Steven R. Semmer

Portable Automated Mesonet II

Abstract The Portable Automated Mesonet II (PAM II) system was developed by NCAR to provide surface mesoscale data for the research needs of the atmospheric science community. The PAM system has 60 remote stations with planned growth to 300. In such a distributed system, data communication is a vital subsystem and, since it dictates some key system constraints, deserves special attention. The NOAA/NESDIS satellite, GOES, is used to link the remote stations to the base stations. This provides very wide areas coverage but limits the data rate. Special attention was given to the design of the sensor subsystems to minimize the possibility for human error and to maintain the calibration in field conditions while using interchangeable modules. This was achieved by using a dedicated microprocessor in the psychrometer and the barometer. The microprocessor in the sensor modules controls the sensors, applies the individual calibration coefficients, and transmits the sensor data to the master data acquisition module...

Minimizing Errors Associated with Multiplate Radiation Shields

Abstract Multiplate radiation shield errors are examined using the following techniques: 1) ray tracing analysis, 2) wind tunnel experiments, 3) numerical flow simulations, and 4) field testing. The authors’ objectives are to develop guidelines for radiation shield and temperature sensor design, to build an improved shield, and to determine factors that influence radiational heating errors. Guidelines for reducing radiational heating errors are given that are based on knowledge of the temperature sensor to be used, with the shield chosen to match the sensor design. A new class of shield called a part-time aspirated multiplate radiation shield is introduced. This type of shield consists of a multiplate design usually operated in a passive manner but equipped with fan-forced aspiration capability to be used when necessary (e.g., low wind speed). A prototype shield reduced radiational heating errors from 2° to 1.2°C. In addition, nighttime low wind speed errors were reduced from 1.6° to 0.3°C. Existing passi...

The NCAR Atmosphere-Surface Turbulent Exchange Research (ASTER) Facility

Abstract 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.

A Field Intercomparison Technique to Improve the Relative Accuracy of Longwave Radiation Measurements and an Evaluation of CASES-99 Pyrgeometer Data Quality

Abstract Techniques for improving the relative accuracy of longwave radiation measurements by a set of pyrgeometers [the Eppley Laboratory Precision Infrared Radiometer (Model PIR)] are presented using 10 PIRs from the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). The least squares–based optimization technique uses a field intercomparison (i.e., a time period during which all the PIRs were upward looking and set up side by side) to determine a set of optimization coefficients for each PIR. For the 10 CASES-99 PIRs, the optimization technique improved the standard deviation of the difference of downwelling irradiance between the PIRs from ±0.75 to ±0.4 W m−2 (for nighttime data). In addition to presenting the optimization method, various PIR data quality checks are outlined and applied to the PIR data. Based on these quality checks, the measured case and dome temperatures of the CASES-99 PIRs were all reasonable. Using the 10 CASES-99 PIRs, simple estimates of the average nighttime net rad...

An Evaluation of Calibration Techniques for In Situ Carbon Dioxide Measurements Using a Programmable Portable Trace-Gas Measuring System

Abstract The construction and deployment of a portable trace-gas measurement system (TGaMS) is described. The air-collection system (dubbed HYDRA) collects air samples from 18 different locations and was connected to either one or two LI-COR LI-7000 gas analyzers to measure CO2. An in situ “field calibration” method, that uses four calibration gases with an uncertainty on the order of ±0.1 μmol mol−1 relative to the WMO CO2 mole fraction scale, revealed CO2 output from the LI-7000 had a slightly nonlinear relationship relative to the CO2 concentration of the calibration gases. The sensitivity of the field-calibrated CO2 to different forms of the field-calibration equation is investigated. To evaluate TGaMS performance, CO2 from collocated inlets, portable gas cylinders, and nearby independent CO2 instruments are compared. Results are as follows: 1) CO2 measurements from HYDRA multiple inlets are feasible with a reproducibility of ±0.4 μmol mol−1 (based on the standard deviation of the CO2 difference betwe...

ASTER: The Atmosphere‐Surface Turbulent Exchange Research Facility at NCAR

The new Atmosphere-Surface Turbulent Exchange Research (ASTER) Facility at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., will support observational research on the structure of the atmospheric surface layer and will provide state-of-the-art measurements of surface fluxes of trace chemical species, water vapor, sensible heat, and momentum. At a time when human influences are perturbing both the atmosphere and the biosphere, it is increasingly important to improve our understanding of the interactions between the atmosphere and its underlying surface. There is a demonstrated need for a facility that can provide measurements of: the structure of the atmospheric surface layer, in support of studies of dry deposition, resuspension, natural emissions and chemical reactions within this layer.