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J. Latham and A. H. Miller

Abstract

When ice-coated spheres suspended from an insulating fiber were rotated rapidly through a stream of steadily falling natural snow crystals they acquired an electrical charge the magnitude of which increased markedly with an increase in impact velocity and degree of surface irregularity. A smooth sphere acquired a positive charge and the spheres of irregular surface structure acquired a much larger negative charge. The sign of the charging is in qualitative agreement with the temperature-gradient theory but the effects of impact velocity and surface geometry are not. Rough calculations indicate that the average charge transfer between a snow crystal and a sphere of irregular surface structure impacting at a velocity of several meters per second is several orders of magnitude greater than predicted by Mason's equations, but a comparison between these results and those emanating from the laboratory experiments of Latham and Stow indicates that the results of this field experiment are entirely explicable in terms of a temperature-gradient theory modified to accommodate these two enhancement processes. It is concluded that the experiments of Reynolds, Brook and Gourley yielded a more representative value for the average charge transfer per collision between an ice crystal and a soft-hail pellet inside a thundercloud than did the experiments of Latham and Mason, and that the Reynolds-Brook mechanism can easily generate charge inside a thunder-cloud at the minimum rate required by a tenable theory of thunderstorm electrification.

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R. A. Kropfli and L. J. Miller

Abstract

No abstract available.

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R. K. Hauser and A. J. Miller

Abstract

Routine estimates of certain atmospheric energetics parameters [AZ, KZ, AE, KE, C(AZ,AE), C(KE,KZ)] have been computed for the National Meteorological Center's routine 0000 and 1200 GMT analyses and for the 00, 12, 24, 36, 60 and 84 h forecasts which were prepared by the six‐layer operational forecast model from 0000 GMT. The verification of these forecast values during October and November 1975 and January and February 1976 revealed well‐defined temporal behaviors. As the forecast cycle proceeds, there are significant losses of AZ and KZ, while AE and KE show a relative increase at 1–2 days. Cross sections for January 1976 show that a 12% increase of KZ, which occurs during the first 12 h forecast interval, is associated with a downward and northward movement of the mean [u] jet stream core.

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M. J. Miller and A. K. Betts

Abstract

The low-level atmospheric transformation associated with a class of traveling convective storms observed. over Venezuela is described. A strong low-level cooling is observed, confined mostly to the subcloud layer, and associated with a deeper layer of drying and acceleration of the easterly flow. A density current model is used to stratify the storm travel speeds, peak surface gusts and the accelerated flow at low levels behind the storm, and to relate these to the low-level flow ahead of the storm. There is reasonable agreement between these atmospheric data and laboratory observations of density currents. The updraft and down-draft structure is discussed using an interesting sounding cross section and trajectories from a three-dimensional numerical simulation. It appears that two distinct downdrafts exist: one driven by precipitation within the cumulonimbus cell, and a second mesoscale feature which is dynamically driven, and associated with descent over the spreading cold outflow.

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Charles A. Knight and L. J. Miller

In attempting to use centimeter-wavelength radars to investigate the early stage of precipitation formation in clouds, “mantle echoes” are rediscovered and shown to come mostly from scattering by small-scale variations in refractive index, a Bragg kind of scattering mechanism. This limits the usefulness of single-wavelength radar for studies of hydrometeor growth, according to data on summer cumulus clouds in North Dakota, Hawaii, and Florida, to values of reflectivity factor above about 10 dBZ e with 10-cm radar, 0 dBZ e with 5-cm radar, and −10 dBZ e with 3-cm radar. These are limits at or above which the backscattered radar signal from the kinds of clouds observed can be assumed to be almost entirely from hydrometeors or (rarely) other particulate material such as insects. Dual-wavelength radar data can provide the desired information about hydrometeors at very low reflectivity levels if assumptions can be made about the inhomogeneities responsible for the Bragg scattering.

The Bragg scattering signal itself probably will be a useful way to probe inhomogeneities one-half the radar wavelength in scale for studying cloud entrainment and mixing processes. However, this use is possible only before scattering from hydrometeors dominates the radar return.

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A. J. Miller and L. M. Leslie

Abstract

Forecast probabilities of rain were calculated up to 12 hours in advance using a Markov chain model applied to three-hourly observations from five major Australian cities. The four weather states chosen in this first study were three cloudiness states (0–2 oktas, 3–5 oktas and 6–8 oktas) and a rain state. Second-order Markov models with time-of-day dependent transition probabilities were fitted after appropriate statistical testing.

Forecasts were made using transition probabilities for summer and winter seasons. The skill of the Markov chain forecast probabilities of rain was evaluated in terms of Brier scores using to years of independent data, and compared with forecasts based upon persistence and climatology. The skill of the Markov model forecasts appreciably exceeded that of persistence and climatology and a real time trial of the procedure is being planned.

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Patricia A. Miller and Michael J. Falls

Abstract

Results of radiometer temperature profile simulations are analyzed in order to examine the hypothesis that knowledge of temperature inversion parameters obtained from other instruments would substantially improve the accuracy of radiometric temperature profiles. Five variations of a statistical retrieval method are used to produce radiometric temperature profiles. These profiles are then compared with radiosonde data under both inversion and noninversion conditions. The best algorithm yields consistently better results than the traditional (pure radiometric) technique, but still fails to correctly reproduce the radiosonde inversions.

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A. J. Miller, J. K. Angell, and J. Korshover

Abstract

Comparison is made between the horizontal wave energy flux at 32.5N for the atmospheric layers 700–500 mb and 300–200 mb, as computed by the approximation of Eliassen and Palm, and the observations of Oort and Rasmusson. It is observed that although the two estimates have similar annual profiles, the values for any particular month may differ by a factor of two. In addition, our computations indicate an inter-year variability of magnitude ±15% about the mean value that may be important in consideration of the energy budget of the sub-tropics and mid-latitudes.

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A. J. Miller, J. K. Angell, and J. Korshover

Abstract

Evidence is presented for an approximate year-to-year variation of the zonal available potential (AZ), eddy available potential (AE), zonal kinetic (KZ) and eddy kinetic energy (KE) in the extra-tropical troposphere between 1962 and 1971. A modification of the oscillations occurs after 1968 due to as yet undetermined causes, but prior to this there is a fairly significant correlation between these oscillations and the quasi-biennial zonal wind variation in the low tropical stratosphere. The results indicate a relative in-phase relationship between the AZ, KZ and AE, KE terms and a relative out-of-phase relationship between the AZ, AE and KZ, KE terms.

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A. J. Miller, J. K. Angell, and J. Korshover

Abstract

By means of spectrum analysis of 11 years of lower stratospheric daily winds and temperatures at Balboa, Ascension and Canton-Singapore, evidence is presented supporting the existence of two principal wave modes with periods of about 11–17 days (Kelvin waves) and about 4–5 days (mixed Rossby-gravity waves). The structure of the two wave modes, as well as the vertical eddy momentum flux by the waves, is shown to be related to the quasi-biennial cycle, although for the mixed Rossby-gravity waves this is obvious only at Ascension. In addition, the Coriolis term, suggested as a source of vertical easterly momentum flux for the mixed Rossby-gravity waves, is investigated avid found to be of the same magnitude as the vertical eddy flux term. Finally, we have examined the mean meridional motion and the meridional eddy momentum flux for its possible association with the quasi-biennial variation.

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