advancing the science and technology related to urban meteorology with input from key end user communities, urban meteorologists can better meet the needs of diverse end users. To continue the advancement within the field of urban meteorology, there are both short-term needs, which might be addressed with small investments but promise large, quick returns, as well as future challenges that could require significant efforts and investments (Boxes S.1 and S.2). END USER NEEDS End users of urban meteorology information have demonstrated impor- tant needs for urban meteorological observations and analyses. They use urban meteorological information either directly or indirectly for planning and decision-making. For example, national security officials may utilize urban dispersion models to plan for an accidental or terrorist release of BOX S.1 Short-Term Needs 1. Maximum access to observational data in different categories from diverse sources 2. Regularly updated metadata of the urban observations using standardized urban protocols 3. Continued and expanded international urban model intercomparisons over urban areas 4. Development and application of best practices to strengthen the dialog between meteorologists and end user communities

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SUMMARY 3 BOX S.2 Challenges 1. How can new capabilities for urban observations be developed and imple- mented, particularly using the network of personal digital assistants (PDAs; includ- ing smartphones) and vehicles and new technologies for measurements in the whole planetary boundary layer? 2. How can weather and climate models be urbanized and how can urban areas be included in the model prediction evaluation and validation metrics? 3. How will the capability for integrated urban meteorology-decision support systems be developed? chemical or radioactive material in an urban area. Another example is how the construction sector uses historical data regarding the occurrence of heavy rain, snow, and icing to prepare for days in which no outdoor work can take place due to such precipitation events. A clear mechanism to help the urban meteorological community better identify user groups, reach out to them, and begin an ongoing dialogue to assess and better meet their needs has yet to be identified. It is important to recognize that there are multiple types of urban me- teorological phenomena that have impacts on different types of users with different types of needs. End users are heterogeneous and cover a vast spectrum of job roles, goals, needs, experience, and understanding. Their needs span a wide range of accurate urban meteorological information, from high frequency and low frequency events that may occur over the short and the long term. To complicate long-term planning, both types of events will be affected by climate change. Acknowledging and understanding this heterogeneity is important for the urban meteorological community to better understand, interact with, and meet the needs of end users. As many participants noted at the workshop, there are numerous end user needs that are not sufficiently being met (Table S.1). In many cases, the urban meteorological community is simply unaware of the precise data and information needs of various user groups. If such information is not pro- vided, there is a risk that disparate groups of end users will start generating (or more fully develop) their own data streams, not necessarily following best practices in data collection, analysis, or interpretation, and produce not only redundant but also inconsistent information. More importantly, if the urban meteorological community does not provide required informa- tion, end users’ needs will not be met, reducing the effectiveness of their decision-making.

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4 4 URBAN METEOROLOGY TABLE S.1 Sampling of Specific Unmet End User Data Needs Sector Examples of Unmet Data Needs Flood Control • Rainfall and snowmelt runoff and storm water datasets (municipal and public • Urban flooding and/or overloading of combined storm water/ safety officials) sewage systems due to localized precipitation and ability/ inability of urban pervious surfaces to store water • Atmospheric river (i.e., narrow corridors of concentrated moisture in the atmosphere that when striking land can produce hazardous storms) information Electric Power • Air temperature for assessing energy demands and related (power producers, grid loads on the grid operators, local utilities) • Wind and solar radiation data for renewable energy assessments Insurance/Reinsurance • Accurate and timely forecasting of extreme events (company officials) • Surface roughness, overland decay, and wind speed Business • Solar radiation, precipitation, and air quality data for agriculture (company officials, public (e.g. for agricultural regions near and/or impacted by cities) and private service • Canyon-level wind flow (e.g. for construction sector) providers) Urban Design • Vegetations stress index for cities/optimization (architects, urban planners, • Urban air quality municipal officials) • Assessment of urban heat island mitigation measures such as green roofs and tree planting campaigns • Development of climate change mitigation and adaptation strategies of cities and regions, • More dense array of first order meteorological stations in and around urban areas • Improved methods for assessing the extent to which rural meteorological stations are subject to the impacts of local land use change Transportation • Canyon-level wind flow Management • Precipitation and its form (i.e., rain, freezing rain, sleet or snow) (officials in departments • Representativeness of surface observations overseeing highways, • High spatial resolution forecasts (e.g. roadway scale) railroads, airports, harbors, • Road surface temperatures and rivers)

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SUMMARY 5 TABLE S.1 Continued Sector Examples of Unmet Data Needs Public Health • Solar radiation, wind, humidity and air temperature at (health department officials, matching scales for health (e.g., heat indices) environmental protection • Consistent urban heat island baseline datasets for agency officials, air quality vulnerability/risk assessments (standardized methods and data) management districts, • Spatially explicit datasets that characterize the urban public safety officials, heat island (i.e., further than just surface air temperature emergency managers) measurements; surface skin temperature, air temperature, humidity, wind and radiation data may provide a more comprehensive assessment of “heat”) • Heat and cold wave and physical stress forecasts with temporal and spatial resolution at city scale • Street-level air quality • Extreme precipitation event forecasts • Extreme localized heat/cold advisories, disease vector, and air quality advisories Security • Higher temporal, vertical, and horizontal spatial resolution (public safety and security data (e.g. urban boundary layer structure and mixing layer officials) heights, vertical profiles of winds, turbulence, temperature of particular importance to dispersion applications) • Dual-use leveraging of data from other applications (e.g., radar-derived precipitation calibrated with rain-gauge data for flood predictions) • Regularly updated urban data (e.g. land-use characteristics, building footprint data) Emergency Response • Street-level detailed flood information (public and industrial safety • High spatial and temporal resolution wind, temperature, officials) and moisture data in and above the urban canopy Although there are many unmet needs, there are some success stories where the needs of the end user are largely met (see Chapter 2 for case studies). A common theme present in these examples is that direct commu- nication throughout the process is key to successful coordination. In some cases, the end users worked directly with researchers to develop tools that are tailored to their specific needs. The hallmarks of effective communica- tion are a better understanding of capabilities on both sides, the successful translation of user needs to the research community and research products to the end user community through collaboration and a better understanding in the research community of institutional constraints that end users face.

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6 6 URBAN METEOROLOGY Coordinated Efforts Many workshop participants noted that it would be useful if there were more coordinated data sharing strategies among various agencies and training of various end user communities on how to utilize existing data. Coordinated data formats, available metadata, quality assurance, and novel dissemination strategies are increasingly essential as urban meteorological data and model output cross the research-to-operations “valley.” Education Several discussions at the workshop focused on the underrepresenta- tion of urban meteorological, climatological, and field coursework and training at all educational levels. Advanced approaches to training and workforce development will be critical in producing the next generation of urban meteorological models and applications. Strengthened training would likely give the community a better understanding of what is available and unavailable in urban meteorology and how and what urban meteorology information is used in decision-making by end users. Ultimately, it is essential that the urban meteorological community under tands what data are needed by end users that are not currently pro- s duced and/or not conveyed in usable ways to end users. OBSERVING, MODELING, AND FORECASTING IN THE URBAN ENVIRONMENT Meteorological observations and forecasting are complex in cities be- cause of the high spatial variability, unique physical characteristics of the urban canopy and its impacts on various processes, and challenges with model initialization. Although cities still pose a number of difficult chal- lenges for both the scientific and end user stakeholder communities that are not adequately addressed by current meteorological observation, forecast- ing, and information dissemination technologies, there has been significant progress in the past 10 to 20 years (Table S.2). Current Capacity One such advancement is ground based remote sensing capabilities, such as scanning radars (e.g., Next-Generation Radar, NEXRAD), radar pro- filers and sodar, lidar (light detection and ranging), and radiometric profilers.

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SUMMARY 7 TABLE S.2 Advances in Urban Forecasting and Monitoring Techniques Mechanism for Forecasting and Monitoring Advances in Technology Monitoring and Observations Urban campaigns Urban observation networks Ground-based remote sensing • Scanning radars • Radar profilers and sodar • Lidar • Radiometric profilers • Lightning detection Airborne/spaceborne remote sensing • Urban land cover • Thermal imaging and UHIs • Aerosols • Hydrometeorological parameters Modeling Systems Urbanization of numerical weather prediction models Atmospheric dispersion and urban air quality models Hydrological models Coastal storm surge-inundation models Urban observation networks with in situ sensors within the urban canopy have been deployed in several cities. However, the design of networks that capture the spatial variability within the urban canopy layer and integrate in situ observations with remote sensing instruments to obtain a three- dimensional (3-d) picture of the urban atmosphere still poses challenges. A need for urban testbeds for testing of observation and modeling strategies and development of end user applications still exists. Urban observation campaigns have provided sustained, accessible data sets for a specific range of environmental conditions and promote initiatives for future urban studies. Advancements related to aircraft and satellite remote sensing have been useful in assessing the extent and magnitude of the urban heat island identifying urban rain- and snowfall anomalies, and determining air quality. Additionally, airborne and spaceborne data help properly characterize urban land cover and morphology, which is important for coupled meteorological systems. Urban treatments have been explicitly included in some numerical weather prediction models. As the model grid size decreases to a few kilometers, cities can cover a significant number of grid cells in the model- ing domain. There are several methods to “urbanize” Numerical Weather

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8 8 URBAN METEOROLOGY Prediction (NWP) models: use of empirical models, implementation of urban canopy parameterization schemes of varying complexity into climate and operations models, and coupling of microscale computational fluid dynam- ics models with NWP models. Atmospheric dispersion and urban air quality models have also been advanced for improving the decision-making capabilities in these areas. Such models are essential in protecting people, the natural environment, and urban infrastructures from negative impacts of elevated air pollution in cities. Several approaches are now available for neighborhood to street-scale dispersion modeling (1-5m), which is especially important to emergency managers who require fast and reliable information on dispersion of toxic airborne contaminants in the event of an accidental or terrorist release. Improved hydrologic models, both conceptual models and physically- based models, provide a methodology for assessing surface water hydrology and water balance changes for an array of surfaces. This is important for cities and their impervious surfaces which modify surface runoff, evapo- transpiration, infiltration, and groundwater recharge. Given that a majority of the U.S. population lives within coastal coun- ties, coastal storm surge models are increasingly important. Such models use population, land cover, elevation, climatological, and oceanographic data- sets to model and animate sea level rise and surges associated with storms. Emerging Technologies There are also several emerging technologies in meteorological fore- casting and monitoring (Table S.3). First, coupled modeling systems are beginning to be used for major U.S. cities. For example, coupling building energy models with urban canopy parameterizations provides tools to study feedbacks between building energy use and urban climate. In general, there has also been significant progress in data assimilation and probabilistic forecasting techniques over the last decade, but applications for urban areas are still largely unknown. Probabilistic information can aid users in their decision making, but end users are not always trained on how to properly interpret model-generated probabilistic information, and model- ers have difficulty in communicating probabilistic information to end users. There are a number of advanced sensing techniques for the atmospheric boundary layer, such as differential absorption lidars, Doppler-lidar systems, and commercial aircraft-based measurement technologies. These emerging technologies have resulted in improvements of weather forecasts, but there is a need for better observations in the atmospheric boundary layer.

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SUMMARY 9 TABLE S.3 Emerging Technologies in Meteorological Forecasting and Monitoring Mechanism for Forecasting and Monitoring Emerging Technologies Modeling Systems Coupling modeling systems • Use of high resolution building data sets in urban weather and climate models • Coupling of atmospheric models from the global down to urban scales • Advanced exposure assessments • Application of weather and climate models for urban planning Data assimilation and probabilistic forecasting techniques Monitoring and Observations Advanced sensing techniques for the atmospheric boundary layer Nontraditional sensor networks (e.g., mobile vehicles for measurements; Twitter, Facebook, YouTube, and text messaging alerts for reporting events and accessing data) Lastly, there is a great potential in utilizing nontraditional sensor net- ontraditional sensor net- works (i.e. the transmission of critical real-time meteorological, hazard, or emergency response data through Twitter, Facebook, YouTube, and text messaging alerts) that are primarily enabled by smart technologies such as Global Positioning System (GPS) or mobile Geographic Information System (GIS) capabilities, which are fairly ubiquitous in urban regions. SHORT-TERM NEEDS Based on the information from the workshop and its deliberations, the committee identified four short-term needs in urban meteorology, which can be addressed with small investments but will likely result in large, quick returns. 1. Many end users would like improved high spatial and temporal resolution observational data, including forcing data for urban meteoro- logical models, observational data to characterize urban areas and deter- mine rban model parameters and urban sources/sinks, observational data u for rban model validation, and long-term observational data. The committee u

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10 10 URBAN METEOROLOGY noted that a variety of data in each of these categories are available from different communities and that another need is to maximize the access to observational data in different categories from diverse sources by • securing access to existing data sets from previous urban campaigns, • assuring that long-term monitoring networks will serve needs of both the global and urban climate communities, and • integrating data sets from various monitoring networks into central data archives that can be easily accessed by the broader science and end user communities. 2. Observational data have maximum value only if they are accompa- nied by comprehensive metadata—information about the data that helps fa- cilitate its understanding and use. Without detailed metadata, observational data over the heterogeneous urban areas could be easily misused by the urban meteorology community and others. A plethora of surface monitoring sites often exist in urban areas, but metadata are typically lacking for these sites, data access is not easily available, and data quality may be question- able. Given the heterogeneous nature of urban areas, the site selection, quality assurance, and management of instruments are crucial. Furthermore, because the urban environment evolves rapidly as development proceeds, another need is regularly updated metadata of the urban observations using standardized urban protocols. 3. Model intercomparison projects help in the discovery of major model deficiencies and the identification of the importance of certain major pro- cesses. Recent initial urban model intercomparisons indicate that based on all statistical measures, the simpler models perform as well as the more complex models, partly due to the lack of observational data for complex models and a lack of physical understanding of urban processes. Model performance generally improves when additional information about the surface is provided. Model intercomparisons for different urban types (e.g., coastal, mountainous, tropical, etc.) in developed and developing countries could be useful for future studies. Given that model intercomparisons can show how to improve urban models, the committee concludes another short-term need is for continued and expanded international urban model intercomparisons over urban areas. 4. Effective communication between urban meteorologists and end users is a challenge. There are several reasons for this, such as few communication

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SUMMARY 11 experiences and the difficulty in understanding each other’s needs, prac- tices, and capabilities. One example is the general difficulty in communi- cating probabilistic information to end users.1 Although some end users only need to know the most likely outcome among different options (e.g., the maximum temperature will likely be 57 degrees Fahrenheit), most users require probabilistic urban meteorological information to make informed decisions. The committee concludes that there is a need for development and application of best practices to strengthen the dialog between urban meteorologists and end user communities. CHALLENGES Although there are some clear short-term and relatively straightforward steps that could be taken to advance the field of urban meteorology and its usefulness to the end user communities, some potentially valuable advances would require significant efforts and investments. Therefore they will likely be challenging to implement and the value of such activities would need to be weighed against costs. Based on the information from the workshop and its deliberations, the committee identified three overarching challenges. 1. Given the challenge in obtaining representative measurements from individual sites over urban areas, it is important to develop new capabilities. First, technologies that integrate the measurements may be available from the network of personal digital assistants, including smartphones, for report- ing and evaluating events, and accessing information. Networks of vehicles with GPS capability (e.g., from U.S. Post Office, United Parcel Service (UPS), Federal Express, and some taxis) also have the potential to be valuable urban measurements (e.g., for temperature and pressure measurements). Secondly, given that urbanization affects the physical and dynamic structure of the planetary boundary layer (PBL)—roughly the lowest one kilometer of the atmosphere—new technologies for critical measurements within this layer are essential. The PBL influences both local weather and the concentration and residence time of pollutants in the atmosphere, which in turn impact air quality. Urban PBL is also the most understudied and undersampled layer in the urban atmosphere, in large part because of the 1 robabilistic forecasts convey the uncertainty, or likelihood, that an event will P occur. They assign a probability to each of a number of different outcomes (e.g. there is a 10% chance the maximum temperature will be 56 degrees Fahrenheit, there is a 75% chance it will be 57 degrees Fahrenheit).

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12 12 URBAN METEOROLOGY difficulty of access, especially over some parts of a city. These measurements are also important for dispersion applications. Challenge: How can new capabilities for urban observations be devel- oped and implemented, particularly using the network of PDAs (includ- ing smartphones) and vehicles and new technologies for measurements in the whole planetary boundary layer? 2. With the increase of horizontal resolution in weather and climate models, it is possible for urban areas to occupy a whole model grid cell or a large fraction of grid cell. Some weather and climate models do not in- corporate urban areas; the assumption is made that these areas are covered by the dominant vegetation type in the grid cell. Other models consider urban areas as a specific land cover type which can occupy a fraction of grid cell or a whole grid cell, whereas some land models are urbanized by considering more detailed urban processes. While an urbanized land model has been integrated into some weather and climate models (e.g., in the UK Met Office Unified Model for weather pre- diction), it is still not considered yet in most forecasting systems (includ- ing the global forecasting system at the National Centers for Environmental Prediction, NCEP). In addition, because observing systems are more or less uniformly distributed, particularly where terrain is not an obstacle, evaluation and validation metrics do not consider urban areas explicitly. Challenge: How can weather and climate models be urbanized and how can urban areas be included in the model prediction evaluation and validation metrics? 3. It can be a challenge for scientists to understand one another, even for scientists in the same discipline. Therefore it is not surprising that there typically is a lack of mutual understanding between meteorologists and the diverse end users found in urban areas. On one hand, it is important for meteorologists to better understand how individuals and organizations interpret forecast information and integrate it with other inputs, such as socio conomic, political, and cultural factors, in decision-making processes. e On the other hand, it is just as important for end users to better under- stand how observational data and models generate forecasts and associated uncertainties.

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SUMMARY 13 A combination of complementary approaches may help meet the chal- lenge of mutual understanding between users and meteorologists. The first approach is the continued development of urban testbeds. The goal of a testbed is to solve operational and practical regional weather-related phe- nomenon and/or forecast challenges through a quasi-operational framework which includes measurement specialists, forecasters, researchers, private- sector, and government agencies with a strong connection to the end users. Testbeds can result in more useful observing systems, improved use of data in forecasts, enhanced services and products, economic and public safety benefits, and eventually, more effective decision making by users. The trans- lation of research and development (R&D) findings into better operations, services, and decision-making can usually be accelerated by testbeds. A second approach is the continued development of applied science projects that involve meteorologists and end users. In particular, interdisciplin- ary projects are increasingly needed to address critical gaps in the interface between natural, biological, and human systems in the urban landscape. It is also mutually beneficial to jointly train graduate students and postdoctoral researchers. For example, a new postdoctoral training program between N ational Center for Atmospheric Research (NCAR) and the Centers for Disease Control and Prevention (CDC) requires the candidate to stay at NCAR for one year to become familiar with weather and climate prediction, and stay at CDC for another year to integrate urban meteorology with public health. A third approach is the development of joint urban meteorology and deci- sion support exercises (e.g., emergency response, climate change and urban planning). These exercises could lead to the successful translation of end user needs to the research community and to help provide a better understanding of capabilities on both sides, including giving the research community a better understanding of institutional constraints that end users face. It could also be beneficial for urban meteorologists and end users to attend joint conferences and each other’s professional conferences. Challenge: How will the capability for integrated urban meteorology- decision support systems be developed through the integration of • support for future, intensive urban research projects, that integrate modeling and observations and focus on improving the fundamental knowl- edge of physics and dynamics in the urban atmosphere, • increased dialogue between urban meteorologists and end users, and • urban meteorology testbeds?

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14 14 URBAN METEOROLOGY FINAL THOUGHTS The field of urban meteorology has grown considerably in the past 50 years, and with the increased growth of cities worldwide, including the United States, there is a pressing need for continued scientific advances within the field. As the capabilities within urban meteorology have im- proved, the uses for urban weather information and its value to decision makers have increased. Users of urban meteorology information need high quality meteorological information available in a wide variety of formats that foster its use, within time constraints set by end users’ decision processes. To help meteorologists provide this tailored information, it is important to foster direct interaction with key end user communities, who can help identify their information needs. By advancing the science and technology related to urban meteorology with input from key end user communities, meteorologists can better meet the needs of diverse end users.