Abstract. A primary objective of the 1995 and 1996 Flatland boundary layer experiments, known as Flatland95 and Flatland96, was to measure and characterize entrainment at the top of the convective boundary layer. The experiments took place in the area near the Flatland Atmospheric Observatory near Champaign-Urbana, Illinois, in August-September 1995 and June-August 1996. The site is interesting because it is extraordinarily flat, has uniform land use, and is situated in a prime agricultural area. Measurements in the entrainment zone are difficult to make due to the time and space scales involved. We will present entrainment estimates derived from budget calculations with data from UHF wind profiling radars and from radiosondes. The results demonstrate that the remote sensing instruments produce results comparable to radiosondes and have significant advantages for boundary layer studies. Surface flux measurements are also used in the calculations. Direct heating by shortwave radiation absorbed by aerosols in the boundary layer is found to be an important component of the boundary layer heat budgets. The entrainment virtual temperature flux and the ratio of entrainment to surface flux found from the budget calculations are somewhat larger than expected. Advection of warm air, which is not accounted for in the budget calculations, is probably a factor in some periods but may not be significant in the full data set. For the full data set, the mean entrainment velocity found from the heat budget is 0.03 + 0.01 m s -l, slightly less than the mean rate of change of the boundary layer height. The mean entrainment ratio A R is 0.51 + 0.12 and the median is 0.43, comparable to results from some other studies in comparable conditions.
A simplified theory has been developed for calculating the effect of satellite potential on the ion current measured by an experiment such as an ion mass spectrometer or an ion trap. The theory is based on the use of a spherically symmetric Debye potential distribution in the sheath around the satellite and is particularly appropriate for use in regions where the Debye length is large, such as in the plasmasphere and magnetosphere. Ion data obtained from the ion trap on the Ogo 3 satellite during a pass through the plasmasphere show excellent agreement with the theory. The inferred ion densities from this analysis are as much as 1 order of magnitude different from what would be inferred from previous analyses.
A large 40.475 MHz Doppler radar has been constructed near Sunset, Colorado, 16 km west of Boulder, in order to study winds, the troposphere, stratosphere, and mesosphere. Early results from the troposphere are presented here. It is concluded that: (1) With a sensitive VHF radar it is possible to detect echoes from throughout the upper troposphere almost all the time, under both clear and cloudy conditions; (2) Horizontal winds derived from the Doppler shifts of the echoes are consistent at heights more than 2 km above the local terrain with rawinsonde winds measured over 55 km away; (3) The reflectivity of the scatterers in the troposphere is not strongly aspect sensitive, up to an angle of 30° from the zenith.
In a recent rocket experiment designed to resolve the slow-wave echoes comprising the resonances excited by topside sounders the plasma, the upper hybrid resonance, and the first three electron cyclotron resonances were observed. In particular, the variation of echo frequency versus delay was observed for the upper hybrid resonance. The good agreement ' found between the observed and the predicted frequency delay function confirms the slowwave propagation nature of the phenomenon. The frequency of all three electron cyclotron resonances are consistent with the Pogo (3/68) magnetic-field model within its stated accuracy. Observations of resonances in the ionosphericplasma have been made by topside sounders since 1961 [Lockwood, 1963; Calvert and Goe, 1963; Calvert and McA•ee, 1969]. The experiments consist of exciting the plasma in the vicinity of the spacecraft with a short RF pulse and observing the response with a radio receiver. A resonance consists of an apparent ringing signal lasting a few milliseconds when the system is tuned to certain frequencies. The principal resonances occur at the plasma frequency f•, the upperhybrid frequency f•, and the electron cyclotron frequency fn and its harmonics nfn. Current explanations of the resonances involve the propagation, reflection, and reception of slow waves [McA]ee, 1968, 1969a, b, 1970a, b; Bitoun et al.A continuum of such slow-wave echoes can be found with a specific frequency delay relationship. This could produce a trace on a topside ionogram analogous to the normal electromagnetic-wave traces. However, the range of frequencies contributing to the resonance during a normal listening period is considerably narrower than either the rec.eiver bandwidth or the spectrum of the transmitter pulse. Therefore the echoes from a single pulse cover all delays to produce the apparent ringing signal. Previous satellite experiments were incapable of observing this frequency variation with delay within the receiver bandwidth because of on board signal detection.Previous evidence favoring the echo .explanation has relied on the prediction that two separate sets of echoes can occur when the appropriate conditions are met. One corresponds to waves launched with propagation vector components generally parallel to the spacecraft velocity, and the other to waves in the opposite direction. These two echoes may have different frequency delay variations, and, furthermore, they would experience different Doppler shifts. The difference in frequency between the two is about a kilohertz, so that an observable beat is produced in a sounder system by using amplitude detection. Quantitative comparisons between predicted and observed beat patterns yield convincing agreement [Warnock et al., 1970; Feldstein and Gra#, 1972]. This paper describes a rocket-borne sounder experiment. designed specifically to observe the slow-wave echoes comprising a resonance and reports the major results. EXPERIMENTThe rocket sounder consisted of a frequency synthesizer, a pulsed transmitter, a receiver, a transm...
A new VHF clear‐air Doppler radar has been constructed in very flat terrain near Urbana, Illinois. This radar, called the Flatland radar, as presently configured measures a profile of the vertical component of the wind velocity every 2.5 minutes. It is found that typical time variances of vertical velocity over this very flat terrain are similar to the small variances observed during “quiet” periods near mountains. The observed absence of extended periods of large variance supports the hypothesis that the “active” periods observed near mountains are mainly due to orographic effects. The absence of such effects at Flatland should facilitate the study of other meteorological processes. For example, in the case study presented here it is suggested that the vertical motions associated with large‐scale baroclinic storms are measurable by the Flatland radar.
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