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EDWARD M. BROOKS

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Ian M. Brooks

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Several recent studies have utilized a Haar wavelet covariance transform to provide automated detection of the boundary layer top from lidar backscatter profiles by locating the maximum in the covariance profiles. This approach is effective where the vertical gradient in the backscatter is small within and above the boundary layer, and where the inversion is sharp and well defined. These near-ideal conditions are often not met, particularly under stable stratification where the inversion may be deep and is sometimes ill defined, and vertical gradients are common. Here the effects of vertical gradients and inversion depth on the covariance transform are examined. It is found that a significant dilation-dependent bias in the determination of the boundary layer top may result when using the published method. An alternative approach is developed utilizing multiple wavelet dilations, and is capable of identifying both the upper and lower limits of the backscatter transition zone associated with the inversion while remaining insensitive to mean vertical gradients in the background signal. This approach enables more detailed information on the small-scale structure of the inversion and entrainment zone to be retrieved than is possible using existing techniques.

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Edward M. Brooks
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Edward M. Brooks

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The purpose of this article is to introduce a substantial short cut in finding the wind flow in an isentropic surface. This is done by utilizing the fact that, at a given level in the isentropic surface, the acceleration potential is uniquely determined by the pressure. From new tables of values for every 5000 feet, values for any level can be quickly obtained by easy interpolation.

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Edward M. Brooks

In the normal use of pilot-balloon reports as an aid to drawing upper-air charts, these reports are usually assumed to represent the mean air flow undisturbed by turbulence. This assumption needs to be justified if the pibals are used in the preparation of wind convergence charts.

The failure of the pibal to represent the mean undisturbed wind is due to (1) inaccuracy and (2) unrepresentativeness of the pibal observation. Most of the inaccuracy arises from incorrectly prescribed heights of the balloon based on a constant rate of ascent. In the cases studied, the average error (Table I) was 4.1° in direction and 3.3 miles/hour in speed. The unrepresentativeness due to turbulence averaged 1.3° and 2.5 miles/hour. The total inaccuracy and unrepresentativeness together averaged 5.6° and 4.1 miles/hour.

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Ian M. Brooks

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A method for determining the angular offsets between measurement axes for multiple motion sensing systems and a sonic anemometer using underway data is demonstrated. This enables a single angular rate sensor to be used with spatially separated accelerometers, collocated with sonic anemometers, for the motion correction of turbulence measurements on a mobile platform such as a ship. Effective motion correction of turbulence measurements at sea is demonstrated. The errors in instrument alignment are considered, and estimates are made of the resulting biases in wind stress estimates.

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S. E. Reynolds and M. Brook

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The time of onset of the initial electrification in a thunderstorm cell has been correlated with the appearance of the initial radar (3 cm) precipitation-echo. The results show that precipitation is a necessary, but not sufficient, condition for the onset of thunderstorm electrification. The presence of radar-detectable precipitation does not lead to thunderstorm electrification, unless the precipitation echo evidences rapid vertical development. When this condition is fulfilled, the appearance of the initial electrification is almost coincident with the appearance of the initial radar precipitation-echo. On days when no precipitation echoes were present, no electric fields significantly different from the fair-weather positive fields were observed, although the clouds noted ranged from small fair-weather cumulus to clouds of considerable depth and active convection.

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Ian M. Brooks and David P. Rogers

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Large-scale horizontal rolls can have a significant influence on turbulent transport across the atmospheric boundary layer. The formation and maintenance of such rolls is dependent on the thermal and dynamic stability of the boundary layer (BL). The authors present aircraft observations of boundary layers, both with and without roll circulations, off the coast of California. The contribution of the rolls to the turbulent fluxes of heat, moisture, and momentum, and the variances of the three velocity components are determined for four cases. The fractional roll contributions to the u and w variances, and the sensible heat and along-wind momentum fluxes, show a near linear increase with altitude, from less than 10% at 30 m to more than 70% at the top of the BL. The variance in υ and crosswind momentum flux are more scattered, although the variance shows a slight increase with altitude from about 40% to 60%. The latent heat flux also shows a great deal of scatter, especially in the lower third of the BL where the total flux is small; above this, values range between about 40% and 85% but show no clear trends. A stability parameter in the form of a bulk Richardson Ri number is calculated for each of 13 profiles through the boundary layer; it is found that the Richardson number successfully identifies those cases where rolls are present, and its value corresponds to some extent with the strength of the rolls. Values close to zero correspond to cases with well-defined rolls; for 0.1 < Ri < 0.25 rolls are found to exist, but they tend to be weak and patchy; and no rolls are found where Ri is greater than the critical value of approximately 0.25. Reynolds numbers are calculated from a number of different definitions and indicate the dynamic instability of the shear dominated boundary layers.

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C. E. P. BROOKS

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EDITH M. FITTON and CHARLES F. BROOKS

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