Cooperative Atmosphere Surface Exchange Study Research Articles

Intermittent and Elliptical Inertial Oscillations in the Atmospheric Boundary Layer

Abstract As a convective boundary layer over land decays in the late afternoon, the atmosphere responds to the release of turbulent stresses. For many years, this response has been presumed to take the form of an inertial oscillation, a horizontal circulation with a frequency equal to the local Coriolis frequency, though published documentation of inertial oscillations in the atmosphere has been rare. In fact, documentation of inertial oscillations has been more associated with frontal passages than with the evening transition of the atmospheric boundary layer. A month of boundary layer wind profiler data from the Cooperative Atmosphere–Surface Exchange Study-1999 field program is analyzed here with the Hilbert–Huang transform (HHT), which allows analysis of intermittent, nonstationary, and amplitude-varying wave events. Inertial motions are found in this dataset, but neither the onset times of these inertial motions nor the preferred levels of occurrence are consistent with the evening-transition hypothe...

Heat Balance in the Nocturnal Boundary Layer during CASES-99

Abstract A unique set of nocturnal longwave radiative and sensible heat flux divergences was obtained during the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). These divergences are based on upward and downward longwave radiation measurements at two levels and turbulent eddy correlation measurements at eight levels. In contrast to previous radiation divergence measurements obtained within 10 m above the ground, radiative flux divergence was measured within a deeper layer—between 2 and 48 m. Within the layer, the radiative flux divergence is, on average, comparable to or smaller than the sensible heat flux divergence. The horizontal and vertical temperature advection, derived as the residual in the heat balance using observed sensible heat and radiative fluxes, are found to be significant terms in the heat balance at night. The observations also indicate that the radiative flux divergence between 2 and 48 m was typically largest in the early evening. Its magnitude depends on how fast the gr...

Turbulence Measurements with the CIRES Tethered Lifting System during CASES-99: Calibration and Spectral Analysis of Temperature and Velocity

Abstract Finescale temperature and velocity measurements with multiple vertically spaced cold-wire and hot-wire sensors on the Cooperative Institute for Research in the Environmental Sciences (CIRES) tethered lifting system (TLS) were produced during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). The various calibration methods are presented as well as algorithms to extract high-resolution estimates of the energy dissipation rate ϵ and the temperature structure constant C2T. The instrumentation is capable of measurements of ϵ ≈ 10−7 m2 s−3 and C2T ≈ 10−6 K2 m−2/3.

Sound Propagation in the Nocturnal Boundary Layer

Abstract An experimental study of sound propagation near the ground in stable, nighttime conditions was performed in conjunction with the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). Low-frequency sound transmissions were continuously recorded at microphones out to a distance of 1.3 km from a loudspeaker during CASES-99 intensive observation periods (IOPs) 6 and 7. Fading episodes in the received signal energy of 10 to 20 dB, lasting several minutes to an hour, were frequently observed. Strong discrete events, such as the density current and solitary wave of IOP 7, were found to have significant effects on acoustical signals, although substantial variability in received sound energy often occurred outside such events. Sound propagation model predictions demonstrate that wind and temperature data from a tall tower, such as the CASES-99 60-m tower, can be used to predict the momentary variations in a 50-Hz sound signal with good success. Tethersonde and rawinsonde data are generally too in...

Intermittent Turbulence in the Stable Boundary Layer over Land. Part III: A Classification for Observations during CASES-99

Abstract In this paper a classification of stable boundary layer regimes is presented based on observations of near-surface turbulence during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). It is found that the different nights can be divided into three subclasses: a turbulent regime, an intermittent regime, and a radiative regime, which confirms the findings of two companion papers that use a simplified theoretical model (it is noted that its simpliflied structure limits the model generality to near-surface flows). The papers predict the occurrence of stable boundary layer regimes in terms of external forcing parameters such as the (effective) pressure gradient and radiative forcing. The classification in the present work supports these predictions and shows that the predictions are robust in a qualitative sense. As such, it is, for example, shown that intermittent turbulence is most likely to occur in clear-sky conditions with a moderately weak effective pressure gradient. The quantit...

An Evaluation of Bulk Ri-Based Surface Layer Flux Formulas for Stable and Very Stable Conditions with Intermittent Turbulence

Abstract High-rate near-surface overnight atmospheric data taken during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) is used to quantify the representativeness of surface layer formulations under statically stable conditions. Combined with weak wind shear, such conditions generate large dynamic stability (Ri > 1.0), intermittency, and nonstationarity, which violate the underlying assumptions of surface layer theory. Still, such parameterizations are applied in atmospheric numerical models from large-eddy to global circulation. To investigate two formulas, their parameterized sensible heat flux and friction velocity (u∗) values are compared, when driven by CASES-99 measurements, to CASES-99 measurements of the same from various heights. Significant inaccuracies in the magnitude and sign of flux are found with 1) a frequent, large underprediction of heat flux for Rib > ∼1.0, 2) an overprediction of negative sensible heat flux and u∗ for ∼0.2 < Rib < ∼0.8, 3) a systematic underprediction...

Extreme Gradients in the Nocturnal Boundary Layer: Structure, Evolution, and Potential Causes

During the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) campaign in southeastern Kansas in the fall of 1999, the University of Colorado's Cooperative Institute for Research in Environmental Sciences (CIRES) made a series of vertical profiling measurements using the CIRES tethered lifting system (TLS). The results reported here began during a period when the nocturnal boundary layer (NBL) was characterized by a low-level jet (LLJ) peaking at 120 m and a temperature profile that increased smoothly with height to a point slightly above the height of the LLJ peak. Then, within a period of less than 30 min, the character of the NBL changed abruptly, with the breakdown of the well-defined LLJ and the appearance of a surprisingly steep temperature change of some 3.5 K around 180–190 m AGL. Part of this inversion was extremely sharp, with the steepest portion showing a temperature change of 1 K over an altitude range of only 5 cm, corresponding to a vertical temperature gradient in excess of 20 K m−1. The general shape of this steep gradient was maintained—albeit with slightly reduced values—for at least 20 min. It is understood that the magnitude of the steepest portion of this gradient exceeds all previously observed atmospheric gradients by over an order of magnitude, although comparable gradients—albeit under very disparate conditions—have been observed in the ocean. A second surprising feature apparent in these results was the steepness of the gradients in turbulence structure at the top of the NBL and within the residual layer (RL), the region above the NBL that is usually slightly stably stratified and extends upward to the height marked by the vestiges of the previous day's capping inversion. At the NBL top, energy dissipation rates and temperature structure parameters dropped sharply by more than one order of magnitude over a distance of only a few meters. At higher altitudes within the RL, a 60-m-thick region of very weak turbulence was observed. This low-turbulence region also exhibited sharp edges, where energy dissipation rates and temperature structure parameters changed by at least an order of magnitude over vertical distances of only a few meters.

Formulation of Turbulent Fluxes in the Stable Boundary Layer

Abstract The mixing lengths for heat and momentum are computed from seven levels of eddy correlation data during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). A number of formulations of the mixing length are evaluated, including surface layer similarity theory, several hybrid similarity theories, a formulation based on the Richardson number, and a formulation based on the local shear. A formulation of the mixing length is examined, which approaches z-less similarity for large z and surface layer similarity close to the ground surface. A generalized version includes a dependence on boundary layer depth, which approaches the usual boundary layer height dependence for neutral conditions. However, for many of the observational cases, a boundary layer did not exist in the usual sense, in that turbulence was generated primarily above the surface inversion layer and occasionally extended downward toward the surface. For these cases, inclusion of z-less turbulence is crucial.

Relationship between Low-Level Jet Properties and Turbulence Kinetic Energy in the Nocturnal Stable Boundary Layer

Abstract In the nighttime stable boundary layer (SBL), shear and turbulence are generated in the layer between the maximum of the low-level jet (LLJ) and the earth's surface. Here, it is investigated whether gross properties of the LLJ—its height and speed—could be used to diagnose turbulence intensities in this subjet layer. Data on the height and speed of the LLJ maximum were available at high vertical and temporal resolution using the high-resolution Doppler lidar (HRDL). These data were used to estimate a subjet layer shear, which was computed as the ratio of the speed to the height of the jet maximum, and a jet Richardson number RiJ, averaged at 15-min intervals for 10 nights when HRDL LLJ data were available for this study. The shear and RiJ values were compared with turbulence kinetic energy (TKE) values measured near the top of the 60-m tower at the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) main site. TKE values were small for RiJ greater than 0.4, but as RiJ decreased to less ...

Cooperative Atmosphere–Surface Exchange Study-1999

Cooperative Atmosphere–Surface Exchange Study-1999

Analysis of Ducted Motions in the Stable Nocturnal Boundary Layer during CASES-99

Abstract Data obtained with multiple instruments at the main site of the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99) are employed to examine the character and variability of wave motions occurring in the stable nocturnal boundary layer during the night of 14 October 1999. The predominant motions are surprisingly similar in character throughout the night, exhibiting largely westward propagation, horizontal wavelengths of ∼1 to 10 km, phase speeds slightly greater than the mean wind in the direction of propagation, and highly coherent vertical motions with no apparent phase progression with altitude. Additionally, vertical and horizontal velocities are in approximate quadrature and the largest amplitudes occur at elevated altitudes of maximum stratification. These motions are interpreted as ducted gravity waves that propagate along maxima of stratification and mean wind and that are evanescent above, and occasionally below, the altitudes at which they are ducted. Modal structures for ducte...

Surface-Layer Characteristics in the Stable Boundary Layer with Strong and Weak Winds

An extensive meteorological dataset obtained from the plumevalidation experiment conducted by the Electric Power Research Institute (EPRI) atKincaid during 1980–1981 is analysed for studying the characteristic differences in thesurface-layer parameters in strong and weak wind stable conditions. The surface-layerparameters are computed using the similarity functions Φm and Φh proposed byBeljaars and Holtslag. The weak winds are characterized using the geostrophic wind speedas well as the wind speed at the 10-m level. The surface fluxes are found to be finitein weak wind conditions.Empirical formulations for the eddy diffusivities of momentum(KM) and heat (KH), and drag (CD) and heat exchange (CH) coefficients, as powerlaw functions of the bulk Richardson number (RiB), are proposed under both strong andweak wind conditions. Results are close to those based on observations taken from the IndianInstitute of Technology low wind diffusion experiment, the Land surface processes experiment,the Hanford diffusion experiment, the Cabauw field experiment and the Cooperative Atmospheric SurfaceExchange Study 1999 (CASES-99) experiment. In addition, the fluxes obtained fromthe proposed empirical relations are in good agreement with those based on similarity theory as wellas the turbulence measurements taken from the CASES-99 experiment.

On the bulk Richardson number and flux–profile relations in an atmospheric surface layer under weak wind stable conditions

For describing the surface fluxes in the atmospheric dispersion models, the commonly used linear universal similarity functions φm and φh for nondimensional wind and temperature profiles (J. Atmos. Sci. 28 (1971) 181; Boundary-Layer Meteorol. 7 (1974) 363) are analyzed under weak wind stable conditions. The stability parameter z/L (z is the height above the ground and L is the Obukhov length) is not known a priori. On the other hand, the bulk Richardson number (RiB) is readily available from profile measurements. Thus, a systematic mathematical analysis is carried out to analyze the applicability of the linear functions in terms of RiB. These linear functions are found to be valid as long as RiB is smaller than Prtγ/β2 where Prt is the turbulent Prandtl number and β and γ are constants appearing in the linear functions of φm and φh. The linear functional forms for φm and φh underestimate the Obukhov length resulting in the large value of the stability parameter when RiB⩾Prtγ/β2 occurring in weak wind stable conditions. Using the data from plume validation field experiment conducted by Electric Power Research Institute at Kincaid, it is found that in 70% of the weak wind cases, the RiB is larger than Prtγ/β2. The similarity functions proposed by Beljaars and Holtslag (J. Appl. Meteorol. 30 (1991) 327) are shown to perform well in weak wind stable conditions for all the values of RiB. The computed surface fluxes are also compared with those based on turbulence measurements taken in Cooperative Atmospheric Surface Exchange Study.

Land Surface Heterogeneity in the Cooperative Atmosphere Surface Exchange Study (CASES-97). Part II: Analysis of Spatial Heterogeneity and Its Scaling

Abstract Multiscale atmospheric forcing data at 1-, 5-, and 10-km scales from the 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) were used to drive three uncoupled land surface models: the National Center for Atmospheric Research Land Surface Model version 1 (NCAR LSM), the Oregon State University Land Surface Model (OSU LSM), and the Japan Atomic Energy Research Institute's Atmosphere–Soil–Vegetation Model (SOLVEG). The data included high-resolution, gauge-corrected precipitation estimates from dual-polarization radar, with the experimental period covering the spring green-up process. The effects of increasing scale on modeled estimates of the domain mean flux were more pronounced when there was greater heterogeneity of the land surface in terms of the surface vegetation, although this result was model dependent. All models made use of prescribed parameters based on dominant land cover at the different scales, not effective parameters, leading to situations where the coarser-scale flux res...

Choosing the Averaging Interval for Comparison of Observed and Modeled Fluxes along Aircraft Transects over a Heterogeneous Surface

Abstract Data from the April–May 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) are used to illustrate a holistic way to select an averaging interval for comparing horizontal variations in sensible heat (H) and latent heat (LE) fluxes from low-level aircraft flights to those from land surface models (LSMs). The ideal filter can be defined by considering the degree to which filtered aircraft fluxes 1) replicate the observed pattern followed by H and LE at the surface, 2) are statically robust, and 3) retain the heterogeneity to be modeled. Spatial variability and temporal variability are computed for different filtering wavelengths to assess spatial variability sacrificed by filtering and how much temporal variability can be eliminated; ideally, spatial variability should approach or exceed temporal variability. The surface pattern to be replicated is a negative slope when H is plotted against LE for a given time. This is required for surface energy balance if H or LE vary horizontally more ...

Land Surface Heterogeneity in the Cooperative Atmosphere Surface Exchange Study (CASES-97). Part I: Comparing Modeled Surface Flux Maps with Surface-Flux Tower and Aircraft Measurements

Abstract Land surface heterogeneity over an area of 71 km × 74 km in the lower Walnut River watershed, Kansas, was investigated using models and measurements from the 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) field experiment. As an alternative approach for studying heterogeneity, a multiscale atmospheric and surface dataset (1, 5, and 10 km) was developed, which was used to drive three land surface models, in uncoupled 1D mode, to simulate the evolution of surface heat fluxes and soil moisture for approximately a 1-month period (16 April–22 May 1997) during which the natural grassland experienced a rapid greening. Model validation using both surface and aircraft measurements showed that these modeled flux maps have reasonable skill in capturing the observed surface heterogeneity related to land-use cover and soil moisture. The results highlight the significance of rapid greening of grassland in shaping the surface heterogeneity for the area investigated. The treatment of soil hydrauli...

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...

Shear-Flow Instability in the Stable Nocturnal Boundary Layer as Observed by Doppler Lidar during CASES-99

Abstract This study investigates a shear-flow instability observed in the stably stratified nighttime boundary layer on 6 October 1999 during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) in south-central Kansas. A scanning Doppler lidar captured the spatial structure and evolution of the instability, and high-rate in situ sensors mounted on a nearby 60-m tower provided stability and turbulence data with excellent vertical resolution. Data from these instruments are analyzed and linear stability analysis (LSA) is employed to carefully characterize the wave field, its interaction with the mean flow, and its role in turbulence generation. The event persisted for about 30 min and was confined within the shear zone between the surface and a low-level jet (LLJ) maximum. Eigenvalues corresponding to the fastest growing mode of the LSA showed good agreement with the basic wave parameters determined from the lidar data. Good qualitative agreement was also obtained between the eigenfunction of the f...

Triple-Hot-Film Anemometer Performance in Cases-99 and A Comparison With Sonic Anemometer Measurements

Two levels of triple-hot-film and sonic anemometers were deployed on a 5.5-m towerduring the Cooperative Atmospheric Surface Exchange Study (CASES-99) in October1999. Each triple-hot-film probe was collocated 50 mm from the sonic sensing path ona common boom. Various problems with using triple-hot-films in the atmosphere toresolve wind components are addressed including the derivation of a yaw angle correction using the collocated sensors. It was found that output voltage drift due to changes in environmental temperature could be monitored and corrected using an automated system. Non-unique solutions to heat transfer equations can be resolved using a collocated sonic anemometer. Multi-resolution decomposition of the hot-film data was used to estimate appropriate day and night averaging periods for turbulent flux measurements in and near the roughness sub-layer. Finally, triple-hot-film measurements of mean wind magnitude (M), turbulent kinetic energy (TKE), sensible heat flux (H), and local friction velocity (u*) are compared to those of the collocated CSAT3 sonic anemometers. Overall, the mean wind magnitudes measured by the triple-hot-film and the collocated sonic sensorswere close, consistent and independent of stability or proximity to the ground. The turbulent statistics, TKE, u*, and H, measured by the two sensor systems were reasonably close together at z = 5 m. However, the ratio of sonic measurement/hot-film measurement decreased toward the ground surface, especially during stable conditions.

The Role of Quantitative Infrared Imagery in Investigations of the Nocturnal Boundary Layer

Abstract During the Cooperative Atmospheric–Surface Exchange Study in 1999 (CASES99), an intensive investigation of the stable nocturnal boundary layer, a versatile and sensitive cryogenically cooled thermal imaging radiometer, the Inframetrics PM 380 Thermal Camera, was deployed. The 60-m-high instrumented tower at the central CASES99 site provided the perch from which to survey the instrumented research field. The field of view of 16° (0.28 sr) and an angular resolution of 1/16° (0.0011 sr) enabled a segment of landscape 500 m distant, of approximately 150-m width, to be viewed with a resolution of approximately 0.5 m. Or, looking down from the 50-m level of the tower, a section of the ground surface 15 m on a side could be viewed with a resolution of 5 cm. The surface brightness temperature of any area could be surveyed with a temperature resolution of 0.1°C and a time resolution of 30 Hz. The information obtained from analysis of these thermal images uniquely complemented the data acquired by the more...