Boulder Atmospheric Observatory Research Articles

Comparison of Wind Monitoring Systems. Part I: In Situ Sensors

Abstract It has recently become clear through advances in both theoretical and experimental meteorology, that improvements in modeling the transport and dispersion of pollutants will require on-site measurements of the atmosphere. This requirement has in turn generated questions about 1) our ability to make such measurements both near the surface and through the first few hundred meters of the atmosphere and 2) the expected accuracy and precision of such measurements using current technology. To help answer these questions an experiment was conducted at the Boulder Atmospheric Observatory to assess the ability of in situ and remote sensors to measure the mean and turbulent properties of the lower atmosphere. Two categories of sensors were tested. One consisted of lightweight in situ sensors of types that have been frequently used in the recent past for boundary layer studies. The other category consisted of four commercially available Doppler sodars, with the capability to measure wind speed, wind directi...

Errors in Second Moments Estimated from Monostatic Doppler Sodar Winds. Part II: Application to Field Measurements

Abstract In this paper we use the theoretical results presented in Part I of this paper to correct turbulence parameters derived from monostatic sodar wind measurements in an attempt to improve the statistical comparisons made with the sonic anemometers on the Boulder Atmospheric Observatory tower. We first show the approximate magnitude of the error due to spatial and temporal pulse volume separation as a function of mean wind angle relative to the sodar configuration and for several antenna pulsing orders. Sodar-derived standard deviations of the lateral wind component, before and after the application of the spatial and temporal volume separation correction, are presented. The improvement appears to be significant. The effects of correcting for pulse volume averaging derived in Part I are also discussed.

Computer demodulation technique for a dual-channel microwave radiometer

The three stairstep output of the video amplifier has now been designed for the NOAA/WPL dual-channel radiometer in order to provide an analog input to one A/D converter instead of to two phase-detectors, and the synchronous detection for data and AGC is done in software. We put the new system into the dual-channel radiometer which was set in Boulder Atmospheric Observatory (BAO) to make experiments for three weeks and found the processor reliable and flexible.

High Temporal-Resolution Infrared-Radiometric Observations of the Atmospheric Boundary Layer

An experiment designed to study the relationship of radiometric fluctuations to those of temperature and associated variables in the atmospheric boundary layer is described. Simultaneous and colocated observations were conducted by a ground-based infrared radiometer, fast response temperature, humidity, and wind sensors at 8 levels on the 300-m meteorological tower of the NOAA Boulder Atmospheric Observatory, an acoustic sounder, and a microbarograph array. The observed events included the arrival and development of a drainage flow, atmospheric waves, and convective plume activity. The analysis of the time series of the radiometer output compared with the directly measured meteorological variables and with the acoustic sounder record points out the potential of the ground-based radiometric technique for remote sensing of atmospheric fluctuation phenomena.

Measurement Errors in Vertical Wind Velocity at the Boulder Atmospheric Observatory

Abstract This paper describes an effort to identify instrumental and flow-related contributions to the mean vertical velocity offsets observed by sonic anemometers on the Boulder Atmospheric Observatory's 300 m tower. A simple technique to remove these offsets, which typically range from 0.1 to 0.2 m s−1, is presented. The correction yields 20 min averaged vertical velocities with good temporal and spatial continuity. Corrections for the vertical velocity standard deviation and the temperature flux are negligible. For momentum flux, the correction could be important when that flux is very small.

A quantitative comparison of horizontal and vertical acoustic sounding with in-situ measurements

In this paper, we present some results on an experiment to test the accuracy and utility of a horizontally-aimed acoustic sounder. A high-frequency, high-resolution mini-sounder was mounted on the mast of the Boulder Atmospheric Observatory aimed in the cross-wind direction. Measurements of C T 2, wind velocity and temperature and velocity variances were obtained under both stable and unstable conditions. These measurements were found to be in agreement with the equivalent values obtained, where appropriate, by the tower-fixed instrumentation and a vertically-pointed sounder, confirming the accuracy of the horizontal sounder. In addition, some information into the horizontal structure of plumes and gravity waves was obtained along with evidence of lack of excess attenuation at least for lengths within the unambiguous range of mini-sounders.

The Frontal Hydraulic Head: A Micro-α Scale (∼1 km) Triggering Mechanism for Mesoconvective Weather Systems

Abstract Measurements from the NOAA Boulder Atmospheric Observatory (BAO) 300 m tower, the National Center for Atmospheric Research (NCAR) Sabreliner aircraft, and the NOAA GOES-5 satellite, give evidence for the cross-front scale collapse of nonprecipitating surface cold-frontal zones to horizontal distances of ∼1 km or less. The leading edges of these frosts possess the characteristic structure of density current flows: an elevated hydraulic head followed by a turbulent wake. Vertical motions at the frontal heads exceed 5 m s−1 at 300 m (AGL). The ascent at the frontal head may act as a (∼1 km-scale) triggering mechanism for the release of potential instability and the formation of intense squall-line mesoconvection.

Some observations of turbulence structure in stable layers

AbstractThree days of measurements in stable conditions (Monin‐Obukhov length L between 15 and 300 m) at the Boulder Atmospheric Observatory tower are presented. Winds came off the Rocky Mountains on two days and off the plains on the other day. Vertical profiles of the mean horizontal velocity and Brunt‐Väisälä frequency N, and the standard deviation of the vertical component, ow, fluxes, spectra, cospectra, quadspectra, correlations and length scales are examined in this study. the measurements indicate that wave motion was present on all occasions, but which varied in strength, from weak to strong. New theoretical arguments are tested, which suggest that where the Richardson number is less than one, the velocity gradient dU/dz rather than N determines the integral scales Lx(w) of the vertical velocity fluctuations, i.e. Lx(w) ≃ ow/(dU/dz). But theory also suggests that it is not this scale, but the larger buoyancy scale ow/N that determines the temperature variance and thermal diffusivity. On this basis, dimensionless parameters are developed for plotting the measurements of temperature variance and thermal diffusivity. They are found to be in reasonably good agreement with other observations and with theory, even though the relative strength of wave motion to turbulence varied considerably. Low frequency wave motion appeared to be associated with significant heat flux, a possibility that has been excluded or overlooked in some previous studies.

Wave and Turbulence Structure in a Shallow Baroclinic Convective Boundary Layer and Overlying Inversion

Abstract Data from the Boulder Atmospheric Observatory (BAO) are used to investigate the wave and turbulence structure of the convective atmospheric mixed layer and the overlying inversion. Three cases are discussed, one in considerable detail, in which the depth of the mixed layer is below the top of the 300 m tower at the BAO and is nearly steady state for several hours. Velocity and temperature variances and spectra, coherences between vertical velocity and temperature, and vertical velocities at different levels on the tower are used to show that although the mixed-layer behavior is for the most part similar to that found in previous studies, there are some significant differences due mainly to the relatively large shear term in the turbulence energy equation compared with buoyancy, both within the mixed layer and in the capping inversion. For example, the wavelength of the spectral maximum for vertical velocity in the upper half of the mixed layer is about three times the boundary-layer height, which...

Buoyancy and Pressure Perturbations Derived from Dual-Doppler Radar Observations of the Planetary Boundary Layer: Applications for Matching Models with Observations

Abstract The technique developed by Gal-Chen in 1978 is used to derive vertical velocities, buoyancy, and pressure perturbations from dual-Doppler radar observations of the planetary boundary layer (PBL). Several approaches to verification are pursued. They include: (a) scan-to-scan temporal continuity of the derived fields; (b) an objective test to find out how well the derived pressure perturbations balance the dynamical equations; (c) comparison of dual-Doppler derived, horizontally averaged fluxes of heat versus in situ measurements and other data sets; and (d) a noteworthy improvement in the quality of the retrieved pressure when tendencies are included. Previous studies indicate that in order for the method to be viable the radars have to resolve the PBL with at least ten vertical levels. One such event occurred on 27 September 1978 during project PHOENIX, conducted at the Boulder Atmospheric Observatory (BAO) 300 m tower. An inversion above a shallow boundary layer of height around 800 m was eroded...

Meso- and Microscale Features of a Colorado Cold Front

Abstract Data from the NOAA BAO (Boulder Atmospheric Observatory) tower and the PROFS (Program for Regional Observing and Forecasting Services) surface mesonetwork have been used to detect the meso- and microscale flow patterns associated with the passage of a shallow cold front over complex terrain. This front moved across the PROFS surface mesonetwork and the BAO tower site on the morning of 3 December 1981. Partial blocking of the cold airflow by the higher terrain to the north led to a westward movement of the cold front in the upper reaches of the South Platte River drainage basin. A meso-β-scale anticyclonic eddy subsequently formed in the lee of this terrain obstruction. The micro-α-scale vertical eddy structure in the cold frontal zone is defined using data from the BAO tower. Updrafts of >6 m s−1 at 200 m AGL occurred at the wind-shift and temperature-drop line. Immediately behind this feature, micro-α-scale eddies entrained air through the frontal surface, diluting the low-level flow which was o...

Meteorological Tower Measurements of a Surface Cold Front

Abstract measurements from the Boulder Atmospheric Observatory meteorological research tower are used to describe the structure and physical processes of a strong surface cold front. Analysis reveals that the horizontal gradients in temperature and wind velocity at the front are concentrated within a 200 m distance and measured vertical velocities exceed 5 m s−1 at the leading edge of the front. Calculations with the Miller frontogenesis equation show the magnitude and relative importance of cross-frontal confluence versus differential vertical motion (tilting) in forcing frontogenesis.

Capability of Surface-Based Clear-Air Doppler Radar for Monitoring Meteorological Structure of Elevated Layers

Abstract Radars and acoustic sounding systems sense properties of the turbulence structure of the atmosphere. If atmospheric turbulence can be related to the mean gradient parameters, Doppler radars and acoustic sounders can provide information about height profiles of quantities such as temperature and refractive index as well as wind in stable regions of the atmosphere. In this paper turbulent and mean quantities were measured on the 300 m meteorological tower at the Boulder Atmospheric Observatory near Erie, Colorado, and the relationships between the turbulent and mean gradient quantities were examined in order to evaluate hypotheses for simplifying the kinetic energy balance and refractive index variance equations. FM-CW radar measurements of backscattered power and Doppler spectral width were also made for comparison with tower-measured refractive index spectra and Doppler velocity spectra. Height distributions of the turbulent dissipation rate within stable layers are shown and viscous cutoff radar...

Meteorological associations with aerosol composition in the boundary layer

The associations of meteorological parameters with aerosol elemental concentrations in the boundary layer have been studied with measurements obtained at the Boulder Atmospheric Observatory from November 29 to December 6, 1979. Air motions were documented by measurements at the tower up to 300 meters and by regional weather data. Aerosols were sampled at 4, 9, 23, and 49 meters by particle size selective cascade impactors and by time sequence filter samplers and were analyzed by proton induced X ray emission (PIXE). Principal sources of fine particle (<1 μ m aerodynamic diameter) components were automotive emissions for Pb and Br and tall stack emissions and atmospheric conversion of pollution gases to sulfate for S. In coarse particles the major earth crustal elements Al, Si, K, Ca, and Fe were soil derived. Pb and Br concentrations and the ratio Br/Pb generally decreased with height. Also, these concentrations varied with ventilation and vertical mixing and were greatest during southerly air flow from Denver. S concentrations did not generally vary with height, ventilation, vertical mixing, or wind direction, suggesting long‐range transport, although 10‐fold temporal variations were observed. Crustal element concentrations generally decreased with height but depended in part on sampler position on the tower structure, indicating microscale shielding effects of coarse particles which were not observed for fine particle elements. Temporal variations of all elements were associated with major meteorological changes, including a cold front passage and a chinook wind.

Vertical variation of roughness length at the Boulder Atmospheric Observatory

Observations near the BAO tower site have suggested a roughness length of about 0.01 m. Wind profiles from the tower yield values between 0.04 and 0.30 m. We now analyze fluctuations of vertical velocity at 10 m, and get intermediate values, differing with wind direction. Thus, inferred z0 values increase systematically with height.

An observational study of instability and turbulence in nighttime drainage winds

A detailed analysis of two nighttime drainage wind events that commenced on the evenings of 7 and 8 October, 1980 is presented. Data on wind velocity and temperature (10-s values), obtained from each of the eight instrumented levels of the Boulder Atmospheric Observatory, are used to construct 10-min means and root-mean-square values of all the variables. Additional information is provided by acoustic sounder data for 8 October.

Quantitative low-level acoustic sounding and comparison with direct measurements

In this paper, measurements of the first 150 m of the atmospheric boundary layer obtained by a high-frequency acoustic mini-sounder are compared with measurements obtained by a full complement of instruments including sonic anemometers mounted on the Boulder Atmospheric Observatory tower. The acoustic mini-sounder, starting as low as 6 m from the ground, measures in the monostatic mode the profiles of the vertical wind speed, w, and of the temperature structure parameter, CT2 with enhanced height resolution of the order of 1 m and time resolution of the order of 30 s. The results of the comparison show that the high-frequency mini-sounder is an effective atmospheric boundary-layer profiler that is also portable and relatively inexpensive.

A study of multiple stable layers in the nocturnal lower atmosphere

The structure of nocturnal inversions in the first 300 m of the atmosphere is analyzed using observational data from the Boulder Atmospheric Observatory (BAO) from March through June 1981. The temperature profiles show more than one inversion layer 41% of the time during the observational period. The vertical distributions of wind speed and moisture also show evidence of stratification during these multiple-layer events.

Wave and turbulence structure in a disturbed nocturnal inversion

Acoustic sounder and tower data obtained at the Boulder Atmospheric Observatory (BAO) are used to examine several features of the wave and turbulence structure associated with a disturbed nocturnal inversion. General features, including mean fields and Richardson number, for the case selected for this study are presented. Spectral analysis of the tower data reveals a separation of energy into wavelike and turbulent fluctuations. Analysis of the heat flux, however, shows upward counter-gradient fluxes in the vicinity of a low-level jet and near the top of the inversion. Cospectral analysis shows that the major contribution to the upward heat flux occurs at frequencies that would normally be considered characteristic of waves. In some cases, the upward flux is associated with a phase shift between vertical velocity w and fluctuating temperature θ different from the quadrature relation that would be expected of internal waves. Time series analysis reveals that these unexpected positive fluxes occur in relatively short bursts. Analysis of time series of θ and w in other cases, as well as inspection of acoustic sounder records, shows that sometimes such upward fluxes can result from a combination of wave motion and horizontal temperature advection. In this case the advection is associated with a shallow cold front.

Analysis of Nighttime Drainage Winds in Boulder, Colorado during 1980

Characteristics of nighttime drainage winds that occurred along the eastern slope of the Rocky Mountains around Boulder, Colorado during the calendar year 1980 are examined. The data used for this study were acquired from the Boulder Wind Network (BWN) and from the Boulder Atmospheric Observatory (BAO). Data were available almost continuously from BWN and less frequently from BAO. BAO is a 300 m tower, instrumented at eight levels, but only surface wind observations are obtained from BWN. However, the combination of BWM and BAO observations represents a relatively unique set of wind data for the examination of drainage flows. Criteria for the identification of drainage winds are used to isolate events that are relatively free from external influences. Eighteen drainage wind events are identified, and some climatological features of the wind regime are established. In addition, the vertical structure of the flow associated with one event that reached the tower is examined in detail. Descriptions of the features of this flow and physical interpretations are presented. It is concluded, on the basis of this analysis, that observed features may be interpreted on the basis of the physical features contained in the model of Rao and Snodgrass (1981).