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Philip R. A. Brown

Abstract

A technique for the measurement of the ice water content (IWC) of cirrus clouds is described. The IWC is obtained by the measurement of the total water content (TWC) and the subtraction of the saturation specific humidity with respect to ice at the ambient pressure and temperature. The method is independent of the measurement of the crystal size spectrum and also of any assumptions about the bulk densities of various crystal habits. Examples of IWC measurements made during the International Cirrus Experiment are presented and compared with conventional measurements from a 2D optical array probe. The prime sources of error are the accuracy of the calibration of the TWC probe and the occurrence of subsaturated air, which invalidates one of the main principles of the technique.

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Philip R. A. Brown

Abstract

The use of the holographic cloud particle imaging system developed by the Cloud Physics Branch of the Meteorological Office and carried on the C-130 Hercules aircraft of the Meteorological Research Flight (MRF) has hitherto been limited by the extremely labor intensive data extraction process. A new image reconstruction system has now been developed that enables numerous holograms from a single flight to be analyzed. A brief description of this system is given, and some of its uses and limitations are demonstrated by examples of both droplet and ice-crystal data. In each case, the holographic data are compared with those from what are now conventional cloud microphysical probes, principally the ASSP and 2-D Optical Array Probe. Results show that the holographic system can measure gross features of the droplet size spectrum in conditions when the ASSP data may be unreliable. Ice crystal measurements confirm the ability of the holographic technique to produce data down to sizes of about 60 μm, well below the practical limit for the 2-D Cloud probe. Holographic ice concentrations appear to be systematically larger than those from the 2-D, typically by a factor of about half an order of magnitude. Some possible sources of error in each system have been examined but the exact cause of the discrepancy remains unproven. The relative unambiguity of the holographic sample volume suggests that this system will give the most reliable results, particularly for columns.

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R. A. Brown

Abstract

The neutral Ekman boundary layer is known to be dynamically unstable to infinitesimal perturbations under typical geophysical conditions. This paper discusses this instability to two-dimensional, simple-harmonic perturbations, for the stratified Ekman layer.

While viscosity and Coriolis forces are generally important in setting up the basic mean profile, the inflection point instability can be investigated in the inviscid, non-rotating system limit. However, the singular nature of the resulting second-order characteristic equation makes it necessary to solve the non-singular sixth-order, viscous stratified equation. Since typically occurring Reynolds numbers are much larger than critical, emphasis has been placed on investigating the behavior of maximum growth rates versus stratification for large Re. The appropriate dimensionless parameters are found to be: ξ=(2/Ro Re)½, and Ra= gSδ4/KmmKh [where δ=(2K/ f)½, Re= V gδ/K m, Ro=Vg/fδ and S=( z+g/ cp)/] for the general case, or ξ and RI=gS/V z for the inviscid case.

Unstable stratification shifts maximum growth rates toward a longitudinal orientation and shorter wave-lengths from the neutral stratification values of leftward orientation angle, ε=17°, and wavenumber, α=0.5. The local Richardson number at the inflection point is found to he the pertinent parameter for the effects of stratification. This instability is damped completely for values of Rii>0.25. Unstable stratification tends to support the dynamic instability such that the growth rate for this mode is dominant significantly into the convective instability regime.

The instability takes the form of counter-rotating circular motions which remain qualitatively similar for a wide range of the basic variables.

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R. A. Brown

Abstract

The equations of motion for a neutrally buoyant fluid are solved to produce an equilibrium flow consisting of a modified Ekman spiral mean flow plus a helical secondary flow. By considering the secondary flow to be a finite perturbation on a mean large-scale flow, approximate equations are obtained for the secondary flow and the modified mean flow an functions of the large-scale parameters. When the energetics of this system are considered, an equilibrium magnitude for the secondary flow can be obtained.

The finite perturbations are assumed to preserve the structure of the infinitesimal perturbation solutions for the dynamic instability of the Ekman boundary layer. In particular, helical rolls occur as finite perturbation solutions. These finite disturbances are found to alter the mean Ekman velocity profile such that it becomes stable. The rolls, with characteristic depths of 5–7 times the Ekman characteristic length and corresponding wavelengths of 4π times this parameter, may be frequent occurrences in both atmospheric and oceanic boundary layers.

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A. R. Brown and N. Wood

Abstract

Numerical simulations are used to investigate the impact on the stable boundary layer of moderate topography (with hill heights in some cases comparable to the undisturbed boundary layer depth). Area-averaged properties of the resulting boundary layers, which are often highly inhomogeneous, are diagnosed. The presence of the hills leads to enhanced turbulence and drag, and a deepening of the area-averaged boundary layers (over and above that due to a simple displacement effect). The ability of well-established formulas for the depth of the boundary layer over homogeneous terrain to predict this deepening is investigated. Finally, the implications of the results for the use in large-scale weather and climate prediction models of effective roughness length parameterizations of the effects of hills are discussed. While not capturing some of the more detailed effects, the simplest approach of using a roughness length independent of stability is found to perform reasonably well in predicting the total surface drag.

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R. A. Brown and Lixin Zeng

Abstract

A method of determining surface pressures in oceanic storm systems using ERS-1 scatterometer data is employed to determine the lowest pressures in 25 storms. This method uses the surface winds as a lower boundary condition on a planetary boundary layer model to determine gradient winds and, thereby, pressure gradients. An optimization scheme referenced to a pressure outside the storm provides a pressure field and an estimate of the low pressure. The values are compared to ECMWF analyses in each case; there is good agreement, with some expected differences.

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K. A. Browning and R. Wexler

Abstract

A technique is proposed for the measurement of kinematic properties of a wind field in situations of widespread precipitation, using a single Doppler radar to sense the motion of the precipitation particles. The technique is an extension of ideas put forward by Probert-Jones, Lhermitte, Atlas, Caton and Harrold, and is based upon the Velocity-Azimuth Display (VAD) obtained by scanning the radar beam about a vertical axis at a fixed elevation angle. Harmonic analysis of the VAD permits divergence to be obtained from the magnitude of the “zeroth” harmonic, wind speed and direction to be obtained from the amplitude and phase of the first harmonic, and resultant deformation and the axis of dilatation to be obtained from the amplitude and phase of the second harmonic. Although limitations to the accuracy of this technique are imposed by inhomogeneities in the horizontal distribution of precipitation fall speed and, in the presence of strong vertical wind shear, by elevation angle errors and reflectivity inhomogeneities, the errors resulting from these effects can be made acceptably small by scanning at appropriate elevation angles and ranges. An optimum scanning procedure is suggested. A short case study is also presented to support the view that meaningful estimates of mesoscale divergence and deformation can be obtained using this technique.

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C. Derksen, R. Brown, and A. Walker

Abstract

A detailed evaluation of snow water equivalent (SWE) and snow cover extent (SCE) derived using the combined Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave Imager (SSM/I) brightness temperature records for the 1978–2002 period was carried out for a longitudinal transect in the continental interior of North America. Comparison with in situ SWE observations showed that the SMMR brightness temperature adjustments are required to produce SWE retrievals with similar bias and rmse as observed during the SSM/I period. Underestimation of SCE in the passive microwave dataset (relative to NOAA snow charts) was identified as a systematic problem, most pronounced in early winter and during seasons with above-average snow extent. The passive microwave data were successfully merged with historical data based on strong interdataset agreement for a 1978–92 overlap period. Analysis of SWE and SCE time series for the months of December through March 1915–2002 provided information on SWE and SCE variability over the interior of North America, but yielded no evidence of significant trends in either variable over the 88-yr period. Anomalies observed during the relatively recent period of passive microwave data acquisition did not exceed the range of anomalies observed in the historical data record.

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H. E. BROWN and R. A. BRINTZENHOFE

Abstract

No Abstract Available.

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D. P. Brown and R. A. Harvey

An integrator, which automatically and periodically records the weighted average solar radiation from the previous period, has been developed. The instrument is used to obtain a measure of the average solar radiant energy per day. This measurement was previously obtained by manual—and often tedious—graphical analysis.

Theoretical considerations involving averaging by passive networks are presented with experimental results which show the accuracy of the instrument.

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