The Cooperative Atmosphere-Surface Exchange Study—1999 (CASES-99) refers to a field experiment carried out in southeast Kansas during October 1999 and the subsequent program of investigation. Comprehensive data, primarily taken during the nighttime but typically including the evening and morning transition, supports data analyses, theoretical studies, and state-of-the-art numerical modeling in a concerted effort by participants to investigate four areas of scientific interest. The choice of these scientific topics is motivated by both the need to delineate physical processes that characterize the stable boundary layer, which are as yet not clearly understood, and the specific scientific goals of the investigators. Each of the scientific goals should be largely achievable with the measurements taken, as is shown with preliminary analysis within the scope of three of the four scientific goals. Underlying this effort is the fundamental motivation to eliminate deficiencies in surface layer and turbul...

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CASES-99: A Comprehensive Investigation of the Stable Nocturnal Boundary Layer

The goal of this study is to assess complications in atmospheric stable boundary layers which are not included in numerical models of the stably stratified boundary layer and to provide a formulation of surface fluxes for use in numerical models. Based on an extensive interpretive literature survey and new eddy correlation data for the stable boundary layer, this study defines two prototype stable boundary layers: the weakly stable case and the very stable case. The weakly stable boundary layer is amenable to existing models. The very stable boundary layer eludes modeling attempts due to breakdown of existing formulations of turbulence and due to features found in the atmosphere which are not normally included in models. The latter includes clear-air radiative cooling, low-level jets, surface heterogeneity, gravity waves, meandering motions, and other mesoscale motions which propagate from outside the local domain. While these mechanisms are not essential to understanding idealized or laboratory versions of the stable boundary layer, they complicate comparisons of numerical models and theories with actual atmospheric boundary layers. Statistics which describe various features of the stable boundary layer are offered for future comparison with modeling results.

Stratified Atmospheric Boundary Layers and Breakdown of Models

Boundary-layer measurements conducted at the Marsta site in Sweden from a winter-time situation (23–25 Feb.) with stable stratification have been analysed. The data comprise wind and temperature profile measurements up to 30 m, turbulence measurements at 2, 6 and 30 m and Doppler acoustic sounder data up to about 150 m. The upwind fetch at the site is flat and free from obstacles to a distance of ca 5 km for the particular sector chosen for the experiment. During the night, a two-layer vertical structure developed. Analysis of power spectra, co-spectra and variances in a shallow and very stable turbulent boundary layer near the ground show that the turbulence is fully developed and follow the universal behaviour. Above, at a height of 30 m, another turbulent layer is produced by increased wind shear near a low-level jet. This turbulent upper layer can be regarded as a layer of free shear flow. At this height, there also exist wave-turbulence interactions at low frequencies which sometimes cause a countergradient heat flux.

Observations of a multi-level turbulence structure in a very stable atmospheric boundary layer

Because turbulent fluctuations in the atmospheric refractive index (n) at a wavelength λ are related to fluctuations in the temperature (t) and the humidity (q) by n = A(λ, P, T, Q)t + B(λ, P, T, Q)q it is possible to estimate the refractive-index structure parameter Cn2 from meteorological quantities. I describe and evaluate two such estimation procedures,one based on the velocity, temperature and humidity scales u*, t*, and q*and a second based on the routine meteorological quantities Uh, Ts − Th and Qs − Qh. The subscript h here denotes the wind speed (Uh), temperature (Th), or humidity (Qh) at a reference height h; the subscript s indicates the surface value. I also develop analytical expressions for the coefficients A and B as functions of λ, the atmospheric pressure (P), and the temperature and the humidity in four wavelength regions: visible (including near infrared), an infrared window, near millimeter, and radio. In a sensitivity analysis of the two estimation procedures, the core of the paper, I demonstrate that the accuracy of the Cn2 estimate is a strong function of the Bowen ratio (Bo). At two Bo values within the interval [−10, 10], one dependent on λ and the other dependent on meteorological conditions, the uncertainty in the Cn2 estimate becomes infinite.

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Estimating Cn 2 over snow and sea ice from meteorological data

Numerous tethered balloon soundings which measured meteorological parameters and ozone concentration during the POLLUMET field experiments are investigated. They give indications of the influence of ozone conservation in the nocturnal residual layer (RL) on the development of the near surface ozone concentration of the next day. Mixing down from the RL to the surface is simulated by a simple model based on transilient turbulence theory, using measured profiles as initial values or model verification. The cases investigated show that the ozone mixed down from the RL contributes 50–70% to the maximum concentration near the surface on the following day, the rest coming from chemical production and possibly advection. Various features of exchange characteristics between the RL and the nocturnal boundary layer, (NBL) are described by the analysis of the soundings. It can be shown that the vertical exchange situation during the night can have a considerable influence on the ozone concentration of the following day.

On the relation between ozone storage in the residual layer and daily variation in near-surface ozone concentration: a case study

A model which simulates the diurnal dependence of the optical refractive index structure parameter within the atmospheric boundary layer is constructed by solving an energy balance equation for the ground surface temperature. This solution yields surface fluxes of sensible and latent heat. Empirical relationships are then used to calculate Cn2. Model results compare favorably with data obtained at a desert location under clear skies. A sensitivity analysis indicates that Cn2 is particularly sensitive to time of year, wind speed, the product of soil heat capacity and soil density, and soil moisture content. This model is applicable over relatively flat surfaces with little or no vegetation.

Modeling the diurnal dependence of the optical refractive index structure parameter

Measurements of the optical refractive index structure coefficient Cn2 and the temperature structure coefficient CT2 were made over the Tularosa Basin of south central New Mexico using both tower and aircraft mounted instruments. The height dependence of CT2 agrees reasonably well with the predictions of similarity theory in the daytime. However, the nighttime profiles disagree significantly from the surface-layer predictions for z/L > 0.05. Two factors can explain this disagreement: 1) for strong stabilities, the boundary layer is not fully turbulent, and 2) in the absence of strong synoptic forcing, mountain drainage flows may modify the profiles. The magnitude of CT2 during the daytime is found to be closely correlated with the difference in temperature between the surface and the air; in addition, there is a small dependence on wind speed. Measurements of CT2 through the depth of the convective mixing layer during the day agree closely with the model of Kaimal et al. (1976). These results imp...

Behavior of the Temperature Structure Parameter in a Desert Basin

C T 2measurements taken over a desert in stable conditions indicate that the atmosphere remains intermittently turbulent for Ri numbers as high as 10. This is in contrast to previous results which suggest that the atmosphere is essentially nonturbulent for Ri > 2. These measurements also indicate that time-averaged C T 2measurements do not scale with the time-averaged mean Ri number in very stable conditions. However, the standard deviation of log10 C T 2does appear to scale with Ri.

D. L. Walters

Papers

Modeling the diurnal dependence of the optical refractive index structure parameter

Behavior of the Temperature Structure Parameter in a Desert Basin

Intermittent turbulence in measurements of the temperature structure parameter under very stable conditions