Abstract

Observations of boundary-layer flow within the Santa Barbara region taken on 20 September 1985 revel the presence of a wide variety of flow features, including mesoscale wind vortices sea/land breezes, and thermally driven upslope/downslope winds. Details of these features, in particular the mesoscale vortices, are documented with dual-Doppler radar, Doppler sodar, aircraft, surface mesonet, and rawinsonde data. Numerical simulations of flow in the region using a mixed-layer model show good agreement with the observations. Model simulations indicate that sea-/land-roughness differences and planetary vorticity are of minor importance in forming the midchannel eddy (MCE), an eddy that is observed in the channel during the early morning hours. MCE formation is, however, shown to be strongly dependent on the initial stratification of the atmosphere, with more intense eddies forming as the stability increases. A second independent mechanism for MCE formation appears to be the interaction of drainage flows with the large-scale flow. A daytime vortex, known as the Gaviota eddy, occurs as the result of surface heating that generates a sea-breeze flow opposing the large-scale ambient flow.

The Second Prospectus Development Team (PDT-2) of the U.S. Weather Research Program was charged with identifying research opportunities that are best matched to emerging operational and experimental measurement and modeling methods. The overarching recommendation of PDT-2 is that inputs for weather forecast models can best be obtained through the use of composite observing systems together with adaptive (or targeted) observing strategies employing both in situ and remote sensing. Optimal observing systems and strategies are best determined through a three-part process: observing system simulation experiments, pilot field measurement programs, and model-assisted data sensitivity experiments. Furthermore, the mesoscale research community needs easy and timely access to the new operational and research datasets in a form that can readily be reformatted into existing software packages for analysis and display. The value of these data is diminished to the extent that they remain inaccessible.

The composite observing system of the future must combine synoptic observations, routine mobile observations, and targeted observations, as the current or forecast situation dictates. High costs demand fuller exploitation of commercial aircraft, meteorological and navigation [Global Positioning System (GPS)] satellites, and Doppler radar. Single observing systems must be assessed in the context of a composite system that provides complementary information. Maintenance of the current North American rawinsonde network is critical for progress in both research-oriented and operational weather forecasting.

Adaptive sampling strategies are designed to improve large-scale and regional weather prediction but they will also improve diagnosis and prediction of flash flooding, air pollution, forest fire management, and other environmental emergencies. Adaptive measurements can be made by piloted or unpiloted aircraft. Rawinsondes can be launched and satellites can be programmed to make adaptive observations at special times or in specific regions. PDT-2 specifically recommends the following forms of data gathering: a pilot field and modeling study should be designed and executed to assess the benefit of adaptive observations over the eastern Pacific for mesoscale forecasts over the contiguous United States; studies should be done over the western Atlantic and Caribbean-Gulf of Mexico regions, particularly during hurricane season; and enhanced observations should be implemented for the mountainous western states and for the Mississippi and Missouri River Valleys.

Data sensitivity tests and observing system simulation experiments (OSSEs) are important tools for gauging the benefit of existing or proposed observing systems. OSSEs involve only model predictions and are essentially self-contained. Data sensitivity tests require the full consideration of modeling infrastructure, namely, observation ingest quality control, objective analysis, and numerical prediction. Sensitivity tests involving both wind and moisture profiles are particularly needed to determine their impact on improved precipitation forecasts. New variational analysis techniques are capable of assimilating so-called proxy observations. These techniques should be fully exploited. Diabatic initialization should be addressed through the assimilation of satellite cloud data and very high resolution WSR-88D radar measurements into very high resolution models with sophisticated cloud microphysics. Success in this area should improve quantitative precipitation forecasts in the first few (model) hours.

There is a pressing need to better understand the interaction of moist convection with large-scale flow. One key is better characterization of the impact of precipitation formation and evaporation on the fluxes of mass, momentum, and heat in moist convection. Humidity measurement in precipitating downdrafts is a crucial measurement, which currently cannot be made reliably. The capabilities of polarization-diversity radar should be explored in a quasi-operational context to determine whether WSR-88D radars should be upgraded. Progress in quantitative precipitation forecasting is impeded by poorly resolved and inaccurate water vapor measurements.

Further improvements in numerical weather prediction demand improved monitoring of Earth surface characteristics so that spatial and temporal variations in air–surface fluxes are realistically simulated. Over land, priority should be given to the coupling of mesoscale meteorological models with hydrological models and to routine assimilation of surface (soil, moisture, and plant) characteristics. Improved air–sea fluxes are essential to proper modeling of marine cyclogenesis. The most important, practical ocean measurements include sea surface temperature, thermocline depth, wave spectra, and ice coverage and thickness.

More than 120 scientists, engineers, administrators, and users met on 8–10 December 2003 in a workshop format to discuss the needs for enhanced three-dimensional mesoscale observing networks. Improved networks are seen as being critical to advancing numerical and empirical modeling for a variety of mesoscale applications, including severe weather warnings and forecasts, hydrology, air-quality forecasting, chemical emergency response, transportation safety, energy management, and others. The participants shared a clear and common vision for the observing requirements: existing two-dimensional mesoscale measurement networks do not provide observations of the type, frequency, and density that are required to optimize mesoscale prediction and nowcasts. To be viable, mesoscale observing networks must serve multiple applications, and the public, private, and academic sectors must all actively participate in their design and implementation, as well as in the creation and delivery of value-added products. The mesoscale measurement challenge can best be met by an integrated approach that considers all elements of an end-to-end solution—identifying end users and their needs, designing an optimal mix of observations, defining the balance between static and dynamic (targeted or adaptive) sampling strategies, establishing long-term test beds, and developing effective implementation strategies. Detailed recommendations are provided pertaining to nowcasting, numerical prediction and data assimilation, test beds, and implementation strategies.

The Atmosphere-Surface Turbulent Exchange Research (ASTER) facility developed at the National Center for Atmospheric Research (NCAR) will support observational research on the structure of the atmospheric surface layer. ASTER will provide state-of-the-art measurements of surface fluxes of momentum, sensible heat, and water vapor, and support measurements of surface fluxes of trace chemical species. The facility will be available to the scientific community in the spring of 1990. The motivation for the development of ASTER and the elements that constitute this new national facility are briefly discussed.

The Aeronomy Laboratory of the National Oceanic and Atmospheric Administration and the Atmospheric Technology Division of the National Center for Atmospheric Research are jointly developing Integrated Sounding Systems (ISS) for use in support of TOGA (Tropical Ocean Global Atmosphere) and TOGA COARE (Coupled Ocean–Atmosphere Response Experiment). Some of the ISS units will have to be operated on research ships during TOGA COARE's intensive observing period in late 1992 and early 1993. The greatest technical challenge in adapting the ISS to shipboard use is to stabilize the UHF wind profiler that is an integral part of the ISS. In June 1991 a UHF wind-profiling Doppler radar was installed on a stabilized platform aboard the NOAA research vessel Malcolm Baldrige on an eight-day cruise in the Atlantic Ocean. The wind profiler was gyrostabilized and profiler winds were corrected for ship motion utilizing the Global Positioning System. During the eight days at sea, CLASS (Cross-Chain LORAN Atmospheric Sounding System) and OMEGA Sounding System balloons were launched onboard ship for wind profile comparisons. Results of the comparisons show excellent agreement between wind profiles, with an rms difference of about 1 m s⁻¹ in wind speed.

Several ground-based remote sensors were operated together in Colorado during February and March 1991 to obtain continuous profiles of the kinematic and thermodynamic structure of the atmosphere. Instrument performance is compared for five different wind profilers. Each was equipped with Radio Acoustic Sounding System (RASS) capability to measure virtual temperature. This was the first side-by-side comparison of all three of the most common wind-profiler frequencies: 50, 404, and 915 MHz. The 404-MHz system was a NOAA Wind Profiler Demonstration Network (WPDN) unit. Dual-frequency microwave radiometers that measured path-integrated water vapor and liquid water content were also evaluated. Frequent rawinsonde launches from the remote-sensor sites provided an extensive set of in situ measurements for comparison. The winter operations provide a severe test of the profiler/RASS capabilities because atmospheric scattering is relatively weak and acoustic attenuation is relatively strong in cold, dry conditions. Nevertheless, the lower-frequency systems exhibited impressive height coverage for wind and virtual temperature profiling, whereas the high-frequency units provided higher-resolution measurements near the surface. Comparisons between remote sensor and rawinsonde data generally showed excellent agreement. The results support more widespread use of these emerging technologies.

During the week 29 October–4 November 1988, a Ground-based Atmospheric Profiling Experiment (GAPEX) was conducted at Denver Stapleton International Airport. The objective of GAPEX was to acquire and analyze atomspheric-temperature and moisture-profile data from state-of-the-art remote sensors. The sensors included a six-spectral-channel, passive Microwave Profiler (MWP), a passive, infrared High-Resolution Interferometer Sounder (HIS) that provides more than 1500 spectral channels, and an active Radio Acoustic Sounding System (RASS). A Cross-Chain Loran Atmospheric Sounding System (CLASS) was used to provide research-quality in situ thermodynamic observations to verify the accuracy and resolution characteristics of each of the three remote sensors. The first results of the project are presented here to inform the meteorological community of the progress achieved during the GAPEX field phase. These results also serve to demonstrate the excellent prospects for an accurate, continuous thermodynamic profiling system to complement NOAA's forthcoming operational wind profiler.

Abstract

Urbanization modifies atmospheric energy and moisture balances, forming distinct features [e.g., urban heat islands (UHIs) and enhanced or decreased precipitation]. These produce significant challenges to science and society, including rapid and intense flooding, heat waves strengthened by UHIs, and air pollutant haze. The Study of Urban Impacts on Rainfall and Fog/Haze (SURF) has brought together international expertise on observations and modeling, meteorology and atmospheric chemistry, and research and operational forecasting. The SURF overall science objective is a better understanding of urban, terrain, convection, and aerosol interactions for improved forecast accuracy. Specific objectives include a) promoting cooperative international research to improve understanding of urban summer convective precipitation and winter particulate episodes via extensive field studies, b) improving high-resolution urban weather and air quality forecast models, and c) enhancing urban weather forecasts for societal applications (e.g., health, energy, hydrologic, climate change, air quality, planning, and emergency response management). Preliminary SURF observational and modeling results are shown (i.e., turbulent PBL structure, bifurcating thunderstorms, haze events, urban canopy model development, and model forecast evaluation).